A Quick Guide to Preventing Efflorescence in Your Concrete Countertops


Severe efflorescence in concrete

Efflorescence. It’s the whitish powdery material that forms on the surfaces of masonry or concrete construction, and also it’s the white blush that can form on sealed concrete floors or concrete countertops. While it poses no threat structurally, efflorescence is an aesthetic nuisance that affects both interior and exterior concrete. This article discusses why efflorescence occurs, how it can be prevented and how to deal with it if it does happen.

There are two kinds of efflorescence: primary and secondary. Primary efflorescence occurs when concrete bleedwater dries on the surface. Secondary efflorescence occurs when soluble mineral salts are leached out of cured concrete.

Efflorescence “growing” on the inside of a concrete block wall.

Primary Efflorescence

Eliminating primary efflorescence begins before the concrete is cast, simply by using basic good concreting practices: Start with a concrete mix that uses well-graded aggregates, a low water-to-cement (w/c) ratio, and fly ash or other pozzolan as a partial cement replacement; use a water reducer to increase workability without adding extra water to the mix.

Extra water in the concrete makes it more porous, weaker, and more susceptible to shrinkage cracking. The extra water is an unwanted internal reservoir that can leach the salts out of the concrete. Concrete made with a w/c of 0.45 or less will produce a relatively strong, dense mix that’s unlikely to have excessive bleedwater.

Making good concrete is just the first step. It does no good to have a well-designed, low w/c ratio concrete if it’s not cured properly. Wet curing under burlap, plastic sheeting or curing blankets allows the concrete to gain strength and density during the critical early days after casting. Well-cured concrete inhibits water movement, and this is one important step to controlling primary and secondary efflorescence. If the concrete doesn’t allow moisture movement, the salts deep within the concrete can’t be leached out.

efflorescence on a polished concrete big-box store floor.

Secondary Efflorescence

All masonry and concrete materials are susceptible to secondary efflorescence, including concrete countertops. Secondary efflorescence is most often caused by moisture or water vapor migrating through a concrete slab, bringing soluble salts to the surface of the concrete. The amount and character of the deposits vary according to the nature of the soluble materials and the atmospheric conditions.

Concrete contains a variety of soluble mineral salts, both from the cement and from admixtures like calcium chloride, and even from chemicals applied to the concrete after it has hardened. It’s those salts that are the “seeds” of efflorescence.

While all concrete has some soluble salts in it, not all concrete will effloresce. Efflorescence will occur only if all of the following conditions exist within the concrete:

  • The concrete must have soluble mineral salts within it.
  • There must be moisture to dissolve the soluble salts.
  • Evaporation or hydrostatic pressure must cause the mineral salt solution to move towards the concrete surface.

If any one of these conditions is eliminated, efflorescence will not occur.

Controlling secondary efflorescence is a more common problem for contractors who have “inherited” pre-existing concrete. The mix itself can’t be changed, so factors that affect water movement into and out of the concrete are where steps can be taken. Identifying and minimizing the sources of moisture are the first step. Reducing the porosity of the concrete to prevent the soluble salts from being leached out is the second step.

This second step to controlling efflorescence is to apply chemical hardeners, also known as densifiers, to existing concrete. These make the matrix less porous by generating calcium silicate hydrates that plug the pores and clog the capillaries.

Curious how repairing efflorescence plays out in a real-world scenario? Read this blog from our backlog that details an example of how to repair a concrete floor with efflorescence that was progressively worsening.

6 Problems with Concrete Countertop Mix Designs and How to Prevent Them

Murphy’s Law states, “Anything that can go wrong will go wrong.” This doesn’t have to be the case with concrete countertops. With some basic knowledge, you can prevent these basic problems with concrete countertop mix designs.

1. Air Bubbles (Pinholes)

Underside of a wet cast ramp sink. The large holes are where air bubbles got trapped under the top cap of the mold.

All concrete will have air trapped in the mix due to the mixing process. Fine aggregates and sand tend to trap air bubbles, and a stiff cement paste won’t allow the air to rise and escape.

The only way to cast traditional concrete so there are no air bubbles (large or small) on the surface is to design the mix so that the fresh concrete is very fluid or can be made very fluid by vibration. Air cannot be made to disappear, dissolve or not get entrapped by adding an admixture. Only a very fluid concrete will allow the air bubbles to push their way through the concrete and rise to the surface.

Often the cast surface will be hole-free but any significant grinding will reveal tiny pinholes just below the surface. Large air bubbles have enough buoyant force to push aside the aggregate and escape, while the smallest bubbles get left behind because they are too small and not buoyant enough to push their way through the concrete.

Defoamers work by preventing stable air bubbles from forming during mixing. Defoamers don’t make air disappear, rather they reduce the cement paste’s surface tension characteristics so bubbles are harder to form. The concrete still needs to be fluid enough to allow the bubbles to escape.

It is often nearly impossible to completely eliminate pinholes. In this case, you can fill them in with a fine cement paste called grout. (This is sometimes called slurry, incorrectly. Slurry is the dirty water produced by wet grinding concrete.)

For a detailed grouting procedure, see this article.

Pinholes in GFRC

When working with GFRC, the energy of spraying the mist coat lays down a thin veneer with no air bubbles. It is still important to use defoamer with liquid GFRC polymers because denser GFRC is stronger GFRC.

It is also possible to direct cast GFRC. This is essentially casting a fluid backer without mist coat. With this technique, it’s possible for fibers to show and for trapped air to cause pinholes to show. The shape of the piece dictates whether you can get a good casting – air rises, so horizontal surfaces may be fine, but vertical surfaces probably will have some exposed air voids. This is also true for traditional wet cast concrete.

2. Curling

Thin concrete beams that have curled (the middle is higher than the ends)

“Concrete mixes that are shrinkage prone will curl more than mixes that are shrinkage resistant.”

Curling is caused by poor curing and storage conditions. When one side of a concrete slab is allowed to dry out while the other side remains moist (or is moister), the concrete will tend to shrink towards the dry side. Prolonged moist curing, and storing the slabs so both sides get even airflow, can control curling.

As a general rule, traditional concrete should moist cure for a minimum of 5-7 days before being allowed to slowly and evenly dry out. However, most concrete countertop artisans use high-performance mixes that allow for a cure time of 1-2 days before proceeding with processing and sealing.

GFRC does not need to be moist cured. GFRC uses a polymer that acts as an internal barrier to keep moisture inside, and that is what achieves “moist curing” over 5-7 days. GFRC should still be kept evenly moist/dry on both sides.

Concrete mixes that are shrinkage prone will curl more than mixes that are shrinkage resistant. Good aggregate gradation, lower cement contents, and low water-cement ratios are the key to making shrinkage resistant concrete. In addition, shrinkage reducing admixtures (SRA) can manage or reduce shrinkage and curling. Some shrinkage reducing admixtures increase the mix water demand and may reduce the strength of the concrete.

For GFRC, the base mix design is extremely shrinkage prone because it is so cement rich. Furthermore, GFRC tends to be used for large, thin slabs which exacerbates the tendency to curl. Make sure that GFRC slabs are well supported so they don’t sag, and ensure there is good airflow under the slab so that moisture doesn’t build up and create differential shrinkage.

3. Hairline Cracks

Hairline cracks (and all cracks for that matter) form when tension forces in the concrete exceed the tensile strength of the concrete. Tension forces in the concrete can be generated from shrinkage, heat or deflection.

Generally, hairline cracks are very narrow and represent a stress-relief response to the excessive tension force. Larger cracks tend to be caused by bending forces that generate large deflections that open up the cracks.

Most hairline cracks form over time as the concrete dries out and shrinkage stresses build up. Keys to managing shrinkage involve good curing, good mix design and possibly the use of SRA’s; see Curling, above.

Heat can cause micro-cracking where the concrete develops micro-hairline spider web or map cracks. Often these can only be seen if water or other liquid is applied to the affected concrete. The use of PVA or AR glass fibers, good curing, and good aggregate gradations can minimize the effects of intense heat. Limiting the temperature and/or duration of heating will greatly reduce the likelihood of thermal cracking.

Bending, due to uneven supports or excessive loading, can cause hairline cracks. If the concrete just cracks but does not open up, and if the load that caused the crack is removed, then the resulting crack is often called a hairline crack. If the load is high enough or sustained, and movement is allowed, then the crack can open up. This is what is called a structural crack.

4. Harsh or Stiff Mix

A very harsh and dry concrete mix

Harsh mixes usually have too much large aggregate or very rough, angular aggregate. This is an issue only with traditional aggregate-based concrete mixes.

Simply adding more cement to the concrete mix can help, but too much cement can cause excessive shrinkage. The better solution is to regrade the aggregates to allow for more fine aggregate. Often blending coarse and rounded aggregate will help with a harsh mix.

Sometimes concrete has low workability or is very stiff because it has a very low water/cement ratio. The common solution to stiff concrete is to add extra water to make it more flowable. This is a very poor solution to a common problem. Cracking, shrinkage, low strength and high porosity often result. This describes common sidewalk concrete.

The better solution is to add water reducer. Concrete countertop mixes typically use high range water reducers that are polycarboxylate based, called “superplasticizers”. Here is a video of how well superplasticizers work:


The equivalent superplasticizer to the one used in the video is WR310. I generally prefer working with a liquid superplasticizer such as WR420.

5. Segregation

“Sometimes a good concrete mix can segregate when too much superplasticizer is used.”

Ideally, all ingredients in your mix are evenly distributed. Segregation is when gravity causes the heavy ingredients such as aggregate/sand to settle, and fluid cement paste and water rise to the top.

Highly fluid concrete mixes tend to segregate if the cement paste viscosity is not stabilized. Segregation occurs when the cement paste is too fluid and not viscous enough to support and suspend the larger aggregate. What happens is that the large aggregate sinks to the bottom of the forms and the pure cement paste forms a runny, scummy layer on top of the concrete. The fluid is not water; rather it is the highly fluid cement paste that has separated from the aggregates due to gravity.

Two common solutions can solve segregation. One is to use a viscosity-modifying admixture (VMA). It’s a stabilizer and thickener.

Another solution is to increase the very fine aggregate content of the concrete. Typically fly ash, microspheres or powdered stone is added, or during the mix design, some of the coarse aggregates are replaced with equal amounts of very fine material. Typically the fine material accounts for about 5% by volume of concrete.

Sometimes a good concrete mix can segregate when too much superplasticizer is used, or, the wrong superplasticizer is used in the attempt to create a highly fluid mix. Polycarboxylate superplasticizers have paste stabilization characteristics that other superplasticizers don’t, and the use of either VMA’s or extra fines can augment the stabilization.

6. Long Set Time/Low Early Strength

Several factors influence the set time for concrete. These include temperature, admixtures, and water content.

Temperature: Colder temperatures slow the hydration rate, increasing set time and strength development. In contrast, high temperatures sometimes speed the set time so much that you can’t work with the concrete fast enough. In this case I use ice as a substitute for some of the mix water. Both are measured by weight.

Admixtures: Chemical retarders, some synthetic pigments, some liquid pigments, low reactivity pozzolans used as cement replacements, and some water reducers can all slow or retard the setting rate of concrete.

Water content: High w/c ratios also slow the setting time somewhat.

These are a few of the issues that could occur with concrete countertop mixes. To understand more about precast concrete countertop mix design, please see the article “The best mix design for concrete countertops”. There is also a huge amount of information about GFRC and its mix design available here.

