MATERIALS
Problems:
Size: While these tiles are ideal for the first layer (since they are small, thin and therefore light), they are too thin and small for the other layers. Smaller tiles means more time and work into sticking them together. And more number of layers to achieve the required thickness.
Shape: Since tiles are rectangular (1:3), when placed in 45deg, there is a lot of wastage and odd shaped residues.
CLAY TILES
Size : 75mm x 230mm x 12mm
Compressive Strength : 5MPa
Weight : 0.387 kg/unit
Density of material :
Price : Rs. 5/unit (shop price)
PLASTER OF PARIS
PoP to water ratio :
1: 0.5 (slightly viscous mix: paste-like) for ends where tiles rest on supports
1: 0.7-0.8 between tiles (runny mix)
Initial setting time : 10 seconds once tiles are held in place (tile will cantilever from previous tile without any support)
Setting time : 20 minutes (for PoP to gain strength)
CEMENT PLASTER/ MORTAR
Plaster layer: 1:2 cement: sand
Mortar between tiles: pure cement mortar
In the usual masonry construction with brick or stone, the mortar's role is to keep the position of the blocks till the keystone is inserted. Once the masonry achieves equilibrium, the mortar is not even taken into account for any compression strength calculation and has a negligible role to play from then on.
Unlike other masonry construction. the cement mortar here plays a significant role and needs to be taken into account for its binding strength. This membrane structure works on the principle of interlocking layers while staggering the joints and the key holding them together is the mortar.
EXPLORING WORKING WITH PoP
It took a few attempts to get a workable mix of PoP. We started out with more dry mixes which set too quickly and lost workability after each or every other tile. We were also mixing large quantities in each batch and most of the mix ended up waste.
However, with practice, we came to the optimum mix and also managed to increase the number of tiles laid per PoP batch from 1/batch to 10/batch.
The first layer requires 3 sets of hands- 1 that mixes PoP, cleans the container for the fresh batch and takes care of other supplies; 1 that applies the mix on the tiles and checks the shape with the guides and 1 that sticks the tiles and controls the slope (while the second directs) and holds the tile till the initial setting of the PoP (10 seconds).
Note that all adjustments in the laying of tiles has to be done in the first 3-4 seconds of laying as PoP sets really quickly. It is harder than seems considering that we’re building in air and have to use a keen eye to ensure the shape of the vault with the help of the guide strings.
Laying tiles in PoP mortar
Note how each tile takes approximately 30 seconds from applying mortar to fixing and adjusting in place to initial setting of PoP.
Laying tiles in PoP mortar (edge)
COSTING
The total cost of construction according to our experiments was Rs 3660 per sq m or Rs 340 per sq ft