Shrinkage of concrete refers to the volumetric change of concrete due to cement hydration (i.e., self-desiccation) and environmental drying (i.e., water evaporation). It creates dimensional instability and induces tensile stress when the concrete is restrained that may be large enough to cause cracking. The most fundamental way to mitigate concrete shrinkage is to decrease the shrinkage of cementitious paste in the mix design. Some major factors of concrete mix design affecting the shrinkage include water-to-cementitious material (W/CM) ratio, dosage of superplasticizer (SP), concrete strength, and cementitious paste volume (CPV). While the effects of concrete strength and CPV on shrinkage are straightforward, those of W/CM and SP are more complicated because they also affect the pore size and distribution that influence the moisture movement. In this paper, it is advocated to study the concrete shrinkage by its wet packing density (WPD). Considering that the shrinkage of concrete is associated with the moisture movement in capillary pores and that the WPD accounts for the void ratio in fresh concrete, it will be shown in this paper that the shrinkage can be correlated negatively to the WPD with the consideration of CPV. The paper thus provides a new insight to the interdependence amongst shrinkage, CPV, and WPD of concrete.

No abstract

No abstract

By electrostatic repulsion and steric hindrance, polycarboxylate-based superplasticiser (SP) decreases the water demand for cement hydration in mortar/concrete production. Subsequently, less water is needed for a prescribed flowability, which improves its concurrent design limits of strength and flowability. Nonetheless, SP also introduces unfavourable dilatancy (or shear thickening) in mortar due to the clustering of mobile SP polymers in interstitial voids and adsorbed polymers on cement. Interestingly, the dilatancy does not increase monotonically with SP dosage. Reversal of dilatancy occurs when a threshold SP dosage is reached, and subsequent addition of SP decreases dilatancy because the fine particles in the mortar are better dispersed with adequate SP. Interstitial voids substantially decrease due to deflocculation of the fine particles, which improves the wet packing density (WPD) of mortar. Based on this, it is believed that by blending the cement with fly ash and/or using multi-sized sand, the dilatancy of mortar can be alleviated. In this research, using a coaxial viscometer, a rheology test programme of superplasticised mortar was conducted to determine the threshold SP dosage when dilatancy reversal occurs, to study the effect of using multi-sized powder and sand on mortar dilatancy and to correlate the dilatancy of mortar to its WPD.