ACI 446.4R

Abstract

This report considers issues related to the fracture of concrete materials and structures subjected to dynamic loading from impact, explosions, and earthquakes. Experimental evidence on rate effects is described in Chapter 2, including mixed mode effects associated with shearing action and bond effects. Analytical modeling of strain-rate effects is discussed in Chapter 3. Models based on micromechanics considerations, damage models that incorporate microcracking phenomena, and strain-ratedependent fracture models are described. Computational modeling of localized failure under dynamic loading is discussed in Chapter 4. <br>Although still the subject of speculation, rate effects on strength are considered to originate in the rate dependence of bond breakage in the mortar at a microscopic crack and in local inertia effects. Indirect evidence is presented suggesting that, at high loading rates, the cement-aggregate bond strength increases to the point that it is no longer the weakest link in the concrete. Another possible explanation, however, is that the large amount of energy per unit time available at high loading rates does not force failure at the weakest link. <br>No increase in bond strength between concrete and smooth wires is found at high loading rates (Yan and Mindess 1994; Vos and Reinhardt 1982). The bond between concrete and deformed bars, however, is greater at high loading rates than low rates because the bond strength depends on two effects: shear and microfracture within the concrete; and bearing between the concrete and the deformations of the bar. The strength of concrete in both the shear and bearing modes increases with increased loading rate. This effect of enhanced bond strength often also leads to reductions in failure ductility for reinforced concrete elements. <br>This report is one of a series of four state-of-the-art reports on fracture of concrete prepared by ACI Committee 446 over the past decade. Two reports, titled "Fracture Mechanics of Concrete: Concepts, Models and Determination of Material Properties (ACI 446.1R-91)" and "Finite Element Analyses of Fracture in Concrete Structures: State-of-the-Art (ACI 446.3R-97)," respectively, are published in the ACI <em>Manual of Concrete Practice</em>. The third report, "Fracture Mechanics Applications and the Code" was at one time published in the <em>ACI Manual of Concrete Practice</em> but was withdrawn in 1998.