1. GENERAL KNOWLEDGE
1.4. Comparison of NDT methods
The summary of NDT methods applicable for concrete is given in Table 1.2.
TABLE 1.2. COMPARISON BETWEEN NDT METHODS OF TESTING CONCRETE
Method Acoustic emission Concrete resistance meter Principle When a material is loaded part of it may
be loaded beyond its elastic limit. Kinetic energy is released. This is known as acoustic emission They are inaudible but can be detected by sensors attached to the surface of a test object.
The resistivity of concrete is related to its moisture content and to a lesser degree to chloride content. Resistivity measurement gives an indication of the rate of corrosion, which may occur if oxygen-moisture or chloride-oxygen-moisture is present at the reinforcement. Resistivity is measured by inserting electrodes into small holes on the surface and passing an alternating current through them. The difference in potential is then measured.
Main applications Continuous monitoring of structure during service life to detect impending failure and monitor performance of structure during proof testing. This method has also been used in recent years to study the initiation and growth of cracks in concrete under stress.
It is used for measuring the ability of the concrete to conduct the corrosion current. The higher the resistance the slower the corrosion process can proceed. The device can also be used to measure moisture contents and to map moisture migration patterns.
User expertise Extensive knowledge is required to plan the test and to interpret results.
Low operative expertise is sufficient
Advantages It monitors response of existing structure to applied load. It is capable of detecting onset of failure and locating source of possible failure. Since acoustical signals come from defects throughout the
structures a few transducers are enough to detect and locate defects over large areas.
The equipment is inexpensive, simple to operate and many measurements can be rapidly made. It is very useful when used in conjunction with other methods of testing, e.g. half-cell potential.
Limitations The equipment costs are high. The method requires means of loading the structure and complex electronic equipment. As the method is not yet fully developed it is still regarded as a laboratory method at present.
It is not reliable at high moisture contents. It needs calibration and precise results are not usually obtained. The electrodes require good contact and nearby bars can affect readings.
Text cont. on page 32
Table 1.2. (cont.)
Method Covermeter Electrical half-cell potential measurements Principle The basic principle is that the presence of
steel affects magnetic field. An
electromagnetic search probe is swept over the surface of the concrete under test. The presence of reinforcement within the range of the instrument is shown by movement of the indicator needle. When the probe is moved until the deflection of the needle is at a maximum, the bar in question is then parallel to the alignment of the probe and directly beneath it. The needle indicates the cover on the appropriate scale for the diameter of the reinforcing bar.
Essentially, electrical potential of steel reinforcement is measured relative to a reference electrode (half-cell). This enables potential contour maps to be plotted. The electrode potential of steel in concrete indicates the probability of corrosion.
Main applications It is used for determining the presence, location and depth of rebars in concrete and masonry components. Advanced versions of covermeter can also indicate bar diameter when cover is known. It is moderately easy to operate. However, some training or experience is required to interpret the results.
The half-cell provides a relatively quick method of assessing
reinforcement corrosion over a wide area without the need for wholesale removal of the concrete cover.
Quantitative measurements are made so that a structure can be monitored over a period of time and any deterioration can be noted.
User expertise It is portable and rugged equipment and gives reliable results if the concrete is lightly reinforced.
Strict adherence to standard procedures is very important to obtain meaningful results. Some experience is required. The user must be able to recognize problems in interpreting results.
Advantages The presence of closely spaced reinforcing bar, laps, transverse steel, metal tie, wires or aggregates with magnetic properties can give misleading results. The meter has several scales for different bar sizes, therefore the bar diameter must be known if a true indication of cover is to be obtained.
It is portable equipment. Field measurements can be readily made and results can be plotted in the form of equipotential contour diagram, which can indicate likely areas of corrosion. It appears to give reliable information.
Limitations The maximum range of the instrument for practical purposes is about 100 mm. It does not give indication of the quality of
concrete cover or the degree of protection afforded to the reinforcement.
The main limitation is that it does not provide information on rate of corrosion. It also requires access to reinforcing bars to make electrical contact.
Table 1.2 (cont.)
Method Fibre scope (endoscope) Gamma radiography Principle It consists of a bundle of flexible optical
fibres with lens and illuminating systems.
It is inserted into pre-drilled boreholes of an element under investigation to examine its condition.
