Slab On Grade Construction for Dummies

Updated: Apr 6

Slab on Grade are terms used by engineers for concrete laid directly on earth. It may be your garage concrete, balcony floor, or the flooring that is set on your living rooms. Most technical people also refer slab-on-grade as S.O.G. (es-ohw-gee).

Please do not confuse it with S.O.G. with sog as I've been asked before what is sog, I answered her that it is the deflection of any materials due to loads. To my surprise, my apprentice were referring to slab-on-grade. My bad!

I'll show you the steps on the proper construction of slab on grade so you'll have an idea what is your engineer is doing or should you want it DIY.

1. Thoroughly compact the soil base

This is important for elevated slab on grade. Say, you put an embankment for your structure. Ask your engineer if the soil base has attained the required minimum compaction percentage for casting of concrete. This is normally at 98% compaction.

2. Prepare slab elevation

Anticipate the finished floor plan elevation after laying out the gravel bed, the concrete, grout and tiling. This is important so you’ll never run out of overhead ceiling. Most non-technical people doing DIY concrete casting for the first time forgets this. It would be best if a surveying team visits the site before the first concrete pouring so elevation will be set like the picture below.

Picture Showing Elevation Points

Surveying team will be able to show what needs to be adjusted, elevated or lowered.

3. Lay out the gravel bed

A simple mistake most people commit when they cast out the gravel bed is not compacting it. Gravel bed needs to be compacted as it may be a cause for future settling. Also, gravel bed may shrink for about 5%, thus, additional 5% of the actual total computation volume of gravel bed is advised.

4. Use Soil Poisoning Treatment

One of the classic things people forget or take for granted when casting permanent concrete is not using pest and termite control. The simple spraying of pest and termite control before the casting of concrete is important especially for termite-prone soils. Ask for your engineer what is best. Still, I advise that you use soil poisoning treatment to protect your home, furniture, wooden stairs for the probable attacks of termite in the long run.

Picture Showing Actual Soil Poisoning Using Spray

Some soil treatment may last ten to several years. Get it as it is a not so expensive additional cost.

5. Install the polyethylene sheet

Anyway, for slab-on-grade that will be installed any tiling works, it is best to use polyethylene sheet or PE Sheet. It is just a sheet of plastic laid after the gravel bed. Oh, by the way, do not forget to use soil treatment before laying your polyethylene sheet. Basically, the use of PE sheet is for vapor barrier. It prevents or minimizes moisture from getting into the concrete which diminishes the possibility of getting a cracked tiles.

Picture Showing Polyethylene Sheet

6. Install the reinforcing bars

Now, see to it that a reinforcing bar is installed as it is based on the designed plan. Some slab on grade may need reinforcing, some may not. I couldn’t advise more about this than to ask a technical person near your area as Standards may differ per countries. Anyhow, if it is just a small slab on grade for gardening or path walk with no heavy loads like cars, then it may be okay to not use any reinforcing bars at all.

7. Cast the concrete

Prepare all the necessary manpower, materials and equipment before scheduling your concrete casting. See to it that enough concrete or ready mixed supply is available on site.

Now, installation of slab on grade come with the use of construction equipment. For thicker S.O.G like the photo above, that is a 0.5 meter concrete slab, the use of concrete vibrator is a must. The use of concrete vibrator is essential so you could prevent hollow or honeycombed concrete structure. Should there be power outage, prepare for a generator. Also, see to it that you cast you slab on grade on a nice weather.

Further, it is best that you cast all your slab on grade at once so the concrete will be homogeneous throughout the structure. Being homogeneous would mean lesser possibility of uneven slab settling or cracks.

Now, for the final finishing, see to it that elevation friendly for water drainage is prepared before the concrete has completely settled. See to it that your slab on grade does not have an area for rainwater to settle on the concrete, else, you will have a small pond susceptible for algae growth.

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