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Several raw coal ashes which were not finely divided, were used as admixtures for high volume coal ash shotcrete. Rheological properties of the concretes before projecting were evaluated by the modified slump cone test. Microstructure of the projected concretes was examined by the quantitative image analysis for backscattered electron images. Differences in the strength development between the shotcretes and conventional cast concretes were discussed in relation with influences of pneumatic projection on the initial packing of binder particles and the subsequent changes in coarse capillary pore structure. The modified slump test was useful for evaluating flow and projection properties of concrete. Plastic viscosity of coal ash concrete depended on types of coal ash even when the same slump value was attained. When a coal ash with a high carbon content was used, the plastic viscosity increased. The increase in viscosity resulted in the increase in pumping pressure for projection. The coarse capillary pore structure in the projected concrete without accelerator was not greatly different from that in the cast concretes. However, when a large amount of coal ash was incorporated, the threshold diameter of coarse capillary pores at early ages was reduced. The development of compressive strength in shotcrete was not notable compared to the conventional compacted concrete. It was suggested from the evolution of capillary pore structures that the accelerator for shotcrete prevented further densification in microstructure at long ages.

Fresh Properties and the Development of Strength in High Volume Coal Ash Shotcretes