The Recipe for Strong Concrete

Concrete walls, floors and sidewalks can last for centuries, but not all concrete lasts that long. Poorly produced concrete will crack, crumble and flake, sometimes at alarming rates.

Many people confuse the words concrete and cement. Concrete is the hard, strong material we walk on and use to construct buildings. Cement is one of concrete's ingredients, the glue that holds it together.

To make good, strong concrete, a crucial part of the mix is water, which moistens the powdered cement and transforms it into a thick paste. This coats all surfaces of the aggregate and sand so they'll stick together. Proper mixing evenly blends the small and large particles so the concrete compacts well.

The correct amount of water causes the microscopic crystals of cement to react, absorbing water in the process, growing closer together to hold the sand and aggregate more tightly. Too much water causes the crystals to grow farther apart and weaken the concrete. The amount of water added to make a good concrete mix is a compromise between strength and workability.

Excess water also causes concrete to lose its thick, syrupy consistency and become soupy. In a soupy mix, the aggregate sinks to the bottom and cement rises to the top. Result: weak concrete as a whole and the exposed surface in particular. A mix that's too wet will cause the surface to crack, chip off and powder.

After pouring, but before smoothing the surface, you have to let the concrete set - stiffen to the point where your foot will sink only about a quarter-inch into its surface if you stand on it. During this waiting period, excess water from the concrete rises to the surface. This bleed water is normal and is soon reabsorbed by the concrete. The more water there is in the mix, the longer you have to wait for the bleed water to be absorbed. Never add more water to the mix than the manufacturer recommends.

Once concrete stiffens and its surface is troweled smooth, curing begins. The longer the cure, the stronger the concrete.

You'll get fewer cracks with longer cures. But there's no escaping cracks because when concrete dries, it shrinks. While troweling, concrete masons control cracking locations by deeply grooving the surface at regularly spaced intervals to make weak spots called control joints. The concrete cracks at those joints and not randomly.