Authorized reprint from: May 2005 Vo 3 No.1 of PPTTII JJoouurrnnaall

PTI Journal Technical Paper

An Introductory Study on VoidDetection Inside Internal PolymericPost-Tension Ducts Using GroundPenetrating Radar

By:

Todd Allen

Dilip Choudhuri

Copyrighted 2005, Post-Tensioning InstituteAll rights reserved.

37May 2005 - PTI Journal

ABSTRACT: This paper is unique as it discusses the appli-cation of Ground Penetrating Radar (GPR) as a non-destructive technique to inspect for the presence of voidsin grouted polymeric post-tensioning ducts as applicableto segmental bridge construction. Due to significant prob-lems with the use of metal ducts in built post-tensionedsegmental bridge infrastructure1 various Department ofTransportations (DOT's) are reviewing their standards andspecifications with regards to corrosion mitigation.Extensive research has been performed concerning voidswithin metallic tendon ducts2,3 that has lead to successfulvoid detection and subsequent repair. The use of propri-etary polymeric post-tensioning ducts in place of metallicducts is being considered by the bridge industry and isalready in use in some states in the U.S3. The advantages ofpolymeric ducts are that they have excellent wear resistanceand are both chemically and electrolytically inert to corro-sion causing acid species. An experimental study on thesuccessful use of the GPR technique for direct void detec-tion within polymeric post-tensioning ducts is presentedherein.

KEYWORDS: polymeric duct, post-tensioning, voids,ground penetrating radar, GPR, reflectivity, dielectric con-trast, and conductivity

PTI Journal, V. 3, No. 1, May 2005. Received and reviewed under Institute journalpublication policies. Copyright 2005, Post-Tensioning Institute. All rights reserved,including the making of copies unless permission is obtained from the copyright pro-prietors. Pertinent discussion will be published in the November 2005 issue of the PTIJournal if received by September 15, 2005.

1 Todd Allen, President, Radarview LLC. Radarview LLC is a specialty non-destruc-tive evaluation company workng in the civil/structural and geotechnical industriesand is headquartered in Houstin, Texas. The author can be contacted via e-mail attodd.allen@radarviewllc.com

2 Dilip Choudhuri, P.E., Principal, Walter P. Moore's Structural Diagnostics ServicesGroup (SDSG). The SDSG group of Walter P. Moore specializes in the engineeringassessment, analysis and the restoration of existing structures. The author can be con-tacted via e-mail at dchoudhuri@walterpmoore.com

1.0 INTRODUCTION

Ground Penetrating Radar (GPR) is a powerful tool for thenon-destructive examination of concrete structures. Itsapplications for concrete structures include locating steelreinforcement, embedded conduits, post-tension cables,voids, and potential areas of deterioration. Slab thicknessmeasurements, concrete cover surveys, and evaluation ofas-built reinforcement for structural assessments are someof the other uses of the GPR technology. The introductionof polymeric post-tensioning ducts provides yet anotheropportunity to utilize the GPR technique for direct voiddetection in bridges.

In the past, complementary non-destructive testing (NDT)methods have been successfully used to locate metal ductsand voids within post-tensioned bridge structures4,5. TheGPR technique has been successfully utilized in locatingmetal ducts prior to making the necessary exploratoryholes for endoscopic evaluation to detect voids. The needto excise exploratory holes for void detection stems fromthe fact that electromagnetic radiation will not penetratemetal ducts. On the other hand, the impact-echo tech-niques have been successfully used to directly locate voidsin metal ducts6, as the metallic material does not affect P-Wave propagation.

The roles of GPR and impact-echo techniques essentiallyreverse themselves when applied to bridges built with poly-meric post-tensioning ducts. Impact-echo cannot locatethe presence of voids within a polymeric ducts due to thelow impedance of polymeric material5. The advantage ofthe GPR technique is that a polymeric material's lowreflectivity does not significantly affect the GPR signal.Hence, voids can be directly detected non-destructivelyusing the GPR technique.

