Paul Kivisto, Minnesota Department of Transportation
Bridges in Minnesota experience harsh conditions with wide temperature extremes, fairly long snow and ice seasons, and many applications of deicing chemicals. The standard bridge deck protection system of the Minnesota Department of Transportation (Mn/DOT) includes epoxy-coated reinforcement, a 7-in. (175-mm) thick conventional concrete structural slab, and a 2-in. (50-mm) thick low-slump concrete overlay. This system has worked extremely well since the mid 1970s and is specified on most bridges. High performance concrete (HPC) bridge decks offer potential benefits to the state including decreased construction time, lower permeability, and cost savings of 5 percent or more compared to decks with low-slump overlays.
HPC Specifications
Mn/DOT’s HPC specifications include a minimum cementitious materials content of 611 lb/cu yd (363 kg/cu m) and the use of 75 percent Type I cement with 20 percent Class C or F fly ash and 5 percent silica fume to reduce permeability. The specified compressive strength is 4300 psi (29.6 MPa) at 28 days. The specifications also require a water-cementitious materials ratio no greater than 0.40, 4-in. (100-mm) slump, and well graded aggregates. The curing specifications require concrete placement when surface evaporation rates are less than 0.1 lb/sq ft/hr (0.5 kg/sq m/hr), prewetted burlap or cotton mats placed within 15 minutes of finishing, and wet curing for seven days. The specifications do not include any values for permeability since adherence to the specifications is expected to produce a concrete mix with a chloride permeability less than 1500 coulombs at 56 days.
Experiences
Fifteen HPC bridge decks have been placed in Minnesota since 1997. Most of the deck placements have gone well, but we have experienced a few problems. On two different decks placed in 1999, significant spalling occurred due to silica fume balls that had accumulated in the concrete. The balling was not evident during the deck placement but manifested itself after one winter. In both cases, the concrete contained silica fume slurry that was added to the mix from an external tank at the concrete plant. We are unsure if the balling occurred because of improper mixing in the slurry tank, the ready-mixed concrete facility, or the concrete truck itself.
The specifications now require that the concrete trucks comply with ASTM C 94 and limit the truck capacity to 75 percent of its rated capacity. The contractor is also required to wet sieve concrete samples on site to detect the presence of any silica fume balls. The specifications still allow the concrete supplier to use either silica fume slurry or a dry densified powder, but in most recent deck placements the concrete supplier has used dry densified powder. Since the specifications were revised, we have not experienced any additional balling problems.
On the two decks that experienced silica fume balling, the deck contractor was allowed to core out the spalled areas and patch with concrete. In both cases, additional spalling occurred after the second winter, and we felt that the patching would not provide adequate long-term protection for the deck. The contractor was required to mill off the top 2 in. (50 mm) of one HPC deck, and replace it with a 2-in. (50-mm) thick low-slump concrete overlay. That deck is performing adequately at this time. The second deck has been patched a second time, and we continue to monitor the patches.
Another problem that we have encountered is cracking in the deck surface due to improper curing practices. The first instance happened in 1999 when the curing specification only required the contractor to fog the deck to keep it wet prior to placing wet burlap. During the deck pour, the wind speed increased, and the manual fogging operation was not able to keep up with the rate of evaporation. Several areas of map cracking were evident in the deck after completion of the curing. In an effort to reduce shrinkage cracking due to improper curing, the specifications were revised to require placement of wet burlap within 15 minutes of finishing. The burlap must be maintained in a wet condition for seven days after placement of the deck.
A second instance of deck cracking occurred in 2002, when the contractor did not have his work bridges set up behind the paving machine for immediate application of the wet burlap. The contractor tried to fog the deck from the ends and sides of the bridge. As the wind increased, the manual fogging was not able to keep up with the surface evaporation. Transverse cracks at 5 ft (1.52 m) intervals have occurred throughout the deck. We plan to flood the deck surface with methacrylate to seal the hairline cracks. The specifications were not modified after this placement, but we will continue to discuss these problems during deck pre-placement meetings.
For the most part, contractors are taking a favorable view of HPC bridge decks in Minnesota. Contractors have requested a change to HPC decks on a few bridges to reduce construction time by two weeks or more compared to conventional concrete decks with an overlay. With only 15 HPC decks in service, contractors are still learning how to best place and cure the concrete. One contractor has placed the last three HPC decks at night. This has vastly reduced the potential for plastic shrinkage cracks.
Future Plans
Mn/DOT has been pleased with HPC bridge decks and continues to look at ways to improve the product and specifications. We are investigating the use of a concrete mix with up to 30 percent Class F fly ash and no silica fume. This will provide reduced permeability as well as easier curing. HPC decks provide options to our standard low slump concrete overlays by reducing construction time and providing a high quality deck with low permeability.
Further Information
For further information, contact the author at [email protected] or 651-747-2130.