Development of a Transducer for Measuring Tensile Strains in Concrete Bridge Girders

Transcription

1 Development of a Transducer for Measuring Tensile Strains in Concrete Bridge Girders By Dr. Jim Richardson, Dr. Michael Triche and Mr. Stephan Skelton Department of Civil and Environmental Engineering The University of Alabama Tuscaloosa, Alabama Prepared by UTCA University Transportation Center for Alabama The University of Alabama, The University of Alabama in Birmingham and The University of Alabama at Huntsville UTCA Report June 30, 2005

2 Technical Report Documentation Page 1. Report No FHWA/CA/OR- 2. Government Accession No. 3. Recipient Catalog No. 4. Title and Subtitle Evaluation of Profiled Pavement Markings 7. Authors Dr. Jim Richardson, Dr. Michael Triche, and Mr. Stephan Skelton 5. Report Date November Performing Organization Code 8. Performing Organization Report No. UTCA Final Report Performing Organization Name and Address The University Transportation Center for Alabama The University of Alabama Box Tuscaloosa, AL Sponsoring Agency Name and Address Alabama Department of Transportation 1409 Coliseum Boulevard Montgomery, AL Supplementary Notes 10. Work Unit No. 11. Contract or Grant No. Alabama DOT Research Project No Type of Report and Period Covered. Final Report: August 15, 2002 August 14, Sponsoring Agency Code 16. Abstract This report describes the design and testing of a transducer for measuring tensile strains in reinforced concrete bridge girders. This transducer was desired by the Alabama Department of Transportation (ALDOT) Bridge Rating and Load Test Section for performing load tests on reinforced concrete highway bridges. ALDOT began its bridge testing program over 15 years ago and successfully removed load restrictions from nearly 100 steel girder bridges. The procedure used by ALDOT (and other bridge testers, typically), is to measure the flexural strains in the bridge due to known truck loads. Flexural strains can easily be measured in steel girders using electrical resistance strain gages. Such gages cannot accurately measure tensile strains in reinforced concrete girders because of the presence of flexural micro-cracks. The purpose of this project was to develop a transducer to measure tensile strains in reinforced concrete bridge girders. The transducer design was based on the experience of ALDOT and the authors with earlier versions of the strain transducer. This report summarizes the literature on methods of measuring bridge response during load tests, describes the transducer design process, and presents results from laboratory and field testing of a prototype of the final transducer design. 17. Key Words bridge, load test, reinforced concrete, transducer 18. Distribution Statement 19. Security Class (of this report) Unclassified 20. Security Class. (of this page) Unclassified 21. No. of Pages 22. Price ii

3 Contents Topic Table of Contents.. List of Tables List of Figures.. Executive Summary.. Page iii iv iv v 1. Introduction Literature Review. 2 Various Devices to Measure Bridge Structural Response.. 2 Measuring Tensile Strains in Reinforced Concrete Transducer Design 4 4. Calibration, Lab Testing and Refinement of Transducer Field Testing Conclusions References 15 iii

4 List of Tables Number Page 3-1 Results from computer analysis of Square-shaped transducer possibilities Summary of predicted and measured transducer characteristics based on a gage length of Force on transducer and calibration factor as functions of transducer pre-tension... 9 List of Figures Number Page 3-1 Trial transducer configurations Deformed shapes of C-shaped and square transducers Force on transducer and transducer sensitivity as functions of width opening (2h) in square transducer Dimensions adjusted to optimize transducer performance Conical and hemispherical pre-tensioning screw-tips and attachment block divots Transducer error (transducer strain actual strain) for conical tipped screw on greased conical divot and flat surface Transducer error (transducer strain actual strain) for hemisphericaltipped screw on hemispherical-shaped divot Square transducer mounted on bottom of concrete bridge girder. Strain gages mounted to two reinforcing bars and an older-version transducer can also be seen Strain measurements due to 72 kip truck from three transducers: (Alabama DOT (AL 09), Square, and Little C) and from strain gages mounted on two reinforcing bars Strain profile from three transducer measurements: Alabama DOT (AL 09), Square, and Little C; and from two strain gage measurements 13 iv

