This article contains detailed concrete countertop mix recipes for GFRC. Before we give a specific mix recipe, it is important to understand several principles of concrete countertop mix design. You should also understand the basics of what GFRC is. If you are not familiar with GFRC, please click here.
Principles of Concrete Countertop Mix Design
GFRC typically uses a fiber-free “mist” or “face” coat that is applied to the visible surface of the forms, followed by a fibrous “backer” coat. The mist/face and backer coats should all use the identical base mix design.
The main ingredients of your mix are sand, Portland cement, Integrity Ultra Adpack, water, and AR glass fibers. The total cementitious material (TCM) represents the weight of the reactive ingredients in the mix: Portland cement plus the reactive components in the Integrity Ultra Adpack. The TCM is reported for each of the mix recipes, listed below.
The proportions of the first three main ingredients (Sand, Cement, Adpack) are important. They have been very carefully selected to produce the strongest concrete possible. Do not change the ingredient proportions of sand, cement or Adpack in the recipes below.
Two other important ingredients that may be varied within a reasonably narrow range are the amount of water in your mix and the AR glass fibers.
- Water cement ratio: w/c = 0.30. Water is a very important ingredient that must be dosed carefully. Click here for more information about what w/c means and the importance of water in concrete.
- A w/c of 0.30 is a good starting point. Vary w/c as needed within the given range. Try increasing it by 0.01 if your mix is a bit stiff or reducing it by 0.01 if your mix looks too wet. The maximum allowable w/c is 0.32.
- Fiber dose: Add to the backer coat 2.5% AR glass fibers by weight of total material. This includes water, pigment, and all other ingredients. Click here for more information about how fibers are dosed in GFRC.
- For lightweight, low-stressed decorative pieces, the fiber content may be reduced to 2% (but no less), and for high stress pieces, it may be increased to 3%.
- Using fewer fibers increases flowability but reduces strength, and it makes your concrete more brittle and fragile. Adding too many fibers (more than 3%) often reduces both flowability and strength because the extra fibers make good compaction difficult, trapping air voids in the concrete that reduce strength.
Other ingredients, like pigments and superplasticizer, can be varied the most. Pigments can have a useful range of 0% to 6%. Integrity UltraFlow superplasticizer has a useful range of 0% to 0.85%.
- Pigments and superplasticizer are always dosed based on total cementitious weight. For example, if the TCM amount is 10 kg, and the UltraFlow dose is 0.50% (equal to 0.005), then the amount of UltraFlow required would be 0.005 x 10 = 0.05 kg (or 50 grams). The TCM for each mix recipe is shown below.
The following recipes use exactly the same ingredient proportions but have different batch sizes depending upon your needs. Note the area of concrete in square feet and the volume in cubic feet each recipe will make. This is useful for splitting up your project into manageable batches.
Note that our GFRC Kits contain ingredients in the correct proportions. This allows you to follow the recipes below with minimal waste of materials.
Concrete Countertop Mix Recipes for Direct Cast GFRC
The direct cast method of making GFRC is popular with concrete countertop professionals. They mix the AR glass fibers directly into concrete and simply pour the concrete into the molds at the total desired thickness of the piece.
The following recipes are based on a volume of concrete. This is useful when you are planning on filling your forms full to make solid concrete. They are also useful for sizing your batches and planning your project ingredient amounts.
This recipe makes a volume of concrete that can easily be mixed in a 5-gallon bucket:
| English: | Metric: |
| Sand – 20.00 lb | Sand – 9.072 kg |
| Portland cement – 15.82 lb | Portland cement – 7.176 kg |
| Integrity Adpack – 4.88 lb | Integrity Adpack – 2.214 kg |
| Water (w/c = 0.30) – 6.00 lb | Water (w/c = 0.30) – 2.722 kg |
| AR Glass Fibers 19mm (2.5%) – 1.20 lb | AR Glass Fibers 19mm (2.5%) – 0.544 kg |
| Total Cementitious Material: 20.00 lb | Total Cementitious Material: 9.072 kg |
| Total Concrete Weight: 47.90 lb | Total Concrete Weight: 21.727 kg |
| Makes 5.55 sq ft @ 3/4″ thick, 4.15 sq ft @ 1″ thick, 0.34 cubic ft | Makes 0.52 sq m @ 20 mm thick, 0.39 sq m @ 25 mm thick, 0.010 cubic meters |
This recipe makes 1 cubic foot of concrete:
| English: | Metric: |
| Sand – 58.02 lb | Sand – 26.320 kg |
| Portland cement – 45.88 lb | Portland cement – 20.810 kg |
| Integrity Adpack – 14.15 lb | Integrity Adpack – 6.419 kg |
| Water (w/c = 0.30) – 17.41 lb | Water (w/c = 0.30) – 7.896 kg |
| AR Glass Fibers 19mm (2.5%) – 3.54 lb | AR Glass Fibers 19mm (2.5%) – 1.605 kg |
| Total Cementitious Material: 58.02 lb | Total Cementitious Material: 26.320 kg |
| Total Concrete Weight: 139.00 lb | Total Concrete Weight: 63.049 kg |
| Makes 16.14 sq ft @ 3/4″ thick, 12.12 sq ft @ 1″ thick, 1.00 cubic ft | Makes 1.50 sq m @ 20 mm thick, 1.13 sq m @ 25 mm thick, 0.028 cubic meters |
This recipe is based on 1 bag (30 lbs) of Integrity Ultra Adpack:
| English: | Metric: |
| Sand – 123.00 lb | Sand – 55.792 kg |
| Portland cement – 97.25 lb | Portland cement – 44.152 kg |
| Integrity Adpack – 30.00 lb | Integrity Adpack – 13.620 kg |
| Water (w/c = 0.30) – 36.90 lb | Water (w/c = 0.30) – 16.753 kg |
| AR Glass Fibers 19mm (2.5%) – 7.50 lb | AR Glass Fibers 19mm (2.5%) – 3.405 kg |
| Total Cementitious Material: 123.00 lb | Total Cementitious Material: 55.842 kg |
| Total Concrete Weight: 294.65 lb | Total Concrete Weight: 133.771 kg |
| Makes 34.22 sq ft @ 3/4″ thick, 25.69 sq ft @ 1″ thick, 2.12 cubic ft | Makes 3.18 sq m @ 20 mm thick, 2.39 sq m @ 25 mm thick, 0.060 cubic meters |
Concrete Countertop Mix Recipes for Hybrid GFRC
The hybrid method for GFRC involves spraying or placing a fiber-free “mist” or “face” coat onto all visible surfaces of the molds, and then hand applying or spraying a fibrous “backer” coat. A mist coat is typically about 1/8″ thick, while a face coat is a bit thicker. The main thickness of the finished piece is due to the backer coat, which is typically made 3/4″ thick. 1″ thick is also possible.
