Research On Monitoring Of Water Transportation In Concrete And Mechanism Of Shrinkage Deformation

The shrinkage deformation is one of the important factors which affect the long-term service performance of concrete structures. Many experimental studies and theoretical researches have been conducted in this area, but the shrinkage mechanism is still not fully understood. It is generally acceptted that humidity gradient caused by moisture migration is the interna l driving force of shrinkage deformation. With the same body surface ratio, the differentce in cross section shape will directly affect the moisture migration and concrete internal humidity gradient. As a result, the shrinkage deformations also have great differences. For currently shrinkage prediction models, the b ody surface ratio or effective thickness is always used to present the effect of cross-section size on the shrinkage, while only B3 model takes the effect of cross-section shape into consideration which give shape coefficients of several specific shapes such as cylinder, slab, etc. In this study, a new piezoelectric-relative humidity monitoring method was proposed and experimental verified. The influence of moisture migration caused by different cross section shapes was studied and the coefficient ks of B3 model was modified. Finally, the concrete shrinkage mechanism was discussed deeply. The contents are as follows:The PZT pitch was packaged with the purpose of water-proof and protection against electromagnetic interference. Their stability were verified when they were embedded in water and used to measure the p-wave amplitude and velocity for 12 hours. Then the packaged PZT pitches were employed to monitor the process of cement hydration. It is found that the development of amplitude can be divided into three stages: slow decrease in 3.5 hours, rapid increase among 3.5~7.5 h, and stable stage after 7.5 h. By embodyed a pair of packaged PZT pitchs into a cement paste, 36 piezoelectric sensor was fabricated to study the influence of PZT’s distance, water cement ratio, air entrainment agent content, and excitation frequency on the monitoring signal amplitude. The results show that, the monitoring singnal amplitude increases with a decrement in PZT ’s distance or water cement ratio. There exits a reasonable content range for air entrainment agent to get greater signal amplitude. While for excitation frequency, it is hard to reach an uniform understanding which describe the amplitude variation with frequency. The proposed relative humidity monitoring method was experimental verified in a constant temperature humidity chamber. It is found that the monitoring signal amplitude monotonically decreased with the relative humidity, therefore monitoring signal amplitude can be regard as an index of measuring the relative humidity.With the same effective thickness, five groups of specimens with different cross-section shapes were design and manufactured. The influence of moisture migration path caused by cross-section shape was experimental investigated and the applicability of currently used prediction models were assessed. A new “the minimum radius of moisture transfer” was introduced and the coefficient ks in B3 model, which was used to describe the effect of cro ss-section shape, was revised. Finally the modified B3 model was experimental verified by test results. It is shown that with the same effective thickness and different cross section shapes, their shrinkage developments may have a significant difference. The modified coefficient Ks can describe the influence of cross-section shape on shrinkage development well.Three concrete specimens were cast and the humidity sensors were embedded in different depths. to measure the different depths’ s humidity. The shrinkage deformation of specimens and relative humidity in concrete were measured. It is found that the shrinkage in linearly related to the relative humidity. The relationship between internal relative humidity and c oefficient ks was further discussed on the base of that linear relationship between shrinkage and relative humidy. The results show that the humidity and temperature varys with depth. and the more close to the surface, the smaller humidity and temperature is. It indicates that there exist humidity gradient and temperature gradient in concrete cross-section. In addition, with the same body surface ratio, the new coefficient Ks can provide a refernce of internal relative humidty with different cross-section shape. A bigger coefficient Ks indicates a greater relative humity and a low shrinkage deformation.