Advanced FRP-Wood Composites in Bridge Applications

Advanced FRP-Wood Composites in Bridge Applications

H.J. Dagher, Ph.D., P.E.*, Melanie Bragdon**

* Director, Advanced Engineered Wood Composites Center, and Professor, Dept. of Civil Engineering,University of Maine, Orono, ME 04469; PH 207-947-0622; [email protected]** Graduate Student, Dept. of Civil Engineering, University of Maine, Orono, ME 04469

Abstract

This paper describes two technologies for reinforcing or strengthening timber bridges with Fiber-Reinforced Polymers (FRP). The first consists of reinforcing the tension side of glulam girders or deckswith unidirectional FRP. This technology allows to significantly increase the flexural strength of girders,particularly when low-grade laminating stock is used. The second consists of post-tensioning stress-laminated timber decks with transverse Glass FRP (GFRP) 7-wire strands. The paper briefly describesthree demonstration bridges constructed in Maine using different aspects of these technologies: (1) theMedway FRP-glulam girder bridge, (2) the Milbridge FRP-glulam deck vehicular pier and (3) theMilbridge GFRP stress-laminated deck.

Introduction

This paper illustrates the applications of two technologies for reinforcing or prestressing timber bridgeswith Fiber-Reinforced Polymers (FRP):

(1) The first technology consists of reinforcing the tension side of glulam girders or decks withunidirectional FRP. This technology allows to significantly increase the bending strength of theflexural element, particularly when low-grade laminating stock is used. UM studies havedemonstrated, for example, that E-Glass-FRP (GFRP) tension reinforcement ratio in the order of3% can increase the allowable bending strength of glulam beams by over 100% (. While the gainsin bending strength are significant, the corresponding gains in bending stiffness are in the order of15%. Therefore, this technology is most appropriate when deflections, rather than strength controlthe design. Two demonstration bridges using different aspects of this technology are brieflydescribed: The Medway FRP-glulam girder bridge, designed for HS 25 loading, and the MilbridgeFRP-glulam deck vehicular pier. The difference between the two projects is that the first useshorizontally laminated girders whereas the second uses vertically laminated deck panels. Both thegirders and the deck panels are reinforced on the tension side.

(2) The second technology consists of post-tensioning stress-laminated timber decks with transverseGlass FRP (GFRP) 7-wire strands. Stress-laminated timber decks offer a simple form of short-span bridge construction in which longitudinal wood laminations are transversely post-tensionedusing typically steel threadbars. The friction generated between the wood laminations causes thedeck to perform as an orthotropic plate. One drawback of the system is the necessity to retensionthe steel threadbars in service. Creep in the wood and drying of the wood laminations often causeprestress losses in excess of 50% in service. To reduce prestress losses and avoid the need toretension the bridges in service, the University of Maine conducted a 6-year study that developed

Copyright ASCE 2004 Structures 2001 Structures 2001

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an alternative stressing system using Glass-Fiber-Reinforced Polymer (GFRP) tendons. TheseGFRP tendons have a modulus of elasticity equal to 1/5 that of steel threadbars; therefore, theyretain a larger fraction of their prestress force than steel when drying shrinkage and creep of thewood deck occur. This paper briefly describes the design, construction and field monitoring of astress-laminated deck located in Milbridge, Maine, and post-tensioned with the new GFRPtendons. It also compares the prestress losses observed in Milbridge with those in two otherstresslam timber decks: one constructed in a University of Maine laboratory in 1996 and post-tensioned with GFRP tendons, and the other constructed in Gray, Maine, in 1991 and post-tensioned with steel threadbars. All three stresslam decks were constructed using #2 North EastLumber Manufacturer’s Association (NELMA)-graded Maine-grown eastern hemlock, allowing forproper comparison of the prestress losses.

