An Introductory Study on Void Detection Inside Internal Polymeric Post-Tension Ducts Using Ground Penetrating Radar

Transcription

1 PTI Journal Technical Paper An Introductory Study on Void Detection Inside Internal Polymeric Post-Tension Ducts Using Ground Penetrating Radar By: Todd Allen Dilip Choudhuri Authorized reprint from: May 2005 Vo 3 No.1 of PTI Journal Copyrighted 2005, Post-Tensioning Institute All rights reserved.

2 An Introductory Study on Direct Void Detection inside Internal Polymeric Post-Tension Ducts using Ground Penetrating Radar By Todd Allen 1 and Dilip Choudhuri 2, P.E. ABSTRACT: This paper is unique as it discusses the application of Ground Penetrating Radar (GPR) as a nondestructive technique to inspect for the presence of voids in grouted polymeric post-tensioning ducts as applicable to segmental bridge construction. Due to significant problems with the use of metal ducts in built post-tensioned segmental bridge infrastructure 1 various Department of Transportations (DOT's) are reviewing their standards and specifications with regards to corrosion mitigation. Extensive research has been performed concerning voids within metallic tendon ducts 2,3 that has lead to successful void detection and subsequent repair. The use of proprietary polymeric post-tensioning ducts in place of metallic ducts is being considered by the bridge industry and is already in use in some states in the U.S 3. The advantages of polymeric ducts are that they have excellent wear resistance and are both chemically and electrolytically inert to corrosion causing acid species. An experimental study on the successful use of the GPR technique for direct void detection within polymeric post-tensioning ducts is presented herein. KEYWORDS: polymeric duct, post-tensioning, voids, ground penetrating radar, GPR, reflectivity, dielectric contrast, and conductivity 1 Todd Allen, President, Radarview LLC. Radarview LLC is a specialty non-destructive evaluation company workng in the civil/structural and geotechnical industries and is headquartered in Houstin, Texas. The author can be contacted via at todd.allen@radarviewllc.com 2 Dilip Choudhuri, P.E., Principal, Walter P. Moore's Structural Diagnostics Services Group (SDSG). The SDSG group of Walter P. Moore specializes in the engineering assessment, analysis and the restoration of existing structures. The author can be contacted via at dchoudhuri@walterpmoore.com PTI Journal, V. 3, No. 1, May Received and reviewed under Institute journal publication policies. Copyright 2005, Post-Tensioning Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the November 2005 issue of the PTI Journal if received by September 15, INTRODUCTION Ground Penetrating Radar (GPR) is a powerful tool for the non-destructive examination of concrete structures. Its applications for concrete structures include locating steel reinforcement, embedded conduits, post-tension cables, voids, and potential areas of deterioration. Slab thickness measurements, concrete cover surveys, and evaluation of as-built reinforcement for structural assessments are some of the other uses of the GPR technology. The introduction of polymeric post-tensioning ducts provides yet another opportunity to utilize the GPR technique for direct void detection in bridges. In the past, complementary non-destructive testing (NDT) methods have been successfully used to locate metal ducts and voids within post-tensioned bridge structures 4,5. The GPR technique has been successfully utilized in locating metal ducts prior to making the necessary exploratory holes for endoscopic evaluation to detect voids. The need to excise exploratory holes for void detection stems from the fact that electromagnetic radiation will not penetrate metal ducts. On the other hand, the impact-echo techniques have been successfully used to directly locate voids in metal ducts 6, as the metallic material does not affect P- Wave propagation. The roles of GPR and impact-echo techniques essentially reverse themselves when applied to bridges built with polymeric post-tensioning ducts. Impact-echo cannot locate the presence of voids within a polymeric ducts due to the low impedance of polymeric material 5. The advantage of the GPR technique is that a polymeric material's low reflectivity does not significantly affect the GPR signal. Hence, voids can be directly detected non-destructively using the GPR technique. The first part of this paper discusses and defines the basics of GPR examination including dielectric constant, conductivity of materials and how they affect GPR energy transmission. A simple example of how the acquired data is graphically presented using a B-Scan (side or elevation view) and the corresponding C-Scan (plan view depth slice) are shown to aid the readers with the analysis of acquired GPR data. The analysis performed for this study was done primarily using the B-Scan presentation. The second part of the paper details the experimental setup, vis-à-vis, the concrete test block used for evaluating May PTI Journal 37

