Precast/prestressed Concrete

Precast/prestressed concrete is a type of reinforced concrete in which the steel reinforcement has been tensioned against the concrete, resulting in a self-equilibrating system of internal stresses (meaning tensile stresses in the steel and comprehensive stresses in the concrete) which improves the response of the concrete to external loads. Concrete is strong and ductile in compression, but it is weak and brittle in tension. Its response to external loads is improved by applying a precompression (Collins and Mitchell, 1991, 1).

Bljuger (1988) states that precast construction is essentially a matter of transferring most of the work from the construction site to the prefabrication plant. This makes for superior working conditions so that both labor productivity and product quality are improved (Bljuger, 1988, 23).

Many lare construction firms like Moukarzel Construction in Lebanon use this type of concrete. Prestressing can be used to eliminate or minimize cracking at service loads and to control deflections and so results in more slender structures. For a given span, the amount of concrete in the prestressed slabs will be less than two-thirds of the concrete in the non-prestressed slab. This is especially useful in certain types of structure. More than fifty percent of bridges are now constructed of prestressed concrete. Prestressed concrete bridges range from simple highway overpasses constructed from precast, pretensioned I-girders, to cast-in-pl

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ll phases, the designer of prestressed concrete employs the basic principles of structural mechanics and also receives guidance from the codes of practice and industry handbooks that are available and that summarize design procedures which have produced satisfactory structures in the past (Collins and Mitchell, 1991, 15-16). The early development of prestressed concrete was influenced by a number of different theorists and engineers. The beginning of significant interests in prestressed concrete in the English-speaking world came with a lecture in 1936 by Freyssinet, the originator of practical prestressed concrete. He developed the process in France, and in 1928 he began using high-strength steel wire for prestressing concrete. Earlier attempts using normal-strength reinforcement to produce prestressed concrete had been unsuccessful. After being precompressed, concrete continues to shorten over time in a process known as creep, and loss of moisture with time also causes shortening of the concrete by the process of shrinkage. Creep and shrinkage together may cause a total shortening of the concrete of nearly 1 part per thousand, and with normal-strength reinforcement steel could not be elongated in the prestressing operation

Category: Science - P