18
The durability of compressed earth-based masonry blocks
Abstract
This chapter focuses on the durability of compressed earth masonry – bricks and blocks. It aims to provide a detailed analysis of the factors at play in the determination of durability. Because the durability aspects of compressed earth masonry and those of concrete are broadly common, and because there is by far a larger body of research work in concrete, the chapter borrows heavily, but carefully, from the factors relevant to the durability of concrete. The durability of a brick or brick system depends on a wide range of factors, including the brick materials used, the adopted brick technology, the prevailing environment, appropriateness of use, and care during service, among other possible factors. The detailed analysis of these factors, together with a view of future trends and developments, is the aim of this chapter. The chapter is aimed to capture interest with researchers, trainers, building and construction materials manufacturers and government and local authority policy makers, all who have interests in sustainable development, low-cost housing, appropriate technology, and care for environment and soil-based technology for community infrastructure.
Keywords
Bricks and blocks; Compressed earth; Low-cost housing; Masonry; Sustainable soil stabilization18.1. Introduction
Building bricks and blocks are common wall material options for housing, although there may be other applications in general construction, such a paving, drains and canal lining, among other uses. The history of bricks is long, and brick technologies also have a long history, including the sustainable use of unfired bricks as well as composite material usage – by, for example, incorporating straw to provide extra strength (particularly flexural strength) and to minimize cracking and disintegration during service (Aubert & Gasc-Barbier, 2012; Böke & Akkut, 2003; Cid-Falceto, Mazarron, & Caňas, 2012; Guinea, Hussein, Elices, & Planas, 2000; Heathcote, 1995; Moropoulou, Bakolas, & Anagnostopulou, 2005; Walker, 2004). The selection of materials used for brick making has tended to depend on both location and environment. Most communities have successfully managed to utilize clays of varying plasticities to provide the basic cementation capability. Egyptians and Romans managed to develop lime-based cementitious systems at an early stage, thus obviating the over-reliance on clay plasticity to provide basic cementation (Aubert & Gasc-Barbier, 2012; Moropoulou et al., 2005). The Egyptians, for example, had dual systems, utilizing straw brick construction, and later developed lime-based technologies by calcining gypsum as the basic raw material for sourcing quicklime. The quicklime would then be used in various ways, the most notable being as binder in clay–lime pozzolanic cementitious systems. The Romans used an alternative approach for sourcing quicklime, by calcining limestone. They then developed further technology by using accelerated pozzolanicity provided by volcanic ash rather than relying on the much slower cementation process using raw clay soil. Volcanic activity inputs energy into rock and earth materials, disordering the microstructure to provide a material structure that is readily broken chemically, resulting in higher reactivity. Other innovations, such as appropriate use of varied material densities to produce durable and complex structural forms, further refined and enhanced the sustainability of the Roman infrastructure. The structures made using these technologies and innovations are still evidenced today by the robust durability and complexity demonstrated by the survival to date of the Coliseum and Pantheon, among other Roman structures and ruins (Aubert & Gasc-Barbier, 2012; Moropoulou et al., 2005).
This chapter focuses on the durability of earth bricks and blocks. It endeavors to provide a detailed analysis of the factors at play in the determination of durability of earth bricks and blocks. It also aims at raising awareness of the durability tests for compressed earth units that are more commonly encountered at present. In addition, is postulates the likely future scenario in terms of materials research and development (R & D) on compressed earth masonry, and the complex interplay of this R & D with a wide range of the most key stakeholders, from government, materials researchers and users to those involved in the entire housing and construction sectors.
Because the durability aspects of masonry and of concrete are broadly similar, and because there is by far a larger body of research work in concrete, this chapter borrows heavily, but carefully, from the factors relevant to the durability of concrete. The durability of a brick or brick system will most probably depend on a wide range of factors, including the brick materials used, the adopted brick technology, the prevailing environment, appropriateness of use, and care during service, among other possible factors. The detailed analysis of these factors, together with a view of future trends, is the main aim of this chapter.
18.2. Factors influencing durability of earth-based masonry
The durability of a brick or brick system will most probably depend on a holistic interplay of many factors (Lanas & Alvarez, 2003). These factors are likely to fall under one or more of the following categories:
1. Materials used
2. Technology and resultant material engineering properties
3. Prevailing environment
4. Appropriateness and care.
18.2.1. Materials used
Materials used and technologies adopted with these materials are perhaps the two most important factors that will determine durability during service. It is well known that cement-based bricks are typically more durable than clay bricks, especially if the latter are made in unfired processes. However, in both fired and unfired brick categories, the material ingredients are critical to durability of the bricks formulated from them. Both fired and unfired technologies for soil-based bricks are common, and both processes require some degree of plasticity, especially for non-cement-based soil bricks. This leaves out some categories of soils, such as low-plasticity silty and sandy soils, which mainly require cement for the provision of cementation.
Soil is virtually inexhaustible as a raw material. The cost and environmental benefits of its utilization in construction far outweigh those of stone or concrete. Unfortunately, in their natural state, soils generally have relatively low load-bearing capacities and often require stabilization. The strengthening can take the form of physical–mechanical approaches such as use of aggregates and/or other granular particulate material (including waste, such as aggregates sourced from waste tires among other industrial wastes) to provide particle–particle resistance, and/or use of natural or synthetic fibers, and/or use of chemical approaches. Traditionally, chemical stabilization and strengthening has been by far one of the most common approaches to strengthening compacted earth, and has been achieved using expensive and environmentally undesirable materials (mainly lime and/or Portland cement (PC)), whose manufacture deplete energy resources and exacerbate pollution.
The effects of the nature of materials used in the formulation of the earth bricks and blocks on durability are very similar to those encountered in ordinary concrete, where cement/binder, aggregates and water are the key ingredients and hence the key determinants of durability. Thus, in compressed earth, the type of soil, the binder used (cement, lime, etc.) and the water used are the key material-related aspects that need detailed consideration.
