Posted: Mon Apr 01, 2019 12:46 pm
Hi All,
First post here. Many thanks to Tony for a great site- always nice to see a guy who takes genuine pride in his work.
Right, so I have priced and won a right bodge of a job to sort. Essentially, it's uncalibrated Indian sandstone in a residental development. It was laid ten years ago, very shoddily. It varies between a half-arsed attempt at a full bed, and dot and dab, all with building sand rather than sharp, and varying strength of mix between reasonable to 'forgot to put the cement in the mixer'. Oh, and no sub base to speak of. Approx 350m2.
It's held together remarkably well considering. Last year they had it jet washed, and the gimp on the pressure rashed just kept going despite the pointing being blasted everywere! On the bright side, barely any joint raking needed.
After this, the resident's association (who we do a lot of work for) asked as to quote for repointing 'with a brush in resin compound' :laugh: :laugh: Having done a test patch a year prior ago (with an eye to doing the whole job) I sent them the following:
Please forward this email to everyone so the everyone understands the technical stuff behind both the problems and the possible solutions.
The pressure washing was not the cause- properly laid patio can be pressure washed ad infinitum. A properly laid patio is similar to a concrete slab- rock solid. The pressure washing simply washed out pointing that had already parted company with the slabs.
A path should be laid with a sub base (compacted granular fill, ie. Type 1/granite scalpings), a cement bedding layer, and then the slabs. As far as I can see, none of Budgenor's slab paths have a sub base.
Parts of your path are laid with four of five blobs under each slab, on the original soil. Other parts have a 'full bed' underneath, but as the slabs were not calibrated (this means sawn completely flat underneath), laying would require considerably more skill than was presumably available- hence a lot of slabs have cement underneath but this is only touching the slab in a few places. This is no better than the aforementioned 'dot and dab'.
Most of the paths are laid upon a building sand and cement mix. Building sand should not be used for laying slabs on- it is too soft, and moves too easily. Think of quicksand and you can understand how rain and ants wash the building sand away under the stones. Sharp sand is load bearing (due to the angular grit contained within it) and is the correct sand to use. Now, were this cement mix a decent mix then the use of building sand might not matter. However, it is almost all a very weak mix, and in parts no mix at all (just building sand)
What all the above means is that the slabs are free to move. However slight the movement, cement has no elasticity and this leads to the pointing breaking away at the sides. This has become progressively worse as the weak building sand mix has been washed way over the years. As I mentioned earlier, the pressure washer simply finished the job.
The 'bodge' to which I refer is the path on the left hand side as you exit the automatic gates, which I did as a test a year or so ago. A very wet mix was injected into the gaps with a pointing gun. This mix then gave support to the edges of the slabs where it was injected. The 'magic ingredient' in this mix was SBR. SBR is a polymer bonding agent, a waterproofer, and also adds a degree of flexibility into the mortar. It's uses are many, in particular laying thin screeds over existing concrete. In your application, these properties provide two things- one, that the mix will bond to the slabs far better. Two, that a small amount of flex (imperceptible to your eye) is allowed, hopefully (along with the extra support by injecting it down the cracks) preventing rocking and the pointing cracking away from the slabs.
The above is not correct practice, and I couldn't offer any warranty upon it's effectiveness. Were the base to continue to erode, the whole lot may again fall apart years down the line. However, the theory is (to my eyes) sound; it's relatively cheap compared to replacing the lot, and there is a test patch to examine which has been down a year. If the slabs laid upon dot and dab upon no base whatsoever are still relatively in place, simply missing the pointing, it sounds reasonable to presume that the above would go a long way to holding it all securely. It's only taking foot traffic, after all.
Now, whilst you have all the information above, there is simply no substitute for a site meeting, with me lifting some slabs and showing the committee for their own eyes the state of the underside. I would also be minded to try to lift some of the repaired section by the gates- test a slab or two to destruction even. If you could arrange a convenient time for the majority of yourselves, I should be delighted to meet you and demonstrate, either on a Sunday or of an evening.
I've since won the job and started work. It's going well- can highly recommend the PnuPoint gun- it's given us no bother and just works.
The mix I'm using is 3:1 with 200ml/kg of SBR, plus plasticiser and colourant.
Can anyone see any way to improve this bodge further?
Secondly, we occasionally (like one batch in five) have issues with cracks appearing after striking off. I'm mixing each batch in a bucket to keep it fresh, one bucket fills the gun twice over. Very sloppy mix, gets it right in there. It's not been too hot, and I'm loathe to wet the joints due to potential staining issues. I'm using 50/50 fast set and normal cement, as this gives us a good setting time for striking off. Any ideas?
