" TRANSPORT A N D ROAD RESEARCH L A B O R A T O R Y

Department of Transport

RESEARCH REPORT 250

NON DESTRUCTIVE TESTING METHODS FOR

CONCRETE BRIDGES

by R J W o o d w a r d

The views expressed in this report are not necessarily those of the Department of Transport

Bridges Division Structures Group Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1989

ISSN 0266-5247

CONTENTS

Abstract

1. Introduction

2. Mechanical vibrations

2.1 Whole structure vibrations

2.2 Pulse methods

2.2.1 Sonic

2.2.2 Ultrasonics

2.2.3 Acoust ic emission

2 .2 .4 Acoust ic pulsing

. Electromagnetic methods

3.1 Radiography and radiometry

3.2 Visual methods

3.2.1 Endoscopy

3.2.2 Optical f ibres

3.2.3 Moire photography

3 .2 .4 Holography

3.2.5 Deformation

3.3 Infra-red

3.3.1 Thermography

3.3.2 Thermoelast ic stress analysis

3.4 Radar

3.5 Nuclear magnetic resonance

4. Electrical and magnetic methods

4.1 Electrical methods

4.1.1 Resistivity

4 .1 .2 Electrode potential measurements

4.1.3 Eddy currents

Page Page

1 4.2 Magnetic methods 13

4.2.1 Magnetic induction 13 1

4.2.2 Magnetic flux exclusion 13

1 5. Other methods 13

1 5.1 Surface hardness methods 13

2 5.2 Pull-out tests 14

2 5.3 Nuclear methods 14

3 5.4 Measurement of prestress 14

3 5.4.1 Stress relief 14

5 5.4.2 Slot cutting 14

5.5 Permeability 14 5

6. Discussion 14 7

7. Conclusions 16 7

8. Acknowledgements 16 7

9. References 16 9

9

9

9

10

10

10

11

12

12

12

12

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12

© CROWN COPYRIGHT 1989 Extracts from the text may be reproduced

except for commercial purposes, provided the source is acknowledged

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I 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.

NON DESTRUCTIVE TESTING METHODS FOR CONCRETE BRIDGES

ABSTRACT

Over the last few years various investigatory techniques have been increasingly applied to concrete bridges. The report describes the test methods and the scientific principles upon which they are based. It includes a description of some of the more recent work carried out under contract to TRRL to develop improved inspection techniques.

1 INTRODUCTION

There are over 50 000 concrete bridges in the United Kingdom, most of which have been constructed during the last sixty years. These structures form a vital part of the highway network and represent a huge capital investment. Over the last few decades they have been subjected to ever-increasing loading with the rise in both traffic f low and maximum permissible weights for individual vehicles. In addition the increased use of sodium chloride as a de-icing salt has caused corrosion of the reinforcement due to the ingress of chlorides into the concrete. Not surprisingly, this has led to the premature deterioration and even failure of some structures (Wallbank 1989, Woodward and Williams 1988).

Systematic inspection of bridges is needed to identify signs of deterioration at an early stage, and bridge owners have developed comprehensive routine inspection and reporting procedures by which defects are reported (Department of Transport 1988~). Visual inspection is the most commonly used method but this generally only identifies deterioration once it has reached an advanced stage. Ideally, techniques are required for detecting defects before they cause serious structural problems.

Over the last few years non-destructive investigatory techniques such as electrode potential and resistivity measurements have been applied increasingly to bridges. The use of such techniques has been to investigate the causes of visual deterioration and the extent of underlying deterioration to allow properly planned remedial work (Department of Transport 1986). This report reviews the techniques that are available, explains the scientific principles upon which they are based and describes some of the more recent work carried out under contract to TRRL to develop improved inspection methods. It gives an appreciation of the capabilities and limitations of

existing techniques and provides a background against which new techniques and equipment can be evaluated.

2 MECHANICAL VIBRATIONS

Techniques using mechanical vibrations can be divided into two groups: whole body motions and pulses which propagate through a material. Whole body motions are usually low frequency, ranging from less than one Hertz to a f ew tens of Hertz, whereas pulses are usually of a much higher frequency. If the frequency is in the range 20 Hz to 20 kHz the sound is audible but above 20 kHz the waves are inaudible and are referred to as ultrasound (Figure 1). Low frequency pulses can be generated by a hammer blow whereas high frequency ultrasonic pulses are usually generated using piezo-electric transducers.

2.1 WHOLE STRUCTURE VIBRATIONS Most bridge structures have clearly defined natural frequencies of vibration which depend on their mass and stiffness, and each natural f requency has an associated mode shape and damping value. If it is assumed that these properties remain constant unless the structure deteriorates, then measurements of dynamic characteristics should enable the condit ion of a structure to be determined. This could be done either by carrying out a one off measurement and comparing the results wi th theory, or by periodically repeating the measurements and looking for changes (Bianchi et al 1988). The dynamic characterisitcs of a bridge can be obtained by measuring its response to impact, traff ic or wind, or by attaching a vibrator and excit ing the structure over a range of frequencies.