Soil aggregates: “Soil” commonly refers to any loose material at the earth's crust, regardless of particle-size distribution, composition, or organic content. Soil has also been defined as “natural aggregate of mineral grains that can be separated by such gentle means as agitation in water” (Grim, 1968). The term “aggregate” is not commonly used with reference to soil – although, strictly speaking, soil particles are indeed aggregates, albeit weak ones. In concrete, the use of aggregates has the dual purpose of providing bulk volume as well as physical–mechanical strength, rather than cementitious (chemical) activity. In soil-based materials, the soil “aggregates” are expected to play the same role as normal aggregates in concrete: bulk volume and particle–particle interaction leading to strength. Therefore, irrespective of whether bricks or blocks are soil- or cement-based, weak aggregates are more likely to fail to provide high strength and hence are not able to impart durability under high stresses. Especially for concrete, reactive aggregates are undesirable, because any reactivity is likely to be gradual and slow, slowly and gradually affecting the robustness of any matrix form that may have already developed strength initially (Debieb, Courard, Kenai, & Degeimbre, 2010). The internal stresses due to these late reactions (for example, alkali–silica reactions) may weaken the compacted material.
Surface texture of aggregates affects both water absorption and adhesion to binder material, while excessive particle size would result in higher binder content requirements. The use of low amounts of binder is therefore very critical in cases of extreme excesses in either fine or oversize aggregates. For this reason, the achievement of a well-graded aggregate matrix is a common objective for optimal performance in cement-based bricks. Flaky and elongated aggregate particles are also susceptible to breakage upon stress and thus impact adversely on durability. From the foregoing, it is clear that for durability in aggregate-containing bricks and blocks, all the aggregate properties mentioned here need a very careful assessment before use. Other aggregate-related factors that may be of concern as far as durability is concerned include excessive water absorption and particle shape. Water absorption in aggregates is usually dependent on the aggregate surface texture. Wet aggregates are known to have lower crushing values, thus decreasing strength. On the other hand, elongated and/or flaky aggregates fracture easily, leading to reduced durability (Oti, Kinuthia, Snelson, & Bai, 2010f; Oti, Kinuthia, & Bai, 2010e). The use of sand in brick making should be seen in the same light as in the use of aggregates as discussed above. To most practitioners, designers and standards, sand refers to the aggregate fraction where the particle size falls between 2 and 5 mm.
Cement/binder: Because cement is ordinarily made under very standard and controlled processes and conditions, it is normal and logical to anticipate durability when the right amounts of cement are used. However, because cement is the most expensive material ingredient in bricks and blocks, it is perhaps the most sensitive ingredient to any abuse. However, in most cases it is usually the amount, rather than the quality, of the cement that is more likely to affect durability. The standards have progressively changed from stipulating restrictive properties on building materials, and the emphasis is slowly shifting towards the need for product manufacturers to state the possible uses, appropriate or ideal situation of applicability, and the consequences of nonadherence, during service, to conditions of use recommended by the manufacturer. This shift has eased pressure that has traditionally been caused by a blanket expectation of product performance. It has also encouraged the uptake of a wider range of innovative materials and products with a wider range of performance characteristics. Thus, as long as the manufacturer has provided insight into the use of the final product, cement bricks and blocks with low cement content can now be used under limited conditions and applications as intended by the manufacturer. The challenge on the user is now to ensure that the material or product is applied as recommended, in order to expect or anticipate the appropriate durability, commensurate with the cement quantity used.
Water: Of the ingredients in bricks and concrete, water is perhaps the most readily available for many regions, in the quantity and quality necessary to ensure acceptable durability. Unless marine water is used, the quality of most other water sources is usually adequate for most cementitious applications, obviating costly water purification processes before use. It is the quantity of water used that is more likely to cause concern. There is a conflict between the amount of water needed for complete or optimal hydraulicity and that needed for material workability. The former requires far less water and thus, for profitability, the manufacturer has to try vibro-compaction or other techniques, depending on plant available, so as to result in a workable material that is as near as possible to the water level needed for optimal hydraulic potential. Excessive water absorption results in excess water in the cementitious system, and it is well known that strength in cementitious systems has an inverse relationship with the water content (or water/cement (or water/binder) ratio).
Other material factors: Most of the material effects observed in concrete can also be traced directly or indirectly in compressed/stabilized soil systems. The effects of excess lime in the jointing mortar or in the stabilization process are the same as excess lime produced during cement hydration (Dow & Glasser, 2003; Miqueleiz et al., 2012). The excess lime, as indeed any excessive salts in the material used, may cause discoloration of the brick or blocks, causing unsightly structures, as shown in Figure 18.1. Repeated wetting and evaporation exacerbates the process, explaining why it is possible to witness more efflorescence on the masonry in certain elevations and not in others. Care or caution must therefore be taken in the use of emergent newly researched materials, as in the attempts to utilize industrial and agricultural wastes. Such attempts must be accompanied by a thorough analysis of ingredients that may cause discoloration or material disintegration. Work by Miquieleiz et al. (2012) has shown that excessive efflorescence can cause material disintegration. However, in most mild cases of efflorescence the damage is limited to the unsightly structure. The use of industrial and agricultural wastes in cement-bound bricks and blocks is dealt with in a dedicated section on sustainability later in this chapter.
18.2.2. Technology and resultant material engineering properties
Fired bricks: Fired bricks form a significant proportion of soil-based construction materials, providing the bulk of the engineering bricks used in wall materials. These bricks are stronger and more durable than their unfired counterparts, such as either stabilized or simply compressed earth (Oti, Kinuthia, & Bai, 2010a,b,c,d). However, blanket application of fired bricks for all situations has attracted criticism. With pressure mounting from the pursuance of sustainability, the cost of fuel for firing bricks has become unbearably critical. Thus, designers and practitioners have steered away from using fired bricks for all cases, preferring to use them only when strength and durability demands warrant their use, and opting for the unfired bricks and/or other cheaper alternatives where environmental and application requirements can allow.
The genesis of the desirable durability aspects of fired bricks is the sintering calcination process, resulting in robustness by way of high compressive strength and low water absorption as a result of low wetting tendency and low capillary suction. The calcination breaks down a clay soil in various stages:
• 100–250°C: Removal of loosely held water (adsorbed or evaporable water).
• 500–600°C: Rapid removal of combined water from the clay. This is the proper dehydroxylation temperature of the clay.
• 600–900°C: Removal of carbon dioxide from the carbonates (if present). This process is commonly referred to as the decarboxylation of the clay (loss of carbon dioxide (CO2) from the breakdown of any limestone (such as calcite) present).
• Above 900°C: New high-temperature phases begin to form, and although these are not desirable as pozzolanic materials for stabilization with for example lime (they are not easily dissolved, even at high pH), the phases are the genesis of the high fused strength that results in durability of fired bricks.
