State of the Science Although research on in-place aluminum bridge decks is scant, bridge failures and deteriorations over the past decade inspired engineers toward determining the structural integrity of aluminum alloys in bridge rehabilitation. Recent studies, examining the corrosive effects of acidic and salt atmosphere on aluminum bridge decks indicates the material is expected to endure for at least 30-years (Li, et al., 2010). Furthermore, the Virginia Department of Transportation conducted a series of tests, including service and ultimate load test using Reynold’s deck system (Dobmeier, 2001). The results of Dobmeier, et al.’s (2001) experiment, “clearly demonstrates that aluminum bridge decks are a feasible alternative to reinforced concrete decks”. When considering alternatives for bridge deck replacements, the structural engineer must evaluate the life cost of materials chosen. Specifically, although the material cost for aluminum bridge decks is higher than for structural steel, the cost for fabrication, construction and length of serviceability prove cost effective when compared to steel (Ghaswala, 2010; Tindall, 2008; Walbridge & de la Chevrotiere, 2012). Specifically, with a per unit weight that is about one-third that of structural steel, construction methods are simplified and time for rehabilitation reduced (Ghaswala, 2010). Furthermore, maintenance costs are greatly reduced with aluminum decks, which eliminate the need for protective coating (beyond that required at points in contact with other materials), and high corrosion resistance (Li, et al., 2010). Beyond the construction advantages of using aluminum decking in bridge rehabilitation, the reduced self-weight of the material allows the design engineer to replace heavier concrete decks and increase load carrying capacity of the structure (Li, et al., 2010; Ghaswala, 2010). Using aluminum deck for bridge rehabilitation projects also allows the designer to widen obsolete structures, as with Virginia Department of Transportation project in 1996 (Dobmeier, et al., 2001; Misch, et al., 1999). Although the advantages of aluminum bridge decks are clear, concerns about design standards and connection methods must also be considered. Recent studies determined cost savings for using aluminum bridge decks in movable bridge design, and found that a bridge using aluminum consumed at least half the power of structural steel (Hoglund, et al., 2010). For the bridge under study, one of the most important design criteria was power demand, which for aluminum totaled 9.17 kW per bridge opening (Hoglund, 2010). Clearly, for movable bridges, aluminum provides an advantage even beyond its fabrication, erection and corrosion resistance. In the Netherlands, an 80-year old steel and concrete bridge required upgrading and rehabilitation. It was determined by designers that aluminum was the only material that could meet the upgrade requirements (Hoglund, et al., 2010). Using aluminum, the bridge’s pedestrian paths were expanded from 2.5 to 4.8 meters (Hoglund, et al., 2010), which was not achievable with steel, concrete or a combination of the two. Using aluminum bridge decks on existing wood, concrete or composite decks has be successful in Europe and the United States over the past decade (Hoglund, et al., 2010). Studies conducted by Siwowski (2009) determined that “aluminum bridge decks panels are [a] feasible alternative to R[einforced] C[oncrete] decks from the standpoint of stiffness, strength and load carrying capacity”. The study also confirmed the behavior of weld joints; in the Heat Affected Zone (HAZ), the panels presented a clear weakness (Siwowski, 2009). Specifically, the HAZ material is the point of failure in each stress test (Siwowski, 2009). The deck panel failed at the weld, “unzipping” the connection and resulting in deck failure at 560 kN and 920 kN under the load patch and of the HAZ material of the bottom deck respectively (Siwowski, 2009). Using FSW process, however, will diminish the failures associated with HAZ and weldment fatigue. More recently, Saleem, et al. (2012) completed strenuous testing of aluminum bridge deck design currently used in Europe and Sweden. Laboratory testing was conducted for flexion, shear strength and uplift capacity of connections, lip test for the tongue and groove connections, fatigue and residual strength test of the system (Saleem, et al., 2012). The research found that the aluminum bridge deck panel design exceeded required design standards for the United States bridge systems. Specifically, a simple span sustained an ultimate load of 312 kN, which 87% higher than required (Saleem, et al., 2012). Furthermore, the two-span continuous panel test’s loading, unloading and reloading test proved minimal stiffness degradation (Saleem, et al., 2012). The shear strength of the connectors (clamps) indicated nearly three times (2.7) required strength, and four times required upload strength (Saleem, et al., 2012). With a target load of 166 kN, the system’s tongue and groove strength was double at 334 kN (Saleem, et al., 2012). Finally, the panels showed no sign of wear during the two million cycle wear test (Saleem, et al., 2012). Using Aluminum in Design of Bridges The structural engineer designing bridges need only familiarize himself with three of the many aluminum alloy families (Tindall, 2008). Specifically, the 5-, 6- and 7-xxx families are used in bridge design. Five-xxx alloys use magnesium for corrosion resistance, and are generally available in plate or sheet form (Tindall, 2008). Six-xxx alloys use magnesium and silicon to enhance extrusion properties, and are available in a virtually infinite number of shapes, plates and sheets (Tindall, 2008). The 6-xxx alloys are considered easily welded and provide “good all-round performance” (Tindall, 2008). The 7-xxx alloys use zinc and magnesium for initial strength and post-weld strength, and stronger than both the 5- and 6-xxx alloys (Tindall, 2008). Although the magnitude of shapes and alloys available can be overwhelming, one of the most important factors in bridge design is lead-time. The structural engineer designing an aluminum bridge must adjust his or her methods to include considerations such as the minimum order for special extrusions (generally one-ton), and lead time for special orders (Tindall, 2008). The basic design principals used for structural steel have been used for formulating aluminum design practices (######).
Tindall (2008) states that structural designers were never taught how to use aluminum in bridge design. With over 23% of aluminum use in construction (Li, et al., 2012), environmental concerns and costs associated with bridge rehabilitation, misconceptions of using aluminum bridge decks must be clarified. The weight of aluminum sections is a well-known advantage. Equally important, a variety of aluminum shapes are readily available (Tindall, 2008). This is particularly important for the structural engineer who is designing a unique structure. The variety of structural shapes is outside the scope of this paper, however, as the use of aluminum and familiarity with design methods expand, it is important for future efforts.
The structural engineer choosing aluminum for bridge deck replacement must consider the following: (1) aluminum has a low E-modulus, which must be considered for determining vibration and instability; (2) the fatigue stress of steel is about twice that of aluminum; (3) the coefficient of thermal expansion of aluminum (twice that of steel) weakens it at welments, and; (4) aluminum in contact with steel or concrete must be protected to avoid galvanic corrosion (Hoglund, et al., 2010). Using modern welding techniques (discussed later) such as Stir Friction Welding (SFW), proper protective coatings and design techniques, each of the concerns is easily overcome.