FRP-Glulam Demonstration Bridges

Two demonstration bridges using different aspects of FRP-glulam technology are described: TheMedway FRP-glulam girder bridge, designed for HS 25 loading, and the Milbridge FRP-glulam deckvehicular pier, designed for HS 20 loading. The difference between the two projects is that the firstuses horizontally laminated girders whereas the second uses vertically laminated deck panels. Both thegirders and the deck panels are reinforced with GFRP on the tension side.

Medway Bridge. This is a 54 ft (16.5 m) long, two-lane, FRP-glulam girder bridge with a glulam deck.The bridge was designed using HS25 loading. Pultruded GFRP sheets were used to reinforce theMedway girders as shown in Figures 1 and 2. The glulam girder design values were: Fb = 2,400 psiand MOE = 1.8 106 psi. The girder dimensions were: Depth = 51 inches; width 5-1/8 inches; andlength = 54 ft. The FRP properties parallel to the axis of the beam were: MOE = 6 Million psi andtension strength = 104 Million psi. The FRP dimensions were: 4.75 inches x 1/2-inch total thickness x54 ft long. The FRP was placed one lamination up from the bottom of the beam (see Figure 1). Theextra wood lamination provided protection for the FRP during shipment and construction. Creosotetreatment was applied after the FRP was bonded to the beam. The FRP reinforcement in thisapplication was sacrificial. The bridge is being monitored to evaluate the degradation of the GFRP aswell as degradation of the GFRP-Wood bond lines.

Milbridge Commercial Pier: FRP-glulam decks. The 175 ft (53 m) long by 16 ft (4.9 m) widevehicular pier was designed using HS 20 loading. It consists of eight simple spans, approximately 21.5ft (6.6 m) long each. Each span consists of four FRP-glulam longitudinal panels 4 ft. wide x 21.5 ft.long x 10.5 in. thick, placed side by side and connected using an underdeck distribution beam atmidspan. Each panel is reinforced on the tension side with a 1/10-inch thick GFRP composite, whichwas applied at the University of Maine (Figure 4). The GFRP consists of a wet-impregnatedunidirectional stitched fabric, 4 ft wide, which was applied to the wood and allowed to cure undermechanical pressure. Four layers of a 26 oz fabric were used to obtain the 1/10 inch thick FRP.Urethane coatings were applied to the FRP for UV protection. The glulam panels were pre-treatedwith CCA. Construction took place in Sept. through Dec. 2000.


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Outer Compression Zone: 10% of depth or 4 x1.5” lams; Grade 2.0E-1/3

Inner Compression Zone: 15% of depth or 6x1.5” lams; Grade 1.8E-1/3

51- 1/4 inch Core: Max 50% of depth or 13x1.5” lams; Grade NELMA #2

Inner Tension Zone: 15% of Depth or 6x1.5” lams; Grade 1.8E-1/3

Outer Tension Zone: 10% of Depth or 4x1.5” lams; Grade 2.0E-1/3 Bottom 2 lams, slope of grain 1/16

5 1/8” FRP: 1/2” thick

1.25 inch wood lam Grade 2.0E-1/3 Slope of Grain=1/16

Figure 1. FRP-Glulam Girder Lay-up for Medway Bridge

Figure 2. Medway 54 ft (16.5 meter) FRP-Glulam Girder Bridge

FRP


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Figure 3. Milbridge Vehicular Pier 175 ft (53 m) and 16 ft (4.9 m) wide

Figure 4. Application of Wet-Impregnated GFRP unidirectional Stitched Fabric on Milbridge Glulam Deck Panel (UMaine Advanced Wood Composites Laboratory).


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The main advantage of the FRP-glulam deck design is its lightweight nature which reduces constructioncosts. The FRP-glulam decks weighed about one-third as much as the equivalent prestressed concretedeck panels. The pier will be monitored for a period of 5 years with particular emphasis to FRPdegradation and the FRP-wood bond integrity.

