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1、BRITISH STANDARD BS 1881-204: 1988 Incorporating Amendment No. 1 Testing concrete Part 204: Recommendations on the use of electromagnetic covermeters UDC 666.972:620.179.1 + 620.179.14 BS 1881-204:1988 This British Standard, having been prepared under the direction of the Cement, Gypsum, Aggregates
2、and Quarry Products Standards Committee, was published under the authority of the Board of BSI and comes into effect on 31 August 1988 BSI 12-1998 The following BSI references relate to the work on this standard: Committee reference CAB/4 Draft for comment 86/11764 DC ISBN 0 580 16571 X Committees r
3、esponsible for this British Standard The preparation of this British Standard was entrusted by the Cement, Gypsum, Aggregates and Quarry Products Standards Committee (CAB/-) to Technical Committee CAB/4, upon which the following bodies were represented: Association of Lightweight Aggregate Manufactu
4、rers Association of Metropolitan Authorities British Aggregate Construction Materials Industries British Civil Engineering Test Equipment Manufacturers Association British Precast Concrete Federation British Ready Mixed Concrete Association Building Employers Confederation Cement Admixtures Associat
5、ion Cement and Concrete Association Cement Makers Federation Cementitious Slag Makers Association Concrete Society County Surveyors Society Department of the Environment (Building Research Establishment) Department of the Environment (Property Services Agency) Department of Transport (Highways) Depa
6、rtment of Transport (Transport and Road Research Laboratory) Electricity Supply Industry in England and Wales Federation of Civil Engineering Contractors Institute of Concrete Technology Institution of Civil Engineers Institution of Highways and Transportation Institution of Structural Engineers Ins
7、titution of Water and Environmental Management Royal Institution of Chartered Surveyors Sand and Gravel Association Society of Chemical Industry The following bodies were also represented in the drafting of the standard, through subcommittees and panels: British Nuclear Fuels Limited United Kingdom
8、Atomic Energy Authority Amendments issued since publication Amd. No.Date of issueComments 6201December 1989 Indicated by a sideline in the margin BS 1881-204:1988 BSI 12-1998i Contents Page Committees responsibleInside front cover Forewordii 1Scope1 2Definitions1 3Applications1 4Basic principles of
9、the method1 5Apparatus2 6Basic calibration of the covermeter2 7Method of use4 8Accuracy5 9Influence of test conditions5 10Report6 Appendix A Method for determining bar size and cover when neither is known7 Figure 1 Typical examples of cover to reinforcement1 Figure 2 Measurements of cover to the bar
10、 for basic calibration of electromagnetic covermeters3 Publications referred to Inside back cover BS 1881-204:1988 ii BSI 12-1998 Foreword This Part of BS 1881 has been prepared under the direction of the Cement, Gypsum, Aggregates and Quarry Products Standards Committee. It supersedes BS 4408-1:196
11、9, which is withdrawn. All aspects of testing concrete are being included as Parts of BS 1881, from sampling fresh concrete to assessing concrete in structures. Part 201 gives general guidance on the choice of non-destructive test methods and should be consulted for advice on methods which complemen
12、t the measurement of cover to reinforcement or are useful as alternatives. Electromagnetic covermeters have now been in use long enough for experience to have accumulated of applications for a variety of types of structure and under different conditions. Development of other forms of covermeter is i
13、n progress but no significant field experience is yet available. A range of suitable devices is commercially available. Since the capabilities of these vary, the choice of instrument may be governed by the particular test conditions and information required. A British Standard does not purport to in
14、clude all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pag
15、es i and ii, pages 1 to 8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. BS 1881-204:1988 BSI 12-19981 1 Scope This Part of BS 1881 gives re
