Essais du b6ton Partie 204. Recommandations relatives g I usage des detecteurs 6lectromagn6tiques d enrobage d armature

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1 BS 1881 :Part204:1988 UDC : ) British Stanar Testing concrete Part 204. Recommenations on the use of electromagnetic covermeters Essais u b6ton Partie 204. Recommanations relatives g I usage es etecteurs 6lectromagn6tiques enrobage armature Priifverfahren ffir Beton Teil 204. Empfehlungen zur Anwenung elektromagnetischer Gerlte zur Messung er Betoniibereckung

2 BS 1881 : Part 204 : 1988 Forewor This Part of BS 1881 has been prepare uner the irection of the Cement, Gypsum, Aggregates an Quarry Proucts Stanars Committee. It supersees BS 4408 : Part 1 : 1989, which is withrawn. All aspects of testing concrete are being inclue as Parts of BS 1881, from sampling fresh concrete to assessing concrete in structures. Part 201 gives general guiance on the choice of non-estructive test methos an shoul be consulte for avice on methos which complement the measurement of cover to reinforcement or are useful as alternatives. Electromagnetic covermeters have now been in use long enough for experience to have accumulate of applications for a variety of types of structure an uner ifferent conitions. Development of other forms of covermeter is in progress but no significant fiel experience is yet available. Compliance with a British Stanar oes not of itself confer immunity from legal obligations. A range of suitable evices is commercially available. Since the capabilities of these vary, the choice of instru- ment may be governe by the particular test conitions an information require.

3 L 0 B8ll II Amenment No. 1 publishe an effective from 22 December 1989 to BS 1661 : Part 204 : 1968 Testing concrete Part 204. Recommenations on the use of electromagnetic covermeters AMD 6201 Correction AMD 6201 Duanber 1989 Clause 5. Apparatus Delete line 1 of paragraph 3 an substitute the following. With covermeters base on the first principle, alternating k-8 CA814 c

4 BS1881 :Part204:1988 Contents Forewor Committees responsible Page Insie front cover Back cover Recomme&ations 1 scope 2 Definitions 3 Applications 4 Basic principles of the metho 5 Apparatus 6 Basic calibration of the covermeter 7 Metho of use 8 Accuracy 9 Influence of test conitions 10 Report Appenix A Metho for etermining bar size an cover when neither is known Figures 1 Typical examples of cover to reinforcement 2 Measurements of cover to the bar for basic calibration of electromagnetic covermeters c c 1

5 BS 1881 : Part 204 : 1988 Recommenations 1 Scope This Part of BS 1881 gives recommenations on an escribes the principles of operation of electromagnetic evices that may be use for estimating the position, epth an size of reinforcement burie in concrete. It also escribes their methos of use an applications, the accuracy to be expecte an the factors which may influence the results. NOTE. The titles of the publications referre to in this stanar are liste on the insie back cover. 2 Definitions (a) Plain bar Inicate cover cm = cl For the purposes of this Part of 8S 1881 the efinitions given in BS 6100 : Part 6 apply, together with the following. 2.1 actual cover. Least istance, c,, between the face of the concrete an the surface of the reinforcement as shown in figure inicate cover. Distance, c,, between the face of the concrete an a notional surface of the reinforcing bar uner investigation as shown in figure 1. NOTE. Where values of cover are require for compliance with specifie cover to a bar of known cross section, the inicate cover has to be converte to actual cover by a metho such as that escribe in 7.2(a). 3 Applications Electromagnetic covermeters can be use for: (a) quality control, to ensure correct location an cover to reinforcing bars after concrete placement; (b) investigation of concrete members for which recors are not available or nee to be checke; (c) location of reinforcement as a preliminary to some other form of testing in which reinforcement shoul be avoie or its nature taken into account, e.g. extraction of cores, ultrasonic pulse velocity measurement or near-to-surface methos; () location of burie ferromagnetic objects other than reinforcement, e.g. water pipes, steel joists, lighting conuits. NOTE. Some evices may also etect non-ferromagnetic metal objects. Such use is outsie the scope of this Part an reference shoul be mae 10 manufacturers publications. (b) Ribbe bar Small eformations. Inicate cover cm = cz W Square twiste bar Large eformations. Inicate cover cm ki + c2 1 Figure 1. Typical examples of cover to reinforcement (see clause 2) 2

