Effect of Set Accelerator on Properties of Wet Sprayed Concrete

Abstract

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.

Effect of Set Accelerator on Capillary Suction and Porosity of Concrete – Cast Samples with Constant Water/Binder Ratio

“…Despite the scatter, Figures 7 -9 confirm previous findings from sprayed concrete specimens cored from panels after full-scale spraying with different accelerator doses [19] -the accelerator alters the pore structure of the sprayed concrete -increasing both suction porosity and the rate of capillary suction. The increase in suction porosity in accelerated sprayed concrete is due to both the additional water affect and the accelerator effect.…”

“…For sprayed concrete we have previously proposed that this difference is entirely ascribed to filling of macro pores due to the special irregular and partly open macro pore structure in sprayed concrete, and hence not to be included as suction porosity in the cement paste [5]. However, in this work on laboratory cast accelerated concrete we observed that the difference in water absorption from one sided to fully submerged is similar to what we observed in sprayed concrete specimens [19]. Hence the mechanisms of filling open and closed macro pores as defined by Equations 4-6 are uncertain and it is not entirely clear whether the extra porosity filled from one-sided to submerged state should be ascribed to what we term suction porosity or macro porosity.…”

“…The method involved adding water to the set accelerator to dilute it, giving more time before rapid stiffening occurred, and to achieve constant water/binder ratio between all the set accelerator doses. The capillary suction rates and porosity of the hardened concrete were investigated to verify that the effects on porosity and capillary suction measured by Trussell et al [19] were due to the effect of set accelerator on porosity, and not merely a result of water/binder variations due to the additional water content with higher doses of set accelerator. We compared measured porosities to a modified Powers model to assess the effect of accelerator on the hydration products.…”

“…Trussell et al [19] varied set accelerator doses in full-scale sprayed concrete experiments and measured increased suction porosity with increased set accelerator dose. They found that Powers' equation linking suction porosity, water/binder ratio and degree of hydration in hardened cement paste [7] gives very low values for degree of hydration for concrete with high doses of set accelerator and concluded that Powers' model is inapplicable for sprayed concrete containing set accelerator.…”

“…Due the uncertainty of the placed concrete composition for concrete sprayed at full scale (uncertainty over the precise set accelerator dose added at nozzle, and the effect of rebound and dust) the laboratory experiment described in this manuscript, where the proportions could be controlled more precisely, was performed to verify the findings of Trussell et al [19]. This will give further understanding of the effect of set accelerators on concrete properties and is important for research and development on the use of sprayed concrete for permanent tunnel linings.…”