A radioactive isotope directs a beam at a member and an X ray
photographic plate is held against the back face. Gamma radiation attenuates when passing through a building component. The density and thickness of the materials of the building component will determine the degree of attenuation.
Photographic film records are usually made, which could be analyzed.
Main applications It can be used to check condition of materials in cavities, concealed piping, electrical wiring in cavity walls, honeycombing in reinforced masonry construction or detect voids along grouted stressed tendons.
This technique is quite established for examination of steel members. It can be used for locating internal cracks, voids and variations in density of materials, grouting of post-tensioned construction as well as locating the position and
condition of reinforcing steel in concrete.
User expertise Interpretative visual skill is required.
Hence experience and training are essential for correct results.
It must be operated by trained and licensed personnel.
Advantages It affords direct visual inspection of otherwise unaccessible parts of an element.
It can be used for field
measurements, simple to operate, relatively inexpensive compared to X ray radiography and is applicable to a variety of materials.
Limitations It is semi destructive in that probe holes usually must be drilled and must connect to a cavity.
In many cases in engineering structures, the method is unusable because it is difficult to place the photographic films in a suitable position. There are also the problems of health and safety both for the operatives and those in the vicinity as it requires long radiation exposure time.
Common site-radiography source such as Ir-192 can only be used for penetration depths of 200 mm in concrete. Areas must be isolated from public.
Table 1.2 (cont.)
Method Ground penetrating radar Neutron moisture gauge Principle Radio frequency waves (0.5 to 2GHz)
from radar transmitter are directed into the material. The waves propagate through the material until a boundary of different electrical characteristic is encountered.
Then part of the incident energy is reflected and the remainder travels across the boundary at a new velocity. The reflected (echo) wave is picked up by a receiver. The transducer is drawn over a surface and forms a continuous profile of the material condition below. The equipment consists of a radar console, a graphic scanning recorder and a combined transmitting and receiving transducer.
It works on the principle that hydrogen retards the energy of neutrons. Water in the concrete contains hydrogen molecules, which scatter the neutrons. The backscatter is measured. Hence the greater the back scatter the more moisture there is in the concrete. Instruments are generally calibrated based on semi-infinite volume and uniform moisture content.
Main applications It is capable of detecting a number of parameters in reinforced concrete structures:
the location of reinforcement
the depth of cover
the location of voids
the location of cracks
in situ density
moisture content variations.
It can be used to measure moisture content of concrete, soil and bituminous materials and to map moisture migration patterns in masonry walls. Their application to concrete testing is very recent and still in the exploratory stage.
User expertise User must have good knowledge of wave propagation behaviour in materials in order to meaningfully collect and interpret results. Training and experience are required.
It must be operated by trained and licensed personnel.
Advantages It can be used to survey large areas rapidly for locating reinforcement, voids and cracks.
The instrument is portable and moisture measurements can be made rapidly.
Limitations Results must be correlated to test results on samples obtained. Any features screened by steel reinforcement will not be recorded. With increasing depth, low level signals from small targets are harder to detect due to signal attenuation. It is expensive to use and uneconomical for surveying small areas.
A minimum thickness of surface layer is required for backscatter to be measured. It measures only the moisture content of surface layer (50 mm). It emits radiation. Results are inaccurate because hydrogen atoms of building materials are measured in addition to those of water. Its use in concrete is limited and requires calibration in order to calculate density or moisture content.
Table 1.2 (cont.)
Method Pullout devices (a semi-destructive test) Rebound hammer Principle The test involves drilling a hole in which
a standard threaded or wedge anchor is placed. This is then pulled until the concrete raptures. With the help of calibration charts the maximum force gives an indication of the strength of concrete. Pullout devices can be inserted during casting of concrete.
It consists essentially of a metal plunger, one end of which is held against the concrete surface while the free end is struck by a spring-loaded mass which rebounds to a point on a graduated scale. The point is indicated by an index rider.
The amount of rebound increases with increase in concrete strength for a particular concrete mix.
Main applications It provides an estimation of the compressive and tensile strengths of hardened concrete.
It measures the surface hardness of concrete and provides an estimation of surface compressive strength, uniformity and quality of concrete.
User expertise User expertise is low and can be used in the field.
User expertise is low and can be readily operated by field personnel.
