RATE EFFECTS ON THE STRESS-STRAIN BEHAVIOUR OF EPS GEOFOAM

The rate-dependency of the stress-strain behavior of EPS (Expanded Polystyrene) geofoam with densities of 19.3 and 28.0 kg/m(3) was investigated by performing unconventional unconfined compression tests. A set of monotonic loading (ML) tests were performed at different constant values of vertical (axial) strain rate, (epsilon)over dot(v) . The (epsilon)over dot(v) value was stepwise changed many times and several sustained loading (SL) tests were performed during otherwise ML at a constant (epsilon)over dot(v) in other tests. A number of SL tests were performed during global unload and reload cycles to infer the stress-strain relation when (epsilon)over dot(v) = 0. The elastic properties were evaluated by applying minute unload/reload cycles during otherwise ML. The rate-dependent stress-strain behaviour observed in these tests was described by an elasto-viscoplastic model (i.e., a non-linear three-component model), for which the vertical (axial) stress, sigma(v) consists of inviscid and viscous components, sigma(f)(v) and sigma(v)(v) while (epsilon)over dot(v) consists of elastic and irreversible components, (epsilon)over dot(v)(c) and (epsilon)over dot(v)(ir). It is shown that the viscous property of EPS geofoam is of Isotach type in that, under the loading conditions where (epsilon)over dot(v)(ir) is always positive, the curous current sigma(v)(v) value is a unique function of instantaneous (epsilon)over dot(v)(ir) and (epsilon)over dot(v)(ir) therefore the strength increases with (epsilon)over dot(v). This viscous property was quantified based on the test results and incorporated into the model. The rate-dependent stress-strain behaviour, including the creep behaviour, observed in the experiment is simulated very well by the proposed model. In particular, the fact that the creep strain becomes significant when the sustained sigma(v) value becomes larger than the inviscid yield vertical stress is well simulated.