To examine the effects of deterioration on the dynamic characteristics of individual structural members tests were carried out at TRRL on two 14 m post-tensioned concrete beams that had been deliberately damaged. One beam was cracked by loading above the decompression moment and the other had 50 per cent of its prestressing wires cut at both mid-span and at its eighth point. Mode shapes and frequencies were recorded. It was concluded that whi lst most of the significant frequencies and mode shapes of the beams were identified, at tempts to identify the damage were unsuccessful.

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- pulse echo m

Acous t i c emission

Acous t i c puls ing m

Fig.1 Summary of techniques using mechanical vibrations

Site measurements have been equally unsuccessful. Work carried out under contract to TRRL has shown that application of the technique is fraught wi th dif f iculty. Dynamic characteristics are sensit ive to changes in support conditions which may have little structural consequence. They are also sensitive to temperature which can affect the st i f fness of the surfacing, and changes in dead weight, for example due to resurfacing.

It must be emphasised that it is st i f fness and not strength that is being measured. Defects can occur which have little ef fect on st i f fness but cause a serious reduction in strength, for example, loss of prestress or Iocalised corrosion of the reinforcement.

2 . 2 PULSE M E T H O D S 2.2 .1 Sonic The simplest application of this method is to strike the structure and listen to the response. This is

commonly used for detecting delaminations in concrete caused by corrosion of the reinforcement. In the UK, hammer tapping is the most popular technique for generating sonic pulses, whereas in the USA and Canada considerable effort has gone into the development of alternative methods. The simplest is to drag a number of chains over the deck and listen to the sound made as they pass over delaminations (Manning and Holt 1983).

A more sophisticated development is an electronic device that responds to the audio frequencies generated in unsound concrete when struck with a small hammer operated by a relay (Milberger 1973). It gives reasonably accurate results on exposed concrete decks but results obtained on decks with asphalt surfacings are questionable as it is not possible to distinguish between delaminations in the concrete and debonding of the surfacing. None of these methods give the depth of the delamination (Ayyub and White 1987).

2

More recent ly an impact-echo method has been developed for detect ing delaminat ions (Sansalone and Carino 1989). A mechanical pulse is introduced into the concrete by a mechanical impact on the free surface. A receiver is mounted on the surface close to the impact point to monitor the arrival of reflected pulses. The f requency content of the ref lected pulses is used to determine both the presence and depth of delaminations. Tests have been carried out on exposed concrete and concrete under asphalt surfacing a l though to-date the technique has been confined to the laboratory.

2.2.2 Ultrasonics

The commonest appl icat ion of ul t rasonic test ing of concrete is measurement of ul t rasonic pulse veloci ty. This is used mainly for determinat ion of concrete un i fo rmi ty and strength, a l though it can also be used to locate defects and measure the depth of surface breaking cracks (British Standards Inst i tu t ion 1986).

Frequencies in the range 20 kHz to 50 kHz are normally used as above 50 kHz the pulses are rapidly at tenuated. A pulse is generated on the concrete surface and the t ime taken for it to reach a receiver posi t ioned elsewhere on the concrete enables its ve loc i ty to be calculated. The presence of reinforcing steel can introduce errors since the veloci ty of the pulses in steel is greater than in concrete (Figure 2). Wherever possible measurements should be made in such a way that steel does not lie parallel and close to the direct line between the transducers. Where th is cannot be avoided correct ions can be made to take account of the presence of the steel (British Standards Inst i tu t ion 1986).

Pulse veloc i ty is related to the elastic constants of the concrete and can be correlated w i th strength. However such correlat ions are dependent on the concrete under test and ideally cores should be taken. A l ternat ive ly variat ions in concrete qual i ty can be determined by taking a large number of measurements over the whole structure. The results can be presented either on a histogram or a contour diagram to enable areas of poorer qual i ty concrete to be identif ied. Tomset t (1980) has suggested that the coef f ic ient of var iat ion of such measurements can be used as a measure of concrete qual i ty. For readings taken over a whole structure he suggests that the coef f ic ient of variation should be between 6 per cent and 9 per cent.

Pulse veloc i ty measurements can also be used to detect large voids and measure the depth of surface breaking cracks (British Standards Inst i tut ion 1986, AbduI -Amir and AbduI-Karim 1987). If the t ransmi t ter and receiver are placed

on opposi te sides of a crack the t ime of f l igh t w i l l be longer than in sound concre te as the pulse has to propagate around the crack (Figure 2). In pract ice erroneous results are obta ined if the crack is crossed by re in forcement , is w h o l l y or par t ia l ly f i l led w i t h water , or opposi te sides are in con tac t , as th is wi l l a l low the pulse to propagate across it.

