Non Destructive Evaluation
The first step towards performing a condition assessment is to get details of the structure with respect to its design, features, and past performance. An initial visual inspection of the structure can reveal useful information about areas that need a closer look. There are many causes for the deterioration of structures, so it is difficult to exactly pinpoint the type of damage that has led to the deterioration. However, all types of damage, whether they be load related, environment related, or earthquake related, lead to similar signs of deterioration, such as cracking, scaling, delamination, discoloration, etc.
Areas that show cracking, discoloration, rust stains, etc. should be investigated closely with the help of visual aids such as magnifying lens and telescopes. The visual inspection helps in planning a detailed strategy to investigate the structure further using more sophisticated techniques.
A number of investigative techniques are available to study the condition of the material in a structure. These include the evaluation of the material in a non-destructive manner, i.e. without causing any damage while testing, and semi-destructive tests such as removal of a piece of the material for evaluation, or even destructive tests where the material is tested to failure without damaging the overall structure.
The non-destructive techniques range in sophistication from simple ones where the quality of sound obtained by striking the surface of the material with a hammer indicates the quality of the material, to complicated techniques where the ultrasonic signals traveling through the material are analyzed mathematically.
Structural integrity can be achieved by providing:
Non-destructive evaluation (NDE) is also sometimes called ND Testing (NDT) or ND Inspection (NDI). Generally, the use of words like ‘flaw’ or ‘defect’ is avoided. Instead, terms such as ‘cracks’, ‘inhomogeneities’, ‘pits’, ‘inclusions’, ‘indications’, or ‘anomalies’ are used.
The failure rate of a material during its service life is not
constant. As shown in Figure 1, the failure rate is high in the initial
stages due to manufacturing defects. In the late stages of service, the
service-induced damage again causes an increase in the failure rate.
Figure 1. Failure rate of materials and components
Some common manufacturing defects in concrete include voids and inclusions (due to improper consolidation), poor surface finish or cracked surface (due to plastic shrinkage), damage and cracking from residual stresses (due to thermal effects), surface weakness, weak bonds between steel and concrete due to bleeding, cold joints etc.
Service-induced damage could be load related, such as fatigue,
impact, residual stresses due to overloading and creep, or environment-related,
such as corrosion, chemical attack, ASR, creep and shrinkage, carbonation,
freezing and thawing, salt scaling, etc. Improper maintenance or repair
could also be classified as a service-induced damage.
A good NDE method should be:
NDE methods operate at their limit for many problems. Thus, it is not possible to obtain 100% accuracy from these methods. Some factors that affect reliability of NDE techniques are listed below.
NDE of concrete
Typical defects in concrete are cracks, delaminations, voids, honeycombing, loss of subgrade support, inadequate member thickness, etc. Compared to metals and composites, NDE of concrete is complicated because of the following:
Failure of concrete is a complex phenomenon because more than one mechanism of damage occurs simultaneously, and it is difficult to diagnose which mechanism caused the initial damage. Hence, it is necessary to have an understanding of the basic underlying causes of damage in concrete and their manifestation. The dominant cause for failure of concrete is corrosion of the reinforcing steel. The other causes are less common, but still critical, agents of material failure. It is important to bear in mind that the failure of concrete structures can seldom be ascribed exclusively to the failure of a material component (cement, aggregate or reinforcement) or to failure of the system (structural or design failure). Table 1 presents the common distress mechanisms in concrete.
Table 1. Causes of distress and deterioration of concrete
Visual inspection is the most important non-destructive test. It forms the basis of all inspections. A detailed visual inspection makes it possible to narrow down the critical areas in a structure that need further investigation using sophisticated techniques. The trained eye of an inspector can often reveal information that is sometimes difficult to pick up using hi-tech instruments.
Use of visual aids
Visual inspections may be performed directly or indirectly (when photographs, radiographs or videos of the damaged areas are analyzed at a later stage). The direct inspection can be aided by a number of tools:
Borescopes are industrial telescopes that give access to closed areas. These can be of three types:
The following items of inspection apply to concrete construction
Other general defects that can be identified by visual observation are:
Limitations of visual observation
Organization of plan for detailed inspection
Visual inspection can reveal the areas in the building that require further investigation. For example, areas in concrete where rust stains are observed need to be checked for the extent of corrosion, in order to assess the residual strength. In steel structures also, the rate of corrosion can be checked using appropriate techniques.
The overall plan for detailed investigation may be drawn up based on the total area of the building, as well as on the extent of damage that the building has suffered. For example, a building with occasional complaints of water leakage would demand lesser priority compared to a building that has just been gutted by fire or damaged by an earthquake. The plan should first cover those areas that are structurally vulnerable and are liable to compromise the integrity of the building.
Before chalking out the plan, an inventory of the equipment available
for testing, whether intrusive or non-destructive, should be prepared.