6 3 BS 1881 : Part204: 1988 L 4 Basic principles of the metho An electromagnetic fiel is generate by the search hea (see clause 5). When a reinforcing bar or other metal object lies within this fiel, the lines of force become istorte. The isturbance cause by the presence of the metal in turn prouces a local change in fiel strength as etecte by the search hea an inicate by the meter (see clause 5). Both the orientation an the proximity of the metal to the search hea affect the meter reaing. It is therefore possible to locate reinforcing bars an etermine their orientation. Cover to a bar may also be etermine if a suitable calibration can be obtaine for the particular size of bar an the materials uner investigation. With some evices it is possible, uner ieal conitions, to estimate both bar size an cover when neither is known. (See appenix A.) embee reinforcing bar. Such instruments are less sensitive to non-magnetic materials than those using the ey current principle. The meter shoul incorporate scales or igital isplay ranges (in this Part calle scales), calibrate in accorance with clause 6, so that the inicate cover to the reinforcement can be rea irectly. The manufacturer s instructions shoul state clearly the conitions uner which such scales are applicable. A reference scale, preferably linear, shoul also be provie for calibration by the user to suit particular conitions of use (see clauses 3 an 8). The inicate cover to steel reinforcement, when the evice is calibrate in the laboratory in the manner escribe in clause 6, shoul be accurate to within f 5 % or * 2 mm, whichever is the greater, over the working range given by the manufacturer. 6 Basic calibration of the covermeter v: * L 5 Apparatus A number of suitable battery or mains operate covermeters exist. They comprise a search hea, meter an interconnecting cable. The concrete surface is scanne with the search hea kept in contact with it while the meter inicates by analogue or igital means the proximity of reinforcement. The search hea may consist of a single or multiple coil system. The physical principle involve can either be one utilizing ey current effects or magnetic inuction effects. With covermeters using magnetic inuction, a multicoil currents in the search coil set up ey currents in the reinforcement which in turn cause a change in the measure impeance of the search coil. instruments working on this principle operate at frequencies above 1 khz an are thus sensitive to the presence of any conucting metal in the vicinity of the search hea. Location of non-magnetic but conucting material is outsie the scope of this Part of BS However, such instruments, being very epenent upon the conuctivity of the material, may be very susceptible to changes in steel type (see 9.2.1) an special calibration may be neee. With covermeters using magnetic inuction, a multicoil search hea is use with a lower operating frequency than the ey current type of evice (typically below 99 Hz). The principle use is similar to that of a transformer, in that one or two coils carry the riving current while one or two further coils pick up the voltage transferre via the magnetic circuit forme by the search hea an 6.1 General Regular laboratory checks on the covermeter shoul be carrie out to establish the accuracy of calibrate scale reaings. The frequency of such checks will epen on the manufacturer s instructions an the conition of use of the covermeter, but it is recommene that they be carrie out at least every 6 months. The ates of calibration shoul be recore an kept with the covermeter. The calibration shoul show that all meter measurements taken lie within the accuracy state in clause 5. Instruments that o not comply shoul be returne to the manufacturer for ajustment. Most covermeters in use are battery operate but, if mains operation is also available as an option, calibration accuracy shoul be checke uner each type of power supply. If ifferent search heas are to be use with the same meter, calibration checks shoul be carrie out for each hea. In some analogue instruments, a single calibrate scale is ivie into sectors, each sector ientifying a given inicate cover for a state range of bar sizes. In such a case, a bar of the largest state size at a given actual cover shoul prouce a scale inication at the en of the relevant sector nearer to zero cover inication. Similarly, a bar of the smallest state size will prouce a scale inication at the en of the sector further from zero cover inication. Such sectors shoul not overlap. The methos escribe in an 6.4 may be use in the laboratory. Suitable methos for routine site calibration checks are given in clause 7. c

