Bridging the Gap
April 27, 2011
During a major, temperature-sensitive construction operation for the New Pearl Harbor Memorial Bridge in New Haven, Conn., one air treatment company teamed up with a construction firm to successfully control the movement of the bridge superstructure during two challenging integral pier cap placements.
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 $2.2 billion mega-project to reconstruct and widen 7.2 miles of I-95 in Connecticut between West Haven and Branford.
One part of the bridge project involved the placement of two integrated pier caps encasing four girders spanning 230' 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 construction firm Cianbro/Middlesex JVIII (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 ~0.1" (2.5 mm). It also required the bridge builders to maintain the steel temperatures at a range of 60 to 70°F (15 to 21°C). 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; that is, they had to maintain the proper conditions until the pier cap concrete reached a compressive strength of 3,000 psi. At that point, the concrete can 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 while the pier cap concrete cured.
CMJVIII contacted Richard Shoemaker, key account manager with Polygon, an air treatment company based in Amesbury, Mass. 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 spent eight weeks formulating an 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.
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°F for the allotted time, Polygon recommended use of eight 30-ton direct expansion (DX) cooling units and two 20-ton 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. Five DX units were placed on both sides of the roadway and ducted into the containment. 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.
CMJVIII directed Polygon to install 20 monitoring points along the bridge and record the steel temperatures in the containment. 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.
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°F on average.