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.

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!

Concrete

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.

 

How to avoid ghostly mistakes in your concrete countertops

Sometimes lines appear in concrete countertops where the reinforcing seems to show through. This is called ghosting. Since it’s October, and we will be celebrating Halloween here in the U.S., I thought I’d write about “ghosts”.

Ghosting is most often caused by changes in the concrete matrix due to aggregate segregation, changes in the water-cement ratio, and other changes in the cement paste. Any changes in the cement paste affect how it cures, and this affects the appearance of the finished product.

The primary cause of ghosting is pushing reinforcing material into the concrete after the forms are filled, which causes a pumping action directly under the reinforcing. This pumping action disrupts and changes the cement paste, which in turn results in ghosting.

Another cause of ghosting is hanging the reinforcing in the forms and pouring the concrete through it. If the stream of concrete is split by the reinforcing but does not get a chance to remix below it, the aggregate gets pushed aside but the paste and cream flow back together. This causes a paste/mortar matrix change directly below the reinforcing that will cure slightly differently, resulting in ghosting.

A third way ghosting occurs is when reinforcing already buried in fresh concrete is moved or vibrated. The movement causes localized segregation, which in turn causes ghosting.

Ghosting demonstrated by deliberately pushing the reinforcing through the concrete

 

Ghosting in an actual installed project from the Hall of Shame. The reinforcing isn’t even correct! Outlining the sink – please!

Other surface marks that resemble ghosting can be caused by leaky forms, excessive form release agent pooled or puddled in the forms, and items left on the curing concrete surface (like sponges, soda cans, polishing discs and hand pads). Anything that traps moisture in or on the concrete in some areas while other areas of the concrete remain uncovered and allowed to dry will cure differently. Concrete that is moist longer will cure more fully and the color will be more intense in those areas.

Ghosting is NOT caused by the reinforcing being too close to the visible surface. The fact that the reinforcing is close to the visible surface does not cause any changes in the curing of the cement paste.

Unfortunately, once these surface marks occur, there is no way to remove them. Prevention is the only way to ensure that ghosting doesn’t happen.

And to think, a ‘professional’ made these so-called concrete countertops

Have you come across a piece of concrete that was barely recognizable as a concrete countertop? In all my years in the industry, I’ve seen entirely too many. Below are just a couple that I’ve come across. It’s appalling. I can only hope the customer didn’t actually pay for these jobs….

There are many things wrong with these pieces of concrete; just one is the sealer. Get a FREE Concrete Countertop Sealer Success Kit that will explain how to use coating sealers without fear of failure. Coatings don’t have to be thick, plasticky, pockmarked, streaky, or uneven. When done right, they can look completely natural and provide excellent stain and scratch resistance. Learn how with this free kit.

Temperature and relative humidity: What they mean for you and your concrete countertops

Drying is an important process that must be managed and understood in order to avoid problems. At certain times drying is to be avoided, whereas at other times it is necessary. Understanding how temperature, dew point and humidity levels work together will shed light on how you can manage drying.

Concrete needs to stay moist in order for it to cure. But some sealers need the concrete to be dry in order for them to cure and stick properly. It’s a delicate balance. Temperature and humidity levels play a major role in whether your concrete or sealer cures properly, or whether your concrete stops curing, develops map cracking (or worse) or if your sealer has bonding and curing problems.

It’s well known that in hot conditions things dry out fast. So during the summer months (and for some of us summer temperatures have already arrived) it’s vital to make sure bare, exposed concrete that’s still curing stays moist. On the other hand, cold temperatures make it harder to dry things out. Cold concrete takes much longer to dry out because water does not evaporate as fast.

I will describe how drying is influenced by temperature, relative humidity and describe what the dew point is and how it relates to relative humidity.

Drying is also known as evaporation. The rate at which water evaporates depends upon several factors, including the temperature, relative humidity and air flow rate. Water evaporates very fast when it’s exposed to hot, dry fast moving air. Conversely water evaporates very slowly when it’s in cold, damp still air.

Temperature describes how much energy is available to drive evaporation. On a warm day more water can evaporate because there is more thermal energy available to do the work of evaporation. In contrast water has less available thermal energy to drive evaporation when it is cold out. Hence drying slows considerably when it’s cold.

