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.

Ancient Romans used cement to construct their aqueducts, roads and buildings, some of which still stand after nearly 2,000 years. The stronger improved cement that we use today, called portland cement, was invented in 1824 and was first used in the United States about 1875.

Ingredients

If you slice concrete open, you'll see a random mix of stones of many sizes called aggregate. Concrete manufacturers carefully grade the stones and mix them in exact proportions so they pack together in the concrete mix.

Grains of sand, which are also graded, lie tightly packed around the stones. You can also see a gray material - cement - filling the spaces between the sand and aggregate, coating their surfaces. Most binding agents, such as glue, harden as they dry. But the chemicals in cement react and harden as a result of getting wet.

Water

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.

Setting

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.

Curing

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

After the concrete sets, water becomes the key ingredient for curing. The longer concrete is wet, the stronger it gets. If it dries, the hardening stops. So it must be watered frequently. Pros sometimes even dam up the edges of the freshly poured project and flood the surface with water.

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.