List of SymbolsThe Mohr-Coulomb (MC) failure criterion is a set of linear equations in principal stress space describing the conditions for which an isotropic material will fail, with any effect from the intermediate principal stress r II being neglected. MC can be written as a function of (1) major r I and minor r III principal stresses, or (2) normal stress r and shear stress s on the failure plane (Jaeger and Cook 1979). When all principal stresses are compressive, experiments demonstrate that the criterion applies reasonably well to rock, where the uniaxial compressive strength C 0 is much greater than the uniaxial tensile strength T, e.g. C 0 /T [ 10; some modification is needed when tensile stresses act, because the (theoretical) uniaxial tensile strength T 0 predicted from MC is not measured in experiments. The MC criterion can be considered as a contribution from Mohr and Coulomb (Nadai 1950). Mohr's condition is based on the assumption that failure depends only on r I and r III , and the shape of the failure envelope, the loci of r, s acting on a failure plane, can be linear or nonlinear (Mohr 1900). Coulomb's condition is based on a linear failure envelope to determine the critical combination of r, s that will cause failure on some plane (Coulomb 1776). A linear failure criterion with an intermediate stress effect was described by Paul (1968) and implemented by Meyer and Labuz (2012). BackgroundCoulomb, in his investigations of retaining walls (Heyman 1972), proposed the relationshipwhere S 0 is the inherent shear strength, also known as

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FOREWORDAn extensive Round Robin test programme on compressive softening was carried out by the RILEM Technical Committee 148-SSC "Test methods for the Strain Softening response of Concrete". The goal was to develop a reliable standard test method for measuring strain softening of concrete under uniaxiat compression. The main variables in the test programme were the specimen slenderness hid and the boundary restraint caused by the loading platen used in the experiments. Both high friction and low friction loading systems were applied. Besides these main variables, which are both related to the experimental environment under which softening is measured, two different concretes were tested: a normal strength concrete of approximately 45 MPa and a higher strength concrete of approximately 75MPa. In addition to the prescribed test variables, due to individual initiatives, the Round Robin also provided information on the effect of specimen shape and size. The experiments revealed that under low boundary friction a constant compressive strength is measured irrespective of the specimen slenderness. For high friction loading systems (plain steel loading platen), an increase of specimen strength is found with decreasing slenderness. However, for slenderness greater than 2 (and up to 4), a constant strength was measured. The shape of the stress-strain curves was very consistent, in spite of the fact that each labora-tory cast its own specimens following a prescribed recipe. The pre-peak behaviour was found to be independent of specimen slenderness when low friction loading platens were used. However, for all loading systems a strong increase of (post-peak) ductility was found with decreasing specimen slenderness. Analysis of the results, and comparison with data from literature, showed that irrespective of the loading system used, a perfeet localization of deformations occured in the post-peak regime, which was first recognised by Van Mier in a series of uniaxial compression tests on concrete between brushes in 1984.Based on the results of the Round Robin, a draft recommendation will be made for a test procedure to measure strain softening of concrete under uniaxial compression. Although the post-peak stress-strain behaviour seems to be a mixture of material and structural behaviour, it appears that a test on either prismatic or cylindrical specimens of slenderness hid = 2, loaded between low friction boundaries (for example by inserting sheets of teflon between the steel loading platen and the specimen), yield.; reproducible results with relatively low scatter. For normal strength concrete, the closed-loop test can be controlled by using I the axial platen-to-platen deformation as a feed-back signal, ] whereas for high-strength concrete either a combination of axial] and lateral deformation should be used, or a combination of] axial deformation and axial load.

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