Says Everett: “We strongly believe that the outcome of the LTBP program will strengthen our ability to deliver, manage and maintain an efficient highway infrastructure,” and will help:
advance deterioration and predictive models;
improve knowledge of state maintenance effectiveness;
demonstrate the effectiveness of sensor technology in bridge inspection/condition assessment;
implement performance-based design; and
foster the next generation of bridge and bridge management tools.
What has changed most in the past five or 10 years in the way FHWA works with bridges?
“We have an improved awareness of the factors that affect a bridge’s performance, from design to construction to preventive maintenance to inspection, and ultimately rehabilitation/replacement,” says Everett. Some examples:
1. Preservation: One of the biggest changes took effect under SAFETEA-LU, when Congress recognized the importance of preserving the existing inventory of bridges and allowed the use of federal bridge program funding for systematic preventive maintenance/system preservation.
2. Improved design codes: In collaboration with AASHTO, FHWA has invested significant resources into developing the Load and Resistance Factor Design (LRFD) specifications and providing training to the states in using LRFD. The states are now using LRFD for all federal-aid bridges.
LRFD is based on technological advances in bridge engineering, sound scientific principles and a systematic approach to ensure safety, durability, economy and aesthetics. The LRFD philosophy is consistent with other major bridge design codes used in other progressive countries around the world.
LRFD-designed bridges are expected to have a long service life with minimal maintenance.
3. Improved load rating codes: The AASHTO Load and Resistance Factor Rating (LRFR) serves as a compendium to LRFD in setting the standards for condition evaluation and rating of bridges. LRFR provides procedures and policies for determining physical condition, maintenance needs and load capacity for highway bridges. LRFR assists bridge owners in establishing inspection procedures and evaluation practices that meet the National Bridge Inspection Standards. FHWA supports AASHTO in adopting and publishing the Manual for Bridge Evaluation (MBE), which provides guidelines for using LRFR.
4. High-Performance Materials for Improving Performance of Bridges: FHWA in collaboration with AASHTO, industry and academia has developed and deployed high-performance materials:
High-Performance Concrete (HPC): HPC is one of several products investigated and advanced under the Strategic Highway Research Program. HPC has enhanced durability and strength not normally attainable in conventional concrete. HPC is denser, stronger and less permeable. (Strategic Highway Research Program 1 studies indicated that chlorides would diffuse through 2 inches of concrete cover in 12 to 20 years. For HPC elements using appropriate quantities of pozzalanic admixtures or slag, the chloride diffusion rate would be 50 to 70 years.) HPC enables longer bridge spans, increased durability (100-year life), improved engineering properties, better long-term performance and reduced life-cycle costs.
High-Performance Steel (HPS): Structural steels have high strengths, enabling engineers to design and build long-span bridges. However, extra care must be taken in welding, corrosion protection and crack prevention. To overcome these concerns in structural steels, a cooperative research program between the FHWA, the U.S. Navy and the American Iron and Steel Institute (AISI) developed HPS for bridges and structures. HPS has improved weldability, and resulted in excellent corrosion resistance, high crack tolerance and very high strengths. The combination of these improved properties of HPS leads to cost-effective applications in bridge design and construction. Many states are taking advantage of these properties in new bridge designs to improve long-term performance, lower first cost and reduced life-cycle cost through lower maintenance and repair costs.
Fiber-Reinforced Polymers/Composites (FRP): FRP has unique properties, such as high strength, light weight, corrosion-resistance, high toughness, etc., which make it very good for strengthening, repair and seismic retrofit of bridges and structures. FRP has great potential for providing engineering solutions for rebuilding our aging infrastructure. It has attracted the interest and attention of the research community, government and private industry to find ways to successfully integrate FRP in structural applications. In recent years, FRP has been used as rebars and prestressing tendons in concrete structures, sheets and laminates for strengthening concrete and steel members, wraps and shells for seismic retrofit of concrete columns, and structural shapes for bridges and decks.
Ultra-High-Performance Concrete (UHPC): A fiber-reinforced cementitious composite, UHPC is the next level of high-performance concrete. UHPC has very high compressive strengths in the range of 20 ksi to 30 ksi, and tensile strengths as high as 1.5 ksi. It has high durability – very low permeability and is highly resistant to abrasion, freeze-thaw and scaling. The cost of UHPC is steep at the present time. It is now used cost-effectively in a few applications, including closure pours between precast panels, precast deck panels and some girders.
5. Self-Consolidating Concrete (SCC): SCC is a concrete that does not require vibration during placement. SCC flows into and completely fills intricate and complex forms under its own weight, passes through and bonds to congested reinforcement under its own weight, and is highly resistant to segregation. SCC offers many advantages for the precast concrete industry and cast-in-place construction. For example, low noise levels in the plants and construction sites, eliminated problems associated with vibration, less labor involved, faster construction, and improved quality and durability. States are using SCC in precast, prestressed bridge elements, and cast-in-place projects.
6. Lightweight Concrete (LWC): Benefits over normal-weight concrete include reduced dead load of structure, enhanced durability (better curing), and reduced handling, transportation and erection costs (although actual material costs are higher).
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