Extending alfresco slab

Hi Rhys,

A couple of things you should consider-they may or may not be relevant to your situation but worth thinking about:

1. The waffle slab and the new slab will likely behave differently in response to changing conditions so it's often better to allow differential movement. A structural engineer would be the person to provide a bit better info but rather than pinning the slab together and trying to 'force' them together, incorporating expansion/slip joints between the two might work better. An example might be that the new slab could be more prone to normal seasonal movement than the waffle slab so it would need to be allowed for.

2. Similar to the above, pinning the slabs together may prevent the waffle slab from moving as it was designed to. A 5m x 5m x 0.2m (say) slab which is behaving differently from the waffle slab might not just result in problems at the join of the two, but problems within the waffle slab itself as one 5m long section is not behaving the same as the rest of the slab.

3. Whats the likely condition of the ground below the area of slab extension? You mention a P site classification. Is it due to fill (controlled or not), trees, old buildings on site, poor ground etc. and could that impact the new slab?

4. Not had much to do with waffle slabs, but generally maintaining uniform moisture conditions under or near slab perimeters is good practice. The construction of the new slab will selectively protect a 5m wide section of the ground next to the existing slab from drying out or wetting up which would likely impact the soils under the slab over time. Not such a worry if the rest of the slab is protected as well (via apron) but if not, the risk is setting up varying moisture contents (and variable degrees of movement) under the slab which can cause problems.