THE EFFECT OF HIGH TEMPERATURE ON REINFORCED CONCRETE STRUCTURES

The article represents the behaviour of reinforced concrete and its components (concrete and reinforcement) under high temperature. The comparing analysis of the experimentally and theoretically obtained results has been performed. The carried out experiment has disclosed that the mechanical properties of concrete alters differently in cases of temperature rise and theoretical reference. The most visible difference has been noticed at a temperature of 100 °C (Fig 4, Fig 5). The main fire resistance calculation basics are discussed. The temperature fields of the reinforced concrete element cross-section are calculated according to the standard fire curve using the program COSMOS/M of the finite element method. Concrete thermal properties, thermal conductivity and specific heat capacity dependence on temperature are taken into account in the model (Fig 10, Fig 11). By means of this model, the corresponding algorithm (Table 2) was made and can be used for obtaining temperature distribution for the reinforced concrete element of different cross-sections. According to the received temperature fields and applying the zone method, the influence of differences in theoretical and experimental results on element load bearing capacity is determined. The residual strength of the element considering the theoretical reduction curve of concrete strength is 5% larger than the results obtained in cases of 30 and 60 minutes heating. 90 and 120 minutes heating indicates that element strength is only 2% larger than the results calculated experimentally. The reduced zone dimension determined due to a decrease in the reduction coefficient at a temperature of 100 °C has affected residual element strength.

Fatigue Crack Tip Plasticity Associated With Overloads and Subsequent Cycling

The plastic zones associated with single overloads of cyclically loaded specimens have been mapped using electron channeling patterns. The zones are asymmetric with respect to the crack tip, and are complex in shape. Crack retardation subsequent to an overload is closely related to the size and shape of the overload zone, but has no apparent relationship to the maximum zone dimension. Following an overload, cracks try to exit from the monotonic zone by moving toward the nearest elastic-plastic boundary. The size of the overload zone is predicted by a plane strain rather than plane stress relationship. The minimum retarded growth rate corresponds to an effective stress intensity factor no greater than the threshold value for Stage II growth. This is caused by crack closure, with minimal crack tip shear strains and an absence of crack tip opening and blunting. Since the crack growth rate quickly approaches the preoverload rate once the crack crosses the overload boundary, it appears that residual stress within the overload plastic zone is the key factor in governing crack retardation.