NDT- Non Destructive Testing

 

The testing of hardened concrete plays an important role in controlling and confirming the quality of cement concrete used at site has developed the required strength. The quality of the product was checked and evaluated by NDT methods. Most material in building, bridges, dams, tunnels, etc., are made of concrete. This construction requires concrete of high quality in terms of strength and durability. NDT has the ability to determine the strength and durability of critical construction without damaging them and the test can be carried on site.

  To monitor the service behaviour of concrete structure over a long period, it was imperative that tests be non-destructive. This approach, though new for the testing of concrete, had long been used in the testing of metals; therefore, a vast amount of knowledge and experience accumulated by the metals industry was readily available for application to concrete technology. Of course the composite nature of concrete did necessitate some modifications and changes in the techniques and equipment being used in the testing of metals; nevertheless, it was a great help in getting started.

There are several NDT methods applicable to concrete structures. The importance of checking certain properties according to the type of structure. The NDT methods applicable for concrete inspection include ultrasonic, rebound hammer and cover meter tests. It is clear that ultrasonic method has a superior capability in the sense that it is capable of providing more information on concrete parameters as compared with other methods.

The main advantage of non destructive method is that the strength and durability and other factors such as corrosion of bars, number of bars, bar spacing, quality of concrete, etc. can be easily determined, without damaging the concrete structure. All these factors are determined with less time and less cost by this method. In other words, we can get complete information of the old and the newly constructed concrete structure.

 

NEED FOR TESTING

 

The need for testing may arise from a variety of causes, which include;

1 Proposed change of usage or extension of a structure.

2 Acceptability of a structure for purchase or insurance.

3 Assessment of structural integrity or safety following material deterioration or structural damage such as caused by fire, blast, fatigue or overload.

4 Serviceability or adequacy of members known or suspected to contain material that does not meet specifications, or with design faults.

5 Assessment of cause and extent of deterioration as a preliminary to the design of repair or remedial schemes.

6 Monitoring of strength development in relation to formwork stripping, curing, prestressing or load application.

7 Monitoring long-term changes in materials properties and structural performance.

 

METHODOLOGY

IS:13311 (part I) specifies Non-destructive testing method using Ultrasonic pulse velocity and part II specifies Rebound Hammer method. But the test selection procedure will be based on a combination of factors such as non-destructiveness, cost, speed and reliability, and may conveniently follow a procedure such as that shown in Table 1.

Table 1: Test selection procedure

INSPECTION SCHEDULE	METHODS
First survey	· History of structure · Visual inspection
Second survey	· Ultrasonic testing · Rebound hammer · Cover test · Arrangement of bars · Other NDT (if necessary)
Third survey	· Core test · Vibration test · Displacement test