Fracture Energy Test for Highway Concrete: Determining the Effect of Coarse Aggregate on Crack Propagation Resistance

Portland cement concrete fracture properties—specific fracture energy, fracture toughness, and brittleness—were investigated for typical Michigan highway concretes containing different coarse aggregates and varying in age: 7,28, and 91 days. These fracture properties can be determined from the complete load-deflection curve of a notched beam. The effective beam is 965 mm long, 100 mm wide, and 200 mm high, with a 100-mm center notch. Results show that the specific fracture energy, which determines the resistance to crack propagation, for a concrete pavement mix is controlled primarily by the coarse aggregate type. Differences of 100 percent were obtained between aggregate types. A glacial gravel yielded the highest resistance (160 N/m), and the dolomitic limestones and blast furnace slag yielded the lowest resistance (80–100 N/m), although the concretes all had similar strength properties. The fracture toughness, resistance to crack initiation, was found to be linear related with concrete strength. Typically this results in improved early age fracture toughness for concretes containing dolomitic limestone and blast furnace slag as coarse aggregate, compared to glacial gravel, because natural aggregate concrete typically has slower strength gain initially. Concrete brittleness, based on the entire load-deflection response, showed that concretes containing stronger coarse aggregate, such as glacial gravel, are significantly less brittle at early ages than are concretes containing weaker aggregate. However, these stronger aggregate concretes become more brittle, and thus crack sensitive, over time.