Cylindrical samples from old adobe were obtained from adobe samples by using a core extractor. An accelerated weathering tester QUV-SE [33] simulates the effect of ambient exposition. Figure 4 illustrates the equipment with a group of samples ready to be exposed to weathering. The equipment is capable of controlling irradiance by a closed-loop control and uses eight UVA-340 lamps simulating the sunlight from 365nm down to the solar cut-off of 295 nm. In addition, the apparatus has a condensation mechanism to simulate outdoor moisture attack by heating a water reservoir to produce a vapor that maintains a 100 % relative humidity. Radiation and condensation alternate cyclically in programmable periods. Exposition of materials to this artificial regime produces degradation in a similar way that natural ambient conditions. Proportionally time factor between natural and artificial exposition is not constant since it depends on several variables that are not fully controlled (e.g., tested material, variation in natural conditions, variables capable of controlling in the equipment). The standard ASTM G154-16 [34] covers the principles and operating procedures for apparatuses designed for UV exposition of non-metallic materials.

Mechanical tests on the samples were done by using the GDS triaxial testing apparatus shown in Figure 5a [35]. The system allows controlling confinement pressure and vertical stress. A universal compression tester machine was used for unconfined compression tests on cylindrical samples (Figure 5b). Additional equipment and accessories include scale, oven, sieves, soil classification instruments and compaction molds.

Figure 4 Figure 4 Accelerated weathering tester QUV-SE Source: Created by the authors.

Figure 5 Figure 5 Mechanic devices: a) triaxial testing apparatus; b) compression tester machine Source: Created by the authors

Stress states at failure from compression tests were represented by Mohr’s circles as can be shown in Figure 7, which includes the failure envelope according to the Mohr-Coulomb failure criterion given by (1). In this criterion, is the shear stress at failure, c is cohesion, σ is the normal stress to the failure plane, and ϕ is the shear strength angle.

(1)

Figure 7 Figure 7 Mohr’s circles and failure envelope of new adobe and no exposition to weathering. Source: Created by the authors.

Similar graphics for different adobes, exposition period and type of weathering were plotted and estimated the corresponding shear strength parameters, ϕ and c. Figure 8 presents the comparison of the different shear strength angles. It is evident that this parameter was not affected by the duration of the exposition, neither by natural nor by artificial regime. As a result of the acceleration in the weathering process, new adobe exposed to artificial conditions had a very little increment in the shear strength angle compared with the same adobe but naturally exposed. It is important to realize that old adobe retained the same shear strength angle despite the long and unfavorable conditions of natural weathering. From a practical point of view, the shear strength angle is 25° for old and new adobe, regardless the exposition conditions.

Figure 8 Figure 8 Shear strength angle of adobe samples. Source Created by the authors.

The patterns observed in the compressive strength analyzed in the preceding section, are reflected in the cohesion parameter (Figure 9). This observation indicates that the growth in the strength is primarily governed by an increase in the cementation of the clay minerals by cause of exposition to natural or artificial ageing. Old adobe is nearly insensitive to accelerated exposition because after approximately 300 years of natural exposition to weathering, almost 3000 hours of additional accelerated exposition are insignificant. Nevertheless, prolonged exposition to ambient conditions without preventive or reconstructive interventions seriously diminished the cohesion of adobe units. Reduction in cohesion values after 1500 hours of accelerated exposition evidenced in new adobe samples could be due to the same mechanism that occasioned the gradual reduction of cohesion in old adobe after many years from its construction until the present.

Figure 9. Figure 9. Cohesion of adobe samples Source: Created by the authors.

A more detailed investigation concerning the incidence of weathering conditions on the shear strength characteristics of the adobe should be addressed to develop a material constitutive model including deterioration features caused by ambient conditions. Research of this kind ought to include a detailed analysis of ambient conditions along historical periods in the specific site of the building under study and use accelerated weathering testers to simulate the ambient conditions on the materials, preferably imposing longer periods beyond that those used in this work. Resistance of adobe materials is presented in literature in terms of unconfined compressive strength [10] and there are no reports of shear-strength parameters.

