Tag Archives: water cement ratio

Creating Concrete Countertops Using GFRC

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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.

Spray-Up

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).

Premix

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.

Hybrid

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.

Superplasticizer in all-sand concrete countertop mixes

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Recently I got the following question:

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:

Answer

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. My favorite is Optimum 380, a liquid polycarboxylate based superplasticizer sold by Fishstone (www.concretecountertopsupply.com).

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

The importance of water/cement ratio in concrete countertop mix design

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Three simple ingredients can be blended and proportioned numerous ways to make concrete:

  • aggregate
  • cement
  • water

In concrete, the single most significant influence on most or all of the properties is the amount of water used in the mix.

In concrete mix design, the ratio of the amount of water to the amount of cement used (both by weight) is called the water to cement ratio (w/c). These two ingredients are responsible for binding everything together.

The water to cement ratio largely determines the strength and durability of the concrete when it is cured properly. The w/c ratio refers to the ratio of the weights of water and cement used in the concrete mix. A w/c ratio of 0.4 means that for every 100 lbs of cement used in the concrete, 40 lbs of water is added.

Typical w/c ratios are as follows:

  • Normal for ordinary concrete (sidewalks and driveways): 0.6 to 0.7 
  • Specified if a higher quality concrete is desired: 0.4

The practical range of the w/c ratio is from about 0.3 to over 0.8.

  • A ratio of 0.3 is very stiff (unless superplasticizers are used).
  • A ratio of 0.8 makes a wet and fairly weak concrete.

Typical compressive strengths when concrete is properly cured are:

  • 0.4 w/c ratio –> 5600 psi
  • 0.8 w/c ratio –> 2000 psi.

The simplest way to think about the w/c ratio is to think that the greater the amount of water in a concrete mix, the more dilute the cement paste will be. This not only affects the compressive strength, it also affects the tensile and flexural strengths, the porosity, the shrinkage and the color.

The strength is reduced mostly because adding more water creates a diluted paste that is weaker. Think of it like over-diluting grape Kool-Aid. The more water you add, the weaker the Kool-Aid is.

grape koolaid

Explained more technically, more water results in larger spacing of the cement particles. As the crystals grow, they are too far apart to knit together and form strong bonds.

cement particles

Concrete with a higher w/c ratio is also more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is a large excess of water, that water bleeds out onto the surface. The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.

Using a low w/c ratio is the usual way to achieve a high strength and high quality concrete, but it does not guarantee that the resulting concrete is always appropriate for concrete countertops. Unless the aggregate gradation and proportion are balanced with the correct amount of cement paste, excessive shrinkage, cracking and curling can result. Good concrete results from good mix design, and a low w/c ratio is just one part of a good mix design.

Curing: An essential step to create high quality concrete countertops

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Curing. We all know it’s important, but what exactly is it, why is it important and what factors affect curing?

Adding water to portland cement starts a chemical reaction called hydration. As hydration proceeds over time, the portland cement and water are transformed into beneficial calcium silicate hydrate (CSH) compounds. These compounds are the glue that hold the aggregates together, creating the hard, solid material we know as concrete. There are other compounds that form during the hydration process, but they are not responsible for strength.

Portland Cement + Water = CSH (provides strength)

Curing is the process of maintaining moisture levels inside cast concrete so that hydration can continue. As long as free moisture and unhydrated cement exist inside the concrete, the strength, hardness and density will gradually increase. Practically speaking, curing is simply the process of keeping the hardened concrete moist so that it can continue to gain strength.

