Effect of chemical structure of polycarboxylate-based superplasticizers on workability retention of self-compacting concrete

In self-compacting concrete (SCC) mixture design, polycarboxylate-based superplasticizers (PC-based SPs) usually guarantee the initial workability. However, the time dependent workability of mixtures principally depends on the chemical structure of PC-based SPs and its compatibility with cement. PC-based SPs are the copolymers of chemical structures, which have the potential to be modified to improve their performances. In this study, three synthetic PC-based SPs were synthesized by using radical polymerisation techniques. The effect of these admixtures on setting time of cement pastes and time dependent workability and strength development of SCCs were investigated, respectively. Test results showed that, from the viewpoint of chemical structure, workability retention performance of PC-based SPs could be manipulated by modifying the bond structure between main backbone and side-chain of copolymer. PC-based SPs with ester bonding was not effective in maintaining fresh concrete workability due to the alkali attack vulnerability of this bond structure. On the other hand, by directly bonding the polyoxyethylene side-chain to the backbone of copolymer, fresh SCCs workability can be effectively maintained at least for a period of 2 h. It should be noted that, in addition to the SP type, water/powder ratios of SCC mixtures were also responsible for the long workability retention performances. Best results were derived from mixtures incorporating 2.3 wt% of P3 type SP. However, some lengthening in setting times and reduction in early strength should also be expected. © 2007 Elsevier Ltd. All rights reserved.