FRP Hybrids Make Encore Appearance at TRB 2014
- Published: Friday, 04 April 2014 08:36
- Written by Tom Kuennen
After a strong showing in January 2013, glass and polymer fiber-reinforced polymer hybrid precast girders again dominated the technical presentations on precast technology and accelerated bridge construction at the 93rd annual meeting of the Transportation Research Board earlier this year in Washington, D.C.
The TRB event kicked off the year for the surface transportation community, drawing nearly 12,000 to Washington from around the world. The program covered all transportation modes, with more than 4,500 presentations in nearly 800 sessions and workshops addressing topics of interest to stakeholders in government, industry and academia.
|PHOTOS: Aghahassani, Elbadry and Moravvej|
For nearly 60 years TRB has been held among a cluster of hotels along Connecticut Avenue near Woodley Park and the National Zoo, but in 2015 it will move to bigger digs at the Walter E. Washington Convention Center. Following are summaries of technical papers at TRB addressing precast applications. Next month we will look at ready-mixed concrete and cast-in-place technology at TRB. For more information, visit www.trb.org.
Perspective view of a typical hybrid FRP-concrete truss girder, along with specimen dimensions and reinforcing details.
Lighter-Weight Bridge System Relies on FRP-Concrete Girders
Glass fiber reinforced polymer (GFRP) tubes filled with concrete play an important part in a new-design bridge system, say Mohammad Aghahassani, Mamdouh Elbadry, Ph.D., P.Eng., and Mohammad Moravvej, Department of Civil Engineering, University of Calgary, in their 2014 TRB paper, Experimental Fatigue Evaluation of Hybrid FRP-Concrete Bridge Truss Girders.
“A novel hybrid bridge system comprising transversely spaced precast prestressed concrete truss girders, and a concrete deck cast or placed on top, has been developed,” the authors say. “The girders have top and bottom concrete flanges connected by precast vertical and diagonal truss members made of glass fiber reinforced polymer tubes filled with concrete.”
|Perspective view of a typical hybrid FRP-concrete truss girder, along with specimen dimensions and reinforcing details.|
The vertical members, predominantly in compression, are connected to the concrete flanges by means of dowels, they write, adding: “The diagonals, mainly in tension, are connected to the flanges using single-headed bars. The concrete flanges provide the flexural capacity of the girder, while the truss members resist the shear forces.”
The flanges are pretensioned during fabrication, and the girders may be post-tensioned by external tendons after erection to balance the slab weight and to provide continuity in multi-span bridges. All reinforcement and prestressing can be made of corrosion-resistant steel or glass and carbon FRP.
“The proposed truss girder is thus light in weight and durable,” the authors say. “The light weight allows for longer spans and reduces the initial cost. The potential enhancement in durability reduces the maintenance cost and extends the structure’s life span.”
The writers describe an experimental evaluation of the behavior of two-panel truss girders under static and fatigue loading. The fatigue tests under different service load levels and amplitudes showed that the truss girders satisfied the bridge code fatigue requirements. Post-fatigue monotonic loading tests showed considerable residual load-carrying capacity of the girders following fatigue failure. “The test results have shown excellent performance of the truss girders under both static and fatigue loading,” Aghahassani, Elbadry and Moravvej conclude.
Accelerated Design Weds Hybrid Girders, Precast Slabs
|PHOTOS: Nguyen, Zatar and Mutsuyoshi|
Pedestrian bridge composed of two HFRP I-girders topped with a bridge deck made up of GFRP gratings constructed in Japan in 2011 replaced corroded steel bridge at right. Authors provide details of girders’ cross-sections (units are in mm).
Composite crossings of hybrid fiber reinforced polymer (HFRP) I-girders topped with precast ultra-high performance fiber reinforced concrete (UHPFRC) slabs offer promise for accelerated bridge construction, say Dr. Hai Nguyen and Dr. Wael Zatar, Marshall University, Huntington, W. Va., and Dr. Hiroshi Mutsuyoshi, Saitama University, Japan, in their paper, Hybrid FRP Girders Topped with Segmental Precast Concrete Slabs for Accelerated Bridge Construction.
“Fiber reinforced polymer composites can be considered as an alternative to conventional materials in bridge construction, as they provide the following advantages: high strength-to-weight ratio, excellent fatigue resistance, high corrosion resistance, low CO2 emissions and competitive life cycle cost,” the authors say.
Carbon fiber reinforced polymer (CFRP) has higher tensile strength and stiffness than glass fiber reinforced polymer (GFRP), but it is more expensive than GFRP, they write. In a previous project, the authors developed HFRP I-girders for bridge applications in which the use of CFRP and GFRP in a girder section with a specific ratio of flange to web width was optimized.