As the concrete gets stronger and denser, its porosity decreases. This is important, because early on the concrete is much more porous than when it’s older and has hydrated longer. Porous concrete loses moisture to evaporation quickly, and this can lower internal moisture levels and stop hydration. If the concrete dries out, it stops gaining strength. This is why it is so important to cover your concrete right after casting and keep it moist. When concrete dries out, it dies, just as a tomato seedling would die if it weren’t watered.

tomato seedlings watering can

When concrete is mixed, all the water needed for full hydration is present in the mix design. Often contractors add more to the concrete than needed for hydration, to make the concrete more workable. This extra water is called water of convenience. This extra water causes the cement particles to be too far apart to knit together into a strong matrix. It results in a longer set time and lower strength.

snowballs

Cement particles that are too far apart can’t knit together.

Very powerful superplasticizers make it possible to remove almost all of the water of convenience, leaving a little bit more than just the water needed for hydration. This is the ideal blend of just enough water for hydration, but not so much water that the cement particles are spaced too far apart.

It’s actually rare that concrete is cured until most of the cement is hydrated. This generally takes months or years to occur. Rather, the concrete is cured for as long as you need it to be to reach the desired strength. The length of curing time can vary widely depending upon the structure or item made out of the concrete, the mix design, the concrete’s temperature and the desired strength at a certain time, to name just a few factors.

For concrete countertops, clients are not willing to wait 28 days for their concrete to be delivered. Because of this, mix designs tailored for concrete countertops have high early strengths so that the concrete can be cast, cured, processed and delivered in a couple of weeks (or less). For example, ordinary construction grade concrete often achieves a compressive strength of 4000 psi in 28 days. It’s not unusual for a mix design for concrete countertops to reach that strength in only 2 days. With some advanced mixes, this can be achieved in a matter of hours. So for a particular design strength, different mixes require different curing times.

Another factor that will influence the curing time is temperature. Colder concrete gains strength much slower than warmer concrete. At 3 days after casting, concrete cured at 45 degrees F only has about 70% the strength of the same concrete cast at room temperature (73 degrees F). In contrast, concrete cast and cured at 90 degrees F has about 10% more strength than concrete cured at 73 degrees F. Over time these differences gradually become smaller, but often it’s the early (2 to 3 day) strength that is more important than the 28 day strength.

So remember, don’t use more mix water than you have to, keep your concrete evenly moist for at least the first couple of days, heat it if necessary. All this will allow your concrete to cure and strengthen. You will end up with higher quality concrete countertops and happier clients.

Good concrete countertop mix design for cast in place

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Key characteristics that define a good cast-in-place concrete countertop mix are finishability and shrinkage resistance. However, these two characteristics are at odds with each other and must be carefully balanced in order to produce a good cast in place concrete countertop mix that is a joy to finish and does not curl or crack.

Other beneficial characteristics worth mentioning include workability and flexural strength. While high compressive strength is not necessary (though it is impressive), high quality concrete, often a byproduct of creating a high compressive strength mix, is also beneficial and desireable. Additionally, adequate work time, high early strength and a good appearance add to the list of desireable characteristics.

The two key characteristics that are very important, finishability and shrinkage resistance, are often determined by the aggregate gradation and the cement to aggregate proportioning.

Finishability, that is, the ease of trowelling the concrete into a smooth, even, high-quality surface relies on a sufficient amount of cement paste and very fine aggregate to create enough cream to trowel. Cream is the fine portion of concrete that is floated to the surface early in the casting process and is worked and reworked during trowelling.

Shrinkage resistance is also influenced by the water-cement ratio, by the cement paste content and by the amount of fine aggregate. Whereas finishability benefits from more cement paste and fine aggregate, shrinkage resistance benefits from less cement paste that has a lower water-cement ratio, since that is what actually shrinks. Minimizing the fine aggregate preserves workability when the cement paste volume is reduced, because fine aggregate (sand) has much more surface area than coarse aggregate, so more cement paste is needed to coat and separate fine sand than is required for a coarse blend of aggregates.

A poor cast in place concrete countertop mix would have large aggregate of one size, say 3/8″, mixed with fine sand. This is a case of “gap grading”. A good mix will have well-graded aggregate.

aggregate gradation in concrete countertop mix