7 4 8s 1881 : Part 204 : Metho A A straight, clean, smooth steel bar of the type esignate by the manufacturer of the evice is cast off-centre in a concrete prism, in such a position that the range of actual covers from the as-cast faces extens through the manufacturer s state range of operation of the evice. The minimum cover is 12 mm (see figure 2). For checking smaller covers irectly use metho I3 or C. The flatness of the moul faces shoul be such that no part of any cast surface of the prism eviates by more than 0.5 mm from the mean surface. The concrete shoul have an orinary Portlan cement content between 300 kg/m an 400 kg/m3 an the aggregate shoul not have significant magnetic properties. No aitions shoul be use in the concrete. Care shoul be taken uring casting not to ben the bar. After curing an emouling the specimen, the actual cover to the bar is measure to an accuracy of _+ 0.5 mm from the working faces at both ens of the prism. If the two measurements from one face to the bar o not iffer by more than 1 mm, their mean is taken as the actual cover. If the ifference excees 1 mm, a new prism shoul be cast. The covermeter is then use in accorance with the manufacturer s instructions to measure the inicate cover to the reinforcement on all moule faces parallel to the bar, within the working range of the evice an uner conitions where extraneous effects on the magnetic fiel are at a minimum. If separate scales are available for ifferent bar sizes it is avisable to carry out the preceing calibration proceure using a range of calibration blocks incorporating bars of representative sizes. In each case the range of actual covers to the bar from the four faces shoul exten over the manufacturer s state working range of the evice, as inicate on the respective scale. Bar projecting not less than,100 mm at both ens Steel Trowelle face Steel +- As-cast faces of concrete prism not less than 50 mm longer than the search hea Figure 2. Measurements of cuver to the bar for basic calibration of electromagnetic covermews (see 6.2)

8 BS 1881 : Part 204 : 1988 L c L 6.3 Metho6 A bar as escribe in metho A is move across a table towars the search hea an the actual cover compare with the reaing on the appropriate calibrate meter scale. It is essential that no metallic materials, e.g. nails or screws, are incorporate in the table top near the search coil. The search hea shoul also be parallel to the bar an both search hea an bar stationary when reaings are mae. The tolerances shoul not excee those for metho A. 6.4 Metho C A series of vertical holes is rille in two opposite faces of a box such that a bar, as escribe in metho A, can be place horizontally at ifferent istances from the top. The search hea is place over the line of holes an the actual covers compare with the reaings on the appropriate calibrate meter scale. It is essential that no metallic materials, e.g. nails or screws, are incorporate in the table top near the search coil or in the box. The search hea shoul also be parallel to the bar an both search hea an bar stationary when reaings are mae. The tolerances shoul not excee those for metho A. 7 Methouse 7.1 Preliminary proceures The covermeter is switche on an the meter ajuste so that the neele on the inicator ial (analogue evices) correspons to the appropriate calibration mark as inicate by the manufacturer ( zeroing the instrument). In the case of igital inicating evices, the manufacturer s instructions on setting up the meter before the test shoul be followe. In all cases this proceure shoul be carrie out with the search hea far remove from the reinforce concrete surface an in such a way that any other extraneous effects on the magnetic fiel are at a minimum. Rapi movement of the search hea shoul be avoie as this can affect the zeroing. A warming up perio may be specifie by the manufacturer after which time a further ajustment shoul be mae. In any case, measurements shoul not be taken before the zero reaing is stable. During the perio of operation, further frequent zero checks shoul be mae. For battery operate covermeters, an initial check on the state of charge of the battery shoul be mae followe by further frequent checks uring the perio of operation. The search hea is then scanne over the surface of the concrete to be examine for the presence of reinforcement. If reinforcement exists below the surface an within the working range of the covermeter, this will be inicate by the meter. 7.2 Works or site calibration It is strongly recommene that calibration checks be carrie out on site using either of the methos escribe in 6.3 or 6.4 an the relevant calibrate scale(s). This is particularly important when the bar type is ifferent from that use for the laboratory calibration. Where this site calibration check is unsatisfactory or where bars are present of size outsie the range of specific scales on the meter, or it is suspecte that the concrete in the structure will significantly affect the results, calibration by either of the following methos will be necessary. (a) A series of test holes is rille to the bars at positions relating to the full range of covers in the structure as inicate by the meter. Care shoul be taken not to amage the reinforcement. The istance between bar an concrete surface is then measure at each point. A measurement is also mae with the covermeter at each position using the linear reference scale an a calibration curve constructe. In situ covers are then calculate using the reference scale reaings an this calibration curve. (b) The calibration proceures escribe in clause 6 are aopte using bars of the known type an size an concrete of the same type as that uner investigation. A calibration curve is then constructe using the linear reference scale. Proceure (a) may be more applicable to on site investigative work whilst proceure (b) woul be more likely to be suite to manufacturing processes such as those in precast concrete works. It may be possible to take avantage of projecting reinforcement to check the performance of the covermeter. 7.3 Examination of the concrete The search hea is traverse systematically across the concrete an, where reinforcement is locate, rotate until a position of maximum isturbance of the electromagnetic fiel is inicate by the meter. In such a position, uner ieal conitions, the inicate cover to the nearest piece of reinforcement may be rea if the bar size is known (see clause 9 for factors affecting reaings). Further, the axis of the reinforcement will then lie in the plane containing the centre line through the poles of the search hea. Where reinforcement is not too congeste, it is possible to map out all bars within the area uner examination which lie sufficiently close to the surface. It may also be possible to etermine positions of laps. If the bar size is known, the cover can be measure. If the cover is known, the bar size can be estimate. It cannot be too strongly emphasize that, for maximum accuracy, interference from other reinforcement or magnetic material has to be avoie. The amount an accuracy of information obtaine will epen on the influencing factors given in clause 9 an the experience of the operator. 5