“The top and bottom flanges of the HFRP I-girder included multiple layers of CFRP and GFRP,” Nguyen, Zatar and Mutsuyoshi write. “The web of the HFRP I-girder was made of GFRP layers because it is subjected to lower flexural stresses than the flanges. This way, the advantages of both CFRP and GFRP were utilized in the HFRP girder. The HFRP I-girders can be used in severe corrosive environments and in accelerated bridge construction.”
Although the HFRP girders may have lower stiffness and strength as compared to conventional bridge girders, the authors say, they can be used with FRP grids/decks to form pedestrian bridges, as appears in the accompanying image.
From FRP grids and decks the authors turned their attention to UHPFRC, or ultra-high performance fiber reinforced concrete, deck slabs. UHPFRC is composed of pre-mixed cementitious powder, sand, water, water reducing agent and steel fibers. The pre-mixed cementitious powder includes conventional portland cement, silica fume and ettringite. The steel fibers have a diameter of 0.2 mm (0.008 in.) and a tensile strength of 2,000 MPa (290,280 psi).
Two fiber lengths were used to uniformly disperse steel fibers in concrete in order to effectively resist cracks and to prevent fibers from bending during mixing. The fibers were 22 mm (0.87 in.) and 15 mm (0.59 in.) long, and were added at approximately 1.75 percent volume ratio with equal quantities of the 15- and 22-mm fibers.
The UHPFRC slabs were precast and cured at 85°C (185°F) for 24 hours. Tests were performed on the UHPFRC cylinders to determine their compressive strength and modulus of elasticity. The average compressive strength of the UHPFRC was 182 MPa (26,420 psi) and the average tensile strength was 11.9 MPa (1,730 psi). The tensile modulus of the UHPFRC is almost the same as its compressive modulus (approximately 46.1 GPa, or 6,690 ksi).
The authors then studied the behavior of composite girders consisting of pultruded HFRP I-girders, and full length/segmental precast UHPFRC topping slabs. Bolt shear connectors and epoxy were used to connect the HFRP I-girders and UHPFRC slabs. They conclude:
Epoxy bonding provides an efficient solution for connecting the precast UHPFRC segments. The stiffness of the girder with the segmental precast slabs connected by epoxy was almost similar to that of the girder with the full length precast slab. The ultimate strength of the segmental girder with epoxy connections was approximately 12 percent lower than that of the full-length precast girder, thus indicating a partial shear transfer across the segmental connections.
Connection methods for the UHPFRC segments should be further investigated to obtain full shear transfer across the segmental connections.
The use of mortar connections is not recommended for real bridge applications as the differences in mechanical properties between the mortar and the UHPFRC result in high stress concentrations at the connections. Since the compressive strength and modulus of the mortar is smaller than those of the UHPFRC, stiffness reduction in girder with mortar connection is attributed to the compressive failure of the mortar.
|Schematic of UHPC waffle deck system.|
Although design of the HFRP-UHPFRC composite girders is governed by stiffness and deflections of the girders rather than their strength, the developed composite girders are promising to apply for pedestrian bridges. They also can be used to rehabilitate existing structures and provide a competitive and sustainable option for accelerated bridge construction.
Their study was limited to short-term behaviors of HFRP-UHPFRC composite girders. Long-term behaviors of these girders under the effects of temperature gradients, fatigue loadings, or hostile environments should be investigated before utilizing this composite system to real bridge applications.
FHWA’s UHPC ‘Waffle Deck’ Bridge Stands Up to Loads
Load testing confirms desirable performance of the UHPC “waffle deck” bridge design developed for the Federal Highway Administration’s Highways for Life Accelerated Bridge Construction program, say Sriram Aaleti, Department of Civil, Construction and Environmental Engineering, University of Alabama-Tuscaloosa, and Sri Sritharan, Department of Civil, Construction, and Environmental Engineering, Iowa State University-Ames, in their TRB 2014 paper, Design of UHPC Waffle Deck for Accelerated Bridge Construction.
FHWA’s Accelerated Bridge Construction (ABC) methods focus on use of prefabricated bridge elements, the authors say, noting: “In the context of ABC, precast concrete deck panels are being increasingly utilized by departments of Transportation in several states for both bridge deck replacements and new structures to decrease construction time. Furthermore, the use of prefabricated full-depth precast concrete deck systems can accelerate the construction and rehabilitation of bridge decks significantly, while extending the service life and lower life-cycle costs of the bridge decks and minimize the delays and disruptions to the community.”
However, transverse connections used previously between precast bridge deck panels have exhibited various serviceability challenges due to cracking and poor construction of connections, they say, adding durable and efficient field connections be developed to implement precast deck panels in practice.
“These connections can utilize high-performance materials such as ultra-high-performance concrete (UHPC) to ensure improved performance,” they say. “While these materials may adhere well to the precast components, it is important to design these connections to prevent cracking and leakage along the connection interfaces between precast elements.”
