Common problems with waffle slabs

Look for these common problems with waffle slabs as part of your investigation:

  • Turf placed around the building that creates a dam under the slab. Look for water draining from the lawn and into the sand layer under the building. Look for turf placed against the building with insufficient fall away from the building. Correlate regions of poor drainage with high slab levels. If the ground under the waffle slab is sloping, look for regions of high slab levels in the bottom edge or corner where water sits and soaks into the ground.
  • Gardens placed against the building and garden edging directing water under the building.
  • Correct placement of the steel reinforcement. This is nearly impossible to diagnose without taking a core through the slab. If the slab steel is too low then it will provide minimal resistance to slab cracking and slightly decreases the stiffness of the slab system. Look for uncontrolled slab cracking – it’s an easier way to diagnose misplacement of the steel than taking multiple cores. Consider arranging an xray analysis or ground penetrating radar analysis if you suspect misplacement of the reinforcement.
  • Look for sources of soil moisture such as dripping taps, air conditioning condensate, hot water system overflow or downpipes discharging to the ground against the building.

Things that are unlikely to negatively affect the performance of a waffle slab:

  • Thicker slab than shown on the drawings. So long as the slab reinforcement is 30mm to 40mm from the surface, a thicker slab is actually better for a waffle slab. I think an 80mm slab is actually to thin to be easy to build  and maintain cover.
  • Concrete spillage around the edge of the slab. This merely indicates the formwork wasn’t as uneven as the ground prior to pouring. A little bit of concrete overspill will not detract from the performance of a waffle slab.
  • Breaks in the waffle slab void formers. If the void formers are broken, concrete will fall through the void when it is being placed. Problem? No. More concrete will be used. The concreter won’t save as much money but the performance of the waffle slab will not be affected at all. In fact concrete in the void is vastly superior than just air!

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