| Plastic shrinkage cracking has been an attened topic in the concrete technique field. There are a large number of factors working on this problem. In this thesis, the experiments were designed with different mix proportions and environmental conditions. Early-age plastic shrinkage was measured by the non-contact method of laser displacement sensor; maximum crack width and total crack area of concrete were tested by way of an end-restrained span mental specimen; meanwhile, 24h compressive stress, water evaporation, and bleeding were also tested to investigate the effect of mix proportions and environmental conditions.In order to explain theoretically crack initiated by deformation, some necessary cracking initiation conditions were discussed and then former parameters used to evaluate the concrete crack grade were contrastively analyzed. After these works, the 24h compressive stress and feature of plastic shrinkage curve were found out to predict and judge the degree of plastic shrinkage cracking. The research results can be concluded as following:①The area of crack firstly increased and then decreased with 24h compressive stress increasing. The most serious situation happened, when 24h compressive stress ranged from 10 to 13 MPa.②Without considering elastic module and creep, the acceleration of early shrinkage deformation of concrete can be found. The earlier and larger of this character occurred, the more serious condition happened.③There is a good relationship between the beginning of plastic shrinkage and the rate of bleeding. The rules of plastic shrinkage and cracking, however, can not be explained by the difference of bleeding and water evaporation.④Water binder ratio plays an important role in plastic shrinkage and cracking. When it increased from 0.35 to 0.54, the area of cracking of control concrete increased, but that of concrete with fly ash and slag first increased and then decreased, holding the slump 18~22cm. A reduction of water binder ratio resulted in a decrease in plastic shrinkage.⑤Fly ash restrained the early-cracking and reduce plastic shrinkage with the dosage of 75～150kg/m3 , when the water binder ratio was 0.54 and the dosage of binding materials was 350kg/m3. And the more fly ash was used, the more obvious depressing effect was. The influence of slag on cracking was not as the same as fly ash, but it also helped reduce plastic shrinkage. Area of cracking of control concrete was larger than that of concrete with fly ash and slag.⑥Plastic shrinkage of high strength concrete developed slowly and 24h comprissive stress was significantly improved with high temperature. The function of RH and wind speed was to influence on water of the concrete surface, so had an impact on plastic shrinkage but did not had a strong effect on 24h compressive stress.⑦The shrinkage of concrete with silica fume appeared more sensitive to environmental conditions, and the more silica fume was used, the larger shrinkage was, the less water evaported. |