5 Executive Summary This report describes the design and testing of a transducer for measuring tensile strains in reinforced concrete bridge girders. This transducer was desired by the Alabama Department of Transportation (ALDOT) Bridge Rating and Load Test Section for performing load tests on reinforced concrete highway bridges. ALDOT began its bridge testing program over 15 years ago and have successfully removed load restrictions from nearly 100 steel girder bridges. The procedure used by ALDOT (and other bridge testers, typically), is to measure the flexural strains in the bridge due to known truck loads. Flexural strains can easily be measured in steel girders using electrical resistance strain gages. Such gages cannot accurately measure tensile strains in reinforced concrete girders because of the presence of flexural micro-cracks. The purpose of this project was to develop a transducer to measure tensile strains in reinforced concrete bridge girders. The transducer design was based on the experience of ALDOT and the authors with earlier versions of the strain transducer. This report summarizes the literature on methods of measuring bridge response during load tests, describes the transducer design process, and presents results from laboratory and field testing of a prototype of the final transducer design. v

6 Section 1 Introduction ALDOT performs load tests on older highway bridges that are still in good condition but were designed for lighter trucks. The procedures for calculating maximum allowable truck weights for a bridge are very conservative due to lack of accurate information on material properties and other factors affecting structural response. In a bridge load test, known loads are applied to the bridge and the response to those loads is measured. This information is used by ALDOT engineers to calculate maximum allowable truck weights for the bridge. If these weights exceed the state legal truck weight limits, then load restrictions can be removed from the bridge. Engineers typically measure bridge response using strain gages. Strain gages work well on steel and prestressed concrete girders but not on reinforced concrete girders due to the existence of flexural cracks in reinforced concrete girders. The flexural cracks produce a non-uniform distribution of strain in the tension (cracked) zone and normal strain gages, including special strain gages made for concrete, do not accurately measure strains in these regions. One solution is to attach strain gages to a length of metal anchored at either end to the concrete surface. The resulting transducer measures the average strain over the length. Such transducers have been used by Bridge Diagnostics, Inc. (BDI) to successfully test many reinforced concrete bridges. However, the transducer design is proprietary, and the Alabama Department (ALDOT) of Transportation desired its own simple yet accurate transducer that it could produce in-house. This report reviews the literature on devices for measuring bridge response during load tests, describes the transducer design, and presents the results from laboratory and field testing a prototype of the transducer. 1

7 Section 2 Literature Review A review of the literature revealed that other investigators have measured several different structural response parameters of reinforced concrete bridges including vertical deflection, dynamic response, and flexural strains. The measurement of flexural tensile strains in reinforced concrete present special problems and several approaches exist. Various Devices to Measure Bridge Structural Response Devices used to measure vertical deflection include Linear Variable Differential Transformers (LVDT) (Casadei, et al. 2005, Schiebel, et al. 2002) and cable-extension transducers. One end of these devices needs to be anchored to a non-moving object, usually the ground. Laser (Fuchs, et al. 2004) and photogrammetric (Jauregui, et al. 2003) devices measure vertical deflections and do not require anchorage to the ground, an advantage when the bridge section measured is over water or traffic. Some researchers have measured the dynamic response of a bridge (Zhao, et al. 2002, Li, et al. 2005) for the purpose of detecting damage or deterioration (indicated by a change in dynamic response.) The most common bridge response parameter measured during bridge load tests is flexural strain of the superstructure. Bridge load capacity is usually controlled by flexural stresses, and these can be easily calculated from a measured strain profile the linear distribution of strain from the top of the girder to the bottom of the girder (see Figure 5-3 later in this report for an example). Strains can easily be measured in steel bridges by attaching electrical resistance strain gages to the top and bottom flanges of the steel girders (Stallings, et al. 1993, Chajes,, et al. 1997). Electrical resistance strain gages can be used to measure strain profiles in a similar manner for prestressed girder bridges (Cai, et al. 2004) because the prestressing prevents the concrete from cracking in the tension zone at the bottom of the girder. Measuring Tensile Strains in Reinforced Concrete In normally reinforced concrete girders, the concrete will crack in the tension zone of the girder (for example along the girder bottom at mid-span for a simply-supported girder). Tensile strains are not evenly distributed in the tension zone. When the tensile stress in the concrete exceeds its tensile strength, the concrete ruptures forming a crack and load is shifted to the underlying steel reinforcement. A common technique is to measure the average strain over a length (gage length) spanning several flexural cracks (Phares, et al. 2003). Guidelines for gage length are provided by Bridge Diagnostics, Inc. on their web site (Bridge Diagnostics, 2005). BDI transducers have been used by many to measure strains in reinforced concrete structures (Schulz, et al. 1993, Jauregui, et al. 2004). Other transducers for measuring strain in reinforced concrete include a 2