9 8S1881 :Part204: Aitional proceures 9.2 Steel Where accurate measurement of concrete cover to reinforcement is require in the range 0 to 20 mm the following proceure is recommene. A 20 mm thick spacer of material inert to the covermeter, such as woo or plastics, is place between the search hea an the concrete surface. An apparent cover for the particular bar size is rea irectly from the meter an the inicate cover obtaine by subtracting 20 mm from this value. This metho is only suitable where the concrete surface is flat an smooth (see clause 9). Where special probes are available for investigating shallow or closely space bars with increase accuracy, the manufacturer s instructions shoul be followe. Depening on the evice use an the reinforcement configuration, other techniques may be applie to extract a greater amount of information, such as the irection of reinforcement or both size an cover when neither is known. One such technique is given in appenix A. Another, referre to as the orthogonal technique, has been escribe by Das Gupta an Tam, an Tam et al (see publications referre to, insie back cover). The instrument manufacturer s instructions may refer to further techniques. It shoul be stresse, however, that only an experience operator shoul use any of these methos. 8 Accuracy Uner the most favourable site conitions, inicate cover can be measure to an accuracy approaching that obtainable in the laboratory when the bar size is known. When the bar size is not known an a proceure such as escribe in appenix A is aopte, this level of accuracy may be reuce. The experience operator may be able to counter the effects of some of the sources of inaccuracy by selecting the most appropriate combination of proceures. While the accuracy of the covermeter shoul be within the limits given in clause 5, the accuracy of measurement likely to be obtaine on the average site is within f 15 % or + 5 mm, whichever is the greater, for reinforcement at covers of less than 100 mm. Where influences occur of the type escribe in clause 9, accuracy of measurements of cover an bar size may be significantly less although the accuracy with which reinforcement can be locate may not be reuce. The level of accuracy in such cases may be improve by making calibration test rillings. 9 Influence of test conitions Type. Calibrate meter scales are generally vali for a particular type of steel (see manufacturer s instructions). The effect of ifferent types of steel on the reaings obtaine is generally small but in special cases, such as high tensile prestressing bars, it may inclue errors as high as f 5 % or more. Where such materials are present, the calibration proceures escribe in clause 7 shoul be aopte Cross section. Calibration curves or scales erive for smooth bars can generally be use for ribbe bars. It shoul be note that the minimum thickness of concrete between the bar an the surface will be the inicate wver less the height of the rib. The inicate wver in this Part is as efine in 2.2 an illustrate in figure 1. Where twiste section bars are present, significant errors may arise unless one of the calibration proceures escribe in clause 7 is aopte (see figure 1 (cl) Shape an orientation. For accurate measurement of wver an size, the bar has to be both straight an parallel to the concrete surface Multiple bars. Closely space bars may have a significant effect on the measurements mae. The manufacturer s rewmmenations shoul be followe in the wse of closely packe perpenicular or parallel bar assemblies. It is essential that measurements uner these conitions are carrie out by experience operators. In the case of parallel bars, the cover at which the reinforcement etail bewmes significant epens on several factors incluing the sensitivity of the eviw an the search hea imensions. As a general rule the accuracy of inicate wver measurements will be influence whenever there is more than one bar within the etection range of the search hea. As the istance between parallel bars ecreases a point is reache at which it is impossible to locate iniviual bars. Special probes may be available to improve the accuracy of inicate wver measurements an ifferentiation between iniviual bars in such cases. In the case of bunle or lappe bars, it is rewmmene that separate calibration is carrie out as escribe in clause Tie wire. Tie wire, particularly where close to the surface, can lea to misleaingly low wver reaings for the main reinforcement. However, an experience operator will be able to istinguish between the lowliza effects of wire, nails, etc., superimpose on the major reinforcement effects. 9.1 General Potential loss of accuracy (which may be minimize by the experience operator) is ue to many extraneous factors affecting the magnetic fiel within the range of the covermeter an to other physical influences. 9.3 Concrete Aggregate. Significant inaccuracies in inicate cover may arise when aggregates with magnetic properties are incorporate in the mix. Similarly, certain specialize floor finishes can rener measurements inaccurate though it may still be possible for iniviual bars to be locate.

