16
The properties and durability of adobe earth-based masonry blocks
Abstract
This chapter deals with adobe technique and its materials for making eco-efficient masonry block. In particular, after a brief historical digression, it presents how manufacturing adobe earth-based masonry blocks, their dimensions, which soil is suitable and which stabilization materials are currently used for enhancing their properties. Then the principal mechanical and hygro-thermal properties of adobe blocks, how to determine them and which parameters influence them are provided. In addition, the present methods of testing adobe blocks for durability are presented and practical advice on maintaining or improving them are suggested. Finally, environmental and economic benefits potentially associated with the use of adobe earth-based masonry blocks, including new ways to reuse bulk industrial waste as stabilizers, are provided.
Keywords
Adobe blocks; Durability; Eco-efficient constructions; Mechanical and hygro-thermal properties16.1. Introduction
Earth is an economical, environmentally friendly and abundantly available building material and is probably one of the oldest building materials known. Nowadays, earth constructions are an area of growing interest, both for rescuing heritage and for a rediscovered eco-sustainability material. However, because earth constructions are a forgotten technique, a lack of skilled people at all levels in this area can be found, from designers to masons, as well as problems of how to carry out compatible conservation works on earthen heritage. This chapter tries to fill this gap for a peculiar earthen construction technique, namely adobe, in which raw earth is mixed and molded to form blocks to make a bearing wall. The following sections, after a brief historical digression, will deal with how manufacturing adobe earth-based masonry blocks, their typical dimensions, which soil is suitable and which stabilization materials are currently used for enhancing their properties. Then, the principal mechanical and hygro-thermal properties of adobe blocks, how to determine them and which parameters influence them will be provided, as well as methods of testing adobe blocks for durability and practical advice on how to maintaining or improving it will be supplied. Finally, environmental and economic benefits potentially associated with the use of adobe earth-based masonry blocks, including new ways to reuse bulk industrial waste as stabilizers, will be addressed.
16.2. Adobe technique and materials
Earth has been extensively used for the construction of walls and buildings for 1000 years around the world (some recorded cases of the use of earth blocks date back to Mesopotamia around 10,000 BC (Quagliarini, Lenci, & Iorio, 2010)), particularly in all hot-dry, subtropical and moderate climates and in those countries where there is not much vegetation, so there is a lack of wood (Ren & Kagi, 1995). Approximately 30% of the world's present population still lives in earthen structures (Cofirman, Agnew, Auiston, & Doehne, 1990). For many centuries, hand-molded unburnt earthen blocks (adobes) have been used for load-bearing masonry structures. In particular, adobe earth construction was prolific in the ancient world, and archaeologists discovered surviving examples in many different places. Earth block buildings were found in Turkistan (dating back to about 8000–6000 BC), in Assiria (from about 4000 BC) and monumental earth structures can be observed still today in upper Egypt (from about 3200 BC). The historical city of Shibam in the south of Yemen is completely built by adobe (about fifteenth century). For centuries, Indians in New Mexico built their houses by earth blocks. During the seventeenth and eighteenth centuries, this technique was commonly used in England and Scandinavia, and European immigrants brought it to the United States, where a large number of earth houses were built between the eighteenth and nineteenth centuries (Minke, 2000). These earthen ancient constructions are precious cultural heritages and their artistic value is immeasurable (Heathcote, 1995). Adobe in fact is a very simple earth building technique and maybe this is the reason why most ancient constructions were made of adobe. The word “adobe” seems to come from the Arab attob, which means sun-dried brick (Pacheco-Torgal & Jalali, 2012). The production of adobe blocks generally consists of manually filling a wooden mold (Figures 16.1–16.3), which is first wet with water and scattered inside with sand so as to reduce water absorption by wood and facilitate the removal of the earth block, with wet earth (this molding reduces the quantity of voids between grains and thus gives a form to the mixture thanks to cohesion) and allowing to dry in natural environmental conditions, under the sun or, preferably, the shade. Usually, adobe walls consist of rectangular blocks arranged next to one another and put with alternate joints. The adobe blocks are generally stacked nearly dry, with a thin layer of clay or mortar made by their same composition (Figure 16.4).
