The Brew Cabin

Posted: **Sat Jul 11, 2009 4:27 pm**

I intend to redo our patio, which is approx 30 years old and about 45 square meters. The patio is adjacent to the rear of the house and extends about 4 meters away from the house, the far edge being supported by a concrete block wall about 1500mm tall. Beneath the 300mm square concrete flags is a full mortar bed, below the mortar bed is a cracked concrete base. The central area of the patio has subsided approx. 75mm due to insufficient support below the concrete. Iâ€™ve removed the flags, and most of the full mortar bed. The patio is only for foot traffic. No vehicles bigger than a dinky car!

I want to retain the existing concrete, even though it has cracked and subsided. I hope that it has finished settling, and should provide a suitable base for new sandstone paving. I also need to raise the height of the new paving near the house to create a suitable slope.

I need to I build up the existing concrete base by between zero and 75mm. Hereâ€™s my plan â€“

MOT1 goes over the areas where the bedding layer needs raising the most. Concrete ballast (20mm) is used in adjacent areas where only 40mm raise is required, 10mm gravel / shingle is used where 20mm raise is needed, sharp sand is used where less than 20mm raise is required.
The material is compacted using a vibrating plate.

Does that seem a reasonable approach?

Thanks.

P.S. Great site - so much great information.

Posted: **Sat Jul 11, 2009 4:38 pm**

Whatever you do without removing the cracked concrete will be a compromise as once cracked it will always be on the move.

Posted: **Sat Jul 11, 2009 6:29 pm**

I think you will get differential settlement over time. I would start with a blank canvas, ie take out the concrete. As you say "there is nothing supporting the slab". It will certainly be on the move. " The choice is yours".

Posted: **Sun Jul 12, 2009 11:55 am**

Could the concrete not be broken up in situ, and then compacted down to reduce the risk of any future settlement before regulating the whole surface with 100mm or so of lean mix concrete?

Posted: **Mon Jul 13, 2009 9:36 am**

Gents, Thanks for your replies, not what I wanted to hear, but you are all correct. I started breaking up the concrete to find a 2 or 3 inch gap between the bottom of the concrete and the "supporting" hardcore. So more breaking up will be done.
Breaking up in situ and then compacted down sounds a good plan.

This leads to my next problem. It appears that the original patio level was above the damp proof course. (see photos). The first photo shows the original patio level, the second shows more detail of the hole in the wall shown in the first photo. There is no sign of damp on the inside of the house. We think the original patio was done in the 1960's. Is it possible that a chemical damp course was injected to allow for the raised patio? Your thoughts would be most appreciated. Thanks.
PatioPictures2009

The Brew Cabin

Variable thickness sub base - Redo existing patio that has sunk