A variety of non-invasive inspection techniques used to evaluate the properties of materials
Importance and Demand of Non-Destructive Structural 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, to those where the concrete surface is slightly damaged, to partially destructive tests. Partially destructive concrete tests, such as core tests and pull off tests, require the surface to be repaired after the test.
Properties Assessed By Concrete Testing
The range of properties that can be assessed using non-destructive tests and partially destructive tests is quite large and includes fundamental parameters:
- Elastic modulus and strength
- Surface hardness
- Surface absorption
- 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.
New and Existing Structure NDT
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. The cost of coring and testing of destructive testing (removing cores for compression testing)may only allow a relatively small number of tests to be carried out on a large structure that may be misleading. Non-destructive testing can be used in those situations as a preliminary to subsequent coring.
Typical situations where non-destructive testing may be useful:
- 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.
Basic methods for Non-Destructive Testing of Concrete Structures
The following methods, with some typical applications, have been used for the NDT of concrete:
- Visual inspection, which is an essential precursor to any intended non-destructive test.
- An experienced civil or structural engineer may be able to establish the possible cause(s) of damage to a concrete structure and hence identify which of the various NDT methods available could be most useful for any further investigation of the problem.
- 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 modeling, 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.
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