Although adobes are most used for lightly loaded single and two-storey residential buildings, adobes have also been used to build up to 10-storey high buildings (Houben & Guillaud, 1994). Nowadays, the material is mostly used for restoration purposes. In addition, adobe is currently introduced in contemporary sustainable architecture (Illampas, Ioannou, & Charmpis, 2014).
The soil used for making adobe generally refers to an in situ sandy loam subsoil. Topsoil is in fact unsuitable mainly due to the significant amount of organic matter present that biodegrades, absorbs water and is highly compressible. Recent studies (Kouakou & Morel, 2009) showed that the suitable soil should contain less than 20% clay minerals. In particular, (Quagliarini & Lenci, 2010) have underlined that a preferable clay content into the bearing adobe block elements should be between 12% and 16% (by weight). Cohesion, indeed, is the most important property that earth should have to be used for construction purposes (Houben & Guillaud, 1994), so the presence of clay is essential because it acts as a natural binder.
Romans usually used in situ raw earth as a building material (Adam, 1984). Vitruvius himself (Barbaro, 1997), in fact, states that earth can be used as a building material if it is cretosa, that is guaranteed by a great presence of clay. Moreover, he suggested producing adobe blocks during spring or autumn because the sun is not so strong and thus there is not a rapid drying process that can produce cracks. Only if it is really necessary to produce them in summer or winter is it good to adopt some precautions: In the first case blocks have to be covered with wet straw to avoid excessive drying whereas in the second case they have to be covered with dry sand in order to allow good seasoning of earth blocks also in humid and cold weather. The standard dimensions of Roman adobe blocks as Vitruvius (Barbaro, 1997) states are 1 foot per 1.5 feet per one palm (0.296 × 0.444 × 0.074 m3), but different, even if similar, dimensions of the blocks can be found in some Roman archaeological sites (i.e. 0.31 × 0.46 × 0.13 m3 (Quagliarini et al., 2010)). In general, their dimension is highly variable, ranging in length from 0.25 to 0.6 m (Houben & Guillaud, 1994). The method of production, in general nonindustrial, enables the manufacturer to vary block size and shape to suit requirements by using mold inserts (Figure 16.2).
Anyway, ancient masons well knew that it was hardly possible to use only earth to produce large adobe bearing blocks because of their excessive shrinkage and consequently cracking and because of the limited workability of the mixture. In this way, sand or coarse sand was added into the mixture (Quagliarini et al., 2010) to “degrease” clay and to allow making it into a mixture. Sand, in fact, reduces the clay fraction relative to other components and allows a lower shrinkage due to the smaller presence of the component (clay) prone to this phenomenon. The components of adobe earth material, in fact, can be considered as analogous to those of concrete: The inert aggregate fraction is represented by granular soils (sand and gravel) and the binder fraction is represented by cohesive soils (silt and clay) (Hall & Djerbib, 2004).
Moreover, ancient masons often put vegetable fibers into the mixture such as straw or dry grass (Houben & Guillaud, 1994). This can reduce hygrometric shrinkage because of both their traction strength and above all their capability of releasing water slowly (Quagliarini & Lenci, 2010). However, this position is not unanimous because vegetable fibers could rot, leading to the appearance of fungi (Pacheco-Torgal & Jalali, 2012). However, fibers, when present, were added randomly but possibly in a homogeneous way to the wet soil and mixed until getting a complete homogeneous composite. It is known that, in practice, straw fiber addition is suggested to be about 5–10 kg/m3; however, as clay content increases, this proportion can be increased, even if some authors recommend that the fiber content should be restricted about 0.5% by weight (Yetgin, Çavdar, & Çavdar, 2006).
