Lime Concrete Flat Slabs in Coastal Saurashtra, Gujarat

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Title

Lime Concrete Flat Slabs in Coastal Saurashtra, Gujarat

Summary

The flat roofs and floors are commonly found in traditional buildings in coastal Saurashtra region. The understructure for the roof is usually made out of timber and at times stone or steel, above which lime concrete is laid. The lime concrete is compacted through ‘Tippani Raas’ (traditional dance form performed by women with particular compaction tool along with traditional musicians playing Manjeera and Nagàrà). Thus, construction of the flat slab not only provides employment but also an occasion for social gathering. The construction mostly involved renewable materials like lime, sand and lime-stone aggregates, having very little impact on environment. There are a few variations found in the construction process of these slabs in different areas of Saurashtra, which are mainly based on ingredients of lime concrete as well as on the finishing materials. The high thermal mass of these slabs keep the inside areas cool. Today, the construction of lime concrete flat slabs has stopped in most parts of Saurashtra¹ due to lack of availability of wood and advent of new materials like cement etc. This has led to the lack of interest in the artisans working in construction industry and there is a lack of skilled artisans who can construct lime concrete flat slabs.

Location

Lime concrete flat slabs are found in all parts of Saurashtra, including its coastal areas.

Fig 1: Map Showing Location of Saurashtra

Case studies presented here are mainly from Veraval, Mangrole, Chorwad and Beyt Dwarka region.

Context

Saurashtra is a dry region and does not attract much rainfall (average annual rainfall of Saurashtra is 75 cms²). Thus, sloping roof is not necessary for this region and flat roofs can work. Flat roofs also have the advantage of being used for sleeping out in hot nights, as some parts of Saurashtra can get very hot reaching the highest temperatures of about 44° Celsius³ in summer. Flat slabs can be used as intermediate floors as well as roofs. Use of these roofs can also ensure the possibility of extending the house by adding additional floors if the support structure permits. These advantages might have made the flat slabs popular throughout the region.

Materials: Limestone is easily available in most parts of coastal Saurashtra and thus was commonly used in building construction. There used to be many lime kilns in Saurashtra to supply lime for local construction. Stones used as papadi were sourced from Ranavav near Porbandar.⁴ Teak wood was more easily available and was a common building material. Choice of additives like guggad, jaggery, methi etc was generally based on local availability and local construction norms in various parts of Saurashtra.

Risk of Disasters⁵: Most of the coastal Saurashtra region falls under seismic zones MSK VII (Moderate risk zone for earthquakes) and some parts including the northern coastline of Jamnagar district fall under MSK VIII (High risk zone for earthquakes) (Fig 2). These zones relates to Richter scale magnitude range of 6.0 to 6.9.⁶ The wind currents are strong, with the average velocity of 50 metres/second (180km/h) throughout the year making this region very high damage risk zone B for wind and cyclone hazards (Fig 3).

Fig 2: Gujarat Earthquake Hazard Map (Source: Vulnerability Atlas of India: Gujarat, 1997, BMTPC and MUD)
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Fig 3: Gujarat Wind and Cyclone Hazard Map (Source: Vulnerability Atlas of India: Gujarat, 1997, BMTPC and MUD)

Housing/ Settlement Typology

Traditional buildings with lime concrete floors and/or roofs are found in most part of Saurashtra. The building typology varies depending on local context and region. The lime concrete flat slabs are constructed over residential structures, commercial buildings and public as well as religious institutions. These are found in buildings usually ranging from one to three storeys with average floor height being approximately 10 ft. The lime concrete slabs mostly cover rectangular areas. As roofs, these are used alone or in combination with pitched roofs.

Knowledge System/ Innovation

Lime concrete flat slabs are generally supported by series of structural timber elements including main beams and joists. Apart from the space requirements, the span of the roof mainly depends on the length of the available timber structural members. In a variation of the lime concrete slab called 'pidiya-papadi', the lime stone or sand stone slabs of 2-3 inch thickness directly rest on joists. In newer constructions, concrete members or steel sections are used instead of the wooden ones for resting the stone slabs.

