It is sometimes important to check and test whether the structure (mostly concrete) is suitable for its designed use and the ideal testing would be carried out without damaging the concrete.
The range of properties that can be assessed using non-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. It is also possible to check the quality of workmanship and structural integrity by the ability to detect voids, cracking, honeycombing and delamination. However, if a destructive test is necessary, a non-destructive testing can be used in this situation as a preliminary to subsequent tests and thus increasing the confidence level of a smaller number of destructive tests.
This report is intended to describe current non-destructive testing methods used on concrete structures including their advantages, errors and difficulties.
Basic methods for NDT of concrete structures
These are the most commonly used NDT methods for measuring and checking different concrete properties and defects.
Visual inspection: it is an essential precursor to any intended non-destructive test. It requires an experienced engineer in the field of concrete structures to be able to identify the possible caused of damage to the concrete element in question and as a result determine which of the wide range of NDT methods could be mostly useful to further investigate the problem.
Difficulties and errors lie within the nature of the problem and the frequency of occurrence. Some of the defects might have been recently studied and analysed and therefore it is highly likely that the expert inspecting the concrete cannot identify the problem and effectively evaluate it. For example, it is quite easy to identify honeycombing and efflorescence and assign the relevant NDT method to check the extent of the problem and the damage caused.
Half-cell electrical potential method: this method is used to measure the electrode potential of steel reinforcement i.e. to detect the corrosion potential of reinforcing bars in concrete.
The potential difference between the reinforcement and the half-cell is measured using a high impedance voltmeter. One major disadvantage is while the method can measure the potential value to estimate the likelihood of corrosion it cannot, however, indicate the rate at which corrosion occurs which would be highly useful in other applications.
Schmidt/rebound hammer test: this is a test used to evaluate the surface hardness of concrete by the rebound principle. A correlation has been established between the hammer rebound number (the rebound distance corresponding to the position of the rider on the scale) and the hardness or the strength of the concrete (or any surface in general).
Advantages include its convenience; it is a very convenient tool that weighs no more than 1.8 kg and it is suitable for use both in a laboratory and in the field. It provides a quick, inexpensive means of checking the uniformity of concrete.
The Schmidt hammer can encounter many difficulties represented in the properties and compounds of the concrete structures. In particular, the results of Schmidt rebound hammer are severely affected by smoothness of the surface, size, shape and rigidity of the specimen, its age, its moisture content, type of aggregate and cement, etc
For example, when a rough surface is tested; the plunger tip causes excessive crushing and a reduced rebound number is measured. In this case a more accurate result can be obtained by grinding a small rough surface area to uniform smoothness with a suitable stone.
It should be emphasised that the hammer must not be regarded as a substitute for the standard compression tests but as a method for determining the uniformity of concrete structures and comparing one against another.
Carbonation depth measurement test: it is used to determine whether moisture has reached the depth of the reinforcing bars and hence corrosion may be occurring. The carbonation effects are more sever in older concretes when the carbonated layer can be several millimetres thick. Its main advantage is that it could be used as an initial step prior to performing the half-cell method as it determines moisture effects on concrete and consequently the possibility of corrosion occurrence.
In order to obtain reliable results, measurements should be taken in more than one spot and in different locations to make sure that no uneven distribution of water is present within the structure and this is where the difficulties can be encountered as it may be hard to access some of surfaces of concrete and hence reducing the quality and accuracy of the measurements.
Cover meter testing: used to measure the location and depth of steel reinforcing bars beneath the surface of the concrete and also possibly to measure the diameter of the reinforcing bars. The basic principle is that the presence of steel affects magnetic field.
Advantages include that it is portable and rugged equipment and gives reliable results if the concrete is lightly reinforced. The meter also has several scales for different bar sizes which can be set independently when the presence of different sizes reinforcement bars can give misleading results; therefore the bar diameter must be known if a true indication of cover is to be obtained.