The limitations and scope of these equipments must be well understood.
The type of test to be carried out and the extent of the investigation
(i.e. whether only specific areas are to be selected or the entire structure
needs investigation) should be properly detailed.
Rebound and penetration techniques
Both the techniques described above are surface techniques. Thus, they are able to assess only the surface condition of concrete. Hence, a quantitative estimation of the compressive strength of concrete may not be possible using these methods. However, a quick indication of damaged areas can be obtained. The application of these techniques also requires the preparation of the surface. These methods also do not give a good indication near edges and corners. Good calibration and training is necessary to produce reliable results from these methods.
Figure 2. Digital rebound hammer
Figure 3. Use of Schmidt rebound hammer to detect the quality of concrete in a slab in a fire damaged building
A qualitative evaluation of concrete can be easily obtained by just sounding it (i.e. tapping it) with a hammer. When the hammer is struck on good concrete, a ringing sound is created. However, on areas where delaminations or cracks occur, the striking of the hammer produces a drum-like sound. The limitation of this method is that it cannot detect defects that exist deep in the member. Also, defects lying under overlays are also difficult to find.
Chain drag is another way of finding out delaminated parts and voids. Compared to sounding with a hammer, chain drag can cover more area in a given time. In this method, the operator passes a heavy chain on the surface of the concrete. The quality of sound generated is picked up using microphones and characterized.
Figure 4 shows a schematic diagram of the use of sounding and
chain drag methods. The areas identified as ‘defective’ in
the sounding technique could be marked using paint for further investigation.
Chain drag is a more effective method as it covers a large area, and has
the potential to be mechanized. For example, to evaluate a concrete bridge
deck, a simple arrangement would be to attach chains at the back of a
vehicle, and keep appropriate sound sensors (microphones) to record the
sound as the vehicle makes a slow pass over the deck. With suitable datalogging,
this can result in complete evaluation of the bridge deck.
Figure 4. Use of sounding and chain drag techniques
The methods based on propagation of stress waves through concrete are most popular, since they are reliable, can give good quantitative data, and able to map and detect defects that lie deep in the concrete member. Ultrasonic methods can be classified into four types:
Properties of sound waves
Three main types of sound waves travel through materials (these are classified depending on the direction of motion of particles):
In a compressional wave, the particles of the material vibrate in a direction parallel to the wave propagation. In a shear wave, the particles vibrate perpendicular to the wave direction. In the case of surface waves, the particle vibration both parallel and perpendicular components.
Other types of waves also travel through materials. Lamb waves are planar waves traveling through thin sheets, while standing waves are created by the interference of two or more waves.
The wave velocities for the main waves are dependent on the stiffness and density of the material. The velocity of compressional waves, Vc α (E/ρ)0.5, while the velocity of the shear waves, Vs α (G/ρ)0.5, where E is Young’s modulus of elasticity, and G is shear modulus or modulus of rigidity. Vs is 0.5 – 0.6 times Vc, while the velocity of surface waves Vr is 90% of Vs.
When sound waves travel from one medium to another, a part of the energy gets reflected and the other part gets transmitted. The amount of reflection and transmittance depends on the acoustic impedance of the media.
Acoustic impedance, Z = ρV, where ρ is the density of the material, and V is the wave velocity. Z values for some typical materials are shown in Table 2. If the acoustic impedances of the media are known, then:
% transmitted energy Et = 4Z1Z2/(Z1+ Z2)2, and
% reflected energy Er = (Z1-Z2)2/(Z1+Z2)2.
The above formulae are valid for normal incidence of the sound
wave. When the incidence is at an oblique angle, reflection and transmittance
occurs in a manner similar to that for light rays (remember your basic
Table 2. Acoustic impedance for some materials
Ultrasonic pulse velocity (UPV)
In the UPV method, the velocity of a pulse traveling through concrete is measured and correlated to its stiffness using the relation mentioned earlier. The velocity of the pulse increases with the stiffness of the concrete, but decrease with increasing density. Wave attenuation increases when concrete becomes denser, because of absorption of energy. A typical set up of the UPV test is shown in Figure 5. A shown in the figure, the test can be used in three modes – direct, semi-direct, and indirect. The direct mode, or the through-transmission mode, is the most reliable, but needs access to both sides of the material. In the indirect mode, a plot is drawn between the travel time of the pulse and the distance between the transmitter and receiver. The distance at which the slope of the plot changes represents a change in the material property (which could be a transition from bad to good concrete, or the other way round).
Figure 5. Various arrangements
of the transducers for UPV
The transmitter and receiver are both piezoelectric transducers. These are able to convert between mechanical impulse and electrical signals. The receiver picks up the pulse generated by the transmitter (since the compressional wave is the fastest, it arrives at the receiver first). If the distance of travel is known, then the velocity can be calculated by a measurement of the time from the waveform. The modulus of elasticity can then be calculated from the expression:
V = (E/ρ)0.5, where &rho is the density of the material.
