1、BRITISH STANDARD BS 1881-206: 1986 Testing concrete Part 206: Recommendations for determination of strain in concrete UDC 666.972.017:691.32:620.1BS1881-206:1986 This British Standard, having been prepared under the directionof the Cement, Gypsum,Aggregates and QuarryProducts Standards Committee, wa
2、s published underthe authority of the BoardofBSI and comes intoeffecton 31October1986 BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference CAB/4 Draft for comment 85/14014DC ISBN 0 580 15387 8 Committees responsible for this British Standard The preparati
3、on 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 Manufacturers Association of Metropolitan Authorities Briti
4、sh 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 Association Cement and Concrete Association Cement Makers
5、Federation Concrete Society County Surveyors Society Department of the Environment (Building Research Establishment) Department of the Environment (Property Services Agency) Department of Transport (Highways) Department of Transport (Transport and Road Research Laboratory) Electricity Supply Industr
6、y in England and Wales Federation of Civil Engineering Contractors Greater London Council Institute of Concrete Technology Institution of Civil Engineers Institution of Highways and Transportation Institution of Structural Engineers Institution of Water Engineers and Scientists Royal Institution of
7、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 Atomic Energy Authority Amendments issued since publication Amd. No.
8、 Date of issue CommentsBS1881-206:1986 BSI 08-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 General guidance 1 4 Common types of strain gauge and their use 1 5 Choice of device 5 6 Report 5 Figure 1 Types of device for measuring strain 2 Publi
9、cations referred to Inside back coverBS1881-206:1986 ii BSI 08-1999 Foreword This Part of BS1881has been prepared under the direction of the Cement, Gypsum, Aggregates and Quarry Products Standards Committee. It supersedes BS4408-2:1969, which is withdrawn. All aspects of testing concrete are being
10、included as Parts of BS1881, from sampling fresh concrete to assessing concrete in structures. Part201gives general guidance on the choice of non-destructive test methods. This Part describes several well-tried and accepted methods of measuring strain in concrete. There are other methods of measurin
11、g strain, some of which were described in BS4408-2. These include: semiconductor element electrical resistance; piezoresistance semiconductor; photoelastic gauges, fibre optics; micrometers, etc. While these methods may have special applications, they are not suitable for general use with concrete a
12、nd so are not discussed in this standard. There is a tendency for results obtained from a strain gauge to be accepted without question. The warning applies generally that it is often necessary to measure stress induced strains of the same order as those produced by changes in ambient temperature. Co
13、nsideration should be given to the characteristics and limitations of the various devices and systems to ensure valid results. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Complian
14、ce 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, pages i and ii, pages1to6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had
15、amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS1881-206:1986 BSI 08-1999 1 1 Scope This Part of BS1881gives recommendations on methods and devices which can be used to determine strain in concrete. Some devices measure a property directly related t
16、o strain in the gauge (e.g. electrical resistance gauges) whereas others actually measure displacement, which can be used to determine strain using a known base length (e.g.displacement transducers). References to strain gauges in this Part include both types of device. The use of the following devi
17、ces is described: a) mechanical (including mechanical/optical); b) electrical resistance (metal and alloy elements); c) vibrating wire (acoustic); d) electrical displacement transducers. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Definition
18、s For the purposes of this Part of BS1881the definitions given in BS6100-6 apply, together with the following. 2.1 strain change in length per unit length in a specified direction, i.e.%L/L, where L is the initial length in the specified direction and %L is the length change in that direction NOTEIn
19、 metric units strain is normally expressed in micrometres per metre, but in this standard the well-known colloquial equivalent “microstrain” has been adopted. 2.2 gauge length length of the concrete over which the initial measurement is made NOTEThis length is usually only part of the whole length o
20、f the concrete unit, and only part of the length of the device. 2.3 dummy gauge nominally identical gauge, not subjected to strain, installed in the same environmental conditions as the gauges used to measure strain. Properly connected in the measuring circuit it will cancel out environmental effect
21、s on the measurement gauges 2.4 discrimination smallest change in strain which can be indicated or estimated from the scale of a device 2.5 gauge factor constant by which the reading from the gauge has to be multiplied to derive the strain 3 General guidance There are several aspects which are commo
22、n to all gauges. The most important consideration for any type of gauge is that the operator should be experienced in its use. Characteristics and limitations of various devices and systems should always be carefully studied before choosing the most appropriate. Careful note should be taken of the m
23、anufacturers instructions. Owing to the different methods of application of these devices, there is no universal method of checking their accuracy. It is essential that the operator both be aware of the factors which will affect the accuracy and calibration of the device and, where possible, check t
24、hat the device is correctly calibrated by a method traceable to the length standards maintained by the National Physical Laboratory. Electrical displacement transducers, for example, can be calibrated using a micrometer complying with BS870. It should always be borne in mind that a strain gauge meas
25、ures the strain in the gauge length only. The necessity for care in locating the gauges onto (orinto) the concrete cannot be over-emphasized, whether they be located on lugs, stuck to or clamped onto the surface, or cast into the concrete. The effects of local strain concentrations and gradients sho
26、uld be borne in mind. For general purpose strain measurement of a concrete structure, a gauge length less than about four times the size of the largest aggregate particle is likely to be affected by local variations in the mix. For measuring strains in different directions at the same location, elec
27、trical resistance gauges are available in the form of rosettes and can be used as single2-way devices or single3-way devices, depending upon whether or not the principal strain axes are known. Similar arrangements can be contrived with other gauges. 4 Common types of strain gauge and their use 4.1 M
