Army Engineer Magazine 6q4r1x

AE Magazine March–April 2010

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he RTCL material selected for these applications is polypropylene (PP) coated fiberglass blended with high‐ density polyethylene (HDPE). Advantages of using RTCL include, corrosion, insect, and rot resistance; no toxic chemical treatments required to increase service life; environmentally friendly; diversion of waste plastics from landfills; reduction of deforestation, green house gases, and global warming. RTCL has many advantages but does behave differently than traditional materials and certain properties must be addressed during the design stage. The materials science advancements enable RTCL to be used in a broad array of applications, including those with strict and heavy load requirements. The first vehicular bridge composed of a PS/HDPE RTCL with typical rectangular cross sections was built at Fort Leonard Wood, MO in 1998 with an initial cost greater than chemically treated wood. To date, this bridge has not required any maintenance and looks new. When viewed on a lifecycle cost basis, this bridge paid for its higher initial material costs in less than eight years (2004). However, initial costs are still most often the deciding factor for material selection (i.e. substituting RTCL for traditional chemically treated‐wood). Since 1998, researchers and engineers have sought construction designs using RTCL that are cost competitive to traditional chemically treated wood designs on a first‐cost basis. Thus, arch and I–beam cross section designs were investigated as a means to reduce the material and installation costs for a given load capacity. The latest demonstration of this sort is a RTCL bridge with an I‐beam sub‐structure and a maximum load capacity sufficient to handle an M1 tank constructed at Fort Bragg, NC. This innovative design is cost competitive with a wood timber bridge designed to carry the same load. However, the RTCL I–beam sub‐structure bridge is virtually maintenance‐free and is not subject to degradation effects of moisture, rot, insects, weather, and corrosion. It is a non–trivial effort to design safe, efficient RTCL structures capable of bearing heavy loads over long time periods, even though these are sophisticated composites. This is due to the nonlinear nature of the mechanical properties of polymers and polymer composites. For example, polymers and polymer composites are subject to creep, which is the permanent deformation that occurs due to a long‐term constant load. The primary project aim was to construct two low–

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AE Magazine March–April 2010 maintenance, affordable bridges using recycled materials in short span applications. The project team was to build and evaluate the structures composed of RTCL as a replacement for a conventional wood timber bridge with an H–25 load rating (based on the American Association of State Highway and Transportation Officials) and designed to maintain the load of an M1 tank. Secondary aims included evaluation of the mechanical performance of the bridge, determination of the costs to complete each bridge and draw comparisons with a wooden structure of the same span and loads, and determination of other benefits of utilizing these types of construction materials and methods. The research team designed, built, and load tested two bridges which satisfied all requirements. The bridges were designed by McLaren Engineering Group using traditional timber bridge design methodology adjusted to accommodate the unique material properties of the RTCL material. The material selected is a RTCL composed of PP coated fiberglass (FG) blended with HDPE, (FG+PP)/HDPE, and is manufactured by Axion International, Inc. The selected material is unique in several attributes. It is a combination of recycled HDPE, which is normally subject to significant creep at very low stress (e.g. 100 psi), and automobile bumper scrap, which is a composite composed of PP and FG. The materials are sourced from the consumer and industrial waste stream. The (FG‐PP)/HDPE material properties are available in the previously referenced patents for specific percentages of (FG+PP) in HDPE, and the current lumber properties are available through the manufacturer, Axion. This RTCL has a specific gravity of about 0.85 and is one eighth the density of steel, but the resulting specific strength (strength/unit weight) is greater than many steels. Degradation due to natural UV direct sunlight does not exceed a rate of 0.003 inches/year. This slow rate of degradation contributes to make this material superior to other material options. The basic construction design included rows of pilings with pins holding I‐beam piling caps to each row, a steel sill plate pre‐drilled with holes to align girders, girders spanning the length of the bridge bolted to the pile caps and placed edge to edge across the bridge width, smaller I‐beams nestled in the web of and perpendicular to the larger I‐beam girders, deck boards affixed to the girders with standard deck screws, and curbing and a railing affixed to the edges of the bridge. The width of both bridges is 16 feet and 6 inches, and the span is 42 feet and 56 feet, respectively. Construction methodology was typical of a timber bridge, with exceptions for the properties and quantity of materials. Standard installation equipment, construction tools, and hardware were utilized. Construction of the RTCL bridges was successful and both performed successfully in load tests. Load testing occurred on June 11, 2009, when an M1 safely crossed and verified the safety validation of the new structure. A second bridge was dedicated on September 18, 2009, during which there was another tank crossing. Field testing and data acquisition on the

bridges post‐construction confirmed that the stresses under tank loadings (71 tons) were below the creep stresses of the material, and the deflection was limited to about 0.5 inches. This deflection is well below the allowable deflection, according to the bridge designs. In addition to the M1 tank, several other types of vehicles crossed the bridges, including a 30 ton steam roller. The steam roller applied 3oo lbs/linear inch along the between the roller and the deck surface, resulting in a load applied to only one deck board at a time rather than an area. The steamroller was not listed in the potential vehicles to cross the bridges and this type of almost point loading was not ed for in the design considerations. This was immediately apparent when the steam roller crossed. Some of the deck boards on the outer edges of the bridge bent upward, as the screws located on the few girder‐deck board connections were partially dislodged. This type of damage between the deck surface and the girders decreases the potential load sharing between the girders and increases the stresses due to the applied load directly under the wheels. This is in contrast to bridge designs, which attempt to optimize the deck board–girder connections to help lower the maximum stresses imparted during loading. Modifications were made on both bridges to ensure future successful crossings. Based upon data obtained from field testing and resident monitoring devices of bridges at Fort Bragg, it is apparent that this technology is both viable and desirable for all structural short span bridge applications. A third bridge will be constructed at Fort Bragg by the summer of 2010. The thermoplastic composite products used for these bridges are made from virtually 100% recycled materials, they are cost competitive on an installed cost basis as compared to traditional materials, the life cycle costs are far lower than traditional materials since RTCL is virtually maintenance free and not subject to corrosion or degradation, and construction time is much faster than traditional materials. This “rapid build” capability of the RTCL materials allows construction completion in four weeks for a bridge design like those at Fort Bragg, as opposed to a steel/concrete bridge that requires significant curing times that require construction delays. The Fort Bragg project received a strong vote of confidence from the DoD due to its success, as stated by Daniel J. Dunmire, Director of Corrosion Policy and Oversight: “This thermoplastic bridge, able to withstand heavy loads with little to no maintenance, expected to last at least 50 years, is no longer the bridge of the future‐it’s the bridge for today.” AE This presentation is based on an original technical and scientific abstract titled, “The Utilization of Recycled Thermoplastic Composites for Civil and Military Load Bearing Applications”, written by Dr, Thomas j. Nosker, Dr. Jennifer K. Lynch, and Richard G Lampo, Rutgers University Materials Science and Engineering Department. It was sponsored by the US Army Corps of Engineers Construction Engineering Research Laboratory.

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