Concrete Countertop Mix Ingredients and Admixtures

In a previous article, I explained basic principles of concrete countertop mix design, such as the role of sand, cement, water, water-cement ratio and aggregate gradation. Click here to read that article and view the videos.

In this article, I provide details about two important specialty admixtures for concrete countertop mixes: superplasticizer and viscosity modifier. These admixtures work hand in hand to create the right mix consistency without compromising strength by adding water.


Superplasticizer (High Range Water Reducer) for Concrete Countertop Mixes

Polycarboxylate superplasticizers are a very powerful type of high range water reducer. This video shows how adding a small amount of a powerful water reducer, instead of water, can take a concrete mix from dry and crumbly to flowable.

Here is a video that demonstrates the dramatic effect superplasticizer has on mix consistency, then explains the science behind how it works.


The video uses a powdered superplasticizer, BASF’s Melflux 2651, in order to demonstrate that it is not the liquid in the superplasticizer that causes the increase in slump. Melflux 2651 is very difficult to find in reasonable quantities for a concrete countertop maker. Buddy Rhodes Concrete Products offers a similar powdered superplasticizer. However, powdered  superplasticizers are extremely powerful and very easy to overdose. For that reason, we recommend and sell a liquid superplasticizer, ADVA Cast 555.

Recommended Product

Water Reducer ADVA555ADVA Cast 555, manufactured by Grace and packaged by Buddy Rhodes Concrete Products, is a powerful high-range water-reducing admixture (superplasticizer) based on the next generation of polycarboxylate technology. ADVA Cast 555 is designed for use in precast, ready-mix and self-consolidating concrete (SCC) applications, and it is excellent for use in GFRC.ADVA Cast 555 provides excellent early compressive strength, faster setting strengths and fluidity in mix designs with substantially reduced water content.

Click Here for details of how to dose and use this superplasticizer, and to purchase it.


Viscosity Modifiying Admixture (VMA) for Concrete Countertop Mixes

The following seminar excerpt explains what viscosity modifying admixtures do. VMAs are used to reduce the slump of concrete, essentially performing the opposite of a high range water reducer (superplasticizer). They also help prevent segregation in aggregate-based mixes.


Recommended Product

Fritz-Pak’s Super Slump Buster is an easy to use, powdered viscosity modifying admixture (VMA).It is available in 8-oz bags, which is enough for dozens of average concrete countertop projects. VMA should not need to be used for every project, as explained below.

The manufacturer sells Super Slump Buster only in cases of 60 bags. We sell it by the bag.


Click Here for details of how to dose and use this VMA, and to purchase it.


Concrete Countertop Mix Design Principles

Here’s some information to help you understand important, fundamental principles of concrete countertop mix design that will help you be successful with make-your-own concrete countertop mixes. Learn about water-cement ratio, how admixtures work, and much more.


How Concrete Works Seminar – primary ingredients (18 minutes):



Admixtures Seminar – secondary ingredients (3 hours):

Did you like what you learned in just 18 minutes in the How Concrete Works seminar? Then click here to get FREE access to the 3-hour seminar “Everything You Ever Wanted to Know About Admixtures for Concrete Countertops”.


Mix Designs

We offer make-your-own mix designs for GFRC and precast, with calculators that remove all the math. These are strong, engineering-based mixes used for years by professionals all over the world. Our precast mixes achieve compressive strengths of over 4000 PSI (27 MPa) in 1 day, 6200 PSI (42 MPa) in 3 days, and 8200 PSI (56 MPa) in 7 days.

Click here to view our available mix designs.

mix calculator



Dry Polymers versus Wet Polymers for GFRC: A detailed analysis and recommendation

NOTE: This article has been modified from its originally published version. The manufacturer of one of the products reviewed made threats of legal action to both to CCI and to another manufacturer named in this report. While we stand behind our test results, and we strongly believe that comparative studies such as this are a helpful and essential service to the whole concrete countertop industry and can only further everyone’s success, we would prefer to spend our time conducting more studies, not fighting a legal battle. Furthermore, note that none of the manufacturers knew that this study was being conducted, and that CCI spent several months and thousands of dollars doing it. We deeply apologize to those who are being denied access to the relevant, useful and truthful information that we worked so hard on.

GFRC is a highly specialized form of concrete designed and optimized for making large, thin panels and lightweight 3D objects. The key property of GFRC that makes this possible is its high flexural (bending) strength. Unlike conventionally-reinforced concrete where compressive strength is important, it is the bending strength of GFRC that is all-important. Not only is ultra-high compressive strength irrelevant, ultra-high compressive strength concrete is well known for being brittle, a characteristic opposite to what makes high-quality GFRC.

Typically GFRC needs to remain internally moist for at least 7 days in order to achieve adequate strengths. Premature drying will slow or halt curing, leaving the concrete soft, porous and weak. Adequate curing can be achieved via traditional wet curing, where the concrete is kept in a 100% humid environment for 7 continuous days. This is impractical for most applications, so instead a polymer curing admixture is used.

The polymer curing admixture’s primary and all-important purpose is to maintain the concrete’s internal moisture levels so that the GFRC’s Portland cement paste continues to hydrate (cure) even when the piece sits in the open air. The polymer does this by essentially forming an internal curing membrane, slowing moisture loss.

As with all concrete, curing is vital to achieving the desired physical properties. Flexural (and compressive) strength, stiffness, porosity and mechanical toughness are all dependent upon the cement paste remaining moist so it can continue to hydrate. We call this curing, and the longer concrete cures the better it gets.

In the United States, the Precast/Prestressed Concrete Institute (PCI) has guidelines and specifications for polymer curing admixtures. The PCI’s Manual for Quality Control for Plants and Production of Glass Fiber Reinforced Concrete Products, 2nd Edition MNL 130-09, Appendix G: “Specification for Polymer Curing Admixture”, has property and performance requirements for polymer curing admixtures. These requirements are to ensure GFRC product quality, and to ensure that the curing compounds are tested by an independent laboratory to demonstrate that:

  1. The recommended quantity of polymer curing admixture in GFRC mix with no moist curing equals flexural properties of GFRC cured 7 days moist when both are tested at 28 days.
  2. The long-term durability of the dry-cured polymer admixture modified composite, verified by aging tests, is equal or greater than the durability of GFRC cured 7 days moist.
  3. The unit weight (density) of a mix design incorporating polymer curing admixture is greater than 120 pcf (1930 kg/m3).
  4. The polymer exhibits durability, ultraviolet stability, and oxidation resistance and stability in a high-alkaline environment.

Essentially, these requirements ensure that the polymer is subjected to independent testing that proves that it is an acceptable substitute for 7 day wet curing.

Some key property requirements mandated by the PCI specification are:

  • Aqueous thermoplastic co-polymer dispersion (water-based liquid polymer)
  • Acrylic-based
  • 45% to 55% solids by weight

The PCI performance and property requirements are very similar or the same as those outlined in the International Glassfibre Reinforced Concrete Association (GRCA) Specification, 4th Edition, Table 2.

The effectiveness of the polymer curing admixture (hereafter referred to simply as “polymer”) at retaining the concrete’s internal moisture level is very important in the early life of GFRC because unless it remains moist enough, the concrete will be weak, it will show crazing and cracking and it will be more porous. Additionally, commercial GFRC polymers have been shown to prevent detrimental aging effects in outdoor GFRC. Without polymer, GFRC tends to become more brittle and weaker over time, and extensive testing (accelerated and real-time aging over many years) has shown the benefits of polymer in GFRC. Some of the main uses for GFRC in the commercial world are for large exterior building panels, where long-term strength and durability are vital to ensure product longevity and the public’s safety.

The two GFRC polymers widely used in the North American commercial GFRC industry are Forton VF-774 and Polyplex. These two polymers comply with the PCI standard.

Over the past several years, the use of GFRC in small scale architectural concrete (such as concrete countertops, furniture, etc.) has grown in popularity, primarily because of its versatility, strength, durability and relative ease of manufacture. The simplicity of forming, the ability to create complex three-dimensional pieces with relative ease, and the significant reduction in overall weight have made GFRC the material of choice for many artisans.

One of the keys to being successful using GFRC is using the right ingredients and understanding their purpose and function. The polymer plays an essential role to achieving the strength and durability expected from GFRC. Using the wrong polymer, or using a polymer incorrectly, can result in inferior concrete that is weak or exhibits cracking or curling.

In addition to a carefully tailored chemistry designed to aid curing and preserve long-term flexural strength, GFRC polymers also contain defoamers and shrinkage reducing additive. These additives improve the strength of the material by reducing trapped air and by eliminating micro cracks formed by drying. These, and the specialized chemistry of the polymer itself, are what sets GFRC polymers apart from the myriad “polymers” used in the concrete industry. Two common uses for other kinds of polymers are surface bonding agents and polymers used in overlays and micro-toppings. The specific formations of those polymers favor surface adhesion over other characteristics, and it’s these formulation differences that make these unsuitable for GFRC. Just because it’s a milky white liquid (or a white powder) doesn’t mean it’s good to use in GFRC.

Recently dry powdered polymers (versus liquid polymers) have become favored in our industry, claiming equal effectiveness, simplicity of use and the cost-saving from not having to ship heavy water-based liquids.

Equal effectiveness and cost savings are addressed later in this article. To address ease of use, consider the following.

Dosing a liquid polymer requires knowledge of the polymer solids content of the bulk liquid polymer as well as skill at calculating how much of the polymer’s water counts as batch water. For instance, Forton VF-774 is a liquid polymer curing admixture and contains 51% solids, and thus is 49% water.

Dry polymers are 100% solids, so batch water calculations are simpler.

To eliminate all math and make using liquid polymer easy, CCI’s GFRC mix calculator automatically adjusts batch water amounts for any liquid polymer, eliminating any perceived complication from dosing liquids. We also now offer a calculator that can also handle the use of dry polymers with equal accuracy and ease.

Liquid versus Dry Polymer Test Overview

CCI performed extensive, detailed testing of PCI requirement #1: The recommended quantity of polymer curing admixture in GFRC mix with no moist curing equals flexural properties of GFRC cured 7 days moist when both are tested at 28 days. We also verified requirement #3. We did not test requirements #2 or #4, and to our knowledge no independent laboratory has performed testing of those requirements.

CCI tested three polymer systems: one liquid and two dry.

  1. Liquid polymer: Forton VF-774
  2. Dry polymer #1: CENSORED’s GFRC Admix
  3. Dry polymer #2: Buddy Rhode’s Concrete Products’ (BRCP) GFRC Admixture (and the BRCP GFRC Blended Mix)

Forton VF-774

Forton VF-774 is a liquid polymer curing admixture. It is an industry standard with a decades-long history of use in the commercial GFRC industry, and it is fully compliant with PCI MNL 130-09, Appendix G requirements. Forton has a 51% solids content and was used at a 5% and 6% polymer solids dosage rate.

Test samples made using Forton also used two different pozzolans which were used as partial cement replacements: Some of the samples used VCAS at a 20% replacement dose, and others used white silica fume at 10% replacement dose.

Dry Polymers

The two dry polymer systems differ in how they are formulated. Both claim to have a dry polymer curing admixture blended with other additives, such as a defoamer, shrinkage reducing admixture and wetting agents, plus other specialized additives unique to each company.

