Sustainable Housing Provision: Preference for the Use of Interlocking Masonry in Housing Delivery in Nigeria

Th is paper reports a study on the preference level for the use of interlocking masonry over the conventional types in sustainable housing delivery in Nigeria. Globally, buildings are the largest energy consumers and greenhouse gases emitters, consuming over 50% in some cases. Common materials used for masonry works in housing delivery in Nigeria such as sandcrete blocks and burnt bricks impact high energy and greenhouse gases on the environment due to the production processes involved. Intelligent choice of building materials capable of reducing energy used in buildings is imperative towards achieving materials efficiency and cost reduction. In this study, a comparative survey was carried out empirically among selected professionals in the building industry from 4 out the 6 geo-political zones in Nigeria through the use of questionnaire, direct observations, and interview schedules. Analyses of Chi-square test for significance of differences between materials price rating and acceptability of interlocking masonry as well as level o f willingness of respondents to use the selected materials for future pro jects were conducted. Findings signify shorter time of construction and reduced cost of construction expended when interlocking blocks are used. The study concludes that interlocking masonry is a good replacement to the conventional types in construction of housing in Nigeria.


Introduction
Build ing materials constitute the largest single input in housing construction. While Adedeji [1] observed that about sixty (60) per cent of the total housing expenditure goes for the purchase of building materials, Arayela [2] averred that the cost of building materials constitute about 65 percent of the construction cost. Ogunsemi [3] opined that bu ild ing materials form the main factors that restricts the supply of housing and ascertained that they account for between 50-60 percent of the cost of buildings. Thus, Adedeji [4] rightly observed that one main barrier to the realisation of effective housing in Nigeria as revealed in successive government efforts has been the cost of housing in the country. He argued that in the early periods, shelter in Nigeria was eas ily affordable as build ing materials were sourced fro m man's immed iate environ ment at affordable costs. Technology also was readily available with commensurate simple techniques. But contact with the outside world through interregional and international training of professionals in foreign countries as occasioned by colonization, brought changes to tastes and hence outlook to house forms. These changes rendered the undeveloped local building materials inadequate while there was an increased demand for exotic ones. Accordingly, Arayela [2] posited that the modern build ing industry lays much emphasis on sophisticated building materials and techniques that are expensive and energy consuming.
Though, housing delivery efforts have evidently been inhibited by prohibitive costs of building materials, this problem cannot be reasonably and reliab ly overco me by merely resorting to the use of locally availab le materials without due considerations to the applicable init iative, the cost of processing and sustainability of the local materials. One of the most impo rtant components of a sustainable building is the material efficiency. Co rrect selection of building materials can be performed by taking into account their co mplete life time (i.e. fro m cradle to grave‖) and by choosing products with the minimal environ mental impacts. For instance, González and Navarro [5] estimated that the selection of building materials with low environ mental impacts can reduce carbon dio xide (CO 2 ) emissions by up to 30%. The use of renewable and recycled sources is widely encouraged as the life-cycle of a build ing and its elements can be closed [6]. The other factors that greatly affect the selection of build ing materials are their costs and social requirements such as thermal co mfo rt, good mechanical properties (strength and durability), aesthetic characteristics and an ability to construct quickly. Ideally, the co mbination of all environ mental, econo mic and social factors can give a clear description of a material, and thus helps in a decision making process regarding the selection of the materials Interlocking M asonry in Housing Delivery in Nigeria suitable for buildings [7]. It is along this line that the building industry in Nigeria is evolving varied kinds of building system adapted to the local materials, environ mental conditions, city developments and levels of techniques of building construction that are in use. One of such system is the adaptation of interlocking masonry into the building industry which forms the focus of this research.