GFRP-Stress-Laminated Demonstration Deck in Milbridge, Maine

Stress-laminated timber bridges offer a popular, simple form of construction, in which longitudinal woodlaminations are transversely post-tensioned using steel threadbars. The friction generated between thewood laminations causes the deck to perform as an orthotropic plate. One drawback of the system isthe necessity to retension the steel threadbars in service. Creep in the wood and drying of the woodlaminations often cause prestress losses in excess of 50 % in service (Dimakis and Oliva 1988,American Association of State Highway and Transportation Officials [AASHTO] 1991).

The objective of this demonstration project was to design, construct, and field monitor a stress-laminated timber bridge that is prestressed with GFRP tendons. The low-modulus GFRP was selectedto reduce prestress losses and possibly avoid the need to retension the bridges in service. The project’sscope included the design and construction of the Milbridge bridge and monitoring of its prestress leveland wood-moisture content for a 2½-year period. During the monitoring, periodic assessment of thecondition of the prestressing system, wood components, and wearing surface of the Milbridge bridgewas performed. The project also compared the prestress losses observed in Milbridge with those in asimilar bridge constructed in Gray, Maine, in 1991 using steel prestressing threadbars.

The Milbridge bridge was completed in summer 1997 and is located on Washington Street in Milbridge,Maine (Figure 5 and 6). The bridge is constructed from 2-x-14-in. Chromated-Copper-Arsenate(CCA)-treated eastern hemlock laminations. The bridge is a single-span, two-lane, stress-laminateddeck structure that is approximately 16 ft long and 25 ft wide and is prestressed with twelve ½-in.-diameter GFRP tendons.

During the first 2½ years of service, the prestress loss in the Milbridge deck was only 14 %: 12 % inthe first three months and 2 % in the following 26 months (Figure 7). In contrast, the steel threadbars ofa similar bridge located in a similar environment in Gray, Maine, were retensioned twice during the first2 months of service (Figure 7). The stress in the steel threadbars continued to decrease, and the barslost 67% of their prestress in the 27 months following the second restressing.

It is concluded that the GFRP tendons used in this study significantly reduced prestress losses instresslam timber decks versus steel threadbars. After approximately 2 ½ years of service, the Milbridgedeck retained 86% of its initial prestress and performed satisfactorily without being restressed.


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Figure 5. Milbridge Deck Modules werePlaced Across Abutments with a Backhoe

Figure 6. Elevation of Milbridge Stress-Laminated Deck withDraped GFRP Tendons Visible in the Foreground


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Gray bridge, steel threaded bars

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Conclusions

This paper described two technologies for timber bridges strengthened with Fiber-Reinforced Polymers(FRP). The first technology consisted of reinforcing the tension side of glulam girders or glulam deckswith unidirectional GFRP. The second consisted of post-tensioning stress-laminated timber decks withtransverse Glass FRP (GFRP) tendons. The paper described three demonstration bridges constructedin Maine using different aspects of these technologies: (1) A 54 ft long FRP-glulam girder bridge inMedway, (2) a 175 ft long FRP-glulam deck vehicular pier in Milbridge and (3) a 16 ft long GFRPstress-laminated deck also in Milbridge. The FRP-glulam deck weighed one-third as much as anequivalent prestressed concrete deck. After approximately 2 ½ years of service, the Milbridge GFRP-stresslam deck retained 86% of its initial prestress and performed satisfactorily without being restressed.

AcknowledgementsFunding for this work was provided by the Federal Highway Administration, the USDA ForestProducts Laboratory, the USDA-WUR program and the National Science Foundation.

ReferencesAASHTO (1991). “Guide specifications for the design of stress-laminated wood decks.” Washington,

D. C.: American Association of State Highway and Transportation Officials.Dimakis, A. G.; Oliva, M. G. (1988). “Behavior of Stress-Laminated Timber Parallel Chord Bridge

Decks: Testing and Analytical Correlation.” Report 88-1, College of Engineering, Univ. ofWisconsin.

Figure 7. Comparison of the prestress force retained in theMilbridge bridge, which is prestressed with GFRP tendons,and the Gray bridge, which is prestressed with high-strengthsteel threaded bars.


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Advanced FRP-Wood Composites in Bridge Applications

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