[Posted on February 25th, 2013 by Ashley Aguirre]
Before you are ready to design your own Houston Concrete pattern, there is a fair amount you will need to learn about the properties of concrete mixes. One of the most important properties of concrete is aggregate size.
Houston Concrete mix is made from three basic ingredients: cement, aggregate and water. Aggregate is the inert filler that is mixed into the concrete to give it structure. Aggregate is perhaps the most overlooked concrete ingredient, but it just so happens to serve a very important purpose.
Aggregate comes in a variety of sizes from roughly 1.5 inches in diameter to fractions of an inch in size. The size of the aggregate you need depends on your ultimate goal for your concrete. Most concrete contains fairly large pieces of aggregate, as big as 1 to 1.25 inches in diameter. This works well for most uses, but it can often be much too large for stamped concrete.
In most cases, large concrete aggregate is ideal because it cuts down on costs. It also makes the concrete less susceptible to shrinkage and cracking as it dries. But large aggregate is not ideal for patterned concrete. Large aggregate interferes with the patterns stamped into concrete. Generally aggregate under 3/4inch in diameter is best for stamped concrete. How low you go beneath that aggregate size is generally up to you and what is best for your pattern.
In general, the deeper the pattern, the smaller the aggregate size should be. A great way to gauge what is appropriate for your pattern is to do a test run on small batches made with aggregates of different sizes. But once you go smaller than 1/2-inch your aggregate may begin to suffer with shrinkage issues. The less inert filler in a mix, the more liquid arises. And liquid shrinks; the more liquid, the more evaporation, and the more shrinkage.
However, you don’t always have to trade shrinkage for aggregate size. Some professionals go to aggregate sizes as small as 3/8-inch or smaller to achieve more uniform color distribution and a smoother aesthetic that allows patterned concrete to mimic other materials.
Those who use this small aggregate must take steps to minimize the likelihood of sever shrinkage and cracks. When working with smaller aggregate, be sure to place control joints in the concrete every 10 feet or less. This will prevent cracking and minimize shrinkage that could otherwise ruin the job you spent so much time shaping and perfecting.
Another great idea for reinforcing concrete with small aggregate size is steel reinforcement. Steel reinforcing bars that are 1/2 inch in diameter help to hold the concrete together to prevent shrinkage. If the concrete does shrink, the steel reinforcing bars help to hold the concrete together and keep the cracks from widening.
As an added bonus, steel reinforcing bars keep adjacent pieces of concrete close together to prevent the degradation of your pattern work over time. And because it is so inexpensive and takes virtually no time at all to install before the concrete goes down, there really is no reason to avoid this additional step to achieve a quality finished project. The more you know about concrete, the better your first concrete job will go.