The deterioration of concrete structures in the last few decades calls for effective methods for condition evaluation and maintenance. This resulted in development of several nondestructive testing (NDT) techniques for monitoring civil infrastructures. NDT methods have been used for more than three decades for monitoring concrete structures; now it has been recognized that NDT plays an important role in the condition monitoring of existing RC structures. NDT methods are known to be better to assess and evaluate the condition of RC structures practically. This paper reviewed several available NDT methods developed and used in the last few decades.
Testing and quality checkup are important at different stages during the life of a structure. To properly maintain the civil infrastructures, engineers required new methods of inspection. Better inspection techniques are needed for deteriorating infrastructure (Rens et al., 1997) [
NDT has been defined as comprising methods used to examine objects, materials, or systems without impairing their future usefulness, that is, inspect or measure without harm. NDT methods are now considered as powerful tools for evaluating existing concrete structures with regard to their strength and durability. NDT methods have been drawing more and more attention, in the sense of reliability and effectiveness. The importance of being able to test in situ has been recognized, and this trend is increasing as compared to traditional random sampling of concrete for material analysis (Shaw and Xu, 1998) [
It is better practice that NDT engineers have the knowledge and training of various NDT methods available for testing a parameter, to select better technique form the available methods according to the condition of structures. Use of different methods for evaluating a single parameter increases the confidence and also validates the results. Combining the results of various NDT methods for assessing the quality of structures has been required for better results; this aspect has been discussed in the present paper.
This paper also presents brief literature review of the recent NDT tests performed on concrete structures, followed by a table describing advantages, limitations, and principle of several NDT methods, and also present status and future aspects of NDT methods have been discussed followed by a table presenting various codes describing these methods.
Nondestructive techniques are useful for evaluating the condition of structure, by performing indirect assessment of concrete properties. These techniques have been improved in last few years and the best part is that NDT avoids concrete damage for evaluation. Several researchers perform NDT tests to evaluate the condition of concrete structures. Methods range from very simple to technical depending on the purpose.
Several mechanical and physical properties of concrete structures can be used to assess the condition and capacity of the structures. Sanayei et al. (2012) [
Several researchers performed different types of NDT tests such as mechanical, chemical, electrochemical, and magnetic methods to evaluate the condition by combining the results. Rens and Kim (2007) [
Propagation of waves or reflection of different rays such as X-ray, through concrete structures, can be used to detect the deterioration level of concrete structures. Impact echo method has been used by many researchers to evaluate the condition of concrete. In this method a spring loaded device is used to generate waves, and these waves are used to detect condition of structures. Kamal and Boulfiza (2011) [
Zhu and Popovics (2007) [
Ground penetrating radar (GPR) is another method to locate the rebars, voids, and other defects in concrete structures. Chen and Wimsatt (2010) [
Ultrasonic pulse velocity is used by many researchers for the assessment of concrete properties by using travel time of longitudinal waves over a known distance. Sharma and Mukherje (2011) [
Several NDT methods utilize electrical properties of concrete structures to assess the condition of structures. El-Dakhakhni et al. (2010) [
Electrochemical methods are also developed and used by many researchers to detect the deterioration level of structures. Sangoju et al. (2011) [
Vibration-based techniques can be used to monitor the concrete structures. Bagchi et al. (2010) [
Permeability or porosity of concrete structures is responsible for the diffusion of harmful agents in the concrete. Deo et al. (2010) [
NDT methods for assessing the concrete structures have been classified by many researchers based on the governing principle. McCann and Forde (2001) [
Different NDT methods and parameter measured.