CENSORED’s admixture does not include a pozzolan as part of its formulation, so CENSORED’s GFRC mix design calls for a separate pozzolan. In addition to white silica fume, VCAS was also tested with the CENSORED admixture.

In contrast, the BRCP GFRC Admixture included a pozzolan pre-blended into the admixture. The CENSORED Admix was dosed at 3% by weight of dry cementitious material, while the BRCP GFRC Admixture was dosed at 14.63%. Note that the BRCP dose was much higher because it includes more ingredients, including white silica fume pozzolan.

All mix formulations used Federal White Type 1 white Portland cement, #30 silica blasting sand, and 19mm AR glass GFRC fibers, and all GFRC mix formulas complied with the manufacturers’ recommended proportions and dosing.

The basic mix designs for the three different polymer systems were:


  • 1 part sand, 1 part cementitious (Portland cement + VCAS pozzolan)
  • 20% VCAS dosed as a partial cement replacement
  • 5% (and 6%) Forton VF-774
  • W/C (water to cementitious) 0.32
  • 3% 19mm AR glass fiber dose

Forton/white silica fume

  • 1 part sand, 1 part cementitious (Portland cement + white silica fume pozzolan)
  • 10% white silica fume dosed as a partial cement replacement
  • 5% (and 6%) Forton VF-774
  • W/C (water to cementitious) 0.30
  • 3% 19mm AR glass fiber dose

Buddy Rhodes Concrete Products GFRC Admixture 0.84:1

  • 0.886 parts sand, 1 part Portland Cement (resulting in a 0.84 to 1 ratio of sand to all cementitious content due to the cementitious content of the GFRC admixture)
  • 14.63% BRCP GFRC Admixture
  • W/C (water to cementitious) 0.32
  • 3% 19mm AR glass fiber dose

Buddy Rhodes Concrete Products GFRC Admixture 1:1

  • 1 parts sand, 1 part Portland Cement
  • 14.63% BRCP GFRC Admixture
  • W/C (water to cementitious) 0.32
  • 3% 19mm AR glass fiber dose

CENSORED/white silica fume

  • 1 part sand, 1 part cementitious (Portland cement + white silica fume pozzolan)
  • 10% white silica fume dosed as a partial cement replacement
  • 3% CENSORED GFRC Admix
  • W/C (water to cementitious) 0.30
  • 3% 19mm AR glass fiber dose


  • 1 part sand, 1 part cementitious (Portland cement + VCAS pozzolan)
  • 20% VCAS dosed as a partial cement replacement
  • 3% CENSORED GFRC Admix
  • W/C (water to cementitious) 0.32
  • 3% 19mm AR glass fiber dose

Sample Preparation

All samples (except for SCC test series) were cast in two layers, with each layer being thoroughly compacted with a bubble buster roller.

SCC samples were made fluid, poured into the molds in one layer and gently shaken to level the mix. Care was taken to minimize disturbance or manipulation of the mix.

casting-test-panel-for-GFRC-flexural-testing-1 casting-test-panel-for-GFRC-flexural-testing-2

Casting test panels.

Two test panels were cast for each candidate mix design (called a test series). The samples were cast, cured under plastic overnight and then demolded the next day. Demolded samples were then allowed to air cure on racks that permitted free air-circulation around all sides of each sample. Prior to 28 day testing, the samples were flattened by grinding, cut into standard size coupons and soaked in water for 24 hours, in compliance with ASTM C-947 testing practices.

cutting-GFRC-samples-for-flexural testing


Cutting samples from test panel.

Flexural Testing

To investigate the strength and effectiveness of two dry and one liquid GFRC polymer systems, The Concrete Countertop Institute conducted an extensive array of flexural tests to determine whether the two dry polymers functioned as polymer curing admixtures. Additionally flexural tests on GFRC containing Forton VF-774 were tested as a control. CCI performed over 200 flexural tests following the guidelines of ASTM C-947, Standard Test Method for Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete, using our own flexural testing machine.


CCI’s flexural testing machine

To validate the testing performed at CCI, select identical samples were sent to an independent test lab in Oakland, CA. These select samples were cast from the same batch of concrete on the same day by the same person and the flexural tests were performed on the same day. The flexural test results from the 18 samples tested in NC and CA were statistically identical, verifying that the test results from the in-house testing at CCI were real and accurate.

Four-point bending tests measuring peak load at break were performed, yielding Modulus of Rupture (MOR) data, which is the peak flexural strength of the material measured at sample failure.


GFRC sample in 4-point bending. Note: applied load is 4 times greater than the digital readout.

The average MOR data for the three polymer curing admixture systems are shown below, along with data for similar GFRC made without any curing admixture and moist cured for 7 continuous days prior to air curing. All data shown are 28 day values.



Each colored bar in the chart above represents the average MOR for a particular test series, and each test series consisted of up to 12 individual flexural test samples cast from the same batch of concrete. The chart summarizes the results from over 200 individual flexural tests.

Variations in MOR for each test series are shown by the error bars which show one standard deviation of variation. Larger error bars represent greater MOR variation within a test series.

It was not uncommon to see MOR values that were a few hundred PSI different for adjacent samples cut from the same GFRC test panel. After careful analysis, we determined that the magnitude and variation of strength was mainly due to the presence and distribution of entrapped air bubbles within the GFRC matrix, and partially to the non-uniform orientation and distribution of the glass fibers within the samples. More entrapped air resulted in lower strength. This adds credence to the notion that denser GFRC is better GFRC. This is in alignment with PCI requirement #3 about unit weight (i.e. density) being greater than 120 pounds per cubic foot.




Variations in the amount of entrapped air in adjacent samples cut from the same test panel.

Each test series is labeled with a shorthand code that reveals details of the particular mix design tested. For example, the first red error bar is labeled “D1 3T, 10SF, 2.55% 19, 0.30”.

  • The “D1” stands for test D, the fourth test series, and the 1 refers to the first of the two test panels cast. (A 7 instead of a 1 refers to the second panel, since each panel had 6 flexural test coupons cut from it).
  • The polymer curing admixture and its dose are next.
  • In this example “3C” refers to a 3% dose of CENSORED’s GFRC Admix.
  • Forton is labeled “F”, and a 5 or 6 refers to a 5% or a 6% dose.
  • BR stands for the Buddy Rhodes Concrete Products GFRC Admixture; 3BR is a 3% dose, and 3+ refers to a dose slightly higher than 3%.
  • One of the tests used the BRCP Blended GFRC mix, which included sand and cement along with the admixture in a single bagged mix. That test is designated “BR Blended Mix”.
  • The BRCP GFRC mix design calls for a sand to cementitious ratio of about 0.84:1, whereas all other non-BRCP mixes used a 1:1 ratio. For direct comparison one BRCP test series was tested at a 1:1 sand:cementitious ratio. This is where the designations “3BR Admix 0.84:1” and “3BR Admix 1:1” come from. A 3% dose of the Admix was used, with various sand to cementitious ratios.
  • “0P” means no (0%) polymer curing admixture was used.
  • Pozzolan and dose are next, so “10SF” means that white silica fume was used at a 10% replacement dosage. “20V” means a 20% VCAS dose.
  • The fiber volume fraction and size are listed next, which were 19mm long AR glass fibers. Fiber volume fraction is defined as the weight of fibers divided by the total weight of concrete, including the weight of fibers plus all other mix ingredients, wet and dry. So “3% 19” means a 3% volume fraction of 19mm fibers was used.
  • The final number is the water to cementitious ratio. So “0.30” means the mix had a 0.30 w/c ratio. Note that all w/c ratios were between 0.30 and 0.32, following good concreting practices and eliminating a poor w/c ratio such as 0.40 as a confounding factor.
  • In addition, some of the tests have the “SCC” designation at the end, signifying that they were cast using a “self-consolidating concrete” method.

Conclusions from Flexural Testing

As seen in the chart above, all three polymer curing admixtures were able to achieve the average equivalent 28 day flexural strength of GFRC moist cured for 7 days: The average performance for each of the 3 polymers dry cured was roughly equivalent to the performance of no polymer wet cured for 7 days (shown by the gray bar).

Some mix parameters (SCC, or a different pozzolan for instance) yielded individual strengths lower or higher than the 7 day moist cure average strength, but each polymer curing admixture was capable of achieving that strength for some of the test series.

No particular polymer curing admixture was significantly superior or inferior in terms of the ability to achieve adequately high flexural strengths. All showed marked variation in strength despite being cast and compacted as identically as possible.

Cost Analysis of GFRC Ingredients

Polymer curing admixtures are not inexpensive, accounting for about 1/3rd (or more) of the material cost GFRC. A common reason cited for using dry polymers is the cost savings due to not shipping water.

To evaluate the actual cost of making GFRC using each of the three different polymers, it is necessary to look at the total cost of making a known amount of GFRC, rather than looking at the cost of obtaining only the bulk polymer curing admixture. This is because each of the three polymer systems use mix formulas which specify different recommended doses, water to cementitious ratios, pozzolans and pozzolan doses, and sand to cementitious ratios.

As previously mentioned, the main purpose of the polymer curing admixture is to maintain the GFRC’s internal moisture levels so that the concrete can cure properly and achieve the desired flexural strengths. Extensive flexural testing revealed that both Forton, the Buddy Rhodes GFRC Admixture and the CENSORED admixtures all performed as expected in terms of PCI requirement #1, when they also meet PCI requirement #3 for density.

Note that continuous wet curing for 7 days with no polymer achieves the same necessary flexural strength. The greatest cost savings would be achieved simply by not using polymer. However, wet curing for 7 days is not realistic for most creative concrete artisans, hence the need for polymer.

Shipping can greatly influence the cost for ingredients, and more importantly, the actual cost to produce GFRC. The influence that shipping has on cost can only be revealed when actual material use rates when making GFRC are taken into account. Simply comparing the cost to ship a bucket of one material versus a bag of another reveals nothing about what your actual costs are. Likewise, simply comparing the bulk price of one ingredient to another provides a skewed perspective on how costly (or inexpensive) it is to buy and use one ingredient versus another.

To reveal the actual cost involved in buying, shipping and using different polymer and pozzolan systems, typical amounts for each ingredient were selected that would be used to make GFRC backer using those ingredients.

The CCI GFRC mix calculator was used to calculate the amount of each ingredient and its individual cost (with and without shipping) needed to make 100 square feet of ¾” GFRC backer. Assumptions were:

  • The sand and white cement used in each of the mixes were locally sourced and are based on real pricing for Federal white cement and bagged #30 silica sand locally available in Raleigh, NC.
  • AR glass fibers, like the polymer and pozzolans, often must be shipped and are not often locally available.
  • Fibers are usually purchased and shipped from the same source as the polymer and pozzolan. The material and shipping costs for the fibers were included in the calculations but are not reported, only the cost differences between the polymers and pozzolans are shown below, since the same fibers were used in both mixes.

United Parcel Service (UPS), a very common shipping method, divides the lower 48 US states into multiple shipping zones. Zone 2 represents a short shipping distance where the delivery address is close to the shipper. Zone 8 represents the farthest distance between shipper and delivery address, equivalent to shipping across the country. Current (as of 3/2015) UPS ground shipping rates within the lower 48 US states were used in the calculations.

The following table shows the current online retail prices for the bulk materials, the combined price for each material including shipping, and the net cost ($/square foot) to make ¾” GFRC backer using those materials. Labor, tax and membership discounts are not included.


