Effect of various liquid admixtures on cracking of plastic concrete

Plastic shrinkage cracking (PSC) is one of the earliest possible defects that can potentially ruin the performance of concrete structures with large exposed surfaces. PSC is mainly attributed to tensile stresses arising in concrete due to a combination of capillary pressure, and restraints provided by reinforcement and formwork. The adoption and use of several admixtures in modern-day concrete has been known to alter properties of both the fresh and hardened concrete. In this research, the adopted admixtures include a minimum and maximum dosage of a glucose-based retarder, calcium chloride-based accelerator, chloride free air entraining agent, lignosulphonate plasticiser, shrinkage-reducing admixture (SRA), poly carboxylate ether (PCE) based super-plasticiser, and a sulphonated melamine formaldehyde (SMF) based super-plasticiser. The experimental tests were conducted in a climate chamber with an ambient temperature of 40 °C, relative humidity of 10% and a wind speed of 20.2 km/h. By understanding the phenomenological and fundamental relationships of PSC when various admixtures are used in conventional and high-flow concretes, it was found that admixtures cause a substantial reduction in PSC severity. The respective admixtures led to a reduction in measured crack areas, capillary pressures, evaporation amount/rate, as well as settlement/shrinkage behaviours in both types of concrete. Compared to conventional concrete, high-flow mixes showed a greater reduction in the severity of PSC. The underlying working mechanism of these admixtures largely relates to the surface tension reduction ability.