For a small community, Greenfield, Mo., was plagued with what appeared to be major inflow and infiltration (I&I) problems. The sewer pipes...
Innovative materials improve service life, reduce service cost
The Innovative Bridge Research and Construction (IBRC) program was established in 1998 within the Federal Highway Administration by the Transportation Equity Act for the 21st Century (TEA-21). The six-year IBRC program provides funding for the primary purpose of helping state and local transportation agencies demonstrate the use of innovative materials for bridge repair, rehabilitation, replacement and construction. Some key goals of the IBRC program include:
The true value of the IBRC program is in advancing technologies that will enable all bridges to last longer at a lower total cost. IBRC funds are being used for special engineering studies, material testing and evaluation, mix designs and for instrumentation and post-construction monitoring of the performance of high-performance concrete (HPC) bridges.
Under typical circumstances, the state DOTs might not be able to conduct these extra efforts because of a shortage of staff, pressure of project schedules or lack of funds. IBRC funds can help overcome these difficulties and are often used to enlist the services of local universities to conduct the necessary studies.
Another important goal is to help foster the widespread use of proven applications of these innovative materials by developing engineering design criteria for using innovative products and materials in highway bridges and structures. These goals are entirely consistent with and support the FHWA’s goal of eliminating bridge deficiencies and improving mobility.
For over 200 years, concrete and ferrous metals (and, to a lesser degree, timber) have been the primary materials for building highway bridges. Bridge engineering has advanced from stone and masonry to reinforced concrete and prestressed concrete; from cast iron to ductile irons to high-yield strength steels; from concrete T-beams and simple slabs to prestressed concrete box beams; from riveted to bolted to all-welded steel members.
Until recently, the emphasis in improvements in bridge engineering has been related to new shapes, cross sections and structural systems and to new or refined design codes such as the AASHTO Load and Resistance Factor Design specifications. Now, we are in the midst of a significant leap forward in the materials used for bridge building.
Concrete, steel and timber have served us well, and the science of bridge engineering is founded on solid principles and proven codes. Still, the advancing age, diminished capacity and deteriorating condition of the nation’s bridge inventory—587,460 bridges on public highways—is the source of some significant concern to bridge owners. Deficiencies on specific bridges also are the cause of considerable inconvenience and additional travel costs to the traveling public and to commercial users of the highway.
The FHWA is committed to improving mobility on the nation’s roads and to lowering the cost of building and maintaining highways and bridges in serviceable condition. Mobility will be improved by eliminating deficient bridges, by speeding up the repair process and by fixing or building bridges that will require less frequent maintenance and repair in the future.
What is becoming increasingly obvious is that improved (i.e., high-performance) versions of traditional bridge materials are critical to the FHWA’s goals of eliminating deficient bridges and making bridges serve longer at lower total (life-cycle) cost.
Follow the money
Bridges on all public roads are eligible for IBRC funding, but applications must be submitted through a state DOT. While funds can be requested for both preliminary engineering and construction work, priority consideration is given to construction work. FHWA also encourages the use of IBRC funds to cover the costs of instrumentation and monitoring and evaluation of the performance of the innovative materials both during and after construction. Other selection criteria include whether the project meets one or more of the goals of the program, incorporates materials that are readily available, is ready or nearly ready to proceed to the construction phase and has a design with potentially widespread application.
The IBRC program is funded for six years through fiscal year 2003 at a total authorized level of $102 million. As of fiscal year 2001, 157 projects had been selected for funding. Seventeen million dollars will be available for construction projects in fiscal year 2002 and a similar amount in fiscal year 2003. The program funding runs out after fiscal year 2003, but under the program the FHWA will continue to evaluate experience on the various projects and will disseminate information on lessons learned.
By the time the program expires, IBRC funds will have supported the construction or rehabilitation of an estimated 250 bridges with innovative materials. The lessons learned from some completed IBRC projects are encouraging and show the potential for considerable improvements in cost, service life and service capacity on bridges regardless of size and traffic volume.
HPC is one of the most important and far-reaching products developed under the Strategic Highway Research Program of the 1980s. The concept—concrete engineered to meet the service requirements for site-specific field conditions—has enormous potential for application in reinforced and prestressed concrete bridge elements as well as for concrete highway pavements. The goal is long-term performance and, specifically for bridges, the enhanced ability to build bridges with service lives of 75 to 100 years.
With funding from the IBRC program, the Virginia DOT has initiated a project to evaluate the use of structural lightweight HPC in prestressed bridge girders. The lightweight aggregate is a rotary kiln expanded slate. The low-permeability mix design also used 40% slag, with a total of 752 lb of cementitious material. The water-to-cementitious material ratio was 0.32.
Three Type II AASHTO girders were fabricated, two with the lightweight mix and one with a normal-weight mix. Deck slabs were cast for composite action. The girders will be tested for development length of the prestressing strands as well as flexural strength. The normal-weight girder will serve as a benchmark for comparison to the lightweight girders.
Two AASHTO Type IV girders, identical to those that will be fabricated for the actual bridge structure, also were fabricated with the lightweight mix. These girders were instrumented with vibrating wire strain gauges, placed at the center of gravity of the prestressing force at mid-span. The concrete strains will be monitored over the next year to determine prestress losses. Also, differential leveling during this period will monitor girder camber. Three of the actual bridge girders will be instrumented similarly, and long-term monitoring of the concrete strains will be performed in situ.
The bridge structure carries Route 106 over the Chickahominy River at the border between Charles City County and New Kent County. It consists of three 85.8-ft spans. The structure is designed to be continuous for live load and consists of six Type IV girders per span. VDOT hopes the study will result in a reduction in dead load as well as the other strength and durability advantages of HPC.
High-performance steel (HPS) is a success story that resulted from the combined efforts of key states, the FHWA, the U.S. Navy, industry and academia. Based on research begun in 1992, the steel industry is now producing grades of steel that have higher toughness and better corrosion resistance and that allow significant improvements in welded connections. High-strength HPS 70W steel is available along with HPS 50W, and bridges are being built using both grades in designs optimized for economics.
The Tennessee DOT was an early proponent of HPS for bridges and in 1996 began the construction of the bridge carrying State Route 53 over the Martin Creek embayment. This two-span bridge with a 28-ft roadway resting on three continuous welded plate girders was designed using the newly adopted AASHTO Load and Resistance Factor Design specification for bridges. The bridge is founded on two integral, pile-supported abutments and one hammerhead pier and has no transverse joints.
The Tennessee DOT originally designed the bridge with 675,319 lb of grade 50W steel. The bridge was redesigned and all girder material in the revised design was HPS 70W. As a result, the total weight of steel in the as-built bridge was 510,994 lb. A savings in weight of 24.2% was realized, and the total cost of the in-place steel was reduced by 10.6%.
Further developments in HPS steel since the construction of the Martin Creek bridge have lowered the cost premium between HPS and conventional steels. Engineers are now learning to optimize the use of HPS 70W where the increased yield strength is truly needed, and both of these factors will lead to higher potential savings in the total cost of steel bridges.
The last solicitation for candidate IBRC projects will be announced on or about March 15, 2002, with project applications due by July 15, 2002. For further information on the IBRC program, visit the IBRC website (http://ibrc.fhwa.dot.gov).