Jim Wild, Vermont Agency of Transportation
Vermont’s awareness of high performance concrete (HPC) began in 1997 at an HPC Bridge Showcase meeting held in New Hampshire. After the meeting, Vermont’s Agency of Transportation (VTrans) began plans to incorporate higher performing concrete into its bridges. A bridge that had already been bid was selected for a trial in 1997. The lessons learned were then incorporated into our first specifically bid HPC project that was built in the 1999-2000 construction season.
In about 1998, alkali-silica reactivity (ASR) became a recognized issue in Vermont. To combat the problem and prior to beginning an aggregate screening process, new concrete mixes containing supplementary cementitious materials were developed. For concretes used in superstructure and substructure elements, repairs, and overlays, the cementitious materials were required to include 40 lb/cu yd (24 kg/cu m) of silica fume and either fly ash at 20 percent or ground granulated blast-furnace slag at 25 percent of the total cementitious materials. Alternatively, the total amount of cement and silica fume could be supplied as a preblended product. The silica fume was included to reduce the permeability of the concrete.
In 2002, VTrans began to require HPC as a preventative measure against potential ASR in all bridge projects for the substructure and superstructure elements except the prestressed concrete beams. Measured concrete compressive strengths are generally in the range of 5000 to 6800 psi (34 to 47 MPa) depending on the class and source of the concrete.
VTrans specification requires a pre-placement meeting prior to any deck concrete placement. The goal is to achieve consistent placement and curing procedures for the concrete. This, in turn, helps us recognize problems that may be mix design related. The importance of proper curing is emphasized to the contractor during these meetings. Unfortunately, some contractors still do not follow the stated procedures for curing. The reasons include crews that do not have enough people or experience, not being organized, or curing is not a priority to them. The specification requires fogging equipment. Application of the wet cure must begin within 10 minutes after the screed machine has passed. Wet curing durations remain the same as conventional concretes: 7 days for substructure elements and 10 days for superstructure elements.
In the beginning, contractors were reluctant to deal with HPC because the silica fume concrete was more difficult to finish and required more effort and attention for curing. During the deck pre-placement meetings, VTrans emphasizes to the contractor that a smooth warehouse floor surface is not needed. The surface should be lightly finished and covered with wet burlap as soon as possible. The contractor must also be willing and able to adjust the finishing technique to obtain the desired results because HPC finishing characteristics change rapidly with the environmental conditions. Most of the major contractors are now familiar with HPC and know what they must do.
The price of in-place HPC is approximately $150 to $250/cu yd ($196 to $327/cu m) greater than conventional concrete. We believe that the increased cost is worth it for the better product.
VTrans is currently updating its prestressed concrete specifications to include a maximum permeability requirement at 56 days and ASR testing of the aggregates. If the aggregates are found reactive, then retesting with the proposed mitigation method is required.
Summary
High performance concrete is still relatively new to VTrans, so long-term data on field performance is not yet available. If HPC performs as well as the laboratory testing indicates, VTrans will have longer lasting bridges with less maintenance for future generations.