Abstract
Abstract The curing reaction of thermosetting resins is associated with chemical shrinkage which is overlapped with thermal expansion as a result of the exothermal enthalpy. Final material properties of the polymer are determined by this critical process. For adhesive anchor systems the overall shrinkage behavior of the material is very important for the ultimate bond behavior between adhesive and the borehole wall. An approach for the in-situ measurement of 3-dimensional shrinkage and thermal expansion with digital image correlation (DIC) is presented, overcoming the common limitation of DIC to solids. Two polymer-based anchor systems (filled epoxy, vinylester) were investigated and models were developed, showing good agreement with experimental results. Additionally, measurements with differential scanning calorimetry (DSC) provided supporting information about the curing reaction. The vinylester system showed higher shrinkage but much faster reaction compared to the investigated epoxy.
Highlights
Adhesive anchor systems are based on polymer matrix systems in combination with different filler materials and additives in order to modify their properties [1] in terms of mechanical stability during their service life [2,3], curing and aging behavior [4,5,6] and compatibility with targeted materials in the application
The reaction of the epoxy is characterized by a maximal temperature difference of 7 °C above room temperature with a relatively high resulting thermal expansion lasting around 120 min, followed by a shrinkage reaction that is connected to a decreasing temperature (Fig. 4a, c)
A small thermal expansion can be detected within the initial period of fast exothermal reaction and after that, a fast shrinkage reaction takes place for the 5–7 min
Summary
Adhesive anchor systems are based on polymer matrix systems in combination with different filler materials and additives in order to modify their properties [1] in terms of mechanical stability during their service life [2,3], curing and aging behavior [4,5,6] and compatibility with targeted materials in the application. Chemical shrinkage during the curing reaction is a well-known phenomenon of thermosets [7,8] but it is usually overlapped with thermal expansion [9] since the crosslinking is an exothermal process. Both contributions, chemical shrinkage and thermal expansion, determine the residual strains and stresses and the final dimensions of the cured material [10]. The performance and life-time of installed adhesive anchors depends on the residual strain state that is reached at the end of the curing process. In case of shrinkage these correspond macroscopically to tensile stresses on the interface between steel anchor element and borehole and will limit the achievable macroscopic shear bond strength
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