TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport IRRL

Contractor Report 156

THE NON-DESTRUCTIVE DETECTION AND MAPPING OF ASR CRACKING IN CONCRETE J

by R L Smith and M J Crook (Harwell Laboratory)

t "

The authors of this report, are employed by Harwell Laboratory. The work reported herein was carded out under a contract placed on them by the Transport and Road Research Laboratory.

The views expressed are not necessarily those of the Deparlment of Transport. F

This report, like others in the series, is reproduced with the authors' own text and illustrations. No attempt has been made to prepare a standardised format or style of presentation.

Bridges Division Structures Group Transport and Road Research Laboratory Old Wokingham Road Crowthorne, Berkshire RG1 1 6AU

1989

ISSN 0266-7045

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation o n 1 st

April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

i.

2.

3.

CONTENTS

.

,

6.

Introduction

Alkali-Silica Reaction (ASR)

Literature Search and Desk Study

3.1 Possible Techniques

31.1 31.2 31.3 31.4 31.5 31.6 31.7 31.8

3.1.9

Radar/Microwave Ultrasonics Thermography X, Gamma Radiography Acoustic Emission Rebound Hammer Electrical Techniques Nuclear Magnetic Resonance (NMR) or Magnetic Resonance Imaging (NMI) Mechanical Resonance

3.1.10 Holography 3.1.11Compton Backscatter 3.1.12 Chemical 3.1.13 Nuclear Tracers

3.2 Technique Assessment

Experimental Work

4.1 Preparation of Test Samples 4.2 Experimental Procedures

Page No

4

5

5

6 6 7 7 7 8 8 8

8 8 8 9 9

ii

ii Ii

4.2.1 Ultrasonic Pulse Velocity ii 4.2.2 Through Thickness Ultrasonics ii 4.2.3 The Ultrasonic Thickness Gauge 12 4.2.4 Pulse Echo at Near Surface 12 4.2.5 Ultrasonic Time-of-Flight Measurements 13 4.2.6 Acoustic Emission 14

Evaluation of Results

Conclusions

7. References

15

15

17

( C ) CROWN COPYRIGHT 1989. Extracts from the text may be reproduced, except for commerciai purposes, provided the source is acknowledged.

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List of Illustrations

Table 1

Table 2

Table 3

Table 4

Measured Compression Wave Velocities in Test Blocks

Ultrasonic Thickness Gauge Measurements

Transducer Location for Acoustic Emission

Acoustic Emission Results

Fig. 1

Fig. 2

Fig 3

Fig 4

Fig 5

Fig 6

Fig 7

Fig 8

Fig 9

Fig i0

Fig ii

Fig 12

Fig 13

Fig. 14

Fig. 15

Details of Test Blocks

Schematic of the Experimental Techniques

Surface Wave Velocity Measurement

Through Transmission Transit Times

Pulse Echo Signals from Test Block

Ultrasonic Scanning Rig

Scanning Rig Probe Arrangement

Ultrasonic C-Scan on Block 4

Oscilloscope Waveforms for Block 3 at 200 mm Probe Separation

Oscilloscope Waveforms for Block 4 at 200 mm Probe Separation.

Time-of Flight Measurements (i00 mm Probe Separation)

Time-of-Flight Measurements (200 mm Probe Separation)

Time-of-Flight Measurements Illustrating Possible Propagation Paths

Schematic Diagram of Acoustic Emission Equipment

Location of AE Transducers on Concrete Test Block

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i. Introduction

The purpose of this study was to investigate nondestructive techniques for the detection and mapping of cracking due to the alkali-silica reaction (ASR) in concrete bridge sections. The requirement was for the development of a nondestructive system which could determine the extent of internal cracking and the orientation and depth of cracks in reinforced concrete. The system should be able to scan the concrete surface, discriminate between cracks, reinforcement and other embedded materials, and be robust and portable. The project was split up ihto identifiable stages, the first stage was a desk study to identify likely methods and laboratory demonstrations of any method judged to merit further development. This report describes the results of the first stage. The first part of Stage One was a literature search and desk study on nondestructive testing methods likely to meet all or some of the technical requirements. These requirements may be summarised as follows.

i. The technique(s) needs to be able to determine the depth and internal structure of existing surface breaking cracks which have been detected visually.

. The orientation of the crack with respect to the surface and internal structures such as rebars should be determined.

. The influence on the application of the technique(s) of rebars or gels from the chemical reaction needs to be assessed.

. The technique should be nondestructive although a small amount of surface damage may be permitted.

5. The technique(s) should be capable of manual or automatic scanning.

6. The technique(s) should be usable on all types of bridge component.

7. The technique(s) should be robust, usable on-site in built-up areas and powered by batteries or 115 V supply.

A further consideration was whether realistic trials could be carried out using existing equipment to choose the systems to be investigated in the experimental phase.

It was accepted at the outset that it was highly unlikely that any one technique would meet all these criteria but that the assessment should be made on the basis of achieving as many of these aims as possible. The use of multiple complementary techniques was not precluded.

The second part of Stage One consisted of some laboratory demonstrations of four ultrasonic techniques and an acoustic emission technique. These had been identified as the most probable techniques to meet the requirements given above and equipment was available for initial trials. Other techniques were identified from the literature survey as having some application to the mapping of ASR cracking. These techniques were radar, thermographic imaging and electrical continuity but no experimental work was carried out on these techniques.

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2. Alkali-Silica Reaction (ASR)

Alkali-Silica reaction has been recognised as a potential problem for concrete constructions since the 1940's but only came to prominance in this country in the mid-1970's. The subject has been extensively reviewed in a recent book by Hobbs (Ref. i) and only some basic features of relevance to the crack detection and sizing are given here. The alkali-silica reaction is a reaction between the hydroxyl ions in the pore water of concrete and certain forms of silica which are occasionally present in significant quantities in the aggregate. The reaction product contains silica, sodium, potassium, calcium and water and its formation produces a "gel" of alkali-metal-ion hydrous silicate. Deterioration of the concrete due to ASR will only occur when three conditions are met simultaneously:

i) A sufficient alkaline solution in the pore structure of the concrete, usually sodium or potassium alkalis from Portland cement.

ii) An aggregate susceptible to attack by this alkaline solution; many types of aggregate contain reactive forms of silica.

iii) A sufficient supply of water.

