Hello
I had a patio laid last Autumn. Over 100 Bradstone silver grey sandstone slabs were laid onto a full mortar bed with mortar joints. Beneath the full mortar bed is a layer of hardcore with a layer of coarser sand/gravel bound with cement on top. I've noticed in recent weeks that slabs in some areas sound hollow when tapped and I'm sure they were all OK earlier in the year. The slabs aren't moving and the mortar is not cracking, so visually it all looks OK. One hollow sounding slab though does have dark grey damp patches that don't seem to dry. This same slab had the worst amount of efflorescence of all the slabs. I'd upload a picture or the slab and the construction layers if I knew how to! I think the builders have pretty much followed the Bradstone install method but I'm quite worried about the hollow sound and puzzled by the one damp looking slab. What could be causing this?
John.
Patio question - Sandstone slab, hollow & one damp?
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Tony McC
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Where, in the Bradstone installation guide, does it suggest "a layer of coarser sand/gravel bound with cement on top" be used over the sub-base? That's a bollix of a construction, completely useless and possibly detrimental to the pavement structure!
As explained on at least a dozen previous occasions, flags laid on a mortar bed act like a drumskin and even the slightest void beneath the flag, a depression in the bed the size of marble, can cause a very, very minor shollow sound to become highly amplified and give the auditory impression of a huge gaping hole beneath. It's not a reliable method of detecting bed construction.
As for the other visual problems, we'd need to see photies before diagnosing possible faults.
As explained on at least a dozen previous occasions, flags laid on a mortar bed act like a drumskin and even the slightest void beneath the flag, a depression in the bed the size of marble, can cause a very, very minor shollow sound to become highly amplified and give the auditory impression of a huge gaping hole beneath. It's not a reliable method of detecting bed construction.
As for the other visual problems, we'd need to see photies before diagnosing possible faults.
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JW1970
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Hi Tony
My bad wording I think. Working from the bottom up there is a layer of MOT, a layer of aggregate and cement laid to create a fall, a full mortar bed with the sandstone slabs on top. The slabs are jointed with mortar. Here's a picture of the offending slab. As I said, many sound hollow but only this one has a damp patch that does not go away, it also had the most efflorescence. Could this be reflective staining?
https://photos.app.goo.gl/V22j1Fwj6ZbpZenz6
John W.
My bad wording I think. Working from the bottom up there is a layer of MOT, a layer of aggregate and cement laid to create a fall, a full mortar bed with the sandstone slabs on top. The slabs are jointed with mortar. Here's a picture of the offending slab. As I said, many sound hollow but only this one has a damp patch that does not go away, it also had the most efflorescence. Could this be reflective staining?
https://photos.app.goo.gl/V22j1Fwj6ZbpZenz6
John W.
John W
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Tony McC
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The mortar bed should be directly above the sub-base and none of that nonsense 'aggregate with cement' in between.
That one flag is, for some unknown reason, affected by picture framing to the edges and there is a hint that there could just be minor hollows nbeneath, but the difference is tonal rather than mineral, so more likely to fade or weather out in a season or two.
That one flag is, for some unknown reason, affected by picture framing to the edges and there is a hint that there could just be minor hollows nbeneath, but the difference is tonal rather than mineral, so more likely to fade or weather out in a season or two.
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JW1970
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Hi Tony, Can I ask why a foundation base as I described is not a good idea please? I've read the Bradstone website again and it recommends a 75mm layer of all in one and cement be laid under the bedding layer. The builders who did my patio are long established and have always used this method. The patio looks great and all slabs are stable but in some areas have begun to sound hollow in recent weeks after being ok at install last Autumn. Wishing I'd read your website before I had the builders in. John.
John W
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Tony McC
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I'll need to check just what Brdastone are publishing these days but going back a few years to when I was doing consultancy for them on installation methods, the suggestion of mixing cement with the sub-base or, even worse, just putting a dusting of cement over a sub-base was very definitely NOT recommended.
Beneath the bedding and paving, you can have a sub-base (unbound) or a base (bound). If it's to be a sub-base, and it usually is, then there is no binder added to the aggregate and the intention is that the layer can move ever so slightly to accommodate ground movement. A base, in contrast, is bound, rigid, inflexible, solid, and the thinking is that, even if there is any minor ground movement, the base is sufficiently strong to avoid being affected - it would bridge minor settlement or ride-out some expansion.
Scattering a bit of cement over a sub-base aggregate does NOT make a sound base. It's not a proper concrete; it's not been designed to cope with the expected stresses; it's neither one thing nor the other: too weak for a base; too inflexible for a sub-base.
It's not that a "foundation base" is not a good idea: it's that any such base has to be of the right form, adequate strength, properly constructed. Scattering cement does not qualify for that description!
Beneath the bedding and paving, you can have a sub-base (unbound) or a base (bound). If it's to be a sub-base, and it usually is, then there is no binder added to the aggregate and the intention is that the layer can move ever so slightly to accommodate ground movement. A base, in contrast, is bound, rigid, inflexible, solid, and the thinking is that, even if there is any minor ground movement, the base is sufficiently strong to avoid being affected - it would bridge minor settlement or ride-out some expansion.
Scattering a bit of cement over a sub-base aggregate does NOT make a sound base. It's not a proper concrete; it's not been designed to cope with the expected stresses; it's neither one thing nor the other: too weak for a base; too inflexible for a sub-base.
It's not that a "foundation base" is not a good idea: it's that any such base has to be of the right form, adequate strength, properly constructed. Scattering cement does not qualify for that description!
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