10-08-2012, 02:05 PM
Guidebook on non-destructive testing of concrete structures
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INTRODUCTION
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. Ideally such testing should be
done without damaging the concrete. The tests available for testing concrete range from the
completely non-destructive, where there is no damage to the concrete, through those where the
concrete surface is slightly damaged, to partially destructive tests, such as core tests and
pullout and pull off tests, where the surface has to be repaired after the test. The range of
properties that can be assessed using non-destructive tests and partially 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. In some cases it is also possible to check the quality of
workmanship and structural integrity by the ability to detect voids, cracking and delamination.
Non-destructive testing can be applied to both old and new structures. For new
structures, the principal applications are likely to be for quality control or the resolution of
doubts about the quality of materials or construction. The testing of existing structures is
usually related to an assessment of structural integrity or adequacy. In either case, if
destructive testing alone is used, for instance, by removing cores for compression testing, the
cost of coring and testing may only allow a relatively small number of tests to be carried out
on a large structure which may be misleading. Non-destructive testing can be used in those
situations as a preliminary to subsequent coring.
Qualification and certification
The qualification and certification of NDT personnel for the inspection of concrete is
not commonly covered by the qualification and certification schemes presently established in
most countries. Usually such schemes are based on the requirements of the International
Standards Organization (ISO) 9712 “The qualification and certification of NDT Personnel”
and cover the use of methods such as ultrasonics, radiography, eddy current testing and
surface methods tests to inspect essentially homogeneous materials such as metals. The
growing interest in the use of NDT for the inspection of concrete may result in a demand for
certification in the future.
BASIC MANUFACTURING PROCESSES AND DEFECTS OF CONCRETE
STRUCTURES
Types of concrete structures
Concrete is a mixture of stone and sand held together by a hardened paste of cement and
water. When the ingredients are thoroughly mixed they make a plastic mass which can be cast
or moulded into a predetermined size and shape. When the cement paste hardens the concrete
becomes very hard like a rock. It has great durability and has the ability to carry high loads
especially in compression. Since it is initially plastic it can be used in various types of
construction; however the forms used to produce the final shape can not be removed until the
concrete has developed enough strength by hardening. Where tensile stresses are imposed on
the concrete, it must be reinforced with steel.
Reinforced concrete
Reinforced concrete is a combination of concrete and steel. Alone concrete is very
strong in compression but very weak in tension. Since concrete bonds firmly to steel
reinforcement the combination acts as one material which offers high compressive strength,
high tensile strength and high shear strength. Reinforcement in concrete also helps to control
cracking such as shrinkage and surface cracking. There are two main types of reinforcement:
deformed bars (i.e. with grooves) and mesh sheets, such as rectangular mesh, square mesh and
trench mesh. The position of reinforcement is always shown on drawings. Steel reinforcement
must be securely fixed in the right position.