While the bulk material and shipping costs for each polymer system varies, the magnitude of the cost for making GFRC using any one of the three polymer systems is quite low compared to the retail price of the products made from GFRC. And the magnitude of the cost differences between any of the three polymer systems is small and relatively insignificant: the greatest cost difference is less than $0.75 per square foot. It is very easy to raise the prices of your GFRC creations by at least $0.75 per square foot to recoup any material (and shipping) costs!

Advantages and Disadvantages of Wet versus Dry

Advantages of Dry Polymer

  • No issue with cold-weather shipping and storage.

Liquid polymer can freeze. It is possible to thaw out and may be usable after one freeze, but precautions should be taken, including no-freeze shipping which increases cost.

  • Less room for error in mix calculations

Dry polymer is 100% solids. Liquid polymer is 45% – 55% solids, and it requires calculating part of its weight as mix water. Manual mix calculations are therefore more complex. However, note that the CCI GFRC Mix Calculator eliminates manual calculations.

  • Less time spent weighing out ingredients.

Because the dry polymers tested include defoamer and shrinkage reducing admixture (SRA), there is the potential for spending less time weighing ingredients, if defoamer and SRA are used.

Indeed, defoamer is an important addition to GFRC, because as demonstrated by the photos above showing that variations in the amount of entrapped air in adjacent samples cut from the same test panel significantly influenced flexural strength. Less entrapped air resulted in greater strength.

Because the Buddy Rhodes GFRC Admix includes pozzolan, it eliminates weighing out one more ingredient.

Because labor costs for producing artistic GFRC concrete creations are vastly higher than material costs, the time savings from weighing out fewer ingredients is a significant factor in our decision of which dry polymer to recommend.

Disadvantages of Dry Polymer

  • Not independently certified to meet PCI standards, nor proven by decades of use in extreme situations

The most important disadvantage is that no dry polymer has been thoroughly tested by an independent laboratory to meet all four of PCI’s requirements. The large-scale commercial GFRC industry that produces building panels and other products where public safety is a concern does not use dry polymers. Most commercial GFRC plants are certified by PCI, and often the specifications for the large projects they are bidding on require PCI certification.

In our much smaller scale industry that produces artistic products, performance of our concrete is typically not a life or death situation. However, without thoroughly understanding the product that you are using, you may use a product with success for years, and then when you push the boundaries and a failure occurs, you don’t know why. Further, if you become involved in a project that includes building panels or some other item that potentially could be a product liability issue, a problem occurs, and it is discovered that you are using methods and ingredients that do not meet PCI standards, there could be serious legal implications.


Our artisan industry is steeped in the home-brew tradition, where continuous change and apparent innovation prevail. However, new ingredients critical to the short and long term performance of the very material we build our businesses and reputations on should be selected and used only after very careful consideration and with clear, business-driven reasons.

CCI takes a conservative approach, preferring to stick with tried and true products and methods unless a clear problem with the existing products or methods calls for a solution. The only significant problem with wet polymer is the possibility of it freezing. This does present a real business problem for those in colder climates who must pay a significant extra cost for no-freeze shipping, and then still have the possibility of a freeze.

However, this shipping issue can be alleviated simply by planning ahead and stocking up before winter. Some sealers also must ship no-freeze in the winter months. Plus, as demonstrated in this article, the biggest reason for elevated material costs is not planning ahead. Forton has a shelf life of about one year. Stock up ahead of winter, and store it in a climate-controlled environment.

Our recommendation is to stick with wet polymers, unless you live in a cold climate or want to save time weighing out fewer ingredients. In that case, we recommend and sell the Buddy Rhodes GFRC Admixture, despite its slightly higher cost, because it is a single component system backed by the strength and reputation of the Buddy Rhodes Concrete Products (BRCP) company. Note that the tests and analysis were completed before we started selling the BRCP admix. (Note also that we made this recommendation before the other manufacturer threatened legal action.)

CCI has always taught from-scratch mixes, and this is the first time we have recommended any kind of pre-blended product. We have never maintained that bagged mixes or pre-blended products are bad, simply that learning from scratch provides an important, foundational understanding of concrete that is necessary to succeed with whatever mix you use. It also allows you the freedom to choose whatever mix you want based on your own personal preferences and needs for convenience, cost, and aesthetics. BRCP also offers from-scratch, raw ingredient options, as well as an array of mixes to meet various needs. While we have varying techniques and personalities, both CCI and BRCP are dedicated to furthering our industry on a platform of quality and integrity. (Again, we said this before the other manufacturer threatened legal action.)

Note: See also this further article with more detail about the testing procedure.

The Many Uses of Fibers in Concrete Countertops

Lately we’ve been talking a lot about GFRC and the alkali resistant glass fibers used in making this unique type of concrete. But, AR glass fibers aren’t the only type of fibers you can use in concrete mixes. Today I’d like to discuss some of the other types of fibers that you can use when making concrete countertops and some of their purposes. When used properly, fibers have many benefits in countertops. Fibers aren’t just for GFRC.

Fibers – Primary vs. Secondary Reinforcing

Fibers have many purposes in concrete countertops. They can be used for reinforcing (think GFRC) or can be used to prevent shrinkage and cracking. When fibers are used for structural reinforcing this is known as primary reinforcing. When they are used for shrinkage control it is known as secondary reinforcing.

Fibers can play a valuable role in both primary and secondary reinforcing. However, the type of fibers and the methods used will vary depending on which type of reinforcing you’re after. Many of the fibers listed below can provide both primary and secondary reinforcing benefits. While most fibers won’t replace steel reinforcing (like AR glass fibers do in GFRC), they can still make concrete stronger and help your concrete countertops to last longer and look better.

The Many Types of Fibers Used in Concrete Countertops

When I say there’s a lot of different fibers to choose from, I’m not kidding. In this post we’ll look at several of your options, but just briefly. I could easily create lengthy posts on the uses and benefits of each type of fiber listed here.

  • Polyvinyl Alcohol (PVA) Fibers – PVA fibers have some structural strength and can also be used for shrinkage control. While they cannot replace reinforcing steel, they improve the mechanical properties of cured concrete, boosting its strength. These are the best choice when the fibers cannot show at all.

PVA Fiber for Concrete Countertops

  • Alkali Resistant Glass Fibers – AR glass fibers are the type of fiber primarily used with GFRC. They can also be used to provide primary and secondary reinforcing in steel reinforced concrete countertops. These fibers are special glass fibers that won’t break down, even when in contact with alkaline concrete. The alkali resistance is achieved by using zirconia. AR glass fibers come in different sizes; the ones used for GFRC are long (13 and 19mm) and large, so they will show if they’re simply mixed in to ordinary concrete. Smaller fiber bundles of AR glass are less visible, but will still show if they happen to be right on the surface.
  • Polypropylene or Nylon Fibers – Polypropylene and nylon fibers are used for shrinkage control; they have no structural strength. These fibers play a valuable role during the curing process, but provide no benefit after. They simply stretch too much to provide any resistance to tensile stresses.
  • Cellulose Fibers – Most of the time synthetic fibers are used for secondary reinforcement, but cellulose fibers are a naturally occurring alternative.

Some fibers are strong and can provide adequate structural strength, but the material they’re made of doesn’t make them a good choice for concrete countertops.

  • Hooked Steel Fibers – Hooked steel fibers possess structural strength. They can help to distribute tensile stresses across the countertop. However they are large, ugly and will show.

hooked steel fibers

  • Chopped Carbon Fibers – Like hooked steel, PVA and AR glass fibers, chopped carbon fibers have stiffness and strengths equal to or greater than steel. Reinforcing is still needed, but the fibers provide a helpful boost of strength and minimize shrinkage during the curing process. But because they are black, carbon fibers will show definitely show in most concrete that’s not black or very dark.

Fibers and Shrinkage Control: How Does it Work?

Fibers thicken and stabilize the cement paste, giving it body. Fibers act like an internal three dimensional net, supporting the aggregate and minimizing settlement.

Aggregate settlement and consolidation are what drive the generation of bleedwater. For bleedwater to reach the surface, it has to form microchannels in the concrete. These microchannels form weak zones in the concrete. In addition, the walls of the microchannels can have much higher water/cement ratios than the concrete itself because of the dilution effect of the bleedwater.

All this results in weaker, more porous concrete that shows greater tendencies for cracking and shrinkage. Adding fibers boosts the properties of the concrete, mostly by preventing the detrimental effects of plastic shrinkage and bleedwater segregation.

Fibers can help reduce shrinkage and cracking.

Fibers can help reduce shrinkage and cracking.

Fibers can also help combat shrinkage by spreading the tensile loads across the concrete. Like I mentioned previously the fibers act as a net, in this case holding small cracks together and transferring stresses across cracks into adjacent concrete. This helps keep any cracks that appear small, often too small to even see. Rather than having one or two large, highly visible cracks, you’re left with a series of small, hard to see cracks spread across the slab. Cracking might be inevitable, but an almost invisible crack is always better than a big one.

One last point: the amount of fibers needed to provide good secondary reinforcement to control micro-cracks is rather low. Typical volume fraction doses range from 0.1% to 0.5%; this translates to 4 lb to 20 lbs of fibers in a cubic yard (4000 lbs) of concrete.

The fiber dose necessary to make GFRC strong is much higher. Primary reinforcing has to resist much more stress and deflection, which is why so many more fibers are needed. The typical (and minimum) GFRC volume fraction dose is 3%, equivalent to 120 lbs of fibers in a cubic yard (4000 lbs) of concrete

Creating the perfect mix for each concrete countertop requires careful consideration of the type of countertop you need to create. I hope this article has helped you to see some of the ways fibers can enhance your mix design.

Can I Use Non-Shrink Precision Grout to Create Concrete Countertops?

Don’t even think about using non-shrink precision grout to create concrete countertops. While its properties may make it seem like an ideal choice at first glance, avoid the temptation to use it. I’ll explain why…

One reason that people are tempted to use non-shrink precision grout (also called NSPG) on concrete countertops is its name.  “Non-shrink” could have some big advantages. Secondly, its compressive strength is often very high. Typical compressive strength values are 8,000 psi to 14,000 psi at 28 days, with 1 day strengths in excess of 3,000 psi. Other factors are its availability and reasonable price.

However, “non-shrink” is a misnomer. This article explains why and what applications are appropriate for NSPG. Concrete countertops are not one of them.

What is Non-Shrink Precision Grout?

NSPG is an all-sand mix characterized by good flowability, very high compressive strength and under ordinary use no net shrinkage. Working time is often short, with set times of about an hour or so, but this can vary with different formulations.

NSPG becomes “non-shrinking” due the use of Type K expansive cement. Type K cement expands soon after setting. The “non-shrink” properties are a bit of a misnomer and can be confusing. All concrete shrinks to some degree or another over time as it dries out. However, the net shrinkage that occurs is what matters in the normal applications of NSPG. The expansive cement expands for a short period of time after it sets. This initial positive expansion exceeds the anticipated shrinkage that will occur over time when the NSPG is used for what it is intended. Thus there is still some expansion remaining. The concrete first expanded a lot and then shrank a bit less, so the net result is nearly no shrinkage. Hence “non-shrink”.

NSPG is used when setting machinery bases, generators, mills, presses, compressors, etc., structural grouting of precast columns, steel columns, and anchoring of sign posts, anchor bolts, and dowels. It is essentially a structural filler in these applications.