The development of ASR is usually a fairly slow process and for expansion and cracking to occur sufficient quantities of the reactive components must be present. However the mechanisms for expansion and cracking are not precisely known, cracking due to ASR often shows a "crazed" pattern and the exact form of the cracking is influenced by the geometry of the concrete member, the presence of reinforcement and the applied stress. Both micro and macro cracking can occur and it is the detection and quantification of the surface breaking macrocracks that is the subject of this study. These cracks can be between 0.i mm and i0 mm in width and are generally located within 25 to 50 mm of the concrete surface, lying perpendicular to the surface. These details were taken as the basic requirements of the nondestructive system although it was also borne in mind that the cracks could go deeper and lie in other orientations including parallel to the surface if the local geometry and reinforcement placement governed the crack growth.

3. Literature Search and Desk Study

The literature search was carried out on the National NDT Centre's database using the free text STATUS system. The prime search words used were ASR cracking, concrete bridges and NDT. The use of ASR produced no direct references. However the combination of NDT and concrete bridges produced a large number of entries of which some 36 have been examined in detail. The following publications provide a general guide to nondestructive testing of concrete and are useful source material for the more detailed assessment of techniques given in sections 3.1 and 3.2.

"The Testing of Concrete in Structures" J H Bunga~v Surrey University Press 1982.

"In-Situ/Nondestructive Testing of Concrete" ed V M Malhotra ACI SP-82 1984.

"Guide to the Use of Nondestructive Methods of Test for Hardened concrete" BS 1881: Part 201: 1986.

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3.1 Possible Techniques

Using the references obtained from the literature search and also the expertise and knowledge of National NDT Centre staff the following list of techniques with possible potential for mapping and sizing ASR cracking was compiled. They are given in no particular order as they will be assessed with respect to the particular problem of ASR cracking in the next section.

3.1.1 Radar/Microwave

Radar techniques rely on the reflection of pulses of electromagnetic energy in the frequency range 100 to 1200 MHz (wavelength 60 to 5 cm in concrete) from discontinuities in a structure. It has been used extensively in the USA for the inspection of pavements and bridgedecks, primarily for delamination type defects or thickness gauging (Ref. 2, 3). Radar has a number of advantages for practical application because it can readily be used for mapping structures and commercial equipment is available. However, in general, data interpretation is difficult and the technique primarily relies on reflecting discontinuities lying perpendicular to the beam, i.e. parallel to the concrete surface, and this is unlikely to be the case in ASR cracking. Penetration is also limited by the wavelengths used and the measurements can be complicated by the presence of conducting materials such as steel rebars and water. For the radar approach to be applied to crack depth measurement a more complex system would have to be adopted than that currently used, for example looking at multifrequency techniques and polarisation effects.

3.1.2 Ultrasonics

Ultrasonic techniques are well established for the mapping of cracks in metallic structures using a variety of scanning and imaging methods (4). Many of these methods rely on the reflection of ultrasonic pulses from discontinuities in an analogous way to the radar approach. The frequencies generally used in concrete are about 50 kHz which gives a wavelength of about 6 cm. Ultrasonic pulse velocities, where the time-of-flight of an ultrasonic pulse from one transducer to another is measured, are sometimes used to give an indication of the quality of the concrete (BS 1881 Part 303). The difficulty in applying such methods to ASR cracks in bridges is caused by the highly attenuating nature of concrete to ultrasound and the possible complications of the rebars. However some ultrasonic techniques have been assessed for inspecting concrete bridges,, including resonance techniques (5) and ultrasonic pulse velocity (5,6). These have met with limited success. In the case of crack detection it is possible that ultrasonic techniques could be adversely affected if the cracks are water filled, particularly when using compression waves. However there has been recent progress in the application of ultrasonics to the inspection of concrete structures by using novel methods of signal generation and reception. These include impact and laser sources, sonic techniques for thick sections and surface wave methods (7,8,9). Ultrasonic (sonic) pulse velocity "~ measurements have been made on a dam suffering from ASR cracking and did provide some guidance on the state of the concrete but was not used for crack sizing (40). The current British Standard for the measurement of ultrasonic velocities in concrete (BS1881 Part 203) includes a basic technique for depth measurement of surface breaking cracks. This technique has been assessed experimentally in the present work along with pulse echo, through transmission and impact ultrasonics.

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3.1.3 Thermography

Thermal techniques rely on the detection of infra-red radiation from the surface of a structure. The pattern of infra-red emissions depends upon the flow of heat through the structure and hence even sub-surface features can be detected. Thermal techniques have been used for the detection of delaminations in bridge decks and for the detection of surface breaking cracks (particularly when filled with water) (i0,ii). They are noncontacting passive techniques which require long measuring periods. Solar heating is often relied upon although applied heat sources could be devised and the application of hot water has been tested (12). This system could be applied to cracking by impregnating the cracks with a hot fluid, however, as in the case of radar it is primarily planar type defects that can be detected. The depth of penetration into the structure of thermal techniques is determined by the diffusivity equation which also governs the likelihood of defect detection. As a rule of thumb the defect must be larger in extent than its depth below the surface to be detected.

3.1.4 X,Gamma Radiography

At the present time high energy X or Gamma Radiography or Gamma source and counter techniques offer the best possibility for inspecting the interior of large reinforced concrete structures. These techniques all rely on the attenuation of high energy (short wavelength) electromagnetic waves as they pass through the structure. Information can be recorded as count rates at individual positions, or on film, or on TV monitors using real time imaging. Extensive work on the development of practical systems has been carried out, particularly in France (13,14,15,16,17,18,19). They can provide detailed pictures of crack networks and have been proved for use in inspecting bridges. Real time systems have also been developed for practical use. However, they obviously pose radiological protection constraints on site application and would usually involve the closure of a bridge and restrictions to the surrounding area for a period of time usually exceeding 24 hrs. Tomographic X or gamma ray systems can give internal structural details which are complementary to normal radiography and can be combined to give complete 3 dimensional information. This technology has been demonstrated on structures such as trees, telegraph poles and concrete columns (20).

