Effect Of Internal Relative Humidity On Interfacial Transition Zone Structure And Shrinkage Of Ceramsite Concrete

The properties of concrete depend upon composition of it and internal structure and changes of structure caused by environmental influence. When the composition of concrete is determined, internal structure and changes of it mainly affected by the outer environmental influence and internal hydration of cement, and the latter have close relations with changes of relative humidity inside concrete.Changes of relative humidity inside concrete mostly affected by internal water diffusion and self-desiccation caused by cement hydration, and are closely relative to internal water and outer curing conditions. Ceramsite has a characteristic of water absorbing and desorption, thus taking three types of ceramsite which has different water absorption and desorption rate as research object, systematic study was carried out on the change law of internal relative humidity with different ages and effect of humidity gradient on the progress of cement hydration and then on microstructure and macro-performance such as shrinkage deformation. And also relationship between relative humidity and shrinkage value of concrete which bring some theoretic basis and experimental foundation for unraveling the nature of shrinkage and cracking was set up.Investigation between different measurement methods for internal relative humidity of concrete shows that experimental results obtained by measuring whenever necessary using thermo hygrometer is more reliable than those obtained by currently used method which is pre-embedded method.Internal relative humidity gradient of concrete with different composition and under condition of one-way drying was studied and result shows that the longer moist curing lasts, the slower the internal relative humidity decreases. Comparing concrete under moist curing conditions for 28days with those under moist curing conditions for 14days, 7days, and the former has a bigger internal relative humidity that the latter at the same position in concrete and the difference comes up to 0.6%~5.4%. And also similar law can be applied to ceramsite concrete. In the same age the internal relative humidity of concrete with pre-wetting ceramsite is higher than that of normal concrete. The bigger the moisture content and replacement rate is the bigger the internal relative humidity of concrete is and the slower it decreases.Time during which the relative humidity is greater or equal to 80% for concrete with ceramsite containing high or medium amount of water is more than one times the length of that for normal concrete. At the age of 360 days relative humidity of centre part is still greater than 80% for QF ceramsite concrete and time during which the relative humidity is greater or equal to 80% can reach 60~90days, but time for normal concrete or YC ceramsite concrete is only 24~28days. Before the age of 56 days, the higher the amount of mineral admixture is, the slower the internal relative humidity decreases. But conditions are completely different after the age of 56 days. At the age of 360 days relative humidity is most high for concrete with 15% or 20% slag. Before the age of 28 days, internal relative humidity of concrete with a W/C of 0.45 is higher than that of concrete with a W/C of 0.3. However, after the age of 28 days internal relative humidity of concrete with a W/C of 0.45 decreases rapidly and is close or even lower than that of concrete with a W/C of 0.3.Microstructure of harden cement past in interfacial transition zone of ceramsite concrete was investigated, and results show that ceramsite has a more significant effect on microstructure of harden cement past in interfacial transition zone of concrete with low W/C than that of concrete with high W/C. Moisture content of aggregate has an apparent effect on the microstructure of harden cement past in interfacial transition zone. Amount of porosity and percent of large pore in ITZ of concrete with normal aggregate or with YC ceramsite containing low amount of moisture content are both higher than that of cement paste, but things becomes the opposite as to concrete with ceramsite containing high or medium amount of moisture. The ratio of CaO to SiO2 in the ITZ of the concrete is higher than that of cement paste and pore structure presents a trend of thinning. Moreover the bigger is pre-wetting rate of ceramsite and the stronger is water absorption and release and the rougher is the surface of ceramsite, the denser is the microstructure of ITZ. Mean pore size of ITZ in QF ceramsite concrete decreases up to 20.6% compared to that of normal aggregate concrete and the amount of pore which is smaller than 50nm accounts for about 80%. Microhardness of interfacial transition zone(0~10μm) of concrete with BY or QF ceramsite pre-wetting for 1hrs is about 43% and 56% higher than that of cement paste respectively and also 79.6% and 96.3% higher than that of concrete with normal aggregate. Microhardness of interfacial transition zone decreases with the increase of the degree of saturation with water. Microhardness of interfacial transition zone of concrete with BY ceramsite is highest. Under on-way drying condition, with the distance from the drying surface increases and internal relative humidity becomes big, the degree of hydration of cement paste and Microhardness of interfacial transition zone and cement paste is enhanced. Meanwhile this trend becomes more apparent when the distance from the surface is not more than 5cm and when the distance is more than 10cm the increase become steady.The degree of pre-wetting and the ability of water release of ceramsite have a significant effect on internal relative humidity and shrinkage deformation. With the moisture content of ceramsite increasing, internal relative humidity of concrete decreases slower under drying or self-desiccation conditions and autogenous shrinkage at early ages and long term drying shrinkage both decrease and the period during which restrained shrinkage stress reach its maximum value becomes long. There is apparent relativity between autogenous shrinkage at early ages or long term drying shrinkage and the decrease of internal relative humidity. At the age of 180 days the total shrinkage of normal aggregate concrete is about 3%~8% less than that of concrete made of YC ceramsite with low moisture content and 2%~15% more than that of concrete made of ceramsite with high moisture content. Autogenous shrinkage and drying shrinkage and total shrinkage of concrete made of BY100 ceramsite are all less than that of normal concrete, and shrinkage-reducing rate of internal curing comes up to 47.7%. Ceramsite sand has more efficient influence on the internal curing.The efficiency of shrinkage reduction of internal curing material can be characterized by two parameters, effective action distance of water in internal curing particles and distance between internal curing particles surface. When Ceramsite sand content is 50%, the distance between two particles surface is 417μm, and effective action distance of water in particles is 256μm, which reaches to the critical spacing, as a result, autogenous shrinkage of the concrete can be reduced 98.2%. The water introduced by internal curing material cannot be used completely, therefore, the water use ratio should be considered in the process of calculation the amount of internal curing material needed to compensate the autogenous shrinkage of the concrete, and the use ratio of Ceramsite sand is 63.7% in this research.