Cost-Effective Housing Technology with Carbon Neutral Blocks

Conventional materials are considered to be major contributors of Green House Gas (GHG) emissions. New cost effective technologies for housing is, therefore, a need of the hour. To make mass housing cost effective in shortest possible time, each of the housing components has to be optimized for cost and production time. Disposal of fly ash is posing a great threat for its disposal problem. There is a dire necessity for alternate materials replacing conventional costly material with cost effective and environmental friendly materials making them energy efficient. This paper presents the technology and advantages of construction of cost effective and energy efficient construction using the developed Carbon Neutral Soil-Cement Fly Ash (SCF) blocks. Construction methods adopted were Arch foundation, Rat-Trap bond using SCF blocks for masonry, precast SCF block Lintel elements and partially precast SCF Slab Panels for flooring/roofing. Overall Cost to the extent of 42% can be saved using this cost effective construction technology over the conventional method. Energy of the buildingcan be drastically brought down from. 36.39 GJ (4.2T of CO2) for conventional construction to 9.51 GJ (1.33T of CO2) for cost effective construction. Energy to the extent of 74% can be saved using this Technology.


Introduction
Housing shortage is the major problem faced by the developing countries. Migration of rural people to urban areas has increased enormously and compelled them to live in unauthorized accommodation and slums. Nearly 38 million rural housing units are required for additional population during next decade [1]. Development of soilcement-fly ash (SCF) blocks and all relevant building components using SCF blocks is an effective solution to meet the challenge of providing cost effective mass housing. Soil is the most abundant and cheaply available low enrgy material traditionally used for construction of houses. It is also known fact, that traditional bricks are manufactured from the locally available suitable soil followed by baking (burning) to develop the desired strength. While on one hand burning of bricks causes air pollution, on the other hand, millions of tones of unutilized fly ash produced by industries pose great threat to the environment. Thus, there is a need to overcome these environmental related challenges while meeting the demand for mass housing.

Need for the Present Work
Adequate Shelter is a basic human need, yet most of the urban population in developing countries still lives in spontaneous settlements as they cannot afford the high cost of building materials. There is lagging in Mass housing programmes due to high costs of conventional building materials. They are also high energy intensive and are becoming scarce giving rise to threats to mass housing. Disposal of waste materials of the industries is becoming a major problem. Hence the need of the hour is to replace conventional construction materials by alternate energy efficient materials with emphasis on alternate technologies. To meet the unprecedented demand for mass housing there is a huge requirement of building blocks. Burnt clay bricks pose threat to environment due to emission of carbon dioxide due to burning of bricks. It is estimated that 24% of green house gas (GHG) emissions [1] is contributed by the construction industry in India. Also, millions of tons of unutilized fly ash produced by industries pose great threat to environment. Thus, the need of the hour is to overcome these environmental related challenges by developing bricks/building blocks which are produced by using local soil, cement, fly ash and water curing without any baking or burning. The present paper disseminates developed SCF blocks which are in turn utilized in casting of cost-effective building components with an emphasis on optimizing construction time.

Carbon Neutral Soil-Cement-Fly Ash (SCF) Blocks
New cost effective and energy efficient technologies for housing are essential to cope up with the ever increasing demand of housing and reduction in carbon dioxide emissions. Cement being the main component of construction activity, construction industry is considered to be the major contributor of Green House Gas emissions. Hence a block had been developed to overcome the above mentioned drawbacks and resulted in the development of Soil-Cement-Fly ash (SCF) blocks which are cost effective and energy efficient. This sustained research work has resulted in to a product with strength and durability, besides making it carbon neutral.

