Issue 50

K. Kaklis et alii, Frattura ed Integrità Strutturale, 50 (2019) 395-406; DOI: 10.3221/IGF-ESIS.50.33 396 cations in the restoration of ancient monuments or historic structures. This particular mortar is often used as a filler or joint material in restoration projects [2, 3], subjected to shear stresses that may develop along the mortar-stone interface. Stone blocks in ancient monuments were typically joined together using metallic connectors in order to provide structural integrity in case of dynamic loading. More specifically, the structural elements of the monuments on the Acropolis of Athens were initially joined together using “I”-shaped metallic connectors placed in appropriate grooves which were filled with molten lead [4]. The restoration process of the Parthenon in Athens, Greece involved the re- placement of a number of marble epistyles [5] where titanium connectors were placed in carved grooves which were then filled with a cement based mortar. Titanium prohibits the use of lead as the “titanium-lead” bimetallic contact forms a strong galvanic element [4]. Pozzolanic mortars can also be used as filler material when seating metallic connectors in grooves carved into the original or restored stone fragments. Apart from this explicit use as filling material, pozzolanic mortars used as rubble, joint, finishing and capping mortars can provide particular durability to the masonry with respect to both external loading and weathering. The longevity of some ancient structures, such as the Pantheon, the Colosseum, Hagia Sofia, etc., can also be attributed to the use of pozzolanic mortars as the main joint material. Therefore, pozzolanic mortars and especially the lime-metakaolin system studied in this research, are very important in the conservation field and particularly when repair mortars are intended to be integrated in ancient structures. This work aims to provide evidence that pozzolanic lime mortars can replace cement-based mortars in historic structure restoration projects, thus mitigating potential problems arising from salt-induced decay and material incompatibility in physico-chemical and mechanical terms, which are typically attributed to the cementitious component of cement- based mortars. Although mortars are typically characterized by their elastic properties and material strength values, it is very import- ant to also include their inelastic or plastic characteristics as the latter are often responsible for their successful long- term performance. Deformations and resulting damage of materials such as concrete and mortars typically follow the elastic-plastic damage model. The elastic-damage and elastic-plastic models differ in that the elastic-plastic models can simultaneously take into account the accumulation effect of plastic strains and elastic modulus degradation [6]. In order to investigate the deformation characteristics and damage evolution with respect to rocks and concrete, it is necessary to perform a series of experiments. Many uniaxial and triaxial experiments under a loading-unloading-reloading regime (cyclic loading) have been conducted and are reported in the international literature (e.g., test on rocks [7, 8] and concrete [9, 10]). The authors have also performed such tests on mortars [1, 11] and have shown that such specimens predominantly exhibit a plastic behavior. Some of these experiments [11] were focused on examining the stress-strain behavior and deformation characteristics of a pozzolanic lime mortar in the pre-peak region of the stress-strain curve. The term “cyclic loading” refers to the loading-unloading-reloading procedure which was utilized to gain a better understanding of the elasto-plastic properties of the material. Details on this test are given by Gatelier et al. [7]. This investigation further analyzes the results from two series of uniaxial and triaxial compression tests under cyclic loading, which were recently completed by the authors [11]. Future work will aim at experimentally investigating the post-peak region behavior of the material in order to suggest a complete damage evolution law. M ATERIAL AND METHODS Composition of the pozzolanic mortar he properties and constitution of the pozzolanic mortar used in the experiments previously conducted by the authors are described in detail in [11]. In summary, a mix design of sand, lime and metakaolin (at 50, 30 and 20% w/w, respectively) was developed utilizing a water to binder ratio of 0.92. Mixing took place at ambient conditions. The specific weight ratio of hydrated lime and metakaolin of 1.5 used, ensured that a fully developed pozzolanic reaction of the metakaolin with hydrated lime could occur. Eventually, any unreacted proportion of the hydrated lime is considered to contribute to the plasticity of the final mortar after its carbonation [12]. Through this process the mortar is assumed to acquire a pore size distribution similar or compatible to porous stone, thus facilitating the homogeneous distribution of water and more importantly allowing the water vapor to escape through the composite system [2, 3]. Additionally, in an effort to improve the performance characteristics of the mortar, the sand used in the mix consisted of equal proportions of carbonate sand passing through the 125 and 63 μm sieves. Preparation of specimens for mechanical experiments The preparation of specimens has been described in detail in [11]. Previous research by the authors [1] indicated that it is better to develop cylindrical mortar specimens by coring mortar blocks compared to direct casting into cylinders. Mortar specimens were cast in prismatic molds constructed out of wooden particle boards. The mold was removed two days after casting and the mortar was allowed to cure in a curing chamber at a relative humidity (RH) of 90-95% T