Claude S. Napier, Jr., Federal Highway Administration
Virginia has used a systematic approach to improve its existing and new concrete bridge structures. The key to success has been close cooperation between the bridge, materials, and construction engineers and the researchers, managers, and Federal Highway Administration (FHWA) staff using the best available technology to solve problems and to implement new technologies. The operations personnel of the Virginia Department of Transportation (VDOT) have worked closely with the Virginia Transportation Research Council’s (VTRC) concrete and bridge research advisory committees and industrial partners.
High performance concrete has been evolving in Virginia over the last fifteen years through extensive laboratory research and field testing with numerous pilot projects to ensure that the performance is acceptable before full-scale implementation. Since 1989, Virginia has made significant changes to its concrete specifications and procedures for high performance concretes including concretes with low permeability, high durability and, when needed, higher early and later-age compressive strengths. The following sections highlight some of the changes that have been made.
Bridge Decks
In 1988, based on recommendations by FHWA, VDOT added requirements for limiting surface evaporation rates for concrete bridge decks. The following year, sawgrooving of concrete bridge decks was specified as a means to improve the quality and durability of the riding surface. And in 1994, a trial special provision requiring 7 days of moist curing for low permeability concrete (LPC) bridge decks was introduced. This is now a standard requirement for all LPC bridge decks.
Prestressed Concrete Girders
Traditionally, VDOT had not used high strength concrete in its precast, prestressed concrete girders. However, in the 1995-1997 construction seasons, five bridges with specified concrete compressive strengths of 7000 to 8000 psi (48 to 55 MPa) were built. In addition, bridges were built in Brookneal and Richlands to demonstrate the applications of HPC. The girders used in the Richlands’ bridge had a specified concrete compressive strength of 10,000 psi (69 MPa) at 28 days. In 1999, VDOT changed its practice to allow the use of compressive strengths up to 10,000 psi (69 MPa) in design, but required approval by the State Bridge Engineer for strengths over 8000 psi (55 MPa). In the same time period, VDOT was working with the Mid-Atlantic Prestressed Concrete Economical Fabrication (PCEF) Committee to develop bulb-tee beam sections that are more efficient than the AASHTO I-beams and permit longer span lengths. These new sections were adopted in 1999 to eventually replace the AASHTO sections. In 2003, bids were received on 13 PCEF bulb-tee bridges. VDOT’s federally funded bridge costs were reduced to $81/sq ft ($870/sq m) from $89/sq ft ($960/sq m) in 2002.
Permeability
In 1994, Virginia developed a permeability special provision for HPC. Maximum permeability values of 1500 coulombs were specified for precast, prestressed concrete, 2500 coulombs for the deck concrete, and 3500 coulombs for the substructure concrete because it was felt that the ready mix industry and the prestressed concrete producers could obtain them consistently. This approach bolstered the confidence of the VDOT operations personnel and the contracting industry in using HPC. The specimens are cured for one week at 73°F (23°C) followed by three weeks at 100°F (38°C) and then tested at 28 days. Seven HPC bridge structures were constructed between 1995 and 1997 and five included the permeability special provision. The requirements were adopted for all HPC projects after 1997.
Materials Technology
In 1992, the need for corrosion protection of strands prompted the inclusion of corrosion inhibitors in prestressed concrete. For LPC containing pozzolans or slag, corrosion inhibitors are used at low dosage rates only for concrete in a marine environment. Also in 1992, VDOT addressed the alkali-silica reaction problem by requiring either cement with an alkali content of less than 0.40 percent or the use of pozzolans or slag with cement having an alkali content up to 1 percent.
In 1998-1999, under an Innovative Bridge Research and Construction (IBRC) Program project, monofilament fibers were used on the Route 11 bridge over the Maury River to control or minimize deck cracking over the piers. In 2003, the total length of cracks was about 25 percent of the length of cracks in a control section without fibers and the average crack width was about half.
Lightweight Concrete
In 1998-1999, another IBRC project was used to implement the use of lightweight HPC (LWHPC) in the prestressed concrete girders and reinforced concrete deck of the Route 106 bridge over the Chickahominy River. Subsequently, LWHPC is being used in haunched spliced bulb-tee beams, PCEF bulb tees, and concrete decks of the Route 33 bridge over the Mattaponi River and in haunched spliced bulb-tee beams with span lengths up to 240 ft (73 m) on the Route 33 bridge over the Pamunkey River.
Summary
By October 2002, 19 HPC bridges had been built in Virginia, 42 were under construction, and 90 under design for a total of 151 projects. Beginning in November 2003, HPC has been used on all bridges that use federal funds. It is expected that HPC will be used on all state-funded bridges sometime in 2005.
The HPC program is progressing successfully based on VDOT’s partnership with industry and FHWA to ensure that the technologies are functionally and economically acceptable. Higher performance concrete structures are cost-effective and are expected to have higher durability, longer service life, and minimum maintenance requirements.
Readers may visit the VTRC website at www.virginiadot.org/vtrc/main/index_main.htm to see reports on a number of the items mentioned above.
Editor’s Note
This article is the fourth in a series that describes how the use of HPC has progressed since it was first introduced into a State’s program. Other articles appear in Issue Nos. 30, 35, and 36.