This paper describes sprayed concrete experiments varying the set accelerator dose. Literature on the hydration of cement with modern alkali-free set accelerators is reviewed and two full scale wet spraying experiments have been conducted, varying the dose of set accelerator in each. The effects on the properties of the hardening and hardened sprayed concrete were investigated by field and laboratory testing. Increasing the set accelerator dose was found to increase the rate of early age strength development but reduce density, long term strength and increase suction porosity of hardened sprayed concrete.

Wet sprayed concrete quality is affected by more production factors than cast concrete, particularly due to the propulsion through the nozzle and the flash set caused by the set accelerator. Practitioners often use the term “sprayability” to describe these factors. We propose a definition of “sprayability” that relates the application to the final properties of the hardened sprayed concrete and review factors affecting it: concrete constituents, proportioning, and application mechanics. These factors affect the hardening and the structure of the hardened sprayed concrete – the porosity, permeability and durability. We consider improving sustainability through proportioning with increased share of supplementary cementitious materials, calculate the placed composition and focus on factors that affect water transport, and hence durability. Due to the spray application and flash-set, irregular compaction voids dominate the macro pore structure of sprayed concrete. Studies of permeability of sprayed concrete have shown that it is possible to obtain low permeabilities given adequate composition and curing. Presumably these samples have been well-cured, uncracked and with non-percolating macro voids. Given observations of cracks in sprayed concrete linings and the macro voids, important further studies will be on the effect of accelerator, compaction porosity and cracking on permeability.

No abstract

No abstract

Alkali-free set accelerators are added at the nozzle to ensure rapid set of wet sprayed concrete. The accelerator affects the strength development, porosity and transport properties, and hence the durability, of the sprayed concrete. We developed a method to cast samples with varying set accelerator doses to measure the effect of the accelerator on porosity, but with a constant effective water/binder ratio of 0.45 for each accelerator dose. Six cylinders of concrete were cast with set accelerator doses of 0, 2, 4, 6, 8 and 10 % of effective binder mass. High workability was achieved to enable mixing before rapid stiffening occurred, though this high workability led to some aggregate settlement in the cylinders. Porosity was measured by capillary suction on dried specimens of hardened concrete and subsequent pressure saturation of macro pores (PF test). The samples cast with higher doses of set accelerator had higher suction porosities and higher rates of capillary suction. Using a modified Powers equation gave very low calculated degree of hydration values for concrete with set accelerator, indicating that the equation is not applicable for concrete with set accelerators, due to the higher suction porosity in accelerated matrices, caused by different hydration products.