Restrained autogenous shrinkage behavior of high strength concrete at early ages

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The fluorescence microscopic examinations were conducted to identify damages induced by self-desiccation and restraining autogenous shrinkage. Characteristics of fluorescent areas detected in high strength concrete at early ages were discussed in relation to the viscoelastic behavior evaluated by the closed loop uniaxial restrained shrinkage testing apparatus. Effects of water ponding, silica fume and saturated lightweight aggregate on the shrinkage behavior were also discussed with emphasis on changes in microstructure observed by the fluorescence microscopy, microhardness measurements and the SEM-BSE image analysis. The major results obtained in this study are as follows ;(1) Sealed concretes were more porous than the concretes cured in water.(2) Water ponding at early ages was effective to prohibit autogenous shrinkage of high strength concretes. Dense microstructure was formed in the water ponded concrete.(3) Weak regions were formed around aggregate grains in sealed concretes. The restraint provided by, rigid aggregate grams against autogenous shrinkage caused invisible microcracks in the vicinity of aggregate grains.(4) There were many remnant core cement particles surrounded by fine gaps in silica fume concrete at early ages.(5) Less effective function of the cement particles with the gaps as an inclusion may account for the greater creep potential of silica fume concrete at early ages.(6) A partial replacement of normal weight aggregate by lightweight aggregate was effective in reducing autogenous shrinkage in high strength concrete. However, effectiveness of replacement was dependent on the initial moisture condition of lightweight aggregate.