Relative humidity is important because the more moisture that’s already in the air, the lower the rate at which water will evaporate and the less moisture that the air can hold.
Relative humidity is a measure of the current amount of water vapor in the air relative to the total amount of water vapor that can exist in the air at its current temperature, and is expressed as a percentage.

A relative humidity of 100% means the air cannot contain any more water vapor at that temperature, whereas a relative humidity of 50% means that the air only has only half as much water vapor as it can hold at the current temperature.

Dew Point is the air temperature at which the air is saturated with water vapor. Warm air can “hold” more water than cold air. When air at a given temperature can’t hold any more water it is fully saturated and is at 100% relative humidity. The air isn’t holding onto the water vapor. What’s being described is really the temperature when water vapor levels reach the saturation point.

Air flow rate is important, because as water evaporates, the layer of air above the water (or damp concrete) gradually becomes more saturated with water vapor. When evaporated moisture levels reach saturation drying essentially stops. Air flow increases the evaporation rate by “flushing” away the stagnant moist air above the concrete.
The following graph shows the relationship between air temperature, dew point temperature and relative humidity.

 

Moist Curing
Concrete needs to remain moist in order for it to cure. Generally the internal relative humidity needs to be above 80% to 85% relative humidity for hydration to take place.

Wetting the surface of bare concrete creates a barrier of water that prevents the moisture from inside the concrete from being drawn out when the surface moisture evaporates. The film of water on the surface is a relatively large reservoir that can evaporate without affecting the moisture within the concrete’s pores. Water on the concrete acts like a buffer.

When concrete is covered in plastic, the air trapped under the plastic quickly becomes saturated with water vapor. When this happens drying essentially stops.

When concrete dries out, the suction forces developed when water evaporates from the pores in the concrete can actually crush weak cement paste. The longer the concrete remains moist, the greater its strength and the greater the resistance to the suction forces. This means fewer cracks and less shrinkage and curling. Longer wet curing also reduces pore size, which means less moisture is available to evaporate and that the moisture has a harder time escaping out of the concrete.

Drying
Some sealers need concrete to be dry in order to stick properly. And some sealers will not cure properly if there is too much moisture in the concrete.

Temperature and relative humidity are important factors to pay attention to so that your concrete dries to the degree you need it to so that the all-important sealer performs the way you and your client expect it to.

If your shop is cold (like most shops are in the winter), the evaporation rate is slower, and because most shops have open sources of water in them (trench drains, wet grinding areas, etc.), the relative humidity is generally higher than outdoor conditions. These limit how much drying occurs. If your shop is at 100% relative humidity it doesn’t matter how long you “dry” the concrete. It won’t lose moisture and dry out because the air surrounding the concrete can’t hold any more moisture.

Remember: wet concrete doesn’t dry out in small, cold shops with still air. Hot shops with moving air that has a low relative humidity will cause rapid drying.

 Happy concreting! 

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.

pozzolans

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.

Conclusion

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 2 of 3

The last article talked about the importance of measuring ingredient precisely, especially water. It is also very important to understand that you must measure by weight, not by volume.

Suppose a mythical concrete countertop mix included cotton balls as one ingredient and golf balls as another. Clearly a much larger volume of cotton balls would be needed to make one pound, versus one pound of golf balls.

There are some ingredients which seem to have consistent weight-volume conversions, such as water. You know that one quart of water is 32 (fluid) ounces. So you could just measure your water using a container with ounce or quart markings on it, right?

Not so. Measuring containers are typically not designed to a high degree of precision. Watch this video to see why using measuring containers can lead to errors and inconsistency.

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.)

Kool-Aid

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.

Efflorescence part 3: Example of repairing a concrete floor

Consider the following scenario: A new urban condo has acid-stained concrete floors finished with an acrylic sealer. The unit downstairs is unoccupied and unheated. The owner notices efflorescence starting soon after move-in and progressively worsening over several months.

We use the example of a floor because concrete floors are generally more susceptible to efflorescence than concrete countertops, and because this actually happened in my condo building in downtown Raleigh, NC.