Buildi ng Materi als and Sustainable Housing Provision
The process of housing development should be based on sustainability principles, wh ich could be applied in the conception, construction and use of the build ings. The goals of the process are to decrease the environ mental costs incurred by inadequate constructive systems and solutions, minimizing the impacts on natural resources, and improving users' co mfort [8]. Gilkinson & Sexton[9] defined sustainable housing as a form of affordable housing that incorporates environmentally friendly and community-based practices. It attempts to reduce the negative impact that homes can have on the environment through choosing better building materials and environ mental design. Sustainable housing provision requires proper defin ition of housing needs, and the participation of the end users to ensure their satisfaction. The general goal of sustainable development is to meet the essential needs of the world's poor while ensuring that future generations have an adequate resources base to meet theirs. It is thus geared towards meeting the needs of the present generation without compromising the ability of future ones to meet their own needs [1]. It further includes the production of materials, wh ich must use resources and energy fro m renewab le sources instead of non-renewable ones. Sustainable building materials are environmentally responsible because their impacts are considered over the complete life t ime o f the products. Sustainable building materials should pose no or very minimal environmental and hu man health risks [10]. They should also satisfy the follo wing criteria: rat ional use of natural resources; energy efficiency; elimination or reduction of generated waste; low to xicity; water conservation; affordability. Sustainable build ing materials can offer a set of specific benefits to the owner of a building such as reduced maintenance and replacement costs, energy conservation, improved occupant's health and productivity, lower costs associated with changing space configurations, and greater flexib ility in design [11].
Achieving sustainability in housing provision requires major societal changes, restructuring of institutions and management approaches. It requires the appropriate political will based on the conviction of the responsibility of government to its citizens, and the need to create hu mane and decent environment for d ignified living [12]. In order to realise sustainable housing provision the housing needs of the Nigerian population have to be put into proper focus, and a coordinated programme to achieve this should be thoroughly worked out. With due consideration given to the input of the local co mmunit ies, government may in itiate aided self-help programmes and low-cost core housing units. It can also facilitate the acquisition of building materials, the cost of wh ich constitutes about 60% of the entire cost of a building. Production of building materials of indigenous origin by private investors should be given logistic and material support by government

Interl ocking Blocks and Energ y Efficiency
Introduction of interlocking or "dry stack" mortarless masonry systems in masonry construction requires the development of efficient, easy to handle, and yet versatile blocks. Varied interlocking blocks developed for use include Sparlock system, Meccano system, Sparfil system, Haener system, and the Solid Interlocking b locks (SIB) or Hydraform blocks, which are an imp rovement over the traditional adobe bricks or unfired laterite blocks that were prevalent in the 20 th century in some African countries [13]. Interlocking blocks can also be of cement and sand content only.
In Nigeria, the Nigerian Bu ild ing and Road Research Institute (NBRRI) developed an interlocking b lock making mach ine meant to produce SIB types. The blocks have geometric size of 225 x 225 x 112 mm. This machine produces solid blocks of laterite co mposition mainly and stabilised with cement material o f rat io 1:20 [4]. The major environmental burdens associated with building materials (conventional and innovative types) include embodied energy of building materials and greenhouse emissions originated fro m each stage of their life-cycle. Embodied energy is defined as the amount of energy required to produce a material and supply it to the point of use. It is an important measure of the effectiveness of build ing materials in the environmental terms [7]. Embodied energy consists of: energy required for the manufacturing of building materials; energy associated with the transportation of raw materials to the factory and of the finished products to the consumer; energy needed for assembling various building materials to form a building. The results presented by Thormark [14] indicate that embodied energy in traditional building can be reduced by approximately 10-15% through the proper selection of building materials with low environ mental impacts. Although the values of embodied energy can vary widely (somet imes by as much as 100%, depending on the number of factors like country, manufacturing processes, recycling technologies, methodology of analysis, fuel costs and destination), they can be considered as reasonable indicators of an overall environmental impact of building materials [15]. The usage of SIB in place of conventional fired ones can significantly reduce the energy use and also cut down CO 2 emissions. Interlocking blocks are manufactured by hydraulically co mpressing a soil and cement mixture (stabiliser) in a block-making machine  The production process involves preparation of soil, preparation of mix, co mpression of mix, stacking and curing of blocks. The results of several studies (Harris, et al. [16], Anand & Ramamurthy [13]) showed that increase in durability and strength over conventional blocks and unfired blocks occurred when cement is added to stabilise solid interlocking blocks. In the production of the latter, a 4M Pa block requires a 1:20 ratio of cement to soil for stabilisation. This means that for one bag of 50kg cement (+-33 litres) you will need about 10 wheelbarro ws (+-65 lit res/wheelbarrow) of soil. This mix yields about 75 blocks, with engineering standards acceptable for wall construction. In Solid interlocking blocks, substantial cost savings can be achieved due to elimination of bedding mortar in the superstructure (except in ring beams and in high gables) accelerates construction, thereby reducing workmanship and cost. Hydraform b locks are three times as efficient as concrete and almost twice as efficient as fired clay bricks in terms of the thermal insulation they offer. Attractive, face brick fin ishes (in a variety of natural colours derived fro m the soil found at individual sites) is also possible with the use of the material [17]. However, Adedeji [17] observed that block strength is affected by cement content quality, curing duration (7 days minimu m) and soil type. Moreover, energy input of interlocking blocks are co mparable to that of unfired clay bricks, which their total energy input was estimated of 657 MJ/ton as opposed to 4,187 MJ/ton for the co mmon fired bricks, while an equivalent output of CO 2 e mission was 41 kg CO 2 /ton compared to 202 kg CO 2 /ton for traditional bricks in mainstream construction [18], [19].
Concrete and cement products are the most widely used for construction of foundations, structural frames, floors, roofs, and prefabricated elements in Nigeria and many parts of the globe [20]. Globally, mo re than 10 b illion tons of concrete are produced annually [21]. Concrete is a durable material with excellent mechanical properties. It is adaptable to different climates, relatively fire resistant, widely available and affordable. Concrete can be moulded almost into any shape and can be designed to satisfy almost any performance requirements [21]. It can be reinforced with either steel or fibres. Moreover, recycled materials can be incorporated into the concrete mix, thus reducing consumption of raw materials and disposal of waste products. The use of admixtures-materials added to concrete-becomes very popular as the final co mposite can have better durability and gains some specific un ique properties [10]. In spite of these advantages, concrete unfortunately has an enormous negative impact on the environment. It is estimated that cement and concrete industry generates up to 7% of g lobal anthropogenic CO 2 emissions, and it is set to increase dramat ically in the coming decades as the Earth's population grows [10]. Apart fro m the emissions related to the combustion of fossil fuels, there is a release of CO 2 associated with unavoidable de-carbonation of limestone (raw material) [22]. Concrete manufacturing is responsible for generating not only carbon dioxide but also other air pollutants like carbon mono xide (CO), sulphur (IV) o xides (SO 2 ), nitrogen (IV) o xides (NO 2 ), hydrogen chloride (HCl), volatile hydrocarbons and particulate matter. Production of concrete causes depletion of non-renewable mineral and water resources required in ext remely large quantities.
Concrete industry must, therefore, take urgent actions in order to reduce the emissions of CO 2 and other air pollutants; to reduce the use of energy; to cut down the use of natural resources (including water); and to minimize the amount of waste generated. One of the effective ways to deal with negative environmental impact of concrete is to reduce the total volume of this material needed for a certain construction process by enhancing its performance [12].

