Tama County s Steel Free Bridge Deck

Transcription

1 Tama County s Steel Free Bridge Deck Mark Dunn Iowa Department of Transportation 800 Lincoln Way Ames, IA mark.dunn@dot.iowa.gov Lyle Brehm Tama County Engineer 1002 East 5 th Street Tama, IA lbrehm@tamacounty.org F. Wayne Klaiber Bridge Engineering Center 418 Town Engineering Building. Iowa State University Ames, IA klaiber@iastate.edu Brent M. Phares Bridge Engineering Center Center for Transportation Research and Education 2901 South Loop Drive, Suite 3100 Iowa State University Ames, IA bphares@iastate.edu Douglas L. Wood Bridge Engineering Center 136A Town Engineering Bld. Iowa State University Ames, IA dwoody@iastate.edu ABSTRACT A major bridge problem in the United States is the corrosion of reinforcing steel and the subsequent deterioration of the surrounding concrete due to deicing salts. There have been efforts in the past to alleviate these problems by using reinforcement that will not corrode, including clad steel reinforcement, fiber-reinforced polymer (FRP) reinforcement, and non-corrosive MMFX steel reinforcement. Another innovative concept is the steel-free bridge deck, which has been developed in Canada. These decks are free of internal steel reinforcement and rely on the internal arching action of the concrete slab, when the slab is confined in both the longitudinal and transverse directions. Using shear studs for composite action between the concrete deck and the steel girders provides longitudinal confinement, while steel straps welded to the top flanges of the girders at regular intervals provide the transverse confinement. FRP reinforcement is included transversely and longitudinally in the decks for temperature and shrinkage reinforcement. The most significant potential benefits of steel-free deck bridges are the decks durability Proceedings of the 2005 Mid-Continent Transportation Research Symposium, Ames, Iowa, August by Iowa State University. The contents of this paper reflect the views of the author(s), who are responsible for the facts and accuracy of the information presented herein.

2 and the material cost savings due to the significantly reduced amount of deck reinforcing. The favorable durability should provide reduced long-term maintenance costs. To the authors knowledge, the first steel-free deck bridge in the United States was constructed in Tama County, Iowa as part of the FHWA Innovative Bridge Research and Construction program. Since the original bridge deck had to be completely removed, Tama County used this opportunity to increase the width of this 41-foot simple span bridge from 24 to 28 feet. Conventional epoxy-coated reinforcement was used in cantilever overhangs. In this bridge, concrete with polypropylene fibers were used to reduce plastic shrinkage cracking and provide post-crack ductility for the slab. The Tama County steel-free bridge deck was designed using the provisions of the Ontario Highway Bridge Design Code. Included in this project are the deck design, construction documentation, periodic load testing, and evaluation. This past summer, the bridge deck was placed with minimal difficulties. Approximately five months after construction, the bridge was load tested using both static and dynamic loadings. Prior to these tests, instrumentation was installed to measure strains and deflections at critical locations. This paper will provide details on the design and construction of the steel free bridge deck, as well as a comparison of the load test performance with the expected design behavior. This bridge is visually inspected periodically and will be tested a second time (approximately a year after the initial test) to determine any changes in its structural behavior. Key words: arching bridge decks steel-free Dunn, Brehm, Klaiber, Phares, Wood 2