Its difficulties or disadvantages comprise its manufacture specifications of maximum range for practical purposes of about 100 mm. It does not give indication of the quality of concrete cover or the degree of protection afforded to the reinforcement.
Ultrasonic pulse velocity testing: mainly used to measure the sound velocity of the concrete and hence the compressive strength of the concrete.
Voltage pulses are generated and transformed into wave bursts of mechanical energy by the transmitting transducer (which must be greased and 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.
The pulse velocity method is a truly non-destructive method as the technique involves the use of sonic waves resulting in no damage to the concrete element being tested.
Factors affecting the pulse velocity include some affecting the concrete properties such as compound of concrete, cement type, water-to-cement ratio, admixtures, etc and others regardless of concrete properties such as acoustical contact, temperature of concrete, moisture conditions, path length, size and shape on concrete and very importantly the presence of reinforcing steel.
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.
One of the advantages of the pulse velocity test being advanced method is that low level of user expertise is required to make measurements. Expertise, however, is needed to interpret the results.
This method is also excellent in determining 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.
Limitation: due to the fact that many factors affect the relation between the strength parameters of concrete and its pulse velocity, the use of the latter alone to estimate the compressive strength of concrete is not recommended unless compared with other tests and previous correlating testing has been performed.
Other tests with similar purposes to some of the tests already explained above are briefly discussed below:
Penetration resistance or Windsor probe test: it is also used to measure the surface hardness and hence the strength of the surface and near surface layers of the concrete.
Basically, a probe is driven into a concrete surface and a measured average value of exposed probe length may then be used to estimate the compressive strength of concrete by means of appropriate correlation data.
Its function is similar to that of the Schmidt hammer, but because the probe can penetrate up to about 50 mm in concrete, the probe penetration results are more meaningful than the results of the rebound hammer which is a surface hardness test only and that is one major advantage it has over the hammer.
However, the test leaves a hole 8 mm in diameter for the depth of the probe and may cause minor cracking. Therefore, it is essential that the hole is backfilled after completing the test although this would not thoroughly better the situation.
Radiographic testing: used to detect voids in the concrete and the position of stressing ducts.
Principle: the intensity of a beam of X rays or gamma rays suffers a loss of intensity while passing through a material due to the rays absorption or scattering by the object being exposed. The amount of radiation lost depends on the quality of radiation and the density and the thickness traversed of the material. The emerged radiation is used to expose a radiation sensitive film so that different intensities of radiation are revealed as different densities on the film.
Tomographic modelling, Impact echo testing and Infrared thermography: these three methods are somehow similar in function in that both are used to detect voids in concrete. Tomographic modelling uses the data from ultrasonic transmission tests in two or more directions to detect voids in concrete. In addition to detecting voids, Impact echo testing van be used to detect delamination and other anomalies in concrete. Infrared thermography can be effectively used to detect water entry points in buildings.
Ground penetrating radar or impulse radar testing: it is used in a similar way to cover meter to detect the position of reinforcing bars as well as stressing ducts on a larger scale.
This section is allocated to the readings and measurements taken in the laboratory experiments of non-destructive testing along with further calculations and comments on the results obtained. It is also to highlight difficulties in non-destructive tests using these data.
The following table represents the times displayed on the pundit box in sec that the pulse takes to travel in the concrete beam for a fixed distance of 300-mm.
The Ultrasonic Pulse Velocity (UPV) could be related to the strength of concrete using the empirical relationship; Strength (N/mm2) = 20 UPV (km/s) - 30. Firstly, the UPV needs to be calculated before finding the corresponding strength.
The next table summarises the strength values of the concrete beam interpreted from the UPV test.
The velocity was obtained using the usual equation of distance travelled (300-mm) converted to km divided by the time taken also converted to seconds. For example, the velocity of travel between 0-mm and 300-mm in the upper edge is found as:
Velocity = (300 10-6) / (71 10-6) = 4.2 km/s.
