Deterioration Mechanisms of Building Materials of Jiaohe Ruins in China
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Journal of Cultural Heritage 14 (2013) 38–44 Available online at www.sciencedirect.com Original article Deterioration mechanisms of building materials of Jiaohe ruins in China a,∗,b b a a c Mingshen Shao ,Li Li , Sijing Wang , Enzhi Wang , Zuixiong Li a State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, China b Chinese Academy of Cultural Heritage, Beijing 100029, China c Conservation Institute, Dunhuang Academy, Dunhuang, Gansu 736200, China a r t i c l e i n f o a b s t r a c t Article history: Almost all defects of earthen buildings such as roughening, erosion, volume reduction, cracking as well Received 6 May 2011 as crazing, etc., have been witnessed in the ancient city of Jiaohe, an earthen architectural heritage in Accepted 19 March 2012 northwest China. In this paper, their long-term durability and deterioration due to prolonged exposure Available online 19 April 2012 to environmental factors were studied, based on the basis of field investigation and laboratory analysis. The results indicated that the deterioration of building materials should be attributed to their basic Keywords: properties, including density, particle size distribution, soluble salts, mineral, mechanical strength, etc., Jiaohe and interaction with environmental factors. And then, four main deterioration modes can be identified, Material properties namely: wind-related deterioration, water-related deterioration, temperature-related deterioration and Deterioration mechanisms Durability chemical related deterioration. It can be concluded that the greatest deterioration was wind-related Reinforcing measures deterioration on west-north facing fac¸ ades, and chemical related deterioration on the surface of building materials. Crown Copyright © 2012 Published by Elsevier Masson SAS. All rights reserved. 1. Introduction natural factors and human factors, some buildings were destroyed completely. Fortunately, from the middle and late parts of 1990s, The ancient city of Jiaohe, located in the Xinjiang Uygur some research workers started to pay attention to these problems Autonomous Region of China, is considered to be the largest, old- and to take effective measures to solve them. Z.X. Li analysed the est and best-preserved earthen city in the world (Fig. 1). The city destroying mechanism of the cliff mass and the classification of was built on a loess plateau atop a cliff of over 30 meters as the deteriorations [1]. Experts from the Nara National Cultural Proper- capital of the state of South Cheshi, one of the kingdoms of the ties Research Institute and the Xinjiang Cultural Relies Bureau have Han dynasty (206 BC–AD 220), and deserted in the Yuan dynasty jointly undertaken archaeological surveys and reinforced protec- (AD 1271–1368). During this period it was of great military signif- tive measures for the site [2]. Since June 2006, Dunhuang Academy icance and played a pivotal role in the economic development of began to take emergency reinforcement measures, such as environ- Western and Eastern countries. After having experienced all kinds ment treatment, anti-weathering consolidants for buildings and of changes and erosion by wind and rain for more than 2000 years, consolidation of the cliffs, with rather satisfactory results, which the main structure of the architecture layout of the Jiaohe Ruins may be regarded as references for similar projects [3]. Based on is still well preserved till the present day, and most architecture fundamental property of the materials themself, this paper aims are clear and distinguishable such as temple, sites of government, to study their mechanisms of degradation exposed to the arid and courtyards, streets, handicraft workshops, watchtowers and so on. semi-arid environment in west of China, to supply the theory sup- The current scale of the Jiaohe Ruins is mainly the architecture of port for preservation of the ancient city of Jiaohe. the Tang Dynasty and the architectures are mainly located in the area of about 1000 meters in the southeast part of the platform. 2. Material properties As one of the world’s architectural wonders, most buildings of Jiaohe city were dug from earth, partially built with one, or a combi- 2.1. Preparation for soils nation of man-made materials including rammed earth, mud brick and cob. However, due to vulnerability of the materials themselves, As one of the world’s architectural wonders, most buildings of they have steadily deteriorated under the combined action of Jiaohe city were dug from earth, partially built with one, or a combi- nation of man-made materials including rammed earth, mud brick and cob (Fig. 2). The three soil samples were taken from Northeast ∗ temple, Great buddhist temple, Pagodas, Government office and Corresponding author. E-mail address: [email protected] (M. Shao). other sites, whose relative positions are shown in Fig. 1. 