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s u r f a c e E n g i n e e r i n g ( R a m L o c h a n T i l e s )

SThe Surface Engineered Block (SEB), with a thin pre-cast tile, can be manufactured in numerous sizes and used in conjunction with diverse materials. It combines a variety of finishes in a manner that brings out the best of the Artist, Architect, Engineer and Artisan - a true reflection of the spirit of Architecture.

The SEB technology is based on a simple rationale; A technology is appropriate, in the true sense of the word, only when it can be adopted for a variety of contexts and can use the local resources to advantage. The SEB was developed after certain aspects of the brick were rationalized and deconstructed as follows:

Soil bearing capacity- 2kg. Width of foundation- approx. 5 x thickness of wall (cm) @ outsetStrength required at base of wall- 10kg/sq.cm.Safety factor is 3 x strength required- 30kg/sq.cm.Brick rating- 100-150kg/sq.cm. Uniformly distributed load over a 9" wall- 1/3kg/sq.cm/floor.

As can be seen, there is a significant disparity between the performance required of the brick and the rate of compressive strength of the brick. Why is brick rated at 100-150kg/sq.cm? The reason for this is to ensure non-erodibility of the exterior surface not because of load bearing reasons. The SEB meets the criteria for walling material by rationalizing load bearing strength, having non-erodible exterior surfaces and adding aesthetic value to a building.

The SEB technology uses local resources, it is adaptable to a variety of contexts, and meets actual structural and environmental requirements, and therefore is an appropriate technology. By understanding this rationale behind the SEB, anyone can adapt this system to different context specific conditions.

The outer surface of the SEB is a thin tile that can be made with different textures, colours, and materials. The tile is made with a 1:2 mix of cement and marble or stone chips that is cast into a simple mould and de-moulded after about half an hour. (Fig. 1) After 48 hours curing the tile is ready to be used. The tile forms the outer impermeable and permanent surface of a SEB. The composition of the tiles backup block can range from mud, lean concrete etc., to a mixture of flyash, gypsum and lime. A simple mould, (Fig. 2), is used to cast the block. The wedge in the tile ensures bonding between the tile and the backup material to make a pre-finished walling block. The SEB can be used on site after de-moulding and 48 hours curing.

Tile Sizes

The size of the tile can vary but must be durable so that it does not break while being transported or during handling by the mason. The size generally varies from 4.5"(115mm) x 9"(230mm), to 7.5"(190mm) x 15"(380mm), although smaller or larger tiles can also be made. As the size of the tile increases the mixture has to be adequately compacted and uniformly distributed. For this purpose, use of a hand-held vibrator or a table vibrator becomes imperative. Corresponding to the increase in size of the tile, its thickness will also increase.

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s The thickness of the tile has to be sufficient to prevent warping and ensure non-breakage during handling. The tile thickness can vary from ¼"(6mm) to ½"(12mm) depending on the size of the tile and the size of the chips being used. For example, a 4.5"(115mm) x 9"(230mm) tile will require a maximum thickness of ¼"(6mm), whereas a 7.5"(190mm) x 15"(380mm) tile would require a thickness anywhere between 1/3"-2/5"(8-10mm). The size of the chips used in the mixture will also be a determining factor in the thickness of the tile. The size of the chips depends on local availability and the kind of aesthetic one wants to develop. Therefore, when locally available chips are large it is more economical to make thicker tiles, rather than spend excessive resources trying to import chips of just the right size.

The width of the wedge should be wide enough for the mixture of cement and chips, to be poured into the mould. The size of the chips being used will be a determining factor in this. The wedge should be durable so that it does not break during handling or transportation. Its depth should be sufficient to ensure a good bond with the backup block material. For instance, if the tile is being used with a Pre-cast hollow-core block then its depth should be less than the thickness of the walls of the block.

The SEB was designed keeping in mind the paradigm that a technology should address engineering principles and simultaneously be aesthetically pleasing and imbibe local talent. Therefore, the SEB can accommodate a variety of finishes that can be integrated while the tile is being cast. The mix of chips and cement, in itself, offers a variety of alternatives. For example, granite or marble chips can be used, or different colour pigments can be added in combination with different coloured and various sized chips (Fig. 4). Another technique involves fixing pieces of glass, bangles, PVC chips, or waste coloured stones etc. on a piece of paper in a pattern. The paper is placed at the bottom of the mould after the mix has been poured in. After the initial setting time, the pieces stick to the tile and the paper can be gently washed off using a wire brush.

The Tile Mould

The simplest and cheapest mould can be made in wood. It is useful when a limited number of tiles have to be made since initial capital investment is very low. Upon excessive use, due to constant contact with moisture, wood tends to warp and certain amount of dimensional error occurs. A more durable alternative for the wooden mould is the steel mould. (Fig. 5) It is resistant to water and can be used for a much larger number of applications. However, due to its tendency to rust care has to be taken to keep it clean and moisture free. If maintained properly a steel mould can be used for a very large number of applications.

