Influence Of Shrinkage Reducing Admixture And Internal Curing On Volume Stability Of Cement Based Materials

With the development of modern high strength and high performance concrete(HSC/HPC), the volume stability of HSC/HPC have been attached widespreadattention. Although HSC/HPC has an outstanding vantage on strength, workability anddurability for HSC/HPC, because of the application of finer cement, lower water tocement ratio and higher content of supplementary cementitious materials, HSC/HPC ismore prone to shrinkage or cracking than normal strength concrete. With the higherearly age strength, denser microstructures, as well as the faster reduction of internalrelative humidity, the traditional methods, such as moisture curing, using expansiveagent or fiber reinforcement, cannot satisfy the requirement of shrinkage or crackingmitigation for HSC/HPC. In addition, shrinkage reducing admixture(SRA), although ithas an excellent performance on the mitigation shrinkage related to capillary tension,because of its disadvantages for strength loss of cement based materials and high cost,it has still not widely accepted in the field yet. Another method, for internal curing withpresaturated light weight aggregate (LWA), it is effective for autogenous shrinkagereduction. However, the higher the LWA content, the more strength loss as well.Therefore, this work focus on the influences of SRA and internal curing with LWA oncement based materials, and a combination of SRA and internal curing is proposed inorder to make use of favorable condition and avoid unfavorable conditions of SRA andinternal curing. Current work provide a more effective and practical method to controlshrinkage cracking of concrete.In this work, the experiments conducted several testing, such as autogenousshrinkage, drying shrinkage, and mechanical property, as well as cement hydration,microstructures development. Based on the results of influence of SRA and internalcuring on the properties of cement based materials, it is found that SRA have anexcellent performance on shrinkage mitigation, regardless of autogenous shrinkage ordrying shrinkage. For example, when the dosage of SRA up to5%by mass of watercontent, the autogenous shrinkage and drying shrinkage of cement mortar with w/cratio of0.3and0.5decrease about70%and50%at28days age, respectively. However,the presence of SRA will affect the cement hydration and microstructures, whichresulted in a5%-20%decrease on strength of cement mortar.Compared with SRA, the internal curing with crushed lightweight aggregate can only mitigate the early-age autogenous shrinkage before28days age, and thelong-term autogenous shrinkage of internal cured cement mortar will close to that ofcontrolled system. When the substitution of LWA for natural sand up to30%, anacceleration of cement hydration and denser microstructures are observed, and whichleads to an approximate or even higher strength for internal cured mixture. However,the drying curing can significantly affect the performance of internal curing sincepremature drying-out for internally cured specimens with section size of40mm×40mm. As a result, under the drying curing condition, the internal curing cannotimprove the properties of cement mixture with small cross-section, instead, more LWAcontent, larger the drying shrinkage and more strength loss.Considering the dominance of shrinkage mitigation of SRA on cement basedmaterials, and its defects on cement hydration and strength, as well as the attribution ofinternal curing on autogenous shrinkage mitigation, cement hydration acceleration, acombination of SRA and internal curing for shrinkage mitigation of cement basedmaterials are proposed, and the influence of this combination on autogenous or dryingshrinkage, strength and other properties of cement based materials are primarilyinvestigated in this work. The results are indicated that the combination of SRA andinternal curing with minimized SRA or LWA dosage can significantly decrease therelative humidity reduction and autogenous shrinkage development. For example, theautogenous shrinkage of cement mortar, which contained1.3%of SRA by the mass ofcement and30%substitution of LWA, is below to100×10-6at28days age. Meanwhile,the influences of internal curing with LWA that presaturated with SRA solution on theproperties of cement based system are also performed. In addition, this combinationcan compensate the adverse effects of SRA on cement hydration and strengthdevelopment by the hydration accelerating by the internal curing. Besides, for thesurface tension reduction and viscosity increase in the presence of SRA, and thecontribution to microstructure optimization from internal curing, the water absorptivityof cement mortar with SRA and internal curing is significantly reduced, this may bebeneficial to the durability enhancement of cement based materials.