Influence Of Admixtures On Shrinkage Property And Cracking Behavior Of Concrete

In this article, influence of concrete admixtures on shrinkage and cracking property of cement-based materials was investigated using the edge constrained plate method, autogenous shrinkage and drying shrinkage test. The main research contents included the effect of superplasticizer type and dosage, unitary and compound mineral admixtures (i.e. fly ash (FA), blast furnace slag (BFS), silica fume (SF)), shrinkage-reducing admixture (SRA) and polypropylene fiber (PPF). And the relationship of mortar shrinkage and concrete cracking was also discussed. Several conclusions had obtained through these research results.(1) At the same slump and w/c, naphthalene and aliphatic superplasticizer increased autogenous shrinkage and drying shrinkage of mortar and greatly increased cracking risk of concrete. However, polycarboxylate superplasticizer (PCA) could inhibit shrinkage of mortar and reduce the risk of cracking problem. When w/c was0.42and dosages varies from0.6%to1.5%, PCA dosages exhibited less impact on autogenous shrinkage. And drying shrinkage of mortar and cracking risk of concrete increased with increasing PCA dosages. The autogenous shrinkage of motor increased with increasing PCA dosages, while drying shrinkage of mortar and cracking risk of concrete decreased with increasing PCA dosages under keeping same slump and dosages varies from0.6%to1.5%. Drying shrinkage is the main reason of cracking for ordinary concrete (w/c=0.50～0.40) with PCA.(2) When content ranged from20%to40%, FA or BFS effectively inhibited mortar free shrinkage, obviously reduced the maximum crack width and total crack area of concrete, and improved concrete anti-cracking property. Autogenous shrinkage and drying shrinkage were greatly reduced with increasing FA or BFS content, concrete anti-cracking property was increased with increasing FA or BFS content. Mortar shrinkage performance and concrete anti-cracking property with FA was better than that of BFS when applied at the same content. When content ranged from5%to10%, SF increased mortar shrinkage and concrete cracking risk, particularly greatly increased autogenous shrinkage before24hours and drying shrinkage before7days. With the increasing dosage of SF, autogenous shrinkage and drying shrinkage significantly increased and concrete cracking risk greatly increased.(3) Replacing30%to40%of cement (by mass) with FA and BFS in the cement-based materials, mortar shrinkage and concrete total cracking area were obviously reduced, and concrete anti-cracking property was greatly improved. BFS and FA compound ratio exhibited little effect on the shrinkage of mortar and the maximum crack width and total crack area of concrete when BFS and FA ratios were in the range of1:2to2:1and content was kept at30%replacement of cement by mass. SF/FA/BFS binary or ternary blends could inhibit mortar shrinkage and greatly reduce24hours autogenous shrinkage growth rate and7days drying shrinkage growth rate. The increasing effect of SF on mortar shrinkage and concrete cracking risk was eliminated in binary and ternary blends in which anti-cracking properties were improved. And the good compressive strength of concrete was observed. A good linear correlation was noted between the concrete total cracking area and the mortar shrinkage, autogenous shrinkage and drying shrinkage is the main factor of cracking for concrete with mineral admixtures.(4) It had been noted that the addition of SRA and PPF could effectively inhibit mortar shrinkage, significantly reduced the total crack area of concrete and improved concrete anti-cracking behavior. SRA could greatly reduce the number of concrete cracks and PPF could significantly reduce crack width. When SRA content was in the range of0.6%to1.2%, mortar shrinkage and concrete total cracking area were lessened with increasing SRA contents, and the concrete compressive strength significantly was decreased. When PPF content was kept at0.9Kg/m3, PPF length had less effect to mortar shrinkage and concrete total cracking area, but had greater effect to concrete crack width. Concrete exhibited better anti-cracking behavior when fiber length was3/5of the coarse aggregate’s maximum particle size. The results showed that the inhibition effect of low content PPF (0.6Kg/m3) was not obvious on shrinkage and cracking. As PPF content increased (0.9Kg/m3～1.2Kg/m3), concrete anti-cracking performance had improved significantly. According to test results and experiment condition, the optimum content of PPF contents was0.9Kg/m3～1.2Kg/m3. The results of correlation analysis between concrete total cracking area and mortar shrinkage showed that the main reason of inhibiting concrete cracking with SRA was that decreasing shrinkage. However, PPF could also inhibited concrete cracking, which was not the main cause of decreasing shrinkage.(5) SRA and PPF used in combination could effectively reduce the mortar shrinkage and concrete cracking area. And the anti-cracking behavior of concrete was obviously improved. When w/c was in the range of0.42to0.34, with the decrease of w/c, mortar drying shrinkage lessened, whereas the autogenous shrinkage of mortar and the maximum crack width and total crack area of concrete increased. A good linear correlation was found between the concrete total cracking area and the mortar shrinkage, autogenous shrinkage was the main factor of cracking for high performance concrete with SRA and PPF at lower w/c.