From a physical point of view, due to the intermittent water sprays, associated expansion and shrinkage of the sample may occur due to the associated temperature changes. These effects in the core region will be somewhat reduced and delayed. Eventually, this results in the formation of distinct stiff skin and soft core that develop different linear coefficients of expansion. For up to 100 h of weathering of SBR Kano et al. observed a 0.5 % volume change due to temperature increase during photo irradiation and temperature decrease during water sprays. This periodic thermal treatment leads to differential expansion and contraction of the skin and the core. In the present study, the long-term weathering of SBR for 504 h and 988 h caused a visual caving-in of the specimen towards the irradiated surface due to contraction and stretching of the rear surface due to expansion. Naturally, the clamped ends did not demonstrate this deformation, leaving the unclamped region to appear like a shallow boat.