Durability and fatigue performance of a typical cold-curing structural adhesive in bridge construction

The science of adhesives and adhesion for structural connections has undergone significant development during the last few decades - and successfully so in a wide variety of industries. Recent applications of structural bonding are being adopted in the construction sector, and particularly in bridge engineering for the rehabilitation and strengthening of existing structures or new constructions. Knowledge concerning structural bonding, however, is rarely specifically intended for the civil engineering domain, where, unlike in aerospace and automotive applications, in situ conditions involve the manufacturing control of often large bonding surfaces and use of cold-curing adhesives, particularly epoxy-based, under uncontrolled weather conditions. Various environmental factors may influence the durability of a bonded bridge connection throughout the long-term service life, with concurrent effects on the development of the physical and mechanical properties of the adhesive that participates in the structural integrity of the joint. In parallel, the curing degree of cold-curing adhesives, which is not completely developed particularly in the earlier age, continues to govern both the physical and mechanical responses of the adhesive. Despite a significant amount of current research on the long-term characterization of cold-curing structural adhesives, precise information concerning the reliability of these materials over their long-term service life, of up to 100 years in the case of bridge applications, is yet to be compiled in order to encourage their wider use by bridge engineers. The objective of this work, based on an extensive experimental campaign followed by analytical and numerical approaches, has therefore been to investigate the long-term effects of curing and aging on the property development of a cold-curing epoxy adhesive under exposure to environmental and cyclic mechanical actions like those typically encountered in bridge applications. Investigations have been conducted concerning adhesive exposures under dry conditions, in normal cases of sealed bridge joints, and wet conditions, when the sealing is distorted or the adherends are permeable, or cases of alkaline attacks when the adhesive is in contact with cementitious substrates. Physical aging (densification) in the earlier age and curing continuation in the later age govern the physical and mechanical property development in the case of dry environments, while curing effects are compensated by plasticization in the later age in wet environments. A 30% reduction of the tensile E-modulus and strength over a 100-year life in immersed conditions has been predicted, but is found to be fully reversible upon drying. The adhesiveâs exposure to alkaline solutions had no detrimental effect. The adhesive environmentally aged over periods of up to 100 years by accelerated conditioning is capable of attaining fatigue lives of ca. 10 million cycles, typical for a bridge lifetime, without failure at maximum cyclic stress levels of higher than 5 MPa. The results of this study are implemented in bridge design concepts and practical recommendations are provided. The moisture ingress into the adhesive layer of two typical bridge joints has been numerically investigated and the corresponding degradation of mechanical properties is derived. Depending on the joint type, this effect may be critical, as for example in the case of the thin bondlines of CFRP strengthening elements.

Thermophysical and Thermomechanical Behavior of Cold-Curing Structural Adhesives in Bridge Construction

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

Fracture and Fatigue of Adhesively-Bonded Fiber-Reinforced Polymer Structural Joints

Ye Zhang

Being good structural replacement for other conventional material, the pultruded glass fiber reinforced polymer (GFRP) profiles are being increasingly used in civil engineering structures. The connection between components is considered the most suspect area for failure initiation. The adhesive bonding is preferred for FRP composite structures, rather than the mechanical fastening, due to the brittle failure nature of composite materials. During past decades, many efforts have been made by researchers to better understand the mechanism of adhesive bonding, to analyze the stress distributions and to improve the strength of composite structural joints. However there is still no commonly accepted design code/standard existing for adhesively-bonded joints in civil engineering infrastructures since several important knowledge gaps are to be filled. Besides the joint strength at failure, the characterization and modeling of the progressive failure process, in particular involving the so-called crack initiation and propagation phases, is also an important concern. By employing the strain energy release rate (SERR) as the fracture parameter, the linear-elastic fracture mechanics (LEFM) approach is considered an efficient method to model the fracture behavior of structural joints. However, due to the uncontrollable crack initiation and the complex geometric configurations, the crack measurement techniques and the calculation method for the SERR are to be validated. In fracture mechanics, the fracture of a material or component can be described by a single mode or the combinations of the following three basic modes: opening mode (Mode I), shearing mode (Mode II), and tearing mode (Mode III). During the fracture of a structural joint, crack initiation and propagation are driven by combined through-thickness tensile (peeling), and shear stresses, thus resulting in a mixed mode fracture. In order to use the fracture results of structural joints to form the mixed fracture criterion for a specific composite material, a feasible analytical or numerical method are to be developed to determine the Mode I and II components of the SERR during fracture. Although many efforts have been made to better understand the short-term behavior of structural joint under quasi-static loading, the long-term performance under fatigue loading and different environmental conditions is a more demanding task when adhesively-bonded joints are applied in a real structure. Most of structural failures occur due to mechanisms that are driven by fatigue loading and for composite structures, the fatigue produced by the repeated application of live load is more critical due to its lighter self-weight, in other words the lower dead load. Besides the fatigue loading, a structure in practice may also experience the combined environmental effects of two basic factors: temperature and humidity. These environmental conditions may directly affect properties of structural joints, including the failure mechanism, the stiffness and strength, the crack initiation and propagation and etc.. Thus, the missing knowledge and confidences in the long-term behavior under cyclic loading and the durability under different environmental conditions are the main obstacles to the further development of FRP composites in civil engineering infrastructures. In this research, the mechanical and fracture behavior of adhesively-bonded double-lap and stepped-lap joints (DLJs and SLJs) composed of pultruded GFRP laminates and an epoxy adhesive were experimentally and numerically investigated under both quasi-static and fatigue loadings. The crack measurement techniques and the calculation methods for the SERR were validated for DLJs and SLJs. The LEFM approach was successfully applied to characterize and model the progressive failure process of structural joints. The Mode I and II components of the SERR of DLJs and SLJs were determined using the Virtual Crack Closure Technique in finite element analysis. Combining with the results of pure Mode I and II experiments, a mixed mode fracture criterion for pultruded GFRP composite was formed. Under fatigue loading, the fatigue behavior of structural joints was successfully modeled by using the stiffness-based and fracture mechanics approaches, besides the F-N curves. Based on the stiffness degradation, a linear and a sigmoid non-linear model were established and the fatigue live corresponding to the failure and allowable stiffness degradation can be predicted. Concerning fracture mechanics approach, the Fatigue Crack Growth (FCG) curves were formed for DLJs and SLJs and the corresponding fracture parameters were obtained. Similarly to stiffness-based approach, fatigue lives corresponding to the failure and allowable crack length can be predicted. The environmental effects on both short- and long-term performances of structural joints were experimentally evaluated and numerically modeled based on experimental results. The temperature-dependent joint stiffness can be predicted using the finite element analysis based on the thermomechanical properties of constituent materials. A relationship between the equivalent quasi-static joint strength under different environmental conditions and the cyclic stresses and the fatigue life was established.

EPFL2010

Fracture and Fatigue of Adhesively-Bonded Fiber-Reinforced Polymer Structural Joints

Ye Zhang

EPFL2010

Thermophysical and Thermomechanical Behavior of Cold-Curing Structural Adhesives in Bridge Construction

Omar Moussa

EPFL2011