Early Age Strength Properties of Shotcrete

Abstract

The early strength properties are reported of shotcrete placed under simulated field conditions using standard wet and dry processes and commercially available cements and accelerators. Bench test results determine fast set agents (FSA) and cements that were used in the shotcrete program. The results were evaluated for effect on rate of setting and strength of motar cubes. The shotcrete program evaluated: one cement used as a control; cement with 2 fast set agents at 1 percent, 3 percent and 6 percent concentrations; and one controlled setting time cement with 0 percent and 1 percent soda ash additive. The shotcrete was placed on vertical and overhead surfaces in 2- and 6-inch thicknesses. Six inch thicknesses were placed in 2 inch layers. The wet and dry processes produced shotcrete with similar strengths. Both fast set agents studied also produced shotcrete with similar properties. The results indicate no significant difference in shotcrete properites when placed on vertical or overhead surfaces or between 2-inch and 6-inch thicknesses. The use of fast set agents or regulated set cement produces shotcrete with higher early strength but also reduced the final strength when compared with shotcrete with Type III cement and no additive. Shotcrete made with regulated set cement produced higher early strength than shotcrete made with the fast set agents. Details are discussed of accelerating admixtures and the conditions under which they are used. The experimental program is reviewed and details are given of the bench tests, the shotcrete program, procedures for making test panels, and the compression tests. A statistical treatment was used to provide a means of summarizing the data collected and to produce equations to predict the strength of shotcrete. Graphs illustrate the compressive strengths of the dry process control mixes, the compressive strength for three concentrations of FSA, and the compressive strength of Regulated Set Portland Cement.