In the ordinary applications that NSPG is intended for, its expand-then-shrink behavior is tolerable. However for countertops, the fact that the NSPG shrinks at all is not tolerable. We will explain this later. First, let us consider the question: Why does “non-shrink” precision grout shrink at all?

How Does It Work?

Most NSPG is a blend of expansive cement, finely graded sands and other proprietary ingredients. The proportions of cement to sand are much higher than in ordinary concrete. When used as a structural filler, this blend creates a very creamy, highly workable (sometimes pourable) mix with high cementitous content (paste). When concrete shrinks, only the paste shrinks, so a higher paste volume results in higher shrinkage. NSPG attempts to counteract the anticipated shrinkage by first expanding.

NSPG and Concrete Countertops

This is part of the problem with using NSPG for countertops. With the oridnary applications that NSPG is intended for, usually not much of the grout’s surface area is exposed to the air, so drying occurs slowly. With a countertop made out of NSPG, there is a huge amount of surface that can dry out. Thus any early expansion that would normally counteract the inevitable shrinkage is overwhelmed by the excessive amounts of shrinkage caused by rapid and continuous drying.

Countertops are large, flat beams with large amounts of surface area exposed to the air, so drying becomes a serious issue. As concrete dries and loses moisture, the capillary tension created by the water in the fine pores causes volumetric shrinkage. It is the loss of internal water that causes the shrinkage. The longer the concrete is allowed to dry, the more shrinkage will occur. For larger pieces of concrete, drying shrinkage can continue for years. Even very thin pieces of concrete will continue to shrink over time, as will be seen below.


To investigate the shrinkage potential for the non-shrink grout, I cast several 5’ long, 1” thick beams with reinforcing steel extremely close to one face (the cast side).

The beam design was chosen to exaggerate the curling tendency of each mix. Long, thin slabs exhibit more curling than short, thick slabs. Shrinkage causes curling, either from differential drying or differential restraint, or both. Differential drying occurs when one side of a slab dries out faster or more than the other side. The side that is drier shrinks more, so the slab curls towards the dry side. Differential restraint occurs when one side of a slab is allowed to shrink more than the opposite side. The reinforcing steel keeps its side from shrinking much, whereas the side that has no reinforcing shrinks more. When both factors are working together, large amounts of curling occurs. (This is why it is important to moist cure slabs properly so that they don’t curl.)

The beams were first allowed to moist cure for 3 days before being stripped and then set out to air dry, reinforcing side down against a smooth impervious surface. These conditions are considered very extreme and were selected to maximize the amount of curl that would occur.

Curl measurements were periodically taken on the back side (the smooth cast side). A 6’ straightedge and a dial caliper were used to measure the growing gap between the slab ends and the straightedge. All measurements are changes relative to the slab geometry measured immediately after stripping.

In the graph, samples “D” and “F” are a commercial non-shrink precision grout mixed and cast according to the manufacturer’s instructions. Sample “D” had a water to cement (w/c) ratio of about 0.30, and sample “F” had a w/c of about 0.43.

NSPG GraphThe other samples, “AA” through “S”, represent comparable beams cast from various commercial concrete countertop mixes and my own from-scratch mix designs. These samples had w/c ratios that varied from 0.35 to 0.45.

Because the non-shrink precision grout had a higher cement to aggregate ratio than all of the other mix designs, the slabs made with the NSPG exhibited the most amount of curl, even though the w/c ratios were similar, or even lower than, the other mixes.

Two things become clear: first, lowering the w/c ratio is only part of solution to reducing shrinkage (and thus curling). Secondly, shrinkage (and therefore curling) occurs for a long period of time after casting, and that significant amounts of curling occur even after 14 days.

Better mix designs (those that minimize shrinkage tendencies through careful aggregate gradation and optimum cement contents) show lower overall shrinkage and stabilize sooner than the NSPG, which has large amounts of cement paste and a narrow range of fine aggregates. For these reasons and all of the reasons explained in this article, I do not recommend NSPG for use in concrete countertops.

The Use of Cenospheres in Concrete Countertops

Cenospheres, also sometimes called microspheres, are used in a wide variety of materials, from paints and finishes to plastics and caulking. And although they have been used in concrete for some time, their use is not widely known. Cenospheres have a couple of uses in concrete countertops, as:

  • a workability enhancer and extra-fine aggregate, and
  • a bulk filler and shrinkage reducer in cement grouts.


Cenospheres vs. Lightweight Expanded Aggregates

My last post discussed using lightweight aggregates like expanded clay, shale or slate to reduce overall concrete weight. While it is possible to create lighter weight concrete with cenospheres instead of lightweight aggregates, doing so presents such mix design challenges that I do not recommend it as a weight-reducing strategy.

In theory, cenospheres can replace some of the normal-weight sand used in concrete. Cenospheres have a density that is less than water (averaging 0.7 vs. water’s 1.0); quartz sand particles typically have a density of about 2.65. This means that 1 pound of cenospheres takes up the same absolute volume as about 3.8 lbs. of sand.

Good concrete mix design practices use a variety of aggregate sizes. The broader the particle size range is, the better the particles pack together and the less cement paste is required to coat the particles and provide excess paste “lubrication” necessary for good workability.

Cenospheres’ particle sizes do vary. However, their extremely small size means that there’s a lot more surface area to coat than an equal volume of much larger sand particles. You will need too much cement to coat the tiny particles, resulting in a crack-prone mix.

The larger the particle sizes, the greater sand replacement percentage possible. Replacements of up to 33% of the sand with the largest-size cenopsheres have achieved concrete weights of around 124 lbs. per cubic foot versus 136 lbs. per cubic foot for normal concrete.

The advantage of cenospheres over lightweight aggregates is that they are essentially “invisible” in the concrete, even when it is ground and polished. However, given the mix design issues above, I do not recommend using cenospheres to reduce weight. They are useful in other ways: for workability and grout.

Workability Enhancer

Cenospheres are very small spherical particles. As such they behave just like microscopic ball bearings in a concrete mix. Adding cenospheres to a conventional weight concrete mix will improve workability due to the ball bearing effect, and since the cenospheres are also structural aggregate, they improve concrete density and strength by providing better packing. In addition, the added fines improve trowelability and finishability. Typically dosages of 1% to 5% by weight of aggregate are added to a concrete mix to enhance workability.

Bulk Filler for Grout

Since cenospheres are very fine and generally light in color, they are ideal for use in the cement grout (aka slurry) used to fill pinholes. Not only does the added bulk from the cenospheres increase the volume of grout without adding more cement, but also the fine aggregate gradation of the particles helps to reduce shrinkage. After all, grout is still a kind of concrete, so the same rules apply. Finally, the spherical shape makes packing pinholes easier and more effective because of the scrubbing action provided by the particles. And the spherical shapes are more likely to “roll” into small holes better than jagged or angular crushed particles. Typical cenosphere dosages for grout are around 10% to 30% by weight of cement.

So What Exactly Is A Cenosphere?

Like fly ash, cenospheres are naturally occurring by-products of the burning process at coal-fired power plants. Unlike fly ash though, cenospheres are lightweight, inert, hollow spheres comprised largely of silica and alumina and filled with air and/or gases. Since they are inert, they are not considered a pozzolan. And because they are very small and have high compressive strengths, cenospheres can be used as a structural lightweight filler.

Where Can You Get Cenospheres?

Now that you understand what cenospheres are and the great things they can do for concrete countertops, how do you get them? Most cenospheres are purchased in large volumes of truckloads or railcars for large industrial projects. So, they can be hard to find in small quantities for concrete countertops.

CCI recommends CenoStar Corporation for their easy to order small quantities and high grade cenospheres. You can buy quart quantities online at www.cenostar.com. If you’re looking for a good product for use in concrete countertops I recommend the EconoStar ES300. It has a maximum particle size of 300 microns (0.3mm). ES300 is light gray in color. If you need white cenospheres try the ChinaWhite CW300 for a similar particle size in a white cenosphere.

Lightweight Concrete – Is It Really Necessary for Countertops?

Any of you that have lugged a precast concrete countertop from your shop to an installation site know that concrete is heavy. It’s just the nature of the beast. But, is it a problem? Should you be creating lightweight concrete? Let’s take a few minutes to learn more about lightweight concrete so you can decide for yourself.

Size versus Weight

First of all, the easiest way to reduce the weight of your concrete slabs is simply to make them smaller. There are 3 ways to do this:

– Make more slabs.

If you used 4 slabs to create a 16 foot long kitchen countertop, each slab would be only 4 feet long and therefore weigh a lot less than a single 16 foot long slab. However, most clients want to minimize the number of seams, so this is usually not practical.

– Make thinner (precast) slabs.

Many concrete countertop makers do not understand reinforcing, and they make their slabs too thick, 2 inches or more, to compensate for their lack of confidence in their concrete. There is no need to make precast concrete more than 1.5″ thick, if you understand how to reinforce it properly. If the client wants a thicker look, you can achieve this with dropped edges.

– Use GFRC.

Glass Fiber Reinforced Concrete can go as thin as 3/4″ for the same kitchen countertop slabs that would need to be 1.5″ thick for precast. This instantly cuts the weight in half.

This article is not about any of these techniques, it is about making the concrete itself lighter weight, so that the same volume of concrete actually weighs less.

What is Lightweight Concrete?

Lightweight concrete is made by replacing some (or all) of the normal weight aggregate (crushed limestone, granite, quartz, etc.) with a lightweight aggregate (expanded clay, shale or slate) to reduce the overall weight of the piece. Often the coarse fraction is replaced with lightweight aggregate and normal weight sand is used. Expanded clay, shale or slate are popular aggregate choices. These are created by heating the parent material to a high temperature causing the stone to “puff”, creating a substance often called foamed rock.

Lightweight vs. Conventional Concrete – What’s the Weight Difference?

  • Lightweight Concrete – approx.. 115 pounds per cubic foot.
  • Normal Weight Concrete – 145 pounds per cubic foot.

One square foot of 1.5 inch thick normal concrete weighs about 18 pounds. The same segment created from lightweight concrete weighs approximately 14.5 pounds. For comparison a square foot of 1.5 inch thick granite is 17.5 pounds.

Choosing Your Lightweight Aggregate

The compressive strength, elastic modulus, splitting tensile strengths and other properties of lightweight concrete are significantly affected by the structural and physical properties of the lightweight aggregate used. The aggregate itself must possess desirable properties such as adequate compressive strength, porosity, appearance, abrasion resistance and good bonding with the cement paste. For this reason you must carefully choose your aggregate if you’re working with lightweight concrete.  

Don’t use:

  • Perlite
  • Vermiculite
  • Styrofoam
  • Air

These don’t have the properties needed for structural concrete. They are better suited for concrete used as insulation or as lightweight filler.

Do use:

    • Expanded clay
    • Expanded shale
    • Expanded slate

Be aware that lightweight aggregate doesn’t polish well due to the porosity and internal voids. You can’t polish air. Even when polished with a 3000 grit diamond pad, the aggregate will remain dull.

expanded shale in concrete

Water and Lightweight Aggregate

The porosity of lightweight aggregate creates some challenges when creating a mix, especially when dosing the water. The increased porosity causes the aggregate to absorb a great deal of water, sometimes for days or even weeks. In general it is recommended that lightweight aggregate be presoaked to achieve a condition known as surface saturated dry (SSD) condition. This ensures that the aggregate will not absorb the mix water.