10 7 BS1881 :Part204:1988 In some wses calibrations carrie out as escribe in clause 7 may assist but results from such methos shoul be treate with caution. The presence of magnetic materials may be etecte by placing the search hea on the concrete at such a position as to be out of the range of influence of the nearest reinforcing bar an noting a reaing on the meter /Warfix. Variations in the magnetic properties of the cement an of any aitions are likely to affect measurements of cover. The comments in with regar to wlibration similarly apply Surface finish. Where rough or unulating surfaces occur, e.g. expose aggregate finishes, the effect on inicate wver will be similar in magnitue to the surface irregularities within the area of the search hea. 9.4 Temperature Some search heas are sensitive to changes in temperature that may be cause by the operator s han. Frequent zero ajustments shoul ba mae an the manufacturer s recommenations shoul be followe in these cases. 9.5 Extraneous effects Interference effects will occur in the neighbourhoo of metallic structures of significant size such as winow fixings, scaffoling an steel pipes, especially when they are immeiately behin the search hea. The egree of influence will epen on the particular wvermeter use but all are affecte by either stray magnetic fiels or electric fiels or both. In such cases reliable use of the instrument may be severely restricte. 9.6 Corrosion of reinforcement Where significant corrosion to reinforcement has occurre, in particular, scaling an migration of corrosion proucts, misleaing inicate wver reaings are likely to be obtaine. 10 Report 10.1 General The report shoul state that the methos use are in accorance with those escribe in this Part of BS Where other specialize techniques are aopte, these shoul be clearly escribe Information to be inue in the test report The test report shoul inclue the following information: (a) ate, time an place of test; (b) escription of the structure or component uner investigation; (c) locations of test areas; () etails of the concrete in the test areas, with reference to possible influencing factors (see clause 9); (e) make an type of wvermeter use an ate of last laboratory calibration; (f) etails of any site calibration proceures; (g) measure values of inicate wver an/or bar size. Where these values are erive by calculation, this choul be state; (h) theestimate accuracy of quantitative measurements; (i) configuration of steel reinforcement, if require, incluing iniviual bar spacings. Sketches may assist here.

11 8 BS1881 : Part204:1988 Appenix Appenix A. Metho for etermining bar size an cover when neither is known This metho can be use with meters having separate calibrate scales for ifferent bar sizes. The range of wvers over which the metho is applicable will epen on the equipment being use. Measurement of apparent wver to the bar is mae using the metho escribe in clause 6, the measurements being recore, however, assuming each of the calibrate scale bar sizes in turn, Xi. This proceure is repeate after inserting a non-metallic spacer of known thickness,, between the search hea an concrete face, yi. The first reaing is subtracte from the secon for each scale (Dr = yr - Xi) an the true bar size taken to be that on the particular calibrate scale for which the conition Di = is approache. A graphical technique may be helpful in increasing the accuracy of this metho. Once the bar size has been establishe, the inicate wver can be rea off from the respective scale. Uner ieal conitions, an accuracy of better than 20 % can be achieve for both bar size an inicate wver using this proceure. The experience operator will, by use of ifferent thickness spacers an repeate measurements, often be able to improve on this level of accuracy. However, the effects mentione in clause 9 may be particularly important here. The metho is unsuitable in the case of closely packe bar assemblies.

