Use of a creep-prone epoxy anchor adhesive was the probable cause of a precast-panel ceiling collapse in an Interstate 90 connector tunnel serving Boston's Logan International Airport, National Transportation Safety Board noted in a hearing last month. On July 10, 2006, a 26-ton section of suspended concrete ceiling and associated hardware became detached from the tunnel roof and fell onto one eastbound vehicle, fatally injuring a passenger.
Summarizing a forthcoming investigation report, the agency noted that epoxy is a polymer whose stiffness is time and temperature dependent. If a load is applied suddenly, the epoxy responds like a hard solid. But, if the load is then held constant, the molecules within the polymer may begin to rearrange and slide past one another, causing the epoxy to gradually deform in the process called creep Û a property typically measured in structural concrete.
Among the I-90 tunnel accident's probable causes, NTSB officials note that the use of an inappropriate epoxy formulation for the ceiling panel hardware anchors resulted from the failure of project principals Gannett Fleming, Inc. and Bechtel/Parsons Brinckerhoff to: a) identify potential creep in the anchor adhesive as a critical long-term failure mode; and, b) account for possible anchor creep in the design, specifications, and approval process for the tunnel's epoxy anchors.
NTSB additionally contends 1) The use of an inappropriate epoxy formulation also resulted from a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems. 2) Those responsible for overseeing the I-90 Boston Central Artery/Tunnel project (CA/T), in design and specifications for the tunnel's ceiling, failed to account for the fact that polymer adhesives are susceptible to deformation or creep under sustained load. 3) Epoxy agent supplier Powers Fasteners, Inc., failed to provide the CA/T project with sufficiently complete, accurate and detailed information about the suitability of the company's Fast Set epoxy for sustaining long-term tensile loads. And, 4) Powers failed to determine that anchor displacement observed in 1999 at a CA/T tunnel ceiling stemmed from creep attributable to Fast Set epoxy.
In addition to investigation findings, the NTSB issued 20 recommendations concerning epoxy anchor adhesives to the Federal Highway Administration, American Concrete Institute, and a host of public and private organizations with a stake in concrete practice, code, and standards development. Those recommendations will be included in a full investigation report soon to be posted at www.ntsb.gov.
Vehemently countering NTSB claims, Powers Fasteners noted that per a distributor's request, it supplied the CA/T project a special order of Standard Set epoxy, a slower setting agent than the Fast Set product, while the Board finding is premised on the notion that the Fast Set epoxy was used on the tunnel ceiling anchors. Powers also notes that it: had no involvement in ceiling design or epoxy selection; did not know that the Fast Set product was used for the ceiling instead of the Standard Set; and, had provided project officials an ICBO evaluation report stating that Fast Set was permitted only to be used for short-term loads. The events that led to the collapse of the ceiling panels cry out for explanation and accountability, the company says. We believe, however, that it would be an untenable conclusion if the federal investigators were to consider Powers Fasteners in any way responsible, since there is overwhelming evidence the fault lies elsewhere.