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    Contractor Report 156


    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


    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.








    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


    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




    6 6 7 7 7 8 8 8

    8 8 8 9 9


    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


    7. References




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

    - 2 -

  • 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

    - 3 -

  • 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.

    - 4 -

  • 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


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