28、echanical gauges 4.1.1 Principle. The movement at the surface of the concrete specimen is magnified by a lever or roller linkage and the magnified movement measured by means of a sensitive dial gauge (the most sensitive gauge complying with BS907), an optical lever, a linear displacement transducer
29、or other device. The gauge can be on a fixed plate or can be of the demountable type and held by hand against locating lugs, studs, balls or recesses stuck to or cast into the specimen see Figure 1(a).BS1881-206:1986 2 BSI 08-1999 Figure 1 Types of device for measuring strainBS1881-206:1986 BSI 08-1
30、999 3 The strain is recorded as either a direct difference between two readings or the difference between the readings and those on a standard bar (usually invar) divided by the gauge length. 4.1.2 Gauge length and discrimination. Types of mechanical gauge are available in a wide range of gauge leng
31、ths, from12mm to2000mm. The discrimination of any type of gauge normally increases with gauge length and can vary from50microstrain to2microstrain per division of the scale. 4.1.3 Mounting the gauge. The studs, etc., should be so fixed that the gauge locates on the reference points symmetrically. Ou
32、t-of-alignment can give misleading results. The adhesive chosen should be such, and applied in such a way, that the movement of the concrete is directly reflected in the movement of the reference points. Sound parts of the surface should be chosen and prepared carefully by roughening and cleaning. T
33、o achieve reliable results, bubbles in the adhesive should be avoided. For long-term tests the reference points should be made of stainless steel and covered with a film of grease when not in actual use. Grease should be removed before taking readings. Under the conditions of exposure, neither the a
34、dhesive nor the reference points should be such as to creep, degrade or corrode. Cleanliness of the gauge and gauge reference points cannot be over-emphasized. 4.1.4 Precautions. For gauges which are clamped to the concrete, the pressure is critical. Most types of demountable gauges should be held w
35、ith two hands, and with all types an even pressure should be maintained. It has been found that different operators can obtain different results and, therefore, if subsequent readings are to be taken at a later date, it is recommended that the same operator should, if possible, take the readings. Te
36、mperature variations have to be taken into account, both in the concrete and in the instrument. Variation can be caused by handling, direct sunlight, etc. For this reason an invar steel bar is generally used either to form the main frame of the gauge and/or to check the gauge each time it is used. T
37、o allow for the effect of possible wear in the instrument or the invar bar, the latter should be checked either after every1000readings or yearly, whichever represents the more frequent use, against a reference invar bar kept solely for this purpose. When the concrete is heated by artificial means,
38、the gauge should be thermally shielded and insulated. 4.2 Electrical resistance gauges (metal and alloy elements) 4.2.1 Principle. This type of gauge is in the form of a flat grid of wires, or etched copper-nickel foil mounted on thin plastics sheet, which is stuck to the test surface see Figure 1(b
39、). Strain is measured by means of changes in electrical resistance resulting from extension and compression of the gauge. The resistance changes can be measured by a single Wheatstone bridge, which can be connected to multichannel reading and recording devices. The relationship between strain and re
40、sistance for these gauges will be approximately linear and defined by the gauge factor. Characteristics will vary according to the gauge construction, but foil gauges will generally be more sensitive than wire gauges and have a higher heatdissipation, which reduces the effects of self-heating. 4.2.2
41、 Gauge length and discrimination. Commercial gauges are available in lengths from0.2mm to150mm, and their discrimination, depending on the measuring equipment, varies from20microstrain to1microstrain. These gauges are more often used than mechanical gauges over shorter gauge lengths. 4.2.3 Mounting
42、the gauge 4.2.3.1 Surface mounted gauges. The location, mounting and protection of the gauge is critical. Care should be taken to ensure that: a) the surface is free of dirt, grease, laitance and loose material; b) the adhesive is carefully applied in a thin layer; c) the adhesive is free of air bub
43、bles; d) the adhesive is fully cured, particularly in cold weather. 4.2.3.2 Embedded gauges. Suitably encapsulated gauges can be cast into the concrete. For guidance on the precautions to be taken see4.2.4. 4.2.4 Precautions. The relationship between strain and resistance will change with temperatur
44、e, and gauges may be self-compensating or incorporate a thermocouple. Alternatively, a dummy gauge can be used. Gauges should be sited away from draughts and direct sunlight, although changes in ambient temperature will not normally affect readings over a small time-scale. Gauges should be water-pro
45、ofed ifthey are subject to changes of humidity, or for long-term use. Background electrical noise and interference will seriously affect the results. This may sometimes be reduced by suitable shielding, wiring systems and instrumentation. Constant calibration is needed to allow for drift.BS1881-206:
46、1986 4 BSI 08-1999 Long-term instability of gauges and adhesives limits their suitability for long-term tests. The strain capacity will also generally be small unless special “post yield” gauges having a high strain limit are used. The use of these gauges requires considerable care, skill and experi
47、ence if reliable results are to be obtained. If the gauge is to be embedded, the elasticity of the encapsulated gauge should be made approximately equal to the elasticity of the concrete for the period over which the measurements are to be taken. 4.3 Vibrating wire (acoustic) gauges 4.3.1 Principle.
48、 A wire stretched between two points has a fundamental resonant frequency. Changes in strain will result in changes in the resonant frequency. This property is used to measure strain. The wire is excited by an electric pulse passed through an electromagnet situated close to the centre of the wire. T
49、he same magnet can then be used to detect and transmit the vibration to a frequency measuring device see Figure 1(c). 4.3.2 Gauge length and discrimination. These gauges are commercially available in lengths from12mm up to200mm and have a discrimination of1microstrain to10microstrain. 4.3.3 Mounting the gauge. The gauge is either securely anchored onto supports fixed in the specimen, or cast into the concrete. In order to protect the wire against damage it is sealed in a tube of material, e.g. acrylic resin. If the gauge is to be cast into the concrete, then a car
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