According to American code ACI: 318-1989, permissible deflection vary from a maximum of span/180 to a minimum of span/480 depending upon type of member and seriousness of damage to adjoining structural element.

Stress condition equations formulated for opposite extreme faces of section or by aid of design uniform or non uniform prestress are solved for determining required cross sectional dimensions and prestress.

The philosophy of limit states was adopted and elaborated by European concrete committee (CEB) to form basis of committee?s original publication in 1964and later in conjuction with international federation for prestressing issued a complimentary report dealing with prestressed concrete in 1996.

The characteristic load F_k which is independent of limit state considered and is seldom exceeded in service is defined as: Characteristic load = Mean load + K x standard deviation, K is a factor so chosen as to ensure that probability of characteristic load being exceeded is small and a value of 1.64 for K ensures probability that characteristic load is exceeded by only 5 percent during intended life of structure.

Short term deflections of prestressed members of class-1 and class-2 types under service loads are influenced by: magnitude of prestressing force and its profile, applied load and self weight of member and flexural rigidity of Un cracked concrete sections.

The Indian and British code methods are limited to under reinforced sections in flexure based on effective reinforcement ratio, American code method provides empirical relations to compute ultimate flexural strength of under and over reinforced sections as well as of sections with compression reinforcement.

When work is resumed surface of concrete previously placed should be thoroughly cleaned of dirt, scum, laitance, loosely projecting aggregates and or soft material using stiff wire brushes and surface should n be thoroughly soaked with clean water for two to three hours before furr concreting using a thin layer of cement slurry.

A successful design should not only satisfy requirements of safety against total collapse of structure due to various causes but also ensures that serviceability of structure is not impaired while resisting normal working loads.

A new philosophy of design, termed limit state approach, which was incorporated in Russian code in 1954 basically limit state design is a method of designing structures based on statistical concept of safety and associated statistical probability of failure.

In structural members, ?_m is a maximum at tension face if h-x is sufficiently small for crack width at tension face not to exceed permissible limit of 0.3mm, it will not exceed limit anywhere, this is reason for smaller crack widths in slabs under service loads, provided thickness does not exceed about 200mm.