A. Concrete shrinks as it dries and will continue to shrink at a very minor pace over time. Millwrights employ specific grouts for setting machines that expand when they cure, although they are not commonly utilized in construction.
All concrete shrinks somewhat as it cures and, once set, expands or contracts depending on the temperature. Cement does not break down at normal temperatures, so it will never disappear completely. However, some of it may be lost to vaporization if the temperature reaches 1400 degrees F (744 degrees C).
Cement also absorbs water during mixing and until it is fully cured - which can take a month or more. This means that concrete will increase in volume as it cures.
The rate of expansion depends on the type of cement used. Ordinary Portland Cement (OPC), which is the most common type used for building concrete, has an average coefficient of thermal expansion of 3 × 10-6 per °F (37.4 × 106 per °C). This means that it will expand by about 3/10 of 1 percent per degree F (1.1 percent per degree C) when it is at a constant temperature. If the temperature changes by 100 degrees F (38.9 degrees C), then the concrete will expand by about 1/8 of 1 percent or 0.125 percent.
Concrete that is used in construction projects is usually provided in a plastic form and then poured into the desired shape.
Concrete does expand and contract—usually in minute proportions—due to the moisture content of the concrete. Does concrete totally dry? The process of "drying" concrete is known as "curing." During curing, water is driven out of the cement paste by heat from the atmosphere or water from an injected gas such as air or carbon dioxide. This process occurs primarily during the first seven days after casting. After this time, the concrete begins to self-cure, or harden, on its own.
If concrete is cut, there is a risk that it will split due to internal pressure changes. This usually happens at night when the outside temperature is lower than the inside temperature. The sun's heat causes the concrete to cure and release some of its internal pressure. If the concrete is cut while it is still wet, it will re-seal itself before morning. Dry concrete does not absorb any more water and cannot be made liquid again without adding water. So, completely dry concrete can never swell or shrink.
Concrete that has been exposed to sunlight or heated during construction may become soft for several weeks after it has set. This is called "green" concrete and must be treated with caution because it may not hold its shape well. Green concrete should be used only where no other type of concrete would be appropriate.
Keeping the concrete wet aids in the curing process. If too much water is evaporated from the concrete, the hardening process slows or stops. Concrete holds moisture after pouring and continues to build strength for as long as it retains moisture, although the longer it moist-cure, the slower the rate of strength increase. Moisture also is required by some types of cement used in making concrete, so any excess water must be removed.
Concrete needs water to set up properly. The amount of water needed depends on the type of concrete and how dry the air is when it's poured. Concrete requires water to prevent cracking caused by heat and gas pressure during setting of the concrete. As the concrete cures, its internal gas bubbles expand, which can cause cracks at or near the surface. Water allows these gases to escape before they cause serious problems.
The concrete mixer adds more ingredients to the batch recipe, mixes them together, and then feeds them into the truck or slab mold. The mixer does this until the mixture reaches the desired consistency. For example, if the mixture is too thick, more water may need to be added; if it's too thin, more cement powder may need to be incorporated.
Curing concrete begins immediately after it's poured. The closer the concrete comes to 100% hydration (the point where all the water has been absorbed), the stronger it will be.