Method of Predicting Concrete Creep

Existing engineering methods of estimating the durability of reinforced concrete constructions are based on one of three main groups of criteria of destruction - by force, by deformation and by energy. Constructive elements having reserves of durability and a ductility limit of 1.5-3 and being often acted upon by eventual loads of mechanical origin undergo elastic deformations only. For characterization of the durability limit in such cases, just force criteria are enough, with evaluation of local and nominal stresses, taking into account the type of limit state. The application of deformational or energetic criteria in such a situation is neither correct in principle, nor reasonable. The modern technology is related to the tendency of reduction of durability reserves by a factor of 1.5-2 relative to previous values. This fact allows economy of materials and energy resources. Reduction of the durability reserves for constructive elements can be allowed only under conditions of application of modern methods of building of various constructions and advanced knowledge of durability. In some cases, when the reinforced concrete construction is permanently charged, the application of force criteria to the evaluation of durability is not correct enough, because prediction of the lifetime of the construction itself is impossible, since durability criteria do not include the time factor. If the 'force approximation' is used, the calculated situation is far from the real one, because several features are not taken into account: in particular, the constructions cannot be modeled for a specified lifetime, because the determining equations do not include the factor of time. Estimation of mechanical characteristics from the second limit state is limited because of the lack of available systematic data on deformability. We also note that creep data are necessary on a long-term basis and should relate to such parameters as stress, humidity, vibrations, etc.