CHARACTERISTIC STRENGTH OF THE MATERIALS AND THE LOADS IN THE STATISTICAL TERMS REQUIRED FOR THE DESIGN CONSTITUTIONS .

CRACKING :-- Cracking of Concrete should not adversely affect the appearance or durability of the structure .The acceptable limits of cracking would vary with the type of structure and envoirnment .The actual widths of cracks will vary between wide limits and the prediction of absolute maximum width  not possible. As a guide the following may be regarded as the reasonable limits:- The surface width of cracks should not,in general exceed 0.33mm ,for particularly aggressive environment, such as exposed to the sea water ,alternative wetting and drying and to freezing whilst wet , subject to heavy condensation or corrosive fumes ,The assumed surface width of cracks at points nearest to the main reinforcement should not in general exceed 0.004 times the normal cover should be not less than 75 mm.The possibility of some cracks being wider may be taken in to account,if necessary. Characteristic strength of materials, means value of the strength of the material below which not more than 5percent of the test result are expected to fall .The characteristic strength for concrete has been given in the working method of the design. For reinforcing steel , the characteristic value shall be assumed as the minimum yield/0.2 percent proof stress specified in the relevant Indian Standard specifications. Characteristic Loads means the value of load which has a 95 percent probability of not being exceeded during the life of the structure .Data to express loads in Statistical terms , for this purpose, the dead loads worked out on the basis of IS :1911 --1967 ,live and wind loads in IS :-- 875--1964 and the seismic forces given IS :1893 --1975 shall be assumed as the characteristic Loads .