Research On Plastic Shrinkage Cracking Of High Flowing Concrete

Presently , concrete especially high flowing concrete can easily crack at early ages. The proportion of plastic shrinkage cracking is rather high in early cracking. The plastic shrinkage cracking would effect construction appearance and promote the propagation of drying shrinkage cracking or other types cracking, but also provide a passage for aggressive agents penetrating and moving to the concrete mass. Thus, there is a theoretical and actual significance to pay attention to the study on plastic shrinkage and cracking. In the paper the effects of the made parameters, the main materials and environmental conditions on plastic shrinkage cracking were investigated. It can be concluded as following: 1) there was a critical w/c ratio of maximum plastic shrinkage cracking between lower and higher w/c ratio of concrete mixtures made with the same cement content per cubic meter, concretes with lower or higher w/c ratio being less prone to plastic shrinkage cracking and the w/c critical ratio critical of maximum plastic shrinkage cracking being about 0.5. The trends of plastic shrinkage cracking in concretes made with the same water or cement paste content per cubic meter are similar with the above case, but the critical w/c ratio is about 0.45-0.4 and 0.4, respectively. 2)To a certainty of sand percentage, concretes with moderate critical sand percentage which changed in about 36%-43% were more sensitive to plastic shrinkage cracking than concretes with lower or higher sand percentage. 3) Concrete made with lower strength grading cement was more prone to plastic shrinkage cracking than that of concrete made with higher strength grading cement. 4)The effects of fly ash and silica fume on plastic shrinkage cracking were related to the actual w/b ratio of concrete mixture. There was a borderline w/b ratio, above which concretes containing fly ash or silica fume were more sensitive to plastic shrinkage cracking than that of concretes without fly ash or silica fume, and below which fly ash or silica fume high performance concretes can restrain the plastic shrinkage cracking. The borderline w/b ratio for fly ash and silica fume concrete is about 0.36 and 0.4, respectively. 5) Polypropylene fiber can make them quite effective in the suppression and stabilization of microcracks in fresh concrete matrix. 6) In laboratory-controlled environment condition, decreasing temperature and wind speed, and increasing relative humidity can decrease the water evaporation rate and consequently, makes concrete mixture less vulnerable to plastic shrinkage cracking, to contrast, there was a rather distinct effect of higher relative humidity on waterevaporation and plastic shrinkage cracking. 7) In outdoor, under the conditions of solar radiation and 8m/s wind speed, the plastic shrinkage cracking of fresh concrete is related to w/c ratio and the borderline w/c ratio is about 0.4. When w/c＞0.4, concretes exposed under this condition were more sensitive to plastic shrinkage cracking than that of concretes under other conditions (solar radiation and no wind or shade and 8m/s wind speed or shade and no wind). When w/c≤0.4, the case is reverse.