As a matter of fact, ancient brick-makers have not had a chance to do scientific experimental investigation on the balance of ingredients and on the optimization of this production, and it is likely that no weight balance was present on-site many years ago; thus, the components were mixed using different volume quantities. In this way, different quantities in volume of sand (or coarse sand) and fibers (i.e. straw) were usually added to the in situ soil. Then a variable volume of water was added to reach a sufficient workability of the mixture: When the sand and fiber content was increased, water content also increased a little. It could be said that addition of sand generally improved workability whereas reducing the sand and vegetable fibers clay fraction became dominant in reducing workability.
Nowadays, a promising way to improve the characteristics of the soil so that the resulting adobe units can bear greater loading and perform better when exposed to weathering is stabilization. Soil stabilization means changing the soil characteristics in order to improve its mechanical or physical behavior. The stabilization processes aim at the reduction of the soil plasticity and improvement of its workability, its compressive strength and its resistance to erosion.
In general, there are three types of stabilization: The first is the mechanical stabilization, the second is the physical stabilization and the third is the chemical stabilization (Medjo Eko, Dieudonné Offa, Yatchoupou Ngatcha, & Seba Minsili, 2012). Mechanical stabilization relates to the densification of soil through compaction. Physical stabilization modifies the soil properties through processes such as soil texture improvement, firing, freezing or electro-osmosis. Chemical stabilization modifies soil properties through mixing with chemical additives such as cement or lime (i.e. combining with 4–10% cement stabilization significantly improves compressive strength and water resistance in comparison with traditional adobe blocks (Morel, Pkla, & Walker, 2007)). The most effective method to modify the adobe is its stabilization with additives. Molasses, bitumen, cow-dung and saw dust could be used as stabilizers (Pacheco-Torgal & Jalali, 2012). In addition, plasticizers used in the concrete industry could be used, such as lignins or naphthalene sulfonates, which are readily available and cheap. Resin or pozzolan can also be used. Natural fibers such as straw, coconut and sisal can be used for additive. Artificial fibers can also be used in adobes, such as plastic or polystyrene fabrics. Bitumen emulsion can be used to prevent the water absorption of fibers (Yetgin et al., 2006).
It is important to underline that if we want to manufacture eco-efficient adobe blocks, the stabilization through, for example cement or lime, could be somewhat inadequate. In fact, this stabilization does not enable the recycling of the material, and this is not good from a sustainable development standpoint. Furthermore, the use of cement or lime (for example) could also meet an economic problem and the availability of these materials in the market in some countries and regions (Bui, Morel, Reddy, & Ghayad, 2009). Anyway, in some specific cases, it could be the only solution for some applications (i.e. low-cost buildings in India subject to the monsoon to avoid having to rebuild every year) (Morel, Aubert, Millogo, Hamard, & Fabbri, 2013).
16.3. Adobe blocks properties
This section provides the principal mechanical and hygro-thermal properties of adobe blocks, how to determine them and which parameters influence them.
16.3.1. Mechanical properties
The adobe does not reach mechanical strength as high as concrete or fired brick. However, some studies observed that the adobe is strong enough, ductile and resistant against earthquakes (Yetgin et al., 2006). In the literature, many studies elaborate on the mechanical properties of adobe bricks (see for example Binici, Aksogan, & Shah, 2005; Bouhicha, Aouissi, & Kenai, 2005; Clementi, Lenci, & Sadowski, 2008; Kumar, Walia, & Mohan, 2006; Lenci, Clementi, & Sadowski, 2011, 2012; Piattoni, Quagliarini, & Lenci, 2011; Quagliarini & Lenci, 2010; Quagliarini et al., 2010; Yetgin et al., 2006). As for stone or brick masonry, the capacity of adobe masonry in compression is strongly related to the compressive strength of its blocks as well as mortar strength, bonding pattern and many other factors. Thus, compressive strength has become a basic and universally accepted unit of measurement to specify the quality of masonry units. The relative ease of undertaking laboratory compressive strength testing has also contributed to its universality as an expression of material quality (Morel et al., 2007).
