This document discusses non-destructive testing (NDT) of concrete. It begins by explaining that NDT assesses the strength and durability of concrete structures without destroying samples. Next, it lists several important reasons for performing NDT, such as quality control, determining crack depth and reinforcement location. Finally, it briefly introduces the Schmidt rebound hammer test, which works by measuring the rebound of an elastic mass to determine the surface hardness of concrete as a proxy for strength.

1. NON-DESTRUCTIVE TESTING OF CONCRETE (NDT ON CONCRETE)
Non destructive test is a method of testing existing concrete structures to assess the strength and
durability of concrete structure. In the non destructive method of testing, without loading the
specimen to failure (i.e. without destructing the concrete) we can measure strength of concrete.
Now days this method has become a part of quality control process. This method of testing also
helps us to investigate crack depth, micro cracks and deterioration of concrete.
Non destructive testing of concrete is a very simple method of testing but it requires skilled and
experienced persons having some special knowledge to interpret and analyze test results.

2. IMPORTANCE AND NEED OF NON-DESTRUCTIVE TESTING
 It is often necessary to test concrete structures after the concrete has hardened to
determine whether the structure is suitable for its designed use.
 The range of properties that can be assessed using non-destructive tests is quite large
and includes such fundamental parameters as density, elastic modulus and strength as
well as surface hardness and surface absorption, and reinforcement location, size and
distance from the surface.
 Non-destructive testing can be applied to both old and new structures.
Typical situations where non-destructive testing may be useful are, as follows:

3.  Quality control of pre-cast units or construction in situ
 Removing uncertainties about the acceptability of the material supplied owing to apparent non-compliance with
specification
 Confirming or negating doubt concerning the workmanship involved in batching, mixing, placing, compacting
or curing of concrete
 Monitoring of strength development in relation to formwork removal, cessation of curing, prestressing, load
application or similar purpose
 Location and determination of the extent of cracks, voids and similar defects within a concrete structure
 Determining the concrete uniformity, possibly preliminary to core cutting, load testing or other more expensive
or disruptive tests

4.  Determining the position, quantity or condition of reinforcement
 Increasing the confidence level of a smaller number of destructive tests
 Determining the extent of concrete variability in order to help in the selection of sample
locations representative of the quality to be assessed
 Confirming or locating suspected deterioration of concrete resulting from such factors as
overloading, fatigue, external or internal chemical attack or change, fire, explosion,
environmental effects

5. SCHMIDT REBOUND HAMMER TEST
FUNDAMENTAL PRINCIPLE
The Schmidt rebound hammer is principally a surface hardness tester. It works on the principle
that the rebound of an elastic mass depends on the hardness of the surface against which the mass
impinges. There is little apparent theoretical relationship between the strength of concrete and the
rebound number of the hammer. However, within limits, empirical correlations have been
established between strength properties and the rebound number.