The first part of this paper discusses and defines the basicsof GPR examination including dielectric constant, con-ductivity of materials and how they affect GPR energytransmission. A simple example of how the acquired datais graphically presented using a B-Scan (side or elevationview) and the corresponding C-Scan (plan view depthslice) are shown to aid the readers with the analysis ofacquired GPR data. The analysis performed for this studywas done primarily using the B-Scan presentation.

The second part of the paper details the experimentalsetup, vis-à-vis, the concrete test block used for evaluating

An Introductory Study on Direct Void Detectioninside Internal Polymeric Post-Tension Ducts usingGround Penetrating Radar

By Todd Allen1 and Dilip Choudhuri2, P.E.

38 PTI Journal - May 2005

the performance of the GPR techniques for direct voiddetection in polymeric post-tensioning ducts. The materi-als and the test block design used for the experimental studywere developed to create a similar substrate environmentthat may be encountered in a field bridge application.

The final part of the paper examines the acquired GPRdata after scanning the experimental test block. The resultsclearly show how GPR data could be used for real-timevoid detection on post-tensioned structures designed andbuilt with internal polymeric post-tensioning ducts.

2.0 GPR BASICS OVERVIEW

GPR has been used in a rudimentary form since 1929.However, it was not until the early 1970s that it becamecommercially available. The applications started in thegeophysical industry and have quickly spread to otherindustries that show promise for new and exciting applica-tions. The use of GPR on concrete is well known and doc-umented7. The GPR equipment consists of a single chan-nel system with two parts - a central processing unit (CPU)and an antenna. The type and range of the antenna is crit-ical to the system and the success of the technique. Itdetermines the resolution, quality, and depth of penetra-tion of the GPR signals. Typically for the evaluation ofconcrete structural systems within the first 18" in depth, a1000 - 1500 Mhz antenna is well suited for use. The anten-na radiates electromagnetic energy in an approximately60-degree cone when coupled to a concrete surface. Areceiver, which is part of the antenna system, receives thereflected signal as a result of differences in the dielectricproperties of encountered materials that the GPR signalstravel through. The CPU converts the reflected signal intoa readable format for interpretation.

The strong dielectric contrast between concrete and mate-rials such as air, reinforcing bars, conduit, water, metalducts or PT cables allows these components to be easilydetected when encountered in a concrete substrate.Virtually any buried object or material that contrasts withthe medium around it can be detected using the GPR tech-nique. This detection of dissimilar materials is limitedonly to the physical size of the embedded object and/or thesize of the electromagnetic radiation wavelength producedby the GPR system. The acquired data can be analyzed inreal-time mode after an operator has become familiar withthe conditions of a particular concrete structure. For com-plex analysis, post-processing software can be utilized topresent acquired information in a coherent manner.

The data representation is typically a B-Scan (elevationview), however a series of scan lines can be used to createC-Scan (plan view depth slice) and three dimensional (3-D) views of the structural element. When electromagneticwaves are radiated into a solid concrete block (without anyvoids or embeds) the radar signal would reflect back onlywhen it reaches the other side (the contrast with air or soilwould produce such a reflection).

Fig. 1 shows how a B-scan image would look with a singlepiece of rebar inside a concrete block. The single rebar hasa higher dielectric value than the surrounding concretecausing a white-black color reflection. A white-blackreflection indicates a positive amplitude response that iscaused when the energy goes from a low dielectric materi-al to a material with a higher dielectric value. Conversely ablack-white reflection would appear when going from ahigh dielectric to low dielectric material such as concrete toair (See Fig. 2). When multiple passes are made over thesame target along its length, the data can be post processedto form an image of reinforcement as shown in Fig. 3.

Fig. 11 - Reflection from a single rebar in concrete,courtesy of GSSI, Inc.

Fig. 22 - Two rebar layers detected in an elevated slab,courtesy of GSSI, Inc.