Considerable variations in the composition of the used soil make the measurement of compressive strength of adobe blocks an important quality control measure for manufacturers and builders. Even if standardized testing methods for evaluating the unconfined compressive strength of cohesive soils are present in literature, they examine earthen materials from the scope of geomechanics rather than in the context of common building applications (Illampas et al., 2014). Thus, quality control strength testing of adobe blocks has often followed procedures developed for fired clay and concrete block units (Morel et al., 2007). However, the suitability of these procedures has largely not been checked by scientific study. The compressive strength of adobe blocks can be many times lower than similar fired bricks. In addition, compressive strength is improved by compaction effort (density) and cement content but reduced by increasing moisture content and clay content. National and international guideline documents have also been developed for earth block test procedures (14.7.4NMAC, 2009; New Zealand Standard 4298, 1998; NTE E0.80, 2000; Standards Australia Handbook 194, 2002). These documents include specific references to testing methodologies and prescribe permissible strength values. Nevertheless, there is little general consensus on test procedure, and some questions, such as how the dimensional effects (i.e. aspect ratio) and platen restraint have to be taken into account, still remain without a unique answer.
Considerable differences also occur in the treatment of samples prior to testing. Specimens may be stored under stable thermo-hygrometric conditions for a certain period of time, oven-dried at a certain temperature up to constant weight or placed outdoors under direct sunlight or in the shade so as to replicate traditional production methods (Illampas et al., 2014).
In general, geometrical effects on single adobe block compressive strength are treated in one of two ways. In many standard test procedures, platen restraint effects are usually neglected. Average or characteristic compressive strength is simply expressed following statistical manipulation of individual test results (Morel et al., 2007), in which these results are often obtained by dividing the net force by the area of the specimen (i.e. by considering the average or nominal stress) and by measuring the vertical displacements only at the center of the block. In this way, the possible nonparallelism between the two platens is missed, as well as the induced combined compressive and bending stresses.
In an alternative approach, used in both Australia (Middleton, 1992; Standards Australia Handbook 194, 2002) and New Zealand (New Zealand Standard 4298, 1998), platen restraint effects are catered for by factoring test values with an aspect correction factor. Correction factors used are typically the same as derived for fired clay units (Figures 16.5–16.6), although other work has suggested alternatives (Heathcote & Jankulovski, 1992). Hence, direct comparison of the test results obtained by different researchers is not always possible.
The methodology adopted in many national standards and codes of practice is similar to that used for fired clay and concrete blocks (Morel et al., 2007). Single units are capped and tested directly between platens. Block surfaces are usually sufficiently flat and parallel to achieve a uniform distribution of the compressive load, and capping with plywood, softwood, mortar, cardboard or a layer of sand (Illampas et al., 2014; Piattoni et al., 2011) can be applied on the specimens' surfaces. Alternatively, smoothening of irregularities by abrasion may be carried out (Illampas et al., 2014). Adobe samples obtained from existing walls, cut from full-size bricks or formed through mold casting have also been examined in the literature (Illampas et al., 2014).
The test units include full size bricks; prisms with length-to-width ratios from 2 to 0.5 and cubes with edges of 40, 50, 100 and 150 mm (Illampas et al., 2014; Piattoni et al., 2011; Quagliarini & Lenci, 2010). The use of cylinders with 1:1 and 2:1 aspect ratios and diameters ranging from 50 to 150 mm is also quite common (Illampas et al., 2014). A scheme is reported in Table 16.1.