There are three t ypes of w a v e propagat ion in sol id bodies. These are long i tud ina l , shear and sur face waves . The techn iques descr ibed so far use longi tud inal waves . A method w h i c h ut i l ises surface waves has been deve loped for de tec t ing sur face breaking cracks (Smith 1984) . The techn ique is sens i t i ve to wa te r f i l led cracks. The tests were carried out on concre te submerged in wa te r but it could be developed for use in air.

Work is being carr ied out at K ing 's Col lege in London to develop an u l t rasonic techn ique to locate re in fo rcement in concre te (Anon 1 1988). This is based on the development of a transducer to initiate a short pulse of ultrasonic energy in the material. Signal processing algori thms are being developed to extract waves ref lected from reinforcement.

The techniques described above rely on waves being propagated in the concrete. An alternative approach is to propagate pulses in the reinforcement or prestressing tendons. Much higher frequencies can be used as the attenuat ion is less in steel than in concrete and this has the advantage that the pulses are more directional. TRRL has placed contacts w i th the Electrical Research Associat ion and City University to invest igate this technique for detect ing fractures in post-tensioning tendons. A transducer is coupled to the end of individual wires or strands and a pulse is injected. If there is loss of section or fracture of a wire, energy is ref lected and detected by the transducer. If the veloc i ty of the pulses is known, the t ime taken for them to travel to the defect and back again can be used to determine the distance of the defect from the anchorage (Figure 3). Unfortunately there is considerable loss of energy into the surrounding material, so the technique can only detect fractures occurring w i th in a f ew metres of the end of a tendon. Another disadvantage of the technique is that it requires access to the anchorage.

2 .2 .3 Acoustic emission Ultrasonic methods are used to gain in fo rmat ion about a material or s t ruc ture by in ject ing a pulse and recording the response. An a l te rnat ive approach is to mon i to r the s t ruc tu re for pulses generated w i t h i n it; they are k n o w n as acoust ic emissions and are produced as mater ia ls deform and part of the stored elast ic s t ra in energy is released as mechanica l or st ress waves . The mechan isms tha t g ive rise to acous t i c emiss ion

3

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include crack growth, plastic deformation and corrosion (Figure 4)•

The technique has been applied to concrete structures and it has been claimed that it could be used to estimate maximum loads applied to structures, and locate and monitor cracks (Uomoto 1987 , Ohtsu 1987) . Application to concrete bridges has been limited• An attempt was made to monitor crack growth, but it was found that it was difficult to distinguish acoustic emission due to crack growth from other sources of emission (Woodward 1983)• Further work is required to identify the mechanisms that give rise to acoustic emission in concrete•

The technique has also been used for detecting wire fractures in post-tensioned bridges (Laloux and Robert 1985)• A number of large events were detected but only one of these was subsequently confirmed as a wire fracture•

There is some evidence that acoustic emission can detect and locate corrosion of reinforcement (Weng et al 1982) . Several mechanisms have been suggested including microscopic cracking and the build up of corrosion products in the

concrete pores• No progress has been made developing this into a method for practical use on site•

2 . 2 . 4 A c o u s t i c p u l s i n g

This is the final var iant on the theme of ultrasonic testing. The idea is to use impressed pulses which are propagated b e t w e e n pairs of t ransducers to obtain an ultrasonic signature of the material through which the pulses have passed (Bartle 1983) • Changes in the signature can then be related to changes in the structure. To-date this technique has only been applied to steel, but once suitable procedures have been developed there is no reason w h y it could not be applied to concrete. Until trials have been carried out it is diff icult to determine whe ther it would be of value.

3 ELECTROMAGNETIC METHODS

The e lectromagnet ic spectrum is shown in Figure 5• It comprises wave lengths ranging from approximately 10 -14 m to thousands of metres• W a v e s with d i f ferent wave lengths interact with

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Fig.5 Elect romagnet ic spectrum

materials in dif ferent ways. A description of how different parts of the electromagnetic spectrum can be used to inspect concrete bridges is given below.

3.1 RADIOGRAPHY AND RADIOMETRY

X-rays and gamma-rays make up the high frequency end of the electromagnetic spectrum. The only difference is their origin. Gamma-rays are produced during radioactive decay of unstable nuclei whereas X-rays result from electronic transitions. This dif ference is reflected in the sources used for test ing concrete. Gamma-ray sources are radioactive isotopes such as Co 6° and Ir 192 whereas X-rays are produced by bombarding special targets wi th high energy electrons and can therefore be switched off. Gamma-ray sources are easily portable although care is required to ensure that they are adequately shielded. The high energies required to penetrate concrete mean that X-ray equipment is extremely heavy and di f f icul t to move. For this reason gamma-rays are the principal radioactive source for site use.

