Effect of magnesium phosphate cement mortar coating on strand bond behaviour

Surface coating on prestressing strand is an effective and economic measure to prevent pre -tensioned prestressed concrete structures from corrosion. The effects of magnesium phosphate cement (MPC) mortar coating on the strand bond behaviour are investigated experimentally in the present paper. Ten pull-out specimens with different MPC mortar coating thickness are designed to study the effect of coating thickness or thickness ratio on the bond failure mode, bond strength and force-slip response. Experimental results demonstrate that the bond failure mode changes from pull-out failure accompanied with strand rotation to cover splitting failure when coating thickness and thickness ratio exceed 10 mm and 0.15, respectively. Compared with the uncoated and epoxy coated control specimens, coating thickness ranging from 25 mm to 35 mm or thickness ratio ranging from 0.37 to 0.52 contributes the most to the increase of bond strength and stiffness. Beyond that range, the facilitation in both bond strength and stiffness gradually decreases. Besides, an analytical model, which considers the composite cover confinement pro-vided by the coating and concrete, is proposed to reproduce the bond strength evolution. Com-parison of experimental and prediction results show that the proposed model can be used to reasonably evaluate the bond strength.