Patricia Miller, Pennsylvania Department of Transportation
In October 2000, the Pennsylvania Department of Transportation (PennDOT) started its research and development of a high performance concrete (HPC) specification for bridge decks. This research was done in conjunction with the Pennsylvania Transportation Institute (PTI) at the Pennsylvania State University. Currently four out of the eleven Engineering Districts of PennDOT are using some aspect of the HPC concept for bridge decks. This article contains a timeline of events leading to our current status.
In 2001 and 2002, a PennDOT concrete mix for HPC bridge decks was developed. This mix had a water-cement ratio of 0.43, a design strength of 4000 psi (28 MPa), and an air content for the freshly mixed concrete of 6%.
In 2002 and 2003, several bridge decks were constructed using this concrete mix. During this time, 25 full-scale concrete mixes were produced and a full battery of tests was performed on them to determine the mixtures’ workability, durability, and other parameters that would impact the long-term performance of the concrete. A Best Engineering Practices Guide for bridge decks was also developed. It included engineering guidelines and design aids on the following concrete properties: compressive strength, strength development, chloride penetration, shrinkage, alkali-silica reaction, freezing and thawing durability, scaling resistance, modulus of elasticity, creep coefficient, and tensile strength. Engineering long-life concrete highway structures was also included in the guide.
During 2004, PennDOT’s specifications book (Publication 408) was revised to include the best practices as identified through the research at that time. Some of the revisions were as follows:
- Design cement-concrete mixes for bridge decks to meet a 28-day to 7-day compressive strength ratio greater than or equal to 1.33.
- Provide the necessary equipment and determine the evaporation rate before starting deck placement and every hour during the placement. Allowable evaporation rate for exposed finished concrete shall not exceed 0.15 lb/ft²/hr (0.732 kg/m²/hr) of exposed surface as determined by Fig. 2.1.5 of ACI 305R-91. Fog cure misting is an acceptable method to mitigate an excessive evaporation rate. Do not leave concrete exposed for an extended duration. Place concrete 5 to 8 ft (1.5 to 2.4 m) ahead of the finishing machine to prevent any premature concrete drying.
- Conduct finishing operations immediately behind the finishing machine or screed from work bridges or rigid construction that are not in contact with the surface of the concrete, are set on rails, and are easily moved. Finish with a 10-ft (3-m) long-handle, straightedge to achieve a smooth, accurate surface. If the concrete surface remains open after the finishing machine operations, make one pass with the float. Do not over-finish the surface.
- Cure the deck as soon as possible. Minimal marking of the fresh concrete is permissible. Maintain wet burlap applications within 10 to 18 ft (3-1/2 to 6 m) of the finishing equipment at all times. Maintain the burlap in a saturated condition during the entire 7-day curing period.
The HPC specification was revised in 2005 based on the research performed and the data collected up to that time. A Best Construction Practices Guide for concrete bridge decks was developed also. The Guide contains the following information on construction practices: quality assurance and quality control, designing performance-based concrete mixes, site preparation, certification of ready-mixed concrete plants and trucks, producing and transporting concrete, placing and consolidating concrete, finishing concrete, and curing concrete. Information on hot weather concreting and successful early and late season placements is also included in this guide.
As part of the research done by PennDOT and PTI, 10 bridges were constructed on I-99 during the 2005 and 2006 construction season. Each bridge was instrumented with temperature sensors, strain gages, and grounding clamps for half-cell potential measurements. Instrumentation was placed on the girders, inside the deck, and in locations surrounding the deck. These instruments were used to monitor the short- and long-term performance of the bridge decks. A weather station was used to document ambient conditions. Construction of these decks was monitored and multiple early age deck condition surveys were conducted.
Between 2001 and 2007, 21 bridges, including the 10 on I-99, were constructed with HPC decks throughout Pennsylvania. Information and data collected from these bridge decks and future decks will be analyzed. These data and other industry data will be used to modify the existing HPC bridge deck specification. Some of the changes that are being considered are:
- Furnish a concrete mix with moderate heat of hydration
- Fineness modulus of the fine aggregate between 2.60 and 3.15
- Minimum 4% total air content according to ASTM C457 for the in-place hardened concrete
- Minimum design compressive strength of 4000 psi (28 MPa) at 28 days with the average 28-day design compressive strength, including overdesign, not to exceed 6000 psi (41 MPa)
- 14-day wet curing of the bridge deck
In conclusion, PennDOT is taking steps to improve the durability of the concrete used in its bridge decks in order to increase the service life of our bridges.
Further Information
For more information about PennDOT’s experiences, see HPC Bridge Views Issue No. 45 or contact the author at [email protected].