Testing the compressive strength of a concrete is a very important process of ascertaining the quality of concrete. There are different types and forms of concrete testing depending on the state of the concrete. Compression testing machine is the most important and popular machine to test concrete compressive strength. Some tests are done when the concrete is still wet (e.g. workability test), some are done when the concrete starts to harden (e.g. cube strength, setting time, etc.), and some are done when the concrete has been erected. Regardless of the time of the test, the major background and idea are to ensure that the type of concrete selected is able to perform optimally.
Notably, testing the compressive strength of concretes when it is still wet has no issue at all. In fact, testing hardened ones with test specimens such as cylinders and cubes is also not an issue. However, testing the compressive strength of a concrete when it is erected is a very huge and difficult task.
In this article, we will focus more on the Non-destructive concrete test with more emphasis on the Rebound Hammer Method.
Non-Destructive Concrete Test
Concrete structures should be tested often as they go toward solidification. This will ensure that the concrete structure has the desired compressibility, right design, and constructed properly. Non-destructive testing (NDT) is a very popular way of testing the compressive strength of a concrete. This is because it does not cause any damage to the material; just as the name implies. NDT can be used fir a number of purposes such as testing for the elasticity, strength, absorption, and hardness of the concrete material. It is also used to detect the presence of voids and cracks. NDT is variously used by engineers for testing both the new and old constructions.
Rebound Hammer Test
The Rebound Hammer Test, also known as Schmidt Hammer Test, is the commonest and most-used NDT for hardened concretes. In fact, it is one of the easiest methods. The major aim of this method is to compare the surface hardness of the concrete surface with the compressive strength of the surface. Whenever the Schmidt hammer is pressed against the surface of the concrete, the scale attached to the hammer will record its rebound mass. This value recorded b the scale is then used to estimate the compressive strength of the concrete by means of a correlation graph which is usually provided by the producer of the hammer.
Be that as it may, it is important to bear in mind that the outcome of this test using the correlation graph is only for checking purpose. Aside from the compressive strength estimation, the Rebound Hammer Test is also used to check the uniformity of the concrete surface. The data gotten from the Schmidt hammer can be used to determine if there is a uniform amount of compaction or otherwise at that corresponding area of the concrete. Generally, a lower rebound number will be observed on areas that are not well compacted when compared to well-compacted areas.
Nevertheless, there are few factors that can affect the value given by the Schmidt Hammer. The first factor is the surface condition of the test area. On the basis of the standard requirements, the concrete surface must be clean and smooth. Secondly, the angle of the hammer when pressing on the is to be tested can also affect the value. Other factors that affect the rebound value include coarse aggregate near the surface, voids near the concrete surface, and so on. Hence, in order to get a fair estimate, it is a usual practice to take an average of 10-12 values from different areas of the concrete surface.