Another alternative is a Fiberglass mould. It is non-erodible, noncorrosive and resistant to dimensional variations while being very durable and lightweight as well. It can, further, be used for a large number of applications without extra maintenance. However, to make the mould cement resistant, only isothelic resin should be used while making the mould.

Procedure For Making Tiles

Material Required:

a) Mould either of wood, steel or fiberglass.

b) Red mud PVC sheet or a smooth plastic sheet upon which the tile can be de-moulded. c) Strips of plastic to place between the mould and the wet mix to prevent adhesion between the two.

d) Fine aggregate of required size and variety.

e) 43-Grade cement. (Note: 53-grade cement may be used to reduce the setting time of the tiles. In this case the tile can be lifted in 24 Hours.)

f) A hand full of Marble dust is added to the mix to allow for intermediate sized particles between cement and chips to increase the bonding. It helps reduce the amount of cement required.

G) Lime powder or marble dust is put on top of the finished tile, before it is de-moulded, to quicken the initial setting time, by absorbing excess water. Any such material can be used such that it absorbs water and does not stain the tile.

Making the Steel Mould:

The mould frame is made by welding together steel sections to attain a box square section of size 1"(25mm) x 2"(50mm). A steel plate, 1/8"(3mm) thick is welded to the frame 1/2"(12mm) from the base. This is done for easy handling of the filled mould by the mason while up-turning it to release the tile. The holes in the plate are for the mason to push the tile out of the mould by using his thumbs. Steel bars of 1/4"(6mm) diameter are turned and welded to the frame as handles.

The size of the mould is determined by the size of the finished block required. For a block of size 9"(230mm) x 9"(230mm) x 4.5"(12mm) material required will include:

a. 2 planks of size 16.5"(412mm) x 4.5"(12mm) x 1"(25mm) b. 2 planks of size 10"(250mm) x 4.5"(12mm) x 1"(25mm) c. 2 mild steel bars

Casting the tile:

The mould is placed in the manner shown in (Fig. 6). A polythene strip is placed to prevent the wet slurry from leaking out of the holes. The fillers are placed along the sides with a gap equal to the wedge of the tile in the centre. Two polythene strips cut to the size of the fillers are placed on top of the fillers to prevent adhesion between the mould and the cement mix for the tile. A 1:2 mix of cement and fine aggregate is put in the mould filling the gap, firmly tapped into place and finished with a

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trowel. The top is finished with lime powder / marble dust and allowed to set for 15-30 min. (Note: 1:1 mix of cement and chips is used for a perfect finish or in saline regions with heavy rainfall like Bombay.)

After the initial setting time, the lime powder is removed. It can be collected, dried and reused. The tile, while it is still in the mould is gently cleaned using a brush to remove the excess cement and expose the grain of the chips used in the tile. The Tile is now ready to be de-moulded.The mould is upturned as shown in (Fig. 7), with the mason's fingers holding the tile in place and his thumbs positioned over the holes in the base plate. The mason gently pushes the tile out of the mould along with the fillers onto a smooth surface. The fillers are slid out of the tile and the plastic strips removed. The tiles can be picked up and stacked the same evening or, in winters, the next day.

To streamline the manufacture process, an assembly line system can be adopted on site to make the tiles and blocks. This allows for an economical and time-saving production. With the help of 3 masons and 5 helpers, almost 200 - 250 tiled blocks can be made per day.

Making The Block:

The block mould is assembled as shown in (Fig. 8). To make a mud block, with one finished side, cement slurry (1:2) is applied on the back of the tile and then placed on the side of the mould. A lean back up material, a mixture of mud with 2% cement, is layered into the mould and compacted using an ordinary hand ram. To make a concrete block, lean concrete of 1:3:6 mix can also be used, in which case 40% in-fill of broken bricks can be added. To make a concrete block the tile is placed on the side of the mould and a vibrator is used to evenly distribute the mixture in the mould. Placing two or more tiles at 90 degrees angle in the sides of the mould, corner blocks can be made for windows, doors, etc.

SAMPLE CALUCLATION (Oct. 1999)-materials required for 100 tiles of size 7.5"(190mm) x 7.5"(190mm) x 3/8"(10mm):Cement 1.2 bags, @Rs.140/bag = Rs.168/- Marble chips 6.6 bags (@23 kg./ bag)@Rs. 30 /bag = Rs.198/- Marble dust 1/2 bags(@20 kg./ bag) @Rs. 20 /bag = Rs. 10/- Mason 1 nos. @Rs. 130 /day = Rs.130/-Labourers 2 nos. @Rs. 80/day = Rs.160/- TOTAL COST OF HUNDRED TILES = Rs. 666/-Cost per tile = Rs.7

THUMB RULES OF SIZE 7.5"(190mm) x 7.5"(190mm) x 3/8"(10mm) WITH WEDGE OF SIZE 7.5"(190mm) x 1"(25mm) x 1.5"(37.5mm):

1 bag of cement gives approx. 80 tiles 1 bag of marble chips (23 -25 Kg) gives approx. 15 tiles 1 kg. of dust is required for 10 tiles 2 masons and 5 labourers can make about 250 tiles a day with 4 moulds.

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