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Publication Number: FHWA-HRT-13-060
Date: June 2013

 

Ultra-High Performance Concrete: A State-Of-The-Art Report for The Bridge Community

CHAPTER 7. FUTURE DIRECTION

This state-of-the-art report identifies the following four primary characteristics of UHPC that distinguish it from conventional concrete:

The compressive strength of UHPC makes it an ideal material for use in applications in which compressive stress is the predominant design factor. The ductility in tension allows the tensile strength of UHPC to be considered in both service and strength design for flexure, shear, and torsion. The durability of UHPC makes it an ideal material for use in an outdoor or severe exposure environment. The higher initial unit cost means that its use needs to be optimized for the intended application and that greater attention should be given to life-cycle costs. In addition, specifiers should consider all costs associated with the use of UHPC on a project, not just the material unit cost. In many cases, the use of UHPC may allow a redesign of the structure thus affecting many aspects of the total cost of deploying the structure. For example, the ability to omit shear reinforcement in a beam can result in a savings of both materials and labor that must be considered alongside the increased material costs. Nevertheless, a number of challenges must be overcome to achieve wide-scale implementation in the U.S. highway infrastructure. These are outlined in the following sections.

OWNER ISSUES

One of the primary advantages of UHPC to owners is its long-term durability. As discussed in chapter 5, the measured durability characteristics far exceed those of conventional concrete. These characteristics should result in structures with a longer service life compared with structures built with conventional concrete, and thus could potentially decreased life-cycle costs. No studies were identified for this report to show that this is the case. When owners began to consider the use of high-strength concrete in bridge beams, a clear case could be made that the initial cost would be less because the number of beams for a given bridge would be reduced. This may not be true with UHPC because the cost differential between conventional concrete and UHPC is much greater than it was between conventional concrete and high-strength concrete. Studies are needed to illustrate the cost benefits of using UHPC for bridges in the United States.

The number of demonstration projects in the United States is limited, with most occurring in only two States. For owners to obtain a reasonable level of comfort in using UHPC, more demonstration projects are needed, and the results need to be disseminated through a variety of channels. These include webinars, in-house seminars, technical symposia, and technical publications. Some of this activity has been ongoing for the past 10 years but more is needed. This is not just for owners but also for bridge designers, contractors, and producers.

There are, however, situations where UHPC can be used to address certain performance issues without a major cost impact. One example is the use of UHPC to fill the connection regions between adjacent prefabricated elements. In this application, the overall cost increment in using UHPC is small because the quantity of material is small. The use of UHPC is reported to eliminate the cracking and leakage that occurs when conventional concretes or grouts are used. At the same time, the use of UHPC can enable the deployment of simplified connection details with shorter discrete reinforcement splice lengths and a reduced number of conflict points.

DESIGN ISSUES

The literature search identified the following national design and construction recommendations for UHPC:

These documents are generally based on the primary document used for bridge design in the individual country. Where sufficient information is not available to support a change or a change is not necessary for UHPC, the documents resort to the provisions of the primary document.

For UHPC to gain greater use in the U.S. highway infrastructure, a design and construction document based on the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications is needed.(76,308) The lack of this document has led to the need to consider each project individually. In most cases, the design has been accepted based on structural tests rather than a rational design basis. A guide specification for construction with UHPC will help owners implement the technology.

Although more research is desirable, it is likely that sufficient information exists today to develop a document addressing the major aspects of structural design according to U.S. practices. These design aspects include material properties, flexural and axial load, tensile load, shear, transfer and development length of prestressing strand, approximate estimates of time-dependent losses based on creep and shrinkage data, some aspects of reinforcement details, and durability. Where information is lacking, the document could use the provisions of the existing bridge specifications. This concept may not immediately result in the most economical design but will generally be conservative. Because several demonstration projects have been completed in the United States, there should be sufficient experience available to identify the necessary provisions in a construction guide specification.

For proper implementation of UHPC, new test procedures that address UHPC are needed for both development of mixes and quality control of the fresh and hardened UHPC. In most cases, these can be adaptations of existing test standards for conventional concrete but modified for the particular properties of UHPC. In addition, generic material specifications are needed to encourage the introduction of competitive materials.

PRODUCTION ISSUES

At the present time, very few producers have experience with UHPC for precast or cast-in-place applications. Information needs to be made available so that they are aware of the differences to expect with UHPC. For example, precasters need to be aware of the need for longer mixing times in conventional concrete mixers, longer set times, and modified curing regimes. Quality control tolerances need to be defined for the standard test methods. For example, the use of small-size cylinders for measurement of compressive strength needs to be established, along with the requirements for specimen preparation and testing machine capabilities.

RESEARCH NEEDS

The largest general design topic area where research is lacking concerns reinforcement details for nonprestressed reinforcement and prestressing strands. This includes development of and splice lengths of bars in tension and compression. Although the existing provisions for conventional concrete could be used, they do not take advantage of the enhanced tensile and compressive strengths of UHPC. A systematic investigation of strand spacing and strand cover is needed for 0.5-, 0.6-, and 0.7-inch (12.7-, 15.2-, and 17.8-mm)-diameter strands to determine whether decreased spacing can be used with UHPC.

Investigations into the use of and reliance upon fiber reinforcement in structural concrete members is also needed. Fiber type, geometry, volume, dispersion, and orientation can all affect the structural performance of the concrete member. Development of interrelated material proportioning methods, component fabrication methods, and structural design concepts are recommended.

U.S. Federal law requires compliance with Buy America provisions. Research is needed into the use of either domestically produced steel fibers and/or the use of non-steel fibers while still producing a UHPC-class material that affords appropriate characteristics.

SUMMARY OF NEEDS

To encourage greater implementation of UHPC in the highway infrastructure, the following activities and documents are needed in approximate order of priority:

AASHTO and FHWA should consider the development of structural design and construction guidelines. This effort should include research to address some of the needed missing information. The current efforts to engage organizations such as the American Concrete Institute, the Precast/Prestressed Concrete Institute (PCI), and ASTM should be extended to AASHTO.(309) PCI should work to develop production procedures for precast UHPC products. The National Ready Mixed Concrete Association should endeavor to address hurdles related to cast-in-place UHPC production, delivery, and casting. The involvement of the AASHTO Highway Subcommittee on Materials and ASTM Committees C09 on Concrete and Concrete Aggregates and C01 on Cement would facilitate the development of test methods and material specifications. The availability of funding to support these activities would accelerate the process.

The need for broader geographic distribution of demonstration projects should be addressed by FHWA in cooperation with the State departments of transportation.

Finally, and perhaps most important, owners need to be convinced that the use of UHPC is a good investment. Without that justification and the resulting demand, UHPC will remain a niche product.

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