Non-destructive testing (NDT) are noninvasive techniques to determine the integrity of a material, component or structure without causing damage. NDT is a commonly-used tool in medicine, forensics, mechanical, electrical and Aerospace engineering. Because NDT does not permanently alter the article being inspected, it is a highly- valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
During the past few years, NDT works had gained attention in the civil engineering/construction industry (both old and new structures). For new structures, the principal applications are likely to be for quality control or the resolution of doubts about workmanship. The testing of existing structures usually relates to routine maintenance inspections and structural integrity assessment.
Various NDT Tests
The following methods have been used for the NDT of concrete:
Half-cell electrical potential method, used to detect the corrosion potential of reinforcing bars in concrete.
Linear Polarization Resistance, used to determine corrosion rate of steel rebars
Schmidt/rebound hammer test, used to evaluate the surface hardness of concrete and indirectly compressive strength.
Carbonation depth measurement test, used to determine whether moisture has reached the depth of the reinforcing bars and hence corrosion may be occurring.
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.
Ultrasonic pulse velocity testing, mainly used to measure the sound velocity of the concrete and hence the compressive strength of the concrete.
Tomographic modelling, which uses the data from ultrasonic transmission tests in two or more directions to detect voids in concrete.
Impulse Response or Impact echo testing, used to detect concrete mobility and eventually voids, delamination and other anomalies in concrete.
Ground penetrating radar or impulse radar testing, used to detect the concrete cover, number of steel rebars as well as 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.
Various Applications & Benefits
Typical situations where non-destructive testing may be useful are, as follows:
Mapping of reinforcement inside structures and determining the position, quantity and rebar spacing as well as concrete cover (used for development of As-Built Drawings)
Identifying and Mapping voids, honeycomb and cracks.
Verifying Crack Depth & Orientation of Crack
Detecting areas of delamination in concrete & masonry elements.
Evaluating the thickness of concrete members and detecting Foundation Depth & Location
Detecting Potential of Corrosion as well as corrosion rate
Assessing the potential durability of the concrete
Measuring of Concrete Resistivity
Locating conduits (metallic and non- metallic) as well as post-tension cables
Scanning prior to coring
Detecting degree of grouting in post-tensioning ducts.
Quality control of pre-cast units or construction in situ
Confirming or negating doubt concerning the workmanship involved in batching, mixing, placing, compacting or curing of concrete
Determining the concrete uniformity, possibly preliminary to core cutting, load testing or
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 (i.e. used to provide indication about concrete quality before proceeding with material sampling)