Studies indicate that steel bridges have microscopic flaws in both the steel itself and at the welds of segments. In determining the potential deterioration rate of a steel bridge, the likelihood of these flaws is evaluated along with load stress and load stress cycles. Data indicates that steel bridges with high incidence of heavy truck traffic are more likely to crack and deteriorate at these discontinuities in the metal. The length of the bridge span and size of the steel components are also factors in how fast deterioration will occur, but in 1979, 86 percent of state highway departments reported to Congress that heavy trucks do have a detrimental effect on the longevity and structural integrity of steel bridges.
HPC, also known as Reinforced Concrete (RC), is used for both bridge decking and entire bridge structures. Studies show that environmental conditions like freezing, thawing, and ice-melt "salt" use have less of an effect on pothole development in HPC decks than does the load stress and load stress cycle impact of heavy trucks. Furthermore, where there are discontinuities in HPC bridge supports, typically occurring where other materials have been included for reinforcement, hairline cracks develop; it has been determined that these cracks grow and develop a grid-like pattern of fatigue with increased loads and load cycles.
Most timber bridges in the country were built before heavy trucks were used for freight, or are built in areas where heavy truck traffic doesn't occur. With timber-built bridges, the primary design specification for bridge life expectancy and safety is weight. Timber bridges are posted with a weight capacity that must be adhered to; exceeding posted weight causes material stress, deterioration and collapse. Even snow loads are known to collapse timber bridges. Heavy truck traffic on wood bridges is not viable.
As of 2010, there is little data available on alternative material construction for bridges. However, because of the need for material recycling of all types, it is anticipated by the U.S. Department of Transportation that alternative materials will become an issue in bridge construction. Analysis of load and load cycling, and the effects of heavy trucks on such materials, will be factored in.