12 Publications referre to ES 1661 BS 6100 Tasting concrete Part 201 Guie to the use of non-estructive methos of test for harene concrete Glossary of builing an civil engineering terms Part 6 Concrete an plaster DAS GUPTA, N.C., TAM, C.T. Non estructive technique for simultaneous etection of size an cover of embee reinforcement. In: British Journal of Non-estructive testing, November 1963 TAM, C.T., LAI, L.N., LAM, P.W. Orthogonal etection technique for etermination of size an cover of embee reinforcement. In: Journal Institute Engineers Malaysia, 22, c

13 BS 1881 : Part 204 : 1988 This British Stanar, having been prepare uner the irection of the Cement, Gypsum, Aggregates an Quarry Proucts Stanars Committee. was publishe uner the authority of the Boar of BSI an comes into effect on 31 August British Stanars Institution, 1988 ISBN X The following BSI references relate to the work on this stanar: Committee reference CAB/4 Draft for comment 86/l 1764 DC British Stanars Institution. incorporate by Royal Charter, BSI is the Inepenent national boy for the preparation of British Stanars. It is the UK member of the International Organization for Stanarization an UK sponsor of the British National Committee of the International Electrotechnical Commission. In aition to the preparation an promulgation of stanars, BSI offers specialist services incluing the provision of information through the BSI Library an Stanarline Database; Technical Help to Exporters; an other services. Avice can be obtaine from the Enquiry Section, BSI. Milton Keynes MK14 6LE, telephone , telex Copyright. Users of Brnish Stanars are remine that copyright subsists in all BSI publications. No part of this publication may be reprouce in any form without the prior permission in writing of BSI. This oes not preclue the free use, in the course of implementing the stanar, of necessary etails such as symbols an size, type or grae esignations. Enquiries shoul be aresse to the Publications Manager, BSI. Linfor Woo, Milton Keynes MK14 6LE. The number for telephone enquiries is O9O an for telex Contract requirements. A British Stanar oes not purport to inclue all the necessary provisions of a contract. Users of British Stanars are responsible for their correct application. Revision of British Stanars. Britrsh Stanars are revise, when necessary, by the issue either of amenments or of revise eitions. It is important that users of British Stanars shwl ascartain that they are in possession of the latest amenments or eitions. Automatic upating service. BSI provies an economic, iniviual an automatic stanars upating service calle PLUS. Details are available from BSI Enquiry Section at Milton Keynes, telephone 0! , telex Information on all BSI publications is in the BSI Catalogue. supplemente each month by BSI News which is available to subscribing members of BSI an gives etails of new publications, revisions, amenments an withrawn stanars. Any person who, when making use of a British Stanar, encounters an inaccuracy or ambiguity, is requeste to notify BSI without elay in orer that the matter may be investigate an appropriate action taken. Committees responsible for this British Stanar The preparation of this British Stanar was entruste by the Cement, Gypsum, Aggregates an Quarry Proucts Stanars Committee (CAB/-) to Technical Committee CABN. upon which the following boies were represente: Association of Lightweight Aggregate Manufacturers Association of Metropolitan Authorities British Aggregate Construction Materials Inustries British Civil Engineering Test Equipment Manufacturers Association British Precast Concrete Feeration British Reay Mixe Concrete Association Builing Employers Confeeration Cement Amixtures Association Cement an Concrete Association Cement Makers Feeration Cementitious Slag Makers Association Concrete Society County Surveyors Society Department of the Environment (Builing Research Establishment) Department of the Environment (Property Services Agency) Department of Transport (Highways) Department of Transport (Transport an Roa Research Laboratory) Electricity Supply Inustry in Englan an Wales Feeration of Civil Engineering Contractors Institute of Concrete Technology Institution of Civil Engineers institution of Highways an Transportation Institution of Structural Engineers Institution of Water an Environmental Management Royal Institution of Chartere Surveyors San an Gravel Association Society of Chemical Inustry The following boies were also represente in the rafting of the stanar, through subcommittees an panels: British Nuclear Fuels Limite Unite Kingom Atomic Energy Authority i Amenments issue since publication Am. No. Date of issue &ie Text affecte a British Stanars Institution. 2 Park Street Lonon WlA 2BS. Telephone Telex CAB14