Abstract
According to the latest UN statistics, more than 2 billion people in the world still live in various forms of earthen buildings, including some in China. The variety of earth-based constructional materials is significant among different regions, with each region influencing the selection of local earth construction materials. In this study, earth materials from four regions of China were collected and sorted, with 10 samples from each source, and cube compressive strength tests were performed to analyze the composition and mechanical properties of the four materials, including northeast black earth, southeast red earth, northwest loess, and Xinjiang yellow sand earth. The results showed that significant differences existed in the composition of earth-based constructional materials from different regions, which have influence on the materials’ compressive strength. The order from large to small of compressive strengths was loess, black earth, yellow sandy earth, and red earth. Material load-displacement curves were influenced significantly by the plasticity index, but the overall failure processes of the various samples were basically the same.
1. Introduction
According to the latest UN statistics, more than 2 billion people in the world still live in various forms of earthen buildings [1]. Earthen buildings are constructed using earth-based materials as the main structural material or the enveloping enclosure structure. As an important part of traditional construction, it has the advantages of low energy consumption, good thermal performance, sound absorption, radiation prevention, and green environmental protection, such that it is a kind of sustained development construction [2, 3]. The New Mexico (USA) government has formulated and implemented the “New Mexico Adobe and Rammed Earth Building Code” and has established detailed regulations regarding the selection of soil quality for load bearing and regenerating earth walls, the manufacturing requirements of raw soil components, and technical indices of raw soil materials [4, 5]. The International Center for Research and Application of Raw Earth Architecture has compiled the “Earth Construction: A Comprehensive Guide” and “Raw-Soil Building” as part of the selection, production, and maintenance of different modified raw earth-based adobe materials [6, 7]. Especially in developing countries, the continued rapid population growth necessitates more construction and the use of industrial building materials [8]. The use of local materials to build housing is an important strategy to counter our worsening global environmental problems [9]. Earth materials are some of the oldest local materials and are widely used in dwelling construction all over the world. In fact, one third of the world’s population still lives in earthen dwellings [10, 11]. In the last twenty years, this original material has found new attention from the construction industry because of its low energy consumption and excellent ability to regulate indoor temperature and humidity [12]. Samples with a small aspect ratio and large compression area are more vulnerable to strong confinement imposed by the loading plate. This confinement restricts the lateral strain of the samples and thus artificially enhances the compressive strength [13, 14].
The selection of earth building materials is also particularly important. In China, the earth quality of different regions varies greatly [15]. Northwest China is the main area of loess distribution, where loess, with typical characteristics, is widespread in Shaanxi, Gansu, Hebei, and Shanxi [16]. Yellow sand earth is distributed in most areas of Xinjiang and contains gravel grain with obvious stratification [17, 18]. Red earth is mostly concentrated in some regions of Fujian, Hunan, Yunnan, Hainan, and Guangxi [19]. Most black soil is distributed in northeast China, including Liaoning, Jilin, Heilongjiang, and eastern Inner Mongolia [20]. Except for red earth, the origin of the other three types of earth is loess [21]. In this study, earth materials were selected from areas of concentrated earthen buildings in China to address the formation conditions, causes, and material composition. Reference to the methods of sample preparation and testing have been proposed previously by Zhang [22], with compressive strength tests performed on 40 cube samples of northwest loess, northeast black earth, southeast red earth, and Xinjiang yellow sand earth, to analyze failure patterns, load-displacement curves, strength of samples, and range of results. This study also further verified the feasibility of the test method for providing a reference for the study and applicability of the mechanical properties of earth-based materials in different regions of China.
2. Experimental Procedure
2.1. Materials
2.1.1. Loess
Loess is a yellow silt sediment transported by wind during the Quaternary geological period. It is native, distributed in continuous, thick layers, covered in low watersheds, hillsides, and hills, and often in contact with bedrock without integration. It has no stratification, often contains paleosol and calcareous nodule layers, in vertical joint development, and often forms steep walls. In China, loess and loess-form earths are mainly distributed in drought and semidrought areas north of Kunlun Mountains, Qinling Mountains, Taishan Mountains, and Lushan Mountain Line. The clay is mainly composed of quartz, mica, feldspar, and carbonate and contains some cementing materials, such as alumina and silica. Native loess in the Yellow River midstream is the most developed, mainly distributed in Shanxi, Shaanxi, southeast Gansu, and western Henan [23–25]. Earth loess samples used here were loess from the Chang’an District, Xi’an city, Shaanxi Province.