Lime concrete can offer an energy efficient option to the cement concrete. Less energy is required to produced lime concrete than cement and lime concrete/mortar reabsorbs carbon over its lifetime (if exposed to the air), offsetting some of the initial pollution.⁷ Moreover, lime has also potential to be re-used, which makes it a relatively eco-friendly material.

In the roofs found in some of the residential buildings and public institutions the timber joist is placed such that while pouring the lime concrete above the joist, it gets into the cavity created by wooden members, making the deeper lime concrete section just above the timber beam. This makes a composite section where lime at top acts as compression member while timber section at the bottom resists the tension⁸. Thus, the overall performance of the structure improves with lesser material⁹.

Disaster Resistance/ Structural System

In a windy or cyclone situation, there is a risk of uplift of the flat roof because of a negative pressure (suction) zone generated on the roof (Fig 4) due to a phenomena known as the Bernoulli principle¹⁰.

Building design in terms of size, shape, openings and enclosure will also have an influence on wind-uplift pressures. Positive internal pressure within the building can combine with negative external pressure to produce extraordinarily high uplift pressure. Uplift forces will also depend on the wind angle and slope. For a conventionally shaped building with rectangular plan, the worst wind direction is 45˚, diagonally across the building. The 45˚ angle produces strong vortices along the roof’s windward edges. Parapets help in reducing the negative pressure as well as put some additional weight on the roof.

Fig 4: Pressures on a Building with a Flat Roof

In a windy situation, weight of the roof will be an important factor in determining the safety and preventing the lifting of the flat roof. Flat roofs, if having thicker slabs are relatively heavy and thus can be stable in cyclones.¹¹ On the other hand, the heavy weight of the roof will be problematic during earthquake, since the centre of gravity will be higher making the building less stable.¹²

Anchoring of the roof with the vertical support elements plays a key role in determining the safety of the building with a flat roof.¹³

Climate response

Basic function of the roof is to provide shelter and protect from sun and rain. Flat roof with thicker slab ranging from a total thickness of x to x inches has high thermal mass which stores the heat for a longer time and thus keeps the inside areas cool.¹⁴ Saurashtra is a relatively dry region with annual rainfall of around 75 mm¹⁵, thus use of flat slab is not problematic during rains.

Construction Methodology

Six main processes involved:

1. Laying of the Under Structure
2. Slacking and Grinding of Lime
3. Preparation of Lime Concrete Mix
4. Laying of Lime Concrete
5. Compacting (Tippani) and Finishing
6. Curing

Laying of the under structure goes on simultaneously with the slacking of lime and preparation of concrete mix. Laying, compacting, finishing and curing of the lime concrete follow these processes .

1. Laying of the Under Structure

The under structure for the lime concrete roof is generally made out of timber members.

Timber Under Structure:

• The concrete slab is supported by a series of timber beams and joists.

• In case of Bhatia’s Dharmashala in Dwarka, the main transverse beam (18 x 15 cms. approx.) is laid first on the masonry wall, followed by longitudinal beam (14 x 14 cms. approx.) that runs between all the transverse beams, dividing the spans of the transverse beams. Longitudinal beams are not required where the spans are manageable with the transverse beams.

Fig 5: Transverse and Longitudinal Beams of Timber Under Structure

• The wooden members (14 x 7 cms. approx.), acting as a formwork for the lime concrete are placed on the masonry wall as well as on top of the beam, and lime is poured till the top of these members encasing concrete.

Fig 6: Members Encasing the Lime Concrete Placed on top of the Beams and Masonry Walls

• The joists (8 x 8 cms. approx.) are laid on the beam at a regular interval of one foot. In order to build a formwork for the concrete, vertical wooden boards of approximately 1-1.5 cm thickness are placed in between every joist. Dovetail joinery is used between the vertical boards and the joists (Fig 8). Additional strips of wood are nailed below the joists to conceal the joints (Fig 9).

Fig 7: Laying of the Joists and the Boards Between Joists	Fig 8: Dovetail Between Joists	Fig 9: Wooden Strip Under Joists

• At this point, the lime concrete is poured on the masonry wall till the level of the top of the joist (Fig 10).