Extra care and attention must be paid when working with air dry lightweight aggregate, or a pre-blended lightweight concrete mix that can only have air dry ingredients in it (otherwise it would prematurely set due to the moisture inside the aggregate). The dry aggregate will readily absorb some of the mix water, requiring continued doses of extra water. It is at this point that it is extremely important that whatever extra water is added be dosed with great care, and that all batches of concrete have identical amounts of water added to it.

Concrete that has different amounts of mix water, and therefore different water to cement ratios, will have different structural, shrinkage and aesthetic characteristics. Concrete that loses mix water to thirsty aggregate during the critical phase when the concrete is setting can exhibit plastic shrinkage, surface map crazing, color variation, mottling and other undesirable and avoidable problems. Undisciplined and uncontrolled additions of unknown amounts of water will significantly affect the performance, durability and appearance of the finished concrete.

Converting a Mix to Lightweight

For concrete countertops, most mixes can be “converted” into lightweight mixes by replacing some or all of the normal weight aggregate with lightweight aggregate. While the surface texture and aggregate shape may have an affect on the workability (rougher and more angular particles make a mix that has lower workability than smooth, rounder particles, all else being equal). Most lightweight aggregates weigh about ½ to 2/3rds the weight of normal aggregate, so on average one pound of gravel can be replaced with a bit more than ½ pound of lightweight aggregate. The volume of aggregate stays the same, but the weight is reduced.

Even though the “conversion” seems simple, the inclusion of lightweight aggregates in a concrete mix will affect its properties and its workability. With appropriate lightweight aggregates, the compressive strength may not be affected, but the workability and the appearance more than likely will. Because the lightweight aggregate readily absorbs water, it is very important to calculate and keep track of all of the mix water added.

Is Lightweight Concrete Necessary for Concrete Countertops?

For most kitchen and bathroom cabinets, little or no modifications are necessary to bear the weight of 1.5” thick normal weight concrete. Lightweight concrete would not convey any significant advantage over normal weight concrete for anything but the largest slabs.

In addition, factors other than slab weight often dictate the maximum slab size and shape. Factors like site access, stairs, corners and general countertop and cabinet configurations, all impact the safe transport, handling and installation of very large slabs. The largest practical slabs may not actually be very heavy and therefore not need lightweight concrete.

What You Missed- Getting Started with Concrete Countertops Seminar

Great training is one of the best ways to ensure great results from your concrete countertops. Here at The Concrete Countertop Institute we offer a variety of different training formats including informative online training seminars. Back in April we hosted our first one- Getting Started with Concrete Countertops. It was a great event for all who attended, but if you missed out, don’t despair. You can view it in its entirety online. It’s free!


Here are a few things you missed if you didn’t attend our “Getting Started with Concrete Countertops” webinar:

  • You Can Be Successful on Your First Project- I made my first concrete countertop back in 1999 when concrete countertops as a concept were just starting to come into form. My first project was for my own home and was created in my garage. It was a success, and your first project can be too. The key to a great project is having a solid technical foundation and the right training. The seminar will give you some great tips for getting started and you’ll even get to see pictures of my very first countertop and forms.
  • Reinforcing- What Not to Do- Properly reinforced concrete is strong and ready to withstand the stresses every countertop must endure. Reinforcing is critical but can be tricky when you’re just getting started. Learn what to do and what not to do when reinforcing a countertop. One quick tip: #3 rebar is not a good choice for a standard 1.5 inch thick countertop. It’s too large, and it’s overkill. I’ll show you what you need to use and how to use it.
  • Your Countertop is Only as Good as Your Form- Nice straight lines make a beautiful form for creating your countertop. Many people that are just getting started think that expensive equipment is essential, but they are wrong. The key isn’t spending a lot on the tools you need to create your form, but rather getting the right ones. I’ve found that a simple circular saw and a good straightedge are more effective than a cheap table saw. Of course if you know someone with a professional grade table saw, enlist their help, but if you don’t, skip the cheap substitutes and use a circular saw. I’ll show you how.
  • Water- How Much Do You Need?- Adding water to concrete is not like adding salt and pepper to your food: you don’t add it to taste. Precise measurements are essential. I’ve seen concrete that looks really stiff that has way too much water. Using too much water can result in reduced strength, increased porosity, increased shrinkage and CRACKING. I’ll teach you how to create concrete that is very workable without adding too much water.
  • Custom Countertop Mixes- Tips for Creating a From-Scratch Mix- Sure, you can buy premade concrete countertop mixes, but they can be very expensive. I’ll teach you how to make your own mixes using easily obtainable materials (like gravel, sand and Portland cement) so you can create 1.5” think countertops for approximately $1.90 per square foot. When you have a fully from-scratch mix, you can customize and you generally have better results. I’ll teach you a basic mix design that you can customize for any project you want to attempt.
  • Installation Made Easy- You’ve got a great concrete countertop- what now? We discuss installation issues in depth in this seminar. For many people installing sinks is one of the most difficult parts of installing a new countertop. I’ll teach you to install anchors and supports in your countertop for easy mounting and installation and show you how to properly support a kitchen sink using the cabinets below. When you anchor a sink to the cabinets it’s a little more work up front, but in the long run you’ll avoid cracks and other problems down the road.

If you want to start making concrete countertops, this seminar is just what you need. I provide tips for every stage of the process from building your forms to creating your mix to sealing and installing the final product. Click here to check out this great seminar today.


Creating Concrete Countertops Using GFRC

In the last two pieces about glass fiber reinforced concrete I’ve discussed the basics of glass fiber reinforced concrete and the importance of fibers. To finish off this series on GFRC I’d like to move into a few of the technical aspects you will encounter when creating GFRC countertops including mix designs, casting and processing.

GFRC Mix Designs

If you’ve worked much with concrete you know that finding the right mix can be difficult and often requires years of experience. Many different factors impact the ideal composition for concrete, and GFRC is no different. Mix design isn’t a concept that can be tackled in one blog post, but here are some of the basic components in a good GFRC mix:

  • Fine Sand– Sand used in GFRC should have an average size passing a #50 sieve to #30 sieve (0.3 mm to 0.6mm). Finer sand tends to inhibit flowability while coarser material tends to run off of vertical sections and bounce back when being sprayed.
  • Cement– Typical proportions use equal parts by weight of sand and cement.
  • Polymer– Acrylic polymer is typically preferred over EVA or SBR polymers for GFRC. Acrylic is non-rewettable, so once it dries out it won’t soften or dissolve, nor will it yellow from exposure to sunlight. Most acrylic polymers used in GFRC have solids content ranging from 46% to over 50%. Consider trying Smooth-On’s duoMatrix-C and Forton’s VF-774, two reliable acrylic polymer choices.
  • Water– Common water to cement ratios range from .3 to .35.  When determining how much water to use make sure to take the water content from your acrylic polymer into account. This can make calculating water to cement ratios difficult unless the solids content of the polymer is known. With a polymer solids content of 46%, 15 lbs of polymer plus 23 lbs of water are added for every 100 lbs of cement.
  • Alkali Resistant Glass Fibers– Fibers are an essential component of GFRC. If you’re using the spray-up method for casting the fibers will be cut and added to the mix automatically by your sprayer at the time of application. If you’re using premix or the hybrid method for casting you’ll mix the fibers in yourself. Fiber content varies but is typically between 5% to 7% of the overall cementitious weight. Higher fiber content increases strength but decreases workability.
  • Other Admixtures– Some other elements you may choose to include in your GFRC mix include silica fume, metakaolin, VCAS and superplasticizers.

GFRC Casting

I talked a bit about casting in the Introduction to GFRC article. There are a few different methods you can use for casting. Since I’ve already discussed the basic concepts behind casting (if you need a refresher check out Introduction to GFRC) I’d like to take a minute to review the pros and cons associated with each casting method.


A concrete mixture is sprayed into the forms using a special device that chops and sprays a separate stream of long fibers. The concrete and fibers mix when they hit the form surface.

  • Pros: Allows for very high fiber loads using long fibers resulting in greatest possible strength.
  • Cons: Requires expensive, specialized equipment (generally $20,000 or more).


Glass fibers are mixed directly into the fluid concrete. The mixture is then poured or sprayed into molds.

  • Pros: Less expensive than spray-up, although a special spray gun and pump is required.
  • Cons: Fiber orientation is more random than when using spray-up and fibers are shorter resulting in less strength.


The hybrid method for casting GFRC uses an inexpensive hopper gun (the same kind used with drywall) to spray a thin face coat into the forms. Once the face coat dries the fiber loaded backer mix is applied either by pouring or hand packing, just like ordinary concrete.

  • Pros: Affordable way to get started with GFRC. A hopper and air compressor run about $400-$500, much less than the spray guns used for spray-up or premix.
  • Cons: Since the face coat and backer mix are applied at different times careful attention is needed to ensure the mixes have a similar makeup to prevent curling.
Spraying GFRC

Spraying GFRC

GFRC Curing

The high polymer content of GFRC often means that long term moist curing is unnecessary. Cover a freshly cast piece with plastic overnight, but as soon as it has gained enough strength it can be uncovered and processed. Many GFRC pieces are stripped 16 to 24 hours after casting.

GFRC Processing

Your skill level, the composition of your mix and the method used will determine how much processing is needed once your GFRC countertop is removed from its molds. Grouting may be needed to fill in bug holes or surface imperfections. Any blowback (sand and concrete that doesn’t stick to the forms) needs to be cleaned or the concrete’s surface will be open and granular. Achieving a perfect piece right out of the mold is very difficult and requires great skill.

Common Questions About GFRC

  • How Thick is a Typical GFRC Countertop?– Typical concrete countertops made with GFRC range from ¾” to 1” in thickness. This is the minimum thickness that a long, flat countertop can be made so it doesn’t break when handled or transported. Smaller wall tiles can be much thinner.
  • Is GFRC Green?– GFRC is roughly on par with other forms of concrete countertops in terms of the “green-ness”. In comparing 1.5” thick concrete countertops to ¾” GFRC countertops, the same amount of cement is used, since GFRC tends to use about twice as much cement as ordinary concrete. This sets them equal to each other. The use of polymers and the need to truck them does make GFRC less green than using ordinary water, which could be recycled from shop use. Both traditional cast and GFRC can use recycled aggregates, and steel reinforcing is more green than AR glass fibers, since steel is the most recycled material, so its use in concrete of all forms boosts the concrete’s green-ness.

I hope you enjoyed this three-part series on GFRC. Before you go check out this short 7 ½ minute video featuring excerpts from my Comprehensive GFRC Self Study Course. Watching an actual GFRC countertop being constructed will help you better understand many of the topics I’ve covered in this series.

You may also view my FREE, 2.5 hour seminar “Step by Step GFRC with Mix Design” by requesting access here.

Understanding Self-Consolidating Concrete

Self-consolidating concrete (SCC) is a highly flowable concrete that can spread through and around dense reinforcing under its own weight to adequately fill voids without segregation or excessive bleeding and without the need for significant vibration. In the decorative concrete arena it has been used to produce castings with a high surface quality, often with few or no pinholes. Less labor, quicker casting times, better surface finish and increased concrete densities are common reasons for choosing SCC.

The flowability of SCC is measured in terms of spread instead of slump, because the material flows so readily. The slump flow test (ASTM C 1611) is similar to a standard slump test (ASTM C 143), but instead of measuring a vertical height change, a horizontal spread measurement is made instead; typical spread values range from about 20” to 30”. The spread in the photo below is over 29”. Note how there is almost no segregation, bleeding or variation.