Method of Manufacture of SCF Blocks
The process of making these blocks are as under: Any locally available soil, other than black cotton soil, can be used for the production of masonry blocks with suitable modification in grading of soil to derive maximum density. Ideally, the particle size distribution in a soil sample should match the Fuller curve [2]. Hence, the soil proposed to be used in production of blocks was made up for the deficiencies in particle sizes so as to bring it close to the Fuller curve requirement. 50 to 85% of the locally available soil is reconstituted by blending and mixed with 1 to 5% cement and 5 to 50% of fly ash. Blending of different soils with fly ash was done to achieve the grading of blended soil which closely matches the Fuller's curve of minimum voids. Optimum fly ash content to be added in the mix was determined from experimental work. The method of blending was computerized and adopted to arrive at the proportions of natural soils/manufactured fine aggregate and fly ash to be blended. Required quantity of SCF mixture was filled in the mould in three layers and the whole assembly of the mould was placed on the machine and was operated to effect the vibrocompaction to get maximum density by a vibrocompacting block making machine. The mould assembly was demounted and the SCF blocks were demoulded. Standard SCF block specimens of size 215x100x75mm were prepared and cured. The SCF blocks were then subjected to compressive strength test and durability test. The strength and Durability Characteristics of SCF Blocks are given in Table 1. There is significant increase in the ratio of wet to dry strength of SCF blocks and reduction in loss of their weight on wetting and drying test with increase in fly ash content, thus reflecting the improved durability of blocks. This may be attributed to addition of fly ash which fills in micro pores and proper grading of blended soil resulting in increased density followed by strength and durability [3,4]. There was extensive studies carried out on energy values of SCF blocks. SCF blocks so developed are carbon neutral (viz. the energy value is zero) with addition of fly ash and cement, whereas the energy content of burnt clay brick is 4.35 MJ per brick. The cost of manufacture of each SCF block is worked out at Rs.4-00 and details are shown in Table 2. SCF bricks are 63% cheaper than conventional burnt clay bricks at prevailing market rate.

Embodied energy of SCF blocks
Embodied energy content of SCF blocks dependent upon cement content and fly ash addition. The embodied energy due to addition of cement is 862.5MJ. In addition 21 MJ of energy is required for operating block making machine having 1 H.P. Transport of the soil to the casting yard consumes 67.5 MJ. Fly ash addition plays a dual role viz., becomes cementitious in the presence of excess lime and has energy saving value of 6.17 MJ per kg. Fly ash to the extent of five percent replaces cement and remaining five percent is taken as otherwise land filled. The energy values of fly ash replacing cement and land filling are -5.966 MJ/kg and 0.617 MJ/kg respectively as specified by EPA [5]. Considering the energy values due to energy intensive cement and energy savings associated with fly ash addition, the net energy value of the block is -0.036 MJ or -0.011 MJ/kg. Since there are no emissions of GHG with SCF block, the block is regarded as Carbon Neutral [6].

Abatement of CO2
If SCF blocks are replaced by burnt clay bricks to the extent 100 billion bricks, the CO2 emission reductions would be 43.12 million Tons. In addition, the reduction in GHG emissions would be 7.12 million Tons due to better surface of wall requiring no plastering and thus resulting in further saving of cement.

Arch Foundation
This foundation was constructed for the research work taking into consideration economy and energy efficiency (low carbon material). Arch foundations are adopted for soils of low bearing capacity with semi-circular or segmental curves so that the ends of arches rest on side abutments and they in turn supported on deeper hard stratum. Since these foundations are cheaper than conventional construction they are best suited for costeffective housing in hard stratum at shallow depths.

Soil Cement Fly Ash (SCF) Mix Foundation
In case the SCF mix which is used for in situ work such as in foundation, there are no energy values associated with transportation and also no casting work takes place. On incorporating above two values, the net energy value of SCF mix would be -0.039 MJ/kg. Since there are no emissions of GHG with SCF mix, the mix also is regarded as Carbon Neutral.

SCF Mix Foundation Construction
This foundation is an alternative to cost-effective arch foundation. This type of foundation is similar to spread foundation, except that the excavated trench is filled and rammed with SCF mix. The soil which is excavated from the trench foundation is sieved and then measured at the same time on the side of the trench. Blending is resorted to as discussed in section 3.1.The mix should be calculated for 1 bag of cement per mix. A team of workers mix and ram manually or mechanically. Usually the top level of the foundation is at the level of the original ground. The section of the foundation should normally be square. Two-floor building section of foundation should be 75 cm x 75 cm. This mix is not only Carbon Neutral but also cost-effective.