3.1.5 Acoustic Emission

Acoustic emission involves the monitoring of spontaneous ultrasonic signals produced by internal changes in a structure which can be used to infer its condition. The acoustic emissions produced by concrete under a variety of conditions has been investigated by a number of workers (21,22). If an active acoustic signal is located then it can be positioned quite accurately by triangulation methods (23). There has been considerable research activity in the use of acoustic emission in concrete and evidence produced that active rebar corrosion produces significant emissions (24). Propagating cracks or cracks under cyclical load (rubbing) would also be expected to produce emissions and hence it may be possible to use acoustic emission to locate ASR crack tips. The use of acoustic emission for concrete bridges has been demonstrated in the past (25). The major problems encountered are the signals due to background noise and the separation of multiple sources (this assumes that the cracks do produce significant emissions).

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3.1.6 RebQund Hammer

The rebound or Schmidt hammer is used to assess the surface hardness of concrete by measuring the amount of rebound from a controlled mechanical impact and although it has some limitations it does provide a quick, inexpensive way of checking uniformity (26). It may therefore have some use in assessing the extent of near surface damage surrounding ASR cracking. A simple impact with a hammer could also give similar information.

3.1.7 Electrical Techniques

Electrical methods can be used to determine the corrosion of reinforcement, moisture content and moisture penetration in concrete. These can be potential measurements, DC resistivity measurements (27) and some recently reported work on the frequency characteristics of the AC impedance (I00 Hz to ii0 MHz) (28). The varying frequency can give a kind of depth profiling and hence may be able to provide information on the subsurface crack profile, particularly if the crack is water filled. However, electrical methods will be very sensitive to the moisture levels in the concrete and will need considerable development to be of practical use.

3.1.8 Nuclear Magnetic Resonance (NMR) or Magnetic Resonance Imaging

(MRI)

This is a technique primarily for moisture content determination and has been shown to work on concrete (29) and hence, as in 3.1.7, it may be useful in mapping water filled cracks. The technique requires the use of large, powerful magnets and sensitive pick-up coils to detect the nuclear resonances, primarily in hydrogen atoms. The penetration depth is therefore limited and the system very sensitive to the moisture

conditions.

3.1.9 Mechanical Resonance

The local mechanical resonance characteristics of a structure will be affected by the presence of cracks and hence an impact (30,31)or forced oscillation device (32) coupled to a suitable transducer could provide information on the extent of damage. This would only give an overall assessment although by detailed signal analysis it may be possible to infer some aspects of the damage.

3.1.10 Holography

Optical holographic images have been used for experimental measurements of frost damage around rebars (33) and cracking in loaded concrete samples and therefore might possibly be used to examine the extent of near surface damage. However more straightforward mechanical or optical mensuration methods could provide the same information and have been used to assess ASR damage. The degree of deformation on the surface may possibly be related to the depth of the cracks.

3.1.11 Compton Backscatter

This technique relies on the detection of backscattered gamma rays from the interior of the structure during irradiation by a gamma ray source. Using a collimator system it allows depth profiling and is

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being developed for location of internal voidage in thick metal structures (34). Its use in concrete has not been demonstrated. It involves similar radiation hazards to the use of gamma radiography and is at present a slow (many hours) process.

3.1.12 Chemical

It may be possible to use suitable chemicals or washing techniques to obtain information on the activity of the ASR. However these approaches are considered to be outside the scope of the present study.

3.1.13 Nuclear Tracers

It may be possible to introduce radioactive tracers into the cracks via liquid penetrants and monitor their migration with detectors to give crack depth information. This would involve detailed calibration work and also has radiation protection implications.

3.2 Technique Assessment

The above list outlines the techniques derived from the literature search and some which have emerged from the desk study of the problem. They represent a wide range of research and development requirements to verify or negate their use for the mapping of ASR cracking. In some cases, for example radar, commercial apparatus is available and hence a proving study would enable the key aspects to be determined and in others, such as NMR, very little has been done.

With the present state of technology there is little doubt that high energy radiography is the most complete solution. This could be coupled with a tomographic system to give complete three dimensional information. However it fails to meet requirements for simple on-site use due to the set-up time and the radiological protection requirements. The same radiological problems apply to the Gamma, Compton and tracer systems also and hence they are not considered viable approaches within the confines of the criteria given in Section i.

The individual assessments given in Section 3.1 have indicated which nondestructive testing techniques may have application to ASR cracking and a number of these are clearly speculative. NMR, Compton backscatter, chemical and tracer techniques fall into this category and will not be considered further. Holography has been applied to a problem with concrete but only in laboratory conditions. The measurement of surface displacement can be carried out extremely accurately with holographic techniques but the surface displacements would have to be related to crack depths to provide useful information on ASR cracking. More straightforward mensuration may well provide the same information, although no general relationship exists between the crack opening displacement and crack depth in concrete and hence such approaches would probably have limited use. However information on the crack opening (width) should be recorded in any ASR cracking survey to provide as complete a characterisation of the crack morphology as possible.

Resonance and rebound hammer measurements have some similar characteristics but neither is expected to give information on crack depths. Resonance techniques give an indication of the overall state of a structure rather than detailed information on individual cracks. The rebound hammer can only give information on near surface conditions and therefore could not be used for crack depth measurement.

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The remaining techniques all have some potential for meeting the criteria given in Section i. The electrical techniques are the least developed as detailed research has only just begun. The measurements are complicated by water and metallic content of concrete structures and there will generally be a depth limitation. Hence at the present time it is unlikely that these techniques could be developed into a practical inspection system in the immediate future.

