Properties of pull-out bond strength and concept to assess ultimate bond stress of NSC and HSC

This study was conducted in two phases. The first phase was a study of bond behaviour using two different tests – the single pull-out (SPO) test, in which a concrete specimen was subjected to compressive stresses, and the double pull-out (DPO) test, in which a concrete specimen was subjected to tensile stresses. This phase of the study aimed to make a comparison of the SPO and DPO tests. To compare the behaviour of these tests, different levels of cube compressive strength (from 18·0 to 96·8 MPa) were considered through the use of different types of coarse aggregate (pink limestone, gravel and dolomite), different water/cement ratios and different cement contents. The second phase of the study focused on the bond strength of high-strength concrete (HSC) using the DPO test to assess the ultimate bond stress. This phase considered the effects of concrete cube compressive strength, bar diameter, concrete cover, embedded length and pre-flexural crack length. The cube compressive strengths of the concretes used were 30, 50, 70 and 90 MPa, the bar diameters used were 16 and 18 mm and the concrete covers were 67, 66 and 42 mm. Based on the test results, a concept to assess the ultimate stress of normal-strength concrete (NSC) and HSC was developed. Equations to calculate the ultimate bond stress and the required development length are also suggested.