I’m as guilty as anyone in making concrete countertop mixes far more complicated than they need to be. Part of it is my background – I’ve spent the better part of 30 years in the concrete and cement industries. Part of it is my personal commitment to make countertops, and now products to make countertops, that will stand the test of time.
I believe in solutions rather than compromise. Some say, “Cracks are an inherent part of the material.” Are they? Should they be? Some say, “You need to keep your countertops in the mold for multiple days.” You do? Why would you want to do that? Some say, “GFRC is better than wet cast.” How about the wonderfully artistic looks that can be achieved using wet cast?
I’ve been spending a lot more time lately working with fabricators, many of them new to concrete. I’ve gotten to the point where a half hour in person or on the phone is enough to get fabricators started with a serviceable mix design; after all we’re only talking about blending four different ingredients. How hard could that be?
So here are some simple formulas you can use to simply the process:
Wet Cast 50-50-38-11 Mix
This formula for wet cast makes about 1 cu. ft.
1. Sand, 50 lbs. – Go to a local readi-mix producer or quarry and purchase some “concrete sand.” That’s right it’s called concrete sand. It is a blend of different sized particles combined to meet the ASTM specification for concrete sand. It’s ubiquitous, and you’ll make good concrete with it. Any other sand with a 20-70 or so gradation will work, make test batches first. Sand with all the particles being the same size will be tougher to work with, often times causing bugholes or other blemishes.
2. Stone, 50 lbs. – Here again find stone 3/8-in. pea gravel from a local ready-mix guy or quarry. Glass, granite or any other sound aggregate will also work. Start with 50 lbs./cu. ft. and adjust from there.
3. Cementitious material, 38 lbs. – You can use straight Portland cement (white or grey) or 30 lbs. Portland combined with up to 8 lbs. of a pozzolanic material such as slag. Either option will work.
4. Water 11 lbs. maximum – If you stay under 11 lbs. of water total (accounting for any water in the sand) for this mix, you will be making about 8,000 psi wet cast concrete. This is strong enough to handle just about any situation. This will not be enough water to make a nice fluid mix without the help of admixtures (see below).
5. Plasticizer (water reducing agent) – This admix is used to increase the fluidity of the material without lowering the strength. When adding this, make sure to follow the manufacturer’s specification. I recommend Trinic’s System 1 Wetcast Admix. Using this admix, start with 2 percent of the cementitious material weight (38*0.02=0.76 lbs.). If this makes your mix fluid enough to cast, great run with it. If you want it more fluid, add more admix, not more water.
6. If you want to make higher strength material, use more of the plasticize and less water. With the System 1 Wetcast Admix, you can make 13,000 psi concrete by using it at 4 percent of the cementitious material weight and backing your water way down. You would also need to pay care full attention to your aggregate gradation to achieve this.
7. You can add fiber if you like or go old school with ladder wire reinforcement.
8. Add heat – If you’re not heating your tops you should be. An electric blanket from any department or discount store works fine. Lay plastic on the tops, then the heating blanket, then an insulating blanket. Put them to bed before you leave the shop and get them more than 110 degrees F at night, and you’ll be fine to strip after 12 to 24 hours.
GFRC 60-60-18 Mix
This formula for glass fiber reinforced concrete (GFRC) makes about 1 cu. ft. Depending on whether you are spraying it or making a self-consolidating mix, the formula will differ a bit.
1. Sand, 60 lbs. – Spray up: You can buy fine sand from a big box store. You can also buy mason’s sand in bulk from a brick supply yard. You may need to screen the mason’s sand for the face coat. Self-consolidating mix: you can use just about any sand including coarser concrete sand. In fact, courser concrete sand, which contains a range of particle sizes, will give you higher strength than fine sand.
2. Cementitious material, 60 lbs. – You can use white or grey cement or a combination of cement and pozzolan.
3. Water, 18 lbs. – If you stay under 18 total pounds of water for this mix, you will be making GFRC in the 8,000 to 10,000 PSI range.
4. Plasticizer – Follow the manufacturer’s directions, but if you are spraying, you will want to use a lower percentage of plasticizer than if you are making a self-consolidating mix. I recommend System 1 GFRC Admixture. If using System 1 and spraying, you should start with 1 percent of the cementitious material weight. If you are making self-consolidating material, you should start with 3 percent of the cementitious material weight and use more as required. You don’t need any additional additives. To make 15,000 psi concrete, dose the System 1 GFRC Admix at 4 percent and back the water way down. You’ll have to play with your aggregates a bit to hit this mark – silica sand in a blended range of sizes would be a good starting point for super high performance GFRC.
5. Fiber – About 3 to 4 percent of the total batch weight should be alkaline-resistant (AR) fiber.
6. Add heat – Once again, if you’re not heating your tops you should be. Lay plastic on the tops, then a heating blanket, then an insulating blanket. Just as with the wet cast, you can put them to bed before you leave the shop, and as long as they reach more than 110 degrees F overnight, you can strip them after just 12 to 14 hours.
And for a sealer, I recommend a silicon-based penetrating concrete sealer. It’s a game changer for fabricators, allowing them to offer concrete with excellent stain-, etch- and scratch-resistance.
There are hundreds of fabricators using these mixes with great success. Sometimes simpler is better.
About the Author
Mark Celebuski has decades of experience in the concrete and cement industries and currently is with Trinic (www.trinic.us). He can be reached at [email protected] or at (800) 475-1875.
