Bao Sheng Zhang

Effect of Saturation Degree on Concrete Deterioration due to Freeze-Thaw Action

Freezing-thawing durability of cement concrete is extremely important in cold weather, to better understand mechanism of frost damage and air-entraining，saturation degree of pores in concrete and its relation with frost resistance were studied in this paper. Concrete specimens with different saturation degree from 0 to 100% were prepared used a sealed tin with a high water pressure pump. Then these specimens were subjected to six freezing-thawing cycles and the relative dynamic modulus of elasticity was examined. The results showed that critical saturation degree of concrete with water- binder ratio of 0.30 and 0.47, air content of 1%, 4% and 6% were from 0.60 to 0.80. When its saturation degree exceeded the critical value, concrete was deteriorated significantly after only six freeze-thaw cycles. The critical saturation degree was mainly related to the air content of concrete mixture, and it decreased with the increasing of air content. The difference between the saturation degree and the critical value can be used to evaluate potential frost resistance of concrete, and its result was consistent with the result of frost tests very well.

Deterioration of Concrete Freezing-Thawing in Different Salts Solution

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.

Microstructure and Mechanical Properties of Shale Ceramsite Concrete

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.

Effect of Mineral Additives on Internal Relative Humidity and Dry Shrinkage of Light Weight Aggregate Concrete

Effect of fly ash and slag on internal relative humidity (IRH) and dry shrinkage of light weight aggregate concrete (LWC) are studied in this paper. As indicated in this test, mixed with mineral additives can reduce dry shrinkage of concrete. The more volume of fly ash and slag ash, the lower decrease speed of IRH is, which contribute to the decreases of dry shrinkage rate too. At the same curing-age and dosage, the relative humidity inside the fly ash concrete is higher than the slag one. As consequence, the rate of dry shrinkage of fly ash concrete is lower than slag concrete. Also, there is a significant linear correlation between the lowered values of IRH and dry shrinkage rate.

Effect of Low Temperature on Mechanical Properties of Concrete with Different Strength Grade

The effect of low temperature (-35°C) on the mechanical properties of concrete with different strength grade such as compressive strength, flexural strength, splitting tensile strength and elastic modulus was studied. The results showed that all of the mechanical properties were improved at -35°C. It was also can be found from the tests, as the strength grade increased, the growth ratios at -35°C of compressive strength, flexural strength and splitting tensile strength decreased. But the growth ratio of elastic modulus increased with the increasing of strength grade.

Mechanics Properties of Concrete at Low Temperature

Mechanical properties at low temperature (-5°C, -20°C and -30°C) including compression strength, flexure strength, splitting tensile strength and modulus of elasticity of concrete with fly ash were investigated. In order to remain the temperature of the specimens unchanged during the test, a new insulation device was designed. In addition, the effects of curing ages on mechanical properties of concrete were studied. The results showed, at low temperature, all of the mechanical properties were improved; as the temperature decreases, the growth ratio increased continuously. The relative growth ratios of compression strength and flexure strength achieved the maximum value when the temperature decreased from -5°C to -20°C, and the relative growth ratios of splitting tensile strength and modulus of elasticity kept increasing during the whole process of cooling. At -35°C, all of the mechanical properties were improved at each curing age.

Effect of Lightweight Aggregate Moisture Content on Pore Structure of Concrete

To understand the impacting law of lightweight aggregate moisture content on the cement paste pore structure and its relationship with humidity and deformation of the lightweight aggregate concrete, it is necessary to study the effect of water absorptivity and pre-wet moisture content of lightweight aggregate on the pore structure interfacial transition zone (ITZ) cement paste. The experimental results indicated that the species and moisture content of lightweight aggregate have a greater effect on the pore structure in interfacial transition zone of cement paste in low w/c concrete than that in high w/c concrete. With the saturation level of lightweight aggregate increased, the overall pore structure shows a refinement tendency. The porosity and the larger pore of ITZ in ordinary aggregate are notably higher than those in the basal body cement paste. Meanwhile, we find that the pore structure of ITZ in lightweight aggregate concrete is much different from that in ordinary aggregate concrete. Especially in the concrete using the lightweight aggregate with a high absorption–desorbtion water rate, the pore aperture in ITZ is significantly lower than that in the cement paste, the pore content below 50nm can reach as high as 80%.

Relation of Permeability by High Hydraulic Method and Chloride Ion Diffusion Coefficient of Concrete

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.

Effect of Inorganic Salts on Pore Structure of Cement Paste

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.

Influence of Lightweight Aggregate on Durability of High Performance Concrete

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.