GB2439993A

GB2439993A - Sensing strain in an elongate reinforcing bar using an optical fibre sensor with integrated Bragg gratings

Info

Publication number: GB2439993A
Application number: GB0613413A
Authority: GB
Inventors: Adrian Ernest Long; Paliakarakadu Assen Mu Basheer; Susan E Taylor; Kenneth T V Grattan; Tong Sun
Original assignee: Queens University of Belfast
Current assignee: Queens University of Belfast
Priority date: 2006-07-06
Filing date: 2006-07-06
Publication date: 2008-01-16
Also published as: GB0613413D0

Abstract

An apparatus and method for sensing axial strain in a reinforcement member of a reinforced concrete structure using at least one elongate light guide such as an optical fibre associated with said reinforcement member, said light guide including at least one integrated Bragg grating. Changes in axial strain applied to the reinforcement member can be detected by detecting a perturbation of frequency, amplitude, phase or polarization of light within the light guide. Preferably one or more optical fibres having a number of Bragg gratings are bonded into an elongate axial groove or slot formed in a side of a steel reinforcing bar or rod before it is embedded in a concrete or similar structure. The light guide may also be located within an axial bore within the reinforcement member. The apparatus may be used to determine cracks and other irregularities in the structure. The system may also be used for monitoring stress and strain in elongate reinforcing bars used as rock anchors.

Description

I

1 pparatus and Method for Sensing Strain in an 2 elongate Reinforcing Bar 4 The present invention relates to an apparatus and method for sensing axial strain in an elongate 6 reinforcing bar and in particular in a reinforcing 7 bar embedded in a reinforced concrete structure in 8 order to determine the strain in the structure and 9 to detect cracks and other irregularities in the structure. :1.1

12 Reinforced concrete structures are commonly used in 13 the construction of buildings, bridges and other 14 civil engineering structures. Such structures comprise one or more bodies of concrete having 16 reinforcing bars, usually in the form of cylindrical 17 steel rods, embedded therein. Such bodies may be 18 pre-cast of fsite or moulded around the reinforcing 19 bars on site. Reinforced concrete structures are inclusive of pre-stressed concrete structures which 21 also incorporate pre-stressed bars as well as normal 22 reinforcement bars.

2 There is a need to monitor such structures during 3 service to ensure that the structure is not exposed 4 to loads beyond safe operating limits and to detect faults or failures in such structures. In 6 particular it is desired to measure the strain 7 imposed on reinforcing bars embedded within the 8 concrete structure.

Changes in the Strain measurements of reinforcing 11 bars, particularly localised strain, can frequently 12 be an indication of the formation of cracks within 13 the surrounding material because the reinforcing 14 bars generally have a high tensile strength and ductility relative to the stiff but relatively 16 brittle surrounding material, particularly in the 17 case of steel reinforcing bars within concrete 18 structures. Early detection and monitoring of 19 strained regions of concrete structures provides engineers with an opportunity to repair the strained 21 sections and avert disaster. Generally, unless a 22 strained concrete structure exhibits some degree of 23 collapse, obvious to the eye, on-site visual 24 inspections are not likely to provide forewarning.

26 Generally visual inspection has been found to be 27 inadequate to diagnose, and provide the opportunity 28 to prevent, catastrophic failure. The alternative of 29 using surface mounted ERS strain gauges on reinforcing bars has proven to be unsatisfactory as 31 the strains measured are often quite different from 32 the actual strain in the reinforcing bar.

2 Reliable test data for the longitudinal strain 3 distribution in reinforcing bars in reinforced 4 concrete structures is a precursor for the development of the cost effective! safe design 6 methods and to ensure that sophisticated finite 7 element modelling (FEM) accurately represent the 8 physical state in a structure. For at least 50 9 years this data had been collected by using surface mounted electrical resistance strain (ERS) gauges 11 mounted on a surface of the reinforcing bars but 12 unfortunately in many instances this approach is 13 flawed as: a) The protection and water proofing for the 16 relatively delicate ERS gauges affected the bond 17 (locally) between the reinforcing bar and the 18 concrete over a much greater length than that of the 19 gauge itself. Thus the accuracy of the results obtained depended on the gauge spacing and the size 21 of the reinforcing bar relative to the gauge.