Rat-trap Bond Masonry
In this bond of brick work, bricks are laid on edge with alternate rowlocks (brick on edge showing its breadth and height) and shiners (brick on edge showing its length and height) leaving 65 mm gap (215-2x75=65 mm) in between bricks. In this bond each alternate course begins with two rowlocks followed by a shiner.

Fig 3 Plans of Odd & Even Courses in Rat-Trap Bond
The intermediate course begins with a shiner succeeded by a rowlock as shown in Fig. 3. Overall wall thickness of 215 can be maintained with around 21% cavities. Economy to the extent of 21% in bricks and 25% in cement mortar can be achieved (lesser number of joints).
This bond is simple to implement and can be employed for fast track construction and has thermal and acoustical comfort due to internal cavity. Plastering can be avoided since SCF blocks have sharp and straight edges that enhance beauty of the building. Details of construction of rat-trap bond masonry is shown Fig. 4.

Quetta bond
This bond is adopted when it is necessary to provide vertical reinforcement in walls for areas subjected to earth tremors. Minimum thickness of wall in this case is one and half brick. With the adoption of this bond, quarter brick by half brick pockets, which are continuous through full height of the wall, are formed along the length of the wall. In these pockets steel rods are placed, and pockets are filled up with cement concrete as the work proceeds.

Vertical reinforcement bars
Bureau of Indian Standards (BIE) suggests provision of vertical reinforcement at corners of walls, junctions of walls, jambs of windows and jambs of doors. Code specifies use of Quetta bond for vertical reinforcement in walls for areas subjected to earth tremors. Since Quetta bond is employed for one and half thick brick wall , for one brick thick wall the Rattrap bond is best suited to provide vertical reinforcement in cavities (65 mm x 65 mm) of walls for areas subjected to earthquake.

Embodied Energy of Conventional Masonry Vis-À-Vis SCF Block Masonry
The embodied energy of burnt clay brick masonry is 2573.2 MJ/m 3 whereas it is 193.1 MJ/ m 3 with SCF block masonry. Embodied Energy of SCF block masonry drastically reduced due to Carbon Neutral block and is only 8% of the burnt clay brick masonry [7].

Precast SCF Lintel
As soon as the SCF block masonry construction reaches lintel level, precast lintels will be placed over the door and window openings and masonry work is further continued above the lintel level. Construction features of SCF lintels are similar to that of partially precast SCF Slab Panels.

Partially Precast SCF Slab Panels for Floor/Roof
For casting purpose a wooden mould was prepared using wooden pieces of cross section dimensions of 75 mm x 50 mm to prepare the mould of internal dimensions of 1805 mm x 350 mm x 75 mm. The wooden mould with an opening at the base was kept on a leveled ground having thin layer of sand. The bricks were wetted and arranged in the mould with a 50 mm gap between them. Steel reinforcement cage (8 mm diameter main reinforcement and 6 mm diameter distribution reinforcement) was inserted in the gap of assembly of bricks with a provision of 15 mm cover. M20 grade concrete was then poured in the gaps and finished flush with the bricks. After demoulding, the slab panel was cured for 14 days in water and 14 days in air. The required number of slab panels were made ready. Figure 9 shows arrangement of blocks and reinforcement in a slab panel. The required number of slab panels of dimensions 1805 mm x 350 mm x 75 mm were produced for construction of a assembled slab system over a room  Fig. 7. R.C Joist Supported over Bearing Walls

Partially precast RC Beam
A partially precast beam was cast to reduce span of SCF slab panel. A mild steel mould of 150 mm x 150 mm cross sectional dimensions and length equal to 3m (room width) was used for moulding a partially precast reinforced cement concrete joist. After placing the mould on a levelled surface, the reinforcement cage was placed in the mould with a provision of 25 mm cover. Steel reinforcement provided for the partially precast joist was designed as per the procedure laid down in IS 14142 -1994 [8]. The concrete was poured in the mould and compacted properly. After demoulding, the joist was cured for 14 days in water and for 14 days in air prior to placing it over SCF block masonry wall for assembly of slab panels as shown in fig.7.

