Experimental Investigation And Theoretical Modeling On Early-age Shrinkage Cracking In Concrete

This paper studies the early age cracking induced by shrinkage of concrete and it is discussed from four aspects including formation mechanics, experimental investigation, engineering actuality and theoretical research. The influences of superplasticizer (SP), shrinkage-reducing admixtures (SRA), expansion admixture, w/c ratio and curing conditions on the early age shrinkage is compared and analysed from the experimental. Some problems about early age cracking occurred in present engineering is represented, and the importance of early age curing and the necessary knowledge on the use of expansion admixture is emphasized. And some try is done on the establishment of theoretical model of early age cracking induced by shrinkage, and the early age shrinkage stress field of concrete and a temporal analytical method of shrinkage stress in the reinforce concrete structure are formed. Which will show an assignable direction for the later research on this thesis.The study in this paper shows that the early age curing will do a marked effect on the shrinkage performance. With a good early age curing, majority of early age drying shrinkage will been avoided and which also do well to the retardancy of cracking induced augogenous shrinkage.The adding of SP will enlarge the early age drying shrinkage sharply, as well as to the autogenous shrinkage. By the adding of it, an obvious autogenous shrinkage well take place in the concrete with a relatively high level w/c ratio. The paper has explored the mechanics about this phenomenon and deemed it is caused by the dramatic reducing of relative moisture in the pore when adding of SP. So the shrinkage is enlarged.The SRA has a good shrinkage-reducing effect to the early age shrinkage no matter to the autogenous shrinkage or the drying shrinkage. However,as to the concrete with a w/c higher than 0.47(no matter SP adding or not), if the early age curing is well done, then the adding of SRA seems not necessary for majority of drying shrinkage has been avoided by a good curing. And as to the concrete with a w/c ratio low than 0.32, then the adding of SRA will reduce a majority of autogenous shrinkage, which is significance in lower the total shrinkage.The shrinkage compensation of expansion admixture will take place only at the premise that a good curing has been done. Otherwise, the use of it will no goodness than not using of it. The paper show that it should be noticed that a large shrinkage will take place after the expansion peak value. And it is suggested that with view to reduce early age shrinkage, compound using of SRA and expansion admixture will make a more steady deformation performance. However the dosage of expansion admixture shouldn't too large. And the reason why the effect of shrinkage compensation of expansion admixture is lower in the present concrete is been discussed and the paper deems that the larger early age shrinkage and restraint cause the phenomenon.As to the autogenous shrinkage, the experiments show that it can been ignored to the concretewithout SP and with a w/c≥0.42. However when the SP is used, the autogenous shrinkage is obvious in the concrete with a w/c at 0.48, so it can not been ignored in this case. And as to the concrete with a w/c low than 0.32, autogenous shrinkage will cause early age cracking.As to the control of early age cracking caused by shrinkage, the paper put forward a conception of "early age curing in leading position while shrinkage-reducing material in secondary". And the establishment of a "prediction model on the cracking induced by thermal deformation and moisture diffusion in the reinforced concrete structure" seems been necessary.