Factors Affecting Shrinkage Of Self-Compacting Concrete At Early Age And Control

Shrinkage cracking of high performance concrete (HPC) with low water to binder ratio (w/b) at early-age has gained more end more attention nowadays. Self-Compacting Concrete (SCC) has accepted as a concept of HPC for its excellent constructability and environment protective property. However, to meet the requirement of high fluidity and stability, SCC recipes are often associated with high contents of binder, fillers, water reducing admixtures, low contents of aggregate and low w/b, which all may contribute to increased time-dependent volume changes as driving forces to shrinkage cracking and/or thermal cracking as well as construction fare. Thus, questions have been raised about such cracking of SCC.In this paper, the factors affecting shrinkage cracking of SCC at early-age and workability including mineral admixture, w/b and binder content were investigated from the point of production technology of SCC. The initial cracking time was measured using a new ellipse-ring shrinkage cracking testing apparatus with multi-channel to evaluate cracking sensitivity of mortar sieving from SCC. Meanwhile, free shrinkage of SCC was also measured to investigate the effect of them on shrinkage cracking on SCC. Besides, the influence of factors affecting shrinkage cracking on the early hydration and hardening of cement pastes was studied using a new non-contact resistivity meter combined with strength test.The results showed that S/G was important factor affecting uniformity and filling ability, 50% was chosen for this paper. Perfect flowability and stability of SCC were gained when combined fly ash (FA) and silica fume (SF), but, high efficiency super plasticizer as well as more dosage of it was needed to reduce huge cohesion induced by SF particles, co-polymer was founded to effective in doing this and compatible well with cement used in my test. The viscosity of mixture was reduced with the increase of w/b, and so SF or viscosity-modifier was useful to improve its cohesiveness. Paste volume was the most important factor affecting workability of SCC. SF was founded again to play an excellent role in maintaining stability under the condition of low binder content.It was founded that the cracking sensitivity of mortar at early-age cured under drying-stored condition was reduced greatly incorporating with FA, the shrinkage rate and value of SCC were also reduced. Compared to sample containing 20% FA, the cracking sensitivity of multiple adding of 20% FA and 5% SF was increased, as well as free shrinkage rate and value. However, fully curing at early-age is important to SCCincorporating with SF. The mortar sample was prone to cracking with the increase of w/b under the experimental condition of this test. So, actual w/c, mineral admixture composition and curing condition must be combined to evaluate cracking sensitivity of SCC at early-age. Besides, when designed with a same strength of 28d, the cracking sensitivity was reduced with the decrease of binder content, especially when it reached 450Kg/m3, the initial cracking time was delayed markedly.The influence of FA and SF on the hydration and hardening of cement paste was reflected by resistivity and strength result, accordingly, the influence mechanism of them on cracking sensitivity of SCC mortar at early-age was explained. The resistivity and strength results of different w/b and binder content implied that actual w/c, the shape and activity of mineral admixture all will have an impact on hydration of paste as well as form of micro-structure and development of hardened property.Finally, the mix proportion with low cracking sensitivity and cost was commended on the basis of production of this paper.