Study On Deterioration Mechanism And Materials Durability Of Earthen Monument In Arid Areas

Hypaethral earthen architecture and Cave temples wall paintings in arid area were treated as a research prototype. And this paper studies the deterioration mechanism and materials durability of earthen monument during cyclic wetting-drying, cyclic freeze-thaw, Salt crystallization and wind erosion. Firstly, moisture migration tests were imposed by temperature field to investigate the effect of moisture migration process on earthen architecture. Secondly, from the point of migration quantity, the infiltration rate has been presented. Once more, as the perspective of auxiliary function, moisture migration process decides the direction, path, location and forms of salt enrichment; simultaneously, the evolution of salt is expressed by a series of disasters. During controlled environments (humidity, temperature) and measured material properties condition, study of salt disasters in earthen monument can provide an effective means to prevent and retard the destruction on monument ontology. As a result, durability of earthen monument is lengthened.Earthen architecture is mostly found in the arid area, which suffers from strong freeze-thaw cycles, wetting-drying cycles and wind erosion process. This study reported the test results of wetting-drying cycles, freeze-thaw cycles and wind erosion on the durability of earthen architecture.Cyclic wetting-drying and freeze-thaw were conducted on the undisturbed and remolded specimens from Jiaohe cultural heritage, Xinjiang, China to understand the durability of earthen architecture to wind erosion and strength attenuation. Test results of wetting-drying cycles showed, under the circumstance of the low original water content, the specimens weight was increased during the beginning three wetting-drying cycles, and then proceeded to a relatively steady state. With the change of moisture content during the condensation-evaporation process, specimen' weight expressed a decreasing tendency. With the increase in wetting-drying cycles, undisturbed specimens showed a decrease in compressive strength and an increase in wind erosion quantity, reflecting a weakening effect of durability. Comparatively, remolded specimens showed an increase in strength and a decrease in wind erosion quantity, or an inhencing effect of durability at the early wetting-drying cycles; but during the later cycles, a decrease in strength and an increase in wind erosion quantity, or weakening durability were found. It was considered the healing of destroyed micro-structure during remolding and the aging of chemical cohesive agents were responsible for the strengthening of remolded specimen at the earlier wetting-drying cycles.Test results of freeze-thaw cycles showed, under low original water content, the undisturbed and remolded specimens weight were increased during the beginning five freeze-thaw cycles. With the change of moisture content during the freeze and thaw process, undisturbed specimen' weight expressed a decreasing tendency, and remolded specimens'weight maintained a level of weight. With the increasing freeze-thaw cycles, undisturbed specimens showed a decrease in compressive strength and an increase in wind erosion quantity due to microstructure damage, that is, a weakening in durability. However, the remolded specimens exhibited an enhancement of microstructure at the beginning cycles, characterized by an increase in strength and a decrease in wind erosion, that is, an improvement in durability. It was considered the healing effect of destroyed micro-structure and the aging of chemical cohesive agents were responsible for the strengthening of remolded specimen at the earlier freeze-thaw cycles. At the further freeze-thaw cycles, the remolded specimens turned to exhibit a decreasing tendency on durability following the increasing damage of microstructure.Soil water characteristic curves and unsaturated hydraulic conductivity were determined using a Ku-pF unsaturated hydraulic conductivity measurement system on the undisturbed and remolded specimens from ancient city of Jiaohe, a cultural heritage site at Xinjiang, China to understand the salt disease in earthen architecture from the viewpoint of unsaturated soil mechanics. Test results showed that the unsaturated hydraulic conductivity had an exponential decrease with the increasing matrix suction. It was found that when the volume water content was less than the water content corresponding to the permeability coefficient of 6×<10-8cm/s, water migration was obviously slow down and the salts in soil solution might be crystallized. The one-dimension salt migration equation could be used to describe the salt migration in earthen architecture using the measured unsaturated hydraulic conductivity from this research. Permeability test with various Na2SO4 solutions indicated that osmotic suction had a less importance on the permeability coefficient than the matrix suction.Taking the simulated base (Coarse plaster and Fine plaster) as the research object, the study utilized the pressure plate extractors and flexible-wall permeameter to research the soil water characteristic curve (SWCC) and permeability. Data were conducted to analysis the variations of water retention capability and unsaturated permeability. Experimental results showed that the general trend of both materials' soil water characteristic curve was similar during the test scale. It was found that the grain fraction of simulated base was the key factor of void size and distribution. Besides, coarse plaster and fine plaster used the same grain fraction to mold specimens, so both materials had similar water retention characteristics. However, owing to the different reinforcement materials, the macroscopic changes of characteristic curve were different. Unsaturated Permeability was exponentially attenuation induced by variations of the suction. Due to the rigorous protection, mural is in an unsaturated condition, so that the permeability given accurately during unsaturated condition could reflect the characteristic of water migration.Moisture adsorption-desorption tests (MAT) were conducted on simulated mural plaster specimen under different air temperatures (AT) and relative humidity (RH) to investigate the possible effect of seasonal alteration and visitors'breath on the deterioration of Mogao Grottoes, Dunhuang, China. Saturated salt solutions were used to maintain a constant RH, and plant growth cabinets were used to maintain a constant or varying temperature in the simulating test. The weight of specimens was periodically measured to determine the adsorbed or desorbed moisture by specimens from or to the controlled air. Test data illustrate that the desorption process is far speedy than that of adsorption one, indicating that it is possible to inhibit the disadvantage effect from visitors, such as shorting the staying time in caves. In case of high humidity, an accumulated moisture adsorption was found to be corresponding to the varying temperature. On the other hand, in case of low humidity, an accumulated moisture desorption was corresponded. Test data imply that opening caves more often to visitors in humid seasons should be avoided so as to prevent continuous wetting of wall paintings.Salt capillary migration experiment shows that, with the capillary rise, samples surface left for sulfate and halide successively. The RH affects water evaporation, and indirectly control the aqueous capillary migration and salt crystallization area. When a solution in equilibrium with its environment has reached saturation, every drop of RH causes evaporation, supersaturation and precipitation. Some solutions reach this critical condition at ordinary RH, and above the critical equilibrium levels of RH only the precipitated salts can be found. But the equilibrium RH changes with temperature. Daily microclimate cycles due to e.g. solar heating or room heating may induce crystallisation cycles in wall paintings. For this reason, in the cold climate, weekly heating-cooling cycles in churches lead to important changes in the wall temperature, followed by severe damage due to crystallisation cycles.