Bryan Spangler, Pennsylvania Department of Transportation and Paul J. Tikalsky, University of Utah
Reducing cracking and creating long-life bridge decks in Pennsylvania has become a critically important strategy in renewing the Commonwealth’s aging transportation infrastructure. In spring 2001, construction commenced on the westbound lanes of the Pennsylvania State Route 56 Kernville Viaduct in Johnstown, PA, using selected concepts of high performance concrete materials. The multi-span curved composite girder bridge is a major artery through a populated valley of the Allegheny Mountains. Throughout the spring and summer of 2001, the deck was cast and cured for 7 days per job specifications. However, following the removal of the wet burlap, extensive transverse cracking was observed over the entire length of the bridge deck. Despite measures taken to determine the source of the cracks throughout the construction season, the early age cracking persisted.
The Kernville Viaduct is a 2700-ft (823-m) long, 27-span, curved, continuous steel girder bridge with an 8-in. (203-mm) thick reinforced concrete deck. The bridge is divided into six units of three to seven continuous spans with isolation joints between units. The galvanized deck reinforcement consists of No. 5 bars at 10-in. (254-mm) centers each way for the bottom mat, No. 4 bars at 12-in. (305-mm) centers each way for the top mat in the positive moment region, and No. 5 bars at 6-in. (152-mm) centers each way for the top mat in the negative moment region. Concrete cover above the top bars is 2.5 in. (64 mm).
A crack survey of the 2001 westbound lanes by the Pennsylvania Department of Transportation (PennDOT) showed that there were 237 cracks at an average spacing of 6.4 ft (1.95 m) in the positive moment regions and 227 cracks at an average spacing of 5.1 ft (1.55 m) in the negative moment region. Cracks had a width generally greater than 0.04 in. (1 mm) with numerous cracks greater than 0.12 in. (3 mm).
Prior to placement of the eastbound lanes in 2002, PennDOT entered into a sponsored collaborative research and implementation partnership with the Pennsylvania Transportation Institute at Penn State University, to identify measures that would mitigate the cracking on the remaining lanes and to assist in implementation and monitoring of bridge construction.
An investigation of the design and construction practices on the westbound lane was conducted and certain deficiencies were identified that were inconsistent with the goals for creating a bridge with minimal cracking. The following changes to the concrete mixtures were recommended:
- Increase the water-cementitious materials (w/cm) ratio from 0.40 to 0.43 and ensure that the contractor is meeting this target. This reduces the modulus of elasticity and stresses at early ages.
- Decrease the cementitious materials content from 650 to 588 lb/cu yd (386 to 349 kg/cu m) to reduce the early age heat generation and the paste component of the mixture.
- Decrease the percentage of ground granulated blast-furnace slag from 50 to 42 percent of the total cementitious material to reduce the volume of paste in the concrete mixture.
- Decrease the target slump from 6.0 to 4.5 in. (152 to 114 mm) to reduce settlement between the bars.
- Reduce the maximum concrete temperature at time of placement from 80 to 75°F (27 to 24°C) to slow the early age hydration and heat generation. The following construction considerations were also implemented:
- Place positive moment regions successively on one day followed by the placement of negative moment regions no less than three days later. This procedure reduces the tensile stresses in the negative moment regions when the deck concrete is cast in the positive moment regions.
- Apply moist curing immediately after concrete finishing and maintain continuously for 10 days with pigmented curing compound applied thereafter to avoid any potential for rapid surface desiccation.
- Increase vigilance in quality control and quality assurance operations to improve concrete production and reject substandard concrete.
Following construction of the eastbound lanes in 2002, crack surveys made immediately after curing and in subsequent months, showed a dramatic reduction in the frequency and width of cracks. Early age cracking, documented immediately after curing, was nearly eliminated by the changes in mixture design and construction practices. At later ages, 174 total cracks were recorded on the eastbound deck, compared to 464 cracks on the westbound deck—a 62 percent reduction. The 7-month survey showed the majority of the cracks in the eastbound lanes were closed or narrower than 0.005 in. (0.13 mm) during cold weather conditions.
The success of these changes has instigated a high performance bridge deck implementation program throughout Pennsylvania. More that 25 additional major bridges have been constructed using the lessons of the Kernville Viaduct to mitigate or eliminate bridge deck cracking and to decrease the permeability and shrinkage of bridge decks. These measures are expected to extend their predicted average service life from 30 to 75 years.
Further Information
Tikalsky, P. J. and Camisa, S. J., “Field Evaluations of Early Age Bridge Deck Behavior,” Report to the Pennsylvania Department of Transportation, February, 2005, 174 pp. Paul Tikalsky may be contacted at [email protected]