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Concrete is a building material used worldwide for its strength, durability, and low cost. The raw materials to make it are readily available. One of the few disadvantages of concrete is that almost universally, the concrete mixture must be made on-site or within a reasonable distance in a concrete batch mixer. Concrete is the resulting material when cement, water, and a mixture of sand and gravel are allowed to react.
This universal building material is the result of an exothermic reaction between the molecules of a cement mixture and water. Cement consists of pulverized alumina, silica, lime, iron oxide, and magnesium oxide that have been heated to a sintering temperature at which surface tension decreases and the particles adhere to one another. The fine, finished powder yields a consistent product susceptible to reaction with water. Consistency of the mix is directly related to the quality of the finished concrete structure.
As water is added to the concrete batch mixer, the cement quickly becomes a paste and finally a mixture of a batter-like compound and rocks, sand, and gravel. The mixture, now called concrete mix, will set quickly in an irreversible reaction. The process, once the water is added, is unstoppable until complete. Concrete typically becomes unworkable to pour three to seven hours after mixing.
For a large construction job, a batch plant may be assembled on the job site. Cement and aggregate are delivered to the batch plant. The reactants are conveyed to the top of the unit, mixed with water, and then either conveyed or pumped to the desired location. A portable concrete batch mixer consists of a large mixing tank and reed-like hosing mounted to a flatbed trailer. The concrete is pumped to the exact pour spot through extensions of the hosing.
Alternatively, the concrete mix will be poured into a concrete truck, which will then transport it. This specialized truck comes with a constantly rotating chamber with a water feed at the top. Concrete batch mixer trucks are outfitted for the possibility of mixing the cement, aggregate, and water directly in the rotating chamber. These trucks are subject to falling over while turning on level ground at speeds as low as 12 mph because the concrete load raises the center of gravity and because it is constantly shifting.
Continuous pour concrete refers to the practice of pouring, settling, and smoothing the concrete in one continuous action. The concrete mix is still mixed in batches. The batches are timed so that the same amount of mixing occurs in each batch and there is no delineation between batches. A wheel-barrow-sized concrete batch mixer may be rented from home improvement stores for small jobs.
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