Advantages In-place strength of concrete can be measured quickly and appears to give good prediction of concrete strength.
It gives accurate assessment of the strength of the surface layer of material. The entire structure can be tested in its 'as-built' condition.
Limitations Pullout devices must be preplanned and inserted during the construction stage, or inserted in hole drilled in hardened concrete. A cone of concrete may be pulled out, necessitating minor repairs. It can only test a limited depth of material.
As it is a surface method, and in
reinforced concrete could only be used to assess the concrete cover quality.
It can be very costly and time consuming as instrumentation is required to measure response. It requires careful planning and can damage structure. The member must be isolated from the rest of the structure prior to the test.
Table 1.2 (cont.)
Method Penetration probe (Windsor probe) Ultrasonic pulse velocity Principle Basically, the test consists of firing a
standard probe into the concrete with a standard cartridge. The extent of the penetration is measured and is related to the concrete strength The strength results are based on predetermined correlation between the type of aggregates used in the concrete and the penetration depth.
Voltage pulses are generated and transformed into wave bursts of mechanical energy by the
transmitting transducer (which must be coupled to the specimen surface through a suitable medium). A receiving transducer is coupled to the specimen at a known distance to measure the interval between the transmission and reception of a pulse. There are three practical arrangements for measuring pulse velocity, namely direct, diagonal and surface techniques. The direct approach provides the greatest sensitivity and is therefore superior to the other arrangements.
Main applications It can be used for estimating compressive strength, uniformity and quality of
concrete. It can also be used for estimating the same properties of mortars.
Determination of the variability and quality of concrete by measuring pulse velocity. Using transmission method, the extent of such defects such as voids, honeycombing, cracks and segregation may be determined. This technique is also useful when examining fire damaged concrete.
User expertise Expertise required is low. Execution is fast and can be operated by field personnel.
Low level is required to make measurements. However, expertise is needed to interpret the results.
Advantages The equipment is easy to use and does not require surface preparation prior to testing. It is good for determining in situ quality of concrete. The results are not subject to surface conditions, moisture content or ambient temperature.
Excellent for determining the quality and uniformity of concrete.
It can rapidly survey large areas and thick members. Path lengths of 10m to 15m can be inspected with suitable equipment.
Limitations It requires minimum edge distance and member thickness. It slightly damages small area. Calibration by manufacturers does not give precise prediction of strength for concrete older than 5 years and where surface is affected by
carbonation or cracking. Calibration based on cover is necessary for improved evaluation.
Proper surface preparation is required. The work is very time consuming as it takes only point measurements. Skill is required in the analysis of results as moisture variations and presence of metal reinforcement can affect results.
The interpretation of ultrasonic test results based on published graphs and tables can be misleading. It is therefore necessary that correlation with the concrete be inspected is carried out. It works on single homogenous materials.
Table 1.2 (cont.)
Method Ultrasonic pulse echo Thermography
Principle Pulsed compressional waves are induced in materials and those reflected back are detected by a hand-held accelerometer.
This is connected to a signal processor that integrates the signal and displays it on an oscilloscope.
An infrared scanning camera is used to detect variations in infrared radiation output of a surface.
Thermal gradients arise because of difference in surface temperature between sound and unsound concrete. Hence delaminations in concrete surfaces can be detected.
The temperature gradients are displayed on a TV screen in the form of colour thermal contours.
Main applications This method has been recently developed for quality control and integrity pile testing. It can detect the type and location of defects or inconsistency of the pile.
It can be used for detecting
delamination, heat loss and moisture movement through concrete
elements especially for flat surfaces.
User expertise High level of expertise is required to interpret results.
User expertise is not high but interpretation of results requires understanding of thermal behaviour and patterns.
Advantages It is portable, simple and cheap to use.
Photographic records can be made.
Internal discontinuities can be located and their sizes estimated.
It is portable and permanent records can be made. Testing can be done without direct access to surface and large areas can be rapidly inspected using infrared cameras.
Limitations It cannot determine net cross section of piles or its bearing capacity. Interpretation of results can be difficult and calibrating standards are required. It also requires smooth surfaces for the probe.
It is an expensive technique.
Reference standards are needed and a heat source to produce thermal gradient in the test specimen may also be required. It is very sensitive to thermal interference from other heat sources. Moisture on the surfaces can also mask temperature differences.