Personnel adjust the ductwork that brings cool air into the containment area. This cooling was needed to control the temperature to restrict the thermal movement of the structural steel while the pier cap concrete cured to sufficient strength. (Click on the image for an enlarged view.)
Scheduled for completion by 2016, the Pearl Harbor Memorial is a 10-lane bridge that will be one of the first extradosed cable stayed bridges constructed in the United States. An extradosed bridge is a hybrid design combining a concrete cable stressed girder bridge with a cable stayed bridge. The structure is the centerpiece of a major $2.2 billion project to reconstruct and widen 7.2 miles of I-95 in Connecticut between West Haven and Branford.
A portion of the bridge project involved the placement of two integrated pier caps encasing four girders spanning 230 feet over a roadway. In its final design condition, these pier caps lock the girders into the concrete, thus restricting movement.
“Structural steel will elongate or contract with changes in temperature. And in our case, if this movement is not controlled while the concrete is curing, the pier caps will crack,” said Aric Dreher, P.E., superintendent with CMJVIII. “To minimize these movements, we had to put together a plan to maintain a set temperature range in each of the four girders. We had to hold these conditions before, during, and after placement for an estimated period of 72 hours.”
The state limited the allowable movement of the steel beams to 2.5 millimeters. It was also required to maintain the steel temperatures at a range of 60 to 70°F. This stipulation made it critical to create an environment that would hold the temperature of the steel to limit movement for a minimum of three days until the pier cap concrete reached a compressive strength of 3,000 pounds per square inch (psi). At that point the concrete is able to withstand the thermal elongation stresses induced by the girders.
The Connecticut Department of Transportation requested that CMJVIII propose a containment system and cooling units to control the temperature to restrict the thermal movement of the structural steel while the pier cap concrete cured to sufficient strength.
“The system proposed and installed by the contractor needed to meet the needs of the project,” said John Dunham, transportation supervising engineer with the Connecticut Department of Transportation.
CMJVIII contacted Richard Shoemaker, key account manager with Polygon, after conducting extensive research on air treatment solutions companies that could respond to and meet this application challenge.
The CMJVIII team and Shoemaker discussed the project requirements, and then subcontracted Munters to handle the temperature control portion. The Polygon team reviewed weather data to identify the climate control equipment needed.
Polygon and CMJVIII then spent eight weeks formulating an extensive work plan before submitting it to the bridge designer and state transportation officials for approval.
CMJVIII also hired Hippwrap, a company specializing in shrink wrap abatement containment, to design a shrinkwrap system to encase the 230-foot bridge span.
“The front-end planning effort was an important phase of this operation,” said Dreher. “Over the course of two months the team put together an extensive plan that detailed system design, start-up, monitoring, enclosure installation and removal, quality control, logistics, and schedule.”
PROJECT SETUP POSES LOGISTICAL CHALLENGES
Due to the complexities of the project, once state officials and the bridge designer approved the plan, all the players - Polygon, Hippwrap, and Cianbro/Middlesex - met on-site frequently to discuss the myriad of details to ensure everyone was in synch. The process then began.To hold the steel beams at 65° for the allotted time, Polygon recommended use of eight 30-ton DX cooling units and two 20-ton DX cooling units - customized air conditioners that deliver high-performance, efficient cooling using refrigerant technology - plus the company’s ExactAire remote monitoring system to measure conditions on an ongoing basis.
The highway below the project area was closed temporarily, and five DX units were placed on both sides of the roadway. They were then ducted into the containment.
“Configuring the cooling equipment over the two-lane highway was quite a challenge,” said Shoemaker. “We had to close the roadway, get the equipment in place, make sure the generators were on-site, and duct into the containment area. All of this work was done during the night time hours.”
Once operational, the 30-ton DX units pumped cold air into the bottom of the bridge and the DX 20-ton units supplied cold air to the top and undersides of the bridge. A measuring system was installed on each pier cap to monitor the steel movement in order to determine if the system was working.
Two steel angles were attached to the steel girders and independently to the column rebar cage. The space between these two angles was closely monitored using digital calipers to the nearest 0.01 mm. This allowed the inspector to evaluate the relative movement of the superstructure and column to a very high degree of accuracy. Once the climate control system was started up, movements decreased to barely-measurable levels.
“The inspector measured steel temperatures 20 feet outside the containment and saw readings in the direct sunlight as high as 120°, while the same steel beam inside the containment was only 65°,” said Shoemaker. “The state was thrilled with these readings, and soon after allowed the team to place the concrete in the pier caps.”
INTENSIVE MONITORING REQUIRED
The concrete pour required intensive monitoring of both the temperature and movement of the steel - only 2.5 millimeters of movement was allowed as part of the control.CMJVIII directed Polygon to install 20 monitoring points along the bridge and record the steel temperatures in the containment. Polygon deployed five of its ExactAire systems with four measuring points each, and the sensors were placed in increments along the steel beams to evenly track the entire span. All of this information was recorded and tabulated and compared with the online ExactAire system.
All of the climatic information was transmitted wirelessly to a remote website where the team could access real-time climatic data for the structural steel. This allowed to team to monitor the conditions and analyze the movement trends with changes in ambient temperatures outside of the enclosure.
PASSING THE TEST
Three days after the pour, the compressive tests were conducted to ensure the curing went as planned. The concrete needed to reach 3,000 psi to pass the test.Testing results indicated strengths exceeding 3,400 psi after only two days of cure, which exceeded the requirements. At that point, all parties agreed to shut down the climate control equipment.
Throughout the project, steel temperatures remained between 63 and 68° on average.
“The hot August temperatures proved to be one of our biggest challenges. This schedule-sensitive operation was on our critical path for the project and we could not afford to wait for the cooler fall temperatures. And at the end of the day the proof was in the results. Polygon was very responsive to our needs and the equipment performed exceptionally,” said Dreher.
“Outside temperatures rose into the 90s during the operation, and yet we were able to keep the steel between 63 and 68° on average - a tough accomplishment given the conditions,” noted Shoemaker.
“This project is an important component of the I-95 New Haven Harbor Crossing Corridor Improvement program and its successful completion is critical to the overall success of the program,” said Dunham. “Hippwrap and Polygon did an excellent job meeting the needs of this technically challenging problem and the implementation of their plan provided the department with a quality product while meeting the aggressive schedule requirements of the project.”
“The successful completion of this operation was a milestone on this project,” said Dreher. “The focus was executing our plan and the day-to-day collaboration between Polygon, Hippwrap, and the DOT was a key component in our accomplishments. It was truly a team effort.”
Publication date:02/14/2011
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