Harry A. Capers, Jr., New Jersey Department of Transportation and Hani Nassif, Rutgers University
High performance concrete (HPC) is now required for use in bridge decks on the state highway system in New Jersey. The use of HPC in prestressed concrete girders is at the discretion of the designer. Specific guidance for HPC is provided in the most recent edition of the Department of Transportation’s (DOT) bridge design manual. To support HPC deployment, the Department initiated a research project through Rutgers University to develop several baseline concrete mixtures suitable for the transportation infrastructure in New Jersey.
The research involved (1) review of existing information, (2) development of mix proportions using local aggregates, (3) evaluation of trial mixtures prepared in the laboratory and at a readymixed concrete plant, (4) evaluation of concrete material properties, (5) investigation of the effects of different curing methods on early age and longterm performance, and (6) preparation of specifications for use in the project special provisions.
Mix proportions for concretes with compressive strengths from 6,000 to 12,000 psi (41 to 83 MPa) were developed. Mineral admixtures consisted of silica fume, Class F fly ash, and ground granulated blast-furnace slag. Chemical admixtures such as high-range water reducers (HRWR) and air-entraining agents were included. The performance of selected HPC mixtures was evaluated by measuring various properties including compressive strength, modulus of elasticity, early age (autogenous) and drying shrinkage, creep, rapid chloride permeability, surface scaling, and freezethaw resistance.
Conclusions of this research were as follows:
- Mixtures that contained at least 5 percent silica fume produced concrete with good mechanical and durability properties. Using more than 10 percent silica fume did not produce significant improvement over the use of 5 to 10 percent.
- Fly ash increased the workability of the concrete, reduced the required amount of HRWR, and reduced the early age compressive strength of the concrete. The optimum range for fly ash was between 10 and 15 percent of the total cementitious materials in the presence of 5 percent silica fume. For concrete decks, where the compressive strength is not as important as durability, adding more fly ash reduced the permeability of the concrete. However, adding more than 30 percent fly ash did not result in any significant reduction in permeability.
The DOT now requires that the HPC mix designs for bridge decks be tested to verify a maximum scaling resistance rating of 3, a minimum freeze-thaw durability after 300 cycles of 80 percent, a maximum chloride permeability at 56 days of 1000 coulombs, and a minimum compressive strength at 56 days of 5400 psi (37 MPa). If the chloride permeability and compressive strength are achieved at 28 days, the performance is considered acceptable.
The specifications require that the concrete be placed 6 to 8 ft (1.8 to 2.4 m) ahead of the finishing machine. Wet burlap for curing of the deck slab must be placed within ten minutes after the concrete is struck off. If it is anticipated that the 10-minute limitation will not be met, the concrete placement operation must be stopped and a cold joint formed. Wet curing is required for 7 days followed by an additional 7 days with a liquid membrane curing compound.
Acceptance of production concrete for the deck is based on a maximum chloride permeability of 2000 coulombs at 56 days and a minimum compressive strength of 4400 psi (30 MPa) at 56 days. Whenever one or more individual test results for chloride permeability exceeds 2000 coulombs at 56 days, the contractor is required to remove the defective concrete or submit a plan for corrective action.
For a copy of the Department’s bridge design manual go to www.state.nj.us/transportation/cpm/ and click on Design Manual – Bridges & Structures and scroll down to BDC02MB-02. For questions, contact the first author at [email protected] or 609-530-2557.