Jim Wild, Vermont Agency of Transportation
The Vermont Agency of Transportation (VTrans) has been using high performance concrete (HPC) for approximately 7 years. Initially, HPC was incorporated into bridge structures to help combat the potential for alkali-silica reactivity.
HPC Mixes
VTrans uses three primary HPC mixes. Each mix contains a ternary blend of cementitious materials including Type II cement, 40 lb/yd3 (24 kg/m3) of silica fume, and either fly ash at 20% or ground granulated blast-furnace slag at 25% of the total cementitious materials content. The superstructure mix, which is used in the deck and for all other cast-in-place concrete above the top of the bridge seats, is designated as HPC-A and contains 660 lb/yd3 (392 kg/m3) of cementitious materials. The substructure concrete mix is HPC-B and contains 611 lb/yd3 (363 kg/m3) of cementitious materials. The third mix is HPC-AA and is used for repairs and overlays. This mix contains 705 lb/yd3 (418 kg/m3) of cementitious materials and 3/8-in. (10-mm) coarse aggregate.
The first 2 years with these materials was a learning period for the contractors to understand the finishing characteristics of the mixes. Because VTrans’ use of HPC is still relatively new, long-term durability studies are not available. So far, HPC has provided a good product. The permeability values for the HPC-A and HPC-B mixes typically range from 600 to 900 coulombs at 56 days. The compressive strengths are typically 70 to 100% above the specified values of 4000 and 3500 psi (28 MPa and 24 MPa) at 28 days for HPC-A and HPC-B mixes, respectively. Because of these excessive strengths, some VTrans field personnel feel there is excessive cracking in the bridge decks and ornamental concrete bridge rails. VTrans has not been able to quantify these perceptions because most of the bridge decks receive a waterproofing membrane and overlay. Plus VTrans has not studied previously constructed bridges built with the conventional cement only mixes to establish a baseline for comparison.
The curing specification for HPC concrete requires 10 days of wet curing for the superstructure and 7 days for other components. For bridge decks, curing must be applied within a maximum lag time of 10 minutes after the screed machine passes.
Modified HPC Mixes
One of the first HPC modifications by VTrans was to the HPC-B mix in 2006 for a 4-ft (1.22-m) thick by 34-ft (10.4-m) high by 34-ft (10.4-m) wide bridge pier. The contractor wanted to construct the pier in one concrete placement instead of four separate ones. We changed the size of the stone from a No. 67 to a No. 467 as specified in AASHTO M43 and decreased the cementitious materials content from 611 to 564 lb/yd3 (363 to 335 kg/m3). Approximately 170 yd3 (130 m3) of this concrete was successively pumped to complete the pier in one placement. We are not aware of any cracks in the bridge pier to date. This mix has been used for mass concrete placements on a few other jobs with good results.
One problem is cracking on the ornamental concrete bridge rails or “Texas rails.” These rails, in the past, have used the standard HPC-A mix. VTrans feels that because of the high strength that this mix achieves, it becomes brittle and doesn’t deflect with the bridge. The mix may also be susceptible to a higher amount of shrinkage. The shape of the rail may also contribute to the cracking because of all the reduced cross sections and blockouts. To reduce the cracking as much as possible, the cementitious content of the HPC-A mix was reduced from 660 to 611 lb/yd3 (392 to 363 kg/m3), the air content was maintained at 6% +/- 1.5%, and the same water-cementitious materials (w/cm) ratio of 0.44 was retained. By keeping the original w/cm ratio, the total water in the mix was reduced. This mix is labeled HPC-A Low Shrink. Shrinkage-reducing admixtures have been specified on a few projects. At this time, we do not have any official data to show if there is a reduction in cracking but our research section has been looking at these ornamental concrete rails and should have a report out later this year.
VTrans primarily puts membranes and overlays on the bridge decks as a waterproofing measure. We have built some exposed concrete decks in the past but their use has been sporadic. However, they still appear to be in good condition. VTrans structures section has begun designing more exposed bridge decks in the past two years. For these, we took our standard HPC-A mix, reduced the cementitious materials content from 660 to 611 lb/yd3 (392 to 363 kg/m3), reduced the maximum slump from 7 to 6 in. (180 to 150 mm), increased the air content from 6 to 7 +/- 1.5% and kept the w/cm ratio unchanged. This modified mix is labeled HPC-A Low Cement. The cement content was reduced in an effort to lower the total water in the mix to help reduce shrinkage and to reduce the actual 28-day compressive strengths. Three bridge decks were completed in the summer of 2008 using this mix with the most noteworthy being on I-89 northbound in Berlin, Vermont. This deck also received the first longitudinal grooving treatment in Vermont. This summer, the southbound bridge on I-89 is scheduled to be reconstructed in the same manner along with three to four other bridges with exposed concrete decks.
VTrans is developing specifications for a self-consolidating, cast-in-place concrete. It was used on one project in 2007 to encase a bridge pier. It was very successful and plans are underway to use it on another bridge project for the ornamental railing.
Vermont will continue to develop HPC mixes based on laboratory testing and field experience to get the most efficient and durable mixes that will maximize the life of our transportation structures for years to come.
Further Information
For more information about Vermont’s HPC experiences, please contact the author at [email protected].