Fig. 33 - Plan view of mild steel reinforcement within aconcrete slab

39May 2005 - PTI Journal

3.0 CONCRETE TEST BLOCK SETUP

For the current study a concrete test block was designed tosimulate some of the conditions that may be encounteredin the field study of a post-tensioned bridge. The overalldimensions of the test block measured 6 ft x 6 ft x 15 in.thick. The test block construction prior to our experimen-tal testing is described in the following sections.

3.1 Post-Tensioning Duct Grouting Configuration

A manufacturer that widely supplies products to the post-tensioning (p-t) construction industry provided samplesof the seven wire strand 5/8 in. diameter tendons, tendonspacers, and polymeric ducts. The ducts are 3 in. diameter,0.1 in. thick polymeric ducts. Four strands using the spac-ers were placed into each duct and the ends were sealed onone end with a special fitting to allow for the grout place-ment (See Figs. 4-7). It is important to note that the area-of-strand is 1.23 in2 and the area-of-duct is 7.065 in2.Additional research would be necessary to determine if theresults of this study change in cases where tendon con-struction (duct and strands) with a higher area-of-strandto area-of-duct ratio are used. A total of four ducts, desig-nated as #RV1 through #RV4 were placed into the testblock formwork at an approximate depth of 6-3/8 in. Figs.8-11 show the test block construction process.

Duct # RV1

Duct RV1 was inclined, to about a 20º angle, during grout-ing operations and was partially filled to create the desiredvoid. The void starts 31 in. from one end of the post-ten-sioning (p-t) duct and develops as a full (diameter of theduct) void at approximately 6" from the p-t duct end. At24 in. from the end, the void size measured normal to theduct is about 1 in. and at a distance of 27 in. from the p-tduct end, the void is about ½ in.

Fig. 44 - Duct with four seven wire strand -5/8� diametertendons

Fig. 55 - Assembled plastic duct with air-vent sealedand grout placement fitting

Fig. 66 -Plastic duct grouting process

40 PTI Journal - May 2005

Duct # RV2

Duct RV2 was slightly inclined to about a 10º angle duringgrouting operations and partially filled to create thedesired void similar to the previous duct (RV#1). This voidalso starts small at about 31 in. from the p-t duct end andincreases in size to ½ in. measured normal to the duct atthe end of the duct.

Duct # RV3

Duct RV3 was placed horizontally during grouting opera-tions and filled with grout to create a series of small voidsalong the length of the duct. The voids are a maximum of1/8 in. in size-measured normal to the duct.

Duct # RV4

Duct RV4 was placed vertically during grouting operationsand completely filled to create a comparison with the otherp-t ducts known voids.

3.2 Reinforcement

Two layers of mild steel reinforcement for the test blockwere placed above and below the p-t ducts. The reinforce-ment was spaced at an approximately 6 in. x 6 in. grid forthe bottom layer and a 12 in. x 12 in. grid for the top layer.The top layer of the reinforcing steel was approximatelydesigned with a 2-½ in. deep clear cover and the bottomlayer is approximately at a depth of 12 in. (See Fig. 8).

3.3 Concrete

High-early strength ready mix concrete (6000 psi) wasplaced within the forms and vibrated to ensure good con-solidation around reinforcement and the polymeric ducts(See Fig. 9). Lifting points were embedded into the blockto facilitate movement of the test block within the testfacility after the concrete cured. The test block was out-doors for 48 days prior to non-destructive testing for voiddetection study.

Fig 77. - Tapered void created on duct RV#1

Fig. 88 - Test block preparation - Note two layers of mildsteel above and below the P-T ducts

Fig. 99 - Concrete placement for test specimen

Fig. 110 - Grout pump

41May 2005 - PTI Journal

The voids within ducts RV1 and RV2 provided excellentcontrasts for detection. GPR resolved the presence of thevoids even where they were less than ½ in. in size measurednormal to the duct (see Fig. 13). Note the strong black-white contrast signifying that the GPR signal is passingthrough a low to high dielectric interface (concrete to airvoid). Duct RV3 has intermittent 1/8 in. voids that did notproduce the strong contrasts of the larger voids in RV1 andRV2; however, RV3's voids were detectable and appear inFig. 14 which shows a faint black-white reflection due tothe small size of the void.