The attenuation of a beam of X-rays or gamma- rays of the same energy passing through a material increases wi th increasing density. Thus in a material of variable density such as concrete, more radiation is absorbed by the dense parts than the less dense parts. There are two techniques which use this property, namely radiography and radiometry. In conventional radiography the emerging radiation is detected by photographic emulsion and the variat ion in photographic density of the applied f i lm provides a measure of the radiation (Figure 6). In radiometry, variat ions in intensity are detected by radiation detectors such as Geiger or scinti l lat ion counters and this technique is used primari ly for measurement of density.

Gamma radiography can be used for locating and sizing reinforcement, and detecting poor quali ty or honeycombed concrete, voids and corrosion in ducts in post-tensioned concrete, but there are many l imitations to its use (British Standards Institution 1986). It is expensive to carry out. The maximum thickness of concrete that can be penetrated is only 0.5 m. Both sides of the member must be accessible. Long exposure t imes mean that it is usually only practicable to take a small number of radiographs, so only small areas can be examined. Finally ionising radiation presents a dangerous health hazard and str ingent safety precautions are required.

To try and overcome some of these l imitat ions, investigations have been made using high energy X-ray sources. The NDT Centre at Harwell has used a linear accelerator to examine a bridge in Hampshire (Pullen and Clayton 1981). French

engineers have examined some of their s t ructures using a portable Betatron (Zouboff 1973). The West Midlands Regional Of f ice of the Depar tment of Transport have used a linear accelerator to examine a concrete v iaduct (Department of Transport 19882). These X-ray sources have enabled th icknesses of concrete in excess of 1 m to be penetrated. However , the cost of the equipment, the d i f f i cu l ty of t ransport ing it to site and the str ingent safety precaut ions required mean that such invest igat ions are very expensive and are rarely carried out.

The invest igat ion carried out by the Depar tment of Transport (19882 ) was to determine whether the technique could be used to detect Iocalised corrosion of the re inforcement. It was found that it was not possible to ident i fy corrosion si tes w i th any degree of accuracy.

Engineers f rom the Blois Laboratoire Regionale des Ponts et Chaussees have developed a technique whereby the in tensi ty of radiat ion passing through the structure is displayed on a te levis ion screen. A conver ter is used to t ransform the gamma-ray f lux f rom the material under tes t into a luminous picture which can be analysed by a te lev is ion camera and displayed on a te levis ion screen (Dufay 1978).

A new approach in the USA is a computer ised tomographic sys tem which works on the same principles as the medical brain scanner. So far the technique has only been used on plain concrete in the laboratory, where a spat ial resolut ion of 1 mm in d iameter has been at tained (Morgan et al 1980).

Al l of the techniques described so far use the t ransmission of energy rather than ref lect ion and refract ion. However backscat ter techniques can be used. The amount of radiat ion scattered f rom a surface can be measured and related to densi ty. The main d rawback of th is method is tha t the mater ial near the surface has a great a f fec t on the results and t h i s leads to errors where there are densi ty gradients (Malhotra 1976).

Despite the advances that have been made, the use of ionising radiat ion for the inspect ion of concrete bridges is expensive, requires s t r ingent sa fe ty precaut ions and is rarely used.

3.2 VISUAL METHODS This covers only a very narrow part of the e lect romagnet ic spectrum but not surpr is ingly there are a w ide range of techniques avai lable.

3.2.1 Endoscopy Endoscopes can be used to inspect areas which are inaccessible to the naked eye. These

Area of d i f f e r e n t dens i ty fo r e x a m p l e , steel o r a i r vo id

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Fig.6 Radiography

instruments consist of a bundle of optical fibres through which light is transmitted to illuminate the area of interest. The image is then transmitted back to the eye either through a lens system or other fibres. Both rigid and flexible endoscopes are available and cameras can be attached to enable photographs to be taken.

Applications include inspecting behind bearings and looking in ducts in post-tensioned concrete. For the latter application 25 mm diameter holes are drilled into the ducts. If voids are found further measurements can then be made to determine their volume (Woodward 1981 ).

3 .2 .2 Optical fibres

Visual inspection is the best method for crack detection, but requires close access to the surface. A technique for the remote detection and monitoring of cracks has been developed by the National Maritime Institute (Hale K F 1984). Optical fibres, etched chemically to control the level of strain to failure, are bonded to the surface to be monitored. If a crack develops it causes the fibre to fracture and the intensity of light passing through it will be attenuated. Thus by periodically monitoring the intensity of light passing through a fibre it is possible to detect the onset of cracking. Tests on concrete have shown that crack widths of 20 to 30/~m can be detected.

Optical fibre gauges might be useful for the detection and monitoring of cracks in inaccessible areas, for example across joints in segmental structures. To function effectively the bond between the gauge and the concrete must not deteriorate. Site trials are required before such gauges could be used with confidence.

