You have a wide variety of aggregate choices. However, we are facing the novice versus experienced architectural concrete maker. Lightweight aggregates such as pumice, expanded shale, porous lava, clay, slate and air-entrained glass beads can be used and they will reduce the weight of the concrete countertop from approximately 140 lbs. per cubic foot to 110 lbs per cubic foot. Lightweight aggregates can create problems with strength and finished appearance and we recommend that the novice craftsman stay with standard aggregates.

Figure C - 3/8 inch white pea gravel

We suggest that you purchase a natural gravel aggregate, sometimes referred to as pea gravel that has a maximum diameter of 3/8 of an inch, as shown in Figure C. This gravel will give you a strong and durable concrete countertop. Natural gravel aggregate has advantages over crushed rock and lightweight aggregates.

Note: When purchasing your aggregate from your local building material supplier always ensure that they are not going to substitute crushed rock for natural gravel. In most circumstances crushed rock is a valid substitute for pea gravel, however it is not the case when building architectural pieces such as concrete countertops.

Additives

There are all sorts of concrete additives available in the market place. Some are valid and others do very little. For architectural concrete products such as concrete countertops you should only concern yourself with three or types of additives:

1. Water reducer
2. Polypropylene fibers (Additional reinforcement)
3. Pigments (Colors)

Water reducer

Water reducers are used to lower the water to cement ratio, increase slump and/or to reduce the cement content. Adding a water reducer allows you to reduce the amount of water in the concrete mix which makes for a stronger concrete and helps to keep the concrete from cracking.

Water reducers come in three basic grades; low range, mid range, and high range. For concrete architectural pieces such as countertops you should use a high range water reducer, such as Glenium 3000 made by BASF that has a super-plasticizer incorporated in the product which will reduce the required water by 10 to 40%.

Additional reinforcement (Fibers)

Fibers, as shown in Figure 20, help to prevent hair line cracks in the concrete. The fibers are made from many materials such as nylon, polypropylene, fiberglass, PVA, cotton, steel and synthetic/steel blends and each manufacturer will tell you that their product is the best. We recommend that you use fibers made from polypropylene such as Durafiber or Fibermesh 150, for concrete architectural products.

Figure 20 - Concrete reinforcement fibers

Note: When adding fibers to the concrete during the mixing stage, they need to be added slowly to the mixture or they will clump in the concrete mix.

Figure 21 - Cement pigments

Coloring the concrete (Pigments)

Professional manufacturers of concrete countertops will blend numerous pigments, as shown in Figure 21, in order to obtain the perfect color. This ability to blend pigments comes from years of experience and experimentation. We recommend that you stick to the standard colors, as shown in Figure 22.

If you do want to create a unique color you should make some samples to ensure that your blending technique is exactly what you want.

Note: There are other things that affect the color of the concrete. The grey color of the portland cement and the brown/tan color of the sand can vary, which will affect the final color of the concrete countertop. As well, the curing of the concrete, how and if it is ground and the sealer used on the countertop will all affect the final color.

Note: The addition of a pigment into the concrete mix does have a negative effect on the overall durability and strength of the concrete. The amount of pigment put into the concrete mix should never exceed 10% of the weight of the cement. For the best results try to keep the pigment weight to less than 8% of the weight of the cement.

Note: Some concrete pigments fall under the guidelines of a hazardous material because they contain heavy metals and hence must be used with caution and with the proper safety gear and the waste by-products must be dealt with as hazardous waste.

Figure 23 - Standard cement pigment colors

Continued - How to prepare the concrete mix for the countertop

How to Make Concrete Countertops - Index