Thermal techniques are more advanced but are primarily designed for delamination type defects. Variations on the current practice, such as using novel thermal input systems e.g. hot fluids, flash tubes, lasers, may allow the techniques to be adopted for crack depth determination, but considerable research is required. A similar position exists for radar techniques where conventional application is orientated to finding delamination type defects but there is scope for development into crack depth measurement.

From the current study the ultrasonic and acoustic emission approaches are considered to be the most viable in the immediate future. Both can potentially penetrate to a significant depth in concrete and provide information on depths and positions of cracks in relation to the surface crack opening. These techniques were therefore the subject of the experimental investigations.

In the light of this conclusion it was proposed to carry out feasibility experiments using the following approaches.

I. Ultrasonic Velocity Measurements - The velocity of ultrasonic waves in concrete is a basic measurement that can be used to estimate the quality of concrete. It is also the prime calibration factor for many of the ultrasonic approaches investigated here. Hence the ultrasonic wave velocities were measured for each of the test blocks.

2. Through Transmission Ultrasonics - For very thick sections it is likely that only a through transmission type of measurement will penetrate the full thickness. The experiments carried out were designed to show the sensitivity of these measurements to internal cracking.

3. Wide-Band Impact Ultrasonics - This technique forms the basis of a concrete thickness gauge and will be most sensitive to cracks parallel to the surface of the concrete to a depth of about 500 mm. Previous work at the National NDT Centre has shown this technique can map an internal simulated void in a test block (9). The current experiments aimed to demonstrate this type of measurement on more realistic crack-like features in test blocks and to determine their depth.

4. Pulse Echo of the Near Surface - Although pulse-echo type ultrasonics has not been demonstrated in concrete it may be possible that near surface cracks could affect the overall response of a transducer placed on the concrete surface. This possible effect was investigated.

5. Ultrasonic Time-of-Flight measurements for crack depth measurement - This type of measurement has been used in the past for crack depth measurement and is included in BS 1881 Part 203. It could be developed into a portable unit relatively easily and hence this technique was investigated in the greatest detail on two test blocks with artificial surface breaking cracks.

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6. Acoustic Emission - A proving experiment on acoustic emission source location in concrete and an assessment of whether acoustic emissions are likely to occur in the case of ASR cracking was carried out.

4. Experimental Work

4.1 Preparation of Test Samples

Four test samples were prepared for this study and their details are given in Figure i. They were constructed from ordinary portland cement in a 6 to 1 mixture containing mixed aggregate up to 20 mm in size. The purpose of these samples was to simulate different sizes of cracks running perpendicular and parallel to the surface of the concrete. The overall size of the test samples was determined by practical considerations for handling in the laboratory. Two of the samples (Blocks 1 and 2) had 80 mm diameter simulated delaminations 1 mm thick at depths of 50 mm and 150 mm respectively while the other two samples (Blocks 3 and 4) had surface opening cracks of I00 mm and 200 mm depth. Both these surface breaking cracks were i00 mm long and 1 mm wide and placed in the centre of the block. The simulated cracks were produced by placing expanded polystyrene sheet, Imm thick, in position during the casting of the concrete. In the case of Blocks 3 and 4 the polystyrene was then dissolved out after the concrete had cured. There was no reinforcement in any of these blocks.

An existing test sample comprising a standard 438 x 213 x 97 mm thick concrete building block was used for the acoustic emission experiments.

4.2 Experimental Procedures

4.2.1 Ultrasonic Velocity Measurements

As most of the ultrasonic techniques used in this study rely on a knowledge of the ultrasonic wave velocity in concrete the velocity was measured in each of the test blocks. Transmission times for ultrasonic pulses at a frequency of 50 kHz were measured through the thickness of the blocks using a commercial instrument (Terratest) and the ultrasonic velocities calculated from the known thicknesses of the blocks. The values obtained are given in Table I. As some of the ultrasonic techniques involve the propagation of waves along the surface of the concrete measurements of the relevant velocity were made on Block 4 using the configuration shown in Figure 2a and the Terratest. This is basically the procedure recommended in BS 1881 Part 203. The results obtained of the transit time as a function of transducer separation are given in Figure 3. The ultrasonic velocity was determined from the slope of the graph and was 2.73 km/s.

4.2.2 Through Transmission Ultrasonics

Measurements of the transit time through the full thickness of Block 1 containing the 150 mm deep delamination were made using the Terratest instrument and 50 kHz transducers. The experimental arrangement is shown in Figure 2b. The transducers were placed opposite each other on a grid pattern of 50 x 50 mm squares and the transit time recorded. The results are given in Figure 4 where the transit time is marked next to the grid position along with the expected position of the defect. It can be seen that the transit time is slightly longer at the position of the defect (position 13). "However the measurements at positions 7 and 8 also indicate the presence of a defect which is

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outside the nominal size of the artificial flaw. One interpretation of this data is that the artificial defect moved position during the casting of the block. Another possibility is that a further defect was introduced by accident at positions 7 and 8 by the presence of the artificial flaw centred at position 13. Subsequent destructive examination of the block demonstrated that the defects were in the planned position. Measurements of signal amplitude through the block proved unreliable due to the problem of maintaining consistent coupling. A high viscosity fluid (honey) was used as a coup!ant for:these experiments.

A variation on this technique was tried on Block 4 to assess the technique on cracks perpendicular to the surface of the concrete. In this case the transducers were displaced from each other by i00 mm and moved in unison across the block. The angled beam was then expected to be interrupted by the crack causing a change in transit time and signal amplitude. Although such an effect was observed it could not be easily quantified in a meaningful way and no further work was carried out.