Materials and Methods
The research method was an abridged form of a parent research carried on materials preference options for sustainable low-inco me housing in selected cit ies in Nigeria in 2007. A mu lt i-stage sampling technique was adopted in selecting the zones and the cities. The stages of the mu lti-stage sampling technique emp loyed were; (i) adoption of the original six geo-polit ical zones and random selection of 4 zones out of the six zones; (ii) random selection of one State per zone and (iii) specific selection of State capitals in the surveyed zones as they were ad judged to be the most urban. A town was randomly selected fro m each o f the geopolitical zones as follo ws Abuja, (North-central zone); Port-Harcourt (South-south zone), Lagos (Southwest zone) and Enugu (Southeast zone). Data were obtained through observations fro m case studies. Also, a well-structured questionnaire, which was designed to investigate 25 variables on housing materials, was used to elicit opinions of professionals and clients on the use of these materials. The variables were structured in question form and responses were required in pre-coded alternatives given. Research assistants, who had earlier been trained by the author, administered questionnaires to selected professionals in the building industry. The selected professionals (Architects, Engineers, Quantity Su rveyors and Builders) d istributed over four out of the six geo-political zones in Nigeria expressed their opinion on the acceptability and willingness to use this material as a replacement fo r the conventional sandcrete blocks. Questionnaires were ad min istered to two hundred respondents in the four geo-political zones as shown in Table 1. Descriptive statistics such as frequency distributions and pie-chart were utilised for the analysis of socio-economic data while Ch i-square (c2) was used to test bi-variate relationships and determine the superiority of the selected materials in terms of cost-efficiency over the conventional type. The chi square model used is given as:

