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 trowelled 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.