Therefore, this section contains four recipes: mist coat at 1/8″ thick, face coat at 3/16″ thick, backer coat at 3/4″ thick, and backer coat at 1″ thick.
The following four recipes make enough concrete to cover 10 square feet of forms. If your forms are bigger or smaller than 10 square feet, simply scale all of the numbers in each recipe by that amount.
Example: Your forms are 25 square feet.
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- You need to use a scaling factor which is found by dividing the area of your forms by 10 square feet: 25 / 10 = 2.5.
- You would then multiply each ingredient weight in the recipe by the scaling factor (2.5 in this case).
- The amount of sand for a mist coat would be 6.17 lbs * 2.5 = 15.43 lbs, Portland cement = 4.88 lbs * 2.5 = 12.20 lbs, Integrity Adpack = 1.5 lbs x 2.5 = 3.75, and water = 1.85 lbs * 4.63 lbs. This makes a total of 36.0 lbs of mist coat.
This recipe makes 10 sq ft of Mist Coat @ 1/8” (2/16″) thick:
| English: | Metric: |
| Sand – 6.17 lb | Sand – 2.797 kg |
| Portland cement – 4.88 lb | Portland cement – 2.214 kg |
| Integrity Adpack – 1.5 lb | Integrity Adpack – 0.683 kg |
| Water (w/c = 0.30) – 1.85 lb | Water (w/c = 0.30) – 0.839 kg |
| Total Cementitious Material: 6.17 lb | Total Cementitious Material: 2.797 kg |
| Total Concrete Weight: 14.40 lb | Total Concrete Weight: 6.532 kg |
| Makes 10.00 sqft @ 1/8″ thick, 0.10 cubic ft | Makes 0.93 sq m @ 3 mm thick, 0.003 cubic meters |
This recipe makes 10 sq ft of Face Coat @ 3/16” thick:
| English: | Metric: |
| Sand – 9.25 lb | Sand – 4.196 kg |
| Portland cement – 7.32 lb | Portland cement – 3.319 kg |
| Integrity Adpack – 2.26 lb | Integrity Adpack – 1.024 kg |
| Water (w/c = 0.30) – 2.78 lb | Water (w/c = 0.30) – 1.259 kg |
| Total Cementitious Material: 9.25 lb | Total Cementitious Material: 4.196 kg |
| Total Concrete Weight: 21.60 lb | Total Concrete Weight: 9.798 kg |
| Makes 10.00 sqft @ 3/16″ thick, 0.16 cubic ft | Makes 0.93 sq m @ 5 mm thick, 0.004 cubic meters |
This recipe makes 10 sq ft of Backer Coat @ 3/4” thick:
| English: | Metric: |
| Sand – 36.00 lb | Sand – 16.329 kg |
| Portland cement – 28.48 lb | Portland cement – 12.918 kg |
| Integrity Adpack – 8.78 lb | Integrity Adpack – 3.983 kg |
| Water (w/c = 0.30) – 10.80 lb | Water (w/c = 0.30) – 4.899 kg |
| AR Glass Fibers 19mm (2.5%) – 2.16 lb | AR Glass Fibers 19mm (2.5%) – 0.980 kg |
| Total Cementitious Material: 36.00 lb | Total Cementitious Material: 16.329 kg |
| Total Concrete Weight: 86.22 lb | Total Concrete Weight: 39.109 kg |
| Makes 10.00 sqft @ 3/4″ thick, 0.62 cubic ft | Makes 0.93 sq m @ 20 mm thick, 0.018 cubic meters |
This recipe makes 10 sq ft of Backer Coat @ 1” thick:
| English: | Metric: |
| Sand – 48.00 lb | Sand – 21.772 kg |
| Portland cement – 37.97 lb | Portland cement – 17.221 kg |
| Integrity Adpack – 11.71 lb | Integrity Adpack – 5.312 kg |
| Water (w/c = 0.30) – 14.40 lb | Water (w/c = 0.30) – 6.532 kg |
| AR Glass Fibers 19mm (2.5%) – 2.87 lb | AR Glass Fibers 19mm (2.5%) – 1.304 kg |
| Total Cementitious Material: 48.00 lb | Total Cementitious Material: 21.772 kg |
| Total Concrete Weight: 114.95 lb | Total Concrete Weight: 52.140 kg |
| Makes 10.00 sqft @ 1″ thick, 0.83 cubic ft | Makes 0.93 sq m @ 20 mm thick, 0.023 cubic meters |