Research On Restrained Deformation Properties Of Reinforced Concrete Members Due To Early Shrinkage During Construction

During the construction period, early deformation cracks are always prevalent for cast-in-place concrete members, which have not been effectively resolved. At home and abroad, a lot of research has been done focused on free deformation of plain concrete. However, in practice engineering concrete members are always reinforced. Under the restriction of internal reinforcement, moulding board, adjacent members, deformation properties of concrete are great difference from that in free shrinkage condition.Because mechanical and deformation properties of concrete are time-dependent during construction, theoretical calculation is difficultly achieved for accurate prediction of early cracking when reinforced concrete members are under the action of deformation and constraint. Therefore, it is necessary to do research on restrained deformation properties of reinforced concrete members due to early shrinkage during construction, which contributes to providing theoretical basis for integrated controlling of early cracks in concrete structure.This dissertation is based on the research results of early age concrete properties and structural properties of concrete during construction at home and abroad, as well as some practical projects. Taking reinforced concrete members that deform mainly in one direction as study object, a systemic study is executed through field investigation, material collection, theoretical analysis, experimental research method. The main work and achievements are summarized as follows:①According to the dominant cracking mode on the single direction of concrete members in initial construction, a combination system of steel-concrete-circumjacent constraints (moulding board, adjacent component) is put forward and simulated. The mechanical model of reinforced concrete deformation under the end and continuous constraints is respectively established, and the calculation equation of constrained stress including the action of reinforcing bars is derived. For the members in coupled state of the steel and external constraints, the effects of the steel on constrained stress due to different deformation factors in reinforced concrete is quantitative analysis. Based on the theory of increment iteration, the time-history relationship of stress-strain under creep effect is established.②By improving the free shrinkage test mold and selecting practical mix ratio for engineering, a new test idea is proposed to do simulation that closing to the actual members in the case of incomplete-constraints status. And the early-age shrinkage deformation properties of concrete members of different reinforced percentage are studied. The quantitative relationship between reinforced percentage and restrained early- shrinkage is advanced in this chapter. By the contrast between plain concrete and reinforced concrete, the influence factors is studied, mix ratio, reinforcement characteristics, etc., exert to shrinkage deformation properties at early age of concrete.③Based on estimation model of concrete properties at home and abroad, forecasting of early mechanical properties of concrete is discussed. The equation that describes the shrinkage value of concrete is established by analyzing the development rate of concrete free shrinkage at early age. According to comparison of shrinkage test results between plain concrete and reinforced concrete, another equation is proposed, which describes the devolpment of early-shrinkage value of reinforced concrete. Quantitative calculation of self tensile stress and creep factors is achieved by the way of incremental iterative algorithm.④Restrained early shrinkage of reinforced concrete is studied by early-shrinkage experiment on-situ test of typical one-way contract components which are frequently used in practical engineering, and internal temperature of concrete, steel and concrete strain curve of the whole development from placement to 28 days early is obtained, namely the distribution of early concrete deformation. Combining with coupling calculation theoretical model of continuous constraints system, an analysis is made to study influence of moulding board and adjacent-members constraints on concrete stress due to shrinkage and temperature deformation.⑤According to results of the locale test, the general idea of integrated control of construction early cracks is established by combination of engineering practice, and the key control period of early crack prevention is proposed. The effect of main construction operation (moulding board removal, curing, etc.) on the restrained stress and strain of concrete members is quantitatively evaluated, and constructional measures and proposals of early crack prevention of concrete structures are proposed at the same time.