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Topic: Stained paving slabs, Loose and stained slabs< Next Oldest | Next Newest >


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Posts: 1
Joined: Aug. 2019
Posted: 18 Aug. 2019,12:56 QUOTE

Help needed
We had a 100m patio four sizes of 20mm cal Brazilian black slate laid  January in the hard frosts. A good fall, drainage channels , compacted base,bed of 3" - 4" sharp sand, 6 and 1 sharp sand mortar bed with SBR painted on the slabs.
The slabs were loose underfoot and white cement stain? came out from the joints filled with easy joint.
The contractor came back recently lifted all the slabs with a trowel no mortar stuck to them. This time they laid another bed of mortar on top with SBR added to the mix and filled the joints again.After the first down pour the cement seems to wash up through the easy joint and  stain all over the slabs, Cleaned it all of with cement remover and it keeps coming out.
When you walk over the slabs the water bubbles up through the easy joint. I expect the slabs have not bonded to the slabs.
I feel if the the jointing grout is replaced with a non permeable grout the water would run into the drainage channel, but would the grout break up if the slabs are loose.
Can anyone give advise why the slabs haven't bonded and the cement is coming through the joints and staining as the contractor has never experience this problem before.
Not sure how to load photos.
Thank you in advance :(
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Tony McC

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Posts: 11095
Joined: July 2004
Posted: 27 Aug. 2019,11:28 QUOTE

You say there was a "bed of 3" - 4" sharp sand" and then a mortar bed. That's a recipe for disaster from the off!

It sounds like the "mortar" is failing, which could be down to one or more of a myriad reasons, from perished cement to improper mixing. The only way to be certain would be to assess the paving on-site and, if necessary, have the "mortar" analysed for cement content.

Changing the jointing will achieve precisely nothing. It's like painting over rust - it might look Ok for a few days, but it's certain to fail again in the short term.

Site Agent - Pavingexpert
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