16、commendations on and describes the principles of operation of electromagnetic devices that may be used for estimating the position, depth and size of reinforcement buried in concrete. It also describes their methods of use and applications, the accuracy to be expected and the factors which may influ
17、ence the results. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS 1881 the definitions given in BS 6100-6 apply, together with the following. 2.1 actual cover least distance, c1, between the face o
18、f the concrete and the surface of the reinforcement as shown in Figure 1 2.2 indicated cover distance, cm, between the face of the concrete and a notional surface of the reinforcing bar under investigation as shown in Figure 1 NOTEWhere values of cover are required for compliance with specified cove
19、r to a bar of known cross section, the indicated cover has to be converted to actual cover by a method such as that described in 7.2 a). 3 Applications Electromagnetic covermeters can be used for: a) quality control, to ensure correct location and cover to reinforcing bars after concrete placement;
20、b) investigation of concrete members for which records are not available or need to be checked; c) location of reinforcement as a preliminary to some other form of testing in which reinforcement should be avoided or its nature taken into account, e.g. extraction of cores, ultrasonic pulse velocity m
21、easurement or “near-to-surface” methods; d) location of buried ferromagnetic objects other than reinforcement, e.g. water pipes, steel joists, lighting conduits. NOTESome devices may also detect non-ferromagnetic metal objects. Such use is outside the scope of this Part and reference should be made
22、to manufacturers publications. 4 Basic principles of the method An electromagnetic field is generated by the search head (see clause 5). When a reinforcing bar or other metal object lies within this field, the lines of force become distorted. The disturbance caused by the presence of the metal in tu
23、rn produces a local change in field strength as detected by the search head and indicated by the meter (see clause 5). Figure 1 Typical examples of cover to reinforcement (see clause 2) BS 1881-204:1988 2 BSI 12-1998 Both the orientation and the proximity of the metal to the search head affect the m
24、eter reading. It is therefore possible to locate reinforcing bars and determine their orientation. Cover to a bar may also be determined if a suitable calibration can be obtained for the particular size of bar and the materials under investigation. With some devices it is possible, under ideal condi
25、tions, to estimate both bar size and cover when neither is known. (See appendix A.) 5 Apparatus A number of suitable battery or mains operated covermeters exist. They comprise a search head, meter and interconnecting cable. The concrete surface is scanned with the search head kept in contact with it
26、 while the meter indicates by analogue or digital means the proximity of reinforcement. The search head may consist of a single or multiple coil system. The physical principle involved can either be one utilizing eddy current effects or magnetic induction effects. With covermeters based on the first
27、 principle, alternating. Instruments working on this principle operate at frequencies above 1 kHz and are thus sensitive to the presence of any conducting metal in the vicinity of the search head. Location of non-magnetic but conducting material is outside the scope of this Part of BS 1881. However,
28、 such instruments, being very dependent upon the conductivity of the material, may be very susceptible to changes in steel type (see 9.2.1) and special calibration may be needed. With covermeters using magnetic induction, a multicoil search head is used with a lower operating frequency than the eddy
29、 current type of device (typically below 90 Hz). The principle used is similar to that of a transformer, in that one or two coils carry the driving current while one or two further coils pick up the voltage transferred via the magnetic circuit formed by the search head and embedded reinforcing bar.