3 the performance of the GPR techniques for direct void detection in polymeric post-tensioning ducts. The materials and the test block design used for the experimental study were developed to create a similar substrate environment that may be encountered in a field bridge application. The final part of the paper examines the acquired GPR data after scanning the experimental test block. The results clearly show how GPR data could be used for real-time void detection on post-tensioned structures designed and built with internal polymeric post-tensioning ducts. 2.0 GPR BASICS OVERVIEW GPR has been used in a rudimentary form since However, it was not until the early 1970s that it became commercially available. The applications started in the geophysical industry and have quickly spread to other industries that show promise for new and exciting applications. The use of GPR on concrete is well known and documented 7. The GPR equipment consists of a single channel system with two parts - a central processing unit (CPU) and an antenna. The type and range of the antenna is critical to the system and the success of the technique. It determines the resolution, quality, and depth of penetration of the GPR signals. Typically for the evaluation of concrete structural systems within the first 18" in depth, a Mhz antenna is well suited for use. The antenna radiates electromagnetic energy in an approximately 60-degree cone when coupled to a concrete surface. A receiver, which is part of the antenna system, receives the reflected signal as a result of differences in the dielectric properties of encountered materials that the GPR signals travel through. The CPU converts the reflected signal into a readable format for interpretation. The strong dielectric contrast between concrete and materials such as air, reinforcing bars, conduit, water, metal ducts or PT cables allows these components to be easily detected when encountered in a concrete substrate. Virtually any buried object or material that contrasts with the medium around it can be detected using the GPR technique. This detection of dissimilar materials is limited only to the physical size of the embedded object and/or the size of the electromagnetic radiation wavelength produced by the GPR system. The acquired data can be analyzed in real-time mode after an operator has become familiar with the conditions of a particular concrete structure. For complex analysis, post-processing software can be utilized to present acquired information in a coherent manner. The data representation is typically a B-Scan (elevation view), however a series of scan lines can be used to create C-Scan (plan view depth slice) and three dimensional (3- D) views of the structural element. When electromagnetic waves are radiated into a solid concrete block (without any voids or embeds) the radar signal would reflect back only when it reaches the other side (the contrast with air or soil would produce such a reflection). Fig. 1 - Reflection from a single rebar in concrete, courtesy of GSSI, Inc. Fig. 2 - Two rebar layers detected in an elevated slab, courtesy of GSSI, Inc. Fig. 1 shows how a B-scan image would look with a single piece of rebar inside a concrete block. The single rebar has a higher dielectric value than the surrounding concrete causing a white-black color reflection. A white-black reflection indicates a positive amplitude response that is caused when the energy goes from a low dielectric material to a material with a higher dielectric value. Conversely a black-white reflection would appear when going from a high dielectric to low dielectric material such as concrete to air (See Fig. 2). When multiple passes are made over the same target along its length, the data can be post processed to form an image of reinforcement as shown in Fig. 3. Fig. 3 - Plan view of mild steel reinforcement within a concrete slab 38 PTI Journal - May 2005

4 Fig. 4 - Duct with four seven wire strand -5/8 diameter tendons Fig. 5 - Assembled plastic duct with air-vent sealed and grout placement fitting 3.0 CONCRETE TEST BLOCK SETUP For the current study a concrete test block was designed to simulate some of the conditions that may be encountered in the field study of a post-tensioned bridge. The overall dimensions of the test block measured 6 ft x 6 ft x 15 in. thick. The test block construction prior to our experimental testing is described in the following sections. 3.1 Post-Tensioning Duct Grouting Configuration A manufacturer that widely supplies products to the posttensioning (p-t) construction industry provided samples of the seven wire strand 5/8 in. diameter tendons, tendon spacers, and polymeric ducts. The ducts are 3 in. diameter, 0.1 in. thick polymeric ducts. Four strands using the spacers were placed into each duct and the ends were sealed on one end with a special fitting to allow for the grout placement (See Figs. 4-7). It is important to note that the areaof-strand is 1.23 in 2 and the area-of-duct is in 2. Additional research would be necessary to determine if the results of this study change in cases where tendon construction (duct and strands) with a higher area-of-strand to area-of-duct ratio are used. A total of four ducts, designated as #RV1 through #RV4 were placed into the test block formwork at an approximate depth of 6-3/8 in. Figs show the test block construction process. Duct # RV1 Duct RV1 was inclined, to about a 20 º angle, during grouting operations and was partially filled to create the desired void. The void starts 31 in. from one end of the post-tensioning (p-t) duct and develops as a full (diameter of the duct) void at approximately 6" from the p-t duct end. At 24 in. from the end, the void size measured normal to the duct is about 1 in. and at a distance of 27 in. from the p-t duct end, the void is about ½ in. Fig. 6 -Plastic duct grouting process May PTI Journal 39