Fig 10: Lime Concrete poured upto the top of the Joist Level

• The masonry wall is continued further and a series of wooden planks (1.5 x 24 x 120 cms. approx.) running perpendicular to the direction of the joists are nailed on the top of the joists covering the slab area (Fig 11).

Fig 11: Wooden Planks Nailed on Top of the Joists

• Before the lime concreting process starts, thin gaps between the wooden planks are sealed with wooden strips (Fig 12 and 13) for prevention of leakage during the concreting process as well as to achieve uniform appearance. These strips are approx 3-4 cm wide and 0.5-1 cm thick.

Fig 12: Bottom View Showing Wooden Strips Covering the Small Gaps Between Planks	Fig 13: Wooden Strips

Pidiya-Papadi :

A variation of the timber under structure is locally known as 'pidiya-papadi',¹⁶ where slabs of stones rest on the timber joists and concreting is done over these slabs.

• Once the walls are erected till desired height (which was approximately 10 ft in most cases), pidiyas (joists) were placed on the walls. The pidiyas (approx 4X6 inches) are laid over masonry. Number of pidiyas and the distance between them is decided based on the span of the papadi. These are levelled and lime concrete is laid upto the top level of pidiyas.

Fig 14: Laying of Pidiyas
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Fig 15: Lime Concrete upto Top of Pidiya Level

• Stone papadis of approximately 2-3 inch (at the max 4 inch) thickness are laid on pidiyas. These papadis are not levelled precisely since the level is achieved by adjusting the concrete layer over Papadis. Once the papadis are laid, the roof is ready for concreting.

Fig 16: Laying of Papadi on Pidiya

• In other variations of pidiya-papadi in places like x and y, we noticed the use of concrete or steel ‘I’-section joists instead of wooden joists under the lime concrete slab.

2. Slaking and Grinding of Lime

• Lime lumps¹⁷ are brought and stored in a pit on site and water is added to start the process of slacking. Slaking is the process of chemical combination of quick lime with required quantity of water, due to exothermic reaction, heat is evolved and the quick lime lumps are formed into powder. This lime is called ‘slaked lime’, or ‘hydrate of lime’, which is essentially Calcium Hydroxide. The water is added for 15 to 20 days as required.¹⁸ If correct proportions are used, the process of slaking of lime converts the lime pieces into powder form. This is then passed through a screen to eliminate unslaked lumps, as unslaked particles of lime in mortar or concrete may cause damage to the construction.

• The slaked lime was traditionally crushed in the channel of ghani¹⁹ (Fig 17 and 18) for further refinement. In a ghani, the lime was churned with water to achieve the desired consistency of lime paste or lime putty. Sometimes the lime was crushed manually. Ghani has been replaced by machines now.²⁰

• When the lime putty is ready, it is taken out of the ghani /machine and kept for 6 to 7 days to mature.

3. Mixing of Lime Mortar/Concrete

• Lime putty is mixed with sand, aggregates and other additives to create desired mixes for plastering, concreting and finishing.

• Below are the tables specifying the proportions of materials for various mixes.

Item	Lime	Sand	Additives
Plaster	2	2	More than average
Average lime mortar for concrete	1	2 - 3	Average
Good lime mortar for concrete	2	2
Lime slurry for floor finish	1	0.5 Fine sand	More than average

Fig 19: Table showing proportions of materials for mixing according to Artisan Manjhibhai Kachrabhai hambhaniya

Average quantity of additives according to Manjhibhai: In 4 tagaras of lime mortar, a pre-mix of 50-100 grams guggad, 100 grams jaggery, and 50 grams of methi in a litre of warm water is added.

Item	Lime	Sand	Aggregates
Lime mortar for concrete	10	3	2
Lime slurry (Neeru) for floor finish	5	2	0

Fig 20: Table showing proportions of materials for mixing according to Artisan Teedabhai Savjibhai, Dwarka²¹

Additives according to Teedabhai: No additives are added to the concrete or neeru mixes, but water mixed guggad and goond is sprinkled after the process of Tippani.

