Non Destructive Tests of concrete is done to determine strength of concrete structures after concrete is hard. 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.
Typical situations where non-destructive testing may be useful are, as follows:
• 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.
Methods for NDT of concrete structures
• Half-cell electrical potential method, used to detect the corrosion potential of reinforcing bars in concrete.
• Schmidt/rebound hammer test, used to evaluate the surface hardness of concrete.
• Carbonation depth measurement test, used to determine whether moisture has reached the depth of the reinforcing bars and hence corrosion may be occurring.
• Permeability test, used to measure the flow of water through the concrete.
• Penetration resistance or Windsor probe test, used to measure the surface hardness and hence the strength of the surface and near surface layers of the concrete.
• Covermeter testing, used to measure the distance of steel reinforcing bars beneath the surface of the concrete and also possibly to measure the diameter of the reinforcing bars.
• Radiographic testing, used to detect voids in the concrete and the position of stressing ducts.
• Ultrasonic pulse velocity testing, mainly used to measure the sound velocity of the concrete and hence the compressive strength of the concrete.
• Sonic methods using an instrumented hammer providing both sonic echo and
transmission methods.
•Tomographic modelling, which uses the data from ultrasonic transmission tests in two or more directions to detect voids in concrete.
• Impact echo testing, used to detect voids, delamination and other anomalies in concrete.
• Ground penetrating radar or impulse radar testing, used to detect the position of reinforcing bars or stressing ducts.
• Infrared thermography, used to detect voids, delamination and other anomalies in concrete and also detect water entry points in buildings.
