Hybrid composite beams arrive.
These topics were among subjects of 4,000-plus presentations offered to the 11,700 transportation professionals from around the world assembled for TRB. In the March 2013 issue of Concrete Products we explored issues in ready mixed and cast-in-place concrete; here are some of the most significant papers involving precast concrete. For more information, visit www.trb.org.
The hybrid-composite beam (HCB)--consisting of a fiber-reinforced polymer (FRP) shell incorporating a tied concrete arch--performs in bridges, say Stephen Van Nosdall, E.I.T., Cristopher D. Moen, Ph.D., P.E., Thomas E. Cousins, Ph.D., P.E., and Carin L. Wollmann-Roberts, Ph.D., P.E., Virginia Tech University, Blacksburg, in their paper, Experiments on a Hybrid-Composite Beam for Bridge Applications.
By concluding that the concept performs, and more research is needed into performance of different configurations, the authors imply that the HCB belongs in the toolbox of concrete beam designs. "[It] offers advantages in life cycle costs through reduced transportation weight and increased corrosion resistance," they write. "By better understanding the system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces."
As such, the beam design fits into the Federal Highway Administration's Highways for Life program, as a means to accelerate bridge construction. "A long term outcome of this research will be a general structural analysis framework that can be used by DOTs to design HCBs as rapidly constructible bridge components," the authors say.
According to the design marketer, HC Bridge Co. of Wilmette, Ill., the HCB is a tied arch in a fiberglass box, where 90 percent of the strength is provided by steel and concrete. Easily transported and mounted, the relatively lightweight fiberglass-reinforced polymer (FRP) outer shell provides shear strength that encapsulates the tension and compression elements.
The compression element is a concrete arch within the shell, constructed when self-consolidating concrete is pumped onsite into the FRP shell. Finally, the tension element is steel reinforcement that runs longitudinally the length of the beam and ties the two bottom ends of the concrete arch together. "The encapsutating FRP shell provides maximum protection from the elements for the steel and concrete, ensuring an extended service life and minimal maintenance," HC Bridge notes.
"The HCB consists of three parts: a shell made out of a fiberglass-reinforced polymer, a concrete arch within the shell, and tension reinforcement tying the ends of the arch," say Van Nosdall, Moen, Cousins and Wollmann-Roberts. "The beam shell includes top and bottom flanges, two vertical webs, and a conduit used to form the concrete arch." The latter is formed by pumping self-consolidating concrete into the conduit, following a parabolic path starting at the bottom at the supports and apexing at the center of the beam, they say, adding, "The arch also has a "shear fin" of concrete extending upward to the top of the beam to hold reinforcing steel for force transfer between a concrete deck and the beam. At each support the concrete fills a block the full height of the beam to anchor prestressing strands that provide tension reinforcement."
Their study focused on identifying the load paths and toad sharing between the arch and FRP shell in an HCB, and testing an HCB with a composite bridge deck. Testing was performed by applying point loads on simple span beams (before placing the bridge deck), and a beam skewed composite bridge system, resulting in strain data for the arch and FRP shell "The test results show that strain behavior is linear elastic at service toads and the FRP shell has a linear strain profile," affirm Van Nosdall, Moen, Cousins and Wollmann-Roberts.
Curvature from strain data was used to find internal bending forces, and the proportion of load within the arch was found. Additionally, a stress integration method was used to confirm the internal force contributions, the authors note. The arch carries about 80 percent of the total load for the noncomposite case without a bridge deck, and the amount of arch bending and axial force depends on the position of loading.
NEW COMPOSITE MELDS HYBRID I-BEAM WITH SLAB
A new composite beam consisting of a hybrid carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) I-beam--combined with a precast ultra-high performance fiber-reinforced concrete (UHPFRC) stab--provides the advantage of high corrosion resistance in the beam, with high strength and durability in the slab. That's according to Dr. Hai Nguyen, Rahall Transportation Institute, Marshall University, Huntington, W.Va., Dr. Hiroshi Mutsuyoshi, Saitama University, Japan, and Dr. Wael Zatar, dean and professor, Marshall University, in their paper, Flexural Behavior of Hybrid FRP-Ultra High Performance Fiber Reinforced Concrete Composite Beams.