As can be seen from table2, the strength varies and fluctuates around a value of 50 N/mm2 apart from a sudden drop to a value of 22.2 N/mm2. This high value of strength strongly suggests the presence of reinforcing steel bars in the top layer of the beam as the pulse velocity in steel is 1.5 to 2 times the pulse velocity in plain concrete as it is much denser than concrete. On the other hand, the velocity values for the lower edge show an approximate consistency for the values of velocity and suggest a plain concrete (no reinforcement steel) or even a different reinforcement arrangement. Overall, the results indication could be interpreted as the concrete beam to be singly reinforced i.e. steel reinforcement is provided for one edge of the cross section.
Moreover, the drop in velocity in the upper edge indicate either a discontinuity in the reinforcement mesh, a corrosion of the steel bars or even a significant crack taking place in that particular region.
As mentioned before, this method although having some advantages it is in practice not mainly used for strength calculations but in fact used to check defects. Strength has to be measured by doing the appropriate calibrations.
The instructions outlined were followed and the meter was used in different very adjacent locations by dragging it to measure the concrete depth of concrete to the reinforcement steel and determine the position of reinforcement bars, if any, within the concrete beam. Admittedly, it was extremely hard to deduce or even guess the arrangement of the steel mesh present within the concrete as the meter reading was changing continuously and suddenly when moving the meter. It was initially interpreted that the beam is singly reinforced with even spacing and no shear links from a range of readings of 8-mm to 12-mm.
However, subsequent readings showed that this cannot be possible as it showed that other reinforcement bars are still present in other spots. At this stage, Mrs Christine Jinks (the Experimental Officer) decided that enough time has been spent on this experiment and it is necessary that the group moves to the next one.
At the end of the experiment, she revealed the actual arrangement for the concrete beam and she agreed that a high experience was needed to find out the layout of the reinforcement in the beam. A quick sketch of half of the beam (because of symmetric arrangement) is attached at the end of this report.
A concrete beam (placed on the floor) was tested with a Schmidt hammer as well as some parts of the laboratory building and below is a summary of the hammer readings:
The strength in N/mm2 was calculated using the relation 1000 psi 7 N/mm2.
For the concrete beam, nearly 10 readings were taken and all were in the range shown in the table. They also showed a consistency in the varying strength from the concrete beam edge (weakest) to the middle of the beam (stronger).
It is worth mentioning that the readings obtained from the concrete beam were not exactly accurate because it was found out that the beam was not completely level and at this stage it cannot be know how much force required to rebound. Readings can only be approved when the beam is bedded.
In general, handbook of NDT of concrete emphasises that it is recommended that a reading should not be taken on the same spot twice and that was another operational mistake during performing the test as no group member was aware of it.
The difficulties of the UPV method were experienced, as can be noticed form the results obtained, on how the different factors of the concrete and the surroundings could affect the pulse velocity and sometimes give readings that cannot be easily interpreted.
Cover meter is the most experiment requiring high level of expertise as an experience with the different arrangements of the steel reinforcement is essential to have an initial picture of the layout of the steel mesh and if any other elements such as shear links and fibre reinforcement are present.
The Schmidt hammer is a reliable method of measuring strength but it should be integrated with other methods for measuring concrete compressive strength.
In conclusion, the non-destructive methods are relatively easy to use and most of the equipment is cheap and readily available but in order to obtain the most accurate possible results a high expertise is fundamentally needed to interpret results.
- Malhotra, V. M. (1991). Handbook on nondestructive testing of concrete. First Edition. CRC Press
- Jones, R. (1962). Non-destructive testing of concrete. First Edition. Cambridge University Press.
- Bungey, J. H. (1982). The testing of concrete in structures. First Edition. Surrey University Press.
- September (2002). Guidebook on non-destructive testing of concrete structures. International Atomic Energy Agency, Vienna.