3 .2 .3 Moire photography This is one of several techniques ,which provide full-field views of surface displacement (Butch and Forno 1982). The surface of the structure is covered with a fine dot pattern, typically 25 dots per cm. For large components patterned paper is glued to the surface. The pattern is then photographed using a specially modified 35 mm camera. Photographs are made on the same film before and after the object is deformed. After the film has been processed the fine pattern on the image behaves as two orthogonal diffraction gratings. A beam of light directed on to the negative will generate two fringe patterns representing in-plane displacements in two orthogonal directions. The displacement sensitivity is typically 1:105 of the width of the field of view (ie 0.1 mm in 10 m) and the strain sensitivity is approximately 100 microstrain at intervals of one tenth of the width of the field of view.

3 . 2 . 4 Holography Holography has been used for detecting surface cracks and frost related damage in concrete (Luxmore 1973, Anon 2 1988). A hologram is produced by il luminating the concrete surface wi th light from a coherent light source and recording on a photographic plate both light reflected from the concrete and a reference beam derived from the same source. With a helium-neon light source a relative displacement of the surface of approximately 0.3/~m can be measured. Whilst the technique has produced good results in the laboratory, it is unsuitable for site use because of its sensit ivi ty to vibration and rigid body movements.

3 . 2 . 5 Deformation As a bridge deteriorates, there may be geometric changes that give warning of the onset of serious structural damage, for example, changes in displacements, slopes and deflections that are contrary to design assumptions (Dausend et al 1980). This type of monitoring is mainly applicable to medium and long-span bridges where the movements are likely to be large enough to be measured with suff icient accuracy. Optical techniques would appear to be the most suitable for detecting such movements, although problems are likely to arise from atmospheric refraction causing the optical axis to bend. Two techniques which are being investigated are described b e l o w : -

(i) Laser al ignment

The property of a laser to produce an intense narrow beam of light enables it to be used as an arbitrary straight line for alignment when used under the correct ambient conditions (Williams 1983). A 'three point system', in which a weak converging lens is placed in the path of the beam to focus it onto a detector has been developed by the National Physical Laboratory (Harrison et al 1972). For site use the lens is mounted on the structure and the laser and detector are positioned at reference points, for example on the supports. The centre of the beam passes through the middle of the lens, so deflection of the structure causes the image to move on the detector. Therefore, if the lens is mounted at mid-span, the movement of the image is equal to twice the deflection.

The National Physical Laboratory have carried out trials at the TRRL in which they showed that over a distance of 250 m, both the short-term spatial stabil i ty and the resolution of the image was better than 1 mm. An accuracy of better than 2 mm over a range of 150 m was achieved, wi th the lens positioned midway between the laser and detector.

9

(ii) Photogrammetry

Photogrammetry is a wel l established technique for mapping the ground surface and measuring the deformation of large structures (Cooper and Shortis 1980). The structure under test is photographed using precision 'metr ic ' cameras which put calibration markers on the fi lm so that coordinates of identifiable parts of the image can be measured. To assist with identif ication targets can be mounted on the structure. With suitable calibration an analysis can be carried out to relate the positions of targets on the image to their positions on the structure; thus, if the structure subsequently deforms the displacements of the targets can be determined• If two photographs are taken from different v iewpoints, it is possible to measure target displacement in three dimensions. Under ideal conditions in-plane displacements can be measured to an accuracy of 1 in 105 of the field of v iew (ie 0.1 mm in 10 m).

The disadvantages are that any deformations which occur whi lst a set of photographs is being taken, for example, due to temperature, wil l cause additional errors. The measurements are also sensit ive to ambient conditions and ideally need to be taken on a bright overcast day. Advantages of the method are the documentary value of the test pictures, the minimal disruption to traff ic, the relative speed and simplici ty of the measurements•

3.3 INFRA-RED 3.3.1 Thermography At ambient temperatures all materials emit infra- red radiation. This property can be used as a means of detect ing defects if a heat f low or

thermal gradient can be established in the material being examined, since the presence of a f law will cause local temperature disturbances (Figure 7). To establish an artificial heat f low through a concrete bridge deck would cause considerable practical problems and there would also be a risk of introducing high thermal stresses, with subsequent cracking of the structure.

Naturally occurring thermal gradients have been used for detecting delaminations caused by reinforcement corrosion. Temperature gradients produced under certain ambient conditions can give rise to differences in surface temperature of up to 3°C between sound and unsound concrete (Manning and Holt 1980, Ayyub and White 1987). An infra-red detection system, with a temperature sensit ivity of better than 0.2°C is used for the remote measurement of surface temperature• The camera is mounted on a vehicle at a height of 4 to 6 m above the ground, as this gives both an acceptable field of view and good definition of the delaminations. It has been claimed that 70 per cent to 95 per cent of delaminations can be detected at depths of 13 mm to 100 mm below the surface (Solomon 1987).

