Low-volume wet-process sprayed concrete: hardened properties

This paper, which reports on part of a 3-year research project into wet-process sprayed mortars and concretes for repair, investigates the hardened performance of wet-process sprayed fine concretes. It follows on from an earlier paper by the authors on the performance of hardened wet-process sprayed mortars and some comparisons with these are made here (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). Work has also been completed by the authors on the pumping and rheology of the fine concrete mixes presented here (Austin SA, Goodier, CI, Robins PJ (2005). Mater Struc, RILEM 38:229–237). Nine laboratory-designed fine concretes were pumped and sprayed through a wet-process piston pump and one through a dry-process pump. The properties measured included compressive and flexural strength, tensile bond strength, hardened density, elastic modulus, sorptivity and drying and restrained shrinkage. In situ test specimens were extracted from 500 × 500 × 100 mm deep sprayed panels. Hardened property tests were also conducted on corresponding cast specimens and, where possible, on specimens that had been sprayed directly into a cube or beam mould. The compressive strengths of the cast cubes, although very similar, were usually slightly greater than the in␣situ cubes, the opposite of what was found for wet-sprayed mortars (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). Inconsistent results for compressive and flexural strengths obtained from spraying directly into a steel mould suggest that this method is not as reliable when using a piston pump as it is when using a low-output worm pump (Austin SA, Robins PJ, Goodier CI (2000). Magz Concr Res 52:195–208). The bond strength of all the mixes exceeded 2.1 MPa at 7 days. The values for modulus of elasticity, when compared with the compressive strength, were similar to published data for this relationship. The sorptivity values showed only a slight relationship with the compressive strength. 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.