Efflorescence is occurring because the moisture in the slab from construction and from acid staining has been locked under the acrylic. Water vapor leaving the slab is drawing soluble salts to the warmer side of the concrete.

Fixing this floor starts with stripping off the sealer and then physically removing the efflorescence. Common means are scrubbing or washing with a dilute acid solution. However, you can’t just mop dilute acid (with a lot of water) all over the place. Doing so would pump more water into the concrete. The best solution is to use an automatic scrubber that washes, scrubs and vacuums in one step. This minimizes the amount of water that penetrates into the concrete.

Then the concrete must be allowed to dry thoroughly before lithium silicate densifiers are applied. Commercial dehumidifiers can speed drying.

The final step to finishing the floor depends upon the floor’s water vapor transmission rate, the aesthetics and the desired level of durability. The simplest solution is either to leave as is or to apply a renewable “wax” to the floor. Unlike acrylics, wax is unlikely to trap efflorescence. If it does, wax can easily be stripped and replaced. This process may not halt 100% of the efflorescence, but it will allow everyday cleaning to remove the slight residue that occurs. In areas such as under a bed that don’t get cleaned regularly, efflorescence may still occur.

For floors that require more protection (like restaurants), first conduct tests to determine the rate of vapor transfer. If the moisture levels are low enough, choose a vapor-pressure-resistant sealer based upon the manufacturer’s recommendations. Impermeable sealers that are able to resist the existing vapor pressure will not develop efflorescence because the water vapor cannot pass through the sealer.

Efflorescence part 2: Secondary efflorescence in concrete countertops and floors

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. Some types of salts simply get dissolved and precipitated onto the surface, while others react with atmospheric carbon dioxide to form mineral crystals.

efflorescende

Efflorescence on a polished concrete big-box store floor.

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.

Knowing that concrete contains soluble salts and that water and the movement of water through the concrete are the cause for efflorescence, then solving the efflorescence problem really boils down to controlling the movement of moisture into and out of the concrete. How this is handled varies depending upon a variety of factors, such as whether the concrete is new or old, cast on the ground or is an interior application.

Since moisture movement into, through and ultimately out of the concrete is how efflorescence forming salts move to and accumulate on the surface, the first step to controlling efflorescence started with the concrete itself, as discussed earlier in the primary efflorescence section.

Managing moisture is the next step to controlling, minimizing or even eliminating efflorescence. Dry concrete is far less likely to effloresce, so identifying moisture sources and then controlling the movement of that moisture into and out of the concrete becomes the key to shutting efflorescence down.

Consider a basic concrete floor slab cast directly on leveled, compacted ground. The concrete itself contains moisture, and if it’s made properly the concrete won’t generate primary efflorescence due to the bleedwater evaporating and leaving salts behind. But secondary efflorescence can, and probably will, occur. That’s because the ground beneath the slab is a moisture reservoir. If water was added to the sand or gravel to assist compaction, that added moisture will eventually migrate through the slab, carrying mineral salts with it and forming efflorescence. The reason vapor barriers are installed beneath concrete slabs is to isolate the concrete from the ground, which represents a large reservoir of moisture.

If the concrete floor is then sealed with an impermeable coating like a urethane or epoxy, vapor pressure or even hydrostatic pressure can cause blistering or sealer failure. Breathable coatings like acrylics allow water vapor to pass through the sealer, preventing blistering. But migrating water vapor can slowly cause salts to accumulate beneath the acrylic sealer, causing unsightly blushing or even sealer failure due to accumulated mineral deposits.

Commercial floors in elevated structures are not cast against the ground, so there’s no moisture reservoir beneath the floor to drive efflorescence. Yet these floors can effloresce too. That’s because water can enter the slab from the top during the course of finishing, acid staining and routine cleaning. Porous concrete absorbs water during mopping, and copious amounts of rinse water pump large volumes of water into the concrete. This moisture then leaches the salts out of the concrete, creating efflorescence.

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. Done properly, they offer a threefold benefit. Reduced porosity is the first. The second is that silicate hardeners consume free lime in the concrete. Thirdly, the silicate gel binds other soluble salts, making them difficult to leach out.