SCC photo

Photo of SCC

 Other tests that are approved or are under consideration are the J-Ring (ASTM C 1621) and the Column Segregation test (ASTM C 1610).

How it’s made

SCC is achieved by designing a mix that has a low yield stress and an increased plastic viscosity (see Figure 1). In other words, the mix should require minimal force to initiate flow, yet have adequate cohesion to resist aggregate segregation and excess bleeding. The yield stress is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity-modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.

scc diagram

SCC diagram

The preferred admixture for reducing yield stress in self-consolidating concrete is a polycarboxylate-based admixture due to its superior water-reduction capabilities and high early-strength gains at low dosing rates. This new generation of synthetic admixture has been specially designed to increase the dispersion of the cement particles, which aids in plasticity, strength and can also help with pigment dispersion.

Avoiding segregation through aggregate gradation and by increasing the amount of fines is possible, but it must be done carefully to preserve the SCC properties. A well graded aggregate distribution minimizes cement paste content as well as minimizes admixture dosage.

Other considerations

Excess moisture in the ingredients, especially the fine aggregates, can have a profound influence on the consistency of the mix. Small fluctuations in moisture content may lead to segregation or affect the mix’s ability to flow. And variations in the aggregate gradation from batch to batch can also cause consistency problems. VMA’s help even variations in aggregate gradations and can account for moderate variations in moisture content. A more robust mix is ultimately created with an enhanced capacity to absorb fluctuations in aggregate gradations and moisture contents.

DIY or Bagged Concrete Countertop Mix?

All concrete countertops have a basic requirement: a concrete mix that provides the structural, physical, and aesthetic characteristics necessary to make a high-quality countertop that meets the client’s needs and wants. Aside from ordering concrete from a ready-mix supplier, there are two basic ways to obtain a concrete mix. One is by using a commercially available bagged concrete countertop mix, and the other is to do it yourself, making a from-scratch mix with basic ingredients. There are pros and cons to both approaches. Which one you use is ultimately a personal preference.

Bagged Mixes 

Bagged mixes offer simplicity and convenience as their key feature. Generally all of the necessary ingredients, except pigment, are pre-blended; all that is required is to add the proper amount of water. Implicit in the offering is that the concrete mix is consistent from bag to bag, that the resulting concrete meets the performance specifications stated, and, most importantly, that the mix itself is appropriate for concrete countertops in general, and specifically for the casting method (cast-in-place versus precast) used.

There are several different concrete countertop mixes on the market. Some come in a single color (e.g. gray cement), while others have gray or white cement bases. Some even come preblended with pigment. Aggregate size, shape, color, and gradation can vary widely. Some bagged mixes have large amounts of coarse aggregate, while others are all-sand mixes with no large aggregate. Some require the addition of polymer admixtures, which are sold along with the dry ingredients.

A bag of dry concrete countertop mix contains a variety of ingredients that the manufacturer has chosen for a specific reason. There might a wide range of factors that influence a particular blend, such as the desired compressive strength, economics, the availability or cost of a particular ingredient, or something more esoteric, such as the concrete’s in-hand feel and workability or satisfying certain textural criteria.

Regardless of whether the bagged concrete mix is originally designed for the do-it-yourselfer or a professional concrete countertop maker, all bagged mixes share a common characteristic: You don’t really know what’s in the bag, and you have to trust the manufacturer’s instructions. Ideally, the mix should always yield the same results, but external variables such as temperature can significantly affect your concrete. So having some control over the mix can be important. If you do need to alter the mix – say by adding accelerator on a very cold day – you don’t know how much cementitious material it contains, so you can’t dose properly.

From-Scratch Mixes 

Control, therefore, is one of the main reasons for using a from-scratch mix. Since all of the ingredients are known exactly, accelerators, superplasticizers, pozzolans, pigments, and decorative aggregates can all be used to tweak the performance and appearance of the mix. However, from-scratch mixes are less user-friendly than bagged products and require an understanding of mix design. Myriad factors such as mineralogy and aggregate particle shape, size, and gradation can have powerful influences on the fresh and hardened properties of the mix. With so many variables it can be difficult to strike a balance between aesthetics, workability, and physical performance.

Making your own concrete also requires you to source, obtain, and batch all of the ingredients. Variations in ingredients, such as color, moisture content, and availability, all come into play and must be considered. Means for precise batching is essential for consistency, and storage of raw materials requires space. 

For more of my articles about from-scratch mixes, click here.

Making from-scratch mix Making from-scratch mix Continue reading

Using different mixes in a single concrete countertop slab

I recently got the following question regarding my blog post “Mix design for cast in place concrete countertop in Cayman“:

Q: Is it possible to use gray cement for the core and white cement as an outer layer?

White and gray portland cement are very similar to each other, and can be safely blended together. You can use one color cement for a core mix and a different color cement for a shell mix, provided the mix designs and the water/cement ratios are identical.

However, I would hesitate to use a gray cement core with a white cement “shell” for aesthetic reasons, especially for projects like the cast-in-place gazebo I did in Grand Cayman, and for regular precast “wet cast” projects. The simple reason is that it’s very impractical to pour a core mix without contaminating the form surfaces, contain the core mix so that it stays where you want it, and to pour the visible shell mix and still end up making good concrete without voids or weak zones. The minor cost savings in cement (white cement in Cayman is about US$20 per bag, vs gray cement at US$10 per bag).

The exception is when an ultra-expensive pigment is involved. In that case it is sometimes worthwhile in cost savings to take the extra trouble to use the pigment only in the outer layer.  I call this “buttering”. It is easier to do with a hand packed precast method, somewhat easy with GFRC, and hardest with wet cast precast or cast in place. Be aware, however, that some exotic pigments could affect the properties of the mix to the extent that you have issues with the 2 layers bonding.

Tips for “buttering”:

  • The first layer should be well compacted, of even thickness and fairly smooth. Loose, clumpy or uneven concrete will create weak zones that could lead to cracking or delamination.
  • The second batch of concrete should be the identical mix (but without glass or stone if used in the first mix).
  • It’s very important to use the same water content, and merely add more superplasticizer to make the second batch more workable.
  • It’s important to place the second batch before the first becomes hard or dries out.

For the gazebo job I did in Grand Cayman (8000 lbs of concrete), the extra price from using all white cement added up to only about an extra $100 in cost. Consider the need for 2 separate mixers, the extra labor and time needed to wrangle two mixes, and it’s easy to see that at times it’s simpler to bite the bullet and pay a bit more up front to make things simpler, and in the end, better and less expensive overall.

Here’s an example of a GFRC piece done with red outer shell (mist coat) and white core (backer). This was done just for illustration purposes. You can see in the second photo that some of the white showed through when the mist coat chipped.

gfrc red mist white backer

gfrc chip in mist coat

Superplasticizer in all-sand concrete countertop mixes

Recently I got the following question:


I have been making concrete counters for a while, but I always have used mixes with aggregate in them. Recently, I started using sand based mixes. The water/cement ratio always ended up being more than desirable. I watched your video on superplasticizers, and although I used the max amount of CounterFlo (by Fritz-Pak), the .34 to 1 water to cement ratio I used produced a really stiff mix. We ended up adding a lot more water and the mix was still fairly stiff.

Here’s the video:


Sand mixes are very versatile mixes, in fact most of the countertops I’ve made over the last 12 years have been cast using an all-sand mix.

By their very nature sand mixes are best for stiffer, hand-packed finishes. They’re not the best choice when making a flowable mix due to the high surface area of the sand. Aggregate based mixes are best for flowable concrete.

With a good mix design that yields high early strengths it’s possible to hone the surface of an aggregate mix and not expose the aggregate. Usually it takes a great deal of effort to expose the aggregate, in large part due to the strength and flowability of the mix.

Getting an all sand mix to become flowable can be a challege, in part because of the type of superplasticizer required but more so due to the fact that sand has much more surface area than gravel does. 1 lb of sand has a lot more particle surface to cover with a fixed amount of cement paste than 1 lb of gravel, so there’s less cement paste separating the sand grains. That means there’s less lubricant, so more friction. That’s why you’ve always needed to add more water to your sand mixes.

When you add water you’re increasing the cement paste volume, thus increasing the particle spacing and adding lubricant. Adding extra water isn’t always bad, provided you know what the w/c ratio is and that it’s below a tolerable level. I’d recommend staying below 0.4.

Fritz-Pak’s Counterflo is a good water reducer, but it’s a very weak one. You’ll have a very hard time making flowable conrete with it. It’s not meant to do that, it’s simply a mild water reducer. It’s only meant to reduce the amount of water needed, not radically change the mix characteristics.

To make flowable concrete you need a powerful high range superplasticizer. CCI sells 2 polycarboxylate-based superplasticizers, WR420 and WR310.

Both my all-sand mix and my aggregate based mix are available in my self-study course Precast Mix Design 101.

Precast Mix Design 101

Mix design for cast in place concrete countertop in Cayman

I wrote recently about the planning stages for an outdoor gazebo bartop I’m doing here in the Cayman Islands. We’ve now poured the project, and I wanted to give you some insight into the mix we used and why.

This project required a large volume of concrete (nearly 8000 lbs, or 2 cubic yards), so I chose to have the local (and neighboring) concrete batch plant mix and deliver the concrete.

loading truck

However, the concrete was based on white portland, and I felt that a standard construction mix would have been too inferior for the demands of a high quality concrete countertop. Therefore I chose to modify one of the batch plant’s mixes to suit my needs.

I spent some time with the plant’s engineer to learn more about their basic slab mix. Because the aggregates were local, I felt that using a mix that they had extensive experience with was smarter than trying to force fit an outside mix design that may or may not have worked the way I wanted it to. All I did was add a few admixtures to modify the base mix (and of course use white portland instead of gray portland). In addition to the base mix, I added pozzolan, a retarder, pigment and a shrinkage reducing admixture. I didn’t even need to use a superplasticizer, although we had one on hand if we needed to tweak the mix at the job.

Cast-in-place concrete needs special attention because it’s more difficult to control the environment during curing, and the concrete is exposed to greater environmental extremes (heat at least, but not cold here in Grand Cayman) than precast concrete used inside a home. Unlike sidewalks and floors, outdoor concrete countertops still have to look good and remain crack free.

I was casting a single slab over 45 feet long that wrapped around a hexagonal concrete gazebo. The thick concrete columns that hold up the roof penetrate the bartop, which wraps completely around each column. Crack control was very important, so I had to use a shrinkage reducing admixture to control the root cause of cracking, rather than hope fibers would do the job. I did not use fibers since ordinary fibers don’t prevent cracking anyway.

Here in the Caymans it gets hot, but not super hot. Average daily temperatures are 88 to 90 every day, all summer. It’s also very humid, which is a good thing, since the concrete doesn’t dry out as fast as when it’s hot and dry, like in a desert climate. To control early set I used a retarder, which gave me plenty of working time.

Although the truck delivered the concrete 10 feet from the job, it still had to be wheelbarrowed up to the bar and hand placed in buckets.

truck parked

The workability of the concrete was around a 4″ slump when we started, and it was unchanged when we finished pouring 2 hours later.

mix consistency

It took another 30 to 45 minutes to begin to firm up, but we left the job before it got to a hard trowel stage. Since we planned on grinding the concrete to expose the aggregate, all we did was screed and float in some decorative coarse aggregate and sliced coral into the top.

adding coral

For more photos of the project, view the entire photo album on the CCI Facebook page.