Blocks are generally tested along the direction in which they have been pressed, which is also the direction in which they are generally laid. Test samples generally comprise between five and ten blocks. Compressive loading is imposed following either a load-controlled (Quagliarini & Lenci, 2010) or a displacement-controlled (Lenci, Piattoni, Clementi, & Sadowski, 2011) procedure. The aim of the latter is to provide information regarding the post-yield behavior of the material. In load-controlled tests, typical applied compression rates are set to between 0.03 and 0.1 MPa/s (Quagliarini & Lenci, 2010; Silveira, Varum, & Costa, 2013). In displacement-controlled tests, constant vertical displacement is imposed at a rate between 0.5 and 7 mm/min (Bouhicha et al., 2005; Ghavami, Filho, & Barbosa, 1999; Kouakou & Morel, 2009; Silveira et al., 2012, 2013; Villamizar, Spinosi Araque, Ríos Reyes, & Sandoval Silva, 2012).
The results of compression tests reported in the literature vary from 0.6 to 8.3 MPa, the most common values being between 0.8 and 3.5 MPa. The lowest strength limits set in national directive documents range from 1.2 to 2.1 MPa (Illampas et al., 2014). Hence, because a single- or a two-storey dwelling usually has a downward thrust from about 0.1 to 0.2 MPa, earthen adobe walls could generally withstand this with a reasonable safety factor.
Apart from the mere determination of the failure stress, constitutive models describing the compressive stress–strain response of adobes have also been developed (Silveira et al., 2013). Recent results have shown how the granular content controls the Young's modulus of the adobe blocks whereas natural fibers control the “plastic” behavior and influence the way they break (Quagliarini & Lenci, 2010).
In addition, stabilization created by stiffening the material will induce a commonly observed behavior in brittle materials as concrete or stone. In contrast, the unstabilized material is likely to be closer to the conventional behavior of soils. In this case, the soil behavior elasto-plastic models are a priori better suited to earthen materials (Morel et al., 2013).
16.3.2. Hygro-thermal properties
Adobe blocks made by unfired clay and straw in different percentages were commonly used in many regions of the world, not only for their mechanical properties, as shown above, but also for the comfort level of buildings made with earth. Because of its high thermal capacity, earth could store heat absorbed during the day, keeping the interior of a dwelling constructed from earth relatively cool (Parra-Saldivar & Batty, 2006). When the outside temperature drops at night, the walls would emit the heat stored during the day inside of the building. The specific heat capacity of the adobe material is considered to be a key factor in its ability to moderate temperature peaks in buildings because adobe materials have relatively high thermal conductivities. In fact, Maniatidis & Walker, (2003) report conductivities of rammed earth of about 0.58 W/m K according to Delgado & Guerrero (2006) and other authors. Some values collected in literature (Acosta, Diaz, Zarazua, & Garcia, 2010) in comparison with other materials are reported in Table 16.2.
The little amount of straw added to the soil in order to reduce shrinkage of adobe blocks does not affect in a strong way the thermal conductivity. Figure 16.7 reports thermal conductivity of adobe block with different straw percentages.
Finally, the ability of adobe blocks to conduct heat also depends on the moisture content (Hall & Djerbib, 2004, 2006a,b) because of the strong relationship between water content and heat conduction (Rees, Zhou, & Thomas, 2001). In moderate and hot climates, the moisture content of adobe blocks could be an advantage because of the phase transition of the water. When the material starts to dry, water evaporates; therefore, heat loss in the form of latent heat occurs, which in turn causes the external surface temperature to decrease. Thus, considering the overall rate of heat transfer, the U-value for a 300 mm-thick earth wall can reach 1.9–2.0 W/m2 K, even if Delgado & Guerrero (2006) report lower U-values (1.5 W/m2 K).
With these data, it is clear that earth could not be considered a good insulation material, but even a poor insulating material can insulate effectively if it is large enough (Acosta et al., 2010) and if it can “absorb” heat (Di Perna, Stazi, Casalena, & D'Orazio, 2011). In fact, several studies show that an adobe house can maintain natural thermal comfort throughout the whole year (Eben Saleh, 1990). The adobe house buildings have a natural air conditioning effect because the temperature of rooms tends to be cool during daytime and warm during nighttime (Chel & Tiwari, 2009; Coffman, Duffin, & Knowles, 1980).
