Effective Initial Crack Depth in Exposed Concrete Surfaces

Abstract

Microcracking at the surface of concrete slabs can result in a reduction in slab strength at the surface and a decrease in the flexural load carrying capacity of the slab especially when undergoing loading that produces top tensile stresses. The majority of this surface cracking is the result of drying shrinkage gradients and slab restraint. The use of an effective initial crack depth (ae) has been previously proposed as a method to model this reduction in slab strength due to surface microcracking. The two-parameter fracture model procedure was modified to make calculations of the effective initial crack depth in three-point bending specimens to examine if reasonable values of ae can be measured in a laboratory setting using fracture mechanics principles. Testing of concrete beams exposed to two curing environments (moist- and dry-cured) indicates that the effective initial crack depth, approximately 50 mm in this study, can be calculated using a dependent relationship between crack depth and compliance values. The procedure was verified by calculating additional fracture parameters of the concrete, which were found to be comparable to values presented in the literature for similar concrete materials and mixtures. The ability to quantify an effective notch depth at the surface of the slab can enable future modeling to determine the effective strength difference of concrete slabs under bottom or top tensile loading conditions.