Experimental Investigation On Fracture Property And Microstructure Of Early-age Concrete

With the development of material science and structure science of concrete, more and more large projects have been constructed, at the same time, high strength concrete and high performance concrete are extensively used. But this also leads to some problems, such as structure and durability failure in massive concrete structures, which were caused by the crack at early ages. It's very necessary to study the microstructure and macro-mechanical property of early-age concrete. Only by knowing the properties of early-age concrete well and taking some steps, we can guarantee the long-term quality of concrete structures.In this thesis, we choose time as indication of early age. The study includes four parts as the following.1. Basic mechanical properties, including the analysis of the compressive strength, tensile strength, stress-strain curve and the stress-volumetric strain at early ages of normal concrete and high strength concrete.2. Fracture properties and material brittleness. Two-parameter model (TPM) was applied to investigate the fracture properties of normal concrete and high strength concrete at early ages. The characteristic length defined in the fictitious crack model (FCM) by Hillerborg to determine the brittleness of concrete.3. Microstructure. The mercury intrusion porosimetry (MIP) was applied to study the development of the porosity and the pore size distribution at early ages. The interfacial transition zone (ITZ) was also studied through the results of scanning electron microscopy (SEM).4. The effect of the curing condition, the maximum size of coarse aggregate and silica fume on the strength, fracture properties and material brittleness of concretewas studied at the same time.In the light of the experimental results and theoretical analysis, there exits certain developing rules and relationships for and between each property of concrete at early age.1. Strength and fracture parameters, as well as other properties of concrete have similar trends in development. The rate of development is large at early ages and slows down beyond early ages. Certain quantitive relationship can be established between the porosity and compressive strength, and between the porosity and the fracture toughness.2. The properties develop faster with decreased water-cement ratio at early ages. That is, high strength concrete has a large mature rate than normal strength concrete.3. The curing condition, the maximum size of coarse aggregate and silica fume have certain effects on the strength, fracture properties and brittleness of concrete. The silica fume strengthens the bond between the coarse aggregate and cement matrix on account of its improvement of the pore structure and composition in ITZ.