S. no. | Parameter measured | NDT method | Advantages | Limitations | Principle |
---|---|---|---|---|---|
1 |
Concrete quality, cracks, defects, and voids |
Visual inspection | Rapid, economical | Expertise is required, superficial, and depends upon skill of viewer | Based on the visual defects on the surface |
Image Pro Plus (IPP) | Simple, rapid, cheaper | Slow results | Comparing colors of different objects | ||
Acoustic emission (AE) | Fast results, detect changes in materials | Costly, defects already present are not detected | Sudden distribution of stresses generates elastic waves | ||
Impact echo | Able to detect condition of concrete accessible from one side only, quick, accurate, and reliable | Interpretation is difficult, reliability decreases with increase in thickness, and accuracy depends on impact duration | Transmission and reflection of electromagnetic waves | ||
Infrared thermography | Easy interpretation, simple, safe, no radiation, rapid setup, and portable | No information about depth or thickness of defects, and results affected by environmental conditions | Surface temperature variation | ||
One-sided signal wave transmission measurements | Used to detect structures accessible from one side only such as pavements | Large thickness affects the results | Propagation velocity of signal waves | ||
Impulse response | Simple, easy to handle | Depends on the skill of user, and deep damages influence the results | Based on stress wave test method | ||
Radiography | Thickness and composition can be easily detected, and rebars can be located | Expensive, hazardous, and limited to low thickness | Velocity of X and gamma rays and its attenuation | ||
Petrographic testing | Provides information about alkali-silica reaction, alkali carbonation reaction, sulfate attack, freezing, and thawing | Required high skill for the interpretation of result | Samples are examined through a petrological microscope using reflected or transmitted light | ||
Lamb Wave Theory (LWT) | Relatively accurate | Difficult interpretation | Based on guided wave theory | ||
| |||||
2 |
Compressive strength, surface hardness, adhesion |
Rebound hammer | Simple, quick, and inexpensive | Not so reliable, smoothness, age of concrete, carbonation, and moisture content can affect results | Rebound of plunger when strucked with concrete indicates strength |
Ultrasonic pulse velocity (UPV) | Quick, portable, large penetration depth, Simple interpretation, and moderate cost | Not very reliable, moisture variation and presence of reinforcement can affect results | Ultrasonic wave velocity and its attenuation | ||
CAPO test | Correlation between pull out force and compressive strength is reliable | Damage to the surface | Expanded ring in the cored hole is pulled out | ||
Probe penetration | Simple, needs less training, and low maintenance | Leave a hole in concrete surface, and coarse aggregates affect the penetration | Penetration of probe is measured and related to strength | ||
microcoring | Good correlation between test results and compressive strength | Depends on the preparation of specimens | Extraction of microcore samples from a concrete structure is used for analysis | ||
Pull off test | Fast results, evaluate adhesion, and tensile strength which can be converted to compressive strength | Damage to the surface | A disc is bonded to the testing surface, and when disc is pulled off, force required is used to obtain pull off strength | ||
| |||||
3 |
Chloride concentration |
Quantab test | Fast and accurate | Expensive, hazardous, limited to low thickness | Reaction of silver dichromate with chloride ion produces white column on the strips |
Potentiometric titration | Reliable | Requires skilled personal | Using acid or water soluble methods, the final volume will indicate chloride content | ||
Rapid chloride test | Portable, simple, and quick | Variation in results by the presence of certain materials | Potential difference of unknown solution is compared with potential difference of solutions with known chloride concentration | ||
| |||||
4 |
Corrosion rate, percentage of corrosion, corrosion progress |
Galvanostatic pulse method | Measures half-cell potential and electrical resistance simultaneously | Unstabilized readings | Based on the polarization of rebar by means of small constant current |
Linear polarization resistance (LPR) | Rapid, requires only localized damage, more detailed information | Measurements are affected by temperature and humidity | Electrical conductivity of fluid can be related to its corrosiveness | ||
Half-cell potential | Simple, portable, results in the form of equipotential contours | Needs preparation, saturation required, not very accurate, and time consuming | Electric potential of rebars is measured relative to half cell and indicates probability of corrosion | ||