1296-2074/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2012.03.006 M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44 39 100 80 Raw soil Ramm ed earth Cob 60 ed percentage /% percentage ed 40 20 Accumulated retain 0 1000 100 10 1 Par ticle size /µm Fig. 3. Particle size distributions of three soils from Jiaohe city. water content, dry density, porosity, liquid limit, plastic limit, etc. are showed in Table 1. 2.2.1. Particle size distribution The particle size distribution test results confirmed that soil was Fig. 1. The ancient city of Jiaohe. composed of all the four main soil fractions: fine gravel, sand, silt, and clay, and the soil type was predominantly silt (Fig. 3). The soil has sufficient proportions of fines (silt and clay), as well as a propor- tion of coarse fraction (fine gravel and sand) for the skeletal frame of the building. Compared to the raw soil and cob, rammed earth has lower amount of silt, about 25%. This is because that rammed earth is usually used to play the part of flattening materials between raw soil and cob, or mud brick, where is subjected to the weathering of water and wind, could not hold on lower adhesion silt. 2.2.2. Plasticity Soil plasticity, the ability of a soil to undergo irreversible defor- mation while still resisting an increase in loading, is indicated by the plasticity index. Plasticity index is the numerical difference between liquid and plastic limits. The plasticity index is an indi- cation of the clay content and characteristics of the soil. The test resulted that the plasticity index of raw soil was 10.4, cob 11.8 and rammed earth 11.7, respectively, which indicates that the soils have active clay mineral and that higher shrinkage would occur when the soils dries. Fig. 2. Wall made up of three materials. 2.2.3. Soluble salts The content of soluble salts in these soils is usually high (Table 2). The average degree is the highest for raw soil and the lowest for cob, 2.2. Soil specification and rammed earth and it distributes more concentrated in wall footing, where salts are susceptible to environmental humidity. 2+ + + 2− The basic physical properties of the soils have been well estab- Cation in the soluble salts is Ca , Na and K , while anion is SO4 , − − 2− lished over the past 6 years of intensive research. The values for Cl and NO3 , almost without CO3 ; some of them are shown in Table 1 Basic physical properties of building materials. Soil Water Density Specific Porosity Liquid limit Plastic Plasticity Slaking speed 3 content (%) (g/cm ) gravity (%) (%) limit (%) index (g/min) Rammed earth 1.76 1.60 2.70 40.74 27.50 18.42 9.08 15.0 2.41 1.56 2.72 42.65 29.00 16.00 13.00 20.6 Cob 1.38 1.55 2.72 43.01 30.60 17.20 13.40 10.5 2.14 1.66 2.72 38.97 31.58 18.15 13.43 11.5 Raw soil 2.65 1.72 2.70 36.30 27.60 17.70 9.90 11.7 3.03 1.68 2.70 37.78 28.00 18.75 9.25 12.4 40 M. Shao et al. / Journal of Cultural Heritage 14 (2013) 38–44 Table 2 Soluble salts in soils from Jiaohe city. Soil Depth (cm) PH Anion (mg/kg) Cation (mg/kg) Amount of soluble salts (mg/kg) − 2− − 2− − 2+ 2+ + + − NO3 CO3 HCO3 SO4 Cl Ca Mg Na +K NH3 N Rammed earth 0 7.37 95 0 227 5627 619 2041 64 733 12 9381 0 7.58 1212 0 228 6579 7656 2892 31 5270 1 24,066 Cob 0 8.05 142 15 192 3787 812 1250 63 921 1 7183 0 7.90 755 0 226 4381 2892 1825 95 2063 1 12,226 Raw soil 0 7.62 1454 0 193 7583 14,094 6243 382 5444 34 35,514 10 7.49 1593 0 229 6015 12,798 7173 352 2861 62 31,106 0 7.80 271 0 193 7197 1454 2201 95 1837 11 13,320 10 7.82 335 0 161 3796 2262 1201 31 2030 1 9832 Table 2. The abundant sulphates of Na, Mg and Ca, as Houben and reduced by 12.39%, mud resistance to wind erosion and rain ero- Guillaud [4] reported, are harmful to soils because they crystallize sion was obviously worse, especially rammed earth, as shown in and make the soils fragile. Fig. 4. 2.2.4. Mineral 3. Deterioration mechanisms in materials The results of X-ray diffraction (XRD) test show that the mineral components are quartz, feldspar, calcite dominantly, with minor As is the case with most other building materials, deterioration dolomite, hematite, magnetite, pyrite, gypsum and other clay min- mechanisms in Jiaohe soils are varied and complex.