With an increase in the characteristics of the cycle, the calculated resistance to endurance increases. The lower boundary of the estimated voltages in concrete 04 under the action of the estimated temporary load is located near zero values. Modern technical conditions, when calculating permanent resistance in the stage of operation of preliminary stressful elements, are not allowed normal stretching stresses in the area of pre -squeezed concrete, which works from external stress for stretching. The ratio of estimated values of the voltage values in concrete, characterizing the cycle of alternating load P, is close to 0, 1. Testing materials conducted by the Research Institute of bridges indicate that under operational conditions the characteristic of the cycle of changes in compressive stresses in the concrete of the lower belts of the beam spans is close to 0, 1. The above brief analysis of the tension of concrete of the zone of the formation of cracks and the main data of the survey of the operated bridges make it possible to make the following assumption about the causes of the occurrence and development of longitudinal cracks.
Even in the process of manufacturing pre -stressed reinforced concrete span buildings in concrete, the zone of the main location of the intense reinforcement can occur in the form of microcracks oriented along the bundles of reinforcement.
The reason for these micro -disclosure may be a combination of a number of factors from which the main ones are: cramped reinforcement or channel -formers of concrete shrinkage, uneven temperature deformations of concrete, mainly in the process of striking, and transverse deformations of concrete of the specified zone from the efforts of preliminary compression. Schematically, the condition for the formation of microcracks in concrete in concrete, oriented along the intense reinforcement, can be represented in the form.
As already noted, the tensile stress in concrete from the exposure of two of the noted factors — compressing concrete with reinforcement and constrained shrinkage have one direction, striving to break concrete with the formation of a crack along the stressful reinforcement.
