of the concrete that is caused by tensile strains that develop due to the Poisson effect. A

new theory that attempts to explain concrete failure based on buckling is offered. The

theory is that as the concrete cracks under axial compressive load, due to the Poisson

effect, micro columns are formed (see Fig. 8-1). These micro columns are not braced

(unconfined concrete) and when the cracks reach a critical length local buckling occurs

which leads to failure of the concrete.

 Radial cracks
 ( C - - - 7 -' )




 Longitudinal
 cracks
















Figure 8-1. Cracked concrete cylinder

 The cracks are not uniformly distributed but are longer and more severe on the

exterior layers than the interior layers (Shah et al.1995; Landis and Shah 1993). However,

for the purpose of this study it was assumed that on average the cracks were uniformly

spaced for each concrete layer considered.

 To develop the stress-strain relationship of concrete based on the assumption that

concrete behavior and failure was due to local buckling of micro columns that form due