The tensile performance of closed cell foam expansion joints: a study on the deleterious effects of compression set
Date
2016
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Closed cell foam is a polyethylene based foam that is used as a gland material for a specific type of bridge deck expansion joint, usually in applications where the total movement to be accommodated is four inches or less. Adhered into the gap of an expansion joint, this material is responsible for maintaining a water tight smooth transition between deck slabs, and for preventing the infiltration of corrosive agents through deck runoff. While currently a viable option for small movement applications, some state agencies have begun shying away from these foams, as reported failures in tension have caused its reliability to become suspect. In determining the cause of these tensile failures, the permanent loss in thickness due to load cycling, also known as compression set, was thought to be a likely contributor. This loss of elasticity will cause unanticipated stresses in tension that could in turn cause failure of the joint, either by tearing of the foam, or a loss of bond with the gap wall. To investigate the possible correlation between compression set and the tensile failure of closed cell foams, the current methods for determining compression set had to first be evaluated, followed by a series of tensile tests that investigated the involvement of compression set in tensile performance.
A series of compression tests were conducted in accordance with the current ASTM standard for measuring compression set. This test is conducted at ambient temperature (73°F) and for a compression cycle duration of just 22 hours. A parallel set of experiments were also conducted to simulate the actual in-service conditions of a bridge joint. This involved holding the specimens compressed for up to 3 months, at both ambient (73°F) and elevated temperatures (110°F). It was discovered that the current standard is likely to produce values of compression set that are significantly inaccurate when considering the lengthy periods of compression and recovery, and elevated temperatures, that these products are likely to experience in actual bridge joints. The results showed that compared to the 9-10% compression set typically reported by manufacturers who test in accordance with the current standard, foam samples exposed to more realistic compression cycles and rebound periods exhibit only half as much compression set (4-5%) at ambient temperatures (73°F), and more than three times as much (30-35%) at elevated temperatures (110°F). Modifications to the duration and testing temperature of the current standard may be necessary for the test to yield results that are appropriate for the bridge joint application.
A series of tension tests were also conducted on specimens subjected to various amounts of compression set, to determine the presence and magnitude of any correlation between compression set and tensile performance. The foam specimens were bonded to steel plates using the manufacturer’s adhesive, subjected to 24 hour and 1 month compression cycles, and loaded in a tension testing machine to failure. The results of these tests showed that for every specimen tested, the elongation at failure was far greater than what is reported by the foam manufacturer, implying a positive tensile performance even in the presence of compression set. All specimens experienced 70-110% elongation in tension, which is much greater than the 20-30% reported by manufacturers. The direct conclusion of these tests is that compression set is not the sole cause of poor tensile performance of closed cell foams. However, these results were entirely contingent on a scrupulous adherence to the installation procedures provided by the manufacturer. Several small instances of negligence during assembly of the bonded specimens had significant detrimental effects on the tensile performance of the foam, implying that improper installation is a more likely cause for limited tensile capacity of closed cell foams. Especially when dealing with the adhesives, diligence in proper installation techniques is essential in creating a strong bond and a high quality joint.