• Different additives were used as per different regions. There were also variations in terms of when exactly this mixture of additives is used in the process. Some used it in the concreting mix, some while tippani and others used it after the tippani was finished. There were also differences in terms of whether the mixture was sprinkled on the whole floor or just corners of the floor.

4. Laying of the Lime Concrete

• Laying of the lime concrete starts after the under structure properly laid and the mix of lime concrete is ready.

• Broken or rejected country tiles and other large aggregates are filled as a base layer in the formwork before pouring of the lime concrete mixture. These tiles increase insulation and heat absorption capacity of the slab.

• Lime concrete is either laid in two layers with the finishing or laid as one layer with the finishing on top.

• Where the concreting is done in two layers, the bottom layer of consists of bigger aggregates, usually pieces of locally available stones like Toda and sea shells. These are placed over wooden boards to create a base and lime slurry²² is poured on top to fill the voids. This layer is approximately 3 to 4.5 inches thick.²³

• After the first layer is settled²⁴, another layer of lime concrete is poured over it consisting of lime with finer aggregates like broken sea shells, corals and fine sand. This layer is 1.5 to 2 inches thick.

• Where the concreting is done as a single layer, a mix of sand, concrete and aggregates is poured on the layer of big aggregates and broken and rejected tiles. This layer is generally 4 inch thick.

• After the lime concrete is laid on timber structure or papadi, a basic levelling is done to remove excess concrete with the help of flat wooden tool called patti.

• The concrete surface and thickness of this finer layer are adjusted by the mason to achieve desired slope for easy run off of the surface water. As an example, if it is a roof slab of 2.5 by 2.5 mts, approximately 1 to 1.25 inch slope is provided. For intermediate floors the slope can be slightly less.

Fig 21: Lime Concreting	Fig 22: Visible concrete on top of the beam at the joist level

Fig 23: Lime Concrete over Papadi	Fig 24: Lime Concrete Floor with Papadi	Fig 25: Lime Concrete Floor with Papadi

5. Compacting (Tippani) and Finishing

• This is followed by process of compaction called ‘Tippani’ mostly performed by women while the concrete is still soft to ensure proper compacting. It began with women singing traditional devotional songs and dancing in tandem while compacting the floor with the rhythm of the song using special wooden tool called 'tippani'.(Fig 50 and 51). A similar tool called ‘Thappi’ was also used for compaction, where workers sitting a row compact the floor moving gradually. Tippani was performed for 3-4 days upto one week. It was performed till the tippani made no further impression on the concrete surface and rebounds on beating. In regions around Dwarka, tippani was done by women from all castes like Sathwara, Kharva (fishermen), Koli etc, while in x, tippani was mainly performed by women of Koli community.

• A diluted lime water mixed with jaggery, guggad and vegetative extracts (extract of methi boiled in water) is sprinkled on the surface while compacting the concrete which provides durability and strength to the concrete layer. Around Dwarka region, only guggad and goond (literally, gum) mixed in water were used.

• Once, the floor is sufficiently compacted, a layer of neeru (slurry) is used (with lime and sand in the proportions of 5:2) as floor finish. Some artisans also used guggad and jaggery along with lime slurry at this stage. To get a glazed finish, shankh-jeeru (powdered sea-shells) is used where a small bag (potali) of powder tied in fine cloth is used for beating the concrete surface. The top is polished/levelled with pattimalo.²⁵ The process of finishing is repeated as necessary till the desired finish is achieved.

6. Curing

The lime concrete slab is cured for 6 to 7 days?²⁶ or until it hardens by covering it with a thin layer of grass, straw or jute bags. This covering layer is kept moistened in order to ensure proper curing.

Variation in Methodology prevalent in Mangrol, Junagadh²⁷

This method is followed in the regions of Mangrol and Chorwad for the concreting after the papadis are laid.

