Wet-process sprayed mortar and concrete for repair
2010-12-02T09:41:48Z (GMT) by
The primary aim of this research was to improve the understanding of the influence of the process and the mix constituents on the fresh and hardened properties of wetprocess sprayed mortars and concretes. The main objectives were: to improve the wetmix spraying process; to specify, measure and optimise in-situ properties; and to disseminate the information obtained in appropriate form to practising engineers to accelerate the use of wet-process sprayed mortar and concrete for repair. The research focused on three types of repair mortars/concretes: pre-blended proprietary mortars (<3 mm aggregate), designed laboratory/site batched mortars and fine concretes (<8 mm aggregate). Thirty mixes were pumped and sprayed using seven pumping/spraying systems. Nineteen types of test were conducted to measure the fresh and hardened properties using three types of specimen production (cast mould, sprayed mould and in-situ specimen). Ten repair scenarios generally encountered in the UK were identified and classified in terms of their characteristics and relevant mixes were identified to satisfy these differing requirements. A rheological audit has been developed and a variety of tests were used to characterise the pumpability and sprayability of each mix, including: rotational viscometers (Tattersall two-point test and Viskomat), pressure-bleed, shear vane, slump, build, fresh density, output, stream velocity (using high-speed video), reinforcement encasement and core grading. A new approach that defines the build in terms of the maximum shear and tensile bending stresses generated at failure was also developed. Hardened properties measured include: compressive and flexural strength, tensile bond strength, drying and restrained shrinkage, elastic modulus, air permeability, sorptivity and hardened density. The hardened performance was generally higher when sprayed with the wet process compared with hand application and lower when compared with the dry process (which was expected), although the values obtained were more than sufficient for normal repair work. All the pre-blended mortars could be pumped and sprayed with a small worm pump. Twelve laboratory-designed mortars were pumped and sprayed in a dedicated spraying chamber constructed at Loughborough and the best of these performed as well as, and produced hardened properties that equalled or surpassed, the pre-blended materials. For worm pumping the grading of a mortar was found to be important and a suitable combined material grading zone has been determined. Two pre-blended and a laboratory-designed mortar were sprayed with a piston pump as were the nine designed concrete mixes, the former producing similar in-situ properties to worm pumping. One pre-blended mix was sprayed successfully with five different wet-process pumps (four worm plus one piston) and three pre-blended mortars and one designed fine concrete were sprayed by the dry-process to benchmark performance, along with data from three repair contracts. The hardened property measurements obtained from spraying directly into steel moulds with a low-volume worm pump were consistent enough to have applications for quality control. The research demonstrated that low-volume wet spraying is a healthier, cleaner and more controllable process (compared with the dry process) which can produce consistently high quality mortars and fine concretes suitable for a range of applications in the UK.