23 b) Closely spaced ERS gauges mounted on the 24 surface effectively transformed a bonded bar into one which was only intermittently bonded to the 26 surrounding material, thus strain gradients could 27 not be reliably monitored (i.e. average strain but 28 not peak strain can be detected).

As a consequence, those using FEM tools could not 31 be sure whether their numerical model satisfactorily 1 represented the complexities of cracked reinforced 2 concrete structures.

4 According toa first aspect of the present invention there is provided an apparatus for sensing axial 6 strain in an elongate reinforcing bar, said 7 apparatus comprising at least one elongate light 8 guide associated with said reinforcing bar, said 9 light guide including a plurality of axially spaced integrated Bragg gratings whereby changes in axial 11 strain applied to the reinforcing bar can be 12 detected.

14 Preferably the at least one elongate light guide is bonded to the reinforcing bar.

17 In a preferred embodiment the at least one elongate 18 light guide is mounted within an elongate groove or 19 recess provided in a surface of the reinforcing bar.

21 Preferably said at least one elongate light guide 22 comprises an optic fibre 24 According to a second aspect of the present invention there is provided a reinforced concrete 26 structure comprising a concrete body having a 27 plurality of elongate reinforcing bars embedded 28 therein, at least one of said elongate reinforcing 29 bar including an apparatus according to the first aspect of the present invention.

1 In one embodiment the reinforcing bars comprise 2 elongate steel rods.

4 According to a third aspect of the present invention there is provided a method of sensing strain in an 6 elongate reinforcing bar comprising a concrete body 7 having a plurality of elongate reinforcing bars 8 embedded therein, said method comprising the Steps 9 of associating at least one elongate light guide with at least one of said reinforcing bars, said 11 light guide including a plurality of axially spaced 12 integrated Bragg gratings whereby changes in axial 13 strain applied to the reinforcing bar can be 14 detected; and detecting a perturbation of frequency, amplitude, phase or polarisation of light within the 16 light guide to determine a change in the strain 17 applied to the reinforcing bar.

19 Preferably said method includes the steps of forming an elongate groove or recess in a surface of the 21 reinforcing bar and locating the light guide within 22 said groove or recess. i

24 Preferably the method includes the step of bonding the light guide to the reinforcing bar.

27 An embodiment of the present invention will now be 28 described by way of example only.

A Bragg grating is a section of an optic fibre or 31 light guide with permanent periodic variation of the 32 refractive index. A Bragg grating reflects light 1 with a wavelength that depends upon the refractive 2 index and the space related period of the variation 3 of the refractive index (the grating period), while 4 light beyond this wavelength will pass through the grating more or less urthindereci. The light reflected 6 by the Bragg grating will exhibit a wavelength that 7 varies as a function of strain.

9 In a pref erred embodiment of the present invention one or more optic fibres (typically 0.125 mm 11 diameter) having a number of Bragg gratings located 12 at relatively close spacing are bonded into an 13 elongate axial groove or slot formed in a side of a 14 steel reinforcing bar or rod before it is embedded in a concrete structure or similar structure to be 16 reinforced by the reinforcing bar.

18 By providing fibre Bragg gratings at close relative 19 spacing, high strain gradients, such as might Occur in the Vicinity of a crack in the concrete structure 21 or at a junction between members, can be accurately 22 detected.

24 The small diameter of the optic fibre compared to an electrical resistance strain gauge facilitates the 26 use of a much reduced size of groove/slot (for 27 example 0.5 x 0.5 mm). Thus only a small percentage 28 of the cross sectional area of the bar (even for a 6 29 diameter bar) is affected which provides a much more convenient surface mounted System and 31 minimising any effect of the strain monitoring 1 system on the properties of the reinforcing bar 2 (e.g. tensile strength of the bar).

4 Either single or multiple optic fibres may be accommodated within the groove/slot and these may be 6 bonded to the bar by a suitable adhesive, such as 7 epoxy resin. Thus the fibre or fibres are protected 8 from the hostile environment! damage during 9 installation of the reinforcing bar and in particular during production of a reinforced 1]. concrete structure containing said reinforcing bar.