First post here. Many thanks to Tony for a great site- always nice to see a guy who takes genuine pride in his work.
Right, so I have priced and won a right bodge of a job to sort. Essentially, it's uncalibrated Indian sandstone in a residental development. It was laid ten years ago, very shoddily. It varies between a half-arsed attempt at a full bed, and dot and dab, all with building sand rather than sharp, and varying strength of mix between reasonable to 'forgot to put the cement in the mixer'. Oh, and no sub base to speak of. Approx 350m2.
It's held together remarkably well considering. Last year they had it jet washed, and the gimp on the pressure rashed just kept going despite the pointing being blasted everywere! On the bright side, barely any joint raking needed.
After this, the resident's association (who we do a lot of work for) asked as to quote for repointing 'with a brush in resin compound' :laugh: :laugh: Having done a test patch a year prior ago (with an eye to doing the whole job) I sent them the following:
Please forward this email to everyone so the everyone understands the technical stuff behind both the problems and the possible solutions.
The pressure washing was not the cause- properly laid patio can be pressure washed ad infinitum. A properly laid patio is similar to a concrete slab- rock solid. The pressure washing simply washed out pointing that had already parted company with the slabs.
A path should be laid with a sub base (compacted granular fill, ie. Type 1/granite scalpings), a cement bedding layer, and then the slabs. As far as I can see, none of Budgenor's slab paths have a sub base.
Parts of your path are laid with four of five blobs under each slab, on the original soil. Other parts have a 'full bed' underneath, but as the slabs were not calibrated (this means sawn completely flat underneath), laying would require considerably more skill than was presumably available- hence a lot of slabs have cement underneath but this is only touching the slab in a few places. This is no better than the aforementioned 'dot and dab'.
Most of the paths are laid upon a building sand and cement mix. Building sand should not be used for laying slabs on- it is too soft, and moves too easily. Think of quicksand and you can understand how rain and ants wash the building sand away under the stones. Sharp sand is load bearing (due to the angular grit contained within it) and is the correct sand to use. Now, were this cement mix a decent mix then the use of building sand might not matter. However, it is almost all a very weak mix, and in parts no mix at all (just building sand)
What all the above means is that the slabs are free to move. However slight the movement, cement has no elasticity and this leads to the pointing breaking away at the sides. This has become progressively worse as the weak building sand mix has been washed way over the years. As I mentioned earlier, the pressure washer simply finished the job.
The 'bodge' to which I refer is the path on the left hand side as you exit the automatic gates, which I did as a test a year or so ago. A very wet mix was injected into the gaps with a pointing gun. This mix then gave support to the edges of the slabs where it was injected. The 'magic ingredient' in this mix was SBR. SBR is a polymer bonding agent, a waterproofer, and also adds a degree of flexibility into the mortar. It's uses are many, in particular laying thin screeds over existing concrete. In your application, these properties provide two things- one, that the mix will bond to the slabs far better. Two, that a small amount of flex (imperceptible to your eye) is allowed, hopefully (along with the extra support by injecting it down the cracks) preventing rocking and the pointing cracking away from the slabs.
The above is not correct practice, and I couldn't offer any warranty upon it's effectiveness. Were the base to continue to erode, the whole lot may again fall apart years down the line. However, the theory is (to my eyes) sound; it's relatively cheap compared to replacing the lot, and there is a test patch to examine which has been down a year. If the slabs laid upon dot and dab upon no base whatsoever are still relatively in place, simply missing the pointing, it sounds reasonable to presume that the above would go a long way to holding it all securely. It's only taking foot traffic, after all.
Now, whilst you have all the information above, there is simply no substitute for a site meeting, with me lifting some slabs and showing the committee for their own eyes the state of the underside. I would also be minded to try to lift some of the repaired section by the gates- test a slab or two to destruction even. If you could arrange a convenient time for the majority of yourselves, I should be delighted to meet you and demonstrate, either on a Sunday or of an evening.
I've since won the job and started work. It's going well- can highly recommend the PnuPoint gun- it's given us no bother and just works.
The mix I'm using is 3:1 with 200ml/kg of SBR, plus plasticiser and colourant.
Can anyone see any way to improve this bodge further?
Secondly, we occasionally (like one batch in five) have issues with cracks appearing after striking off. I'm mixing each batch in a bucket to keep it fresh, one bucket fills the gun twice over. Very sloppy mix, gets it right in there. It's not been too hot, and I'm loathe to wet the joints due to potential staining issues. I'm using 50/50 fast set and normal cement, as this gives us a good setting time for striking off. Any ideas?