8 demountable strain transducer (Cook, 1980), fiber-optic Bragg-grating sensors (Kim, et al. 2004) and LVDTs mounted horizontally on the bridge girder (Schiebel, et al. 2002). It should be noted that it is theoretically possible to measure a strain profile in a reinforced concrete girder by measuring strains at two elevations in the compression zone. The compression zone in a simply-supported girder is on the top of the deck, however, and transducers mounted in this region would be struck by traffic or the load trucks. In this report, the design and testing is presented for a simple yet accurate transducer for measuring tensile strains in reinforced concrete bridges. Lessons learned from two earlier versions of the transducer, the first of which was developed nearly 10 year ago (Gambrell, et al. 1996), were incorporated into the design constraints and criteria. The following sections present the transducer design process, its calibration and lab testing, and its performance in the field. 3

9 Section 3 Transducer Design The first step in the transducer design process was to select the shape of the transducer. Four different transducer configurations are displayed in Figure 3-1. All of the configurations provide a pinned connection (i.e. a connection that transmits force but not moment) between the transducer body and the attachment block glued to the surface of the concrete girder. Pinned connections allow the transducer to behave consistently (i.e., the same in the field as in the calibration device) even if the mounting blocks were slightly tilted or out-of-plane due to small irregularities on the girder bottom surface. The pinned connections required that the transducers be pre-tensioned, however, to accommodate the load reversals that frequently occur during dynamic (moving vehicle) tests. All of the transducer configurations shown in Figure 3-1 provide a mechanism to pre-tension the transducer. The mounting blocks for all of the transducer configurations are glued to the concrete surface. Earlier versions of the transducer used expansion bolts to fasten the mounting block to the concrete surface. Gluing the blocks has several advantages over expansion bolts: it is faster, it does little damage to the concrete surface, and it does not initiate tension cracks. Configuration #4, the square-shaped transducer, was eventually selected as the preferred transducer configuration because the square transducer did not cause any in-plane relative rotation between the transducer attachment point and the mounting block under load, as seen in Figure 3-2. The in-plane rotation of the non-square transducers occurred in the field and in the calibration procedure, and had to be accounted for. However, slight misalignment of the transducer in the attachment block divot, the presence of dirt, or other factors could cause the transducer to respond inconsistently and thereby introduce error. Next the dimensions of the transducer were determined. The transducer dimensions affect the stiffness and sensitivity of the transducer. A stiffer transducer applies a larger force to the attachment blocks, making a glue failure between block and concrete more likely. But a stiffer transducer is also more sensitive, where sensitivity is defined as the ratio of strain measured by the transducer to actual strain (ε transducer / ε actual ). This relationship is shown in Figure 3-3, which is a plot of transducer force and calibration constant as functions of the width of the square. Calibration constant is defined as the reciprocal of sensitivity, or ε actual /ε transducer. 4

10 Figure 3-1: Trial transducer configurations 5

11 Figure 3-2: Deformed shapes of C-shaped and square transducers Figure 3-3: Force on transducer and transducer sensitivity as functions of width of opening (2h) in square transducer. The force and sensitivity of the transducer was calculated for different combinations of transducer dimensions by modeling the transducer with a commercial structural analysis program. The transducer dimensions and their labels are shown in Figure 3-4 and the force and sensitivity for 23 sets of transducer dimensions are shown in Table 3-1. The dimensions shaded in gray in the table were selected as the best trade-off between a desired low attachment force and a desired high sensitivity. A prototype of this transducer was fabricated out of 6061-T6 aluminum. The gage length was increased from 18 inches to 28 inches to increase the likelihood of crossing at least one flexural crack. This was based on an assumed maximum spacing for flexural cracks equal to the girder depth, and a girder depth of 27 inches for the cast-in-place T-beam bridges commonly tested by ALDOT (Standard Drawing No. 714). It should be noted that the bending moment in a bridge girder is not constant over the gage length it varies linearly from a maximum at the midpoint of the gage length. Therefore the longer transducer consistently underestimates the maximum 6

12 bending strain. However, a correction factor could be calculated to correct the underestimation since the loading is known. The force and sensitivity predicted during the transducer design phase were reasonably close to the values measured in the prototype, as seen in Table 3-2. Figure 3-4: Dimensions adjusted to optimize transducer performance 7

