Report on Dynamic Fracture of Concrete (ACI 446.4R-04)

Description

Impact, explosions, and earthquakes impose dynamic effects on concrete structures. Impact loading on a parapet can occur if it is struck accidentally by a crane. Seismic loading produces significant strain rates in concrete shear-walls and other lateral force-resisting elements. Explosive loading, due to accidental detonation of industrial vapor clouds or terrorist bombing, produces high strain rates in floor slabs and columns. These possibilities have prompted experiments on plain concrete specimens to investigate basic properties of concrete under various states and rates of loading. Under dynamic loading (rapidly applied loads of short duration), both structural and material responses depend on theapplied loading rate. Although both the geometry of thestructure and the material properties control the rate of cracking,this report is concerned primarily with the material effects.

Common practice for evaluating the resistance of concretestructures to dynamic loading is to:a) Estimate the transient state of stress in the structureusing an elastodynamic analysis; andb) Evaluate the resistance of the structure using strengthproperties for the concrete and steel that are enhanced bystrain-rate-dependent factors. For the failure modes of aconcrete structure controlled by yielding of the reinforce-ment or crushing of the concrete, common practice usuallyprovides reliable design information. For those failuremodes controlled by crack propagation, however, such asdiagonal tension or splitting failures, and where resistance tofracture is of fundamental importance for computations ofenergy absorption and energy dissipation, common practicedoes not usually yield reliable information.

This inadequacyis due primarily to the fact that dynamic fracture of concretestructures does not involve instantaneous fracture, butcontinuous dynamic crack propagation under dynamicloading. Reliable dynamic failure analyses of concrete structuresrequires knowledge of the dynamic fracture properties of theconcrete as well as its strain-rate-dependent properties.Therefore, this report concerns not only strain rate effectsbut also consideration of the dynamic fracture propertiesof concrete in general.