The initial slope of the curve shear stress against shear strain is suitable to estimate the shear modulus. Despite the deficiency in the resolution of the sensors regarding the small strain range in which the material is elastic, results presented in this section are useful for comparative purposes. Figure 10 compares the evolution of the shear moduli for the samples studied. As other results presented in previous sections, shear moduli from old adobe are not significantly affected by additional weathering.

Figure 10 Figure 10 Shear modulus of adobe samples. Source: Created by the authors.

Regarding new adobe, naturally exposed samples were stiffer than artificially exposed samples for all the periods of exposure to weathering. This fact could be due to the condensation cycles that allow more flexibility to the material as a consequence of the gain in moisture. New-adobe samples artificially exposed reached a kind of stabilization at the end of the weathering period imposed by the accelerated tester.

Comparing shear moduli of new adobe against old adobe, it is clear the significant difference along the entire period studied. Moreover, the ratio between new-adobe modulus and old-adobe modulus from samples artificially weathered is around two for all the exposition periods. The substantial reduction of the modulus observed in old-adobe samples could be caused by the careless conditions of the structure throughout the last century during which the walls were non-protected with a roof.

Reduction of the stiffness accelerates the degradation process of the structure because it increases the deformations under the constant load applied by the self-weight of the walls. In turn, additional deformations induce further stresses that could exceed the strength of the adobe and consequently causing failures, cracks, and detachments. The vicious circle completes given that the more material surface exposed to ambient factors, the fastest the degradation affect the surface of the material.

Considering results show in Figures 6 to 10, Deterioration Factors (DF) can be proposed to consider them in vulnerability analysis of structures built of new adobes with typical degradation of the resistance properties. Inevitably, modification of the physical properties with the time will be occur, thus, the most rational decision is to quantify this diminution. Then if the compression strength is needed, a deterioration factor of 2 (DFcs=2) could be used. The deterioration factor is 1 for shear strength angle (DFϕ=1), the deterioration factor for the Shear Modulus is proposed as 2 (DFG=2) and the deterioration factor for the cohesion is 4 (DFC=4). Deterioration Factors (DF) are proposed to be used as a first approximation for the quantification of the impact of the climatic conditions over the mechanical response of the adobe units.

An experimental study concerning the influence of ambient conditions on the evolution of mechanical properties of adobe materials was carried out. An accelerated weathering machine was used to simulate the exposition of the materials to the main ambient factors that could deteriorate the mechanic properties.

A restoration project of the heritage buildings should include the analysis of the structural performance of the building as well as the study of materials that best fit with the characteristics of the existing materials. Old adobe recovered from the Tausa Chapel (Colombia) and new adobe obtained from a local soil deposit, are the two materials included in this research.

Ambient factors affect the compressive strength of adobe material. Initially, this property increases to a maximum value, after which, it gradually reduces until reaching values such as those found in the old adobe of the rear sector of the chapel. After many years of natural exposition to tropical ambient conditions, additional accelerated exposition does not modify the strength of the adobe.

Ageing of adobe affects the cohesion and the shear modulus while the shear strength angle remains practically intact whether in the old adobe or in the new adobe. However, more experimentation and analysis are required to propose a constitutive model that could reproduce the ambient effect on the mechanic behavior of adobe materials. Degradation of material properties by ambient factors accelerates the structural damage of the buildings, which in turn, increments the exposed surfaces to ambient effects. If it is necessary to consider the degradation of the mechanical properties across the time, Deterioration Factors (DF) are proposed to be used as a first approximation for the quantification of the impact of the climatic conditions over the mechanical response of the adobe units.

Preventive actions on adobe and rammed earth structures cut out the vicious circle of ambient exposition, mechanical degradation, and structural damage. In most of the cases, modest interventions like drainage control, maintenance of roofs or protective painting could be sufficient to reduce the negative impact of the ambient on the state of conservation of heritage constructions.