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A、Concrete a、Plain Concrete Concrete is a stone-like material obtained by permitting a carefully proportioned mixture of cement, sand and gravel or other aggregate, and water to harden in forms of the shape and dimensions of the desired structure. The bulk of the material consists of fine and coarse aggregate. Cement and water interact chemically to bind the aggregate particles into a solid mass. Additional water, over and above that needed for this chemical reaction, is necessary to give the mixture the workability that enables it to fill the forms prior to hardening. Concretes in a wide range of strength properties can be obtained by appropriate adjustment of the proportions of the constituent materials. Special cements (such as high-early-strength cements), special aggregates (such as various lightweight or heavyweight aggregates), and special curing methods (such as cream-curing) permit an even wider variety of properties to be obtained. These properties depend to a very substantial degree on the proportions of the mix, on the thoroughness with which the various constituents are intermixed, and on the conditions of humidity and temperature in which the mix is maintained from the moment it is placed in the forms until it is fully hardened. The process of controlling these conditions is known as curing. To protect against the unintentional production of substandard concrete, a high degree of skillful control and supervision is necessary throughout the process, from the proportioning by weight of the individual components, through mixing and placing, until the completion of curing. The factors which make concrete a universal building material are so pronounced that it has been used, in more primitive kinds and ways than at present, for thousands of years, probably beginning in Egyptian antiquity. The facility with which, while plastic, it can be deposited and made to fill forms or molds of almost any practical shape is one of these factors. Its high fire and weathe