Findings and Discussion of Results
Observations from case studies on comparative cost of interlocking blocks with conventional types were obtained fro m the four (4) selected locations. Prices of interlocking blocks and conventional blocks were also obtained fro m the market. While conventional sandcrete blocks (225 x 225 x 450) sells at N120.00, interlocking block (225 x 112 x 225) mm sells at N25.00 as at October, 2007. Taking into consideration that conventional block requires the use of mortar for the laying o f the b locks and associated non-contributory activities that affect its cost and the net output. Some non-contributory activities associated with the use of conventional sandcrete blocks include: sorting of blocks, taking of blocks, breaking of blocks, laying of blocks, levelling of blocks, taking of mo rtar, mixing of mortar, laying of mortar, spreading of mortar and waiting for materials. These activities together with the use of mortar are eliminated in the operation of interlocking blocks [13]. Besides, interlocking blocks offer several advantages such as design flexib ility, reduced construction time, environ mental friendly and solution to space shortage. This resulted to the significant cost savings observed with the use of interlocking blocks in the study area. Though four (4) units of interlocking blocks will co mbine to make a unit of the conventional block, the cost of 4 units of interlocking blocks is still lesser than that of a corresponding conventional masonry. Also, interlocking blocks are designed and produced in varied sizes in such a way that it does not require cutting into sizes during setting operations. This further reduces the time for setting operation and eliminates associated wastages. The production of SIB does not require firing as in the case of burnt bricks nor expensive factory processes associated with cement products. Hence, energy consumption is reduced considerably. Besides, the cost of using interlocking blocks in construction is lower than that of conventional blocks as its operation does not require special skilled labour as it is in the case of conventional blocks. It was also observed that while a gang of 1mason + 1 labour could achieve a productive hours 6.5m 2 /h with interlocking masonry, a gang of 1mason + 1 labour could only achieve a productive hours 1.55m 2 /h with conventional masonry. This further corroborates an observation made by Anand & Ra ma mu rthy [13] on a study carried out on comparison of output from different types of masonry works, where a crew of one person, achieved the productivity of 4.1 m/h with the use of hollow-interlocking blocks.
The results obtained from the various zones were not significantly different fro m each other, imp lying that the locations of the selected projects did not significantly affect the willingness of respondents to use these materials.
Consequently, respondents favoured the use of interlocking masonry in housing construction based on its shorter time of construction, reduced cost, high energy efficiency and high acceptability index as against the use of the conventional types.

Res pondents' Willingness to Use Interlocking Blocks
The tendency towards the preference for the use of interlocking-blocks masonry was further studied when testing the opinions of respondents about the willingness to use the material in Figure 4. Majority of the respondents (83.4%) were willing to use the products. Only 6.6% claimed that they were not willing to use this material for construction. These respondents were unwilling to use the material based on the fact that the material is not popularly used in the building industry in Nigeria. Few developers and indiv idual ho me owners in the study area exhibited the use of the material. Ten percent of the respondents were undecided on the choice of masonry they could use for house construction. The decision of this group may be affected positively toward the use of interlocking blocks as the material beco mes more popular in the building market. Industrial production of the product can also facilitate reduction of cost and its acceptability.

Chi -Square Test for Significance of ifferences Between Materials Price Rating and Acceptability of Interlocking Masonry
Chi-square model was used to test for association between the materials' p rice rating and acceptability of inter-locking masonry by the respondents. The respondents' opinions on materials price rating were found to depend on their acceptability of interlocking masonry.
Based on the result in Table 2, there is association between acceptability of interlocking masonry and price of materials. The c2 results shown in the table indicated a significant level (P≤0.05) fo r the variables used to assess acceptability and material price rat ing. This implies that the high acceptability preference shown by respondents for the material was dependent on the cost efficiency of the material over the conventional sandcrete blocks used for housing in the study area. Added to this, could be the other advantages of interlocking blocks discussed earlier. Majority of the respondents would want to use SIB as a better alternative to the conventional blocks.
Where: V E = Very Expensive; E = Expensive; M C = Moderately Cheap; Cheap; V C = Very Cheap; NA = Not Acceptable; Rarely Acceptable; Moderately Acceptable; Acceptable; Very Acceptable.
Since the c2 result showed a significant level of association between material p rice rat ing and acceptability of interlocking blocks based on comparative cheaper cost of the material in the locality of respondents, the material is therefore reco mmended for use as a better alternative to conventional blocks in housing delivery in Nigeria.

Conclusions
The goal of this paper is sustainable housing provision through the adaption of interlocking blocks for housing delivery in Nigeria. This paper examines factors for the preference of interlocking masonry over the conventional type in construction of housing in Nigeria. Though the survey covered 4 out of 6 geo-political zones in Nigeria, the respondents' opinion on the variables investigated did not show significant differences fro m one location to the other but in the willingness of respondents to use interlocking masonry as a better alternative to conventional masonry. This is predicated on the cost-efficiency, shorter period of setting, design flexibility, environmental friendliness and energy efficiency of the material. The field data obtained fro m four different locations were analysed, wh ich showed 83% respondents' preference for the use of interlocking masonry as against the use of the conventional type. The analysis of measure of association and their significance of interlocking-blocks masonry based on variables such as material price rat ing and acceptability / willingness of respondents indicated results of significance level at (P ≤ 0.05) of the association of variab les measured. Thus, the result of the research has strongly indicated that interlocking blocks are preferred as better alternatives to conventional sandcrete blocks and should therefore be used as replacement for the former in housing delivery in Nigeria. This will facilitate cost efficiency and make housing provision sustainable.