Concrete has long established itself as one of the most durable and trustworthy of all paving materials. In addition, thanks to decorative techniques developed in recent decades, contractors can now install concrete with a wide range of color, texture, and pattern options. Yet even the most attractive concrete slab may succumb to problems that undermine its structural integrity.
Unfortunately, many home owners fail to appreciate the importance of installing and maintaining concrete correctly. Don't make the mistake of assuming that concrete is indestructible. This article takes a closer look at two common causes of concrete damage as well as the ways a professional contractor can help keep them at bay.
1. Plastic Shrinkage Cracks
Concrete consists of three key ingredients: stone aggregate, cement, and water. During the curing phase, water and cement undergo a hydration reaction, forming hard, crystalline structures that bind the aggregate together. In order to make concrete easy to pour, however, most concrete contains more water than needed for hydration.
As the concrete cures, excess water evaporates out of the surface, leading to small changes in the overall volume of the slab. So long as the rate of evaporation does not exceed the concrete's bleed rate, this phenomenon does not pose any serious problems. However, when evaporation happens too quickly, water rises up out of the slab and the concrete develops networks of random cracks.
Such cracks go by the name of plastic shrinkage cracks. Concrete stands a much greater chance of developing such cracks when installed during periods of hot and/or low-humidity weather. Smart contractors carefully consider such environmental considerations prior to pouring the concrete.
A number of techniques exist for keeping plastic shrinkage cracks at bay. Contractors may erect wind or sun shields to reduce environmental exposure. Misting machines offer an effective way to prevent cracks, by ensuring that the surface of the concrete remains moist. Finally, contractors can include certain additives in the concrete mix to retard the rate of evaporation.
2. Alkali-Aggregate Reaction
Concrete may also develop unattractive and structurally weakening cracks after it has fully cured. In some cases, such cracks form as the result of chemical reactions provoked by the aggregate used in the mix. Aggregate that contains excessive amounts of silica may fall prey to damaging alkali-aggregate reactions.
Most of the aggregate used in concrete today consists of crushed granite, which for the most part has a chemically inert composition. Yet depending on economic and aesthetic considerations, builders often choose to use other types of stone aggregate. Likewise, glass aggregate, often used for decorative purposes, contains naturally high silica levels.
The presence of such silica leads to the highly problematic alkali-silica reaction when wet. As the silica interacts with alkali from the cement, it forms a type of gel capable of absorbing large amounts of water. As the gel sucks up more and more water, it creates expansive pressures inside the concrete, which often cracks as a result.
Good concrete contractors possess a thorough knowledge of the chemical composition of the aggregate used in a particular mix. If high silica levels exist, contractors can prevent the alkali-silica reaction by incorporating certain additives into the mix. Such additives include silica fume, fly ash, and blast-furnace slag.
A second type of alkali-aggregate goes by the name of the alkali-carbonate reaction, which takes place when aggregate contains significant amounts of dolomite. Fortunately, contactors rarely encounter this issue, since dolomite-containing rocks usually don't have qualities that make them suitable for aggregate.
Incorrect mix design, installation, and maintenance can lead to unwanted concrete damage. To learn more about how install a concrete pavement that will last for years to come, please contact the building experts at Supreme Concrete LLC.