Development and Calibration of an Efficiency Factor Model for Recycled Aggregate Concrete Struts

In the strut-and-tie (STM) method of design, the internal mechanism of flow of forces is represented by hypothetical truss in which the behavior of the beam is controlled by the strut connecting load and support points. The strength of such strut is correlated to the shear capacity of the deep beam through a factor called the strut efficiency factor. Different efficiency factor models have been recommended by various internationally accepted codes. However, none of the codes takes into account the effect of recycled aggregates in concrete. Although some codes yield conservative results, these predictions are not sensitive enough to the recycled aggregate content. Therefore, an efficiency factor model sensitive to recycled aggregate concrete and easy to operate is much desired. In this work, published results of laboratory tests on deep beam specimens made of concrete consisting of recycled aggregates were considered for the analysis, employing a suitable strut-and-tie model. All these deep beams were originally designed by sectional or empirical method. Based on regression analysis of the outcomes of the STM analysis, an efficiency factor model has been proposed which takes into account the effect of recycled aggregates in concrete. Subsequently, scaled deep beam specimens containing recycled aggregate concrete were cast and tested in the laboratory in order to calibrate the proposed strut efficiency factor model. The yield of proposed efficiency factor model was compared with the predictions of the selected internationally accepted code provisions. It is found that the predictions of proposed efficiency factor model give consistent and comparable results.