4.2.3 Wide-Band Impact Ultrasonics

The ultrasonic thickness gauge developed at Harwell (and now marketed by Industrial Ultrasound Ltd under the name Concordant) was used to assess this type of approach for the detection of delamination type cracks. The gauge comprises a sender/receiver head unit and a signal capture and analysis unit controlled by a microprocessor. The unit requires the ultrasonic velocity of the concrete to be input to calibrate the instrument. The head unit contains a high energy impact device which floods the concrete section under test with ultrasound, producing a broad band of frequencies in the range i0-I00 kHz. The receiving transducer is placed close to the impact source and is coupled to the surface with a suitable medium to ensure good signal response. The output of the transducer is digitised and analysed using a Fourier transform routine in the microprocessor. The processor then selects the fundamental resonance frequency and converts this to a thickness measurement using the ultrasonic velocity. The results of the tests using this instrument on Blocks 1 and 2 are given in Table 2. The position numbers correspond to those indicated in Figure 4 for the through transmission measurements. The readings at position 13 should correspond to the depth of the artificial defect. Although the absolute values obtained were not within the expected accuracy of the instrument (i0 per cent) the difference in the depths of the defects was consistently recorded. The readings around the periphery of the defect were very inconsistent and this was expected to some extent as to function efficiently the gauge requires an area free of interfering factors equivalent to the depth to be measured. In these blocks reflections from the side wall of the block confused the gauge. This is a limitation to its application in some constructional parts. The variation in the readings did not correlate with the defects known to be in the blocks and hence the technique is not recommended for ASR cracking.

4.2.4 Pulse Echo at the Near Surface

Pulse echo type measurements were made on one of the test blocks using transducers operating at 82 kHz and 250 kHz (Fig. 2d). The transducers were driven by a Harwell designed pulser/receiver'(Type 0870) and the output viewed directly on a digital storage oscilloscope.

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Typical signals obtained are shown in Figure 5. Because of the long "ring-down" time of the transducers and the high attenuation of concrete it was not expected that discrete signals from the defect would be obtained but rather that the character of the signal envelope might change signifying the presence of a crack. However, it was found that the signal did not change significantly in any way. The effective depth of penetration of this approach was estimated at 370 mm from the time lapse of the transducer ring down but due to the negative results this approach was not pursued further.

4.2.5 Ultrasonic Time-of-Flight Measurements

The Time-of-Flight system uses two transducers in the configuration shown in Fig. 2e and 2f which are then scanned over the block and the time of arrival of the ultrasonic pulse recorded at each position. The scanning system used is illustrated in Fig. 6 and comprises;

(a) A data acquisition system and control unit which contains the ultrasonic transducer drivers, a delay generator, sample and hold unit, A to D converter, stepping motor interface, two stepping motor drivers and a microprocessor which controls all the units and acquires the data.

(b) An operator interface, data storage and display unit based on a BBC microcomputer equipped with a dual disc drive and colour monitor. Commands from the BBC to start, stop and define the type of data gathered are sent via a RS 232 interface to the system controller. The acquired data are passed along the same interface to be displayed on the monitor in map format which can also be stored on disc. The microprocessor controls the scanning frame via the stepper motor drives and a joystick can also be used to manually position the scanning frame.

The system can be used either to measure the time at which the first signal arrives at the receiving transducer or the amplitude of the first cycle of the signal. For all the measurements reported here the arrival time mode was used. The two ultrasonic probes are held in mounts on the scanning frame which have water jets built in to couple the ultrasound into the concrete as shown in Fig. 7. This enables the transducers to be freely scanned over the surface. The separation of the transducers can also be varied.

A series of scans parallel to the line of the crack in Block 4 were carried out using 82 kHz transducers separated by 200n~n. The transducers were initially placed s~,,,etrically about the crack (ie 100mm either side of the crack) and then scanned in both the transverse and longitudinal directions with a 2mm step size as indicated in fig. 2(f). A map of the change in ultrasonic time-of-flight is shown in fig. 8. The light areas indicate the presence of the crack greatly magnified due to the separation of the probes and also extends completely along the longitudinal direction due to the limited size of the block. The time delay is recorded in the grey scale and indicates the depth of the crack. White indicates the longest time delay and black is the shortest time-of-flight. The black areas are where no crack lies between the ultrasonic probes. The random variability in the picture noise is due to the inherent attenuation of the concrete causing a variation in the signal level.

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To quantify the depth measurement a series of single line scans parallel to the crack with the probes set either side of the crack on Blocks 3 and 4 were carried out. The probes were moved manually using the joystick control in 50 mm steps and the pulses arrival time was measured directly on a digital oscilloscope. Two transducer separations of 200 mm and i00 mm were used. Typical oscilloscope waveforms for a separation of 200 mm on Blocks 3 and 4 are shown in Figs. 9 and i0. The complicated nature of the arrival pulse can be clearly seen but the shift in time of arrival as the system moves over the crack (100mm to 200mm from edge of block) can be measured. This shift in time is related to the crack depth. These data are given in Figs. ii and 12. The data in Fig. 12 is relatively easy to interpret because the shift in arrival time is related to the depth of the crack and the extent of the crack on the surface is measured by scanning the transducers parallel to the crack. This could therefore form the basis of a practical system for mapping ASR cracking in bridge structures.

The data presented in Fig. ii is more complex to analyse for although the crack is clearly detected there is no apparent sensitivity to depth. We believe that these results are due to the combination of transducer separation used and the surface length of the crack. The first arrival signal is not the one that has propagated underneath the crack but one that has travelled on the surface around the end of the crack and hence there is no depth sensitivity. This is illustrated schematically in Fig. 13 and the interpretation is backed up by simple calculation. This indicates that the system would be ~ifficult to use on short, shallow cracks but these are not the type geherally expected

with ASR.

4.2.6 Acoustic Emission

The objective of this experiment was to demonstrate that acoustic emission pulses could propagate sufficiently well in concrete to enable accurate source location. If this is possible it opens up the possibility of crack depth measurement from acoustically active ASR cracks. The standard concrete block described in section 4.1 was used for this experiment. The equipment used is shown schematically in Fig. 14. Four acoustic emission receivers were placed in fixed positions on one Side of the block as shown in Fig. 15. Each transducer output was passed through a charge amplifier and a 30 kHz high pass filter and amplifier to a four channel digitiser. The digitiser displayed the signal from each of the transducers and for each experiment the digitiser was triggered from the channel with the first arrival signal. The faces of the test block were divided into a grid coordinate system as shown in Fig. 15. The acoustic source used was a 2H x 0.3 mm diameter pencil lead which was broken on the opposite face of the concrete block to the receiving transducers. This is a simulated acoustic emission source commonly used for calibration work.

