Prediction of Compressive Strength for Heterogeneous Mortars from Drilling Resistance Data

Abstract

ABSTRACT The characterization of historic mortars applied in construction is required in many situations, namely for their conservation. In this sense, the drilling resistance test has been being explored by the scientific community, since it has the unique feature of assessing the material strength in depth with very low intrusion. However, the drilling profiles returned by heterogeneous and weak mortars are very irregular which has been considered a limitation of the technique. This article discusses the reasons for the reported irregularity, showing that this supposed drawback is an inevitable outcome in the case of heterogeneous materials and that the statistical analysis of the drilling data can provide important information on the material characteristics. Additionally, it is shown that compressive strength obtained in conventional tests is also very affected by the material’s heterogeneity. A prediction model for compressive strength is established from drilling data collected on 16 low-strength mortars of diverse strength and heterogeneity degrees. This model, applicable to mortars with compressive strength values below 4–5 MPa, showed a mean error of less than 0.5 MPa. Thus, it is possible to predict the compressive strength for a large number of lime-based mortars with a reasonable confidence. The accuracy of the model is then demonstrated on a different set of mortar specimens, composed of six test cases.