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Frost damage is a key deterioration factor for concrete structures in cold and wet areas which has been studied for several decades. For reinforced concrete (RC) structures, bond between reinforcement and concrete plays an important role and its degradation under freezing-thawing environment would affect the service life of the RC structures. This paper aims to develop a mesoscale simulation approach which could estimate and investigate the bond behaviors under the effect of frost damage. Based on the axisymmetric discrete element model -Rigid Body Spring Model, the micro-mesoscale mechanical strengthening/damaging effects by frost action were implemented to non-air entrained concrete and the bond interface. One-way pulling-out test was simulated and the calculated bond strength was compared with experimental results where good agreement was found.

A new element based on the concept of Rigid Body Spring Model (RBSM) that inherently incorporates Poisson effect is developed and utilized to simulate the nonlinear behavior of concrete. Initially, the behavior of element in linear state is successfully verified. For the nonlinear applications in simulation of concrete, nonlinear material models are implemented based on mesoscale modeling technique that does not suffer from complexity of macroscopic models based on volumetric and deviatoric stress and strain tensors separation. Beside capability of incorporating the Poisson's ratio of the material, with the new material models implemented, the new element can represent the complex behavior of concrete.The uniaxial compressive and tensile test beside splitting tensile test on concrete specimen were simulated successfully and similar cracking patterns to the experiments were observed in the simulations. K E Y W O R D Sconcrete simulation, discrete model, mesoscopic, Poisson's ratio, RBSM

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Abstract. The research, presented in this paper, aims to investigate the behavior of a High-Strength Steel Fiber-Reinforced Concrete (HSSFRC) mesoscopic nite element model at compressive high strain rates. In order to produce a three-dimensional meso-scale nite element model, a computer code is developed to randomly produce mesoscopic models of SFRC specimen. The specimen is assumed to be reinforced by 0.6 percent volume fraction of hooked steel bers (Dramix RC-65/35-BN) with random positions and orientations. Aggregates of the compound are assumed to have spherical shape and are produced according to Fuller grading curve. Based on the initial mesoscopic model, a nite element model is produced and used in an explicit dynamic simulation. The contribution of inertial con nement to the dynamic strength enhancement of concrete at high strain rates was investigated, and its e ective role was observed. Accordingly, de ning a Dynamic Increase Factor (DIF) for mortar matrix led to overestimation; nevertheless, the inertial con nement by itself could not justify the increment of specimen strength under the dynamic loading. Obtained results also show that steel bers have a negligible in uence on the strength, strength enhancement ratio (DIF), and post-peak behavior of the model at high strain rates.