3 INTRODUCTION In areas where the use of deicing chemicals for winter roadway maintenance is common, corrosion of reinforcing steel in bridge decks is widespread (Purvis and Babaei 1994). Every year, this reinforcing corrosion causes deterioration to the concrete in bridge decks, resulting in millions of dollars being spent on repair, rehabilitation, and replacement (Gannon and Cady 1992). The elimination of corrosion in reinforcement could result in savings of millions of highway construction and maintenance dollars. Because this reinforcement corrosion is difficult and costly to abate, the most cost-effective method for elimination of the corrosion is to remove the reinforcing steel itself (Thorburn and Mufti 2001). A bridge deck design method developed by researchers from Dalhousie University, Halifax, Nova Scotia and the Ministry of Transportation of Ontario, Canada has eliminated the reinforcing steel from bridge decks. This method takes advantage of the natural arching action present in deck slabs on girders. Tensile restraint is provided externally to the concrete deck slab and away from the harmful chlorides that initiate corrosion (Ventura and Cook 1998). This tensile restraint is provided by attaching steel straps transversely across the top flange of the girders. This paper documents the load testing and monitoring of a steel-free bridge deck in Tama County, Iowa, the first such structure constructed in the United States. Tama County received funding through the Federal Highway Administration s Innovative Bridge Research and Construction program for the design, construction, and monitoring of an experimental bridge deck replacement with a reinforcing steel-free deck on a single-span 40-foot long steel girder bridge. The Bridge Engineering Center at Iowa State University assisted in securing the funding and is responsible for the documentation of the design and construction of the steel-free bridge deck, load testing of the bridge, and evaluation of the field monitoring. BRIDGE LOCATION AND DESCRIPTION The bridge is located in Tama County, Iowa, on County Highway E64, approximately 2.5 miles east of US 63, over a branch of the Iowa River. This segment of roadway carries approximately 450 vehicles per day. The bridge deck was placed on the Tama County, Iowa, bridge on July 27, 2004 and the bridge was load tested on November 16, The project was a bridge deck replacement of a 41-foot long, 0 skew, steel I-beam bridge (see Figure 1). The deck was widened from 24 feet to 28 feet. The bridge cross-section (Figure 2) consists of seven beams. The existing beams were reused on this project. The five interior beams (S 24x79.9) were spaced at 3 feet, 10 inches, with the exterior beams (S 24x105.9) spaced at 5 feet. The exterior overhang was 1 foot, 6 and 1/2 inches from the edge of the slab to the centerline of the exterior girder. The beams were seated on stub abutments. The bridge deck concrete is an Iowa C-4WR-C15-S35 mix with synthetic structural fiber added. TUF- STRAND SF structural grade polymer synthetic fibers from Euclid Chemical Company were added to the concrete mix at a rate of six lbs/yd 3. The structural fibers were not added to provide strength to the concrete, but to assist in controlling cracking and crack propagation. Dunn, Brehm, Klaiber, Phares, Wood 3

4 Figure 1. Elevation view of the Tama County bridge Figure 2. Cross-section view of the Tama County bridge The steel-free bridge deck was designed using the Canadian Highway Bridge Design Code (CSA International 2000), with the exception of the cantilever portion. The bridge deck cantilever portion and the remainder of the bridge was designed using the American Association for State Highway Transportation Officials (AASHTO) Standard Specifications for Highway Bridges (1996). The only reinforcement steel placed in the deck concrete was located in the outer four feet of the deck. This epoxy-coated steel was used to provide tension reinforcement in the overhang portion of the deck, where external steel straps were not feasible. Corrosion-resistant fiber reinforced polymer (FRP) bars were placed in the entire deck for temperature and shrinkage reinforcement. The FRP bars were Hughes Brothers, Aslan 100, vinyl ester matrix, glass fiber-reinforced polymer rebar. Figure 3 shows the placement of the bridge deck reinforcement. Tensile reinforcement for the deck was provided by welding 1/2-in. x 2-in. steel straps transversely across the top flange of the girders. Steel strap placement details are shown in Figures 4 and 5. The reinforcement straps provide tensile restraint below the deck and away from exposure to deicing chemicals. The straps provided lateral restraint to the girders, preventing them from spreading outward, and developing the tensile stresses that form in the deck (Ventura and Cook 1998). The straps were attached at four-foot intervals longitudinally along the girders. Dunn, Brehm, Klaiber, Phares, Wood 4

5 Figure 3. Placement of deck reinforcement Figure 4. Plan view of the Tama County bridge Dunn, Brehm, Klaiber, Phares, Wood 5

6 Figure 5. Tensile restraint strap details INSTRUMENTATION AND EVALUATION METHODOLOGY Girder deflections were recorded using ratiometric displacement transducers and an Optim Megadac data acquisition system. Strains were measured using the Structural Testing System and Intelliducers from Bridge Diagnostics, Inc. Figure 6 shows the location of the strain gages and displacement transducers. Strain gages were attached to the underside of the top and bottom flanges of each girder at mid-span. Strain gages were also epoxied to the bottom of the tensile restraint straps at mid-span and at other various locations of interest. Concrete strain was measured in the bottom of the slab in various locations. By observing the level of strain in the straps compared to the level of strain in the slab, the effectiveness of the straps ability to resist the tensile forces was determined. Figure 7 shows the placement of the strain gages on the structure. Deflection transducers were also placed on the underside of each girder at midspan. Dunn, Brehm, Klaiber, Phares, Wood 6

