From Exp 2 it is clear that the arch with 3 layers of tiles is stable under self load (without centering support) and can take some live load. Now we shall try to build the vault with first layer in PoP mortar and succeeding layers in cement mortar to achieve the required thickness.
Aim:
1. Test the vault’s load bearing capacity in terms of
i) form
ii) thickness
iii) material strength
2. To understand and practice the technique of construction of timbrel vault
i) to develop systems for construction that can be used for efficiency
ii) to train masons and labor
Step 1:
Adding buttresses to the vertical support walls.
Reasons:
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Possible failure of Exp 2 due to failure of support walls in taking the arch thrust under loading. Loading in Exp 3 will be much higher than in Exp 2.
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To serve as an experimental replacement for RCC ring beam which has not been provided in this case.
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Support walls have no foundations. Buttresses will help preventing the lifting of walls under loading condition.
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We want to ensure that the arch fails NOT because the supports fail in bearing thrust. It should serve as a control so we can test, for real, the performance of the vault in terms of its form, thickness and material strength when it is loaded.
EXPERIMENT 3
Step 2:
Fixing Centering
Centering in 3/4” ply is fixed such that it can be dismantled without disturbing the vault (lesson from Exp 1). Centering serves the following purposes:
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To fix guide strings- the arch is drawn on wall at the other end (wall independent of structure. Corresponding points on the drawn arch and centering are linked with guide strings that help building the vault in the right shape.
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Support for first line of tiles. Tiles laid in header course with 20mm (out of 75mm tile width) resting on the centering and the remaining cantilevering.
Step 3:
Laying first tile layer in PoP
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First line of tiles is laid resting on the centering in header course with 20mm resting on the centering and the remaining cantilevering. Ends rest on the support walls at an angle.
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PoP mortar is used for fast setting with the same technique as in Exp 1 & 2. With more practice, our masons have become more efficient with the use of PoP- measuring the right quantity and consistency for each batch.
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The entire line is completed from support to support. 2nd, 3rd, 4th, 5th tile lines are then laid, cantilevering from preceding tile line and with staggering joints.
Laying tiles in PoP mortar
It is important to note here that each successive line is laid only after the preceding line is completed from support to support to form its own arch.
Once an arch is complete, it bears its own weight to the support walls and is, in effect, no longer cantilevering from the previous tile line. This prevents any unnecessary loading on the weak PoP joints and avoids cracking.
Guide strings are used between points marked on centering and corresponding points marked on the arch drawn on the wall to aid in obtaining the curve perfectly.
First 5 lines of tiles of the first course are laid. Note how only the first tile line is supported.
After 5 lines, we move to Layer 2 directly instead of completing Layer 1 because Layer 1 by itself is prone to breaking or being knocked off accidentally since the PoP joints are weak and brittle. Laying successive courses adds strength and stiffness to the vault.
Step 4:
Laying Layer 2 in cement mortar
Layer 2 is laid in a herringbone pattern (45 deg to previous tile line). This breaks maximum joints between the courses.
We were explained by structural engineer – Dilip Thummar that laying tiles in 45deg is better than parallel to the vault span. The primary forces in a vault transfer along the span, hence cracks occur perpendicular to the span along the joints (since they are the weak points). Laying tiles in 45deg breaks these weak lines more effectively.
Tiles are arranged at 45deg and residue pieces are cut to necessary shape and size.
Rich cement mortar (1:2) is used to lay tiles. Cement slurry is used to fill the thin joints in between.
Step 4:
Continue and complete Layer 1 in PoP and Layer 2 in CM
Back to Layer 1. Note the back and forth between Layers 1 and 2.
These 2 layers are completed this way before beginning Layer 3
It would have been ideal to also start Layer 3 while 1 and 2 are still being constructed. This adds strength and stability to the vault as it is being constructed and masons can stand on the vault to build the successive layers. However, in this case, our length is only 1.2 and we could build the vault from both sides. Also, since our tiles are much thinner, we decided to lay a plaster layer between the succeeding tile layers to add thickness. It was thus essential to finish each tile layer completely before starting the next layer.
Curing cement mortar to avoid shrinkage cracks is important. However since the vault is still weak, we did not want to overload it with water. To solve this dilemma, we used a sprayer to keep the cement joints hydrated.
We learnt that this is actually the most ideal way to cure mortar- neither too dry nor soaked in water, just moist enough to avoid cracks.
Valuable insights from structural engineer Dilip Thummar who helped us understand the importance of a tension tie rod between the support walls in terms of opposing thrust and for seismic performance of the vault (considering that we are in Seismic Zone 4)
Also, laying tiles at 45deg as opposed to along the vault span seems sensible since failure in such vaults occurs longitudinally and laying tiles at this angle will provide extra strength.
Step 5:
Plaster layer 12mm thick laid above layer 2
Since our tiles are only 10-11mm thick, we decided to add plaster layers between the tiles to achieve our required thickness of 75mm. Plaster in 1:3 cement: sand
Step 6:
Laying tile layers 3 and 4 with plaster layer in between
Layer 3 is laid over the plaster layer in a reverse 45deg angle to break the joints of Layer 2. Lines are marked on the plaster to stagger the 45deg lines from the previous layer.
Surface of plaster roughened for increased adhesion of the successive tile course.
Tile lines are marked on the plaster in the following way:
a. Set-square used at one corner to mark the 45 deg angle.
b. Aluminium line patta and trovel used to draw the first line.
c. Successive parallel lines drawn at tile widths.
d. Tiles are then placed to trace residue sizes that need to be cut
Laying the tile @45deg in pure cement mortar
Tiles are laid in pure cement mortar. With each tile, it is important to check that the thickness of the vault is uniform. Guide strings and makeshift thickness checkers are used.
While tiles laid in 45 deg angle is best in terms of strength and stiffness, it is practically very tedious as each tile at the end needs to be separately measured and cut. This is time consuming and amounts to increased tile wastage.
Using square tiles might be more useful.
Course 3 is completed.
Course 4 is laid in the same manner with 12mm thick plaster in between
Necessary thickness (75-80mm) is achieved
Removing Centering
Step 7:
Removing centering one day after completion
After Exp 2, we were almost sure that removing the centering would do no harm to the vault. The centering was dismantled without disturbing the structure.
No deflection in the vault seen after removal of centering.
The timbrel vault spanning in all its glory without any centering supports!
Initial Load Test- Ramp Walk!
First load test. Even with apprehensions, our first load test was done with a 50 kg person walking on the vault. Further testing after a period of minimum 21 days.
Deliberations:
a. Need for a tie rod at the base of the vault.
b. Need for transverse walls/ diaphragms. Multiple sources indicate their usefulness in terms of providing alternate paths for load transfer in case of asymmetrical loading.
c. Optimum thickness for the span/rise and loading and minimum number of layers of tiles required for stiffness of material.
d. How to obtain strength of masonry in this case and how does it affect stability of the vault
Experiment to check whether an arch can stand stable under self weight without centering.
Our first set of trial and errors, understanding working with PoP and its challenges. Learning how to build a vault in air using guides and no shuttering.
MATERIALS & COSTING
Specifications of materials used in the experiment with quantities and approximate costs to extrapolate the economics of such construction.