Hello Professionals,
I hope u can help !
I want to lay a new patio using pavers on an existing concrete base approx 8m x 5m. The new patio'd area is to cover roughly the same area ( maybe extended about 2 ft on the lawn side ) and I am wondering whether this existing concrete base is suitable to pave over or whether it will have to come up and I start again from scratch.
The concrete looks as if it is between 1.5 - 3 inches thick in different places, it is pretty solid but has some cracks of approx 3-8mm running thru it in various places. Where these cracks occur the levels of the concrete can change by up to about 10mm. I would also say that the levels across the whole 40 sq.m may vary by about an inch.
What I need to know is can I flag over this and if so, how should I prepare this ready for flagging ?
Existing concrete base?
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84-1093879891
As this is only a patio, I reckon you can safely flag over the existing concrete, but you need to check that there won't be a problem with the "150mm below dpc" rule.
You can lay the flags on the usual 10:1 semi-dry mix of grit sand with cement, and ensure there is adequate fall away from any buildings.
Essential preparation includes setting up string lines to act as level and line guides, and to check that you have sufficient room on top of the existing concrete base to accommodate 30-50mm of bedding plus the thickness of the flags you plan to use. Other than that, just make sure you clean off any crap that's on the concrete, such as weeds, litter or detritus. There's no need to clean it with a power washer or to use a bonding agent - it's acting as a rigid base layer, and adhesion is not essential.
You can lay the flags on the usual 10:1 semi-dry mix of grit sand with cement, and ensure there is adequate fall away from any buildings.
Essential preparation includes setting up string lines to act as level and line guides, and to check that you have sufficient room on top of the existing concrete base to accommodate 30-50mm of bedding plus the thickness of the flags you plan to use. Other than that, just make sure you clean off any crap that's on the concrete, such as weeds, litter or detritus. There's no need to clean it with a power washer or to use a bonding agent - it's acting as a rigid base layer, and adhesion is not essential.
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danensis
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I would have some concerns over this. If the existing concrete is over clay, and has already settled, there is a real risk that the concrete will continue to move, and the patio will end up with differences in level following the lines of the original cracks. This will especially be the case if the movement is caused by tree root dessication as the matter will get worse if the offending trees are not kept lopped.
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84-1093879891
It depends on the age of the existing concrete, and whether there's been any recent movement. If the ex. conc is realtively satble, laying over it is a cheap'n'cheerful option, as the cost of bringing in a skip and breaking out the concrete can be prohibitive on small, patio jobs.
Reflective cracking is not likely to occur on a flagged patio - if there was any movement of the concrete base, then the flags and the bedding would move with the base, relying on the jointing to absorb such movement. Only if the flags were well and truly bonded to a bedding medium that was, in turn, well and truly bonded to the concrete base, would I expect to see any reflective cracking, and even then, I'd be surprised!
If we assume that the concerete base is stable, or relatively so, then taking a chance that any future movement will be small and would either be insignificant, or of such a small magnitude that simple 'lift and relay' in the area of any movement would be sufficient to rectify is probably a better option that splurging out x hundred quid to break out and cart away what's already there.
Reflective cracking is not likely to occur on a flagged patio - if there was any movement of the concrete base, then the flags and the bedding would move with the base, relying on the jointing to absorb such movement. Only if the flags were well and truly bonded to a bedding medium that was, in turn, well and truly bonded to the concrete base, would I expect to see any reflective cracking, and even then, I'd be surprised!
If we assume that the concerete base is stable, or relatively so, then taking a chance that any future movement will be small and would either be insignificant, or of such a small magnitude that simple 'lift and relay' in the area of any movement would be sufficient to rectify is probably a better option that splurging out x hundred quid to break out and cart away what's already there.