2.1.2. Black Earth
Earth from Northeast China is an important grain production base, as black earth has high fertility, with good physical, chemical, and biological characteristics, and reputed as “King of Earth.” Black earth is mainly distributed in the Songnen Plain, Sanjiang Plain, Daxing’anling Piedmont Plain, Liaohe Plain areas, especially from the Nenjiang River basin and the right bank of the Heilongjiang River to the Xiaoxing’anling, and Changbai Mountain areas. Black earth is composed of concentrated and connected black earth and chernozem [26, 27]. Researchers consider that black earth is an independent category of earth. The general belief is that black earth is formed by long-term degradation of plant material in a temperate zone. The parent material is mostly loess clay, and the soil texture is sticky and weighty with poor water permeability. Due to seasonal differences in Northeast China, winter is cold and summer warm and rainy such that plants grow luxuriantly and then die back to the ground and underground. Seasonal temperature differences do not allow the organic matter time to decompose, thus forming a deep and thick humus layer, a material that is basically the same as loess, with SiO2, Al2O3, and CaO content all slightly less than loess but contents of Fe2O3, MgO, and organic matter all higher than loess [28, 29]. Black earth used here was derived from Daqing city, Heilongjiang Province.
2.1.3. Red Earth
Red dense clay earth called “Red Earth” was discovered by F. Buchanan in 1807 in the Malabar Mountains of India [30, 31]. Experts and scholars consider that the definition of red earth is the sediments formed by long-term weathering and agglomeration of parent rocks rich in iron. The three processes of red earth formation include the action of consolidation pressure, the action of decomposition and hydrolysis of fine aggregates after earth formation, which yields free iron, silicon, aluminum, and carbonate, and reaggregation processes that form cemented connections, which form structural units as a skeleton connected through bound water and contacted cements, forming red earth in the modern sense. Red earth’s main components include SiO2, Fe2O3, Al2O3, R2O3, and large amounts of bases [32], and the red earth is mainly distributed in regional portions of Fujian, Hunan, Yunnan, Hainan, and Guangxi. The red earth has the characteristics of high porosity, high viscosity, high moisture content, high liquid plastic limit, strong plasticity, and low compactability [33]. With the relative enrichment of free iron and aluminum, the red earth used here was obtained from Fuzhou, Fujian Province.
2.1.4. Xinjiang Sandy Earth
Xinjiang is a vast territory that is geologically complex, composed of different geomorphologic units of mountains and basins with deep and large fractures as a dividing line. It is one of the important distribution areas of eolian loess in China [34]. From space, the loess in Xinjiang is mainly distributed in the Tacheng area to the west of Junggar Basin, the northern foot of Tianshan Mountain, and the northern foot of Kunlun Mountain-Arjinshan Mountain southern margin of Tarim Basin and Yili. Loess distribution is very limited at the southern foot of Tianshan Mountain and the southern foot of Artai Mountain. The basic minerals of loess in Xinjiang include quartz and feldspar, heavy minerals (72–83%), verdite, and a high content of opaque minerals. The macroscopic structural characteristics of loess in Xinjiang include more coarse mineral particles, large voids, loose components, poor cementation, and large sand content. Xinjiang is located inland and has a dry climate with little rainfall, strong sunshine, and large temperature variation. In dry and less corrosive environments, dust does not easily form earth particles. However, wind transport causes loess material movement and accumulation. Loess in Xinjiang has been judged to be the product of eolian sand particle deposition [35, 36]. The yellow sand earth used here was from Urumqi, Xinjiang Autonomous region.
2.2. Sample Preparation
Taking loess as an example, the optimal loess moisture content is 18.2% and maximum dry density is 2.04 g/cm3 according to “Standard of Geotechnical Experiment Methods” [37] (GBT50123-1999), with the plastic limit at 15%, liquid limit at 26%, and plastic index at 11.3. Before experimentation, an earth was passed through a 5 mm sieve and mixed to optimal moisture content. Using a mold developed by this research group, 10 samples of 100 × 100 × 100 mm cubes were prepared from each earth by a jack forming material method. The preparation device and sample are shown in Figure 1.
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From the same source as above, the indices of black, red, and yellow sand earths were gathered (Table 1). Thirty samples of 100 × 100 × 100 mm cubes were produced, with 10 from each of the three earths (Figure 2).