The comparison of the thermal behavior of an adobe house with a modern concrete house in Yemen showed the benefit of mud as construction material for energy saving in houses (Algifri, Bin Gadhi, & Nijaguna, 1992). Bahadori & Haghighat (1985) and Chel & Tiwari (2009) predicted the room air temperature and energy saving potential of adobe houses with a vault roof structure. Martín, Mazarrón, & Cañas, (2010) analyzed the comfort conditions inside earth buildings in Spain. Finally, Parra-Saldivar & Batty, (2006) analyzed the effects of window size, the position of and the presence or absence of internal walls and type of roof constructions on the thermal performance of adobe constructions. They show that the adobe walls grant decrements of the indoor temperature with respect to external temperature; however, the percentage decrement is strongly related to the window dimension.
The thermal performance of adobe blocks is measured in a number of different ways. Millogo, Morel, Aubert, & Ghavami (2014) report conductivity values of pressed adobe blocks measured in a transient state with a hot wire probe, which is also a heating resistor. Govaer (1987) uses a similar procedure with cylindrical specimens. However, more accurate measurement in a steady state can be realized with a hot-guarded plate apparatus (Gonçalves & Bergmann, 2007) according to ASTM C 177 or ASTM E1225-13. Another method is the calibrated hot box method according to ASTM C1363-11. This method provides laboratory measurement of heat transfer through a specimen under controlled air temperature, air velocity and radiation conditions established in a metering chamber on one side and in a climatic chamber on the other side.
Another advantage of adobe blocks is related to their ability in moderating changes of indoor relative humidity. An unfired clay block is, in fact, much superior to burnt brick as a humidity buffer, and its hygroscopic behaviour can be more effective in reducing the indoor air relative humidity than the use of ventilation (Padfield, 1998). Some investigations show that earth blocks can absorb 10 times more weight moisture than ceramic bricks and that an earth construction is capable of keeping the relative humidity of indoor air between 40% and 60%, which is the optimum range for human health (Pacheco-Torgal & Jalali, 2012).
16.4. Durability of adobe blocks
This section outlines the present methods of testing adobe blocks for durability and suggests practical advice on how to maintain or improve it. The durability of adobe structures is testified by the fact that some of these buildings last for hundreds of years. It usually depends on appropriate maintenance and repairs that are compatible with the original construction. In general, adobe structures may be protected by correctly designing the roof or protecting them (i.e. by plaster). Anyway, water is the main potential drawback for adobe blocks.
Adobe walls can, in fact, erode under rain impact and can collapse when exposed to continuous rain for several hours. In addition, water absorption may cause the swelling of clay minerals whereas evaporation of water from the clay gives rise to shrinkage and cracking. Therefore, adobe walls that are not protected suffer greatly from durability problems due to water erosion, penetration and evaporation.
In order to improve the durability of exposed adobe blocks, cement has been used to stabilize them by mixing up to 15% cement with soil, and bitumen emulsion has also been used to stabilize earth blocks and to reduce the water absorption of them; up to 20% bitumen emulsion has been mixed with the soil (Ren & Kagi, 1995). However, the use of either cement or bitumen emulsion can be, in general, impractical because of the mixing problems and the cost of the large quantities of these materials needed to increase the durability of the adobe. Materials such as soluble silicate, ethyl silicate, silanes or siloxanes, isocyanates and various polymers have been also used as consolidation or waterproofing agents to treat the earth-block surface through impregnation. However, these treatments may be applied with caution, because of their costs, to the use of large quantities of undesirable organic solvents (an environmental problem) and to the often unsatisfactory waterproof results (Ren & Kagi, 1995).
