How do we design Filler Slabs

What are filler slabs?



Filler slab is supposed to be a cost effective, efficient, simple and very innovative construction technology for slab construction if it is effectively designed and constructed.

As the name suggests, in a filler slab, part of concrete in the bottom of the slab (in the tension zone to be specific) is replaced by filler materials. Conventionally we have been using Mangalore tiles and clay pots as filler. Broken pieces of cement blocks, coconut shells and glass bottles are also sometimes used. Recently I have seen an architect using damaged computer keyboards as a filler material in a media room. Any cyber waste can be used as long as it is aesthetically pleasing. Sometimes clients choose filler purely for aesthetic reasons and don’t care for local availability. Otherwise, the filler materials used are generally less costly.

The reason for the cost effectiveness is due to the use of local materials in combination with the reduced use of concrete. At one point in time, filler slabs were really economical. Now we cannot always say it is economical. It depends on a lot of factors. As compared to reinforced cement concrete slabs, filler slabs consume less concrete and steel. So surely this is greener based on what filler you choose.



The structural principle of using filler.


As we all know, for simply supported RCC slab or any flexural member for that matter, lower part of the neutral axis experiences tensile forces and we have compression on top of the neutral axis. We also know that concrete is not great in tension and rebars take the entire tensional forces. In fact, we could have removed the concrete while placing had it been not fluid. However, concrete is fluid in nature and we cannot replace it with out an effective means. Filler material act as a means that help us to remove this concrete from the bottom face of the slab where only rebar is needed. Please note that we will need adequate cover to the rebars and stringent quality control needs to be adopted during the design and construction stage to ensure proper binding of bars, holding of bars thereby ensuring that strength, serviceability and durability is not compromised in any manner.

You can read more about the tensile zone in a flexural member in another blog here.


History of Filler slab in India



Filler slab technology has been used across India from a long time in different forms.

Indo –British architect Ar. Laurie Baker has popularized this filler slab technique in India especially in South India and particularly in Kerala. It was so popular in Kerala some time back that there was a period when everyone wanted to build a ‘’Baker model home’ ’His style was unique and revolutionary.

He was an outstanding Architect in sustainable architecture and green buildings, incorporating these design principles even as early as in 1960s.Concepts of rain-water harvesting, minimizing use of energy consuming materials and building homes suiting the site and environment were his primary focus. Filler slab technology is another cost efficient and sustainable technology introduced by “Gandhiji of Architecture”- Ar. Laurie Baker


(All photo credits in this blog to Ar. Laurie Baker, his junior fellow Architects and internet.)


Advantages of Filler Slabs


  • Due to the replacement of nonfunctional cement concrete with lighter filler materials, dead load of the slab is reduced considerably. And this helps in reducing the foundation size, steel required in structural members. The reduction in weight as much as 20 to 30%. Many fail to note a very important point that reduction of dead weight has a very compounding positive effect. When the dead weight is reduced the seismic demand is also reduced. Note that seismic force is nothing but inertia. Watch the video in this blog to know more about this.

  • It is cost effective at least by 20% compared to the conventional RCC slab due to the usage of less costly filler materials and decreased amount of steel and concrete.

  • Filler slabs does not compromise on the strength or durability if designed and constructed with stringent quality control.

  • Filler slabs creates comfortable temperature conditions inside the living rooms as the air pockets between the filler materials has heat resistant property. The thermal comfort provided by filler slabs makes it an excellent option in humid hot climatic conditions.

  • When materials like cyber wastes like key boards, waste plastic substances especially bottles are used as fillers, this method by default does a waste management as the otherwise harmful substances to the soil and earth are being reused.

  • Filler slab reduces the carbon footprint of a building by around 20%.

  • Correctly chosen patterns in the filler slabs increases the aesthetics of the ceiling.

Disadvantages of Filler slabs


  • The filler slab technology needs some experience. Unless we have people, who have built a few similar homes, there are chances that you mess up in its construction

  • The rebars can corrode if it comes in contact with clay products that are used as fillers. Enough care should be taken to ensure sufficient cover to reinforcement.

  • For the same reasons above, the slopes at the terrace have to be meticulously planned and all the rain water down pipes has to be clog free. Enough care to be taken to ensure maintenance of terrace.


Selection criteria of filler materials and generally used filler materials


  • Filler materials should be non-reactive to cement and water and other materials used like rebars.

  • The filler dimensions, size and shape have to suit with the reinforcement spacing. This shall be well coordinated between all stake holders like client, Architect and Structural Engineer. Any last-minute realization can in fact encourage a compromise and you end up with corroded rebars and leaks after a few years.

  • Its mandatory to ensure the water absorbed by the filler is minimal and better to check this before construction.

  • Local materials to be chosen to keep the costs in control.

  • Light weigh materials to be preferred

  • Ceiling patterns and the functional use of the room also decides the material selection. For example, we cannot propose a computer keyboard in a dinning room. However a keyboard as filler perfectly suits a media room

Filler materials used


  • Mangalore tiles

  • Clay pans

  • Bricks

  • Waste bottles

  • Coconut shells

  • Thermocol

  • Cyber wastes like keyboard

  • Stabilized mud Blocks

  • Terracotta tiles

Filler Slab planning and Casting


Filler slabs are as normal slabs, cast in site. Formwork is laid and the rebars that are designed are laid. Generally, a max allowable spacing of 3d or 300 is adopted. Minimum needed steel is also needed to be provided as per codes. If a better-quality control can be provided and if crack widths can be checked by design, we can adopt wider spacings than 3d or 300. This may be needed if the size of the material demands more spacing than 300. See the video in this blog for more clarity.

Note that if the filler material is some thing that absorbs water, it should be soaked in water to ensure that while construction, it don’t absorb water from the concrete.


Analysis and Design of Filler Slab


There is no need to have much difference in the load calculation, distribution, analysis and structural design of filler sabs used in short spans from a normal slab. The load calculations have to be accurate to take benefit of the lesser weight. Even live loads have to be refined. See civilera blog here for more info on how you can refine Live loads. Based on if the slab is one way or 2 way, design it as a conventional solid slab. Based on the edge conditions, the moment coefficients can be adopted from IS 456. Deflections can be satisfied meeting the L/d ratios.

See the video for more clarity. Also read more about redistribution to understand what happens if we consider the slab as simply supported


Conclusion


This blog would have given you an understanding of Filler slabs including its design. There are many simple but alternative technology that helps us structurally and architecturally to build green homes. Some of the past technology might need a bit of innovation to suit the current requirement. Knowing these points are very important to improve up on these existing practices.

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