Rebound surface hardness of concrete: Introduction of an empirical constitutive model

Surface hardness testing of concrete is a long established NDT method for in situ strength estimation. Nowadays, the rebound hammer is the surface hardness testing device for concrete of the most widespread use. Based on a comprehensive literature review it was realized that no general theory was developed in the last more than 50 years that could describe the relationship between surface hardness and compressive strength of concrete. The diversity of the numerous empirical proposals that can be found in the technical literature needs to be explained. It can be even found in some publications that the method is suitable only for assessing the uniformity of concrete. There is long a time need for a model that can clarify the rebound surface hardness of concrete. Present paper introduces a phenomenological constitutive model (SBZ-model) that can be formulated for the surface hardness of concrete as a time dependent material property. The generating functions of the model are based on the time dependent development of the capillary pore system of the hardened cement paste in concretes that is characterised by the water–cement ratio as a practical simplification. The modelling assumptions and the use of the model would add to the fundamental understanding of the rebound surface hardness of concrete. An extensive experimental verification study clearly demonstrated the reasonable application possibilities of the SBZ-model.

Research highlights
► Empirical surface hardness – compressive strength relations for concrete are diverse.
► A phenomenological constitutive model of concrete surface hardness is introduced.
► The time dependent generating functions are based on the water-cement ratio.
► An experimental verification study demonstrated the potential of the model.
► The model adds to the fundamental understanding of the rebound hardness of concrete.