Cob has been used in building shelters from ancient times. It is one of the earliest and most sustainable building technique discovered. Advent of cob at Sacred Groves happened when we couldn’t afford skilled masons and materials for earth-crete any more because of shortage of funds. Cob was chosen over other materials because it did not require high skilled labor and any volunteer who joined the project could learn to do it with a very minimal training.
WHAT IS COB?
• The literal meaning of cob is “A lump” or “Rounded mass”.
• Cob is a mixture of subsoil, sand (optional),fiber while adding water to bind them effectively.
• Sub soil is the soil taken from at least 2ft below the ground. The top layer should not be taken for cob because it is rich in nutrition and is meant for agriculture. And the soil that should be used for construction should ideally be devoid of any organic substance.
• From subsoil we get sand, silt and clay. Sand (optional) for the compression in the building and clay acts as binding agent.
• Fiber such as coconut fiber and rice straws is for the tension in the wall and is used to bind and hold the structure together.
At site initially we used both Coconut Fibers for micro tension and rice straws for macro tension in cob mixbut, coconut fibers attract termites. And in time with experience we found out that coconut fiber is not necessary. So, instead of that on site we started using only rice straws. Rice straw also provides termite resistance because of presence of high silica content.
In humid conditions cob can lose its strength because of bacterial growth. So turmeric was added when required. Ideally 10%-20% of clay is required in the Cob mix, but if the clay content is more sand (optional) is required to balance the ratio.
To check the quality of the sub soil and to arrive at the proportion of sand (optional), soil and fiber to be mixed in the cob mix.
• Crunch test:
It is a very simple test that depends on the experience and intuition. This is done by making a ball of sub soil (moistened with water but before adding straw) and pressing it in the palm and judging the amount of sand (optional) and soil by the appearance of cracks. If there is excess of cracking it means that the sample has more sand.
• Drop ball test:
It is done by rolling a ball of moistened sub soil (before adding straw) in hand and dropping it from an elevation of one meter. The lower portion of the ball will go flat on the surface but it should not lose the spherical shape of the upper half. If there are cracks in the edge of the lower flattened side it means that the soil has more sand (optional). If the upper dome shape deforms then it means that the soil has more clay.
• Cigar test:
Cigar test is done to check for the tensile strength of cob. It is done after cob mix is prepared. The cob mix with fiber is shaped in the form of a cigar. It should be held at the two ends with fists and should be pulled apart. If it can’t be pulled apart into two pieces it means that the cob mix is made well. This test is sone as the final test of quality of cob.
HOW TO MAKE COB?
1. Dry mix the subsoil: Lay the tarp. Put sub soil and sand (optional) in their respective proportions. Flip the tarp as many times as required to make a homogenous mix. Sand is added only when the soil has more clay than the required amount.
2. Add some water: Make the dry mix into a heap and create a crater in the middle. It’s always better to add too little water than to add too much of it. There is not really a standard for how much water to add. You’ll figure out how much water to put in through experience. Remember not to add too much water though. It might be easier to mix, but it will not hold up as well when you start building and will slump sometimes. A wet mix of cob can also crack more as it dries. However, if you do add too much water you can either leave the batch to dry out in the sun, add more straw to soak up some excess water, or add more dry ingredients.
3. Mix with your feet: Push the outside of the dry material towards the center of the crater with your feet. Once the crater is covered with dry mix you can start stomping. To make sure all the dry materials are mixed together well we can also jog on top of it. According to convenience you can use your toes or heels and jog or stomp on it in your own style. To get a good mix flip the mix and repeat the stomping. Once it starts taking shape somewhat like a burrito the cob is almost done and it is ready to be mixed with straw. At this sttage do crunch test and drop ball test and proceed.
4. Add the straw mix and complete: Flatten the mix. Take a handful of straw and spread it. Stomp the mix again until all the straw gets covered. Use the tarp to gather the mx and turn it over again. Stomp some more to make it flat. Add more straw and repeat the process. There is no exact amount of straw to use. Keep repeating the process until all of the straw is thoroughly distributed and mixed into the cob. At this stage do cigar test to test the tensile strength of cob.
TECHNIQUES TRIED ON SITE :
We started with foot cob which we used to make in cob pits. But the cob production was very slow although the quality was very good. And the walls of the houses were going very slow so we had to look for other alternative as well.
Few volunteers suggested that we make the cob mix in JCB. We did that and the production was huge but after the wall dried we noticed huge structural cracks in the wall. We had to take down the walls because of it and start from the scratch.
Then we came up with the technique which we implemented for the construction of major portion of the walls which is mixing the subsoil, sand (optional)and water with the JCB, getting the mix ready after checking the Crunch Test and Drop Test. Then we take the mix out and spread it out on the tarp and mix straw with feet. Like this production is also fast and quality is also in control. We get the mix ready for one week and keep working with it throughout the week. One point to be noted with this technique is that we found more cracks than the manually prepared cob as it was wet cob but they were not structural cracks.