Impregnating the adobe block with soluble sodium silicate followed by water-based organic silicone emulsion has been tested for its consolidating and waterproofing. The durability of the treated blocks was found to be significantly increased compared with that of the untreated ones; in addition, very low concentrations of both sodium silicate and silicone emulsion were used, and there were no organic solvents involved in the treatment. Therefore, the cost of the treatment was low and could be acceptable for increasing the durability of structures.
Several types of durability tests (Cid-Falceto, Mazarrón, & Cañas, 2012) are proposed for earthen materials, obtaining dispersed results because of the variability of their technical test specifications (Hall, 2007; Mbumbia, Mertens de Wilmars, & Tirlocq, 2000; Ogunye & Boussabaine, 2002; Ola & Mbata, 1990; Reddy & Jagadish, 1987).
However, recent studies have shown that these tests could be too severe and unrealistic because unstabilized earth blocks could often not pass these tests whereas existing traditional unstabilized earth walls have undergone more than 100 years of weathering (Bui et al., 2009). That is why the development of new suitable durability tests accounting for, for example, the different climatic conditions is a main issue and further studies need to be carried out. This could be also important to decide when the stabilization is needed.
The most common durability tests currently used are the spray erosion test and the drip erosion test. These are mentioned for adobe blocks in several national standards (Cid-Falceto et al., 2012).
These tests have been also proposed in numerous documents and are usually considered empirical: The spray erosion test is usually referred to as a direct replica of the erosion originated by rain water, studying its application on real conditions, whereas the drip erosion test is a good, cheap test for testing blocks in areas of little rain (Cid-Falceto et al., 2012).
All of the tests are based on the same ground of subjecting a test tube to a pressure spray for a certain amount of time or until specimen is penetrated (spray erosion), or to a constant waterfall for a certain amount of time (drip erosion), in order to evaluate afterward the damage caused in both cases.
In the spray erosion test, parameters such as tested side, number of samples to be tested, exposed area, spray time, observations, application distance (nozzle-specimen) and pressure are defined. The height of the waterfall or the slope of the sample to be tested are instead provided for the drip erosion test.
16.5. Future trends for eco-efficient constructions
This section deals with the environmental and economic benefits potentially associated with the use of adobe earth-based masonry blocks, including new ways to reuse bulk industrial waste as stabilizers. Nowadays, the use of nonindustrial materials has been rediscovered because of environmental problems caused by the excessive use of the industrial materials. The concept of nonindustrial building materials means local materials manufactured using a simple, quick process with low embodied energy using raw materials from the site or nearby (Kouakou & Morel, 2009). The use of soil as a raw material can be considered a way to perform it. The soil should be taken from the construction site or nearby in order to limit transportation and must contain clay particles to reduce or avoid the use of industrial binders such as lime and cement (see previous sections). Thus, similar to dry stonemasonry, soil can represent one of the few construction materials that can be easily reused and does not generate waste (Kouakou & Morel, 2009). At the end of a building's life, an unstabilized adobe block can be easily reused by grinding or wetting, or it can be returned to the ground with no environmental hazard involved; in fact, it is able to be returned to its initial state (as a soil) by simply wetting.
Even when the soil is stabilized with cement or lime, it could potentially be reused for this type of construction (Pacheco-Torgal & Jalali, 2012), even if this is not a unanimous standpoint in the literature (Morel et al., 2013). By using adobe blocks, an economic impact may be also pursued: The slight cost of raw material, local skills and employment can be developed because of their simple manufacturing, and auto-construction can be promoted.
Another advantage of using adobe blocks could be associated with no indoor air volatile organic compounds; thus, the dwellers should have better indoor air quality (Pacheco-Torgal & Jalali, 2012). Although data on waste-material-reinforced unfired blocks are still limited, new ways to reuse bulk industrial waste as stabilizers for adobe blocks can be followed too. Replacing natural soils, aggregates and typically employed stabilizers with industrial wastes is highly desirable so as to offer a substantial environmental contribution to society. In some cases, a by-product could be inferior to traditional earthen materials, but, because of its lower cost, this could be an attractive alternative if adequate performance can be obtained.