For each pencil break event the coordinates were recorded and the time of arrival of the compression wave signal from each channel was measured on the digitiser. Using these times, the known velocity of ultrasound in the concrete and the coordinates of the transducers, the location of the event was calculated using an optimization algorithm. The results are given in Table 4 and show good correlation with the known positions.

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5. Evaluation of Results

From the above results it can be seen that the ultrasonic techniques investigated had various degrees of success in meeting the requirements given in Section i. The time-of-flight approach clearly demonstrated the potential for a practical method for mapping and sizing surface breaking cracks whereas the near surface pulse echo technique showed no sensitivity to the internal or near surface defects. The other two ultrasonic methods have showed some potential for assessing some aspects of internal damage in concrete. In terms of practical application the most promising technique is the time-of-flight approach as it can provide both crack depth and length information. The possibility of scanning surfaces has also been demonstrated. In practice it would probably be of most use as a portable manual instrument to provide crack depth information as part of an overall survey procedure such as that described in Refs. 40-42. The limitations of this technique are primarily that it will only work for surface breaking cracks and some lower limit on the crack length and depth. From the results shown in Refs. 40-41 these would not appear to be major restrictions.

The through transmission and thickness gauge methods have shown limited potential for finding interior cracks and their use would be restricted to specific instances where interior information was required. Their application would depend upon the size and geometry of the member to be inspected and its internal structure.

Acoustic emission has considerable potential for accurate emission source location. The applicability of the technique depends crucially on the type and duration of acoustic emissions from ASR. To fully assess the technique a programme of fundamental study would be required in conjunction with a materials testing programme on realistic ASR such as that being run by BRE (39).

6. Conclusions

The results from the desk study on possible nondestructive methods for mapping ASR cracking have indicated that a number of techniques may be able to provide all or some of the required information. The practical demands for a portable robust instrument rule out some of the approaches considered, in particular X-ray or gamma-ray radiography and tomographywhich are considered to be the most likely techniques to solve the problem using current technology. The nondestructive techniques which are considered to have some potential to fulfil the requirements in the immediate future are as follows:

i. Ultrasonics

Ultrasonics provide a very powerful tool for crack detection and sizing and is now widely used in industry for both inspection during manufacture and in-service. In this study four different ultrasonic approaches to mapping ASR cracking were considered as it was believed that to cover the whole range of application complementary ultrasonic methods would be required. The experimental work demonstrated that some of these methods could indicate the presence of defects in concrete but that the approach which would provide accurate depth information was the time-of-flight technique. It is believed that this technique, which has been used to a limited extent in practice, provides the most appropriate solution to ASR crack depth measurement. It is therefore recommended that this technique be further developed.

- 15 -

2. Acoustic Emission

The experimental work has shown that acoustic emission can provide source location data in concrete and therefore has the potential to find crack tips in the concrete if they are acoustically active. There is no available data to confirm that ASR cracks do emit acoustic pulses but as the cracking occurs usually by expansion of the reaction products it would seem likely that these sites will generate acoustic signals. The duration of activity cannot be estimated at the present time and therefore further fundamental studies on acoustic emission due to the ASR are required before

the technique could be applied.

Three further techniques were considered to have some potential for the ASR cracking problem but require significant development work. No experimental work was carried out using them for this report.

3. Radar

Radar is the most developed of these techniques and offers an attractive approach as it is a non-contact method. However the standard methods are designed for delamination type defects and different methods would need investigation for crack depth measurements.

4. Thermography

This could be a rapid method for mapping the extent of near surface damage, as it has been applied to detecting delaminations in bridge decks. It is, however, affected by the presence of moisture and its possible capabilities for crack depth measurement have not been demonstrated.

5. Electrical continuity

This technique is at an early stage of development and is unlikely to be a practical technique in the immediate future. It is also affected by the presence of moisture in the concrete.

As described above the technique judged to be most promising to provide quantitative information on ASR cracking is the ultrasonic time-of-flight approach. From the experimental work carried out for this report it is concluded that the technique could be developed into a portable instrument. More extensive research would be required to develop the acoustic emission or radar techniques into practical instruments.

- 16 -

.

I.

2.

°

4.

.

.

.

.

.

i0.

ii.

12.

13.

14.

15.

16.

17.

18.

19.

References

Hobbs D W, Alkali-silica reaction in concrete, Thomas-Telford Lid 1988.

Clemena G G, Evaluation of overlaid bridge decks with ground penetrating radar, FHWA/VA-82/42 Feb 1982.

Harry Stanger Ltd, Impulse radar service, Elstree UK.

Krautkramer J and Krautkramer H, Ultrasonic testing of materials, Springer Verlag.

Scholer C F, Performance of ultrasonic equipment for pavement thickness measurement and other highway applications. JHRP-16 Jul. 1970.

Whitehurst E A, Soniscope tests concrete, American Concrete Inst. J. Proc. 47 Feb 1951 433-444.

Sansalone M and Carino N J, Impact-echo: A method for flaw detection in concrete using transient stress waves, NBSIR 86-3452 Sept 1986.

Smith R L, The use of surface scanning waves to detect surface-opening cracks in concrete, NDT International 17(5) Oct 1984 273-275.

Tasker C G and Smith R L, Ultrasonic techniques for the NDT of concrete, Annual Conf. on NDT Sept 1988 to be published by Pergamon.

Clemena G G, Application of infrared thermography in the detection of delaminations in bridge decks, FHWA/VHTRC 78-R27 Dec 1977.

Holt F B and Maming D G, Infrared thermography for the detection of delamination in concrete bridge decks, 4th Biennial infrared information exchange, St Louis Aug 1978

Arnold R H, Infrared detection of concrete, NASA-CR-II0869

Dufay J C, Television system using high energy radiation for NDT in prestressed concrete bridges, 9th World Conf. on NDT paper 5A Melbourne 1979.