Arrangement of SCF slab panels over the RC Joist and wall
The partially precast joist (beam) was placed over SCF block wall with a 15 mm thick cement-coarse sand (1:4) mortar beneath it. The alignment/ placement of joist was checked and then propped at one third span locations. The brick panels with overall size of 1805 mm x 350 mm x 75 mm , shown in Fig. 8, were placed over the joists/wall, side by side after laying a 6 mm thick layer of cement coarse sand (1:4) mortar over the joists/walls. Fig. 9. The gaps between the panels were filled up with rich mortar. Shrinkage and temperature reinforcement of 10 g x 10 g -100 mm x 100 m m welded wire mesh was placed over the panels. A 35 mm thick screed concrete layer was poured on the top of the assembled slab panels. The roof was finished with a floating coat of 1:3 cement fine sand mortar of 6 mm thickness just after laying the in-situ concrete and cured for two weeks. The prefabricated SCF brick slab panel was designed as simply supported between a joist and a wall as per IS: 14142-1994 [8].

Arrangement of brick panels over masonry wall and precast joist (beam) is shown in
Extensive deflection recovery test was carried out by K. Ravande and N. Sudom [9] as specified in clause 17.6.3 of IS: 456-2000 [10] on the entire assembly of SCF slab panels and joist provided over a room of dimensions 3mx3m.

Embodied Energy of Conventional RCC Slab and SCF Slab Panels
The embodied energy of conventional RCC (Reinforced Cement Concrete) slab is worked out to be 272.32MJ/m3 whereas the embodied energy with SCF block slab panel [7] is 2539.47MJ/ m3. The energy content of SCF slab is 59% of the energy content of the RCC slab.

Cost Analysis
Cost comparison of the Cost-effective housing using CNB vis-a-vis the conventional construction is given in Table 3 and is evident that there is a saving of 42% by adopting CNB with partial prefabrication. It is evident from

Embodied Energy of Conventional Load Bearing and SCF Load Bearing Construction
The embodied energy of conventional brick load bearing with RCC slab is worked out to be 36.39 GJ for a plinth area of 16.6 m 2 whereas the embodied energy with SCF block load bearing using SCF slab panels is 9.51 GJ.
Energy to the extent of 74% can be saved using SCF Block Technology in the construction of a house. Carbon dioxide emissions can be brought down from 4.2 T for a conventional construction to 1.33T for cost effective house using SCF Block Housing Technology [7].

Saving in Cement Compared to Conventional Construction
In mass housing programme with a plinth area of 16.6 m 2 , the saving in cement with SCF blocks construction would be around 1150 kg compared to conventional construction. At national level considering shortage of housing in rural and urban areas at 20 million housing units the saving in cement would be 23 million Tons. The benefit arising out of saving in cement is Rs.115 billion ($1.65 billion). The saving in cost due to GHG emission reduction in terms of Carbon Credits is Rs.32 Billion ($ 460 million).

Time of Construction
With partial prefabrication and using precast units for DPC, Sill and Lintel Level the time of construction can be drastically reduced. With proper scheduling, SCF Block residence can be completed within 30 days.

Climate Change Mitigation and Sustain -able Development
GHG emission is the reason for global warming. Burnt clay bricks pose threat to environment due to emission of GHG due to burning of bricks. While construction industry cannot do away with the bricks or building blocks for meeting the housing demand, the concern for the environment, particularly embodied energy associated with the bricks and its reduction is the need of the hour. The alternate construction technologies play a vital role not only in cost reduction but also in the reduction of GHG emissions [7] and help in protection of environment which lead to sustainable development. The construction of houses using SCF block and housing elements would mitigate climate change considerably.  Provision is made for Electrical, water supply and sanitary, painting and miscellaneous @22% on cost of conventional construction