Time domain reflectometry (TDR) | More robust, easy, locates corrosion, and identifies extent of damage | Less sensitive | By applying a sensor wire along side of the reinforcement a transmission line is created. Physical defects of the reinforcement will change the electromagnetic properties of the line | ||
Ultrasonic guided waves | Identifies location and magnitude of corrosion | Not very reliable | Based on propagation of ultrasonic waves | ||
X-Ray diffraction and atomic absorption | Simple and reliable | hazardous | Intensity of X-ray beams reduces while passing through a material | ||
| |||||
5 |
Carbonation depth, pH of concrete | Phenolphthalein indicator test | Simple, quick, and inexpensive | Inappropriate for dark aggregates, results affected by saturation | Carbonation reduces pH of the concrete |
Rainbow indicator | Quick, descriptive, and easy to use and interpret | Requires drilling of concrete surface up to rebar depth | Carbonation reduces pH of the concrete | ||
| |||||
6 |
Pavement inspection and subsurface condition |
Ground-coupled penetrating radar (GPR) | Low cost, portable, effective | Complex results, difficult interpretations | Propagation of radiofrequency (0.5 to 2 GHZ) |
Hammer sounding | Simple, easy to handle | Depends on the skill of user, and deep damages influence the results | Surface is striked with hammer and hollow or dull tone indicates the existence of delamination | ||
Acoustic tomography | Useful results, moderate | Requires skill, high cost | Waves were received on opposite side, and wave velocity depends on material properties | ||
Falling weight deflect meter (FWD) | Useful results | Can provide misleading results and requires experience for interpretation | Load is produced by dropping a large weight to detect concrete | ||
| |||||
7 |
Flaw detection inside decks, delamination, location, and extent of damage in bridges | Chain drag | Simple, portable | Time consuming, tedious | Chain is dragged over surface for flaw detection |
Vibration based damage identification (VBDI) | Easy to implement, cost effective | Environmental factors, errors in measurements, and nonunique solutions | Based on changes in the dynamic characteristics of a structure | ||
| |||||
8 |
Entire depth of damage, percentage of damage, identification of deteriorating infrastructure |
Seismic refraction method | Calibration is not necessary, more reliable | Valid for high speed for large depths | Seismic waves travel outward from a source and reach a detector |
Ultrasonic longitudinal waves (L-wave, P-wave) | Inspect also at large depths | Not appropriate as primary investigation method | Transmission and reflection of ultrasonic waves | ||
Ultrasonic continous spread spectrum signal | Improved sensitivity | Difficult interpretation | Signals are received by detectors and signal speed depends on defect | ||
| |||||
9 |
Permeability, water absorption |
Water permeability test | Preparation and skill required, time consuming | Semidestructive type test | Assesses the ease with which water penetrates in concrete |
Initial Surface Absorption Test (ISAT) | Consistent results in laboratory | Problems in using in situ, affected by increase in temperature | Rate at which water is absorbed in concrete is measured | ||
Covercrete absorption test (CAT) | Not influenced by local surface attacks | Sensitive to W/C ratio, curing time and moisture content | Rate of absorption of concrete is measured | ||
| |||||
10 |
Concrete cover, rebar diameter, location of reinforcement | Cover meter | Portable | Slow, affected by deep cover and closely spaced bars | Electromagnetic induction |
Radioactive methods | Simple | Hazardous | Generates images of the structure of RC and steel | ||
| |||||
13 | Load bearing capacity of bridge | Static truck load test | Reliable | Dangerous | Response of strain sensors under truck load indicates load bearing capacity |
| |||||
14 | Relative conditions of brick masonry side walls | Multichannel Analysis of Surface Waves (MASW) | Reliable, fast and economical | Expensive, time consuming | Uses multiple sensors to record wave field |
| |||||
15 | Detecting disbands | Microwave NDT method | Detects from one side, rapid, noncontact, and robust | Costly | Reflection and transmission coefficients are measured and related to material properties |
| |||||
18 | Stress/strains sensor for monitoring composite beams | Fiber optic Bragg grating sensors | Suitable for long-term tests | Slow response | Monitors the response of structure subjected to full load |
Several national and international codes of practice accepted and include the NDT methods: List of tests and different codes accepting the test methods are shown in Table
Different codes describing NDT methods.
