Army Engineer Magazine

AE Magazine March–April 2010

The RTCL material selected for these applications is polypropylene (PP) coated !berglass blended with high"density polyethylene (HDPE). Ad-

vantages 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 land!lls; reduction of deforestation, green house gases, and global warming. RTCL has many advantages but does behave di#erently 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 require-ments. The !rst vehicular bridge composed of a PS/HDPE RTCL with typical rectangular cross sec-tions 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 mate-rial 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 !rst"cost basis. Thus, arch and I–beam cross section designs were investigated as a means to reduce the mate-rial 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 su$cient to handle an M1 tank constructed at Fort Bragg, NC. This innova-tive 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 degrada-tion e#ects of moisture, rot, insects, weather, and corrosion.

It is a non–trivial e#ort to design safe, ef-!cient 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–

maintenance, a#ordable 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 O$cials) and designed to maintain the load of an M1 tank. Secondary aims included evaluation of the me-chanical 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 bene!ts of utilizing these types of construction materials and methods.

The research team designed, built, and load tested two bridges which satis!ed 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 mate-rial selected is a RTCL composed of PP coated !berglass (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 signi!cant creep at very low stress (e.g. 100 psi), and automo-bile 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 avail-able in the previously referenced patents for speci!c percent-ages of (FG+PP) in HDPE, and the current lumber properties are available through the manufacturer, Axion.

This RTCL has a speci!c gravity of about 0.85 and is one eighth the density of steel, but the resulting speci!c strength (strength/unit weight) is greater than many steels. Degrada-tion 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 a$xed to the girders with standard deck screws, and curbing and a railing a$xed 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 proper-ties 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 veri!ed 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 con!rmed that the stresses under tank loadings (71 tons) were below the creep stresses of the material, and the de%ection was limited to about 0.5 inches. This de%ection is well below the allowable de%ection, accord-ing 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 contact 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 ac-counted 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 dam-age 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. Modi!cations were made on both bridges to ensure future successful crossings.

Based upon data obtained from !eld 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 construct-ed 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 comple-tion in four weeks for a bridge design like those at Fort Bragg, as opposed to a steel/concrete bridge that requires signi!cant curing times that require construction delays.

The Fort Bragg project received a strong vote of con!dence 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 mainte-nance, 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 scienti!c abstract titled, “The Utilization of Recycled Thermoplas-tic 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.