This expression is valid when the wavelength of the pulse is larger than the specimen width. However, when the specimen width is greater than the wavelength, then this expression should be modified to:
V = k(E/ρ)0.5, where k is a function of the Poisson’s ratio of the material.
Some typical UPV scenarios are shown in Figure 6. When the concrete is of uniformly good quality, the pulse is able to travel through without any disturbance. When there is reinforcing steel in the vicinity, the pulse will travel faster as it goes through the steel. When the pulse has to travel through a region full of voids and microcracks, the time of travel is increased. Because of the presence of cracks, the pulse may not have a direct path, and has to go around the tip of the crack. Thus, the travel time is further increased. Sometimes due to a large crack the pulse can get completely reflected back to the transmitter and no signal is received (From acoustic impedance data, it can be seen that a pulse traveling from concrete into air, which in this case is the crack, would get completely reflected).
Figure 6. UPV scenarios
In order to get reliable data from UPV, it is thus essential to obtain access to both sides of the structure. Adequate training and calibration is necessary to use the semi-direct and the indirect arrangements. It is difficult to point out the exact location of defects in UPV tests, although an overall assessment of the quality of concrete can be obtained.
IS 13311 – Part I (1996) gives a guideline for the analysis
of velocity measurements. This guideline is presented in Table 3.
Table 3. Velocity criterion for concrete quality grading as per IS 13311 – Part I
* Either quality is poor or more tests necessary
Impact Echo / Pulse Echo Method
In the impact echo method, and impacting device such as a hammer is struck on the concrete surface. The sound waves that reflect off defects or other features are picked up by a receiving transducer, and conveyed to a signal processor. The waveform is analyzed at the signal processor. From this analysis, the amplitude and travel time of the waves can be evaluated. A schematic description of this system is shown in Figure 7. Also shown in the same figure is a schematic of the pulse-echo system. In this system, the pulses are generated by a pulsing transducer. The same transducer can then act as a receiver, or an alternate receiver may be provided. The signal is again relayed to a signal processor.
Figure 7. Schematic diagram of the impact-echo and pulse-echo techniques
Figure 8 depicts the measurement of defects using these systems. The travel time of the pulse can indicate the depth of the defect, if the overall depth of the member and the corresponding travel time are known. In addition to such simple analysis, the signal received by the waveform analyzer can be analyzed. The main bang (MB) is what is perceived immediately on impact. The back echo (BE) is the signal that is reflected from the back wall, while the flaw echo (FE) is the signal from the crack or flaw. The time of arrival of the pulse can be analyzed to obtain the depth of the flaw relative to the overall depth. In the example shown in Figure 8, in case (1), the wave is reflected off the back wall. In case (2), a part of the wave is reflected off the tip of a crack, and the other from the back wall. In case (3), the wave is completely reflected off a large crack, and does not propagate on to the back wall, while in case (4), the wave is reflected off a lesser depth of the wall.
Figure 8. Schematic depicting
the use of pulse-echo technique
Reflections from corners and edges, as well as from reinforcement should be accounted for in the analysis. Frequency domain analysis is sometimes performed in order to account for multiple reflections between the surface and interface (flaw or back wall). A fast Fourier transform (FFT) is used to convert time domain pulses into frequency domain data. The discussion of frequency domain analysis is beyond the scope of this course.
Since the exact location of defects can be pinpointed using the impact- and pulse-echo techniques, a comprehensive 3-D mapping of defects is possible by doing area scans, as shown in Figure 9. In this figure, both linear scan and area scan are shown. In the area scan, the different shades represent different reflection times.
Figure 9. Scanning with
the pulse echo technique
Some limitations of this technique are that defects lying under other defects are not easy to detect. Also, reflections from sides, edges, and corners can confuse the data. A sufficient difference in acoustic impedance between the two media is necessary.
Spectral analysis of surface waves (SASW)
Surface waves consist of a spectrum of frequency (or wavelength) components. The penetration depth of these components is proportional to the wavelength. The speed of these waves depends on the elastic properties of the material that they are traveling through. Thus, when surface waves travel through a layered structure, such as a pavement, they get dispersed, i.e. break up into various components. Using various geometries of receiving transducers, all these components can be collected and analyzed using a signal processor. One can draw an analogy to white light, which is also made up of different wavelength components, each component representing a different colour.
A schematic of the SASW technique is shown in Figure 10. A hammer generates the impact. Two transducers are placed at various distances to receive the signal and relay it to the spectral analyzer. The spectral analyzer performs complicated signal processing and generates stiffness profiles for the various layers depending on the wave speeds. A limitation of this technique is that it can work well only for layered strata, where there is a substantial difference between material properties. Reflections from boundaries and corners can also lead to problems in analysis.