The technique was initially developed for use on unsurfaced bridge decks but subsequent work has shown that it can be used on asphalt covered decks but with reduced sensitivity (Manning and Holt 1983, Manning 19851).

3 . 3 . 2 Thermoelast ic stress analysis

Changes in surface stress may be obtained from measurements of the minute changes in temperature that occur when solids are subjected

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10

to adiabatic tension or compression. The temperature changes are very small; a temperature rise of 0.2°C to 0.3°C is usually all that can be measured before material failure, thus it is not practicable to measure this effect under static stress. However, SIRA Institute have developed equipment for detecting changes due to dynamic stresses and stress changes of the order of 1N/mm 2 (0.001 °C) can be distinguished in steel over an area of less than 1 mm 2 (Baker and Webber 1982).

Work at TRRL has shown that the technique can be applied to concrete, but the stress sensitivity is only of the order of 1.5 N/ram 2. This would be of limited value for concrete testing, although a significant improvement (2 or 3 times) might be possible through further optimisation of the test parameters (Woodward et al 1984).

3.4 RADAR High frequency radio waves were f irst used on concrete for measuring moisture content (Watson 1963). More recently low power ground penetrating radar, operating at a frequency of 900 MHz, has been used for detecting delaminations in concrete bridge decks (Clemena 1983).

A pulse of low power radio frequency energy, is directed into the bridge deck and echoes are received from boundaries between materials having different dielectric constants (Figure 8). Initial trials on surfaced decks were not very successful, only 51 per cent of the delaminations were detected and there were also some false indications. However, it was reported that the technique has considerable potential, if

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11

interpretation of the waveforms could be improved (Manning 19851). Advantages of the method are that it permits accurate measurement of the thickness of the asphalt surfacing and, except for the presence of moisture in the deck, the technique is independent of constraints imposed by the weather.

I t has been claimed that it could also be used to detect the depth of reinforcing bars, determine the thickness of members, and locate voids in slabs (Moore 1980). With current systems it is not possible to examine reinforcement in suff icient detail to determine the extent of corrosion damage. To achieve the required increase in resolution, at least a decade increase in frequency would be needed but the attenuation in concrete at such frequencies would be prohibitive.

3 .5 NUCLEAR MAGNETIC RESONANCE (NMR)

NMR is based on the absorption of radio frequency energy by certain atomic nuclei in a magnetic field. The relationship between the strength of the applied field and the frequency at which energy is absorbed is a unique characterist ic of the nucleus. Thus, for moisture the amplitude of the hydrogen NMR signal can be used to determine the amount of moisture in the material.

Equipment has been developed for making non- destruct ive measurements on bridge decks (Matzkanin and De Los Santos 1987). It can be adjusted to allow measurements to be made at selected depths down to 75 ram. Trials were carried out on decks under a variety of overlays including asphalt. It was concluded that the NMR system provided a reliable method for determining moisture content with an accuracy of approximately + / - 0 . 3 per cent moisture.

4. ELECTRICAL AND MAGNETIC M E T H O D S

These techniques relate to the previous section since any alternating magnetic or electric field generates electromagnetic radiation, but for convenience they wil l be treated separately.

4.1 ELECTRICAL METHODS Electrical methods are l imited to resistance and potential measurements, and some covermeters use eddy currents for detecting the depth of concrete cover. High frequency capacitance techniques for detecting moisture and polarisation techniques for measuring corrosion rates are not used on structures.

4 .1 .1 Resistivity Resistance measurements have been used for detecting leaks in waterproofing systems. The resistance is measured between a wetted sponge placed on the surface of the deck and the reinforcement. Measurements are repeated over a grid and areas of low resistance are taken to be indicative of failures in the membrane (Sepllman and Stratfull 1971 ).

Resistivity measurements have also been used for measuring the moisture content of concrete. Over the last few years the interest in this technique has increased as resistivity is one of the factors that controls the rate of corrosion of steel in concrete (Vassie 1980). Measurements are normally made using the four-electrode method. Four small holes are drilled into the concrete surface, and electrodes inserted and fixed in place to provide electrical contact with the concrete. An alternating current is passed between the outer electrodes and the potential difference between the inner electrodes is measured and used to calculate the resistivity of the concrete.

4 . 1 . 2 Electrode potential measurements The electrode potential of steel in concrete is an indicator of corrosion activity. It is not possible to measure this potential directly, but the potential difference between steel reinforcing bars in concrete and a standard reference electrode (half- cell) can be measured using a high impedance voltmeter. This is done by connecting one lead of the voltmeter to a reinforcing bar and the other to the reference electrode. A copper-copper sulphate half-cell is normally used because it is cheap, rugged and reliable. The potential difference at any point can be measured by moving the half-cell to that point and the results are normally presented as an electrode potential contour map of the concrete surface, such as that shown in Figure 9.