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2.3. The Loading Device
Molded specimens were cured in a standard curing room and then tested for compressive strength. Here, a test piece was placed in the room at 25–30°C and 55–60% humidity to cure for 28 d. Using an MTS-500 pressure testing machine (Manta Testing Systems Inc., Mississauga, Canada), the testing value of the relationship between displacement and load was automatically collected. The P-Δ curve was drawn using MS Excel software and a leveling ruler used to check sample smoothness, which was polished if needed. A sample was centered and placed horizontally on the ball support, with the sample in close contact with the machine before test initiation. The loading rate was set to 1 mm/min and 30% of the peak load was taken as the final condition of a test after peak loading, which ensured normal machine operation. Testing steps were as follows:(1)Sample appearance was checked before testing until adequate. Sample surface as well as the upper and bottom press plates was cleaned.(2)The sample is placed on the spherical hinge support of the loading device and is adjusted to ensure that the loading direction is perpendicular to the bearing surface.(3)The test machine was started, when the upper press plate was close to the sample or steel plate, such that there was balanced contact with displacement at zero.(4)Loading was added uniformly and continuously at 1 mm/min.(5)Crack load, peak load, and peak displacement of the sample were recorded as the basis of data check.(6)The sample failure was observed and photos taken.
3. Results and Discussion
3.1. Test Process and Phenomena
3.1.1. Loess Cube Sample
In the initial loading stage, cracks did not immediately appear on soil cube surfaces with no visible cracks. With increased load, the specimen corners formed cracks, which developed along the specimen’s stress direction, but the specimen did not immediately break. The cracking load was 60–70% of the peak load, and as the load increased to peak load, many vertical cracks appeared in the test piece surfaces, along with swelling cracks on middle surfaces and portions of skin soil shed. With further increased load, cracks continued to develop until the specimen was completely destroyed, with rapidly decreased load, bearing capacity loss, and failure mode appearing as a typical “hourglass” (Figure 3).
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3.1.2. Black Earth Cube Sample
The entire processes of failure for the various samples were nearly the same as for the loess sample, but the cracking loads observed were different. At the initial loading stage, fine cracks appeared only at sample ends, when the cracking load was ∼26% of the peak load. Cracks first appeared at the four corners, and as loading increased, the cracks increased in width and extended downward in a “八” (truncated) shape. When loading peaked, cracks at corners were penetrated, with different degrees of vertical cracks penetrating the whole sample appearing toward middle, and the sample surface became expansive and shed material. With further loading increase, sample progressed to complete failure, with heavy skin surface shedding toward the center axis. The final failure pattern was similar to an hourglass shape (Figure 4).
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3.1.3. Red Earth Cube Sample
The whole failure process of red earth cube sample was nearly the same as loess but different in crack loading. No change took place in initial load, and only initial fine cracks were found at upper and bottom load ends; with a load increase to 35% of peak, cracks began. The whole test process is shown in Figure 5.
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3.1.4. Yellow Sand Earth Cube Sample
The whole failure process of yellow sand earth cube samples was nearly the same as for loess but different in crack loading. In the early loading stage, fine cracks appeared at both load ends, accompanied by shedding of particles. Because of this earth’s sand content, there was less stickiness among its particles, which made initial cracks appear at sample corners when loading was ∼30% of the peak. This was different from samples of yellow, red, and black earths, in which crack locations were close to sample edges in penetration. The whole test process is shown as Figure 6.
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3.2. Test Results and Analysis
3.2.1. Compressive Strength Results and Analysis
In testing, the peak load, peak displacement, and time to reach peak displacement were recorded and processed according to
The calculated compressive strengths of the four sample kind results are shown in Tables 2–5.
Earth cube samples from different regions exhibited different characteristics under the same conditions of preparation, maintenance, and testing (Tables 2–5). Among them, the loess compressive strength of 3.84 MPa was the highest because it was basically fine-grained powder clay, providing the sample with better cohesiveness, low plastic index, and high in material compactness in preparation. Loess earths are high in material density and specific gravity, such that these samples exhibited comparatively better strength. Black earth showed a compressive strength of 3.66 MPa, which was 95% of and slightly less than that of loess. The large organic content of black earth played a role in material modification, with a plastic index of 14 as well as higher compactness and density in sample preparation. However, black earth’s lower content of Al2O3 and CaO and gelatinous mass gave it less particle cohesiveness.