As part of FHWA’s ABC project, a full-depth precast, ultra-high-performance concrete waffle deck panel and appropriate connections suitable for field implementation of waffle decks were developed. The UHPC waffle deck system for a given thickness has the same or higher capacity—and is 30 to 40 percent lighter—than a comparable solid precast full-depth panel made of normal strength concrete, due to the improved structural properties of UHPC, note Aaleti and Sritharan. The decreased weight of the UHPC panel has significant benefits, including increase in span length for a given girder size, increase in girder-to-girder spacing, improvement in bridge ratings when used for deck replacement projects, and reduction in seismic, substructure, and foundation loads when compared to solid precast deck panel systems.
The presence of the steel fibers in UHPC and very minimal shrinkage of UHPC after steam curing of the precast elements also decreases the reinforcement requirements when compared to traditional precast deck panels, the authors affirm.
“Following a successful full-scale validation test on a unit consisting of two panels with three types of connections under laboratory conditions, the waffle deck was installed successfully on a replacement bridge in Wapello County, Iowa,” they write. “The subsequent load testing confirmed the desirable performance of the UHPC waffle deck bridge.”
Using the lessons from the completed project and outcomes from a series of simple and detailed finite element analyses of waffle decks, a design guide was developed to help broaden the design and installation of the UHPC waffle deck panel cost effectively in new and existing bridges.
Avoiding Internally Cured, Prestressed Deck Panels
Internally cured concrete is recommended for reducing the occurrence of early-age cracking in concrete bridge decks, but its use with prestressed concrete panels is not optimal, say W. Spencer Guthrie, Ph.D., Brigham Young University, Provo, Utah, and Joseph M. Yaede, E.I.T, Applied Research Associates, Champaign, Ill., in their peer-reviewed paper, Evolution of Early-Age Cracking in Concrete Bridge Decks Incorporating Pre-Stressed Concrete Panels and Internally Cured Concrete.
“Deck cracking can be minimized by using a variety of techniques affecting structural design, materials selection, and construction procedures,” Guthrie and Yaede say. “Unfortunately, some approaches used for accelerated bridge construction, which are desirable to minimize disruptions to the traveling public, may compromise long-term deck performance by increasing the probability of deck cracking.”
Prefabricated bridge elements for speedy bridge construction are advantageous because they minimize traffic interruption, lower construction time, improve construction safety, and are less disruptive to the environment, the authors say. To accelerate bridge construction and eliminate the need for conventional formwork between the bridge girders during the concrete bridge deck pour, precast, prestressed, half-deck panels are often used. The prestressed concrete panels together with the cast-in-place portions are designed to act compositely with the prestressed girders and behave similarly to a monolithic concrete deck.
However, several reports have noted that the use of prestressed concrete panels has led to transverse cracking in the concrete bridge deck, where the cracks in the C-I-P deck surface correspond with the butt joints between adjacent underlying prestressed concrete panels. Although these cracks are not believed to significantly affect the structural performance of the deck, such cracking may accelerate deck deterioration by allowing moisture and chloride ions to penetrate the concrete and initiate corrosion of the embedded reinforcing steel.
The objective of Guthrie and Yaede’s research was to monitor, document, and quantify early-age deck cracking on four newly constructed bridge that incorporated features related to minimizing cracking and accelerating construction. Incorporation of pre-wetted lightweight fine aggregate (LWFA) to promote internal curing helps maximize cement hydration, minimize self-desiccation, and minimize deck cracking, the authors write. Therefore two bridge decks were constructed using a conventional concrete mixture, and two were constructed using a concrete mixture containing pre-wetted lightweight fine aggregate to facilitate internal curing and thereby reduce cracking.
“Each of the four bridge structures incorporated prestressed concrete panels placed between the girders to accelerate bridge construction,” note Guthrie and Yaede. “Deck distress surveys were conducted to quantify and compare the degree of surface cracking among the bridge decks at five months, eight months, and one year following deck construction.”
The authors conclude:
The use of pre-wetted lightweight fine aggregate to promote internal curing within concrete is recommended for reducing the occurrence of early-age cracking in concrete bridge decks.
However, internally cured concrete will not achieve its maximum potential in terms of crack reduction when prestressed concrete panels are used in deck construction; as documented, both conventional and internally cured concrete decks are susceptible to reflection cracking from the butt joints between underlying prestressed concrete panels.
Use of internally cured concrete in a monolithic concrete deck with conventional formwork may yield significantly better performance in this respect, although the benefits of precast concrete panels to accelerated bridge construction would be lost.
Comparing the loss in deck service life from premature cracking with the benefits of accelerated construction resulting from the use of PCPs is recommended for future research.