These measurements indicate the risk of corrosion but give no information about its rate. It is not recommended that this technique be used on its own. Preferably it should be used in parallel with two or three other methods, such as measurement of chloride ion concentration, depth of cover, depth of carbonation and concrete quality to give a more reliable indication of whether corrosion is occurring or is likely to occur (Vassie 1980).

4 . 1 . 3 Eddy currents Alternating currents produce eddy currents in nearby conductors and this phenomenon is the basis of some covermeters (British Standards Institution 1988). A search coil sets up eddy currents in the reinforcement which in turn change the impedance of the coil. The magnitude of the

12

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change is a function of the size and posit ion of the reinforcement. Work has been carried out under contract to TRRL to t ry and calibrate this method for detecting loss of section of reinforcement due to corrosion but there was insuff icient sensit iv i ty.

4.2 MAGNETIC METHODS 4.2.1 Magnetic induction A search head which consists of an inverted U- shaped magnet wi th a coil wound on each arm is placed on the concrete surface. An alternating potential is applied to one coil and the voltage induced in the other is affected by the presence of reinforcement. Some covermeters use this principle (Shirley 1971).

4.2.2 Magnetic flux exclusion The disturbance of a magnetic field in the v ic in i ty of f laws in prestressing steel forms the basis of a technique developed in the USA (Kusenberger and Barton 1981). A constant magnetic field is applied to the prestressed concrete beam under test and a Hall ef fect probe is then used to scan the surface of the beam parallel to the direction of the prestressing steel. An observed change in the field indicates a fracture or discontinuity in the steel. However, distinguishing between these defects and other causes of disturbance, such as pieces of

t ie wire embedded in concrete, presents problems. Tests on laboratory specimens have shown that f ractures in a 13 mm diameter strand can be detected at depths of 5 0 - 7 5 mm from the surface (Ghorbanpoor et al 1989) .

5 OTHER M E T H O D S

There are a number of techniques that do not f i t into any of the previous categor ies and these are described below.

5.1 SURFACE HARDNESS METHODS Numerous a t tempts have been made to relate the hardness of the concrete surface to its strength. The most popular technique for measur ing surface hardness is the Schmid t hammer. A plunger is held in contact w i th the concrete surface, a spring loaded mass str ikes the free end and rebounds and the extent of the rebound is measured. The measurements are inf luenced by many factors including surface f in ish and carbonat ion, and the accuracy is no bet ter than + / - 15 per cent (Malhotra 1984). Howeve r as a comparat ive tool the Schmidt hammer is useful for assessing the un i fo rmi ty of concrete and locat ing weak areas (British Standards Inst i tu t ion 1986).

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Other techniques include an indentat ion method and the Windsor Probe. In the former method, a known impact ing force is applied to a steel ball and the d iameter of the indentat ion produced in the surface of the concrete is measured. In the lat ter method, a steel probe is dr iven into the sur face of the concrete and the depth of penetrat ion measured. The accuracy of these methods is no better than that of the Schmidt hammer (Manning 19852).

Chaussees (Abdunur 1985). A 4 mm wide slot is cut into the concrete in 10 mm increments to a depth of 80 mm (the depth profile of the slot is a segment of a circle 150 mm in diameter). After each increment a specially designed f lat jack is inserted into the slot and the pressure required to restore the original strain distr ibution around the slot is measured. Field tests on the column of a bridge have yielded results claimed to be within 0 .2 N/ram 2 of the applied stress.

5 .2 PULL-OUT TESTS Several tests have been developed to measure the in-si tu strength of concrete by apply ing a direct tens i le force to an anchor locked into a hole dril led into the concrete. Var ious shaped anchors have been devised. The idea is to measure the force required to pull the embedded anchor out of the hole (Maihotra 1984). Pul l-off tes ts have also been used. These invo lve bonding a disc to the surface and measuring the force required to pull o f f a piece of concrete (Long and Murray 1981).

5.3 NUCLEAR METHODS The principal method is neutron backscat ter (Malhotra 1976) . The number of neutrons backscat tered f rom a concrete surface depends to a large extent on the amount of hydrogen present. Thus neutron backscat ter can be used to determine the moisture content of concrete. Isotropic neutron sources are used and commercia l equ ipment is avai lable. However , in pract ice this techn ique is more w ide ly used on soi ls than concrete.

Radiographic images of mater ia ls can be obtained using neutrons, protons and electrons. However , these techniques are conf ined to the laboratory and very l i t t le work has been done on concrete.

5 .4 M E A S U R E M E N T OF PRESTRESS 5.4.1 Stress relief Var ious techniques have been used to determine the level of prestress in concrete by measuring the re laxat ion in strain on and around cores dril led f rom the concrete (Lindsell and Buchner 1987). There are numerous sources of error. The surface concrete may di f fer f rom that w i th in the bulk of the mater ia l , there may be residual strains on the sur face, for example due to shr inkage, and it is d i f f i cu l t to relate strain to stress.

