Non-destructive testing is done on hardened concrete. In non-destructive testing methods, some properties of concrete are used to estimate strength, durability, elastic parameters, crack depth, micro-cracks and progressive deterioration of concrete.
Such properties of concrete are hardness, resistance to penetration of projectiles, rebound number, resonant frequency, ability to allow ultrasonic pulse velocity, ability to scatter and transmit X-rays and gamma rays, its response to nuclear activation and acoustic emission. Various non-destructive methods have been developed using one or more of the above properties. Some of the important methods in use are explained below.
8.13.1 Schmidt’s Rebound Hammer
This is a commonly adopted equipment for measuring surface hardness. It consists of a spring control hammer which slides on a plunger and is housed in a tube. Once the plunger is pressed against the surface of the concrete, the mass behind the spring rebounds. After impact, the spring control mass rebounds and takes the rider along the guide scale. Based on the position, the reading is taken. Considering the reading and the calibration, the actual strength can be determined.
8.13.2 Frequency Method
It is another important non-destructive method used to determine the compressive strength and other properties. The fundamental principle on which the method is based is the velocity of material through a material. A mathematical relation could be made between the resonant frequency of the material to the Modulus of Elasticity of the material. The property of homogeneous material can be made use of heterogeneous materials like concrete with judgement.
8.13.3 Nuclear Method
This is a new technique which is used to determine the moisture content and the cement content. This method employs the scattering of neutrons directed towards the concrete and the number of neutrons returned. With a standard relationship number of neutron and water content/cement content, the required water or cement content can be obtained.
8.13.4 Radioactive Method
Here X-rays and gamma rays are used. When X-rays and gamma rays are passed through concrete, the electromagnetic spectrum penetrates concrete but undergoes attenuation in the process. The degree of attenuation is a function of the kind of matter traversed, its thickness and the wavelength of the radiation. Further, the intensity of the incident gamma-rays and the emerging gamma-rays after passing through the specimens is measured. These two values are used to calculate the density of concrete.
The gamma-rays transmission method is particularly used to measure the thickness of concrete slabs of known density. This is achieved by passing gamma rays of known intensity to penetrate through the concrete. The thickness of the concrete is measured based on the intensity of gamma rays measured on the other end.
8.13.5 Pullout Test
Here a rod is embedded in concrete blocks. These are pulled out and the strength of the concrete is determined. The ideal way to use the Pullout test in the field is to incorporate assemblies for pull out in the structure itself. These could be pulled out and the strength determined.
8.13.6 Pulse Velocity Method
It consists of two parts, viz., the mechanical ionic pulse velocity method and the ultrasonic pulse velocity method.
The mechanical sonic pulse velocity method consists of measuring the time of travel of longitudinal or compressive waves generated by a single impact hammer blow or repeated blows. The ultrasonic pulse velocity method consists of measuring the time of travel of electronically generated mechanical pulses through the concrete. Of these two, the ultrasonic pulse velocity has gained popularity throughout the world.
The pull velocity methods have been used to evaluate the quality of concrete, concrete strength, durability, Modulus of Elasticity, detection of water, etc.
High pulse velocity readings in concrete are indicative of concrete of good quality. Table 8.15 gives the pulse velocity range of quality of concrete (Leslie and Chessman, 1949, reported by Shetty, 2006).
Table 8.15 Suggested pulse velocity for concrete
Pulse velocity techniques have been used successfully for the detection of cracks. This is possible only when the width of the crack is of considerable depth and of appreciable width. The basic principle in such a situation to detect the crack of the depth is that no signal will be received at the receiving transducers, the pulse will pass around the end of the crack and signal is received at the transducers. However, the pulse would have travelled a distance longer than the straight line path upon which pulse velocity computations are made. The difference in the velocity of pulse is used to estimate the path length and therefore the crack depth. Figure 8.9 illustrates the principle of crack detection.
Figure 8.9 Pulse velocity technique
Leave a Reply