GB2257528A

GB2257528A - Locating the position of ferrous reinforcement in a structure

Info

Publication number: GB2257528A
Application number: GB9212858A
Authority: GB
Inventors: Philip Wilmott
Original assignee: Colebrand Ltd
Current assignee: Colebrand Ltd
Priority date: 1991-06-18
Filing date: 1992-06-17
Publication date: 1993-01-13
Also published as: GB9212858D0

Abstract

Apparatus 1 for measuring and recording the distance 2 separating a ferrous reinforcement member 3 which is embedded in a structure 4 and a surface 5 of the structure 4 comprises an inductive coupling circuit 6 operating at a resonant frequency of 400 Hz. The circuit comprises first and second electrically isolated multi turn coils 7 and 8 respectively, wound on a common core 9, the first coil 7 being supplied by a sine-wave generator 10 at a frequency of 400 Hz. The second coil 8 is connected to a synchronous demodulator 11, the output of which may be connected to a display, an alarm, a printer, means for storing the values for use in a computer or an indicator of the trend of variation of the distance 2. <IMAGE>

Description

POSITION LOCATOR This invention relates to a position locator, particularly for locating the position of steel in reinforced concrete structures.

Steel corrodes in concrete by an electrochemical process, which involves the development of anodic, or corroding, and cathodic (passive) sites on the steel surface. The likelihood of early corrosion in steel reinforcement members embedded in concrete will be partly dependant on the depth of concrete cover over such members. There are already measuring devices available in the civil engineering field which give accurate surface measurements of the level of concrete cover over the reinforcement. Such devices often measure on the principle that the proximity of a ferrous reinforcement member will increase the coupling effect in an inductive coupling circuit, an effect which is directly proportional to the distance of the circuit from the ferrous member and which effect is easily measurable in terms of an output voltage.

If a device including such a circuit is applied to the exterior surface of a concrete structure, the measured output voltage will be proportional to the distance from the surface of the ferrous reinforcement member. None of these devices of the prior art however contain means for permanently storing such measured voltage values in the device so that they may be accessed at a later time.

It is the object of this invention to seek to provide an apparatus which seeks to overcome these disadvantages.

According to the invention there is provided apparatus for locating the position of a ferrous reinforcement member which is embedded in a structure, comprising a search head including coil means in an electrical circuit the characteristics of which vary depending on the distance of a member from the coil means, and conversion means for monitoring said change in characteristics.

The coil means may comprise a single coil.

Using the invention it is possible to induce eddy currents in the coil as the member in the proximity of the coil alters the impedance thereof and therefore the voltage across the coil. For a member in the form of a bar of substantially constant cross-section, this change in voltage is proportional to the distance of the bar from the coil, which if said coil is at the surface of a structure can lead to a relatively easy way to locate the precise position of the bar with respect to the surface.

Also, it will be understood that by providing the search head with a single coil, the need in use for the head to be perfectly aligned with a reinforcement bar before measurement is commenced is removed, because a (circular) coil moving across a reinforcement bar must, at some point, regardless of bar orientation, pass "through" the diameter of the bar at which point a measurement can be taken.

Using the invention it is possible to take measurements automatically. As the search head is moved across the surface of the structure, usually concrete, the apparatus substantially constantly monitors the output, i.e. the change in characteristics of the circuit, both high and low values. As the indicated cover (depth of concrete) increases from a minimum, the value may be stored in a memory of the circuit (which memory may be remote), and a signal, preferably an audible signal, may be given.

The apparatus may include a trend indicator means.

The trend indicator means may provide an operator with a relatively simple visual indication as to whether the cover thickness is increasing, decreasing, or remaining substantially constant, for example by a display as follows: increasing A decreasing V constant - The operator in this mode would normally be required to keep detailed notes regarding co-ordinates of measurements. In order to seek to obviate this onerous requirement, the apparatus may include a distance measurement sensor, preferably whereby co-ordinates identifying each cover value can be stored 'in memory'.

The co-ordinates providing this data can then be used to prepare contour plots or line graphs of a surface being measured using the apparatus.

The circuit may include alarm means, such as an audible alarm, if the cover, i.e.

depth of concrete above a reinforcement member, falls below a depth or thickness limit previously set by the operator.

There may also be sensor means incorporated with, in or on the search head.

This sensor means allows the distance between reinforcing bars (rebars) to be stored in memory together with the cover value.

Also, there may be means for rapidly scanning an area of reinforced concrete.

This can assist an operator in scanning, and determining the minimum cover within an area being surveyed.

The apparatus may include an inductive coupling circuit adapted to operate at resonant frequency.

The current may have an input alternating current voltage which may provide an induced output alternating current voltage which increases in use in proportion to the cover distance, conversion means for converting said induced output alternating current voltage to direct current voltage and means for measuring said direct current voltage and recording same for future reference.

