Concrete is basically a mixture of portland cement, sand, gravel and water. Its strength increases with age through a curing process called hydration. This process can take years to reach maximum strength as the cement and water chemically react to bind the sand and aggregate into a continuous, solid form.
For this reason, proper curing is a critical step that ensures a particular mix reaches full potential for strength and durability. Proper curing depends on two factors: Temperature range and available water. After concrete has been poured, the surface needs to be kept moist to allow unhindered hydration. If the weather is too hot, the concrete can lose water to evaporation; if allowed to freeze, hydration will stop altogether–in either event, hydration is compromised and full strength is never reached.
Concrete is often tested for strength to ensure a particular mix complies with an engineer’s specifications.
Factors That Contribute To Concrete Quality
Air is a little know ingredient in concrete, but one we pay close attention to. Known as “Air-entrained” concrete, this terms speaks to the presence of homogenous, microscopic voids throughout the mix. Due to the nature of its composition, concrete will always contain some moisture, however slight. When this moisture is allowed to freeze, these voids provide a place for the freezing moisture to expand; this results in a more stable concrete that will be less susceptible to cracking over its life. When properly entrained, air makes up roughly 4-6 percent of the total volume.
Not just freezing water, but also temperature and moisture content can be factors in the changing volume of cured concrete. The amount of changing volume, although quite small (perhaps only 1/16 inch in 10 feet) is enough to cause cracking. One way to minimize this effect is through “expansion” joints. These joints may be introduced while the mix is still wet; or may be cut into the cured concrete later, using a saw. Either way, these surface joints encourage the concrete to crack along nice, straight lines when volume changes–rather than breaking randomly and causing unsightly faults.
When the surface of concrete flakes-off, this is known as spall–and is often caused by insufficient entrained-air in climates susceptible to freeze-thaw cycles. Another cause of spalling is too much water in the mix. If the water/cement ratio is too high, this can also weaken the surface. When finish troweling, wait until the water sheen is gone and the surface looks dull–this gives the cement more time to absorb the available water and will reduce the chance that spall may occur later.
The water and cement are what make the “glue” that combine with everything in the mix to form a solid mass. The ratio of water to cement is an important one. Formulating a weaker or a stronger mix is as simple as increasing or decreasing the amount of cement. More cement means stronger concrete. One way to cut expenses when mixing concrete is to add “fly ash”. Fly ash is an industrial by-product of burning coal that is finely powdered and light gray in color. By adding it to concrete, it is possible to lessen the cement content and thereby lessen the overall cost of the mix. Although fly ash has its uses in the concrete industry–strengthening concrete is not one of them. At Concrete Advantage, we never cut our mix with fly ash to reduce cost and use only 100% portland cement for a full-strength water/cement ratio.