Resumen de Advanced mechanical characterization of lime mortars and other materials of the civil and architectural patrimony
The objective of the thesis is to provide an advanced and a deeper mechanical characterization of the lime mortars commonly used in the restoration field, which would be helpful for the numerical simulations of historical masonry structures. For the purpose, several aspects concerning the fracture and deformability behavior of lime mortars have been studied.
In the first place, it is found that there is still a lack of standardization on the dosage methodology of lime mortars. Thus, seven types of natural hydraulic and aerial lime mortar were fabricated and five factors which have an influence on their properties have been studied, in particular the water/lime ratio, the mold material, the aggregate size and type and the different curing conditions. Furthermore, an advanced mechanical characterization has been performed on these mortars, including the measurement of the fracture energy. Finally, some empirical equations for determining the relationships between these mechanical properties were proposed, which could be helpful when simulating the numerical models of historical constructions.
In the second place, the analysis of the time effect on the properties of lime mortars at early ages (up to 7 days) is analyzed. It is especially relevant the study of the deformability of lime mortars at early ages as it enables accommodation movements of masonry before cracking. However, the references focusing on the measurement of such property at early ages are inexistent. Then, in the thesis a novel technique developed recently for cement based materials is applied for the first time to natural hydraulic lime mortars. It is named as Elastic Modulus Measurement through Ambient Response Method (EMM-ARM) and it allows the automatic and continuous evaluation of the elastic modulus immediately after casting without demolding the specimen. For the purpose, various sealing and compacting procedures were studied through flexural and compressive strengths, density, ultrasound pulse velocity, open porosity, etc., in order to compare the procedure adopted for EMM- ARM (sealed and vibrated) with the standard one (unsealed and compacted). The range of elastic modulus obtained among the different mortars was between 2.5 and 4.1 GPa on day 7, which shows feasible potential of application of EMM-ARM to natural hydraulic lime mortars at early ages (under 3-7 days). It was also found that sealed specimens led to 50% and 25% lower compressive and flexural strengths, respectively, compared to unsealed ones.
Related to the previous aspect, it is also studied the time effect on the properties of natural hydraulic and aerial lime mortars in the long-term. This is of relevance because it is known that the flexural and compressive strengths of lime mortars continue evolving beyond 28 days as they require higher periods of time than cement-based materials to reach their maximum strengths. However, to our knowledge, there are no studies focusing on the measurement of the fracture energy, the splitting tensile strength or the static elastic modulus in the long-term in such materials. Thus, the third purpose of the thesis is to study the time effect in the mechanical properties of lime mortars. Moreover, these measurements are related to the evolution of the carbonation depth through the phenolphthalein method on prisms to study the influence of the carbonation process on both lime mortars. The results show that there is a faster increase of the mechanical properties in both mortars up to 56 days, which ranges between 60% and 90% of their corresponding values at an age of 448 days depending on the mechanical property and type of mortar. After this age, there is a more moderate but progressive evolution up to 224 days. However, from this age to 448 days, the evolution of the mechanical properties is very slow for the aerial lime mortar and shows a slight increase for the natural hydraulic one. Furthermore, some empirical equations of such behaviors with time are proposed for both mortars.
The forth objective of the thesis is to analyze the loading rate effect on the fracture properties of lime mortars. This aspect is getting more attention recently as many historic masonry structures are situated in zones of seismic activity. For the purpose, three-point bending tests are performed on both natural hydraulic and aerial lime mortars under three various loading rates (loading-points displacement rates, 5.0 × 10^(−4) mm/s, 5.0 × 10^(−1) mm/s and 1.6 × 10^1 mm/s). The results show that the peak load and the fracture are rate sensitive. The maximum dynamic increase factors (the ratio of the dynamic properties to their corresponding quasi-static values) of the peak load are 1.4 and 1.6 for the natural hydraulic and the aerial lime mortars, respectively, whereas it is 1.9 for the fracture energy for both mortars. Moreover, six specimens were dried and tested under the lowest and highest loading rates to study the phenomena of the rate sensitivity. It is found that it is mainly due to viscous effect of free water in the natural hydraulic lime mortar. However, for the aerial lime mortar, the rate effect is related chiefly to the crack growth and velocity. Moreover, through scanning electron microscope (SEM) analysis, it is observed intergranular failure.
Finally, among the mechanical properties measured on the natural hydraulic lime mortars, it was found a significant difference between the compressive strengths of standardized prisms (with 40 mm in depth) and cylinders (with 150 mm in height). Our hypothesis was that this difference was due to geometry and size effects. Then, in the thesis a numerical simulation of the compressive test on prisms is performed but assigning as intrinsic material compressive strength the one of the cylinders. The obtained numerical curves fit very well with the experimental ones, which means that the difference in the compressive strength of prisms and cylinders is due to geometry and size effects and that the compressive strength from the cylinders is roughly the intrinsic compressive strength of the material. Furthermore, two more numerical models were performed by doubling the size of the standard prisms once and twice. With the peak loads of the three models, it is possible to obtain the size effect laws of two natural hydraulic lime mortars. Furthermore, a cohesive simulation of the three-point bending test on such mortars is performed to verify that they behave as cohesive materials.
To conclude, this thesis provides improvements in the techniques to measure the mechanical properties of lime mortars. The analyses performed could be useful to define with more realism and precision the numerical simulation of masonry structures built with lime mortars. The techniques proposed in this research could also be applied to other cohesive materials of the civil and architectural patrimony, such as compressed earth blocks, rammed earth, stones or bricks.
