Influence Of Chemical Admixtures On The Early Cracking And Drying Shrinkage Of Concrete

Chemical admixtures have become the fifth component of modem concrete and have been widely used in the engineering works. However, there is little study on the effect of chemical admixtures on the early shrinkage and cracking of concrete, which is blank for making high performance concrete and research on shrinkage and cracking of concrete. Although much study has been done on the influence of superplasticizer(SP) on drying shrinkage, there is too little on the study of drying shrinkage of high slump concrete with SP and composite SP.In this paper, all components of concrete are fixed apart from the dosage and type of SP. The effect of SP and composite SP on the drying shrinkage of concrete is studied at varied w/c ratio. The influence mechanism of chemical admixture on drying shrinkage is also discussed by SEM and pore structure analysis.It is indicated by the test that the dosage of SP has evident influence on the drying shrinkage of concrete. Initially, the drying shrinkage of concrete rises with the increase of SP dosage. Then it tends to decline when SP is excessive, which will lead to the bleeding of concrete. Drying shrinkage of concrete increases when SP is compounded with set-retarding agent, early-strength accelerating agent and air-entraining agent.Effect of SP and composite SP on the early shrinkage cracking of concrete is also studied. The result shows that the increased SP dosage lowers the early shrinkage cracking resistance of concrete at higher w/c ratio, while it improves the early shrinkage crack-resistance of concrete at lower w/c ratio. Early cracking resistance of concrete decreases when SP is compounded with early-strength accelerating agent. When SP is compounded with set-retarding agent, early crack-resistance decreases at higher w/c ratio and increases at lower w/c ratio. Early cracking resistance of concrete is improved when SP is compounded with air-entraining agent of suitable dosage.