Research On Effects Of Early-age Shrinkage And Plastic Cracking Of High Strength Concrete

In recent years,high-strength concrete has been applied widely in construction engineering. Experience of engineering practice reveal that HSC is easy to induce early-age cracking problem. The problem has been a hot research topic of the concrete technique field. Many research achievement have been acquired mainly on the studies of after-hardened volume stability of high-strength concrete and it's early-age cracking.The proportion of plastic shrinkage cracking is rather high in early cracking, but no systematic study has been done on early-age shrinkege and plastic cracking of HSC.Thus,there is a theoretical and actual significance in production and construction of HSC to pay attention to the research on early-age shrinkage and plastic cracking. In this paper, initial cracking time, maximum crack width and total crack area of concrete are tested by way of an end-restrained span mental specimen to evaluate qualitatively the trend of conrete plastic cracking.The no-contact method of laser displacement sensor has been introduce to solve the problem that early-age plastic deformation of conrete is difficult to measure quantitatively.Based on these,main effects of early-age shrinkage and plastic cracking of HSC have been studied.The research results can be concluded as following: 1) The early-age shrinkage and plastic crack trends of HSC are relatively higher than those of normal concrete.Plastic shrinkage and plastic crack degree of conrete are influenced mianly by environment. The proportion of plastic shrinkage is quite high in early-age shrinkage under the environment with high evaporation . 2) When water binder ratio varieties between 0.28 and 0.34,effect of w/b on early-age shrinkage and plastic crack trend are relative to state of fresh concrete. To a certainty of superplasticizer content,early-age shrinkage and plastic crack area of HSC are higher as the w/b increasing.When slump of concrete mixing is kept a certainty by adjusting superplasticizer content,the influence of w/b on plastic shrinkage and plastic crack trend of HSC are not distinct.To a certainty of w/b, the early-age shrinkage and total plastic crack area are lower when fresh concrete is super workable or very dry. When mixing slump is between 100mm and 200mm,HSC is easier to induce plastic cracking. The degree of HSC plastic crack is the highest when sand percentage is about 38%.HSC with lower or higher sand percentage which is not more sensitive to plastic cracking. Early-age shrinkage of concrete is higher as sand percentage increasing.Thus there is no entire positive relations between plastic crack trend and plastic shrinkage.Keeping other mix proportions parameters same,early-age shrinkage and plastic crack area of HSC are lower as maximum aggregate sizes decreasing. Concrete with accordant slump whose early-age shrinkage and plastic crack area are higher as cement amount increasing. 3) It can decrease the degree of plastic crack when the range of fly ash and slag content is 0%~40% and 0%~20% respectively in HSC. Fly ash and slag content exceeding the range,the trend of HSC plastic cracking is lower as their content inceasing.Early-age shrinkage of HSC whose fly ash content is between 0% and 20% is lower as the content decreasing. However, Early-age shrinkage of HSC whose fly ash content exceeds 20% is higher as the content increasing.Early-age shrinkage of HSC is lower as slag content increasing. Early-age shrinkage and plastic crack area of HSC is lower as fineness of fly ash or slag decreasing.There is a critical silica fume content to make degree of HSC plastic crack highest.The critical SF content is different under different w/b which is between 6.5% and 8%.Below the critical SF content,plastic crack area of HSC is lower as SF content increasing.Plastic crack area of HSC is higher as SF content increasing when SF content exceeds the critical content.Early-age shrinkage of HSC is higher as SF content increasing. 4) Early-age shrinkage and plastic cracking trend of HSC is lower as environmental relative humidity increasing or environmental wind speed decreasing. Under the environment with higher temperature,early-age shrinkage of HSC is higher, but it's plastic crack area is lower. 5) Polypropylene fiber can suppress and stabilitate plastic crack in fresh concrete effectively. To different mix proportions,the best compound proportioning of Polypropylene to descrease plastic crack area is different. The control means of HSC plastic cracking--wet maintenance and spatulating surface again are very effective ,especially the latter.