Effect Of Shrinkage-reducing Polycarboxylate Superplasticizer On Early-age Shrinkage And Cracking Of Cement-based Materials

High-strength and high-performance concrete usually uses a low water-to-binder ratio and a high amount of cementitious material,which increases high risks of early concrete shrinkage and cracking.Shrinkage-reducing Polycarboxylate Superplasticizer(SR-PCA)has been designed and developed based on the diversity of the molecular structure of polycarboxylate superplasticizer.SR-PCA has promising developments due to its combined water and shrink reduction effects,low dosage requirement,and low impact on the strength of concrete.Current research on the performance and mechanism of SR-PCA mainly focuses on the hardening stage of cement-based materials.Its effect on the shrinkage and cracking of cement-based materials at the plastic stage and its mechanism,however,are scarcely investigated.At the same time,the study on the influence of SR-PCA on the shrinkage and cracking of high strength and high performance cement-based materials such as UHPC at the plastic and hardening stages still lacks.In this thesis,shrinkage and cracking of plain mortar and high-strength mortar mixed with SR-PCA at both the plastic stage and hardening stage were studied.The mechanism of SR-PCA mitigating the shrinkage and cracking was also discussed.Firstly,the plastic settlement,plastic shrinkage and plastic cracking properties of SR-PCAadded plain mortar and high-strength mortar at the plastic stage were studied.The mechanism of shrinkage and crack resistance was explained from the perspectives of surface tension,water evaporation,hydration and pore pressure.The results show that SR-PCA can significantly reduce the plastic shrinkage and plastic settlement of plain mortar and high-strength mortar,thus reduce the risk of plastic cracking.Mechanism research shows that the incorporation of SR-PCA can significantly reduce the moisture evaporation and minimize the plastic settlement of cement-based materials.Besides,the incorporation of SR-PCA can significantly impede the surface tension development of the pore solution,reduce the rate of moisture evaporation and hydration,thereby delaying the development of pore pressure and mitigating plastic shrinkage and cracking of cement-based materials.Secondly,the evolution of drying shrinkage,autogenous shrinkage and mechanical properties of SR-PCA-added plain mortar and high-strength mortar at the hardening stage were studied,and the influence of SR-PCA on the pore structure was studied.The results show that the incorporation of SR-PCA can significantly reduce the drying shrinkage of plain mortar and the autogenous shrinkage of high-strength mortar.For the drying shrinkage of the plain mortar under the water-binder ratios of 0.3 and 0.4,the shrinkage-reducing rates of 0.5% SR-PCA at60 days are 17.9% and 19.4% respectively.For the autogenous shrinkage of high-strength mortar,the shrinkage-reducing rate of 1%SR-PCA can reach 49.6% at 7 days.In addition,the incorporation of SR-PCA has no significant negative impact on the compressive strength and flexural strength of plain mortar after 3d and on the tensile strength of high-strength mortar.In addition to SR-PCA can reduce the surface tension,delay the development of pore pressure,thereby reducing the shrinkage of the hardening stage,the pore structure analysis also shows that the incorporation of SR-PCA increases the total porosity of plain paste and high-strength mortar.It reduces the quantity of capillary pores in the pore size range of 5～50 nm that is closely related to shrinkage in plain paste,which is beneficial for reducing shrinkage stress of capillary pore.The incorporation of SR-PCA increases the porosity of each pore size range of highstrength mortar,which is beneficial to promote the creep of high-strength mortar.Finally,the effect of SR-PCA on the cracking performance of high-strength mortar was studied by Temperature Stress Test Machine(TSTM).The results show that SR-PCA significantly prolongs the cracking time of high-strength mortar under the constraint degree of100% and 50%.With 50% constraint degree,SR-PCA can also increase the fracture stress.The stress development rate slows down with the increase of the content and it significantly reduces the cracking potential of high-strength mortar.SR-PCA can reduce the crack index of highstrength mortar,and as the content increases,SR-PCA crack index decreases significantly.With50% constraint degree,SR-PCA-added high-strength mortar creeps more than SRA-added high-strength mortar at the same stress level,thereby obtaining higher stress relaxation,which is beneficial to reduce high-strength mortar stress development and to improve the ability of crack resistance.