Because yellow sand earth in Xinjiang contains sand, which is comprised of hexahedral crystals that do not cohere well with earth particles. A certain content of sand can play a role of modification of loess strength of loess and although yellow sand earth density was high, at optimal moisture content, its sample preparation had high compactness and a plastic index at 11.9. However, yellow sand earth, due to its coarse mineral content, was low in cohesiveness and frictional resistance among its particles, which was why its compressive strength was 3.22 MPa, only 84% that of loess. Red earth had the lowest compressive strength at 1.65 MPa, only 43% of loess cube samples. This was because of a large base content, which forces materials to expand when mixed with water, such that these samples had high porosity after hardening, with a plastic index at 19, indicating, in sample preparation, high plasticity and compactness, less compactness, low material density, high free Fe and Al content, high natural instability, and low cohesiveness among particles. From these data, loess, yellow sand earth, and black earth appeared high in stability, with variation coefficients all <0.1 and reasonable standard deviations. However, red earth had a variation coefficient of 0.14, greater than 0.1 and a bad reliability indicator.
3.2.2. Load-Displacement Curve Results and Analysis
For the four kinds of earth, their strong similarity appeared in their load-displacement curves. The basic curve trend was divided into four stages. The first stage was an initial pressure rise observed as a concave function with the same reverse bend and inflection points. Due to load action, material compression among the different earths appeared in different degrees, as different earth quality and structure yielded red earth the maximum plastic index and higher compressive performance, while black earth was next and loess and yellow sand earth in Xinjiang lower and nearly the same. The second stage was a middle section of the curve with growing pressure, during which materials were assumed to be elastic. Among them, loess and yellow sand earth had the same curve slope and apparent increase in material stiffness with increased loading, leading to shorter displacements. Black earth material was the next and red earth material had the lowest stiffness increase. These analyses were believed to be influenced by the materials’ particle porosity, as loess and yellow sand earth had the lowest cohesive porosity and highest cohesive force amongst their particles and a better load-resistance toward material deformation. In black earth, due to its large organic content, in sample preparation, organics filled the material porosities when meeting water, but after gradual material hardening, these organics played a role in fiber modification of the earth material. This was why these samples exhibited better deformation resistance in loading and reached a high peak load. Red earth, due to its high compactness and porosity, exhibited a curve slope smaller than the former and thus retaining its strength. The third stage of testing involved material elastoplasticity. Loess material had a clear elastoplastic performance with a smaller curve curvature, with the black and yellow sand earth materials with a larger curvature and red earth with the largest curvature. This meant that red earth had a failure process that was slower than the other three materials. The fourth test stage was the loss of material load capacity with the curves of the four materials going downward at different degrees. The load-displacement curves of loess, yellow sand earth, and black earth were all characterized as convex functions, with decreasing slope, and material stiffness slowly degenerated and possessed residual stress. However, red earth exhibited characteristics of a concave function with a fast slope decrease and stiffness degeneration, which meant a severe lack of load capacity. Load deformation capacity, peak displacement, and curve dispersion of earth-based cube samples of different areas increased with their plastic indices (Figures 7–10).
4. Conclusions
In China, the compressive strengths of black earth in the northeast, red earth in the southwest, yellow sand earth in the northwest, and loess were not identical under the same conditions of sample preparation, size, maintenance, loading machine, and loading method. From low to high, the compressive strengths of loess, black earth, yellow sand earth, and red earth were 3.84, 3.66, 3.22, and 1.6 MPa, respectively. However, the value of black earth in agriculture is higher than its industrial value while loess and yellow sand earth in the northwest appear to be favored as an earth-based construction material. These results can serve as a reference for the selection and research of earth construction materials.
The failure patterns of compressive tests of soil-based materials from four regions were basically the same in that all initial cracks appeared at sample cube corners. Under loading, all samples showed expansive cracks at their middle surfaces, and the final failure patterns are all similar to an hourglass shape. Influenced by the nature of the materials, the crack load and crack displacement of samples in different areas were different, with the load-displacement curve significantly influenced by the materials’ plastic index.
The material content, formation condition, and nature of earth material varied by region. Through analysis of the compressive strength of earth materials of different areas, the trends of load-displacement curves were found to be almost same, presenting very similar stress testing performances.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was supported by the National Natural Science Foundation of China for the Study on the Standard Test Method of Materials and Masonry Based on Raw Soil (51478043). Their financial support is highly appreciated.