New Lightweight Concrete Bridge Deck Facilitates Widening

South Elevation Rte. 608 over Rte. 95 NB and SB
(Photo courtesy of Thomas Miller at VDOT)

By Reid Castrodale, PhD, P.E., Carolina Stalite Company, and Steven C. Kerr, P.E.

Project Name: Route 608, Massaponax Church Road, over Interstate 95

Owner: Virginia Department of Transportation

Design Consultant: Whitman, Requardt & Associates, LLP, Richmond, VA

Contractor: Corman Construction, Richmond, VA

Concrete Supplier: Rowe Materials

Lightweight Aggregate Supplier: Carolina Stalite and Solite, LLC

Material Quantity: 330 cubic yards of lightweight concrete in deck and parapet

Material Specifications: VDOT A4, f’c=4,000 psi, air entrained 1,500 coulombs permeability

New Bridge Deck Dimensions: 245’ length x 40’ width x 8” thick South Elevation Rte. 608 over Rte. 95 NB and SB

Year Completed: 2009

The Route 608 bridge over Interstate 95 near Fredericksburg, Virginia was originally opened in 1964. It was designed in accordance with the standards of the time period with a design live load of H 20, and a normal weight concrete deck that was 20’ wide. In recent years, the vicinity has seen significant growth, necessitating a wider deck and an upgrade to an HS 20 design live load to bring the widened bridge to existing VDOT standards.

Due to the high and rising cost of steel beams, VDOT wanted to accomplish the upgrade with minimal additional steel superstructure. Utilizing lightweight concrete for the deck and the parapet enabled the reuse of the existing steel beams.

The original design contained 2 concrete columns per pier. Doubling the deck width from 20’ to 40’ and increasing the service load required only one additional concrete column per pier and two additional plate girders.

Had normal weight concrete been used for the deck, the existing steel beams would have been overstressed and not accomplished the increased service load rating.

SLWC Pic 2
East Approach Rte. 608 over Rte. NB and SB
(Photo courtesy of Thomas Miller at VDOT)

According to Thomas Miller, P.E. of VDOT’s Fredericksburg Structure and Bridge Department, “The design intent in going with a lightweight concrete deck was to take advantage of the weight savings to increase the bridge capacity and while reusing the existing steel beams.”

Lightweight concrete was specified by a note on the plans that read “Concrete in superstructure including parapets shall be Class A4 (lightweight) having a maximum unit weight of 120 pcf.” In correspondence, Thomas Miller mentioned that if he had another project with lightweight concrete, he would modify the general note to read “maximum equilibrium density of 120 pcf” to clarify the intent. He also indicated that special provisions were not required for lightweight concrete because the bridge was a pilot project for VDOT’s Hydraulic Cement Concrete – End Result Specification. Lightweight aggregate was already addressed in Section 206 of the VDOT Standard Specifications.

The specified minimum compressive strength for the deck and parapet concrete was 4,000 psi. The plan quantity for deck concrete was 288 CY with 490 linear feet of standard parapet. The average compressive strength of the lightweight concrete delivered to the project was 5,930 psi. The average of the ASTM C 1260 rapid chloride permeability test results for the lightweight concrete used on the project was 1,015 coulombs, well below the specified maximum limit of 1,500 coulombs.

From the perspective of sustainability, lightweight concrete permitted the reuse of existing elements while minimizing additional construction materials.

As-built Conditions from 11/04/2009 Inspection Report:

BR06-088-117, C501; (UPC 76515)
Rte. 608 (Massaponax Church Road) over Rte. 95, Spotsylvania County

Bridge Replacement over Rte. 95 (Structure #6027) TSM