13 Table 3-1: Results from computer analysis of Square-shaped transducer possibilities L, in h, in w, in w2, in t, in t2, in Force 500µє Sensitivity Notes: Based on a transducer gage length of 18" Sensitivity = ε trans / ε actual Shaded row shows parameters selected for prototype Table 3-2: Summary of predicted and measured transducer characteristics based on a gage length of 28 Force (lbs) Sensitivity Predicted Measured

14 Section 4 Calibration, Lab Testing and Refinement of Transducer The transducers were calibrated in a device that applied a known displacement to one end of the transducer. The displacement was measured with either a micrometer or a digital dial indicator. A straight line was fit to a plot of the transducer strains (output of the Wheatstone bridge circuit connected to the strain gages on the transducer) versus the actual strains (measured displacement divided by gage length). The slope of this line (ε actual /ε transducer ) was called the calibration constant. Surprisingly, the calibration constant was affected by the level of pre-tension, as shown in Table 4-1. Several modifications were made to the connection between the transducer and the mounting block (described below), but the dependency of the calibration constant on the level of pretension remained. Therefore the same pre-tension level (500 micro-strain) was always used for both calibration and field mounting of the transducers. Table 4-1: Force on transducer and calibration factor as functions of transducer pre-tension Calibration Pre-tension, µε Force in Pad, lbs Sensitivity Constant Notes: Gage Length = 28 inches Calibration Factor = 1 / Sensitivity Shaded row = pretension recommended for bridge testing The prototype transducers were mounted on the bottom of a steel beam in the lab to ascertain the accuracy of the transducer. Two equal concentrated loads were applied to the beam, producing a constant moment region over the central portion of the beam span. Strain gages were mounted on the top and bottom flanges of the beam. The strain at the elevation of the transducer was calculated from the strain gages mounted to the steel beam. The transducer error was calculated as the difference between this strain (called ε beam ) and the strain measured by the transducer. The transducer error was unacceptably high--80 microstrain for an actual strain of 500 microstrain. The error may have been caused by stick-slip friction between the pre-tensioning screw and the divot on the attachment block (see Figure 4-1a). Applying grease to this interface made no difference, but pre-tensioning against a flat surface (rather than a conical divot) reduced the error markedly, as shown in Figure 4-2. Some type of divot was needed, however, to hold the transducer in position as the bridge vibrated. A hemispherical-shaped screw tip and divot (Figure 4-1b) worked very well, as seen in Figure 4-3. The divot had a larger radius than the screw tip. The first prototype hemispherical screw tip and hemispherical divot were made using 9

15 hand tools. This set performed slightly better than the screw tip and divot made by the machine shop. ALDOT may wish to finish the screw tip and divot by hand to achieve the best transducer perform ance. Figure 4-1: Conical and hemispherical pre-tensioning screw-tips and attachment block divots Figure 4-2: Transducer error (transducer strain actual strain) for conical-tipped screw on greased conical divot and flat surface 10

16 Figure 4-3: Transducer error (transducer strain actual strain) for hemispherical-tipped screw on hemispherical-shaped divot 11

17 Section 5 Field Testing Prototype transducers (both square and C -shaped) were mounted on a concrete bridge girder alongside other instrumentation during a bridge test (Figure 5-1). The new transducers performed very well during the test, and have consistently performed well on subsequent bridge tests. Strain measurement histories from the following instrumentation are plotted in Figure 5-2: Strain gages mounted on reinforcing bars (R2 and R3) ALDOT strain transducer (AL09) UA strain transducers (Square and Little C) Strain measurements recorded at a common time (175 seconds in Figure 5-2) for each instrument are plotted in Figure Each measurement is plotted at its elevation on the girder to produce a strain profile. As shown in the figure, a straight line can be fit to the data reasonably well. This line can be used to extrapolate the strain in the concrete at the top of the girder. ALDOT is in the process of building 30 square transducers. The aluminum transducer blanks are milled in a machine shop, strain gages are attached by department technicians, and the strain gages and small lead wires are coated for protection with a flexible rubber coating used to coat tool handles. Figure 5-1: Square transducer mounted on bottom of concrete bridge girder. Strain gages mounted to two reinforcing bars and an older-version transducer can also be seen 12

