Estimation of concrete strength combining rebound hammer and Windsor probe test methods Kursat Esat Alyamac 1 , Yavuzhan TAS 2 , Zulfu Cinar ULUCAN 1 , and Merve Acikgenc ULAS 3 1 Firat University, Engineering Faculty, School of Civil Engineering, Elazig, TURKEY 2 Ministry of Transport, 8 th Regional Directorate of Highways, Elazig, TURKEY 3 Firat University, Architecture Faculty, Elazig, TURKEY ABSTRACT: Strength estimation of existing buildings is a key challenge for civil engineers who need to feed structural computations with material data. Specifically, it is critical to determine the in-place strength of concrete to establish the safety of the concrete structures. Both destructive and nondestructive test (NDT) methods are utilized to determine the concrete strength of existing buildings. Among these, NDT techniques are sensitive first to physical properties and provide only an indirect way towards material mechanical performances. NDTs are currently used in combination with the destructive test (core-drilled) or another nondestructive test, which provided more direct information. Rebound Hammer and Windsor Probe Tests are among the most widely used NDT methods regarding concrete strength assessment. In this paper, Rebound Hammer and Windsor Probe Test results were combined to determine the compressive strength of concrete using respond surface method. As a result, a combined NDT method was obtained to estimate the strength of concrete. Furthermore, this combination can be useful for urban regeneration studies, which include a lot of reinforced concrete buildings. 1 INTRODUCTION Since the late 1800s, structures such as houses, roads, and bridges, has been started to construct using the concrete. In the first years, the most important parameter for concrete was thought to be the amount of cement in the mixture. Later, when the effect of the water-to-cement ratio on the concrete strength was noticed, it was understood that the most important key issue for determining the concrete quality was the compressive strength (Abrams, 1927). The concrete compressive strength could be easily determined with the samples taken during the pouring concrete. However, as the time passed by, the requirements and productions have improved, and estimation of the in- place strength of the concrete has become an absolute necessity (Jones, 1949). The core drilling technique is the first method to obtain the in-place strength of the concrete. This technique is reliable and has high accuracy; however, it is not an efficient test method due to not practical and economical (Wagner et al., 1977). For this reason, especially after the 1940s, different tests and devices have been developed to determine the in-place strength of the concrete (Kolek, 1958). These tests, named nondestructive test (NDT), are often, even today, using by combined with core drilling method. These combined methods are more effective than each of NDT method since each NDT method alone cannot estimate the strength of the concrete with high accuracy sufficiently (Malhotra, 1984).