High-performance concrete (HPC) could change the shape--and life span--of portland cement concrete (PCC) bridges. HPC is a performance-based approach to specifying concrete mixes that are more durable and, if necessary, stronger than conventional concrete. HPC's improved durability translates into bridges that hold up better to traffic, climate and other conditions. The material's greater strength allows bridge designers to use more efficient girder cross sections, and possibly to eliminate some girder lines or piers, which can cut overall costs. The result is longer-lasting and potentially less expensive bridges.
The Louetta Road overpass in Houston, Texas, shows how highway agencies can benefit from HPC. The structure which the Texas DOT (TexDOT) says is the first bridge in the country to fully utilize HPC for all aspects of design and construction, could have a useful life of 75 to 100 years, about twice as long as the average bridge. Yet the cost to build the bridge was comparable to similar bridges constructed with conventional concrete.
Using HPC will require highway agencies and industry to change their procedures for designing concrete mixes and designing and constructing bridges. One of the most important tasks before the lead states team for HOC bridge implementation is providing the information and tools to materials engineers, bridge designers, and others so they can take advantage of HPC. The team, which was created last fall under an initiative conceived by the American Association of State Highway & Transportation Officials (AASHTO) Task Force on SHRP Implementation, is involved with several projects that will help state and industry implement HPC for highway structures.
One of the team's projects is to work with AASHTO on new standards for PCC bridge design and mix design.Team leader James Moore, New Hampshire DOT, explains, ÒThe current AASHTO bridge design standards are adequate for concrete as strong as 10,000 psi (69 Mpa), but they may be too conservative for the stronger grades of high-performance concrete.Ó Likewise, Moore adds, AASHTO materials standards do not apply to HPC mixtures. Moore will meet this summer with the AASHTO bridge and materials subcommittees to discuss changes to the standards.
To help state highway agencies understand the characteristics and performance of HPC, the lead states team is refining the current working definition of HPC developed by Charles Goodspeed, Suneel Vanikar and Ray Cook and published in the February 1996 issue of Concrete International. The definition is essentially a set of performance criteria and associated tests that will allow states to specify concrete that meets certain performance grades. The team has asked states that build HPC bridges to amend and expand the definition based on what they learn from the projects.
The strength and durability of concrete mix depends in large measure on the cement and aggregates locally available. Thus, materials engineers and bridge designers must keep in mind what can be accomplished with locally available materials Òso that they can effectively use HPC to achieve longer life cycles and lower costs,Ó says Moore.
Another of the team's projects will help highway agencies track the life-cycle cost of HPC bridges. The team is developing a standard form that will help bridge design departments and maintenance departments accurately assess the benefits versus costs of HPC bridges.
ÒNot too many states have taken the time to work on the life-cycle costs of bridges. The form will make that easier,Ó says Moore. The form should be ready this fall.
To ensure that information about HPC gets to state highway agencies, the team is working with the Federal Highway Administration (FHWA) to hold at least three regional showcase workshops on HPC each year.
The workshops provide participants with a chance to see how highway agencies are using HPC. At the workshop in August, for example, Washington State DOT will conduct a field review of its recently constructed HPC three-span girder bridge. The workshops also cover the advantages and disadvantages of HPC, mix proportioning, structural design considerations and other topics. Three workshops are scheduled for the next several months.
On going projects
There are now about a dozen HPC projects under way in the U.S., and the lead states team wants to add to that number. The team is working closely with FHWA's HPC structures technology delivery team, which is charged with identifying good candidates for HPC demonstration projects and directing HPC research, to have at least five HPC projects in progress at all times.
Although the focus has so far been on HPC structures, HPC also has potential for use in pavements. In addition to improved durability, HPC mixes can also be designed to provide faster curing times, allowing highway agencies to more quickly open a new PCC pavement to traffic. HPC rigid pavements will soon receive more attention. The FHWA continues to solicit states to participate in HPC pavement projects.
For more information on HPC bridge implementation, contact James Moore of New Hampshire DOT, at (603) 271-2731, fax (603) 271-7025.