早期养护时间对混凝土早期收缩的影响

目的研究早期养护时间对混凝土早期收缩的影响,并定量地阐明早期养护对减小收缩和抑制早期裂缝的重要性.方法以掺和不掺减水剂的4种混凝土为研究对象,采用自行设计的一套试验装置测量混凝土在不同早期养护时间下的早期收缩.结果不掺减水剂的混凝土,无论是始终敞开或全程密封养护,总的早期收缩变形都很小.而掺减水剂的混凝土,早期收缩显著增大,集中产生于初凝后的8h以内,且收缩率随水灰比减小急剧增大;在相同水灰比条件下,早期收缩随着早期养护时间的增加而显著减小,但在此之后养护对收缩的抑制作用逐渐减弱.结论对不掺减水剂混凝土,单从抑制收缩的角度来考虑,并不需要采取什么特别的早期养护措施.对掺减水剂混凝土,当不养护时,初凝后8 h内的收缩值远远超过混凝土开裂极限;而有效的早期养护能显著抑制这类混凝土早期收缩,因此,掺减水剂混凝土必须在初凝前开始及时养护,且确保初凝后持续8 h以上的养护时间.

Influence of Early-age Curing Time on Shrinkage of Concrete

The influence of early-age curing time on shrinkage regularity of concrete was studied,and the importance of early-age curing to reduce shrinkage and control early-age cracking was quantificationally clarified as well.Based on 4 kinds of concretes with and without superplasticizer,early-age shrinkage of concrete with a different cure time was measured by means of self-designed and manufactured early-age shrinkage testing set.The results show that early-age shrinkage of concrete without superplasticizer is low,while expansion is observed under sealed curing condition.However,early-age shrinkage of concrete with superplastisizer increases sharply,even more,it increases with the decrease of water-to-cement ratio,especially in 8h after initial setting time.And when in the same water-to-cement ratio,it decreases remarkably with the increase of early-age curing time.For concrete without superplasticizer,there is no demand for special curing measurement to control shrinkage.For concrete with superplasticizer,when no early-age curing is supplied,early-age shrinkage during the first 8h after initial setting time is much bigger than ultimate strain of cracking.However,an effective early-age curing can restrain early-age shrinkage of this concrete remarkably.Thus the curing of concrete with superplasticizer should be started before initial setting time,and even more,the curing time should sustain more than 8h after initial curing time.