HOW TO MAKE COB WALL?
• Once the cob is ready after passing all the tests, we make cob bricks.
• We use rib and spine techniques to make the cob wall.
• Cob spine holds the cob ribs with itself that’s why merging of 2 bricks is necessary .We put construction rubble that is soaked in water (If rubble is not soaked in water then it absorbs water from cob) in between every 2 ribs and alternatively in each layer.
After putting cob ribs and spine we walk on them with spiked wooden shoes to merge all the bricks in a way that they are not individual bricks anymore but a single layer instead and then in the gap between every two ribs we put rubble. When we walk with spiked shoes on the wall the straw from the layer goes down and ties up with the straw in the lower layer which binds the structure together gives tension. We can also wear wooden thimbles to do the job. The binding of straws happens majorly along the spine. At the ribs the cob and rubble alternate and the cob of the top layer merges with the rubble in the lower layer. We merge the layers so that there are no joints anywhere and the wall is one homogeneous monolithic structure. We have no joints in the corner of the wall also but instead we have curves connecting different planes of walls.
We don’t need straight forms or rectilinear molds, cob lends itself to organic shapes: curved walls, arches and vaults. Building with cob is a sensory and aesthetic experience like sculpting with clay.
WHY IS COB WALL USED?
• Energy Efficient:U-value of earth is high and thus it is very effective in heat insulation.
• Inexpensive: easily accessible and cheap resources to acquire.
• Easy execution: No skilled labor is required.
• Healthy: Cob “breathes” through its tiny surface pores and keeps air fresh and clear.
• Strong and earthquake resistant: Straw is used in cob mix, acts like a natural re-bar to hold the whole structure together as one monolithic piece. This makes cob homes extremely solid and resistant to earthquakes.
• Durable: Cob homes have lasted for hundreds of years with minimal upkeep.
• Fire Proof : Cob does not catch fire.
• Acoustic Property:Earthen walls have very good acoustic properties and eliminate noises.
• Freedom of design: Cob is a technique where in different shapes and sculpturesque walls can be done.
CRACK FILLING WITH LIME:
Cracks are common in cob walls! Cracks are a part of cob walls but most of the cracks that occur are surface cracks and hair-line cracks that dont spread or widen as soil is a flexible material and there is plenty of fiber along with it to hold the soil together. Ideally cob walls be left to dry for a year. In the initial period of drying structural cracks may appear because of flaw in the techniques like making cob very soft and wet or not having a homogenous mix of earth and thus there can be formation of structural cracks. We have to take care of these cracks well so that it will cause dangers to the wall and thereby to the people who will live in it.
The cracks can be stitched using crack filling bricks made of lime. Lime has a self-healing property. It means that lime fills up the cracks… also the ones that can occur after the wall is complete. How does lime heal the cracks?
Although lime attains strength in a week the setting (carbonation) goes on for very long periods of time. It can go on for hundreds of years too! For example the lime mortar used in Taj Mahal is not yet set and is still getting stronger!
The chemical equation of carbonation or setting of lime with carbon-di-oxide:
Unlike cement lime should not be wet with water for it to set ell. Rather lime needs air to set. When lime is used as plaster it is in its active state (Calcium hydroxide). When it starts to set lime absorbs carbon dioxide from air and becomes calcium carbonate (lime stone). This process where in calcium hydroxide becomes calcium carbonate is called as carbonation or setting of lime or crystallization. When anything crystalizes it expands in volume. The same manner when lime sets slowly it crystallizes and expands in volume and fills into all the cracks that occur in the wall and it heals the wall whenever cracks are formed.
PROCESS OF CRACK FILLING:
1. Cutting box on the cracks
When structural cracks are found rectangular boxes of size 12″ X 6″ and a depth of 6″ are dug out from the wall in line with the crack. The box should be centered with the crack and a gap of 6″ should be left between each block craved out along the crack. In the same manner the boxes should be dug out on the other side of wall too so that the crack will be stitched from both sides.
2. Preparation of crack filling bricks: Bricks are made in the ratio where in 4 parts of sand are added to one part of lime mix and one part of rice husk. Lime mix is a mix of hydraulic lime and fermented kadukkai and jaggery water mixed in a chakki or by grinding manually.
All the three ingredients are mixed into a homogenous mix by stomping on the mix and then a layer of human hair is added for better tensile strength.
The bricks are made just like adobe bricks with lime as binder, sand as aggregate and rice husk and human hair as fibers. The mix is then put into brick molds of size 12″X6″ with a depth of 6″.
After taking the moulds out the bricks are dried. As the setting pace of lime is very slow the bricks take quite some time to be ready to be used. Thus it is ideal to prepare the crack filling bricks in advance so that they can be used as and when required.
3. Laying the crack filling bricks: Dust is taken out from the boxes carved out along the cracks.
The surface of the box should be moistened before putting the mortar. Then a layer of mortar is put in the cracks and the boxes. The mortar is made of the same mix that the crack filling bricks are made of. Then the bricks are laid in the boxes.