S. no. | Tests/parameters | Codes |
---|---|---|
1 | Alkali aggregate reactivity | IS 2386 (Part 7): 1963 |
2 | Petrographic examination | IS 2386 (Part 8): 1963, ASTM C856-77 |
3 | Pull out test | IS 2770: 1967, ASTM C900-94, E DIN EN 12399 (July 1996), ISO/DIS 8046 |
4 | Water soluble chlorides in concrete admixtures | IS 6925: 1973 |
5 | Ultrasonic pulse velocity | IS 13311 (Part 1): 1992, ASTM C597-97, BS 1881: Part 203: 1986, BS 4408: pt. 5, NDIS 2416-1993 |
6 | Rebound hammer | IS 13311 (Part 2): 1992, ASTM C805-97, BS 1881 Part 202: 1986, EDIN EN 12398 |
7 | Abrasion resistance | IS 9284: 1979, ASTM C779-76, ASTM C944-80 |
8 | Permeability | IS 3085: 1965 |
9 | Testing drilled cores | ASTM C 42-87 |
10 | Infrared thermography | ASTM D4788-88 |
11 | Ground penetrating radar | ASTM D6087-97 |
12 | Density by nuclear methods | ASTM D2950-91, ASTM C1040-93 |
13 | Impact echo method | ASTM C1383-98a |
14 | Half-cell potential | ASTM C876-91 |
15 | Penetration resistance | ASTM C 803-82 |
16 | Dynamic modulus of elasticity by electromagnetic methods | BS 1881: Part 102:1983 |
17 | Radiography | BS 1881: Part 205: 1970, BS 4408: pt. 3, NDIS 1401-1992 |
18 | Water absorption | BS 1881: Part 122: 1983, AS 1012.21-1999 |
19 | Electromagnetic covermeter | BS 1881: Part 204: 1986, BS 4408: pt. 1 |
20 | Concrete strength by near to surface methods | BS 1881: part 207: 1992 |
21 | Strain gauges for concrete investigation | British Standard Institution, London, 1969, (83) |
22 | Density with gamma rays | TGL 21 100/01 |
23 | Determination of chloride and sulfate in hardened concrete | AS 1012.20-1992 |
24 | Visual inspection | NDIS 3418-1993 |
25 | In situ monitoring of concrete | NDIS 2421-2000 |
26 | Surface hardness method | BS 4408: pt. 4 |
An NDT method provides indirect results which can be related to various properties of concrete structures. In the last few decades NDT methods have been developed form rebound hammer to new sophisticated techniques based on propagation of waves in the concrete. With the development in software technologies and battery operated small computers, NDT methods are getting popular among researchers and engineers for quick evaluation and interpretation of results. In the future NDT methods can be useful for the various purposes such as for identifying deterioration levels and modeling the life of structures, extracting indepth information about material properties, and developing methods for combining the results of different NDT methods for better evaluation of condition of concrete structures.
Combining several methods for assessing the structures is now required for better assessment. Normally an NDT engineer uses single method for evaluating a parameter, but sometimes combining several methods for better assessment has been required. So to use and combine different methods knowledge about principle, advantages and limitations of different available methods are required by engineers. Combination of several methods has been required to strengthen the results of each other. Several properties of structures can affect the same measurement, and it is difficult to distinguish the effect of each property on a measurement. Based on experience and knowledge the better combination of NDT techniques can be selected for the diagnosis of concrete structures.
NDT methods inspect or measure without any harm to the structure; no damage of specimens is required during testing. It can be applied for inservice inspections, and this is major advantage of NDT methods, and they are capable of detecting flaws and defects early. By using NDT methods very precise assessment of the defect extent can be evaluated, and when combined with laboratory tests they give good results. Also they are capable of monitoring structures continuously.
NDT results are complex and provide detailed information for concrete tested. It has been difficult for the engineers to understand the results of NDT, and so expertise and experience are required for handling NDT equipments and for the interpretation of results. Sometimes results are not satisfactory, due to wrong selection of method or equipment, so training is a must for using NDT. If assumed physical condition of structure is different from real condition then results and its interpretation are not as expected.
Various NDT methods based on different principles, with their individual merits and limitations, have been discussed. It has been recognized that NDT plays an important role in condition assessment of existing structures, and there has been an urgent need for developing standards for performing NDT methods and for interpretation of NDT results.
Major advantage of NDT methods has been recognised as their capability to test in situ. Great deal of expertise is required for interpretation of NDT field observations and test results. NDT provides useful information by revealing hidden or unknown defects, and repair or replacement of RC structures can be planned according to NDT results. Combination of different NDT methods available is a better way to assess the structures.