【作者】 严耿升；

【导师】 张虎元；

【作者基本信息】 兰州大学 ， 地质工程， 2011， 博士

【摘要】 本论文以干旱地区土遗址及石窟寺壁画地仗为原型,研究干旱气候条件下土质文物的干湿、冻融、盐类风化及风蚀耐久性问题。本研究以水分迁移为主线,围绕温度场的变化和可溶盐的发生发展,模拟土质文物的劣化过程,并提出土质文物耐久性的评定方法。具体研究中,首先在水分迁移过程中施加温度场,通过控制温度场的变化,研究水分迁移过程中土质文物产生的水力学效应；其次,从量的角度出发,研究土质文物的持水特性及非饱和情况下的水分渗透、迁移速率问题；再次,从水分作为载体辅助作用的角度,分析盐分迁移的方向、路径、聚集位置和形式,及盐分通过一系列病害形式表达其发生发展的过程。在可控环境条件(湿度、温度)及已知材料特性条件下,研究土质文物中水盐的迁移聚集和表现形式,可以采取有效手段延缓或者阻止其对土质文物本体的破坏,增强土质文物的耐久性。我国西北地区,强烈干湿循环、冻融循环、风蚀等对性质脆弱的土遗址安全构成极大威胁。本文选择新疆交河故城原状生土试样(固结较好)和重塑土试样(欠固结)进行了干湿和冻融循环试验,之后进行了风蚀试验、强度试验和微观结构观测,来研究土遗址的干湿耐久性和冻融耐久性。干湿循环试验结果表明,在试样原始含水率比较低的情况下,试样首先经历了一个质量增大的过程,3个干湿循环后,试样的质量变化进入一个相对稳定的范围。随着增湿与脱湿过程中伴随的水分迁移,试样质量的总趋势是减小的。且随干湿循环次数增大,原状试样的强度减小,风蚀量增大,即耐久性持续变差。相反,随干湿循环次数增大,重塑样的强度反而有所增大,风蚀量减小,即耐久性有所提高。重塑样由于重塑过程中其微观结构受到破坏,强度弱化。干湿循环在某种意义上对重塑土起到“陈化作用”,具有愈合损伤微结构的能力。初期重塑土强度随干湿循环次数的增加逐渐增强,而后强度逐渐衰减抗风蚀能力减弱。冻融试验结果表明,在试样初始含水率较低的情况下,前5个冻融循环,试样经历了一个质量增大的过程；在随后的冻融过程中伴随着冻干和吸湿,原状试样质量的总趋势是减小的,重塑样质量的变化趋势维持水平。随冻融循环次数增大,原状试样的微结构出现损伤,强度减小,风蚀量增大,即耐久性持续变差；相反,冻融循环初期,重塑样的强度有所增大,风蚀量减小,耐久性得到了提高。随着冻融循环的增加,重塑样的强度开始下降,这与干湿循环后土体耐久性的变化趋势一致。冻融循环同干湿循环一样可以引起重塑土的“陈化”,愈合了土体的结构,但随着冻融进行,矿物颗粒的微结构损伤不断增大,试样的耐久性降低。使用Ku-pF非饱和导水率测定系统,测试了交河生土原状样与重塑样的土水特征曲线和非饱和导水率,从非饱和角度分析了土遗址的盐害机理。试验结果表明,遗址土的非饱和导水率随基质吸力的增大呈指数衰减,当生土的体积含水率小于渗透系数为6×10-8cm/s所对应的含水率时,水分及孔隙盐溶液向土体表面的相对运移速率明显减慢,易溶盐可能发生结晶作用。根据已得到的非饱和导水率,利用一维渗流的盐分运移方程可以描述遗址土中的盐分迁移趋势。不同浓度的Na2SO4溶液渗透试验发现,渗透过程中溶液浓度变化比渗透系数的变化要剧烈的多,间接说明渗透吸力相对于基质吸力对渗透系数的影响要弱。使用压力膜仪和柔性壁渗透仪,进行了壁画模拟地仗(细泥层FP和粗泥层CP)的持水特征曲线测量试验和渗透试验,分析了细泥层和粗泥层的土水特征曲线及有机质(麻和麦秸秆)对壁画地仗持水能力影响程度。试验结果表明,在试验尺度内,两种材料制成地仗的土水特征曲线趋势相近,这是因为颗粒组分是决定模拟地仗孔隙大小与分布状态的主要因素,细泥层和粗泥层均采用同种配比和粒径的土样。但由于加筋材料的不同,特征曲线的细观变化亦有所不同。数据显示,壁画模拟地仗的非饱和导水率随基质吸力的增加成指数衰减。壁画不同于土壤,在其受保护的情况下,本体不可能达到饱和,壁画在非饱和情况下的渗透性,才能揭示其真实的水分迁移规律。季节的改变和游客的呼吸作用都会对壁画中的水分迁移产生影响。通过对壁画模拟地仗在不同温湿度条件下的吸放湿试验(MAT),壁画与水分迁移相关的劣化机理得以说明。试验使用不同类型的饱和盐溶液控制试样所处的湿度环境,利用植物生长室提供恒温和变温环境；并通过定时称量试样的质量来判断试样和特定空气之间的吸放湿过程。试验结果表明,脱湿过程较吸湿过程快,即较长时间积攒在壁画中的水分可以较快的蒸发脱出壁画,这就暗示我们可以采取措施减小空气流通如抑制游人进出、或季节变化对壁画产生的不利影响,如缩短游客在洞窟中逗留的时间。在高湿度环境下,温度的变化可以引起模拟地仗吸湿和放湿,且壁画地仗中水分呈渐进性的积累；反之,在低湿度环境下,循环变化的温度在引起壁画地仗吸放湿的同时会导致水分总量渐进性减少。试验数据表明,在相对潮湿季节,应尽量避免洞窟对游人开放,从而避免壁画变潮而劣化。盐分的毛细迁移试验表明,试样的毛细上升带,自下而上依次为硫酸盐和卤化物。空气湿度影响着水分的蒸发,间接控制着溶液的毛细迁移和盐分的结晶聚集区域(即盐华带)。盐华带标志着毛细水上升的平均高度,该带内水分蒸发,盐分停止迁移并残留下来,在壁画表面发育成大量盐华及内部盐沉积。可溶盐的存在、渗透效应和盐分结晶强化了盐害的破坏。结晶盐因为晶体生长、水化和热膨胀产生的压力,会导致壁画的破坏。壁画地仗中存在大小不一的孔隙,与外界环境相接触的大、中孔隙中发生蒸发时,盐分开始结晶。相邻小孔隙中的水被抽汲出来补充已蒸发的水分,以此联动发生至平衡状态。当孔隙中稀溶液暴露于低的相对湿度条件时,溶液中水分蒸发,盐浓度提高。浓度越高,平衡蒸汽压越小,直至达到一个临界极限,即最低平衡相对湿度时,盐溶液达到饱和状态。环境RH低于此极限值时,盐晶体开始聚集并生长。更多还原

【Abstract】 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.更多还原