20-09-2012, 11:00 AM
DEVELOPING CORRELATIONS BETWEEN NDT METHODS AND ACTUAL CONCRETE STRENGTH
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ABSTRACT
The Non Destructive Testing (NDT) of concrete has a great technical and useful importance. These techniques have been grown during recent years especially in the case of construction quality assessment. The main advantage of non-destructive testing method is to avoid the concrete damage or the performance of building structural components. Additionally, their usage is simple and quick. Test results are available on the site and the possibility of concrete testing in structures is demanding in which the cores cannot be drilled and the use of less expensive equipments.
The Schmidt rebound hammer (SRH) and the ultrasonic pulse velocity (UPV) tests, are useful non-destructive tests, which are so familiar recently and they are useful when a correlation can be developed between hammer/ultrasonic pulse velocity readings and the
strength of the same concrete. This non-destructive measurement method has proved to be of real importance in all constructions serving the purpose of testing and as an effective tool for inspection of concrete quality in concrete structures.
INTRODUCTION
Concrete is the most commonly used construction material in structures. Determination of compressive strength has become the most important concern of researchers since its usage and usually regarded as the main criteria to judge the quality of concrete. Various destructive and non-destructive test (NDT) methods have been developed for determining the compressive strength.
The aim of these tests is to control concrete production and determine under service loads deteriorations in buildings on time. Nevertheless, the destructive methods are expensive and time consuming. In addition, cube and cylinder concrete specimens prepared in laboratory are not represented in situ concrete. Furthermore, getting core specimens from structural element reduces the load carrying capacity of construction elements.The NDT is a direct method to find in situ compressive strength of concrete.
Rebound Hammer (Schmidt Hammer)
The Schmidt Rebound Hammer (SRH), known as the Rebound or Impact Hammer test is considered as a non-destructive method widely used for assessing rock quality materials considering surface rebound hardness that is related to the compressive strength. This test is fast, cheap and an important guide test for rock material description. The methodology of the SRH test is expected to ensure the trustworthy data achievement and on site or the laboratory analysis (Amasaki, 1991).
Principle:
The method is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. When the plunger of rebound hammer is pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
SRH includes a spring loaded piston with steel mass (Figure 3) as explained In British Code (BS1881 part 202, 1986). The SRH as a hardness test works in a way that the rebound of an elastic material is related to its surface hardness against the hitting material. Based on the standard, the energy attracted by the concrete is according to its strength rebound of an elastic material. The kinetic energy equals to the energy released by the key spring of the piston in the straight impact direction which it is released onto the hammer) even if this test involves impact problems and the related stress-wave propagation.
Concrete surface should be carefully selected and prepared to be used by polishing so that the test surface is then ground smooth. A fixed power then applies by pushing the hammer against the surface. The slope angle of the hammer affects the result. After the impaction, the rebound readings should be recorded. There is not any distinctive relationship between hardness and strength of concrete but experimental data relationships can be obtained from a given concrete. A common normalization procedure which could be used for any type of Schmidt hammer with the same nominal design fired in any direction.
Path Length:
The path length (the distance between two transducers) should be long enough not to be significantly influenced by the heterogeneous nature of the concrete. It is recommended that the minimum path length should be 100mm for concrete with 20mm or less nominal maximum size of aggregate and 150mm for concrete with 20mm and 40mm nominal maximum size of aggregate. The pulse velocity is not generally influenced by changes in path length, although the electronic timing apparatus may indicate a tendency for slight reduction in velocity with increased path length. This is because the higher frequency components of the pulse are attenuated more than the lower frequency components and the shapes of the onset of the pulses becomes more rounded with increased distance travelled. This apparent reduction in velocity is usually small and well within the tolerance of time measurement accuracy.
Effect of Reinforcing Bars:
The pulse velocity in reinforced concrete in vicinity of rebars is usually higher than in plain concrete of the same composition because the pulse velocity in steel is almost twice to that in plain concrete. The apparent increase depends upon the proximity of measurement to rebars, their numbers, diameter and their orientation. Whenever possible, measurement should be made in such a way that steel does not lie in or closed to the direct path between the transducers. If the same is not possible, necessary corrections needs to be applied. The correction factors for this purpose are enumerated in different codes.
Shape and Size of Specimen:
The velocity of pulses of vibrations is independent of the size and shape of specimen, unless its least lateral dimension is less than a certain minimum value. Below this value, the pulse velocity may be reduced appreciably. The extent of this reduction depends mainly on the ratio of the wavelength of the pulse vibrations to the least lateral dimension of the
specimen but it is insignificant if the ratio is less than unity. Table given below shows the relationship between the pulse velocity in the concrete, the transducer frequency and the minimum permissible lateral dimension of the specimen.