5.0 CONCLUSION

In conclusion, the results of the testing confirm the poten-tial value of utilizing GPR as a direct void screening toolfor bridges and other p-t structures that are constructedwith polymeric ducts. The post-processed images fromGPR techniques can be used for quality assurance of newlygrouted ducts and for assessment of existing ducts thatneed rehabilitation and repair. Additional research is nec-essary to determine the void size vs depth resolution capa-bilities of GPR when used as a void detection tool in poly-meric ducts.

6.0 ACKNOWLEDGEMENTS

The authors thank Felix Sorkin, President and JoeHarrison, Vice President of General Technologies, Inc. forproviding the labor, materials8 and their facilities for thetesting phase of the aforementioned study. We thank GSSIfor the use of images in Figs. 1 and 2 in this study.

REFERENCES

1. Poston R.W., Frank K. H. and West J.S., “EnduringStrength, Civil Engineering,” September, 2003, pp 58-63.

2. Klaus Mayer, Alexander Zimmer, Karl J. Langenburg;Christoph Kohl and Christiane Maierhofer,“Nondestructive Evaluation of Embedded Structures inConcrete: Modeling and Imaging,” InternationalSymposium for Nondestructive Testing in CivilEngineering 2003.

3.4 Grout

A flowable non-shrink, cementitious grout productdesigned for post-tension duct use was used for the exper-imental study. For testing purposes, the ducts were pre-grouted using a controlled flow grout pump (See Fig. 10)prior to placement into the test block forms. This wasdone to intentionally produce voids of different sizes andfor ease of documentation for this study.

3.5 Completed Test Block

The completed test block was stripped of the forms allow-ing the ends of the ducts to be seen. Ducts RV1 throughRV4 are placed from right to left. The voids in the post-tensioning ducts RV1 and RV2 show up visibly from theend as seen in Fig. 11.

4.0 RESULTS

The test block was scanned using a 2 in. X 2 in. grid. Theresults were analyzed in both B-Scan (side/elevation view)and C-Scan (plan view depth slices). The B-Scan imageswere the most useful for displaying the voids within the p-t ducts. Fig. 12 shows C-Scan depth slice images of therebar on the left hand image; and strands and/or voidsinside ducts on the right hand image ducts for referencepurposes only.

Fig. 111 - Completed test block ducts RV#1, RV#2,RV#3, RV#4 (from right to left)

Fig. 112 - Depth slices of the test block

42 PTI Journal - May 2005

6. Odile Abraham, “Comments on void depth detection intendon duct with the impact echo method based on finiteelement computations,” International Symposium forNondestructive Testing in Civil Engineering 2003.

7. M R Shaw, S G Millard, T C K Molyneaux, and M JTaylor, “Location of Steel Reinforcement in Concreteusing Ground Penetrating Radar and Neural Networks,”Structural Faults + Repair-2003.

8. General Technologies, Inc., Product information sheetsfor Fully Encapsulated Corrosion Protection Systems.

3. American Segmental Bridge Institute, “GroutingCommittee, Interim Statement on Grouting Practices,”December 4, 2000.

4. DePinna M.A., “Focus: Application of High Energy X-ray Inspection of Post-Tensioned Concrete Structuresand Cable Stay Bridges,” PTI Journal, August 2003, pp43-48.

5. Christiane Maierhofer, Martin Krause and FrankMielentz, “Complementary Application of Radar,Impact-Echo and Ultrasonics for Testing ConcreteStructures and Metallic Tendon Ducts,” TransportationResearch Record, No. 1892, 2004.

Fig. 113 - Voids detected in ducts RV1 and RV2

Fig. 114 - Ducts RV3 and RV4