NDT AT A GLANCE

It is important to verify whether concrete structures have achieved their design objectives after the concrete has hardened. It is prefered that any test conducted on hardened concrete should result in minimal or no damage to the structure. It is important to examine hardened concrete structure for their suitability for its intended use. In such cases, Non-Destructive Test methods are used for testing of concrete structures. Non-Destructive Tests and techniquesare increasingly being adopted on concrete structures for the following purposes.

• To estimate the in-situ compressive strength of concrete,
• To identify/map reinforcement profile on slabs, beams, columns and othere structurak elements,
• To measureconcrete cover, re-bar diameter, spacing etc.
• To estimate the uniformity and homogeneity of concrete,
• To estimate the quality of concrete in relation to standard requirements,
• To identify areas of lower integrity in comparison to other parts,
• To detect presence of cracks, voids and other imperfections,
• To monitor changes in the structure of the concrete which may occur with time,
• To stablish the extent of corrosion on prestressed/reinforcement steel,
• To establish chloride, sulphate, alkali contents or degree of carbonation,
• To measure Elastic Modulus,
• To stablish the condition of grouting in prestressing cable ducts,

NDT Tests 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. 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.

Typical situations when Non-Destructive Testing of Concrete may be useful include;.

• 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, honeycombing 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
• 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
• Assessing the potential durability of the concrete
• Monitoring long term changes in concrete properties
• Providing information for any proposed change of use of a structure for insurance or for change of ownership.