The hybrid fiber-reinforced polymer (HFRP) I-beam design for bridge applications optimizes the combined use of carbon fiber-reinforced polymer and glass fiber-reinforced polymer in a beam section with a specific ratio of flange to web width. "Fiber-reinforced polymer (FRP) has outstanding mechanical properties over conventional materials, such as high specific strength, light weight and corrosion resistance," the authors write. "Thus FRP materials have been applied in the construction field to repair and retrofit existing structures or even newly construct structural members of many pedestrian and road bridges."
While CFRP has higher tensile strength and stiffness, it is relatively expensive, they add. GFRP is comparatively less expensive but its mechanical properties are lower than those of CFRP. "In a beam subjected to bending moment, the top and bottom flanges are subjected to high axial stress," the authors note. "In the HFRP beam, these flanges are fabricated using a combination of CFRP and GFRP layers. On the other hand, the web is composed entirely of GFRP because it is not subjected to the same high stresses. The HFRP beam therefore utilizes the advantages of both CFRP and GFRP for strength, stiffness and economy."
This makes the HFRP design useful in severe corrosive environments or where lightweight rapid construction is required, say Nguyen, Mutsuyoshi and Zatar. The application of HFRP beams could also contribute to a reduction of life cycle costs and environmental load due to its lower carbon dioxide emissions. Now, the new composite beam consisting of a hybrid CFRP/GFRP I-beam and precast ultra-high performance fiber-reinforced concrete (UHPFRC) slab has been introduced from Japan, the authors report. In this composition, hybrid FRP (HFRP) provides the advantage of high corrosion resistance, while UHPFRC has high strength and durability.
"The combination of these two materials is expected to benefit structures subjected to severe environmental conditions and wherever there is a need for accelerated bridge construction," say Nguyen, Mutsuyoshi and Zatar. They studied the composite behavior of the HFRP I-beam and ultra-high performance fiber-reinforced concrete (UHPFRC) topping slab, noting "UHPFRC has high ductility in both tension and compression due to the crack-bridging effect of the high strength steel fibers included in UHPFRC. Therefore, steel bars are not necessary to reinforce the UHPFRC slab for shrinkage and temperature effects, thereby reducing the slab thickness and overall self-weight of the HFRP-UHPFRC composite beam system."
UHPFRC is also durable, they say:
* It has the characteristics of densely packed microstructure in which the water-cement ratio is lowered to near the hydration limit (0.24 or less), and voids are therefore reduced to the limit;
* It's highly resistant to mass transfer because the coefficient of water permeability and the diffusion coefficient of chloride ion in the UHPFRC are about 1/106 and 1/300 those of ordinary high strength concrete;
It can be used for more than 100 years without special repairs or reinforcement as specified in the Recommendations for Design and Construction of Ultra High Strength Concrete Structures [Draft] issued by the Japan Society of Civil Engineers.
"It is expected that the new composite beam system will increase beam stiffness, prevent buckling and delamination in the HFRP compressive flange, and more effectively utilize the high tensile strength of CFRP in the HFRP tension flange," observe Nguyen, Mutsuyoshi and Zatar. "Since the UHPFRC slab will carry the majority of compression, it is no longer necessary to include CFRP in the HFRP top flange. However, it is necessary to reinforce the top flange of HFRP beam which is connected with UHPFRC by shear connectors."
In their work, three full-scale composite beams with varying UHPFRC slab widths were tested under four-point flexural loading. Bolt shear connectors with and without epoxy bonding were used in the tested beams. The bolt shear connectors and epoxy were used to resist the horizontal shear flow at the interface between the HFRP I-beam and the UHPFRC slab. The composite action between the HFRP I-beam and UHPFRC slab was investigated.
