Non - destructive testing of concrete and its method of testing .

Non - destructive testing of concrete :

      The test which are carried out for existing completed structure without destructing the member to investigate , crack ,depth , durability , strength etc. are called as non -destructive testing .

Advantages of non destructive testing :-

1. Strength of existing structure can be found out .
2. No wastage of concrete .
3. NDT is quicker method of inspection of fault .
4. The test component is not destructed , hence the same is used for several tests.

Limitations :-

1. Result depend on moisture condition.
2. Skilled expertise are required .
3. Process should be strandised .
4. In some cases , instrument are too costly.

Importance of testing :-

1. Non - destructive test helps in testing of quality of concrete of completed structure I.e finished work .
2. With this type of testing it is possible to investigate the crack depth , durability , strength without destructing existing structure .
3. Since this test are non destructive no wastage of concrete takes place .
4. This are simple in use and with this test strength of existing old concrete structure can be detected other than this the moisture content thickness and cement mirror content of concrete can also be found out with this test .
5. Non destructive testing of concrete is relatively recent in origin .Mainly , these tests are used for determine strength of structural member of already existing building or other construction .
6. The test are simple but the interpretation of result requires expertise and knowledge about the test .

 Types of non - destructive testing .

1) Rebound hammer test .

2) pulse velocity test .

Method of Non -destructive testing :-

1) penetration method
2) Rebound hammer test
3) pull out test method
4) Ultrasonic pulse velocity method .
5) Radioactive method .

Rebound hammer test :-

Principle of rebound hammer :-

   The rebound of rider weight from the plunger due to hammering action is measured . 

  This distance of rebound of rider weight is measured on a scale , which is proportional to the surface hardness of concrete .

Factors affecting rebound hammer test :-

1) Smoothness of surface 

2) Size shape rigrdin of the specimen .

3) Age of specimen 

4) Type of coarse aggregate 

5) Type of cement 

Determination of Compressive strength from the rebound index :

1) To determine compressive strength first       area should be selected properly .

2)  The plunger is held perpendicular to the concrete surface and the body pushed          towards the concrete

3) This movement extends the spring      holding the mass to the body . When the maximum extension of the spring is reached , the latch realease and the mass is pulled towards the surface by the spring .

4) During rebound the slide indicator travels with the hammer mass and stops at the maximum distance the mass reaches after rebounding .

5) The distance travelled by the mass , expressed as a percentage of the initial extension of the spring , is called rebound number .

6) A button on the side of the body is pushed to lock the plunger into the retracted position and the rebound hammer is read from a graduated scale fitted on the body.

7)  With the help of this curve , it is possible to estimate and to find out the compressive strength of concrete with an accuracy of 10% .

8) The relation between flexural strength and rebound number is found to be similar to those obtained for compressive strength , except that the scatter of the result is greater .


2) Ultra - sonic pulse velocity method :- 

Principle : 
Working principle of pulse velocity method is the relation between time taken for the pulse of ultra -sonic sound to travel through concrete and coming back and the compressive strength of concrete.

There are three ways of measuring pulse velocity :-

1) Direct transmission
2) Indirect transmission
3) Surface transmission

Factors affecting the measurement of pulse velocity :-

1. Presence of steel in concrete 
2. Temperature of concrete 
3. Surface of concrete 
4. Presence of moisture in the concrete 
5. Material and mix proportion of concrete 
6. Age of concrete.

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