The mix for concrete countertops is a lot like Mom's apple pie recipe. There are basic ingredients that are common and additional ingredients that depend largely on taste or personal preference. The rising popularity of concrete countertops has encouraged concrete manufacturers to formulate blends specifically for interior countertops. These may take some of the guesswork out of the job, but they can also limit the amount of control you have over the finished product.
For a concrete countertop to be strong and have an attractive finish, it must be built in two layers. These layers are made with different mixtures of concrete. The base layer provides structural strength to the countertop. The upper layer is formulated to provide a smooth, hard finish. These layers are mixed and poured separately.
The mix for the base layer of the countertop consists of concrete mix, a small amount of polypropylene fibres to reduce cracking and Portland cement for increased strength. Enough water is added to bring the mixture to a doughy consistency. It is then spread inside the mould. Metal diamond mesh and rebar are used to increase the strength of the concrete and reduce the chance that any future cracks will expand beyond hairline fractures.
Fine aggregate concrete, sand and Portland cement are used for the topcoat. This mixture should be doughy, able to hold its shape, as well. Polypropylene fibres are not used for the finished surface of the countertop as the ends of the fibres will stick up above the finished surface leaving a fuzzy finish that cannot be removed. Pigments and decorative aggregates like marble chips may be added in this mixture. This should be applied while the base layer is still wet.
Coat the exposed edges of the top of the counter with a water-thinned mixture of the topcoat. The thinned mixture will fill in small air bubbles and gaps in the edges of the concrete. Once the concrete sets to the point where a finger pressed into the surface leaves a noticeable print, it can be finished with a trowel.
The exact formulation of concrete varies between bags. The texture, strength and curing time of concrete varies with the formulation of the concrete as well as the heat and humidity at the job site. Making small test pieces is the only accurate way of judging a particular mixture.