Abstract:
It is often necessary to test concrete structures after the concrete has hardened to determine whether the structure is suitable for its designed use. Ideally such testing should be done without damaging the concrete. There are different testing procedures used for concrete strength detection in structures. These testing procedures are categorized into destructive and nondestructive testing. For the nondestructive testing of concrete strength, the mostly used testing methods are the Rebound Hammer and Ultrasonic Pulse Velocity (UPV) method. However, various factors affect the results of the Rebound Hammer and UPV results. This research aimed to investigate the effects of concrete age, moisture and aggregate size on the Rebound Hammer and UPV results. In order to achieve this, experimental program was carried out in Engineering Workshop of the Federal University of Technology Akure (FUTA). Crushed granite of sizes 10 mm, 13 mm, 19 mm and 25 mm were used as the coarse aggregate in casting one hundred and two (102) standard 150x150x150 mm concrete cubes. Seventy-two (72) of these concrete cubes were tested with Rebound Hammer, Ultrasonic Pulse Velocity tester and Compression Machine at 7, 28, 35, 42, 49 and 56 day curing age. Fifteen (15) concrete cubes at 28-day curing age, out of the remaining thirty (30) and the other fifteen (15) at 56-day curing age were subjected to varied moisture content and then tested with the same instruments as stated above. The Rebound Hammer and UPV values increased as the concrete age and coarse aggregate size increased, but as for the moisture content, the UPV values increased as the moisture content in the concrete cubes increased while the Rebound Number (RN) decreased. Concrete cubes made of 10mm coarse aggregate gave the highest RN of 24 and also exhibited an increased UPV values ranging from 3.52 – 4.86 km/s. However, a range of 3.52 – 4.81km/s were gotten for the UPV when the concrete moisture content increased from 0% to 25% at 28 – day curing age, while the RN showed a decreased trend from 16 – 10. At 56-day curing age, the value of UPV results exhibited an increase in percentage ranging between 8.82 – 63.3 % while the RN increased from 20% - 31.3% compared to the 28-day concrete cubes. Moreover, this research provided the relationships between the destructive testing results and the RN with the UPV data of concrete for different concrete age, moisture and aggregate size. Concrete with higher moisture content gave higher UPV result; concrete made of smaller coarse aggregate size showed higher values for both RN and UPV. Furthermore, the RN and UPV data of the concrete increased as the concrete age increased.
