Papers by Author: Bao Sheng Zhang

Abstract: Inorganic salts are important admixtures usually used in cold weather concrete. As research basic of influence of salts on concrete durability, effects of inorganic salts on pore structure of cement paste were studied in this paper, and possible implications of concrete property with pore structure was also analyzed. Pore structure of paste added CaCl2, NaCl, Na2SO4, NaNO2, Ca(NO3)2 and Ca(NO2)2 curing for 3 days and 28 days were tested through mercury intrusion porosimetry (MIP). The results showed that no matter 0.3 or 0.5 water-cement ratio, the pores whose diameter <50nm in paste with salts increased at 3 days, which was harmful for the control of concrete shrinkage and cracking at early age. Adding Ca (NO3)2 increased coarse pores (>200nm) of paste at 3 days, but these coarse pores turned into fine pores and reduced significantly at 28 days. Adding NaCl and Na2SO4 into cement paste raised coarse pores with size>1000nm at 3 days and 28days, which were harmful for the pore structure.

Abstract: The frost resistance of concrete subjected to 3.5% NaCl solution, 5%, 7%, and 10% Na2SO4 solution and seawater were investigated by quick freeze-thaw test. There were two criteria, the relative dynamic modulus of elasticity (RDME) and the mass of scaling, for evaluating the frost resistance of concrete. The results showed that scaling was the main characteristic of deterioration when concrete subjected to freeze-thaw cycles in 3.5% NaCl solution and seawater, whereas RDME had little change. When freezing and thawing in Na2SO4 solution, concrete failed because of internal damage and surfaces of lower strength concrete specimens appeared local swelling and scaling before failure. Concentration of Na2SO4 solution had influence on frost resistance of concrete: deterioration of concrete in 5% or 7% Na2SO4 solution was severe but slower when the concentration reached to 10%. Entraining air into concrete was the best method to improve the durability of concrete subjected to freeze-thaw cycles in all solutions.

Abstract: Permeation length, permeation coefficient of water permeated into concrete and chloride ion diffusion coefficient of C30, C45 and C60 concrete were studied by high hydraulic method (0.5MPa~25MPa) and quick method of testing chloride ion diffusion respectively, and relationship of concrete permeability tested by two methods was analyzed too. High hydraulic method can distinguish permeability of concrete with different strength or air content and it is a quick and effective method to fast test and evaluating permeability of concrete. The results showed that high hydraulic method is applicable for evaluating permeability of concrete, with advantages of short testing period, easy operating and simple pretreatment of concrete specimens. Permeation coefficient or permeation length of water permeated into concrete exhibited linear relationship with chloride ion diffusion coefficient of concrete. It is applicable for evaluating permeability of concrete using quick method of testing chloride ion diffusion.

Abstract: High performance concrete (HPC) with a water/cement ratio (w/c) of 0.32 and different lightweight aggregate (LWA) contents (0%, 25%, 50%, 75%, 100%) were prepared, and the influence of LWA on concrete frost-resistance and impermeability at different ages were studied, as well as the hydration degree, hydrated product, pattern and pore structure of the paste around aggregate. The results show that, by replacing normal weight aggregate (NWA) with 50% and 100% volume contents of pre-wetted LWA respectively, the chemical bound water of the cement paste surrounding aggregate are increased 12.1% and 22.7% as compared to concrete mixed without LWA. And at 28 days, lightweight aggregate concrete has the highest Ca(OH)2 content, whereas the 90-day Ca(OH)2 content of normal weight concrete is the highest. This proves that, with the increase of LWA content in concrete, both of the internal curing effect of pre-wetted LWA and secondary hydration effect of fly ash (FA) are strengthened, this can also be verified by the SEM study. Furthermore, the pore structure of the cement paste around aggregate can be improved consequently. The performance of frost-resistance of HPC can be improved by mixing LWA, the 90 day-frost-resistance of lightweight aggregate concrete is about 2.5 times of that of concrete mixed without LWA. The influence of LWA on the impermeability of HPC is different from normal concrete. When LWA content is more than 50%, the HPC impermeability decreased obviously, however at later age the difference between them becomes minor.

Abstract: The influence of pre-wetted time on strength of shale ceramsite concrete is studied.Microstructures of both cement paste and interfacial zone between aggregate and paste of concrete are observed by scanning electronic microscope (SEM). The results show that, if the pre-wetted time of the shale ceramsite increased, the decline degree of early strength of concrete will be greater than strength at a longer age, and the growth rate of concrete strength may obviously increase. The pore characteristics of concrete at different ages is examined. The change tendency of concrete strength at different ages is revealed from the point of view of microstructure.

Abstract: Frost resistance of plain concrete and air-entrained concrete subjected to freeze-thaw cycles in fresh water and 5 % and 7 % sodium sulfate solution are investigated in this paper. The test results show that the frost resistance of concrete is different in the different medium. The properties of concrete frozen in 5 % and 7 % sodium solution are different from that in fresh water, and entraining air into concrete properly can increase the frost resistance significantly whether in fresh water or in sulfate solution. Higher strength concrete could resist the degradation of freezing and thawing cycles in water, but some of them failed suddenly in midspan of specimens under the sulfate solution.