减水剂对水泥基材料早期开裂影响的研究

为研究减水剂对水泥基材料体积稳定性及其早期开裂的影响,采用多通道椭圆环收缩开裂测试手段、自由收缩和强度试验等方法测试了掺聚羧酸类、萘系和木质素系减水剂砂浆的早期收缩开裂和开裂敏感性.研究表明,掺减水剂均延长了砂浆初始开裂时间,从而降低了砂浆的开裂敏感性,且高效减水剂比木质素系减水剂更能有效地降低砂浆的开裂敏感性.掺高效减水剂增大了砂浆的自由收缩值,且萘系比聚羧酸类减水剂更会增大砂浆自由收缩值,而掺木质素系减水剂能稍微降低砂浆自由收缩值.减水剂控制砂浆体积稳定性的效果为:木质素系＞聚羧酸类＞萘系.掺聚羧酸类和掺木质素系减水剂都减小了砂浆的最大裂纹宽度,而掺萘系则加快了砂浆最大裂纹宽度的发展速度.在干燥养护条件下,掺高效减水剂比掺木质素系减水剂更能有效地提高砂浆抗折和抗压强度,且掺聚羧酸类减水剂又比掺萘系高效减水剂更为有效.

Influence of Water-reducing Admixtures on Early-age Cracking of Cement-based Materials

For understanding the effect of water-reducing admixtures on early-age cracking and volume stability of cement-based materials,early-age shrinkage cracking and cracking sensitivity of mortars with polycarboxylate-type superplasticizer(PC),polynaphthalene sulphonate superplasticizer(PNS) and calcium lignosulphonate(CLS) were investigated by means of multi-channel ellipse ring shrinkage cracking test,free shrinkage and strength test under drying conditions.The general effect of PC,PNS and CLS is to delay initial cracking time of mortars,and decrease cracking sensitivity of mortars.As for decreasing cracking sensitivity of mortars,PC>PNS>CLS.When keeping the constant W/B ratio and the content of cement pastes,superplasticizers can increase free shrinkage of mortars,but CLS can slightly decrease free shrinkage of mortars. As for the effect of controlling volume stability of mortars,orderly,CLS>PC>PNS.Maximum crack width of mortars with PC and CLS would be reduced,but the development rate of maximum crack width of mortars with PNS is faster in comparison with control mortars.To incorporate superplasticizers is to increase flexural and compressive strength of mortars at 28 days under drying conditions.Superplasticizers are more effective than CLS,and PC is superior to PNS in the aspect of increasing strength.