13 Fibre Bragg gratings may be wave length encoded thus 14 eliminating the problems of amplitude or intensity variations that plague many other types of optic 16 fibre sensors. Due to their narrow band wavelength 17 reflection they can also be conveniently multiplexed 18 in an optic fibre network. The high strain 19 sensitivity of Fibre Bragg Gratings (FBG) has enabled them to be embedded in composite materials 21 for smart structures monitoring and incorporated in 22 civil structures to monitor load levels. Optical 23 compensation techniques are seldom needed and the 24 measurements have long term stability, eliminating the needs for recalibratjon and yielding improved 26 quality assurance.

28 Two diametrically opposed grooves may be formed in 29 the bar, one or more optic fibres having fibre Bragg gratings being bonded into each groove, so that 31 axial strains can be accurately monitored even where 32 transverse bending is present.

2 While the present invention has been described with 3 particular relevance to reinforced concrete 4 structures, it is envisaged that the invention has application for monitoring strain in elongate 6 reinforcing bars used as rock anchors and in other 7 comparable applications.

9 Various modifications and variations to the described embodiments of the inventions will be 11 apparent to those skilled in the art without 12 departing from the scope of the invention as defined 13 in the appended claims. Although the invention has 14 been described in connection with specific preferred embodiments, it should be understood that the 16 invention as claimed should not be unduly limited to 17 such specific embodiments. I.

Claims

1 Claims 3 1. An apparatus for sensing axial strain in a 4

reinforcement member of a reinforced concrete structure, said apparatus comprising at least one 6 elongate light guide associated with said 7 reinforcement member, said light guide including at 8 least one integrated Bragg grating whereby changes 9 in axial strain applied to the reinforcement member can be detected.

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2. An apparatus as claimed in claim 1, wherein the 13 at least one elongate light guide is bonded to the 14 reinforcement member.

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3. An apparatus as claimed in claim 2, wherein the 17 at least one elongate light guide is mounted within 18 an elongate groove or recess provided in a surface 19 of the reinforcement member.

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4. An apparatus as claimed in claim 2, wherein the 22 at least one elongate light guide IS integrated into 23 the reinforcement member.

5. An apparatus as claimed in claim 4, wherein the 26 at least one elongate light guide is located within 27 an axial bore within the reinforcement member.

29 6. A reinforced concrete structure comprising a concrete body having a plurality of elongate 31 reinforcement members embedded therein, at least one

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1 of said elongate reinforcement member including an 2 apparatus as claimed in any of claimed 1 to
5.

4 7. A reinforced concrete structure as claimed in claim 6, wherein said reinforcement members comprise 6 elongate steel rods.

8 8. A method of sensing strain in a reinforcement 9 member of a reinforced concrete structure comprising a concrete body having a plurality of elongate 11 reinforcement members embedded therein, said method 12 comprising the steps of associating at least one 13 elongate light guide with at least one of said 14 reinforcement members, said light guide including at least one integrated Bragg grating whereby changes 16 in axial strain applied to the reinforcement member 17 can be detected; and detecting a perturbation of 18 frequency, amplitude, phase or polarisation of light 19 within the light guide to determine a change in the strain applied to the reinforcement member.

22 9. A method as claimed in claim 8, including the 23 steps of forming an elongate groove or recess in a 24 surface of the reinforcement member and locating the light guide within said groove or recess.

27 10. A method as claimed in claim 8, including the 28 steps of forming an axial bore within the 29 reinforcement member and locating the light guide within said axial bore.

1 11. A method as claimed in any of claims 8 to 10, 2 including the step of bonding the light guide to the 3 reinforcement member.

12. An apparatus for sensing axial strain in a 6 reinforcement member of a reinforced concrete 7 structure substantially as herein described with 8 reference to the accompanying drawings.

13. A reinforced concrete structure substantially 11 as herein described with reference to the 12 accompanying drawings.

14 14. A method of sensing strain in a reinforcement member of a reinforced concrete structure 16 substantially as herein described with reference to 17 the accompanying drawings.