Effect of polyurea coatings on spalling and breach of reinforced concrete slabs under contact explosion

In recent years, polyurea-coated reinforced concrete (RC) slabs have been extensively studied both experimentally and numerically for structural strengthening against contact explosions. However, theoretical investigations remain limited, particularly concerning the impact of polyurea on the local damages of the RC substrates. In this paper, an analytical model based on stress wave propagation theory was proposed to investigate the reflection of compression waves at the backside of the RC substrate slab and predict the spalling depth. Utilizing this analytical model, a quantitative and detailed discussion was presented regarding the effect of the polyurea on the critical spalling and breach of the RC substrate slab. Furthermore, the applicability of the empirical breach prediction, originally developed for uncoated RC slabs, was validated through existing experiments to predict the breach of polyurea-coated RC substrate slabs. The results indicate that polyurea affects the spalling process of the RC substrate slabs. Specifically, the net stress wave adjacent to the concrete-polyurea interface is a compression wave, while it transitions to a tensile wave in the deeper concrete. Polyurea primarily impacts the first spall of the RC substrate slab; subsequent spalling processes after the first spall align with those observed in uncoated RC slabs. Upon the occurrence of critical spalling, polyurea enhances the critical spalling resistance of RC slabs, although it significantly increases the spalling depth. Conversely, when a breach occurs, polyurea reduces the number of spalls but minimally affects on the total spalling depth. Based on these findings, the empirical method for predicting breaches of uncoated RC slabs can effectively be applied to predict the breach of RC substrate slabs coated with polyurea. The test results from more than twenty contact explosion experiments are consistent with the predicted outcomes, thereby validating the effectiveness of the analytical model and providing a method for estimating the breach of polyurea-coated RC substrate slabs.