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Non-destructive testing (NDT) is a wide group analysis techniques used to evaluate the properties of concrete without causing damage. So, instead of absolute values, an estimate of its strength, durability and elastic parameters are obtained. Though these test are easy to perform but their analysis requires special knowledge. These test are conducted to keep a quality control of construction. Monitoring of strength development in relation to formwork removal, curing, pre-stressing load application or similar purposes.
Non-Destructive test (NDT) can be performed on fresh (green) concrete as well as hardened concrete with equal ease and are describe as follows
Fresh Concrete
Maturity Test
Ultrasonic Test
Hardened Concrete
Concrete Core Test
Pullout Test
Penetration Test
Ultrasonic Test
Schmidt Test Hammer
Maturity Test
Radioactive Methods
The following describes some of the important and widely used methods in the constructions industry.
1. Maturity Test :
Concrete having equal maturity will have equal compressive strength, it is on this principle that the maturity test has been formulated. Maturity meter is used to determine the maturity of the concrete at its initial ages. The yielded results are highly reliable provided the initial temperature of the mixed concrete should be maintained between 15-26° C and during the curing period losses of water should be kept minimal.
2. Ultrasonic Pulse Velocity Test :
This test is based on the principle that velocity of sound in a solid material is a function of Sq. root of (E/p) where 'E' is modulus of elasticity and 'p' is density.
An ultrasonic pulse apparatus consists of a transmitter and a receiver which are held against two face of concrete.
The apparatus generate pulse of ultrasonic frequency which are transmitted through concrete by the transmitter. On the other face, the receiver receives the pulse and the apparatus record them.
The velocity of the pulse is found which is correlated to the strength of the concrete.
Higher the velocity of pulse, greater the strength of concrete.
At present the ultrasonic pulse velocity method is the only one of dynamic test type that shows potential concrete strength in-situ.
Table: Quality of Concrete and Pulse Velocity
General Condition | Pulse Velocity km/sec |
Excellent concrete | 4.5 |
Good concrete | 3.5-4.5 |
Medium concrete | 3-3.5 |
Doubtful concrete | Below 3 |
Some major applications:
Estimation of strength of concrete.
Establishing Homogeneity of concrete
Determination of Dynamic Modulus of Elasticity
3. Rebound Hammer Test (Schmidt Hammer Test):
It is done to find out the compressive strength of concrete by using rebound hammer.
The principle of the test is that rebound of an elastic mass depends on the hardness of the surface against which it strikes.
When the plunger of the rebound hammer is pressured against the top surface of the concrete, the spring controlled mass rebounds and the extent of the rebound is dependent upon the surface hardness of the concrete which is being tested.
The surface hardness and therefore the rebound are taken to be related to the compressive strength of the concrete.
The rebound value is read from 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.
The procedure:
Apply light pressure on the plunger: it will release it from the locked position and allow it to extend to the ready position for the test.
Press the plunger against the surface of the concrete keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts.
The spring controlled mass when rebound, it takes with it a rider which slides along a graduated scale. It can be held in position on the scale by depressing the locking button.
The test provides useful information for surface layer up to 30 mm depth and is suitable for concrete having strength of 20-60 MPa.
The concrete surface must be smooth and loose material should be ground off.
Some other commonly employed NDT methods:
Surface hardness tests
Dynamics or Vibration tests
Radioactive and Nuclear methods
Magnetic and Electrical methods
Conclusion
The remaining methods of Non-Destructive Testing are to be discussed further in the coming blogs and here we discussed the most widely used in the present construction industry.
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