Click here for a free seminar including a specific cast in place mix design.

Achieving color consistency in concrete countertops: Part 3 of 3

You have learned that water strongly affects the color of a concrete countertop mix, as does precision of measuring and measuring by weight, not volume. This final article in the color consistency series lists several other reasons you might encounter inconsistency in your concrete countertop mixes.

  • Variability in the ingredients themselves

From year to year, the ingredients you use can vary, even if you buy the same ingredient from the same manufacturer. Pigments tend to be very consistent, but cements, especially gray cements, are not. Your colors that have less pigment, and therefore obtain most of their color from the cement, will exhibit more variability thann heavily pigmented colors.

  • Use of pozzolans

You may use pozzolans such as metakaolin, VCAS, fly ash, slag or silica fume in your mixes. Some of these pozzolans, especially waste products such as slag, can be highly variable in color. And, pozzolans such as silica fume can impart a very distinct color to the concrete. Besides the fact that different pozzolans have different actions and properties, you should not vary which pozzolans you use in your mix designs if you want to achieve color consistency.


Various pozzolans. Photo courtesy www.cement.org

  • Use of admixtures such as superplasticizer

Admixtures like superplasticizers can influence color, not because they add color, but because they can act like dispersants, aiding in cement and pigment dispersion and the resulting color strength. Other admixtures usually have little effect on the color, like fibers, accelerators or retarders, however calcium chloride accelerators are an exception and should be avoided for this and other reasons.

  • Not blending ingredients thoroughly

Thorough and complete blending of all of the concrete ingredients is very important to achieving a uniform and consistent color. All of the pigment added to the mixer should be uniformly blended. If pigment is stuck to the sides of the mixer or in lumps or streaks then the resulting concrete will not be consistent with other batches nor will the color of that batch be uniform.

  • Adding liquid pigment to the mix water

Adding liquid pigment to the mix water before adding the now pigmented mix water to the concrete can cause color inconsistency because a significant amount of the pigment usually remains in the bucket.

Liquid pigments are really ultra fine pigment particles suspended in a liquid. If the liquid pigment is added to mix water, the pigment particles will quickly settle out because the suspension fluid is now greatly diluted by the mix water. No amount of stirring will suspend all of the pigment particles, so much of the color remains in the bucket rather than going into the mixer.

When done carefully, it is possible to add the liquid pigment to a portion of the mix water and then use the remaining mix water to rinse out any pigment residue.  But this involves extra work and requires extra attention. A simpler process would be to add the liquid pigment directly to the mixer and then to rinse out the pigment container with the mix water as it is added.

  • Inconsistent curing practices

Curing the concrete has an effect on concrete color. Curing the concrete after casting helps “lock in” the color. If some slabs are allowed to wet cure for longer than others, the slabs that dry out sooner will appear lighter.

  • Forms

Forms for fluid concrete mixes must be watertight in order to achieve consistent colors. Otherwise, color variations will show where some of the concrete leaked out of the forms. Form materials themselves (the texture and porosity) can affect the color of the concrete too.


Everything that goes into making concrete has some effect on its appearance. Discipline, attention to detail and knowledge of good concrete practices will make your concrete countertops as consistent as possible.

Achieving color consistency in concrete countertops: Part 1 of 3

Integrally colored concrete countertops can show color inconsistency for a variety of reasons, but the primary cause is lack of ingredient control: One or more of the ingredients in each batch of concrete were not carefully proportioned.

Most often the culprit is water. Adding too much water – often to increase the workability – will alter the color of the concrete, making it lighter than a similar batch that has less water in it.

Think of grape Kool-Aid. The more water you add, the lighter the color will be. The same applies to concrete. (The concrete will also be weaker, just like the Kool-Aid will taste weaker.)


Remember, water is the most critical ingredient in concrete, and casually adding water without keeping track of the exact amount will almost guarantee an inconsistent appearance. To ensure color consistency, ALL of the ingredients, including water, must be accounted for.

The simplest way of doing this is to generate a batch report, where each ingredient amount is listed next to a check box. The batch report ensures consistency in ingredient amounts, and the check boxes ensure that none of the ingredients gets left out.

The CCI mix calculators, both the Precast Concrete Countertop Mix Calculator and the GFRC Concrete Countertop Mix Calculator, print out batch reports. Whatever method you use to calculate your mix, make sure that you are diligent about using batch reports.

The best mix design for concrete countertops

Whether you use a bagged mix specifically designed for concrete countertops, or mix your own, mix design is critical for concrete countertops. Unlike sidewalks or foundations which are slabs on grade, concrete countertops are generally long, slender, thin beams that not only behave very differently structurally from slabs on grade but also have very different aesthetic requirements. For example, color is not an important consideration in structural concrete mix design, but it is in concrete countertop mix design.

Before we get into mix design considerations, note that you can either use a bagged mix specifically designed for concrete countertops, or mix your own from scratch. Our preference at CCI is for mixing your own, since Jeff is an engineer as well as a thrifty Yankee. However, there are pros and cons to both approaches. (More on that in another blog entry.) We do believe that even if you choose to use a bagged mix in your business, you should fully understand how concrete really works, so that you are better able to troubleshoot or adjust for your climate and working conditions.

What you need from a concrete countertop mix:

  • High early strength so you can process and finish faster
  • High flexural strength for greater crack resistance
  • Low shrinkage potential which minimizes curling
precast mix design


High Early (Compressive) Strength
Jeff is always saying, “Compressive strength is not as important as you think.” However, high early (compressive) strength is important early on when you need to get the concrete out of the forms, flip it over and start grinding it as soon as possible to get it into your client’s home. Concrete that develops high compressive strength quickly is going to be harder than concrete that develops strength more slowly. This means that the cement paste between the hard sand grains and aggregate will be harder, and the concrete can be ground and polished sooner.

High early strength is accomplished by using a low water to cement ratio, proper pozzolan loading and cement contents higher than construction grade concrete.

High Flexural Strength
Steel reinforcing is still essential, since the flexural strength of concrete is always much, much lower than the compressive strength. For example, the predicted value of flexural strength for ordinary construction concrete that has a very high compressive strength of 12,000 psi is only about 900 psi! But, if the flexural strength of your concrete is as high as possible, it is going to better withstand bending (flexural) forces along with the steel reinforcement, and show less cracking.

High flexural strength is achieved through both mix design and proper reinforcement. Steel reinforcing effectively boosts flexural strength values many times that of unreinforced concrete. GFRC uses a special mix design and high glass fiber loads that create high flexural strength.

Low Shrinkage Potential
Shrinkage can cause either cracking for restrained slabs or curling for unrestrained slabs. Shrinkage occurs when the cement paste dries out. Moisture evaporating from inside the concrete causes strong capillary suction forces in the cement paste that cause it to shrink. If the shrinkage forces are high enough, the concrete cracks. The underlying causes of this can be poor curing practices (allowing the concrete to dry out too soon before it’s strong enough to resist the suction forces), too much mix water, too much cement in the mix, or poor aggregate gradation that requires too much cement paste to achieve good workability.

Curling occurs when one face of a countertop shrinks more than the other side, and the result is that the countertop curls towards the side that shrank more. Curling can occur if one side of the slab remains wet and the other side is dry. Curling is a symptom of shrinkage. Concrete mixes that don’t exhibit significant amounts of shrinkage don’t curl much or at all.

Shrinkage reducing admixtures (SRA’s) are chemicals that reduce the suction forces generated during evaporation. This helps reduce the root cause of cracking and curling: the suction forces in the cement paste.

There are many different styles of concrete countertop mixes:

  • all-sand mixes designed to be stiff and hand packed
  • aggregate-based mixes designed for vibration or cast in place
  • polymer-based mixes that flow like pancake batter
  • GFRC mixes

Regardless of the style of mix, the basic principles discussed above apply.

stiff concrete countertop mix

Stiff mix, all sand

fluid concrete countertop mix

Flowable mix, aggregate based


More Details
Want to know more? Both of the above from-scratch mix designs, as well as a handy mix calculator, are included in our Precast Mix Design 101 self-study course. Questions about this article? Submit a comment below. And happy concrete mixing!

precast mix design2

Concrete Countertop Mixes: Stiff versus Fluid, part 2 of 2

This article concludes the discussion of stiff versus fluid concrete countertop mixes, with the final three differences.

Difference #3: Mix Design

Stiff Mixes

Stiff concrete mixes can be either all-sand mixes (no coarse aggregate) or coarse-aggregate based mixes. Generally they are all-sand mixes due to the ease of spreading and packing a fine-grained clay-like concrete. Coarse aggregate makes the mix “chunky”, often making hand-packing difficult or uncomfortable.

All-sand mixes have far more surface area, so require more cement paste. But the fine grained nature of sand inhibits movement due to the increased particle friction generated from many times more contact points. All-sand mixes are typically stiff, zero slump. It is possible to make anall-sand mix fluid with a combination of a powerful superplasticizer and strong vibration, but this is rarely done.

Fluid Mixes

Fluid concrete mixes are generally based on coarse aggregate.  Aggregate based mixes are more easily made fluid because of the large amount of coarse aggregate in the mix. Pound for pound, coarser aggregate has less surface area than fine sand. A mix that has less surface area requires less cement paste to coat the particles, and any excess paste in the mix acts as lubricant, allowing the particles to move past each other more easily.

Difference #4: Forming Techniques

Stiff Mixes

Because of the nature of stiff mixes, watertight forms are not necessary. As such, careful caulking of the seams isn’t required. If the form edges are caulked, it is because a cast rounded edge is desired instead of grinding a rounded edge. Forms should be tight, but the added step of caulking is not necessary.

Fluid Mixes

It’s a different story with fluid concrete. The forms must be watertight with fluid concrete because any leakage will result in discoloration of the concrete and loss of material.

Fluid concrete also requires that reinforcing steel be tied to the sides of the forms, since the steel will sink if simply placed in the form. However, it is very important not to pour the concrete over the steel. Doing so will result in ghosting. To prevent ghosting, either tie the steel after the forms are mostly filled with concrete, or very carefully pour the concrete through the holes between the steel, taking care not to pour onto the steel. Also, if the concrete is to be vibrated, the steel must be tied in place so that it does not move.

Comparison of Forms

Stiff mixes require less form work than fluid mixes when complex or 3 dimensional pieces are being cast. This is especially true for integral sinks cast into countertops. On the other hand ordinary flat countertop slabs use essentially the same forms for both stiff and fluid mixes.

form for fluid concrete countertop mix

Fluid concrete mixes need more complex forming for an integral sink.

form for stiff concrete countertop mix

Stiff mixes require less forming because the concrete is self supporting.

Difference #5: Casting Effort

Stiff Mixes

Stiff mixes take more effort to cast, and if care is not exercised in placement, air will get trapped between the concrete and the form. This results in large, shallow “craters” that often are on the order of 1/16”deep. The resulting surface is ugly and inconsistent in appearance, and it is difficult to repair by grouting.

craters in stiff cast concrete

Fluid Mixes

Fluid mixes are quick and easy to cast, as long as you take care with the reinforcing steel as noted in Difference #4: Forming Techniques.


As you can see, the choice of a stiff mix versus a fluid mix has many implications for all steps of the concrete countertop production process. Understanding the interplay of these implications is important for producing a high-quality concrete countertop.

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