After the papadis are laid, the gaps between the papadis are sealed with lime mortar (lime and sand proportion of 1:2). Bela stone dust is sprinkled on top of the papadi layer and then a layer of mixture of lime and Bela dust/sand is laid. This layer is 1.5 - 2 inches thick. Then a 5 - 6 inch thick layer of coarse stone aggregates is laid. Water is sprinkled on top and then lime powder is sprinkled. This is then crushed by sambelu. After the crushing process, guggad water (approx. 2 kgs of guggad boiled in a pot of water) is poured in the corners of the slab which helps in sealing the joints. Then a 2.5 - 3 inch thick layer of coarse Bela dust is sprinkled on top. For finishing, a 0.5 inch thick layer of mixture of fine Bela dust/sand, lime and bits of fine stone aggregates is laid. On top a 2 mm layer of lime slurry is applied. Then the process of tippani starts in which about 12 to 14 women perform tippani for approximately 10 days under the supervision of an artisan. Tippani process is repeated till the slab leaves water on surface. Once the access water appears on the surface, the slab is considered fully compacted. On the next day the floor is rubbed and polished with a brush made out of palm leaves. The slab is then covered with banana leaves for a minimum of 15 days upto 1-2 months. Finally, the slab is polished with a mixture of shankh jeeru (powdered sea-shells) and fine lime powder.

Key Issues with the Building Practice

1. Traditional building techniques like lime concrete slab construction are quickly disappearing in Saurashtra. There are very few remaining builders who have the experience, knowledge and skills required for traditional buildings. Most of them do not even build with such techniques anymore, which pauses threat to the survival of these traditions.
2. Constructing a lime concrete slab is labour and time intensive work.
3. People’s perception of modern materials and construction as more durable and easy in terms of construction and maintenance.
4. Cement is widely produced and easily available which has also impacted on use of lime as a base material for concrete.
5. Increased prices and lack of availability of wood impacts on use of wood for construction of the under structure of roofs.
6. Many chuna bhattis have closed down in the region which makes it difficult to get lime locally.

Artisans

• Bhojabhai Rajabhai Gohil

Address: Shahpur Darwaja, Vankarvas, Mangrol

Bhojabhai (Bhojabapa) is an artisan specialising in Pidiya-Papadi roofs. Last cocrete roof he constructed was eight years back. His house is good example of vernacular lime conctere construction.

• Palabhai Khetabhai

Address: Kumbharwado, Baro, Harijanvas, Prashnawada

Palabhai was a labourer constructing Pidiya-Papadi roofs with lime concrete.

• Manjhibhai Kachrabhai Bhambhania

Address: Vishwakarma Society, Surajkardi, Dwarka

Manjhibhai is a Civil Contractor. He has stopped working with lime now, but he used to work with lime and has sufficient experience and knowledge about limestones, concreting, lime plaster and lime masonry.

• Narsinh Jivraj Yadav

Address: Chamunda floor mill, Narsang tekri, opp. Rameshwar Temple, Dwarka
Mobile No: 9879566853

Narsinhbhai used to be a lime contractor. He is around 75 years old and has stopped working with lime. He specialised in preparing lime mortar, which he used to sell to other contractors.

• Teedabhai Savjibhai

Address: Songara, Bhathan Chawk, Next to Vagher Boarding, Next to Mathura Bhavan, Dwarka

Teedabhai used to be an artisan mainly constructing lime concrete roofs.

• Sejiben Kishaan, Veraval

Sejiben used to do tippani work.

Special Tools/ Equipments

Tippani

Tippani is approximately one foot long stick like tool with a broad base. The handle of the tippani is made of bamboo while the base is made of goondi or teak wood. It is used for finer compaction of the surface of lime concrete (Fig 26 and 27). There are also other type of tippanis used. A kind of tippani Teedabhai described is a two inch thick rectangular wooden block (of approximately 35 cm by 25 cm) connected with a handle. This kind of tippani can be used while sitting.

