Omar Moussa
The use of the structural adhesive bonding technique is well established in the aircraft and automotive industries where, in most cases, joints can be fabricated indoors under controlled conditions. In the civil engineering domain, however, load-bearing structures are normally erected on-site in outdoor conditions, i.e. joint fabrication is exposed to varying outdoor temperature and humidity profiles and particularly in winter, when long periods of very low temperatures are frequent, the on-site joining of structural components must remain possible. Consequently, and due to the generally large scale of the structural components, cold-curing adhesives are used, unlike in other fields where hot-curing adhesives are used. Although it offers many potential advantages in bridge construction, structural adhesive bonding is not yet widely used in civil engineering structures. This can be attributed, amongst other things, to the lack of knowledge regarding adhesive behavior during exposure to varying environmental conditions. Despite significant research efforts concerning the characterization of structural adhesives, the question of the changes that occur in the thermophysical and thermomechanical properties of cold-curing structural adhesives during their service life, and particularly at early age and low temperatures, is yet to be addressed. The objective of this work was therefore, based on experimental and analytical investigations, to understand the thermophysical and thermomechanical behavior of cold-curing structural adhesives from the time of the mixing of the different components and throughout the long-term service life. Investigations of curing at early age and low temperatures (during winter) showed that curing already takes place at temperatures slightly above 0 °C. The process is very slow however and several days of curing are required to attain significant curing degrees. The glass transition temperature, Tg, develops even more slowly due to early initiation of vitrification. A new and practical experimental method was developed to establish the relationship between glass transition temperature and curing degree for different types of cold-curing adhesives. Furthermore, cure kinetics (developed for hot-curing adhesives) proved its applicability for cold-curing adhesives at low temperatures. In contrast to thermophysical properties, mechanical properties start developing significantly only after the onset of material vitrification. Lower curing temperatures also significantly decelerate the development process. An empirical model to predict strength and stiffness as a function of simple or complex curing temperature profiles particularly for curing at low temperatures was developed, taking the temperature-dependent vitrification into account. During summer, the temperature at specific locations of certain joints (e.g. joints below the asphalt) may exceed Tg, which may lead to a significant drop in mechanical properties. The investigations showed that when cooled to temperatures below Tg, the mechanical properties fully recovered and even a significant increase in properties is achieved due to post-curing. An existing model to predict temperature-dependent mechanical properties was extended to predict the change in stiffness and strength resulting from the exceeding of Tg and subsequently after recovery. Over the long term and during ambient curing, a significant increase in mechanical and thermophysical properties occurs over the years and decades due to the completion of curing. A model based on the similar long-term increase of concrete properties was revised to also predict the long-term strength and stiffness of cold-curing structural adhesives. Finally several case studies demonstrate how the outcome of this research can be applied, particularly to predict mechanical properties at early age and low temperatures. Such results can be used to plan construction stages or estimate the required waiting periods prior to a bridge being brought into service.
EPFL2011