Woodward R J, Inspecting concrete bridges, Physics Bulletin 35(4) Apr 1984.

Pullen D and Clayton R, The radiography of Swaythling bridge, Brit. J. NDT Sept. 1981.

Television inspection system using high energy radiation for NDT of prestressed concrete bridges, DGZFP Mainz 1978.

Translation of Ref 17 prepared by Windscale Laboratories UKAEA.

Nichues F, Radiographic inspection of prestressed concrete up to 1600mm using a 9 MeV linear accelerator, llth World Conf. on NDT 1985.

Dufay J C Scorpion (real time radiography) Qualitative Revue Practique de Control Industrial 22(121) 1983 35-42.

- 17 -

20.

21.

Sauerwein K, Wiacker H, Habermehl A and Ridder H-W, Mobile computerized tomographic systems MCT3 and MCT%. Giornale delle Prove non Distractive 2 (25) 1987.

Ohtsu, M, Acoustic emission in concrete and diagnostic applications, J. Acoustic Emission 6(2) 1987 99.

22. Maji A and Shah S P, Process zone and acoustic emission in concrete, Experimental Mechanics 28(1) Mar 1988 27.

23. Scruby C B, Stacey K A and Baldwin G R, Defect characterisation in three dimensions by acoustic emission, AERE R 12040 Feb 1986.

24. Hutton P H, Acoustic emission for flaw detection in steel in highway bridges, FHWA-78-97 1978.

25.

26.

27.

Woodward R J, Cracks in a concrete bridge, Concrete July 1983.

Muller K F, Economic use of NDT demonstrated on a reinforced concrete bridge built 50 years ago, Materialpruf 22 1980 286.

Bungay J H, Nondestructive testing of concrete bridges, 20th Annual Brit. Conf. on NDT 1985 EMAS.

28. McCarter W J and Garvin S, Determining the near surface properties of concrete, Annual Brit. Conf. on NDT 1988 to be published by Pergamon.

29. Matzikanin G A, Nondestructive measurement of moisture in concrete using pulsed NMR, 13th Symposium on Nondestructive Evaluation, 1981.

30. Forde M C and Batchelor A J, Interpretation of sonic waves through masonary bridges, 20th Annual Brit. Conf. on NDT 1985 EMAS.

31. Akishika T, Nondestructive examination with shock waves in civil engineering, Nondestructive Testing Journal Japan 1 (2) May 1983.

32. Savage R J and Hewlett P C, A new NDT method for structural integrity assessment, NDT International April 1978.

33.

34.

35.

36.

37.

38.

Rastogi P K and Pflug L, Holographic study of frost damage, Physics Bulletin 39(4) Apr. 1988.

Bridge B, Gunnell J M, Imrie D C and Olso N, The Use of Compton backscatter imaging for the detection of corrosion pitting in offshore structures, Nondestructive Testing Communications 2 1986 103.

Abdul-Amir A N and Abdul-Karim M A H, Microprocessor application to concrete crack depth measurement, J. of Nondestructive Evaluation 6(2) 1987 67.

Abdul-Amir A N and Abdul-Karim M A H, Dual slope analogue-to-digital converter for nondestructive evaluation of concrete crack depths, ibid 81.

Hayward G, Strathclyde Univ. Private communication.

Ultrasonics used to test concrete, Engineer 20 Oct 1988 (Kings College, London).

- 18-

39. Assessing the effect of ASR on structural performance, BRE News of Construction Research Aug 1988.

40. Hammersley G P, Alkali-silica reaction in dams and other major water retaining structures: diagnosis and assessment Proc. ICE 84 paper 9370 Dec 1988.

41. Sims G P and Evans D E, Aklaki-silica reaction: Kamburu spillway, Kenya, case history, ibid. paper 9371.

42. Cole R G and Horswill P, Alkali-silica reaction: Val de la Mare dam, Jersey, case history, ibid. paper 9372.

- 19 -

ACKNOWLEDGEMENTS

The authors would like to acknowledge the assistance of Dr D Buttle MP&MD in the acoustic emission work and Dr C R Wilding MDD for advice on ASR.

- 20 -

Table 1

Measured Compression wave Velocities in Test Samples

Sample No

1 2 4 5

U/S Compression Wave velocity kms -I

3.9 3.3 3.7 3.4

Table 2

Ultrasonic Thickness gauge measurements

Sample No

2

Position No (see fig. 4)

3 8

13 1 2 3 8

13

Thickness Readings

i00 178: 242 i05,115,141.157 i05,115,173,250,366 i00,245,350 92 i02,125

- 21 -

Table 3

Transducer location

Transducer No

Tra~sducTr locatiO~y.

40 120 85 5

2 0 0 120 284 40

0 0 0 0

Table. 4

Acoustic Emission results

Test No

1 2a 2b 3a 3b 4 5 6

S°xrC I

120 160 160 120 120 160 200 200

loc~tion Y|Z

80 97 40 97 40 97 40 97 40. 97 80 97 80 97 40 97

Measured I(~cation Diff~rencq: (mm) X / Y Z X / Y Z

121 80 159 41 159 41 118 43 121 51 159 80 193 76 195 40

92 1 0 -5 95 -I i -2 97 -i 1 0 112 -2 3 15 83 1 ii -14 94 -i 0 -3 87 -7 -4 -10 80 -5 0 -17

- 22 -

W 3 0 0

&

_ 1

- I L 300 _1

.1¢ U o

m

{ U o

w nn

Plan v iew of b locks 1 a n d 2

E l e v a t i o n of b locks 1 and 2

L 3 0 0 = l I - - - - I

P l a n v i ew of b l o c k s 3 and 4

o O

L 3 0 0 , I - 10,, . j . .0L 0 -7

End e l e v a t i o n of 3 and 4

b l o c k s

4

FIG. 1. DETAILS OF TEST BLOCKS.

