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Masonry arch bridge evaluationby means of GPR

Paper byMercedes solla, Henrique Lorenzo

Presenting by

N.G.MALLESH

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ContentIntroduction

Devices used

Survey methodology

Case study of historical BIBEIbridge

Case study of LONA bridge

Advantage of GPR

Conclusion

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INTRODUCTION

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Some masonry arch bridges in Galicia(Nw spain) weresurveyed with GPR.

The main goal of this work was to perform analysis ofhistorical bridges.

Obtaining information about filling materialhomogeneity, detecting inclusions of different materials,faults.

GPR is one of the most recommended NDT method inbridge inspection.


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What is GPR?????

Ground Penetration Radar is a device which make useshigh frequency radio wave Of range 250Hz-500Hz.

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The computer measuresthe time taken for a pulseto travel to and from thetarget which indicates itsdepth and location.

The radar unit emits and receives reflected signals up to athousand times per second.


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By using GPR we can obtain continues high resolutionacross ground profile.

The type of GPR equipment required is dependent on thedepth and size of the target to be located.

Which images the subsurface ,objects , change inmaterial, voids and cracks.

The reflected signals are interpreted by the system anddisplayed on the unit's LCD panel.

[cont]


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3D Laser scanner

Consist of highly accurate andfast scanner and camera.

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Field view of 80 vertical and360 horizontal.

Operated by any PC or notebook.


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[cont]

Photorealistic 3D view.

Automatic generation of textured meshes.

Exact identification of details.

Position and distance measurements.


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Survey Methodology

Topographic survey using total station.

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With the help of RISCAN PRO operating andprocessing software Itself generates photos withdepth formation.

This 3D models and GPR data are used to createradargrams by using Gprmax software.

GPR is used to obtain subsurface ,objects,etc

3D scanner used to obtain data of external geometry.


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BIBEI BRIDGE

This is a Roman bridge across the Bibei river.

It presents three barrel arches of 6.09-18.51-8.77meters span.

Restoration of the first arh from the left margin tothe right margin at the upstream side in the 19thcentury.

The pathway was filled to obtain a horizontal profile.9


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Bibei bridge in the upstreram side generated by laser scanning data

It has been found that velocity of the GPR signal rangesfrom 8.5 to 13.6 cm/ns.( average velocity varies from 14 to 15cm/ns)

Which indicates moisture in the structure.

Possible to observe reflections from the arch-air, air-water and air-ground interfaces as well as the typical cornerreflections.

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Pattern of reflections just above the first arch causedby the refilled materials.

These materials may have been used for a restorationtask as previously mentioned.

Reflections above the arches caused by a pathwayrestoration. 11


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The accurate metric data of relate to the externalgeometry.

3D model generated by laser scanning methods.

Has been useful to locate the reflection produced inthe arch-air interface, distance between the arch-airand air-water interfaces.

Lona bridge generated by laser scanning data

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The reflections produced by two sewers placed in the

bridge pathway were also observed. 14


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ADVANTAGES

It is a non distructive method.

Usefull in revealing structural detail.

Detection and accurate positioning of cracks, hollows

or faults.

Cost reduction.

Risk reduction.GPR helps to verify the structural stability of thestructure and to take steps for their preservation andrestoration.

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Disadvantages

Although GPR can usually distinguish between differentsubsoil zones depending on their composition, the specificidentification of the materials involved is more Difficult.

Requires good software handling skill.

Require someone with field experience to interpret them

accurately.


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Conclusion

The results obtained for these surveys confirmed thatGPR is an effective NDT method for historical bridgesinspection.

It proved useful for revealing structural details.

This is an interesting aspect to explore because it canprovide useful information for engineers in developing

strengthening measures.


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References

J. Martin, M.S.A. Hardy, A.S. Usmani, M.C. Forde,"Accuracy of NDE in bridge assessment," in EngineeringStructures.

C. Maierhofer, S. Leipold, "Radar investigation of masonry

structures, in NDT&E International, vol. 34.

M. Solla, H. Lorenzo, F. I. Rial, A. Novo, B. Riveiro, "GPRSurveying of Historical Bridges in Spain," in Proceedings of

5th International Workshop on Advanced GroundPenetrating Radar (IWAGPR'09), pp. 150-154, 2009


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