Influence Of Polvcarboxylate Superplasticizer On Early Shrinkage Of Concrete

The adding of superplasticizer increases the shrinkage of concrete,especially the phenomenon of early shrinkage cracking,which has become an urgent engineering problem to be solved.In this issue,polycarboxylate superplasticizer,which is widely used at present,was selected.The study mainly focused on the influence of the superplasticizer dosage on early shrinkage at different mixing proportion of concrete.The influence of the superplasticizer dosage on evaporation and surface moisture content was also tested.At the same time,the issue analysed the main factors affecting the early shrinkage of concrete and its mechanism,combined with surface tension test and mercury intrusion porosimetry.The influence of the superplasticizer dosage on early shrinkage at 3 kinds of mixing proportions was investigated by non-contact measurement method.The test results showed that the early shrinkage of concrete would increase significantly when the dosage of superplasticizer exceed a certain threshold at the same mixing proportion of concrete,but not for concrete with low dosage of superplasticizer.If the slump remained unchanged,no matter water-binder ratio increased or paste-aggregate ratio changed,the early shrinkage of the concrete would increase with the dosage of superplasticiser increasing.Hence,the dosage of superplasticiser is a key factor for the early shrinkage of concrete,which must be strictly controlled in the mix proportion design.The results of measuring water content variations showed that the dosage of superplasticiser had little effect on evaporation,surface moisture content and the water proportion of the surface layer concrete when the dosage was low.When the dosage of superplasticizer exceed a certain threshold,the evaporation increases significantly,the time when surface moisture content began to decrease obviously delayed,3 days surface moisture content decreased,and the water proportion of 3 days surface layer concrete decreased marked.At the same mixing proportion of concrete,when the superplasticizer dosage increased,the larger slump made the mortar enrichment of the surface concrete,the initial surface moisture content increased,which led to a significant increase in water loss.The main factors affecting early shrinkage and the mechanism were studied.The results showed that surface tension decreased with the increase of superplasticizer dosage,but the change was very little,which meant the influence of surface tension on early shrinkage was limited.The microscopic analysis showed that the most probable pore diameter reduced with the increase of superplasticizer dosage when the dosage was low,the pore distribution of concrete was refined to some extent.But when the superplasticizer dosage is high,the most probable pore diameter increased rather than decreased.The pore distribution of concrete had no obvious relevance to early shrinkage,and the pore distribution was more like the result of evaporation and hydration.On the other hand,there was a good correlation between evaporation and early shrinkage,that was,the effect of superplasticizer dosage on evaporation loss and early shrinkage of concrete showed a relatively consistent law,whether in the early stages of rapid shrinkage,or in the complete early shrinkage testing process.The water loss and the early shrinkage of concrete did not change obviously when the dosage was low.When the dosage exceeds a certain threshold,the water loss increased under the same curing conditions,which led to a significant increase in early shrinkage.