"The test results showed that all the composite beams exhibited significant improvements in stiffness and strength properties, above those of simple HFRP I-beam without the UHPFRC slab," say Nguyen, Mutsuyoshi and Zatar. "A fiber model was developed to predict the strength and stiffness of the tested beams and the model accuracy was verified. A fairly good agreement between the experimental and analytical results was found."
The study revealed that HFRP-UHPFRC beams are efficient and can provide a very competitive, cost-effective and sustainable solution to bridge structures.
ARAMID FRP BARS MEET AASHTO LRFD SPECS
Aramid fiber-reinforced (ARFP) bars may successfully replace conventional steel and prestressing strand, providing sufficient strength per AASHT0 LRFD Bridge Design Specifications (2010), and considerable deformability when compared to the conventional system, say Shobeir Pirayeh Gar, Stefan Hurlebaus, John Mander, Zachry Department of Civil Engineering, Texas A&M University, and Monique Head, Morgan State University, Baltimore, in their paper, Comparative Experimental Performance of Bridge Deck Slabs with APRP and Steel Prestressed Precast Panels.
Full-depth precast concrete panels expedite the construction process, enhance safety and quality control, and reduce the on-site labor requirements for bridge deck slab application, the authors write. However, corrosion-induced deterioration of conventional steel during the lifetime of the structure is a serious concern, affecting the durability and serviceability of the deck panels.
"Although replacing conventional steel with fiber-reinforced polymer (FRP) bars has become more prevalent over the past few decades to overcome corrosion issues," the authors say, "there is still need for a comprehensive experimental study to investigate the structural performance of a FRP concrete bridge deck slab with precast prestressed panels at full-scale, and to address constructability issues."
A full-scale bridge deck slab consisting of full-depth precast panels reinforced and prestressed with aramid fiber-reinforced polymer (AFRP) bars was experimentally investigated in terms of constructability and overall structural performance. Then it was compared to a similar system, but reinforced with conventional steel and prestressing strand as a control specimen.
Aramid fibers are a class of heat-resistant and strong synthetic fibers, used in aerospace and military applications, for ballistic rated body armor fabric and ballistic composites. The full-scale, AFRP concrete bridge deck slab with full-depth precast, prestressed panels constituted an ideal structural system, which not only provided an efficient construction technique, but also increased the durability of the precast panels, say Pirayeh Gar, Head, Hurlebaus and Mander, adding, "Compared to carbon or glass fiber-reinforced polymer (CFRP or GFRP) bars, AFRP bars have acceptable fatigue and creep-rupture characteristics, as well as a reasonable cost. All the construction stages were implemented in a laboratory environment to resemble the offsite precast plant conditions. Dimensions, boundary conditions, structural details, and loading configurations were all physically modeled to reflect an actual bridge deck slab; constructability issues and structural performance were realistically evaluated."
The AFRP concrete bridge deck slab with full-depth precast prestressed panels was tested in the High Bay Structural and Materials Testing Laboratory of Texas A&M University. The results were compared to the control specimen reinforced with conventional steel and prestressing strand, and the authors concluding:
* By substituting for the conventional corrosion-prone bridge deck slab steel reinforcement with AFRP bars of equivalent capacity that may be either prestressed or non-prestressed, this research showed that the load carrying capacity and deformation performance was not impaired, and is well above AASHT0 load demands. Therefore, the long-term performance or serviceability of the proposed system is expected to be superior, as the deck will not be subjected to corrosion;
* The failure load of the interior span and overhang was found approximately 4.8 and 1.8 times the service load, respectively, which confirmed adequacy of the toad capacity of the AFRP specimen per AASHT0 LRFD. The deflection of the interior span under service load was found to be about 1 ram, which is less than the allowable amount, as per AASHT0 LRFD,
* At the slab interior span and overhang, the average load capacity of the AFRP specimen was found about 1.2 and 0.63 times that of the control specimen, respectively. The cracking pattern under the axle load looked very similar for both specimens--an elliptical shape at the slab interior span and a trapezoidal shape at the slab overhang.
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|Date:||Apr 1, 2013|
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