7 Figure 6. Location of strain gages and displacement transducers Dunn, Brehm, Klaiber, Phares, Wood 7

8 Figure 7. Placement of strain gages Live Loading The bridge was loaded using two loaded tandem-axle dump trucks provided by the Tama County Secondary Roads Department. Trucks 1 and 2 weighed 51,680 lbs and 52,860 lbs, respectively. The bridge was loaded under seven different load cases. Two stationary load tests were conducted with both loaded trucks. Five load tests were performed with Truck 2 only, crossing at crawl speed. Stationary Loading The stationary loading was performed using both loaded trucks simultaneously. Load Cases 1 and 2 were applied once each. Load Case 1 was applied with the wheel line of each truck placed two feet from the edge of the slab. The two-foot offset was selected because the current AASHTO specifications (AASHTO 1996) dictate this as the minimum offset of the vehicle wheel line from the edge of the slab. The same two-foot offset was used for Load Cases 3 and 5 for single truck loading at crawling speed. Load Case 2 was applied with both trucks placed as close as possible to the center of the bridge, in order to induce the maximum load possible into the structure. Crawling Loading The crawling loading was performed with a single loaded truck (Truck 2). Load Cases 3 through 7 were each run twice. In Load Cases 3 and 5, the truck was again placed at a location two feet from the edge of the slab. The truck was centered on the bridge for Load Case 4. Load Case 6 was selected to maximize the Dunn, Brehm, Klaiber, Phares, Wood 8

9 load between two girders. The dual tires from the tandem were centered between the girders. Load Case 7 was used to place the duals directly over a girder for the maximum loading of a single girder. RESULTS Bridge Deflections The maximum deflection for each girder ranged from 0.07 inches at Girder 7 to 0.09 inches at Girder 4. The AASHTO Specifications (1996) stipulate that the deflection due to service live load plus impact should not exceed 1/800 of the span length. The deflection limit for this bridge is 0.62 inches. The deflections recorded during the load test were well below the specification limitation for all girders under all load cases. Deck Strain The design compressive strength of the concrete was 3,500 psi. Test cylinders broken at 56 days show that the actual compressive strength was over 6,100 psi. The bridge was tested at an age of 120 days, indicating that even higher compressive strength was likely. The maximum tensile strain measured in the bottom of the concrete slab was 36 microstrain. The maximum tensile strain occurred under Load Case 4 in a bay adjacent to mid-span and adjacent to the centerline of the bridge. When converted to stress (assuming E c =3,372 ksi), the slab experienced a maximum tensile stress of 121 psi, which is roughly half of the modulus of rupture for the concrete (237 psi) (CSA International 2000). Tensile Restraint Strap Strain The maximum tensile strain in the tensile restraint straps was 71 microstrain, resulting from Load Case 2. The maximum tensile stress in the straps (assuming E s =29,000 ksi) was 2.1 ksi, which is less than 6% of the yield stress of the straps (36 ksi). The relationship between concrete strain and tensile restraint strap strain at girder mid-span is shown in Figure 8 for Load Case 6. The strain gages were located at the girder mid-span between Girders 3 and 4. Dunn, Brehm, Klaiber, Phares, Wood 9

10 Gage Strain, Microstrain (Strap) 4803 (Concrete) Time Figure 8. Tensile restraint strap strain vs. concrete strain at girder mid-span Girder Top Flange Strain The maximum mid-span tensile flexural strain (14 microstrain) in the girder top flange occurred under Load Case 1 in Girder 1. The maximum mid-span compressive flexural strain (5 microstrain) in the girder top flange occurred under Load Case 7 in Girder 2. Converting to stress shows that the maximum midspan flexural stress in the girder top flange is 0.4 ksi compression, which is negligible in relation to the yield stress of the girders (30 ksi). Girder Bottom Flange Strain The maximum mid-span flexural strain (107 microstrain) occurred in the bottom flange of Girder 1 under Load Case 1. The maximum tensile stress in the girder bottom flange (assuming E s =29,000 ksi) was 3.1 ksi, which is approximately 10% of the yield stress of the girders (30 ksi). Lateral Live Load Distribution The approximate distribution factors were calculated from the measured deflections in the structure at mid-span. Lateral load distribution factors were approximated from Equation 1 using test data from the bridge. Dunn, Brehm, Klaiber, Phares, Wood 10