The losses of free and chemically bound water reduce the swelling potential associated with clay soils. Considering that some soils can exhibit very excessive swelling potential as high as 2000%, as can be seen in the section on unfired bricks, the calcination process is therefore beneficial in imparting volume stability on a target soil material. The strong bonding reduces the effects of porosity and imparts chemical resistance (Benavidez et al., 200; Molina et al., 2011).
Unfired bricks: When soils are used in an unstabilized form, serious volume instability may be encountered through excessive swelling. This is because hydrated cations (such as Li, Na, K and Ca) can usually find their way into the clay inter-layer spaces by various mechanisms (adsorption, diffusion, osmosis, etc.) (Kinuthia & Wild, 2001; Kinuthia, Wild, & Jones, 1999), as schematically illustrated in Figure 18.2. This is the origin of the inherent differences in the swelling potential of different clay soils further demonstrated in Figure 18.3.
Traditionally the high volume changes have been mitigated by enhancing flexural strength by incorporating fibrous constituents (Moropoulou et al., 2005; Khedari, Watsanasathaporn, & Hirunlabh, 2005). This is an old technology, and unfired bricks have been strengthened with straw in practices that are centuries old. Soils can be strengthened by other unfired processes, such as soil stabilization techniques. When soils are stabilized with lime and/or Portland cement, a colloidal product predominantly composed of a calcium silicate hydrate (C-S-H) gel is formed, although aluminum phases may also be present. The complex gel gradually changes with time by partially crystallizing, resulting in strength gain in a mechanism very similar to that in Portland cement hydration. The composition of this colloidal product is dependent on material ingredients used: the compounds of calcium, silicon and aluminum coming from the lime or Portland cement used as stabilizer; aluminum and silicon from the soil; and finally the water added for the stabilization process. This colloidal CaO-Al2O3-SiO2-H2O system is beneficial for strength development, although it is prone to ingress of water and other elements. The quantity and long-term development of this colloidal product influences the total porosity and affects strength and volume stability. The blocking of pores is obviously beneficial to strength in construction materials (Benavente, García, Fort, & Ordoňez, 2004; Molina et al., 2011). When well protected, all goes well, and hence we have roads, foundations, brick walls and other products of unfired soil stabilization. Depending on prevailing environment, things could go wrong, as both the residual unbound ingredients and the hydration products (from lime and Portland cement) and the complex C-A-S-H gel are prone to attack in aggressive solutions (Beaudoin, Catinaud, & Marchand, 2001; O’Farrell, Wild, & Sabir, 1999; O’Farrell, Wild, & Sabir, 2000; Snelson & Kinuthia, 2010; McCarthy, Csetenyi, Sachdeva, & Dhir, 2014; Wild, Kinuthia, Robinson, & Humphries, 1996; Wild, Kinuthia, Jones, & Higgins, 1998; Wild, Kinuthia, Jones, & Higgins, 1999).
As will be observed in section 17.4 on durability, some engineering material properties (such as strength, density and porosity, for example) are commonly used as indicators of durability. Water absorption affects all these parameters; hence water ingress will feature prominently in this section. Guetalla et al. (2006) has observed that smooth faces on which water will have less action enhance water resistance. However, in general, any aspect of the masonry that affects water intake – particle size, surface finish, total porosity and void size distribution (affects capillary suction) – plays an important role in determining durability.
18.2.3. Prevailing environment
The prevailing environment is very critical to the durability of masonry. For this reason, regular inspection of masonry cannot be overemphasized. Problems should be pinpointed long before they become severe. Early micro-cracking that may go unchecked could lead to very expensive remedial measures if left unattended.
Damage due to water ingress is perhaps the most common in masonry, as materials in general lose strength upon wetting (Adedeji, 2002; Baojian, Chisun, & Caijun, 2013; Forth, Brooks, & Tapsir, 2000; Kelman & Spence, 2004; Walker, 2004; Obuzor, Kinuthia, & Robinson, 2011a,b; Obuzor, Kinuthia, & Robinson, 2012). Water ingress can be due to flooding or underground water flows or by way of capillary rise. Capillary rise is much dependent on pore structure of the masonry. Kelman & Spence (2004) have reported that capillary effects occur in pores between about 0.1 and 100 μm diameter. Not only does the water weaken the compressed/stabilized material, but any freezing and thawing of the water tests any masonry to the limit. A thorough inspection after every freeze–thaw cycle or season is recommended, and this should be accompanied by a detailed description of any cracking or spalling and any other observations. Whenever possible, photographic evidence or record of the condition of the masonry should be maintained. The inspection should not merely consider the brick or block system, but should be comprehensive, assessing covings, damp proof courses, pointing mortar, adjacent ground slopes, and differential damage due to orientation and wall heights, among other relevant details (Jansen et al., 2012). It is anticipated that masonry zones near the ground are more likely to be more moisture prone and hence more at risk of freeze–thaw and other moisture-related damage.
There are other mechanisms by which water can reduce the durability of otherwise robust brick and compressed earth masonry. The CaO-Al2O3-SiO2-H2O colloidal product mentioned earlier can be very water absorbent under certain environments. In sulfate environments, for example, the “good” strength-enhancing colloidal material can absorb sulfate ions if these gain access to the masonry or any stabilized material, gradually replacing the silica in the “good” system. This is when compounds of calcium (from lime or Portland cement), aluminum and silicon from the soil (or Portland cement), and sulfate (if in excess in the Portland cement or from any other sources, such as ground water, gritting salts, industrial effluents, etc.) all come together in the presence of excess water. Eventually, within this colloidal product, a compound with little or no silica in it, commonly known as ettringite, is precipitated. Ettringite may be represented either in the structural chemistry notation as Ca6[Al(OH)6]2(SO4)326H2O, or in the cement chemistry notation as C3A3CSH32 or C6AS3H32 (i.e. 3CaO.Al2O3.3CaSO4.32H2O) [where C = CaO; A = Al2O3; S = SiO2; S = SO3; H = H2O].