A scenario of SASW application is presented in Figure 11. Two
concrete specimens are investigated. The first one is of good quality,
while the second concrete has a deteriorated layer underneath the sound
layer. The resultant dispersion curves clearly indicate the depth of distress
in the concrete.
Figure 10. Spectral Analysis
of Surface Waves technique
Figure 11. Application of
SASW to detect the depth of deterioration
When cracks grow inside a material, there is a change in free energy. Due to the creation of a new surface, energy is released. This release may be in the form of heat or sound. Sound bursts emanating from growing cracks can be detected by the means of sensitive transducers. As shown in Figure 12, using well-placed sensors, the location of the crack growth can also be determined.
Figure 12. Placement of sensors in acoustic emission
A severe shortcoming of this technique is that it is extremely sensitive to any external sound disturbances. The other problem is that only growing cracks can be detected. Thus, it is quite a useful tool for early detection of growing shrinkage cracks.
Other NDE methods
The detection of heat flow through a body can indicate the presence of flaws or defects. In the infrared thermography technique, heat is passed through the material, and an infrared detector detects the heat patterns emanating from the body. As shown in Figure 13, when a defect is present in the body, it would show up as a cold spot when heat is flowing inward, and as a cold spot when the heat is flowing outward.
Figure 13. Application of infrared thermography technique
This technique is very useful in detecting delaminations in bridge decks. Complete thermal scan of the bridge deck may be obtained to identify defective areas. The limitations of this technique are that its accuracy is somewhat limited to the near-surface areas, and the application necessitates the presence of clement weather. Also, surface conditions might have a bearing on the result.
Radio Detection and Ranging (RADAR)
The various radar techniques are ground-penetrating radar, impulse radar, or short-pulse radar. The principle of radar detection is similar to that of ultrasonic pulse-echo techniques. The speed of radio waves traveling in materials depends on the relative dielectric constant of the material. Thus, when a layered system is present, radio waves, while traveling from one medium to the next, can get reflected or transmitted based upon the difference in properties of the media.
The applications of this technique are in detecting rebar location,
pavement thickness, asphalt overlay thickness, and voids beneath pavements.
The presence of water increases the ‘visibility’ of defects.
However, the rebars can cause interference in the signal. The presence
of chloride ions in moist concrete increases the signal attenuation. When
the technique is used to determine rebar diameter, complex image reconstruction
methods are employed.
In concrete, electromagnetic techniques are typically used to detect the depth of rebar (in other words, concrete cover over rebar). As shown in Figure 14, this can be done by (1) measurement of magnetic reluctance, (2) Eddy current techniques, or (3) Radar.
Figure 14. Determination of concrete cover using electromagnetic techniques
In the magnetic reluctance method, the presence of rebar increases the electromagnetic flux in the U-magnet and this is detected by the meter. In the Eddy current technique, the magnetic field in a coil (which is a part of the Eddy current probe) induces Eddy currents in the rebar. This Eddy current generates a magnetic field of its own that interferes with the main magnetic field. The change in inductance of the coil is then measured using the meter. Such probes are commonly used for rebar locating devices such as the Pachometer. A schematic diagram showing the use of a Pachometer is shown in Figure 15.
Figure 15. Use of a Pachometer for detecting steel in concrete
Eddy current techniques are also widely used to test defective metallic elements. Another electromagnetic technique called the magnetic particle method can also be used for conductive metallic elements. In the technique, a magnetic field is applied using a coil to the test piece. Magnetic particles (such as iron filings) are then sprinkled upon the test piece. These particles line up along surface cracks that are perpendicular to the orientation of the applied magnetic field. This makes visual detection of the surface cracks possible. One obvious limitation of these techniques is that they are limited to surface defects.
The visibility of surface cracks in metallic and non-metallic elements can be increased by penetrating a fluorescent dye into these cracks, and then observing the element under black (UV) light. This technique can also be used when cut sections of concrete are studied under a microscope in order to clearly indicate the voids and defects.
In radiography, X-rays or neutrons are passed through the test object, and the resultant image is captured on a film. This film is then studied to find the location of defects. The transmittance of X-rays or neutrons depends on the density of the material. Defective areas will show a larger transmittance. Radiography can be used to obtain a 360 degree image reconstruction, with techniques such as the CAT (computerized axial tomography) scan, which are commonly used in medical studies. Internal flaws are easy to detect using radiography.
One major limitation of using radiography is the hazards associated
with such techniques. They can be used effectively only if the source
can be placed out of contact with the operator. Such can be the case for
example in pipes, where the source can be placed inside the pipe, and
the film outside. Another limitation is the extremely high costs associated
with these techniques.
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