UNDERSTANDING CONCRETE
Concrete is made by mixing aggregates of varying sizes (sand, gravel, or crushed stone), Portland Cement, and water. The water and Portland cement mix together to form a new chemical compound that is referred to as paste. This paste – which is initially in liquid form – is then mixed with the aggregate. It flows into the spaces that occur between all of the individual pieces of aggregate. Curing begins during the initial combining of the ingredients that make up concrete and continues long into the future. The process of curing is defined as the chemical reaction that starts to occur when the Portland cement and water are combined. Curing causes the paste to harden, binding the aggregate together. Any water leftover from the original mixing that is not used in the chemical reaction remains as water droplets distributed throughout the paste portion of the concrete.
Although the terms curing and drying are often used interchangeably, it is important to note that they are quite different. Drying refers to the evaporation of water from the exposed surfaces of the concrete that occurs immediately after it is poured. Curing refers specifically to the chemical reaction described above. It is not unusual to have very high interior moisture levels in a concrete floor slab that is many months or possibly several years old. The water-to-cement ratio that occurs during the initial mixing is the primary factor that influences the amount of residual moisture contained in the concrete. The amount of water required to chemically react all of the Portland cement is fixed by the quantity of the Portland cement in the mix. It is common practice to add extra water making the concrete “soupy” so that it flows and levels more easily and makes it easier to place the freshly mixed concrete when pouring a floor, or in the case of high-rise construction extra water is added to make it easier to pump the concrete to the upper floors. Since the chemical reaction of curing occurs over time, some of the residual water trapped within the concrete becomes part of the cement paste as it continues to react or cure. In fact some of this additional water is important to the curing process that gradually increases the strength of the concrete. During the initial curing period (7 to 10 days), additional water must be added to the surface to replace the water that evaporates from the surface or a sheet of plastic is placed over the surface to inhibit surface evaporation. Curing compounds are also sometimes applied to the surface to inhibit surface evaporation.
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