This ettringite compound stands out for its variable behavior, ranging from desirable strength-enhancing aspects to what may be associated with the genesis of adverse loss in material strength and volume stability. If formed during the early malleable stages of stabilization, such as soon after compression, ettringite may be beneficial to strength development as long as its formation is readily stopped by lack of feedstock. If, however, excess water absorption continues, or the reaction starts in some shape or form after the material has already set or gained strength, the now ‘delayed' ettringite formation and subsequent excessive water absorption can be very disruptive. This can occur especially when the prerequisite reactive raw material ingredients necessary for ettringite formation are introduced late, when the cementitious material has already hardened. Common sources of this late ingress include flooding, underground water flows, unforeseen industrial effluents, or any other late introduction of sulfate-bearing contaminants (Kelman & Spence, 2004; Wild et al., 1998, 1999). Under flooding conditions, salt water, sewage, and industrial chemicals, among other deleterious contaminants, are a significant risk to masonry. Any bricks made from soil stabilization and in these environmental conditions are susceptible to reduced durability. This has been visually demonstrated in Figure 18.4, which shows linear expansion that takes place when a non-sulfate-bearing kaolin clay soil of high purity was artificially dosed with sulfate (gypsum). Figure 18.5 graphically confirms this expansion potential, demonstrated using lime stabilization of a natural sulfate-bearing clay soil (Kimmeridge clay from Oxfordshire in the United Kingdom) (Higgins, Kinuthia, & Wild, 1998; Higgins, Thomas, Kinuthia, 2002; Kinuthia, Nidzam, Wild, & Robinson, 2003; Wild et al., 1998, 1999).
In the advent of usage of various industrial and agricultural waste and by-product materials in brick manufacture in the pursuance of sustainability, it is easily feasible to recreate the situations narrated here, leading to strong unfired brick systems, but that are susceptible, in the special environmental circumstances already mentioned, to sulfate attack depending on material ingredients and service conditions. Only with a thorough understanding of the brick material constituents, chemical compositions and the chemistry involved is one able to design mitigating steps to enhance the durability of bricks incorporating industrial waste and by-products. For this reason, this chapter includes a section dedicated to the use of waste and by-products in brick manufacture, to showcase only a few experiences.
18.2.4. Appropriateness and care
The analysis of the influence of the material ingredients that constitute masonry systems, together with the prevailing environmental conditions, on the durability of masonry has suggested that the limits of applicability and care during service are very critical considerations. Masonry systems exposed to the elements have far higher expectations in terms of robustness compared to those used for internal walls or other protected environments (Dias et al., 2013). On the other hand, the level of care during service is also critical. Careful maintenance regimes are recommended for any masonry. Biological actions including microorganisms and plant growth thrive in damp conditions, particularly molds and fungi. Other effects such as torn or inelastic sealants, cracked pointing mortar, blocked weep holes, standing water, missing damp-proofing, and worn coatings, among other critical issues, are key factors, and mitigating or remedial steps should be taken as early as possible, together with a strict adherence to manufacturer recommendations (Kelman & Spence, 2004). Any reported cases of flooding of masonry should be carefully analyzed to characterize the flood’s depth, salinity, contaminants and duration, among other factors. To prevent the biological actions mentioned, thorough disinfecting of flooded buildings is necessary to prevent mold growth. In case of problems, a thorough problem analysis is necessary before any remedial action. Covering problems (for example, cracks, loose material and effloresced salts) with render or paint only helps to delay and exacerbate the problem, leading to more expensive remedial measures later. In addition, expert help may sometimes be found necessary, as inappropriate remedy or workmanship can appear an attractive repair and maintenance option but may have consequences at a later date, including serious ones such as loss of life.
18.3. Use of industrial and agricultural wastes and by-products
Due to various forces such as pursuance of environmental care, low cost, technological advances and/or other drivers, there are no longer what may be considered conventional or classical materials for masonry. Changes have been encountered either in the materials used or in the nonclassical applications. For this reason, marginal natural materials that have hitherto not been considered in building and construction, such as expansive clays and weathered, flaky and/or elongated aggregates, as well as contaminated soils, have been considered in ambitious research programmes for potential sources of materials for masonry. In addition, industrial or agricultural waste and by-product materials have also found their way into masonry.
Marginal natural materials can have applications in masonry. The use of nonindustrial harvesting of stone, coral limestone, chalk and gravels has been attempted for sourcing materials for masonry. Use of marginal naturally occurring materials does not, however, attract much attention compared with the use of industrial and agricultural waste streams, primarily because of the negative environmental impact of these waste and by-product materials.
Ground granulated blast-furnace slag (GGBS): This is an industrial by-product material that results from the manufacture of steel from iron ore in a blast furnace. The material has successfully been applied in the concrete industry, where it results in reduced use of Portland cement, an energy-intensive material with a significant negative environmental impact. Use of GGBS also results in improved durability in concrete. The material has had very little impact in masonry until recent times. The reduction in swelling potential in lime-stabilized clay soils by gradually replacing the lime used in the stabilization process with GGBS was demonstrated earlier in Figures 18.4 and 18.5. Figure 18.6 further shows that higher compressive strength values were obtained using a combination of 3% lime and 5% slag (GGBS), i.e. combination 3(5), as opposed to using the traditional lime on its own (combination 8[0]). Considering that slag is a by-product material that is in most cases less expensive compared to lime or Portland cement, there are benefits in using a blended binder that results in the use of reduced amounts of traditional binders in masonry.
In an attempt to exploit the strength enhancement and volume stability imparted by the incorporation of GGBS in stabilized soils, a large research team (comprising Kinuthia et al., 1999; Wild et al., 1996, 1998, 1999; Kinuthia & Oti, 2012; Oti & Kinuthia, 2012; Oti, Kinuthia, & Bai, 2008a,b; Oti et al., 2010a,b,c,d) has comprehensively researched the use of lime–GGBS blended binders, including in the development of a robust masonry system. Figure 18.7 shows unfired building bricks made with a sulfate-bearing clay soil (Lower Oxford Clay from Oxfordshire in the United Kingdom), stabilized using combined lime and GGBS, obviating the brick firing process and also reducing the use of traditional binders of lime or PC. The bricks were made in a trial at Hanson Brick Company Ltd at their fired clay brick plant at Stewartby, in Bedfordshire. Hanson Brick Company Ltd is one of the largest manufacturers of the well-known “London” (fired) clay brick in the UK, whose mold was used in the trials for the bricks shown in Figure 18.7. The bricks have shown resistance to repeated freezing and thawing, which is one of the harshest tests for robustness of construction materials (Oti et al., 2010c).
