Filler slab roofs: An alternative roofing technology

A.   INTRODUCTION TO RCC FILLER SLAB

Filler slab technology is a simple and a very innovative technology for a slab construction.

The reason why, concrete and steel are used together to construct RCC slab, is in their individual properties as separate building materials and their individual limitation. Concrete is good in taking compression and steel is good in tension. Thus RCC slab is a product which resists both compression as well as tensile.

Fig 1: Simply supported slab cross section


Knowing this much if we want to move further to understand the “Filler slab” technology, we will have to further study the cross section of a typical simply supported RCC Slab. Under its own load and applied load, the slab will try to bend as shown in the Figure 1.

 

Fig 2: showing unwanted tension concrete

If we refer Figure 2, which indicates the neutral axis and also tension concrete in the bottom fibers of the slab which is in tension but the top fibers will be in compression.  Knowing this much is the key to understand the filler slab technology. Tension in a slab is on the bottom fiber and compression on the top fiber. That means if we want to optimise the structure we can remove concrete from the tension zone where it is not much needed. That’s the key behind filler slab construction.

This is a very cost effective roofing technology. Knowing the way slab is constructed on site (w.r.t. Gujarat, India), it is not easy to remove, the concrete from the tension zone, hence we try and replace (partially); that part of concrete using light weight and low cost filler material. This method of construction is called filler slab.

Filler slab technology is being used across India, but substantial amount of work on the successful promotion and transfer of this technology was done by Ar. Laurie Baker in South India. It is one place where filler slab has crossed the boundary of research and controlled implementation to being one of the regular options of construction by both government and private sector and also architects and designers have been promoting this technology.

These filler materials are so placed as not to compromise the structural strength, stability and durability, resulting in replacing unwanted and non-functional tension concrete, from below and thus resulting in economy of high energy material consumption and respective cost savings and decreased dead load of the slab.

An internal cavity can be provided between the filler material which adds an extra advantage; other than cost savings and energy savings; improved thermal comfort for the interiors. Also an added advantage of lower dead weight transferred to the supporting elements and finally onto the foundation to further adds up cost saving in design of these elements.

These filler materials are so placed as not to compromise the structural strength, stability and durability, resulting in replacing unwanted and non-functional tension concrete, from below and thus resulting in economy of high energy material’s,  consumption and considerable cost saving and decreased dead load of the slab.

 

B.    MATERIALS SELECTION AS PER NEED AND DESIGN

Light weight, inert and inexpensive materials such as low grade Mangalore tiles, Burnt Clay Bricks, Hollow Concrete blocks, Stabilized Mud blocks/ Hollow Mud blocks, Clay pots, Coconut shells etc. can be used as filler materials. These materials are laid in the grids of steel reinforcement rods and concreting/concrete topping is done over them.

fig 3: Filler material arrangement

The following points to be kept in mind for filler material selection:

  • Filler material should be inert in nature. It should not react with concrete or steel in RCC slab constructed.
  • Filler materials water absorption should be checked for as it will soak the hydration water from concrete.
  • Filler material should be light in weight, so that overall weight of the slab reduces and also the dead load onto the foundations is reduced.
  • Filler material should be low cost so that it cost is much lesser then the cost of the concrete it replaces. This is very important to achieve economy.
  • Filler material should be of a size and cross-section, which can be accommodated within the spacing of the reinforcement and also thickness wise could be accommodated within the cross section of the slab.
  • Moreover the Filler slab also provides insulation (compared to a conventional RCC slab) from the hot climate outside the building, providing thermal comfort to the user. 
  • Filler material texture should match with the desired ceiling finish requirements so as not to provide an ugly ceiling pattern.

 

C.    INSTALLING FILLER MATERIAL IN THE SLAB

Size of the filler material will be very crucial. It is must to decide the filler material before the slab is designed. The dimension of filler material will help decide the spacing of reinforcement and accordingly the depth of the slab and other structural details will be worked out.

Also, filler material should be properly soaked in water so that it does not absorb any water from the concrete. While installing the filler material, one will have to decide the concreting sequence of the slab i.e. from where the concrete will be started and how will it progress to cover the whole slab.

Filler materials like Manglore tiles/Clay tiles can be installed in two layers (2 nos. one over the other) entrapping an air cavity between the two tiles. A sketch showing cross section of a filler slab with manglore tile is shown below. This will improve the thermal properties of the slab.

 

Figure 3: Filler slab section - Manglore tile filler material.

 

D.    ADVANTAGES OF FILLER SLAB TECHNOLOGY:

 

  • By adopting RCC filler slab construction compared to a RCC solid (conventional) slab in case where manglore tiles are used as a filler material, you can save on approximately 19% of the total concrete and including the cost of filler material, you can save around 5-10% of your concrete cost.
  • Another advantage is, if the filler material is just a waste i.e. for ex temporary manglore tiles that are removed from the roof to construct a pukka roof, you can save upon nearly 15% on your roof concrete construction cost.
  • Building a 25 sq. m slab can save you approx. Rs. 5000 from your concrete cost.
  • Filler slab technology can also be applied to mass housing projects and township projects to gain high cost saving and also saving in high energy consuming materials.
  • Another advantage can be of a better thermal comfort if a cavity is kept between the filler material or the filler material itself has a cavity. For example tow manglore tiles/Clay tiles can be kept one over the other to form an air cavity thus keeping the interiors of your house remain cooler in summer and warmer in winters.
  • Filler slabs can be kept exposed (with proper workmanship) to create aesthetically pleasing ceiling with a view of filler material from below and thus the cost of plastering and/or painting also can be avoided.
  • RCC being made of cement, steel, sand and aggregates, is a very high energy intensive material. So reduction in concrete quantity compared to conventional slab construction, adds this technology to the list of sustainable and environment friendly technologies and corporating green building features.
E.    COST SAVINGS
Material saving assuming a 100 mm thick slab 2.54×3.86 m, and calculating the material and cost savings as per market material rates of Ahmedabad, Gujarat, August, 2011 and comparing the savings

 for 1 m3 : RCC Filler slab vs. Conventional Solid RCC Slab.

  • 1.61 bags (19% saving)                       =         Rs 418/ m3 saving in cement cost.
  • 0.09 m3 less sand (19% saving)            =          Rs 21/ m3 saving in sand cost.
  • 0.18 m3 less aggregates (19% saving)  =       Rs 127/ m3 saving in aggregates cost.
  • 10 kg less steel/m3 of slab casted       =       Rs 500/m3 saving in reinforcement cost.
The table below shows the consumoption of materials and cost savings for 1 mquantity of the slab:

Table 1: Cost saving/cu.m of concrete slab

The figures in the table seem less as the slab taken for calculation is a small size slab. Elaborating this savings, assuming a normal MIG house area of 100 sq.m ground area and of two stories. Assuming the slab area comes to be 160 sq.m. for this house, the total savings in concrete cost can be Rs. 30,000. Also if the filler material is not a virgin material for the slab construction, you can save about Rs. 90,000 from your concrete cost.

E.    EMBODIED ENERGY SAVINGS

The table below shows the embodied energy calculations for a 200 sq.m house slab. Slab dimensions to be same as above in D.   Cost Savings.

Cost Comparision: Conventional RCC solid slab v.s. RCC Filler Slab

 

 

 

 

 

 

 

 

 

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