The inductive coupling circuit may comprise first and second electrically isolated multi-turn coils wound on a common core.

The input alternating current may be supplied by a sine-wave generator at a frequency of 400 Hz.

The coupling circuit may be tuned for resonance at 400 Hz.

The conversion means may comprise a synchronous demodulator.

The means for measuring said direct current voltage may include means for displaying the measured voltage value.

The recording means may include means for printing the stored voltage values or means for downloading the stored voltage values to a computer.

According to another aspect of the invention there is provided a method of measuring and recording the distance separating a ferrous reinforcement member which is embedded in a structure and a surface of the structure to which the apparatus is applied comprising providing an inductive coupling circuit operating in use at resonant frequency, supplying said circuit with an input alternating current voltage, applying said apparatus to a surface of the structure, converting the induced output alternating current voltage to direct current voltage, measuring said direct current voltage and recording same for future reference.

An embodiment of the invention is hereinafter described by way of example, with reference to the accompanying drawing, which shows a schematic front elevational view of an apparatus according to the invention.

Referring to the drawing, there is shown an apparatus 1 for use in measuring and recording the distance 2 separating a ferrous reinforcement member 3 which is embedded in a structure 4 and a surface 5 of the structure 4 to which the apparatus 1 is applied comprising an inductive coupling circuit 6 operating in use at a resonant frequency of 400 Hz comprising first and second electrically isolated multi turn coils, 7 and 8 respectively, wound on a common core 9, said first coil 7 having an alternating sinusoidal current flowi n use, said sinusoidal current being supplied by a sine-wave generator 10 at a frequency of 400 Hz.

As the two coils 7 and 8 are brought nearer to the ferrous reinforcement member 3, the voltage induced in the second coil 8 by alternating current flow in the first coil 7, will increase as a result of the increased magnetic flux caused by the proximity of the ferrous member 3. This increase in voltage value will be directly proportional to the distance 2. The apparatus may also be modified for measuring the distance between reinforcement(s), as well as the cover over a reinforcement. The increase in voltage value may be measured by the device 12 and displayed if desired, once the AC voltage has been converted to a DC voltage by a synchronous demodulator 11. The apparatus 1 includes storage means which may be utilised by an operator to record a voltage value which has been measured by the device 12. This voltage value may then be recalled at a later date and printed by the apparatus or downloaded from the apparatus to a computer for further assessment.

Claims

1. Apparatus for locating the position of a ferrous reinforcement member which is embedded in a structure, comprising a search head including coil means in an electrical circuit the characteristics of which vary depending on the distance of the member from the coil means, and conversion means for monitoring said change in characteristics.

2. Apparatus according to Claim 1, comprising a single coil.

3. Apparatus according to Claim 1 or Claim 2, comprising a substantially circular coil.

4. Apparatus according to any preceding claim, comprising a trend indicator means.

5. Apparatus according to any preceding claim, including alarm means.

6. Apparatus according to Claim 5, comprising audible alarm means.

7. Apparatus according to any preceding claim, comprising sensor means associated with the search head.

8. Apparatus according to any preceding claim, comprising means for substantially rapid scanning of an area to be scanned.

9. Apparatus according to any preceding claim, comprising an inductive coupling circuit adapted to operate a resonant frequency.

10. Apparatus according to Claim 9, the circuit comprising an input alternating current voltage which provides an induced output alternating current voltage which increases in use in proportion to the cover distance, conversion means for converting said induced output alternating current voltage to direct current voltage and means for measuring said direct current voltage and recording same for future reference.

11. Apparatus according to Claim 9 or Claim 10, the first and second electrically isolated multi-turn coils wound on a common core.

12. Apparatus according to Claim 11, the input alternating current being supplied by a sine-wave generator at a frequency of 400 Hz.

13. Apparatus according to Claim 12, the coupling circuit being tuned for resonance at 400 Hz.

14. Apparatus according to Claim 13, the conversion means comprising a synchronous demodulator.

15. Apparatus according to any of claims 9 to 14, the means for measuring said direct current voltage comprising means for displaying the measured voltage value.

16. Apparatus according to any of claims 9 to 14, the recording means comprising means for printing the stored voltage values or means for downloading the stored voltage values to a computer.

17. A method for measuring and recording the distance separating a ferrous reinforcement member which is embedded in a structure and a surface of the structure to which the apparatus is applied comprising providing an inductive coupling circuit operating in use at resonant frequency, supplying said circuit with an input alternating current voltage, applying said apparatus to a surface of the structure, converting the induced output alternating current voltage to direct current voltage, measuring said direct current voltage and recording same for future reference.

18. Apparatus for locating the position of a ferrous reinforcement member which is embedded in a structure, substantially as herienbefore described with reference to the accompanying drawing.

19. A method according to Claim 17, substantially as hereinbefore described.