Bijan Khaleghi and Jerry Weigel, Washington State Department of Transportation
The need for a superior and durable concrete capable of resisting environmental distress resulted in the development of high performance concrete (HPC) for bridge decks. This article focuses on the Washington State Department of Transportation (WSDOT) standard practice of using HPC for bridge decks.
Construction Practices
Durability of concrete is one of the most important factors determining the service life of concrete decks. Fly ash has been incorporated into the HPC bridge deck mix to increase concrete’s resistance to chloride penetration. Air-entrained concrete is required for all WSDOT concrete decks to provide the necessary freeze-thaw resistance when exposed to harsh environmental conditions.
For finishing, a self-propelled finishing machine is used to level the concrete and to pan drag the surface. If necessary, the concrete surface is floated by hand prior to combing it with metal tines.
Curing is one of the most important factors in achieving a superior and durable concrete. WSDOT requires two coats of curing compound, in accordance with AASHTO M 148, followed by continuous wet curing for 14 days. Two coats of liquid membrane-forming curing compounds are required immediately after finishing or as soon as the visible bleed water has evaporated. The surface is covered with presoaked, heavy, quilted blankets or presoaked burlap as soon as the concrete has hydrated sufficiently to support foot traffic. When wet curing was first initiated, we did not require the curing compound.
The length of time between concrete placement and start of wet curing was too long; hence, the need for the curing compound. Wet curing is usually maintained by the use of soaker hoses placed between a clear plastic cover and the curing blankets. Intermittent use of the hose is sufficient to keep the blankets wet. Delayed application of curing results in unacceptable surface cracking and the potential for excessive deterioration at later ages.
Concrete temperature must be between 55 and 90°F (13 and 32°C) while it is being placed. The contractor is required to maintain the concrete temperature below 90°F (32°C) during the curing period.
The contractor cannot mix or place concrete while the air temperature is below 35°F (2°C) unless the water, aggregate, or both have a temperature of at least 70°F (21°C). Concrete placed when the air temperature is below 35°F (2°C), must be immediately surrounded with a heated enclosure. Air temperature within the enclosure must be maintained between 50 and 90°F (10 and 32°C) and the relative humidity must be above 80 percent. These conditions must be maintained for a minimum of 14 days.
Use of Fly Ash
Initially, contractors expressed concerns over the addition of fly ash and the required wet curing. These concerns diminished rapidly because the fly ash improved workability and the wet curing was not the problem originally envisioned.
The first benefits realized were improved workability, reduction of aggregate segregation, and less bleed water. Additional benefits include reduced alkali-silica reactivity and increased resistance to sulfate and seawater attack.
Specifications Enforcement
As we all recognize, once a concrete element is placed, corrective action is very costly and removal of substandard concrete can actually cause more damage than accepting the substandard element. For this reason, a preconstruction conference is required five to ten days before a scheduled deck slab concrete placement. The purpose of this meeting is to ensure that the contractor has appropriate supervisory personnel, that the concrete production and placement rates will be adequate to meet the placing and finishing deadlines, that an acceptable self-propelled finishing machine is available and properly set up, and that enough finishers will be provided.
Test Results
The deck concrete on the Washington State HPC Showcase Bridge* used the HPC bridge deck concrete, Class 4000D, with the following 28-day test results.
Further Information
For further information, the lead author may be contacted at [email protected] or 360-705-7181.
*See HPC Bridge Views, Issue No. 2, March/April 1999