Characterization of Cracking Restraint at Saw-Cut Joints Using the German Cracking Frame

Cracking of concrete slabs at saw-cut notches under a given combination of weather and slab conditions is a major concern in the early stages of construction. Several factors affect crack initiation such as material strength parameters, the method and quality of curing, slab–subbase stiffness, and concrete shrinkage and creep. Early-age creep and shrinkage also appear to be key factors in stress and crack development at the tip of the saw-cut notch. These two factors were analyzed and characterized by experiments that provide some insight relative to how shrinkage and creep strain affect crack development. These results can be useful in evaluating the behavior of concrete and the development of cracking at saw-cut notches. The German cracking frame was used to provide restraint and the means to initiate cracking at an artificial notch during early-age strength development in the concrete while the concrete was undergoing creep and shrinkage strains. The cracking frame tests were conducted under constant temperature and relative humidity conditions in the laboratory. A fracture mechanics approach was followed to determine concrete strength at the saw-cut notch. It can be concluded that the development of creep strain in the concrete affects the cracking tendency of the concrete. The data seemed to suggest that a greater cracking tendency at the saw-cut notches did not result until after the incremental creep strain in the concrete had diminished sufficiently. The role of creep and shrinkage under field conditions relative to the control of cracking due to saw cutting in newly constructed concrete slabs is also considered indirectly.