Study On Effect Of Admixtures On Early Deformation Of Cement-based Materials

Shrinkage cracking is one of the popular subjects in the domain of cement-based materials research all along. In recent years, shrinkage cracking caused by materials deformation at early age has a special attention. Chemical admixture has been extensively used in cement-based materials and has become an indispensable ingredient in high performance cement-based materials, but no systematic study has been done on effect of chemical admixtures on shrinkage cracking at early age and cracking sensitivity. So it becomes a problem needed to solve urgently.The paper presents a relatively systematic study on effect of admixtures on shrinkage cracking at early age and cracking sensitivity. The results show the tests, which are used in this paper, including ellipse ring test, free shrinkage test and chemical shrinkage test, could do well in evaluating the shrinkage cracking developing trend of cement-based materials at early age and cracking sensitivity. From the point of strength, free shrinkage and cracking sensitivity of mortars, cementitious material blended 5% silica fume and 40% fly ash is the best constitutions in this materials system.Chemical admixtures have improved initial cracking time of mortars and have decreased cracking sensitivity. High range water-reducing agent is more effective on decreasing cracking sensitivity of mortars than calcium lignosulphonate, polycarboxylate type superplasticizer is more effective than polynaphthalene sulphonate superplasticizer, and high thickness type polynaphthalene sulphonate superplasticizer is more effective than common thickness type polynaphthalene sulphonate superplasticizer. As for decreasing cracking sensitivity, polycarboxylate type superplasticizer and high thickness type polynaphthalene sulphonate superplasticizer are quit effective. Shrinkage reducing admixture could decrease cracking sensitivity significantly.In the circumstance of fixed water-binder ratio and cement pastes content, high range water-reducing agent could increase free shrinkage of mortars. Compared to control sample, free shrinkage of mortars with calcium lignosulphonate was more at early age, but it was less after 14 days. Shrinkage reducing admixture could reduce free shrinkage. Resistivity and chemical shrinkage have also been tested. It was found that polycarboxylate type superplasticizer and shrinkage reducing admixture could retardtime of microstructure form of pastes, and could reduce chemical shrinkage of pastes at early age. Both resistivity and chemical shrinkage of pastes at early age have a good relativity. The paper has analyzed function mechanism of chemical admixtures on shrinkage cracking of cement-based materials at early age, and comes up with an analytic model of shrinkage cracking resulted from dry shrinkage and autogenous shrinkage.