Transmi t te r Rece iv ing t r a n s d u c e r

r'g/" r'~", ,r'~'l L_.J L_ . . ,.__..,

i • • •

posi t ions

o) . S u r f a c e w a v e v e l o c i t y

Transmit ter

r ~ m ~

[ o o ~ ,

r a , oa

r oqmJl . - - . . I

L ~ o .

. Imm, - , . - . -

Defe c t

- r a m , , I

I m~o ,J

...J

D ~ o ~

i . I ~ J

R e c e i v e r

b). T h r o u g h t r a n s m i s s i o n m e a s u r e m e n t

Ultrasonic t h i c k n e s s gauge r-1 r-1 r-1 I I I ! I I I . . j I . . J I...,11

Defec t

J J

c). ULt rasonic concre te t h i c k n e s s g o u g e

FIG. 2. SCHEMATIC OF THE EXPERIMENTAL TECHNIQUES.

r - ' 1 f - - ' 1 r - - i I I I I I I t . . J L . J I....=1

U~ U

d). Pu lse echo measurements

Transmi t ter Rec e i v e r

I-1 D

T r a n s d u c e r s scanned ove r s u r f a c e a t a f ixed d i s t a n c e a p a r t

e). Time of f l i gh t measurement (elevation)

j ~ C r a c k

Transverse

T r a n s m i t t e r Receiver

scan direct ion

gitudinal scan di rect ion

f). Time of fl ight measurement ( p l a n )

FIG. 2. (cant'd) SCHEMATIC OF THE EXPERIMENTAL TECHNIQUES.

I 0 0

T 111

E

U 0

"0

13

u

0 L)

(sd) ~w!~,

! 0

+

-I" +

0 ~ 0

N

n"

L. .

e~ 0

0 0

I1

),isuoJJ.

mm~: 50 _1_ I

50 __1= 50 __1__ 50 __1__ 50 __1 I I I - - I

C )

C:)

O

O

C )

i

j E E

74.7 74.8 (11 121

I I 75.8 .76-7

(61

75,5 (11)

75.8 74..6 (3) 151

75-5 (4)

I 75.6 (91

~ , - . Ex tent

76-7 74-2 171 ..(8)~ (10)

I " I " I / ~ of defect

74.91 I 77.1 175.0 74-5 112) ~ (131 I 1141 (151

I 74.9 1171

'~ L "' J

75-6 75.1 74"8 75"0 ( 1 6 1 (18) (19) ~ (20)

76-6 75.5 75.4 • 75-1 74.5 (21) (22) (23) (24) (2 5)

T i m e s a r e in ps ( ) Pos i t ion i d e n t i f i c a t i o n numbers

FIG. 4. THROUGH TRANSMISSION TRANSIT TIMES ON BLOCK.

Freq. 82 kHz

L r 3 7 0 m m - ~

Freq. 250 kHz

I... J I - 370mm

FIG. 5. P U L S E - E C H O S I G N A L S F R O M T E S T B L O C K .

FIG. 6. ULTRASONIC SCANNING RIG.

FIG. 7. SCANNING RIG PROBE ARRANGEMENT SHOWING PROBES ON EITHER SIDE OF CRACK.

c- O

i m

m m

C

¢.)

m

| m

e m

0 . J

C 0

u m

I N s m

C

> ul

i - -

o 0 . . I m

Z 0 Z

0

_o

r r

d m u_

t l l t~ T , / , w i I tr = . ~ l - .

t~t i t tt

ot t tO tit ,f' /~tP tt.l~ttltot

,'o Fl i t i l l o ~. i o I I i

I D

O.2U ' H a s l ~ d le~

50mm 1 7 5 m m

125mm 200ram

150mm 250ram

FIG. 9. OSCILLOSCOPE WAVEFORMS FOR BLOCK 3 AT 200 MM PROBE SEPARATION.

50mm 175mm

125mm 200mm

150mm 250mm

FIG. 10. OSCILLOSCOPE WAVEFORMS FOR BLOCK 4 AT 200 MM PROBE SEPARATION.

I o o

u u

I-- Im u u

nO

E E E E 0 0 0 0

+ 0

+

I 0 o

(sd) ew!), I!$uo~1

0

N

o o

E

0

Ul

~3

0

o o

Z 0

l--

r~-

O. W

ILl O0 0 r7~

O.

E E 0 0

u~

Z

bJ

~E W rr Z~

W ~E

-r

c.D

i,

b_

0

W

~E

b_

I 0 0

Ii

0

u u

o o I.. L.

u u

"I0 "0

E E E E 0 0 0 0

x o

X

0

X

I 0 0

(sd) aw!l l!suo~1

0

N

0

E E.

0

u

0

0

"0

E 0

m 0 U • -- ¢.

0

oI

c)

0

0 0

A

Z 0 I

I.-

n a,i U~

nn

0 rr

E E C) C) N

V

U~ I-- Z W

~E

rr

U'}

W

I-- "I-

__I b.

b_ 0

W

~E I

I---

N

Receiver T ransmi t te r

I~ / I ~

I ~ / P - I

f D e f e c t

FIG.13 . TIME OF FLIGHT M E A S U R E M E N T S POSSIBLE PROPAGATION P A T H S .

I LLUSTRATING

I. L,.

o l ;,=

~ E

N III 5 ~ L

0 ~" ~ - - - " F " -

Z U.!

n

W

Z 0

W

~.)

U3

0 (.# <

LI_ 0

0 ::3 n m

, "l I ,, |

i i g

~-E.-1 Fq F4 Fq

< n -

<

r ~

I--.- <

i .d " l - ( J U3

-4"

J

I ,

IA r

4 3 8 m m ..J v I

E E

N

L

0 40 80 120 160 200 240

®

280 320 360

®

x Po, s i t i o n of t r a n s d u c e r

0 T r a n s d u c e r i d e n t i t y

T h i c k n e s s ~f b |ock 9 ? m m

FIG. 15. LOCATION OF A - E TRANSDUCERS ON CONCRETE TEST B L O C K .