18 Figure 5-2: Strain measurements due to 72 kip truck from three transducers: (Alabama DOT (AL 09), Square, and Little C) and from strain gages mounted on two reinforcing bars Figure 5-3:0. Strain profile from three transducer measurements: Alabama DOT (AL 09), Square, and Little C; and from two strain gage measurements 13

19 Section 6 Conclusions The outcome of this project was a design for a transducer that accurately measures flexural tensile strains in reinforced concrete bridge girders. Project success was due largely to the careful work of the third author on this report, Stephan Skelton, working in collaboration with personnel from the ALDOT Bridge Rating and Load Test Section. The transducer design was based on UA and ALDOT experience with earlier versions of the transducer. The transducer is pin-connected to the attachment pads. This requires the transducer to be pre-tensioned to accommodate occasional compression strains that occur during dynamic loading (moving load trucks). The advantage of the pin connection is the transducer will always be in pure tension as it is during the calibration process even if the girder surface is uneven. Laboratory and field tests of the transducer indicate it meets all ALDOT requirements: (1) measure strains accurately between 10 µε to 1000 µε, (2) inexpensive to fabricate, and (3) easy to install. Since the completion of this project, ALDOT has fabricated over 30 transducers and used them to test nearly a half-dozen reinforced concrete bridges. Installation time for a typical bridge span has been reduced from approximately one day to approximately one hour. The transducers have performed reliably and have produced accurate strain measurements. Shop drawings of the transducers are available from the authors, with ALDOT s permission. 14

20 Section 7 References Bridge Diagnostics, Inc., Strain Transducer for Structural Testing, (accessed May 15, 2005). Cai, C., and Shawawy, M., Predicted and Measured Performance of Prestressed Concrete Bridges, Journal of Bridge Engineering, ASCE, January/February 2004, pp Casadei, P., Parretti, R., Nanni, A., and Heinze, T., In Situ Load Testing of Parking Garage Reinforced Concrete Slabs: Comparison between 24 h and Cyclic Load Testing, Practice Periodical on Structural Design and Construction, ASCE, February 2005, pp Chajes, M., Mertz, D., Commander, B., Experimental Load Rating of a Posted Bridge, Journal of Bridge Engineering, ASCE, February 1997, pp Cook, C. F. (1980). An Electrical Demountable Strain Transducer Strain, V 16, No. 6, Fuchs, P., Washer, G., Chase, S., and Moore, M., Laser-Based Instrumentation for Bridge Load Testing, Journal of Performance of Constructed Facilities, ASCE, November 2004, pp Gambrell, S., Richardson, J., Triche, M. (1996) Development of a Strain Transducer for Bridge Testing., The University of Alabama College of Engineering, Bureau of Engineering Research., BER Jauregui, D., and Barr, P., Nondestructive Evaluation of the I-40 Bridge over the Rio Grande River, Journal of the Performance of Constructed Facilities, ASCE, November 2004, pp Jauregui, D., White, K., Woodward, C., and Leitch, K., Noncontact Photogrammetric Measurement of Vertical Bridge Deflection, Journal of Bridge Engineering, ASCE, July/August 2003, pp Kim, N., and Cho, N., Estimating Deflection of a Simple Beam Model Using Fiber Optic Bragg-grating Sensors, Experimental Mechanics, Society for Experimental Mechanics, Vol 4, No 4, 2004, pp Li, Z., Swanson, J., Helmicki, A., and Hunt, V., Modal Contribution Coefficients in Bridge Condition Evaluation, Journal of Bridge Engineering, ASCE, March/April 2005, pp

21 Phares, B., Wipf, T., Klaiber, F., and Abu-Hawash, A., Bridge Load Rating Using Physical Testing, Proceedings of the 2003 Mid-Continent Transportation Research Symposium, Iowa State University, August 2003, 9 pp. Schiebel, S., Parreti, R., Nanni, A., and Huck, M., Strengthening and Load Testing of Three Bridges in Boone County, Missouri, Practice Periodical on Structural Design and Construction, ASCE, November 2002, pp Schulz, J., In Search of Better Load Ratings, Civil Engineering, ASCE, Sept 1993, pp Stallings, J., and Yoo, C., Tests and Ratings of Short-span Steel Bridges, Journal of Structural Engineering, ASCE, Vol 119, No 7, 1993, pp Zhao, J., and DeWolf, J., Dynamic Monitoring of Steel Girder Highway Bridge, Journal of Bridge Engineering, ASCE, November/December 2002, pp