30、Such instruments are less sensitive to non-magnetic materials than those using the eddy current principle. The meter should incorporate scales or digital display ranges (in this Part called scales), calibrated in accordance with clause 6, so that the indicated cover to the reinforcement can be read
31、directly. The manufacturers instructions should state clearly the conditions under which such scales are applicable. A reference scale, preferably linear, should also be provided for calibration by the user to suit particular conditions of use (see clauses 3 and 8). The indicated cover to steel rein
32、forcement, when the device is calibrated in the laboratory in the manner described in clause 6, should be accurate to within 5 % or 2 mm, whichever is the greater, over the working range given by the manufacturer. 6 Basic calibration of the covermeter 6.1 General Regular laboratory checks on the cov
33、ermeter should be carried out to establish the accuracy of calibrated scale readings. The frequency of such checks will depend on the manufacturers instructions and the condition of use of the covermeter, but it is recommended that they be carried out at least every 6 months. The dates of calibratio
34、n should be recorded and kept with the covermeter. The calibration should show that all meter measurements taken lie within the accuracy stated in clause 5. Instruments that do not comply should be returned to the manufacturer for adjustment. Most covermeters in use are battery operated but, if main
35、s operation is also available as an option, calibration accuracy should be checked under each type of power supply. If different search heads are to be used with the same meter, calibration checks should be carried out for each head. In some analogue instruments, a single calibrated scale is divided
36、 into sectors, each sector identifying a given indicated cover for a stated range of bar sizes. In such a case, a bar of the largest stated size at a given actual cover should produce a scale indication at the end of the relevant sector nearer to zero cover indication. Similarly, a bar of the smalle
37、st stated size will produce a scale indication at the end of the sector further from zero cover indication. Such sectors should not overlap. The methods described in 6.2, 6.3 and 6.4 may be used in the laboratory. Suitable methods for routine site calibration checks are given in clause 7. 6.2 Method
38、 A A straight, clean, smooth steel bar of the type designated by the manufacturer of the device is cast off-centre in a concrete prism, in such a position that the range of actual covers from the as-cast faces extends through the manufacturers stated range of operation of the device. The minimum cov
39、er is 12 mm (see Figure 2). For checking smaller covers directly use method B or C. The flatness of the mould faces should be such that no part of any cast surface of the prism deviates by more than 0.5 mm from the mean surface. BS 1881-204:1988 BSI 12-19983 The concrete should have an ordinary Port
40、land cement content between 300 kg/m3 and 400 kg/m3 and the aggregate should not have significant magnetic properties. No additions should be used in the concrete. Care should be taken during casting not to bend the bar. After curing and demoulding the specimen, the actual cover to the bar is measur
41、ed to an accuracy of 0.5 mm from the working faces at both ends of the prism. If the two measurements from one face to the bar do not differ by more than 1 mm, their mean is taken as the actual cover. If the difference exceeds 1 mm, a new prism should be cast. The covermeter is then used in accordan
42、ce with the manufacturers instructions to measure the indicated cover to the reinforcement on all moulded faces parallel to the bar, within the working range of the device and under conditions where extraneous effects on the magnetic field are at a minimum. If separate scales are available for diffe
43、rent bar sizes it is advisable to carry out the preceding calibration procedure 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 should extend over the manufacturers stated working range of the devi
44、ce, as indicated on the respective scale. 6.3 Method B A bar as described in method A is moved across a table towards the search head and the actual cover compared with the reading on the appropriate calibrated meter scale. It is essential that no metallic materials, e.g. nails or screws, are incorp
45、orated in the table top near the search coil. The search head should also be parallel to the bar and both search head and bar stationary when readings are made. The tolerances should not exceed those for method A. 6.4 Method C A series of vertical holes is drilled in two opposite faces of a box such
46、 that a bar, as described in method A, can be placed horizontally at different distances from the top. The search head is placed over the line of holes and the actual covers compared with the readings on the appropriate calibrated meter scale. It is essential that no metallic materials, e.g. nails o
47、r screws, are incorporated in the table top near the search coil or in the box. The search head should also be parallel to the bar and both search head and bar stationary when readings are made. The tolerances should not exceed those for method A. Figure 2 Measurements of cover to the bar for basic
48、calibration of electromagnetic covermeters (see 6.2) BS 1881-204:1988 4 BSI 12-1998 7 Method of use 7.1 Preliminary procedures The covermeter is switched on and the meter adjusted so that the needle on the indicator dial (analogue devices) corresponds to the appropriate calibration mark as indicated
49、 by the manufacturer (“zeroing” the instrument). In the case of digital indicating devices, the manufacturers instructions on setting up the meter before the test should be followed. In all cases this procedure should be carried out with the search head far removed from the reinforced concrete surface and in such a way that any other extraneous effects on the magnetic field are at a minimum. Rapid movement of the search head should be avoided as this can affect the “zeroing”. A warming up period may be specified by the manufacturer after which time a