The performance of hardened wet-process sprayed mortars

This paper, which reports on part of a three-year research project into wet-process sprayed mortars and concretes for repair, investigates the hardened performance of wet-process sprayed mortars. Seven commercially available preblended repair mortars were pumped and sprayed through a worm pump, three through a piston pump and two through a dry-spray machine. A laboratory-designed mortar was also worm and piston pumped. The properties measured included compressive and flexural strength, tensile bond strength, hardened density, modulus of elasticity, air permeability, sorptivity, and drying and restrained shrinkage. In situ test specimens were extracted from 500 mm x 500 mm x 100 mm deep sprayed panels. Tests were also conducted on corresponding cast specimens and, where possible, on specimens that had been sprayed directly into a cube or beam mould. A new test to quantify the degree of reinforcement encasement has been developed and an initial investigation into the measurement of the restrained shrinkage of in situ repairs is presented. The compressive and flexural strengths of the laboratory mix were comparable to those of the best of the commercially available preblended mortars. The values for modulus of elasticity, when compared with the compressive strength, were lower than published formulae for this relationship would suggest, especially at lower strengths. the air permeability of most of the mortars was lower than that for normal wet-cured concrete and decreased with an increase in compressive strength. The sorptivity values showed no clear relationship to the compressive strength. The type of wet-process pump was found to have little effect on the in situ compressive and flexural strengths but did affect the bond strength, although this was mainly because of the stream velocity and w/c ratio rather than the pumping process. The pump type also affected the reinforcement encasement, with higher stream velocities producing better encasement. The mixes exhibited a wide range of drying shrinkage, but the data from the restrained specimens suggest an actual repair is influenced as much by ambient conditions as it is by the mix proportions.