New Developments in In-Place Testing of Shotcrete

Abstract

Present methods of quality control are briefly reviewed and one method, the pullout test method, is discussed in detail. The testing of cores from the lining which is the most representative of the present methods has some shortcomings: damage in drilling small cores; cores cannot be drilled at early ages. Two commercially available testing devices are described. The Windsor Probe method consists of shooting a probe into the shotcrete with a calibrated explosive and measuring penetration which is then correlated to compressive strength. The rebound hammer measures the rebound of a rod that is forced against the shotcrete or concrete surface by a calibrated spring. These rebound values are then related to compressive strength. The apparatus used in the pullout test method is illustrated. A bolt with a washer brazed to the head is embedded in the fresh shotcrete or concrete. At the time of testing this bolt is pulled from the material through a steel ring and a truncated cone of shotcrete is extracted. Pullout failure stress which can be correlated to compressive strength is discussed. The three parameters which are of importance in obtaining the pullout failure stress are: the force required to pull the cone, the surface area of the cone, and the cone angle. Figures are presented which summarize the results of tests on concrete slabs and shotcrete panels. A graph illustrates the relationship between compressive strength on 4-inch cubes and the pullout method. The correlations between the pullout method and conventional compressive strength test methods are good. However, statistical interpretation is needed. Modifications to the pullout equipment to make it more portable and other investigations planned for the future are briefly indicated.