Pulverized fuel ash (PFA): For many years, coal has been a dominant source of energy in America, Europe, Asia, Australia and parts of Africa. The waste from this big industry ranges from the unusable mining debris (collectively referred to in various terms, such as simply coal mining waste, colliery spoil, colliery waste, coal mine tailings, and possibly other terms) to the waste resulting from the burning of coal as a fuel. The burnt waste is predominantly the fine particulate material collected from the flue gases, mainly by electrostatic precipitation. It is commonly referred to as fly ash (FA) in America and other places, or as pulverized fuel ash (PFA), as in the United Kingdom and some other places in Europe and beyond. There is also the relatively coarser waste referred to as bottom ash, which, as the name suggests, is collected at the bottom of the coal-burning boilers. Although PFA's classical application is in concrete – where the benefits include enhanced workability, reduction in the amount of Portland cement used, improved later strength, and enhanced durability such as increased resistance to sulfate and chloride forms of attack – its use in masonry is not very widespread. As with GGBS, it has been demonstrated that PFA can also be used in the manufacture of durable masonry (McCarthy et al., 2014; Rahmat, Ismail, & Kinuthia, 2011). The compressed clay–PFA blocks shown in Figure 18.8 have not been commercially tested, but work (Rahmat et al., 2011) has suggested that durable compressed masonry block can be made using this industrial waste or by-product material that is readily available worldwide.
Wastepaper sludge ash (WSA): Most paper is normally coated with clay and limestone to create the smooth surfaces that we are all familiar with. When paper is recycled, the clay and the limestone remain in the sludge from which the recyclable cellulose fibrous material is removed during recycling of paper. When this sludge is combusted to reduce volume of landfilled waste, the clay and limestone are heated in the process. Combusting wastepaper sludge inadvertently produces a waste material that has undergone more or less a similar heat process as Portland cement, albeit at lower temperatures. For many years, this wastepaper sludge ash material has ended up in landfill, without the knowledge that this is weak cement. Successful development of novel cement utilizing wastepaper ash and blast-furnace slag has been reported by Nidzam & Kinuthia (2010, 2011a,b). The cement was developed by combining an industrial waste (WSA) with a by-product material (GGBS) for replacement of Portland cement. This has enabled the development of “green” cement for masonry. The “green” cement has performed very well in terms of strength and durability, and sometimes better in terms of appearance compared with the traditional Portland cement. The demonstration of success has been carried out using concrete block making, as shown in Figure 18.9. However, as has been demonstrated using GGBS and PFA, it is also possible to utilize WSA, and indeed any other waste that has shown success in concrete trials, in compressed earth masonry (Kinuthia et al., 2003; Nidzam & Kinuthia, 2011b). Figure 18.10 shows successful suppression of swelling potential using WSA–GGBS blended binder on the same expansive Lower Oxford clay used in demonstration projects earlier.
In the examples shown here, utilization of a waste in combination with a by-product material has resulted in superior performance relative to the use of a classical or traditional building material. Because such hybrid binders do not usually have excess lime, the common discoloration in the typical use of lime and/or Portland cement (efflorescence) (seen earlier in Figure 18.1) was not observed in the lime–GGBS, lime–PFA, or WSA–GGBS clay systems. Indeed, GGBS, PFA and WSA all thrive by utilizing any excess lime for their activation to form additional cementitious products. Similar approaches are possible elsewhere, using waste and/or by-product materials that are available in significant quantities, including agricultural waste. Other examples of commonly encountered industrial waste and by-product materials that have applicability in both soil- and cement-based building and construction materials and components include waste glass (Chidiac & Mihaljevic, 2011); waste tires (Snelson, Kinuthia, Davies, & Chang, 2009); shale, slate, colliery waste and other forms of recycled aggregates (Baojin et al., 2013; Bryson, Gutierrez, & Hopkins, 2012; Corinaldesi, 2009; Debieb et al., 2010; Kinuthia, Snelson, & Gailius, 2009; Oti et al., 2010f,e); brick dust (Kinuthia & Nidzam, 2011; O’Farrell, Wild, & Sabir, 2000, 1999).
Agricultural waste: For many countries, the major industrial activities are directly or indirectly related with agriculture. For these countries, any major breakthrough in the development of sustainable infrastructure cannot afford to ignore waste from the agricultural sector. Initial focus has been in activities that produce large agricultural waste streams, including the growing of palm oil and sugarcane (Mofor, Kinuthia, Melo, & Djialli, 2009), rice (Billong, Melo, Kamseu, Kinuthia, & Njopwouo, 2011), and fibrous materials for fiber-based soil–cement blocks – e.g. bagasse, straw, kenaf, bamboo, jute, coir, durian (Khedari, Watsanasathaporn, & Hirunlabh, 2005). The research and commercial application of masonry products made by incorporating agricultural materials and agricultural waste and by-product materials is in its formative years. Most research groups have only managed to demonstrate potential. Preliminary work by Mofor et al. (2009) on bagasse ash from the growing of sugarcane has demonstrated possible potential savings in Portland cement of about 50%, as demonstrated in Figure 18.11. However, the durability of masonry made using both fibrous and nonfibrous agricultural waste is ongoing, particularly on waste ash resulting from the burning of waste from the growing and processing of cereals, such as rice husks and bagasse from sugarcane (Khedari et al., 2005).
18.4. Tests and indicators of durability
For a long time, compressive strength has been employed by many users as an indicator of durability. While to some degree this does reflect possible outcome during service life, it has also been established in both soil- and cement-based materials that strength alone is only one of the many factors determining durability. In concrete, for example, tests involving ingress of water and chemicals, especially for applications of concrete in either marine or buried environments, are among the severe test environments under which durability of concrete may be assessed. Unless there are freezing conditions anticipated during service life, pure water often does not pose any significant danger to well-designed concrete. For compressed earth, however, even without the presence of any chemicals, soaking in water has a significant influence on the durability of the masonry. For this reason, applications of buried compressed earth masonry are rare and are strongly discouraged. Indeed, there are practically few or no examples of applications of compressed earth in wet buried conditions. The water absorption capacity tests are therefore very strong indicators of durability. Indeed, all the common tests for durability of compressed earth units are water-based or water-related. Broadly, these tests fall under four overlapping categories, viz:
2. Category 2 – Laboratory-based tests with little or no regard for particular rain characteristics,
3. Category 3 – Laboratory-based tests commonly of a very high degree of severity, usually attempting to adopt tests more suited for other materials (e.g. rocks, concrete) or components, and
4. Category 4 – Both laboratory and, more commonly, nonlaboratory tests attempting to simulate prevailing environmental conditions (holistic variations in weather: sunlight, rain, wind, temperature, flooding etc.).