References

Dobmeier, J. M., Barton, F. W., Gomez, J. P., Masserelli, P. J., & McKeel, W. T. (2001). Failure study of an aluminum bridge deck panel. Journal of Performance of Constructed Facilities, 15(2), 68-75. DOI http://dx.doi.org/10.1061/(ASCE)0887-3828(2001)15:2(68).

Ghaswala, S. K. (2010). Resurgence of Aluminum in Structural Engineering. NBM Media. Retrieved from http://www.nbmcw.com/articles/bridges/5026-resurgence-of-aluminium-in-structural-engineering.html.

Hoglund, T., Soetens, F., Rothe, F., Hirsh, J., Ryckeboer, M., & Lundberg, S. (2010). Case Study: Aluminum Bridges. aluMatter. Retrieved from http://www.nbmcw.com/articles/bridges/5026-resurgence-of-aluminium-in-structural-engineering.html.

Li, H., Ban, H., Shi, Y., Wang, Y., & Zhang, Z. (2010). Experimental research on the wearability, corrosion resistance, and life assessment of an aluminum alloy bridge. Tsinghua Science and Technology, 15(5); 566-573.

Misch, P. C., Barton, F. W., Gomez, J. P., Massarelli, P. J., & McKeel, W. T. (1999). Experimental and analytical evaluation of an aluminum deck bridge. Virginia Transportation Research Council.

Saleem, M., Mirmiran, A., Xia, J. & Mackie, K. (2012). Experimental evaluation of aluminum bridge deck system. The Journal of Bridge Engineering, 17; 97-106. DOI: 10.1061/(ASCE)BE.1943-5592.0000204.

Siwowski, T. W. (2009). Structural behavior of aluminum bridge deck panels. Engineering Structures, 31(2009); 1349-1353.

Tindall, P. (2008). Aluminum in bridges. In ICE Manual of Bridge Engineering (pp. 345-355). Retrieved from http:// www.icemanuals.com. 345 ice | manuals doi: 10.1680/mobe.34525.0345.

Walbridge, S., & de la Chevrotiere, A. (2012). Opportunities For The Use Of Aluminum In Vehicular Bridge Construction. Aluminum Association of Canada.