Concrete is a material few woodworkers spend a lot of time with. Knowing its basic properties before you get out the garden hose and mixing bucket may help you avoid a few beginner's mistakes:
• Concrete is made from three basic elements: cement, water, and aggregate. The active ingredient is cement, made from calcium carbonate (limestone) and aluminium silicates (clay). The second ingredient is water. Mixed in, the water starts a chemical reaction which hardens the cement. Aggregate (sand or stone) is a passive filler ingredient that dramatically affects concrete's strength and durability.
• Concrete for building construction contains both sand and crushed stone to act as aggregates. In fact, some would say our countertop isn't concrete at all unless there's stone in it. We asked concrete countertop builders, and they agreed: call it concrete. The bigger faux pas is to call it cement (which is just one ingredient of concrete).
• Cement shrinks as it cures. Aggregate strengthens the mix by reducing overall concrete shrinkage, thereby reducing voids and cracking. A common proportion of aggregate to cement is three to one. This proportion maximizes the bonding power of cement and the non-shrink properties of the aggregate.
• The water necessary to pour the concrete is always more than is necessary to harden it'as you know if you've ever left a bag of cement on a damp floor'and too much water weakens the final product. The professional's most important measure of concrete strength is its 'slump,' which is simply a measure of its water content. We found a good ratio was 4 litres of water per 30 kg bag of sand mix.
• The longer the curing time, the harder the concrete. Drying too quickly weakens concrete. That's why keeping it damp with wet burlap, or occasionally soaking it with a sponge will give you a better, harder finish. Most concrete takes up to a month to harden completely.
• Concrete is very strong in compression, and weak in tension. A countertop doesn't need a lot of tensile strength because it is so well supported by the cabinet base.
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