Experimental determination of sectional forces in impact tests: Application to composite RC-HPFRCC beams

Abstract

The impact response of concrete structures differs significantly from the quasi-static behaviour due to strain rate influence on mechanical properties, formation of inertia forces and local damaging mechanisms. The time-variable acceleration of the impacted element leads to inertia forces which produce a time-dependent distribution of shear forces and bending moments. The failure mode of impacted elements is significantly affected by the distribution of sectional forces and a detailed study of such forces is therefore convenient for a full understanding of impact mechanics. One of the most extended approaches to study experimentally the impact behaviour of reinforced concrete (RC) beams is with the help of instrumented drop weight testing machines. Such facilities usually incorporate dynamic load cells to measure the support reactions and the impact force. Though the resulting inertia force can be derived by difference of reactions and impact force, its distribution cannot be determined, which is necessary for the estimation of sectional forces. In the present paper, an experimental methodology based on digital image correlation (DIC) supported by a high-speed and high-resolution camera is presented. The proposed methodology provides an accurate estimation of inertia forces which allows deriving the time-dependent evolution of shear forces and bending moments. The methodology is applied to an experimental campaign consisting of RC beams strengthened with a thin layer of high-performance fiber-reinforced cement composite (HPFRCC). The impact response of composite RC-HPFRCC elements is analyzed by examining the shear forces and bending moments at the critical sections. The implemented DIC-based methodology allows understanding the critical instants of the tests which lead to shear cracking. Moreover, a shear strength model is presented to estimate the capacity of studied RC-HPFRCC elements.