The worldwide development of the agroindustry annually produces large volumes of agricultural wastes, and their disposal causes major challenges and serious economic and environmental problems (Villamizar et al., 2012). During recent years, alternative uses of these wastes have been carried out to reduce their amount, and some first results have also been provided to develop their use in the manufacture of earth blocks with initial good results. In fact, several types of organic fibres can be produced from a relevant number of agroindustrial wastes. Thus, they could be included within a mixture so as to reinforce adobe blocks, although they do not provide adequate durability alone (Villamizar et al., 2012).
A huge quantity of straw is produced every summer, and farmers often burn this material, giving rise to ecological problems: instead of burning, this material could be used in adobe block production. However, locally available materials such as barley straw, bamboo, sisal, coconut and Hibiscus cannabinus fibers were also tested in earth construction (Bouhicha et al., 2005; Ghavami et al., 1999; Millogo et al. 2014). The rice hull ash was also studied as an earthen stabilizer, and the incorporation of craft paper fibers from discarded cement bags was studied for the production of earthen blocks (Lima, Varum, Sales, & Neto, 2012). In addition, other studies suggested that synthetic fibres recovered from various waste streams, such as plastic fibers and polystyrene fabric or salvaged steel fibers, rather than deteriorating the environment could be used for the reinforcement of earthen blocks (Medjo Eko et al., 2012).
16.6. Sources of further information and advice
Suggested general standards or handbooks on adobe block are the following:
• ASTM C177-13. Standard Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus, ASTM International, West Conshohocken, PA, 2013.
• ASTM C1363-11. Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus, ASTM International, West Conshohocken, PA, 2011.
• ASTM E1225-13. Standard Test Method for Thermal Conductivity of Solids Using the Guarded-Comparative-Longitudinal Heat Flow Technique, ASTM International, West Conshohocken, PA, 2013.
• Earth Building Association of Australia (EBAA). Earth Building Book. Draft for Comment. Draft Code 05/01. Wangaratta, Australia: Earth Building Association of Australia, 2001.
• Houben, H., Guillaud, H. (1994) Earth construction. A comprehensive guide. CRATerre-EAG, London, ITDG publishing
• Middleton G.F., 1987 (revised by Schneider L.M.) Fourth Edition. Bulletin 5. Earth Wall Construction. North Ryde, Australia: CSIRO Division of Building, Construction and Engineering, 1992.
• Minke G. (2000) Earth construction handbook, WIT press, Southampton, Boston.
• MOPT. Bases Para el Diseno y Construccion con Tapial. Madrid, Spain: Centro de Publicaciones, Secretaría General Tecnica, Ministerio de Obras Publicas y Transportes, 1992.
• NMAC New Mexico Administrative Code 14.7.4. 2009 New Mexico Earthen Building Materials Code. Santa Fé, NM: Construction Industries Division (CID) of the Regulation and Licensing Department, 2009.
• SAZ. Standards Association Zimbabwe Standard (SAZS) 724:2001: Standard Code of Practice for Rammed Earth Structures. Harare: Standards Association of Zimbabwe, 2001.
• SENCICO. Norma Técnica Edificación NTE E 0.80 Adobe. Reglamento Nacional de Construcciones. Lima: SENCICO, 2000.
• SNZ. New Zealand Standard 4297:1998. Engineering design of earth buildings. Wellington: Standards New Zealand, 1998.
• SNZ. New Zealand Standard 4298:1998. Materials and workmanship for earth buildings. Wellington: Standards New Zealand, 1998.
• SNZ. New Zealand Standard 4299:1998. Earth buildings not requiring specific design. Wellington: Standards New Zealand, 1999.
• Standards Australia Handbook 194. The Australian earth building handbook. Standards Australia, Sydney, Australia; 2002.
• Standards Australia and Walker P. HB 195: The Australian earth building handbook. Sydney (Australia): Standards Australia, 2002.
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