Issue 50

K. Kaklis et alii, Frattura ed Integrità Strutturale, 50 (2019) 395-406; DOI: 10.3221/IGF-ESIS.50.33 404 attributed to the completely different stress fields that are developed in each test, as a result of the application of lateral pressure in the triaxial compression tests. Note that the proposed damage evolution equation applies only to the pre- peak region. As presented experimentally [16, 17], the degradation of the elastic modulus becomes more pronounced in the post-peak region and to the specific mortar design. Therefore, additional experimental work should be carried out in the future in order to investigate whether the Young’s modulus degradation process can be utilized to lead to a complete damage evolution law. (a) (b) Figure 10 : Comparison of experimental results and the proposed model for damage evolution on triaxial compression cyclic tests with (a) 2.09 MPa and (b) 6.06 MPa confining pressure. C ONCLUSIONS n the present study, uniaxial and triaxial compression tests under unloading-reloading (cyclic loading) conditions were conducted to elucidate the stress-strain and the material deformation behavior, as well as to derive a damage evolution relationship in the pre-peak region of a pozzolanic lime mortar. Linear equations can be used to describe the relationship between the plastic strain and total strain, while exponential equations can be utilized to correlate the plastic strain and deviator stress in the case of both uniaxial and triaxial com- pression cyclic tests. The ratio ε pl / ε is very high (0.89) for the triaxial compression tests, while it becomes lower (0.61) for uniaxial compression. In addition, a power relationship was derived between the ε pl / ε and the confining pressure σ 3 . A strain-softening behavior can be used to describe the uniaxial test and the triaxial compression test at low confining pressure (1.15 MPa). Triaxial compression tests at higher confining pressures can be described by a strain-hardening behavior. These different types of deformations are also reflected in the macroscopic failure mode of the pozzolanic lime mortar specimens subjected to uniaxial and triaxial cyclic compressive loading. The macroscopic crack pattern of mortar specimens under uniaxial compression are mainly shear failure and, in some cases, split failure, while the specimen subjected to triaxial compression cyclic test with relatively low confining pressure (1.15 MPa) failed along a single shear plane. Under triaxial compression tests with higher confining pressures, failure occurs along a single shear plane or conjugate shear planes combined with lateral expansion. Given that the Young’s modulus is related to stress, strain and the susceptibility to crack initiation, propagation and coalescence, the quantification of changes in the elastic modulus can be used to describe the effects of cyclic loading on material deformation. The observed decrease of Young’s modulus with increasing strain is attributed to the propagation of initial defects and microfractures in these mortar specimens. Increasing the confining pressure in triaxial compression tests affects the degradation of the elastic modulus which can be expressed using a damage index. A mathematical expression is proposed which can calculate the damage index as a func- tion of the confining stress for the case of pozzolanic lime mortars under triaxial compressive cyclic loading. This ex- pression was derived based on a specific pozzolanic mortar design but can be easily expanded to similar mortar designs. The predicted elastic moduli for different unloading-reloading loops are in good agreement with the experimental values. As the damage evolution relationship quantifies changes in the elastic modulus of the material as a function of the con- fining stress, a full characterization of the pre-peak behavior of the material under different loading scenarios can be obtained. This information can be fully exploited when modeling the behavior of these mortars for a specific application. A CKNOWLEDGEMENTS This research was co-financed by the European Union (European Social Fund-ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework 0 2 4 6 8 10 12 14 0.001 0.003 0.005 0.007 0.009 Deviator stress, σ 1 - σ 3 (MPa) Axial strain 2.09 MPa 0 2 4 6 8 10 12 14 0.001 0.003 0.005 0.007 0.009 0.011 0.013 Deviator stress, σ 1 - σ 3 (MPa) Axial strain 6.06 MPa I