11 Distribution factor, DFi = n Δi i= 1 Δi (1) Where, DF i = distribution factor of the ith girder (lanes/girder) Δ i = deflection of the ith girder Δ i = sum of girder deflections n = number of girders Load distribution factors were compared to bridge design code distribution factors (AASHTO 1996). The experimental distribution factors for the exterior two girders were higher than the design code assumptions for Load Cases 3 and 5. The AASHTO distribution factors (1996) assume equal girder spacing. The unequal spacing of the exterior girders could account for some of the differences between the design values and the measured values. Another possible cause for the differences is the relatively low deflections that were measured on the bridge. Because the loads for Load Cases 4 through 7 were moving, there may have been some influence of vibration or impact of the load on the distribution of the loads. A small change in girder deflection due to vibration or impact could be significant enough to change the distribution factors computed from the data. Due to the low girder stresses calculated from the load test data, the girders are adequate, despite the slightly un-conservative estimates of load distribution. SUMMARY AND CONCLUSIONS This report summarizes the load test and load response of a single-span steel girder bridge with an innovative steel-free bridge deck system. The bridge was tested under seven load cases, two stationary loadings with two loaded trucks and five loadings with one loaded truck moving at crawling speed. The performance of the structure was monitored for strains and displacements at various locations. The response of the structure was compared to the design parameters used for the bridge (CSA International 2000; AASHTO 1996). The bridge performed well under the loading. Girder deflections at mid-span were well below the limitations for serviceability in the AASHTO design specifications (1996). The measured strains in the girders and tensile restraint straps were all 10% or less of the yield strain of the steel. Concrete slab strains did not approach the cracking strain of the concrete. Overall, the single-span steel girder bridge with a steel-free deck system performed well under the load test conditions. Dunn, Brehm, Klaiber, Phares, Wood 11

12 ACKNOWLEDGMENTS The authors wish to acknowledge numerous Bridge Engineering Center graduate students who assisted with the bridge testing. Particular thanks go to Van Robbins, M.S. graduate student and currently a bridge engineer for HNTB Corp., who helped develop the initial bridge testing plan and provided significant contributions to the literature study associated with the project. Bridge engineering staff members, particularly Ahmad Abu-Hawash, Chief Structural Engineer with the Office of Bridges and Structures at the Iowa Department of Transportation, are greatly acknowledged. Ed Engle, Secondary Road Research Coordinator with the Iowa Department of Transportation, is thanked for his assistance with the mix design for the bridge deck. Curtis Monk, Bridge Engineer with the Iowa Division of the Federal Highway Administration, is thanked for his contributions to the project development and coordination and for his technical input and encouragement during the project. Finally, the authors wish to thank Rod Vance of Calhoun-Burns and Associates, Inc., Project Manager, for assistance with various aspects of the project. REFERENCES AASHTO Standard Specifications for Highway Bridges. 16th Ed. Washington D.C.: American Association of Highway and Transportation Officials. CSA International CAN/CSA-S6-00 Canadian Highway Bridge Design Code: A National Standard of Canada. Gannon, E. J. and P.D. Cady Condition Evaluation of Concrete Bridges Relative to Reinforcement Corrosion, Volume 1: State of the Art of Existing Methods. Report no. SHRP-S/FR Washington, DC: Strategic Highway Research Program, National Research Council. Purvis, R. L. and K. Babaei Life-Cycle Cost Analysis for Protection and Rehabilitation of Concrete Bridges Relative to Reinforcement Corrosion. Report no. SHRP-S-377. Washington, DC: Strategic Highway Research Program, National Research Council. Thorburn, J. and A.A. Mufti Design Recommendations for Externally Restrained Highway Bridge Decks. Journal of Bridge Engineering July/August, pp Ventura, C. E. and S.E. Cook Testing of a Steel-Free Concrete Bridge. Experimental Techniques November/December, pp Dunn, Brehm, Klaiber, Phares, Wood 12