5 Fig 7. - Tapered void created on duct RV#1 Fig Grout pump Duct # RV2 Duct RV2 was slightly inclined to about a 10 º angle during grouting operations and partially filled to create the desired void similar to the previous duct (RV#1). This void also starts small at about 31 in. from the p-t duct end and increases in size to ½ in. measured normal to the duct at the end of the duct. Duct # RV3 Duct RV3 was placed horizontally during grouting operations and filled with grout to create a series of small voids along the length of the duct. The voids are a maximum of 1/8 in. in size-measured normal to the duct. Fig. 8 - Test block preparation - Note two layers of mild steel above and below the P-T ducts Fig. 9 - Concrete placement for test specimen Duct # RV4 Duct RV4 was placed vertically during grouting operations and completely filled to create a comparison with the other p-t ducts known voids. 3.2 Reinforcement Two layers of mild steel reinforcement for the test block were placed above and below the p-t ducts. The reinforcement was spaced at an approximately 6 in. x 6 in. grid for the bottom layer and a 12 in. x 12 in. grid for the top layer. The top layer of the reinforcing steel was approximately designed with a 2-½ in. deep clear cover and the bottom layer is approximately at a depth of 12 in. (See Fig. 8). 3.3 Concrete High-early strength ready mix concrete (6000 psi) was placed within the forms and vibrated to ensure good consolidation around reinforcement and the polymeric ducts (See Fig. 9). Lifting points were embedded into the block to facilitate movement of the test block within the test facility after the concrete cured. The test block was outdoors for 48 days prior to non-destructive testing for void detection study. 40 PTI Journal - May 2005

6 The voids within ducts RV1 and RV2 provided excellent contrasts for detection. GPR resolved the presence of the voids even where they were less than ½ in. in size measured normal to the duct (see Fig. 13). Note the strong blackwhite contrast signifying that the GPR signal is passing through a low to high dielectric interface (concrete to air void). Duct RV3 has intermittent 1/8 in. voids that did not produce the strong contrasts of the larger voids in RV1 and RV2; however, RV3's voids were detectable and appear in Fig. 14 which shows a faint black-white reflection due to the small size of the void. Fig Completed test block ducts RV#1, RV#2, RV#3, RV#4 (from right to left) 3.4 Grout A flowable non-shrink, cementitious grout product designed for post-tension duct use was used for the experimental study. For testing purposes, the ducts were pregrouted using a controlled flow grout pump (See Fig. 10) prior to placement into the test block forms. This was done to intentionally produce voids of different sizes and for ease of documentation for this study. 3.5 Completed Test Block The completed test block was stripped of the forms allowing the ends of the ducts to be seen. Ducts RV1 through RV4 are placed from right to left. The voids in the posttensioning ducts RV1 and RV2 show up visibly from the end as seen in Fig RESULTS The test block was scanned using a 2 in. X 2 in. grid. The results were analyzed in both B-Scan (side/elevation view) and C-Scan (plan view depth slices). The B-Scan images were the most useful for displaying the voids within the p- t ducts. Fig. 12 shows C-Scan depth slice images of the rebar on the left hand image; and strands and/or voids inside ducts on the right hand image ducts for reference purposes only. 5.0 CONCLUSION In conclusion, the results of the testing confirm the potential value of utilizing GPR as a direct void screening tool for bridges and other p-t structures that are constructed with polymeric ducts. The post-processed images from GPR techniques can be used for quality assurance of newly grouted ducts and for assessment of existing ducts that need rehabilitation and repair. Additional research is necessary to determine the void size vs depth resolution capabilities of GPR when used as a void detection tool in polymeric ducts. 6.0 ACKNOWLEDGEMENTS The authors thank Felix Sorkin, President and Joe Harrison, Vice President of General Technologies, Inc. for providing the labor, materials 8 and their facilities for the testing phase of the aforementioned study. We thank GSSI for the use of images in Figs. 1 and 2 in this study. REFERENCES 1. Poston R.W., Frank K. H. and West J.S., Enduring Strength, Civil Engineering, September, 2003, pp Klaus Mayer, Alexander Zimmer, Karl J. Langenburg; Christoph Kohl and Christiane Maierhofer, Nondestructive Evaluation of Embedded Structures in Concrete: Modeling and Imaging, International Symposium for Nondestructive Testing in Civil Engineering Fig Depth slices of the test block May PTI Journal 41

7 3. American Segmental Bridge Institute, Grouting Committee, Interim Statement on Grouting Practices, December 4, DePinna M.A., Focus: Application of High Energy X- ray Inspection of Post-Tensioned Concrete Structures and Cable Stay Bridges, PTI Journal, August 2003, pp Christiane Maierhofer, Martin Krause and Frank Mielentz, Complementary Application of Radar, Impact-Echo and Ultrasonics for Testing Concrete Structures and Metallic Tendon Ducts, Transportation Research Record, No. 1892, Odile Abraham, Comments on void depth detection in tendon duct with the impact echo method based on finite element computations, International Symposium for Nondestructive Testing in Civil Engineering M R Shaw, S G Millard, T C K Molyneaux, and M J Taylor, Location of Steel Reinforcement in Concrete using Ground Penetrating Radar and Neural Networks, Structural Faults + Repair General Technologies, Inc., Product information sheets for Fully Encapsulated Corrosion Protection Systems. Fig Voids detected in ducts RV1 and RV2 Fig Ducts RV3 and RV4 42 PTI Journal - May 2005