The Mechanism And Action Requirements Of Shrinkage Reducing Admixture Of Concrete

Based on the disc method, the drying shrinkage and mass loss of hardened cement pastes under gradient humidity drying conditions are measured in this paper, aiming to analyze the impact of adding methods(mixing, coating and immersing), dosage and species(SBT-SRA, GRA-SRA) of shrinkage reducing admixture(SRA) etc on the drying shrinkage of hardened cement paste. By testing the surface tension and evaporation rate of aqueous solution or saturated limewater containing SRA and the pore structure of the hardened cement paste, the mechanism of SRA under different relative humidity is revealed. Meanwhile, the shrinkage reducing effect of SRA is evaluated in various water to cement ratios(0.3, 0.4, 0.5), relative humidity(100%RH~12%RH), ages(7d, 28 d, 56d) and the compatibility of SRA with high efficiency water reducing admixture is studied, thus determines the effective conditions of SRA. The influence of SRA on the compressive strength of hardened cement pastes is also studied.The results show that, for the early age(7days), when inner mixed, shrinkage reducing admixture can significantly reduce the drying shrinkage of hardened cement paste, its reduction percentage for the drying shrinkage in the 0.3 water to cement ratio is up to 47.15%, while the other two methods, that is, coating on the surface of samples and using as a curing compound, have much lower reduction percentage for the drying shrinkage of hardened cement paste, that is, 6.64% and 8.27%, respectively. After curing long enough time(56 days), inner mixing and immersing methods give remarkable reduction of drying shrinkage,that is, 45.53%, 30.88%, respectively, while the coating method gives 15.72% only. Adding the shrinkage reducing admixture in the mix leads to a drop of the compressive strength of hardened cement paste, and the more the SRA uses, the more the reduction is. When dosage of SBT-SRA is 0.2%, the compressive strength of hardened cement paste(w/c=0.4, 28d) has only 84.5% of the reference group; nevertheless, the other two ways can result in more or less increase of the compressive strength of hardened cement paste, and higher than the compressive strength of the hardened cement paste without SRA.In terms of reducing mechanism, SRA greatly decreases the surface tension of pore solution, which is the most dominating factor to reduce the shrinkage of hardened cement paste. Besides, SRA can reduce the shrinkage by decreasing the evaporation rate of pore solution in the lower relative humidity condition. SRA decreases the pore size of hardened cement paste, thus reduces the quantity of the harmful pores, resulting the reducing of drying shrinkage of the hardened cement paste. The inner mix way increases the total porosity while the other two ways decrease the total porosity. All the three adding methods have refinement effect on the pore structure. Under different relative humidity conditions, SRA has different reduction mechanisms: in higher range of relative humidity(85%, 75%, and 55%), SRA reduces drying shrinkage mainly by reducing the surface tension of pore solution, leading to a lower stress(caused by the evaporation of pore water) on the pore walls; in the lower range of the relative humidity(25%, 12%), the SRA reduces the evaporation rate of pore water thus decreasing the total shrinkage stress caused by capillary water evaporation, as a result, reducing the amount of drying shrinkage.When GRA-SRA works together with the polycarboxylate superplasticizer, the hardened cement paste expands to some extent in the range of 100%RH to 85%RH, while in the range of 55%RH to 12%RH, GRA-SRA along with polycarboxylate superplasticizer reduces the shrinkage but the reducing effect is less than that of separate mixing in cement paste, this means the polycarboxylate superplasticizer inhibits the reducing effect of GRA-SRA for drying shrinkage.By understanding the mechanisms of SRA covering almost all humidity range and evaluating the reduction effect in different conditions, the paper gives useful scientific guidelines for the development of SRA product and its practical application in civil engineering.