Category 1: It is common to first test for the durability indicators of compressive strength and water absorption. For this reason, these tests are more or less routine tests for compressed earth masonry. Compressive strength tests are in this regard commonly carried out in the basic mode without test specimen confinement and without mortar capping, although the use of soft leveling materials such as cardboard is sometimes common. For water absorption, water uptake by capillary suction and/or by fully soaking test specimens in water is common. Due to the large variations in soils and additives used in compressed earth masonry, together with an equally large variation in the methods of manufacture, it is neither possible nor helpful to attempt to quote en masse test results for water absorption and for compressive strength in this chapter. For compressed earth blocks, these values vary greatly, and the situation is in many cases exacerbated by lack of standards for these materials, for either their manufacture or the testing procedures. This situation is perhaps due to the already mentioned variability in soil types and additives to compressed earth units. Notwithstanding, Guetalla et al. (2006) report a recommended maximum value for water absorption of 15%, while that for capillary suction is 2.5%. This value is not far from the recommended value of less than 14% by Heathcote (1995) for stabilized well-graded sandy soils. The work reported by Guetalla et al. (2006) shows total water absorption values for stabilized earth masonry within the range of 5.3–9.9%, which was 2.5–4 times the corresponding values for water absorption by capillary action. In contrast, work by Oti et al. (2009) reported on very robust bricks of compressed earth stabilized using a blended lime–GGBS binder, despite observing water absorption values within the range of 16–22%, outside the recommended value of 15% as reported by Guetalla et al. (2006). It is precisely for this reason that this chapter has deliberately opted for generic coverage, preferring generic factors and trends, as opposed to over-reliance on specifically quoted values on durability in the literature.
In terms of damage caused by absorbed water, compressive strength test results in the soaked condition have been used, relative to unsoaked strength values, as indicative pointers to durability. The water spray method (see typical set-up in Figure 18.12) is also fairly common for assessing water damage and durability (Guetalla et al., 2006; Obonyo, Exelbirt, & Baskaran, 2010; Cid-Falceto et al., 2012, among others). Typical erosion rates for various masonry materials upon water spraying of unfired masonry are provided in Table 18.1, after the research work by Obonyo et al., 2010), while Table 18.2 shows typical strength reduction values for compressed earth stabilized using a blended binder consisting of lime and GGBS, from work by Obuzor, Kinuthia, & Robinson (2012). It is reiterated that these typical values by Obonyo et al. (2010) or by Obuzor et al. (2012) can change significantly upon changes in material composition and small changes in test conditions, among other factors, and should be interpreted, used or extrapolated with great caution and care, as the following analysis will demonstrate. The work by Guettala et al. (2006) used a target material with the following soil characteristics: 64% sand, 18% silt, and 18% clay, with a chemical composition analysis showing 5.81% sulfate as SO3. Using these values alone, an experienced researcher in this field may be able to approximately establish that this material is best suited for cement stabilization, with possible good results being also expected from an alternative blended binder comprising cement and lime. The cement would target the 64% sand, while the lime would target the 18% clay content present. The results in Table 18.3 (after Guetalla et al., 2006) confirm this argument. Guetalla et al. (2006) ranked the holistic performance on durability of the material as (1) cement, (2) cement + lime, and (3) lime, a further confirmation of prediction. A similar order of performance was observed by Oti et al. (2009) with a completely different composition, using a fine-grained clay soil with no sand at all (Lower Oxford clay). This shows that different compressed earth materials can show identical behavior, and thus extrapolation of results in the literature without due care or consideration of the fine details of the circumstances of the data capture may be risky.
Some users are prepared to use unconfined compressive strength and/or water absorption by either capillary or soaking as indicators of durability. However, with some modifications, durability tests have been formulated to advance these indicators to full-fledged durability tests. As argued earlier, it is rare to find durability tests that are based on ingress of chemicals such as chlorides or sulfates for compressed earth masonry. This is due in part to the care taken in the selection of situations for applications of compressed earth units. A significant section of this chapter has been devoted to the causes of volume changes that can occur when sulfates are present in the masonry material ingredients. This attention is primarily because the masonry need not be in a buried situation for sulfates to affect the masonry. Normal water ingress is capable of kick-starting sulfate-induced expansion when all the other prerequisite ingredients are present, either during service or during manufacture. As was demonstrated by Figure 18.4, sulfate-induced water absorption and expansion can be so severe in terms of magnitude and disruptive capacity, compared to normal water-induced expansion, that significant expansion and disintegration is bound to be picked up during the ordinary analysis for water absorption by capillary or by full soaking. There is perhaps no need, therefore, for further testing of compressed earth masonry using ingress of chemicals.
Table 18.1
Brick erosion test results at a spray pressure of 4.13 MPa
| Type of brick | Time (minutes) | Depth of erosion | Rate of erosion (mm/minute) |
| Soil-cement-lime-fluid | 15 30 45 60 | 17.5 20 25 30 | 0.50 |
| Interlocking bricks | 15 30 45 60 | 0.1 0.2 0.2 0.2 | 0.003 |
| Soil-cement | 15 30 45 60 | 0.5 0.6 0.7 0.8 | 0.013 |
| Soil-cement-lime | 15 30 45 60 | 17.5 18.5 19.5 20 | 0.333 |
| Soil-cement-fiber | 15 30 45 60 | 25 35 45 55 | 0.917 |
Source: Obonyo et al. (2010).
Table 18.2
Loss of strength upon soaking a sulfate-bearing clay soil (Lower Oxford clay) stabilized with lime–GGBS blended binder (4 lime–12 GGBS)
| Curing time (days) | Original (unsoaked) samples | Partially soaked samples | Completely soaked samples | |||
| 16L (Control) | 4L-12G | 16L (Control) | 4L-12G | 16L (Control) | 4L-12G | |
| 7 | 1.30 | 2.00 | 0.20 | 2.30 | 0.15 | 1.80 |
| 14 | 1.400 | 2.50 | 0.25 | 2.40 | 0.08 | 2.00 |
| 28 | 1.75 | 2.60 | 0.50 | 2.70 | 0.50 | 2.30 |
| 56 | 1.80 | 4.05 | 0.60 | 2.80 | 0.50 | 2.50 |
| 91 | 1.60 | 4.70 | F0.75 | 4.40 | 0.50 | 2.90 |
Source: Obuzor et al. (2012).
