Christopher M. Waszczuk, New Hampshire Department of Transportation
With the state and country’s infrastructure needing greater attention, the New Hampshire Department of Transportation (NHDOT) has become proactive in the use of better quality, higher performing materials. The goals are to reduce the maintenance and repairs typically necessary during a structure’s life and to try to increase the service life of structures.
To help achieve the above goals, NHDOT has become involved in the use of HPC as part of the Federal Highway Administration’s (FHWA) High Performance Concrete (HPC) Bridge Showcase Program. The first HPC bridge constructed in the region specified 8000 psi (55 MPa) concrete for the girders and 6000 psi (41 MPa) concrete for the deck. In addition to the benefit of increased strength, durability properties such as low permeability and increased freeze-thaw resistance were considered extremely important. Considerable research and background investigations were performed to ensure that the specifications addressed the practicality of all aspects of the concrete construction process including batching, placing, finishing, and curing.
The following provides a brief description of NHDOT’s first HPC structure with emphasis on the HPC deck construction experience. To ensure a longer life with little to no maintenance, it was essential to install a highly impermeable, crack-free, freeze-thaw resistant concrete deck.
Bridge Background
The HPC bridge is located in Bristol, New Hampshire and carries Route 104 over the Newfound River. The bridge is a 65 ft (19.8 m) single span structure with a total width of 57.5 ft (17.5 m) and was designed to accommodate three lanes of traffic and a sidewalk. The superstructure consists of a cast-in-place concrete deck composite with five precast, prestressed AASHTO Type III girders. The girders are spaced at 12.5 ft (3.8 m) on center. The deck utilizes the bare concrete as the final riding surface.
Deck Construction
The specifications necessitated that the concrete supplier perform several trial batches to refine the concrete deck mix proportions. Once the mix proportions were approved by NHDOT and prior to the actual placement of the deck concrete, a 5 cu yd (3.8 cu m) trial pour was made simulating the actual placing, finishing, and curing conditions. This trial pour was considered very important for the purpose of allowing the contractor the ability to fine tune the admixture dosage to ensure a workable mix. It also allowed the opportunity to ensure that the proper equipment was being used for good finishing practices and that the curing methodology was adequate. The criteria for the deck concrete mix are shown in Table 1. The approved mix design is listed in Table 2. A silica fume mix with a maximum water-cementitious material ratio (w/cm) of 0.38 was chosen based on research conducted at the University of New Hampshire (UNH). A 6 to 9 percent air content requirement was instituted to provide an air-void system for freeze-thaw resistance. Higher concrete strength was needed to minimize the deck thickness required to span the 12.5-ft (3.8-m) girder spacing. A permeability of 1000 coulombs or less was targeted to achieve a dense, highly impermeable concrete. A corrosion inhibitor was specified instead of NHDOT’s standard practice of protecting concrete decks with a barrier membrane and an asphalt overlay. Four days of wet cure with cotton mats were specified.
The deck was built using standard deck construction techniques. The concrete was pumped for ease of placement. To limit air content loss and to prevent freefall of the concrete, the end of the hose was positioned horizontally during pumping. A standard self-propelled finishing machine was used to strike off the concrete surface. Hand finishing was performed only in the areas adjacent to the curb line and screed rail supports. A finishing pan and burlap drag were attached to and followed behind the screed machine to simultaneously finish and texture the concrete surface. Within 15 minutes after a section of the deck was dragged, it was covered with dry cotton mats and then wetted. Bullfloating and over-finishing the surface were strongly discouraged.
Curing consisted of maintaining the wet cotton mats in direct contact with the concrete surface for a period of four days. The timely placement of the cotton mats prevented surface drying and eliminated the initiation of shrinkage cracks. Requirements to limit water evaporation based on climatic conditions at the time of the concrete placement were strictly enforced. No concrete could be placed if the evaporation rate was greater than 0.1 lb/ft2/hr (0.45 kg/m2/hr) or if the ambient temperature was above 85°F (29°C). Once adequate concrete strength was achieved, the hardened finish was transvrsely saw-cut on 1.5 in. (38 mm) centers. The grooves were 0.125 in. (3 mm) wide by approximately 0.25 in. (6 mm) deep. Tining or raking the surface was not considered due to the possibility of tearing and exposing the surface to the drying elements.
Each truck load of concrete was tested for slump, unit weight, concrete temperature, and air content. Consistency and uniformity in the values was of prime importance. Some difficulty maintaining the required air content and a consistent slump was encountered during the deck placement. A higher dosage of superplasticizer than that used in the trial pour was needed at the site in order to achieve the desired workability in the concrete. This phenomenon was largely unexplained, however it is the author’s belief that the interaction of the corrosion inhibitor with the other admixtures during the travel time to the site may have contributed to the inconsistent air content and slump results. Regardless, all the other specification requirements were met.
The NHDOT deck concrete test results are listed in Table 3. The average 28-day cylinder strengths were well in excess of the 7200 psi (50 MPa) requirement and the results from the 56-day rapid chloride permeability tests performed on deck cores were outstanding. Even though the air content of the concrete fell below the specification requirement, the freeze-thaw durability tests performed on the prisms revealed excellent results after 300 test cycles.
Summary
Trial batching and the trial pour played an integral role in optimizing the development and placement of the HPC deck. Modifications were made throughout the pre-pour process to refine the mix proportions and eliminate any foreseeable problems. The concrete was placed using standard deck construction techniques and equipment. Over-finishing and bullfloating the surface were strongly discouraged. Proper curing practices were implemented immediately and were considered vital to ensure a good end result. The concrete surface was immediately covered with cotton mats and wet cured for a period of four days. The final product exceeded expectations. No visible cracks in the deck were found during several post construction reviews conducted by research, construction, and design personnel. UNH conducted an extensive “wet study” of the deck surface and concluded only microscopic longitudinal flexural cracks existed in some areas over the girder lines. No shrinkage cracks or transverse cracks were evident. The 28-day concrete strength exceeded the specification requirement. The freeze-thaw durability, chloride ion permeability, and scaling tests also produced excellent results. Based on preliminary evaluations, the concrete deck will be highly resistant to chloride intrusion and freeze-thaw deterioration and should provide superior long-term service with minimal maintenance .
Further Information
Further information about Route 104 bridge may be obtained by contacting the author at 603-271-6675 or [email protected] and in the following publications:
Waszczuk, Christopher M. and Juliano, Michelle L., “Application of HPC in a New Hampshire Bridge,” Concrete International, February 1999,
Vol. 21, No. 2, pp. 61-62.
Fratzel, Todd M., “Evaluation of High Performance Concrete Slabs Including In-Situ Testing at a Bridge Deck Testing Facility,” Graduate Thesis, University of New Hampshire, May 1996, Durham, NH.
Wilson, Cheryl R. and Cook, Raymond A., “Measuring Performance: Preliminary Use of High Performance Concrete in New Hampshire Bridges,” New Hampshire Journal of Civil Engineers, Vol. 1, No. 2, Autumn 1996.