Evaluation of compressive strength of low and normal strength concretes using non-destructive testing methods

Most of the buildings in Turkey consist of reinforced concrete structures. These structures are at risk of encountering many disasters, especially earthquakes. Urban transformation activities have accelerated in our country to make our structures more secure against possible disasters. The safety of reinforced concrete structures against natural disasters varies depending on many parameters. These can be listed as material and workmanship quality, design of reinforced concrete elements and construction techniques. One of the most important parameters is the quality of concrete. The most reliable way to measure the quality of concrete is to obtain compressive strength by applying uniaxial compression test to cores taken from the structure in laboratory environment. This method requires a lot of cost and time for developing countries such as Turkey. Much cheaper and faster non-destructive testing methods can be used instead of this method. But the reliability obtained from these experiments is a matter of debate. There are many correlation curves developed for the ultrasonic pulse velocity method and surface hardness methods, which are the most common ones of these experiments. TS EN 13791 is a standard that assists in the creation of correlation curves for these experiments. However, the standard has lower and upper limits for these curves. Studies show that there are existing structures in Turkey that has 4 MPa strength of concrete. Therefore, it was observed that the correlation curves provided by the standard were insufficient due to the lower limitations in the determination of the strength of structures by non-destructive test methods. In this study, experiments were carried out to determine the strengths of low and normal strength concrete using non-destructive test methods and correlation curves over a wide range of strength are obtained by statistical analysis of the test results. Main purpose of study is contribute to the determination of new basic curves that will allow the basic curves which are given in the standard but valid only to normal concrete strength classes to be modified to include low-strength concrete classes frequently encountered in our country. xx As a consequence of examination of previous studies on non-destructive testing methods, it is estimated that the maximum aggregate size of the concrete may effect ultrasonic pulse velocity test results. Hence it has been decided to produce concrete with the different maximum aggregate sizes. In this study, 39 different productions were carried out to obtain a total of 21 different concrete mixes in 7 different strength classes with 3 different maximum aggregate diameters. For each maximum aggregate diameter, it is aimed to obtain concretes with characteristic strengths of 4, 8, 12, 16, 20, 30 and 35 MPa. Firstly, a set of test specimens produced from blends designed in accordance with TS 802 standard in a limited strength range. Then by using the srength values obtained from that specimens, blends for the targeted strengths were designed and produced by the method of Generalised Abraham law. Throughout the production, aggregate obtained from the same quarry, same brand cement and plasticizer were used. In each sample set, 6 pieces of 15 cm cube and 4 pieces of 10 x 20 cm cylinder samples were produced. Slump and unit weight values of fresh concrete were measured. The samples kept in the curing pool for 28 days were stored in laboratory conditions until the hardened concrete experiments afterwards. As the hardened concrete tests, first ultrasonic pulse velocity test performed on the cube samples. Before this experiment, the samples were allowed to dry in the laboratory environment, test was applied after the samples became air dried. Then, schmidt hammer and silver schmidt hammer tests were conducted. In both experiments, 12 rebound number readings were taken from the two opposite surfaces of the cube samples. Finally, cube samples were broken in the compression machine to obtain compression strength. Also, elasticity modulus values were calculated by calculating the slope of the stress-strain curve of cylindrical samples. In the last stage, valid correlation curves were determined over a wide range of strength by statistical analysis of the strength values obtained from the compression tests and the results obtained from the non-destructive test methods. It is considered that the proposed curves will contribute to determination of the basic curves which will allow the use of non destructive test methods in accordance with TS EN 13791 standard in order to determine the in-situ strength of low strength concrete which is frequently encountered in our country.