Table 18.3
Loss of strength upon soaking a sulfate-bearing silty-clayey sand stabilized with various binders
| Bricks characteristics | Different wall treatments | |||||
| Cement (%) | Lime (%) | Cement + lime (%) | ||||
| 5 | 8 | 8 | 12 | 5 + 3 | 8 + 4 | |
| Compressive strength in dry state (MPa) | 15.4 | 18.4 | 15.9 | 17.8 | 17.5 | 21.5 |
| Compressive strength in dry state (MPa) | 9 | 12.7 | 10.1 | 11.7 | 12.3 | 15.6 |
| Water strength coefficient | 0.58 | 0.69 | 0.64 | 0.66 | 0.63 | 0.7 |
| Capillary absorption (%) | 2.35 | 2.2 | 3.7 | 2.9 | 2.3 | 2 |
| Total absorption (%) | 8.27 | 7.35 | 9.8 | 9.02 | 8.1 | 7.9 |
| Weight loss (wet–dry) (%) | 1.4 | 1.25 | 2.3 | 2.1 | 1.2 | 1.0 |
| Weight loss (freeze–thaw) (%) | 2.35 | 2.23 | 3.7 | 2.9 | 2.3 | 2.0 |
Source: Guettala et al. (2006).
Category 1 tests also assess the depth of erosion, material loss and/or disintegration or closely similar assessments. Some approaches attempt to simulate wind-driven rain in a water spray and/or water drip erosion methodology. They aim to focus on what may be considered to be the relevant or appropriate rain characteristics, such as intensity/speed, angle of approach or impact or raindrop size, among other rain characteristics (Ogunye & Boussabaine, 2002a,b; Obonyo et al., 2010; Aubert & Gasc-Barbier, 2012; Erkal et al., 2012). Obonyo et al. (2010) assessed the rate of erosion of compressed earth units upon varied pressure of water spray. Two spray pressure levels were adopted, at 2.07 and 4.13 MPa. Table 18.1 shows the reproduction of the test results obtained at the higher spray pressure of 4.13 MPa. The table also gives an indication of the variability of material ingredients for compressed masonry units, suggesting, as argued earlier, the complexity in any over-reliance on typical strength or durability values in a generic publication such as this chapter without due attention to the relevant test conditions used in each case. For unstabilized soil blocks, some researchers have shown preference to total material loss rather than depth or erosion. Cid-Falceto et al. (2012) observed that the maximum erosion produced on unstabilized compressed earth blocks in a water drip test was not due to the direct fall of the drop of water, but rather to the run-off that takes place. They recommended weight losses within the range of 5–10% as the limiting performance criteria for unstabilized compressed earth blocks. In comparison, observation by Oti et al. (2009) on the weight losses for lime–GGBS stabilized earth blocks using the much more severe freeze–thaw tests were within 1.5–2%, demonstrating the wide durability range that is likely to be encountered in practice when dealing with both stabilized and unstabilized compressed earth units.
Another test approach within this category focuses on strength loss either upon varying compaction moisture content or upon water absorption by water spraying, capillary action and/or full immersion in water. Varying compaction moisture content may be considered as a mix design issue and not necessarily an indicator for durability. Such variations in moisture content have been exercised by many researchers during materials design (Rahmat et al., 2011; Obuzor et al., 2011a,b, 2012, among others). In the assessment of strength loss upon water absorption of cured test specimens as a durability test, Obuzor et al. (2012) established significant strength loss of lime-stabilized sulfate-bearing clay soils. In a rare occurrence, the researchers also observed strength increase upon soaking compressed earth units, when the lime binding material was partially replaced with a lime–slag blended binder. Sample results have been reproduced in Table 18.2 and also in Figure 18.13. This work clearly shows that water need not always be a bad thing in some well-researched formulations. Similar observations are reported by Guetalla et al. (2006) for work carried out using earth masonry with microsilica (silica fume) addition. The beneficial cementitious qualities of GGBS have been discussed in slightly more detail in Section 18.3. It is hypothesized that both GGBS and microsilica thrive in a water-cured environment, producing more C-S-H gel such that the high water-cured strength results enable significant water resistance and hence better durability. In another research work by Taallah, Guettala, Guettala, & Kriker (2014) on the mechanical properties and hygroscopicity behavior of compressed earth blocks incorporating date palm fibers, a series of cement-stabilized compressed earth blocks were fabricated at various compacting stresses. The addition of fibers was observed to have an adverse effect on the properties of some of the blocks. The fibers were observed to be of low tensile strength and very high water absorption. The use of fibers in this case would not be conducive for durability.
Category 2: This category of durability tests has little or no regard for rain characteristics, focusing rather on the amount of water absorbed, materials loss and/or disintegration (see Figure 18.14, after Oti et al., 2010c) and/or visual block integrity, and possibly other related or relevant observations (edge wear, cracking, efflorescence, etc.), relative to the variations in environmental situations and conditions simulated in the test. The test recognizes that it is not easy to design reproducible environmental situations and conditions, and it is also not easy to overcome the hardships in the attempt to model rain and/or in the adaptation of tests meant for rocks or concrete. Whenever possible or available, test facilities with carefully controllable environments (temperature, humidity etc. against time, among other relevant factors) are preferable and commonly adopted. Some freeze–thaw tests offer such conditions, using computer-controlled chambers (Oti et al., 2010c; Pan, Li, & Liao, 2014).
Freeze–thaw tests come in many guises, with all manner of variations in test specimen preparation, curing regimes before testing, water or moisture introduction methodologies and cyclic freeze–thaw cycles. It is therefore not appropriate to discriminate and cite here any particular regime or its test results. For this reason, only the nature of factors leading to the attraction to, and common features of freeze–thaw cycles need be analyzed.