Fig 26: Tippani Tool	Figure 27: Tippani Tool

Bibliography

Credits and Acknowledgements

Documentation

• Rushank Mehta ('People in Centre' Consulting)
• Chintan Gohil ('People in Centre' Consulting)
• Ashish Parmar (CEPT University)
• Shakeel Khimani (CEPT University)

Information

• Babubhai Vadgama for telling the history of Beyt Dwarka and the use of construction materials and for the help in getting permissions for measuring the case study.
• Hemubha Vadher for the help in collection of information about the buildings and building materials. He was born on the Island and has lived all his life there, so he had some information on where the materials were imported from in earlier days.
• Mihir Parmar for the help in measuring the case studies.
• Zikarbhai Adam Sodha for telling the process of lime construction.
• Mr. Ker from Gram Vikas Trust, Dwarka for useful information about villages of Dwarka region and contacts of artisans.
• Sadurbha from Dwarka (Mobile: [phone]) for providing us information on Tanks as well as contacts of artisans in Dwarka region.
• Varsing Mala from Rabari Vaas, Beyt Dwarka for information on Beyt Dwarka’s history.

Comments

Footnotes

1. Most artisans informed that they used to construct the lime concrete flat slabs in the past but for last ten years or so they have not got any assignment. The last lime concrete slab found was constructed in around 2001.

2. Need to confirm this data.

3. Need to confirm this data.

4. Which other places in Saurashtra is it available and which stone is it ?

5. Information on history of disasters e.g. magnitudes of particular places in Saurashtra during last few major earthquakes, wind speeds in cyclones etc would be useful.

6. The relationship between the earthquake magnitude and the maximum intensity caused is not a precise one. This approximation of magnitude is based on Earthquakes. (Reference: Don de Nevi, Celestial Arts, Calif., 1977, P.102)

7. Pure lime stone or chalk is composed of carbonate of lime (CaCO₃) When this is heated in a kiln it yields quick lime or calcium oxide (CaO) and carbone dioxide (CO₂). When water (H₂O) is added to quick lime (CAO) for slaking, calcium hydroxide (Ca(OH)₂) and heat are produced. Lime concrete has the active ingredient of Ca(OH)₂ and inactive ingredient of sand/silica. When it is exposed to the atmosphere, it hardens and during that process water is evaporated and carbon dioxide is absorbed from the air. (Reference: Building Construction Volume Two, W.B.McKay, Longman Group Ltd., London, 1970)

8. Inputs required from structural experts to confirm how much thickness of lime and timber elements would make this work and how it will function.

9. Though, the depth of the wooden joist is more than 8 inches on average, making the total depth of the beam more than 12 inches. For composite section it could have been much lesser. Hence, this needs to be checked and commented upon by structural expert

10. Bernoulli principle: The pressure exerted perpendicular to flow direction by a moving fluid drops with increased velocity and rises with decreased velocity to maximum pressure when the fluid is stationary. As a simple demonstration, hold two vertically suspended sheets of paper in parallel planes about 2 in. apart and blow between them. The two sheets will come together, demonstrating the reduced pressure accompanying faster airflow. (Reference: Manual of Low-slope Roof Systems By Charles William Griffin, R. L. Fricklas, Pg. 136)

11. More information required on what thickness and weight can make them safe

12. // what about structural elements in the earthquake situation.//

13. How will an intermediate floor slab differ from a roof in terms of performance in disaster situations.

14. Information on thermal mass/capacity is missing

15. Confirm this data.

16. Papadi is a slab (usually stone but could be of other materials) and pidhiya are joists, which are the wooden structural members of flat roof.

17. These are produced from limestone which is burnt in a kiln.

18. The information on the proportion of water to lime to be sprinkled is missing

19. Ghani is a ground well, having a grinder turning round with the help of bullocks. It is approximately 40 feet in diameter. Traditionally, ghanis were used for crushing seeds and making oil.

20. Confirm wether the sand is added in the ghani or after it is taken out.

21. Need to check proportions

22. Confirm if this is lime slurry (without sand) or lime concrete

23. Confirm if tippani is done after this layer as well as after second layer.

24. Need to specify how much time it will take for the first layer to settle

25. More information on pattimalo - how does it look/photo required

26. According to some artisans, it is cured for 15 days

27. Based on Discussions with Bhojabhai or Mangrol. His contact details are mentioned in the section of artisans.