While chemical hardeners seem like the ideal solution, not all hardeners are effective. In fact, using the wrong type can actually cause efflorescence. Explaining this involves a little chemistry, so bear with me.

Three common forms of chemical hardeners are sodium silicate, potassium silicate and lithium silicate. Lithium silicate hardeners are the most effective and least likely to effloresce due to a combination of factors. Lithium ions are smaller, and compared to sodium or potassium silicates, there are fewer lithium ions for each silicate molecule. It’s the silicate portion that actually does the work, so in effect lithium silicates are more concentrated. Once the silicate molecule reacts with the available calcium or free lime in the concrete, the sodium and potassium ions are freed and become soluble. In contrast, lithium ions are not freed. Free ions can react with other substances in the concrete to form salts. These salts can then leach out and form efflorescence.

Finally, high concentrations of carbon dioxide (CO2) can cause or accelerate efflorescence. Concrete located in areas with gas, wood or oil-fired heaters will develop efflorescence faster than concrete stored in low CO2 concentrations.

Preventing Efflorescence in concrete countertops and floors: part 1

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.

efflorescence on a concrete wall

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

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. This post will cover Primary Efflorescence.

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 around 0.45 will produce a strong, dense mix that’s unlikely to have excessive bleedwater.

Fly ash adds workability and replaces some of the cement. Since it’s a pozzolan, it consumes the calcium hydroxide produced during cement hydration. Calcium hydroxide, also known as free lime, is a key efflorescence-producing compound. Pozzolans consume the calcium hydroxide and produce calcium silicate hydrates, which make the concrete stronger, denser and less porous. This reduces the likelihood of efflorescence by shrinking capillaries and plugging the pores within the concrete.

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. In young concrete, the capillary and pore structure is open and well-connected. As the concrete cures, the pores and capillaries get filled in and closed off, yielding a dense, more impermeable matrix. 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.

Conversely, concrete that dries out quickly soon after finishing is sponge-like, filled with cracks and interconnected pores that allow moisture to move into and out of the concrete. Rapid evaporation of moisture draws efflorescence-causing salts to the surface through the porous, micro-crack filled, weak concrete matrix. Not only will efflorescence happen, it will continue to happen because the concrete never had a chance to cure into a dense, solid mass.

Next post: Secondary efflorscence

Rebar size in Concrete Countertops: It can be too big.

The size of reinforcing steel in a countertop is an important consideration. Concrete countertops that are made with rebar that is too large in scale for the slab thickness are susceptible to telegraph cracking.

When concrete shrinks due to drying out, high tensile stress levels develop around proportionally oversized reinforcing. These stresses cause cracks that run along the length of the steel. Smaller diameter reinforcing has less of an affect on the concrete, so the same amount of concrete shrinkage develops much less tensile forces, thus the likelihood of cracks caused by the steel itself drops dramatically. Because all concrete shrinks to some degree, and shrinkage occurs over a long period of time, the occurrence of telegraph cracking may not appear until long after a countertop is installed.

In addition, oversized reinforcing occupies so much space inside a thin countertop slab that there is very little cover between the steel and the slab surface. Generally, reinforcing is fabricated in a grid arrangement, with strands running along the length of the slab and overlapped strands running across the width of the slab. When stacked, larger rebar can take up half of the total slab thickness, while smaller rebar takes up much less space.

 Oversized rebar can cause telegraph cracking

The sheer size of rebar that is too big places a significant amount of the steel closer to the visible surface of the countertop instead of down near the bottom of the slab. For example, a grid made from 3/8” diameter rebar held only 1/4” away from the bottom (the bare minimum cover for such a size) would place the top of the rebar at the midpoint of the slab, leaving only 1/2” of concrete cover between the steel and the visible surface. Contrast this with a grid made from 3/16” diameter wire held ¼” away from the bottom of the slab, and now there is 7/8” of concrete between the surface and the steel, a 75% increase in cover.

In addition to causing large stress concentrations in the concrete (and therefore increasing the likelihood of telegraph cracking), using oversized reinforcing actually decreases the load capacity of the concrete.

