Cementing down flags

[Sore Arms]

Hi Tony

Have been reading the site as I've dug for Britain in the back garden... have now managed to confuse myself.

Clay and topsoil dug out (70 sq m), DTP1 is in (7 tonnes!) and now I'm just gearing up for the patio itself

Am looking to use Stonemarket Trustone Torvale and was planning on a wet cement mix a la ground force et Mr Walsh. Read some other topics in this forum which implied more of a dry screeding based approach....

so.. am now confused as to what is the best approach.

any help/enlightenment would be great

Thanks

[84-1093879891]

Some contractors prefer a wet bed, but I prefer a semi-dry because it's easier to work and less messy.

Also, we've recently been looking at a problem with 'cement leaching' with some of the imported sandstones, and there is some evidence that wet bedding is implicated in the problem to some extent.

This effect occurs a few weeks after the flags have been laid - an efflorescence-like staining starts to appear along the joints, just the first 10-15mm of the flags, in from the edge of the mortar pointed joint. It would seem that calcium carbonate and associated ions are being leached from the mortar used to bed/joint the flags, and that these CaCO3 ions are passing through the rather porous stone, ending up on the evaporating surface, ie. the 'tops' of the flags.

We noticed last year that the problem was nowhere near as common on imported sandstone flags that were laid on a semi-dry bed and pointed with a very stiff mortar or dry-grouted. The problem seemed worst with the same flags when laid on wet beds with wet mortar joints, and particularly in early-mid spring.

Now, all of this is observation, and not been subjected to proper lab testing, mainly because there's no room for any more flags in the high tech research facility at the bottom of me garden, or at least, not until I get all them damned kids' bikes out of there!

I've been looking at the problem in conjunction with a certain company and we're hoping to find a solution, either by using some chemical jiggery-pokery, or by developing a laying methodology that minimises or eliminates the issue. However, our experiences so far do suggest that semi-dry bedding is a 'safer' method than wet bedding.

Now, this might be as much of a problem with the Torvale, as it's not a sandstone - it's what is known as a Dolomitic Limestone, and we've not tested that particular stone, so I can't really comment.

All I can say is that you should choose the method that suits you best. Try the semi-dry, and if you think it's abit of a struggle, try a couple of metres using wet bed, and see if that's any easier. Neither method is 'wrong' or 'better'; they both work fine, so it's a matter of personal choice.

[Sore Arms]

Thanks Tony


So following on from this, a semi-dry mix seems more workable for a novice to get right and level, but a couple of questions:

1. can you get this delivered from a company?
2. how long (subject to weather conditions of course) does it take to go off? hours? days? could I says spend saturday putting down the mix and sunday laying the slabs? (I have read the concrete background archive on the site and there seems to be a lack of water in the semi mix)


Thanks - great site btw.... you know, you could make a living out of this!! :-) ;-)

[84-1093879891]

You can get a semi-dry mix delivered by almost any reday-mix supplier, but the cost is usually prohibitive, as they charge for each cubic metre of material delivered, plus a charge for each cubic metre of wagon capacity not used.

So, let's say it's 80 quid for a cubic metre, and you order 2 cubic metres, which would be enough to cover around 40 square metres. That's 2 x 80 quid = 160 quid for the concrete, plus 4m³ of empty wagon @ 25 quid = another 100 quid, all plus yer VAT.

Using up 2 cubes of mix is no mean feat. I would expect a professional flagger to manage, no problem, but for a diyer, I reckon you might be struggling. Hence, it's usually better to 'mix on demand' and just churn out another barrowful/mixerfull as and when you need it.

The big prob is, that, even with no added water, the semi-dry mix starts to harden after about 4 hours. It doesn't harden as quickly as would a wet mix, so you do get an extended working period, but, as you can see, taking 2 cubic metres and trying to use it up in 4 hours is probably asking a bit much!

[mouldmaker]

The problem mentioned - the 'picture framing' effect also manifests iself on concrete paving when laid on a too-wet base, or where there is insufficient drainage underneath.

[84-1093879891]

What type of concrete paving, though, Mouldmaker? We tend not to see the picture-framing problem with low water content pressed flags, but it does occur with some of the wet-cast/open-moulded products which we use for patios and gardens.

What's your experience?

[whareaglesdare]

Hello Toni,

As a 'new' user of this fantastic site, just want to say well done with it.

Getting straight to my question, I am interested in 'cementing' down my slabs, like the ground force method.
The slabs I wish to lay are small redish terracotta tile lookalikes (made from concrete).

Is the bed system the same as the spot system? could accidents with the wet mortar mark these reproduction tiles ?
I'd be grateful if you could give this diyer as much advice as possible.

Keep up the good work,
kind regards,
patrick

[84-1093879891]

The full bed system is not the same as spot bedding. Spot bedding leaves voids in the bed whereas a full bed covers the entire base of the flags or whatever else is being laid.

However, the picture-framing issue can occur with the Terracotta-like jobbies, just as it does with some other wet-cast products, so I'd suggest keeping the water content of the bedding mix to an absolute minimum. Because these "tiles" are relatively small, they need to be anchored fairly securely. A professional contractor could/should be able to bed them onto a semi-dry mix, but for DIYers, the moist mix, which is just that bit damper, gives a degree of adhesion without saturating the flags/tiles, as would a wet mix.

One 'trick' that's being tried out by one manufacturer is to 'seal' the base of the flags/tiles in the hope that this will prevent the units sucking-up moisture from the bedding by capillary action. I'm not sure how succesful this has been, as it's early days yet, but I'll be writing about it in another month or so, when we've had a chance to review the results.

[mouldmaker]

Sorry to be so long getting back re: pictureframing. Wetcast paving specifically, the likes of Stonemarket, Town & Country (Remember them?) et al.

[mouldmaker]

Sorry to be so long getting back re: pictureframing. Wetcast paving specifically, the likes of Stonemarket, Town & Country (Remember them?) et al.