5.5 PERMEABILITY There are a number of tests which are used to obtain a measure of the permeabil i ty of concrete to water, oxygen and carbon dioxide. The most commonly used method for measuring absorption of water is the Initial Surface Absorption Test (ISAT) (British Standards Institution 1970). A cap w i th a minimum surface area of 5000 mm 2 is sealed to the concrete surface and fi l led wi th water and the rate at which water is absorbed into the concrete is measured.

An al ternat ive method was developed at the Building Research Establishment during the 1970's (Figg 1973). A small 5.5 mm diameter hole is dri l led into the concrete to a depth of 30 mm. The hole is then fi l led wi th water under a small head of 100 mm and the t ime taken for a known volume of water to be absorbed is measured. The air pressure in the hole can also be reduced and the rate at which the pressure increases gives a measure of permeabi l i ty to air. A number of techniques based on this principle have subsequently been developed (Concrete Society 1988).

A disadvantage of techniques for measuring permeabi l i ty to air and water is that they are affected by the init ial moisture content of the concrete and the weather during the test. To minimise these ef fects the British Standard for the ISAT test specif ies that it should be dry for at least 48 hours before the test.

An indication of the depth of carbon dioxide penetration into concrete can be determined by spraying phenolphthalein onto a freshly fractured surface. This changes colour at a pH of about 9, above 9 i t is pink, below 9 it is colourless. Carbon dioxide reduces the pH of concrete so the transit ion from colourless to pink gives an indication of the depth of penetration (Vassie 1980).

5.4.2 Slot cutting A technique tha t measures force d i rec t ly has been deve loped at the Laboratoire des Ponts et

6 DISCUSSION

The applications for which the techniques described can be used are summarised in Table 1.

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TABLE 1

Application of techniques

Application Techniques Section

Concrete strength

Concrete quality

Detection and monitoring of cracks

Location of reinforcement

Detection of reinforcement corrosion

Detection of corrosion in post-tensioned tendons

Detection of voids in concrete

Detection of voids in post-tensioning ducts

Measurement of residual stress

Measurement of moisture content

Structural surveillance

Ultrasonics Schmidt hammer Windsor probe Pull-out

Ultrasonics Schmidt hammer Radiometry Gamma-ray backscatter Permeability

Optical fibres Acoustic emission Moire photography Thermography Radar

Covermeters

Radiography Radar

Electrode potential Resistivity Magnetic flux exclusion Thermography Sonic

Endoscopy Radiography Ultrasonic pulse-echo

Radiography Ultrasonics Radar

Endoscopy Radiography

Stress relief Slot cutting

Resistivity Neutron backscatter Nuclear magnetic resonance

Dynamic response Acoustic emission Acoustic pulsing Deformation

2.2.2 5.1 5.1 5.2

2.2.2 5.1 3.1 3.1 5.5

3.2.2 2.2.3 3.2.3 3.3.1 3.4

4.1.3 4.2.1 3.1 3.4

4.1.2 4.1.1 4.2.2 3.3.1 2.2.1

3.2.1 3.1 2.2.2

3.1 2.2.2 3.4

3.2.1 3.1

5.4.1 5.4.2

4.1.1 5.3 3.5

2.1 2.2.3 2.2.4 3.2.5

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7 CONCLUSIONS

A wide range of techniques have been used to invest igate concrete bridges and others are under development. The information provided, although useful, still falls short of that required by the engineer. The heterogeneous nature of concrete prohibits the application of well established techniques that have been developed for homogeneous materials, for example, the use of ultrasonic methods for detecting f laws in metals. Site work ing imposes constraints on the development of new techniques. For example disruption to traff ic and damage to the structure caused by the investigation must be minimal, and public safety must not be endangered. In addition the equipment used must be tolerant of vibration, temperature and humidity variations, function in the damp and dirty condit ions usually found on site and, because of the large number of bridges involved, it must be cheap and simple to apply.

Despite the constraints listed above, there has been a large increase in the successful application of test ing techniques to concrete bridges over the last decade. Useful information has been obtained by applying a number of methods and collating the results to form an overall picture of the condit ion of the structure-.

Most of the techniques used test only a small part of the structure. The ideal method of monitoring bridges would be to periodically apply an overall surveil lance technique, to identify those structures which are in need of further investigation. Unfortunately, there are no suitable methods for doing this at present.

Visual inspection is likely to remain the mainstay of bridge inspection, but investigatory techniques will increasingly be used to supplement visual inspections. In addition they will continue to be used for special investigations on structures which are known to be in distress or where problems are suspected.

It is hoped that some of the new techniques described in this report will help meet the demand for more effect ive identif ication of defects.

8 ACKNOWLEDGEMENTS

The work described in this report was carried out in the Bridges Division of the Structures Group of TRRL.

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