In the drawing above, the same concrete is reinforced with equal amounts of steel reinforcing (based on cross sectional area). The top drawing shows one piece of 3/8” (#3) diameter reinforcing steel while the bottom drawing shows four pieces of 3/16” structural reinforcing wire. The cross sectional areas are the same, so the tensile capacity of the steel is the same.

But because the four pieces of wire can be located lower in the slab, the load capacity of the wire-reinforced concrete is now 13% (lower reinforcing layer) to 78% (upper reinforcing layer) greater than the slab with the single #3 rebar, even though there is the same amount of steel in the concrete. In actuality, the structural wire has a higher strength than the rebar, so the difference in capacity is even greater.

Are concrete countertops still at risk of cracking if they have high compressive strength?

“My concrete has a high compressive strength. That means I don’t need reinforcing to protect against, right?”

Not necessarily, and not in all circumstances. All concrete, regardless of the mix design or the magnitude of the compressive strength, is much weaker in tension than in compression. And cracks are caused by tensile failures of the concrete.

The tensile strength of concrete is often only about 10% that of the compressive strength, but this is a rough average. Tensile strength is highly variable, difficult to predict, and is dependent not only on the ingredients and their proportions, but on the casting technique, the curing history, the amount, size and distribution of voids and defects (microcracks), etc. Even the amount of pigment or other admixtures can significantly reduce the tensile strength of the concrete.

Unreinforced countertops rely completely on the tensile strength of the concrete itself to hold everything together. Since it is already known that the tensile strength is variable and difficult to predict, relying solely on it is tantamount to gambling with a finished piece.

Often experimenters will initially try out small pieces with success, because small pieces don’t develop large tensile stresses when handled. However, success leads to bolder ventures, and longer slabs result. Eventually the sheer weight and size of the slab generate tensile stresses that overcome the concrete. Because there is no reinforcement to resist the tensile load after the concrete cracks, the slab suddenly fails and snaps in half with little or no warning.

Reinforcing steel adds ductility, the ability for a material to absorb deflection and overloading without falling apart. Unreinforced concrete fails in a brittle fashion. That is, it fails suddenly and without warning; one moment it’s fine, the next it’s broken. Don’t take these kinds of risks with your concrete countertops.

What causes cracks in concrete countertops?

As the old adage goes, all concrete cracks. What’s important to a client is that those cracks are not visible nor do they impact the performance of the countertop. Well-made concrete countertops should not develop structural cracks, however hairline cracks are possible and not a sign of poor quality.

Hairline cracks and larger structural cracks are signs of stress relief. A crack forms when tensile stress builds up in the concrete and exceeds the material’s capacity to resist those stresses.

Most large, structural cracks in countertops form because of flexing, either because a faucet was tightened too much or, as is the case in this picture, the house settled:

Flexural crack from house settlement:

crack in concrete countertop

Cracks in granite from over-tightened faucet:

crack in granite countertop due to faucet over-tightening

Multiple flex cracks in a overloaded cantilever beam:

crack in GFRC countertop due to flexing

Hairline cracks often occur because of shrinkage, either from drying or from heat. These types of cracks are more difficult to control because they generally occur near the surface, so reinforcing doesn’t help prevent them. The best preventative is to use a good mix design that has low shrinkage tendencies. However, hairline cracks can and do occur, and are often located near areas of moisture (sinks and dishwashers), where dry concrete repeatedly absorbs moisture and then dries out. Over time this wetting and drying cycle will cause the concrete to crack, much in the same way a piece of steel will eventually crack if it’s bent back and forth enough times.

Hairline crack at sink:

hairline crack by sink in concrete countertop

Heat also can cause hairline cracking. Crock pots are a common source of heat related hairline cracks in countertops. Often it’s not the intensity of the heat but the length of time the concrete is heated. Crock pots don’t get very hot, but they sit in one spot for many hours. As the concrete heats it expands, and the more concrete that does heat up and expand, the greater the thermal stress that develops. Generally it’s not just the heating that causes cracking, it’s also the subsequent cooling. As the concrete cools it shrinks, and it’s the shrinkage that causes cracking.

Thermal cracking in solid surface material from a crockpot:

thermal crack in solid surface countertop

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