Freeze–thaw tests have been adopted either for zones subject to freezing and thawing conditions in nature or as standalone tests for soaked material fatigue or robustness, irrespective of whether the region in mind is prone to freezing conditions. The freeze–thaw effects have been shown to be very severe tests for any construction material or component (Oti et al., 2010c, 2009; Pan et al., 2014). For relatively weak materials such as compressed earth, the freeze–thaw tests are perhaps among the severest tests for mechanical stresses and response to water ingress. This observation notwithstanding, not all freeze–thaw test conditions have proved to be very severe or disruptive. Research work by Aubert & Gasc-Barbier (2012) on freeze–thaw effects on compressed earth masonry sprayed with water resulted in hardening of the clayey soil blocks during freezing and thawing cycles. The decision to spray the test specimens rather than to immerse them in water was based on the weak nature of the test specimens. The specimens were not rehumidified after the initial spray, and this led to their desiccation and hardening. It is arguable that under such circumstances, unless a one-off water spray in a freeze–thaw test is seen as the best simulation of the likely prevalent environmental conditions, such a “soft” approach may be considered risky, due to the possibility of misleading apparent durability. Aubert & Gasc-Barbier (2012) have noted this risk in their research, by stating that freeze–thaw cycles on clayey soil blocks will need to be representative of the real risks encountered by these blocks during their service life, and setting up such tests still requires thought and discussion among experts on the topic.
Category 3: This category of durability tests attempts to adopt tests that are more suited to other materials, particularly and mainly tests for rocks and concrete. The previous section highlighted the hardships encountered in the attempts to model real-life environmental situations and conditions. However, with caveats and care in results interpretation and extrapolation, it is possible to have meaningful test regimes for durability of compressed masonry units. It is arguable that water absorption tests, compressive strength tests on noncylindrical test specimens, and most freeze–thaw cycles adopted by many researchers are adaptations of tests for concrete, carefully varied to suit conditions for tests on earth-based materials.
Other innovative adaptations are also common under this test category. Aubert & Gasc-Barbier (2012) and Aldaood, Bouasker, & Al-Mukhtar (2014), for example, both used wave velocity for the assessment of durability of masonry units upon freeze–thaw and wetting and drying cycles, respectively. The wave technologies were adopted from approaches commonly used in tests on rocks and concrete, for the assessment of the effects of water ingress. It is generally observed that the higher the degree of saturation, the more significant is the increase in wave velocity. Aubert & Gasc-Barbier (2012) and Aldaood et al. (2014) used this observation to monitor the degree of water absorption in compressed earth blocks, noting that any cracking due to freezing and thawing or wetting and drying cycles is bound to lead to more water absorption, and hence to changes in wave velocity.
Category 4: These are both laboratory and, more commonly, nonlaboratory tests, attempting to simulate the naturally and actually prevailing environmental conditions to which the compressed earth masonry under test is likely to be subjected. The tests usually aim for the actual holistic variations in weather, leaving nothing behind and looking, for example, to take into account exposure conditions of prevailing sunlight, various rain characteristics (duration, drop size, intensity, etc.), wind, temperature, and flooding, among other details. This category of tests has been found appropriate by some researchers (Guettala et al., 2006, among others) who have either facilities for out-of-laboratory capacity for construction of full or sample masonry units for exposure, or a strong belief or view that immersion of masonry blocks under simulated laboratory conditions is very severe and perhaps unrealistic. They argue that soaking tests sometimes do not adequately reflect the equivalent of the actual environmental conditions of the regions where the masonry units are to be applied. Some water spray or wet and drying methodologies may equally be very violent, with tests involving freezing and thawing being the most severe (Guettala et al., 2006).
Most other accelerated laboratory tests are seen as too severe and hard to validate to natural aging processes. Guettala et al. (2006) are of the view that such tests often cause controversy as one cannot simulate in the laboratory the complex succession of the multiple climatic phenomena. Due to the flexibility of the water spray method, and the fact that it is widely seen as realistic in nature, some researchers have come to the conclusion that among the accelerated tests for the durability of compressed masonry, this is the method that stands the best chance of a close simulation of natural aging.
18.5. Future trends
There is an observable increase in the researched use of compressed earth masonry units. These units are increasingly being seen as industrialized materials, and are no longer considered as only appropriate or applicable to traditional approaches for self-construction of infrastructure in the built environment (Cid-Falceto et al., 2012). Many examples exist of the increasing use of industrial and agricultural waste and by-product materials in compressed earth masonry, and it is not possible to cover or quote all of them here. However, a brief survey of the materials, techniques and applications of today's masonry suggests a thriving ongoing research and innovation-led development of compressed earth masonry technology. There is also a gradual shift from the traditional fired clay masonry categories to unfired categories. While the fired categories adopted a blanket approach of firing clay irrespective of clay composition, there is a far wider variety in material selection and technology adopted for the unfired masonry category. This is because the durability imparted by the firing process is harder to mimic or achieve in unfired systems, resulting in the pursuance of alternative approaches to materials usage to include industrial and agricultural waste and by-product materials. There is also increasing use of synthetic (industrial) and natural fibers for reinforcement, chemical cementation, attention to particle matrix configurations and variable and careful selection of applicability scenarios. Fortunately, as mentioned earlier, there is also a gradual shift in emphasis away from a blanket stipulation of standard properties and material expectations, relying rather on recommendations by the manufacturer for the appropriate usage, and also for the possible consequences of lack of adherence by the end user to these recommendations. The future will therefore most likely witness enhanced end-user sensitivity to global changes in materials technology and materials failure mechanisms, and better or closer liaison with material manufacturers, coupled with a keen(er) awareness of relevant government policies, guidance and legislation. This development or trend is likely to enhance reliability of compressed earth masonry than witnessed in the past, and it is hoped that the mortgage lenders and insurance industries will be quick to respond to the fast-changing landscape, resulting in an ever-increasing adaptation of compressed earth masonry in future social and commercial housing. These scenarios suggest an urgent need for a near-universal harmonization, consensus building and characterization and/or classification of compressed earth-based building and construction materials, taking into account the wide range of materials likely to be encountered and variable manufacturing methodologies. In terms of the further research work needed, tests on durability need to be clearly and well delineated, with those for fired systems adopting considerations that suit fired systems and separate from those best suited for the unfired systems. Those for the unfired systems need, in turn, to differentiate between stabilized and nonstabilized compressed earth systems. Both test categories should embrace and embed the indicative tests of compressive strength and water absorption. The more severe cyclic freeze–thaw and wet–dry tests should be more in reference to stabilized systems, with the water and spray and drip test methodologies being in specific reference to unstabilized systems. This task is beyond the remit intended for this chapter, but the chapter has managed to cite most of the factors that determine the durability of compressed earth units and a few of the tests that are commonly encountered at present.
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