The Brew Cabin

Hi,
I recently had a patio laid with Weathered Bronze Yorkstone Paving. The contractor dry filled the joint with a fine (block) sand and cement mix and watered it afterwards.

Now a month later I have many slabs that have a "dark stain" just on their edges. In very hot weather they do "slowly" dry out but it looks unsightly.

Is this the wrong type of mix causing the problem? Any ideas of what remedial action I can take to cure the problem.

Thanks

Richard
Worcs, UK

Hi Richard,

it sounds as though this could be either cement staining at the edges of the flags or a manufacturing effect similar to that discussed in another thread (the one about dark circular stains on riven flags).

How wide are these marks and are they exactly parallel with the jointing or do they wary in width? If you have a picture, send it to me via info at pavingexpert.com and I'll take a look.

The only other comment I should make at this time is that block jointing sand is less than ideal for making what should be a cement mortar. The grain size distribution is not that of a building sand, but then, for a patio, it really shouldn't matter.

Who manufactures the Weathered Bronze Riven flags?

Hi Tony,

Thanks for the reply. They are StoneMarket slabs. The dark stains are mostly parallel to the edges....but not always....

In the StoneMarket brochure they say they don't recommend the use of "modified" sands due to water retention that can lead to dark staining. I don't think these are mortar stains as when the areas do dry the slab is then fine. Perhaps rake out everything and wet-point with a 4:1 mix as suggested on your web-site????

Thanks

Richard

Modified sands means the Geo-Fix sort of stuff, also known as polymeric sands.

I'd leave the paving for a while longer. Let it have a couple of months of summer and see if it gets any better. If there's no improvement, you could try re-pointing it with a proper mortar in early autumn.

In the meantime, if you do get a photo of the markings, I'd like to see it.

O.K , thanks for the advice, I'll try and borrow a digital camera and get some photo's to you....

Cheers

Richard

Hi Tony,

I've managed to take some digital photos and have e-mailed them to you.....

Cheers

Richard

Gottem, thanks. :)



This looks very much like retained moisture. For some reason, moisture or surface water is being held in these spots and is making the flag edges look darker that usual.

The most common reason for this phenomenion is that the section of jointing immeditely adjacent to the 'staining' is lower than the rest, or it is more permeable and is 'wicking' groundwater from the bedding layer. It's not a 'stain' as such; it's only water and shouldn't have any long term effect, but then again, it shouldn't be as obvious and widespread as it seems to be from the pix you sent.

Is the jointing 'soft' at these wet spots? Try pushing a screwdriver into the jointing. It really shouldn't be able to penetrate, but if it does, it indicates the jointing mix is knackered (technical term) and should be replaced.

However, if the water is being held there because the affected sections are lower than the surrounding paving, there's not a lot you can do. It may well disappear in time, but it really isn't worth losing any sleep over.

See how it goes over the summer. :)

O.K , thanks for the analysis and your time...much appreciated

Hope its ok to butt in on conversation. Interesting comments on the dark edges to slabs as seen in photos.
We are a garden design and build company and have in recent years carried out the instalation of a number of paving display areas for various suppliers and manufacturers.
This dark staining seems to be becoming more common, along with the dark staining blotches etc as you mention.
We understand this to be caused by differential curing within the concrete, resulting in undisolved carbonates from the cement to appear on the surface.
There seems to be various explainations and 'excuses' given, but many different manufactuers of paving slabs all suffer.
There seems a link between moisture under and between the slabs that is drawn to the surface bringing these dark stains with it, along with curing time allowed by the makers before wrapping the pallets etc, thus traping moisture in the slab.
We keep trying different methods of laying, pointing etc to try and find a best way. Having last week replaced a whole section of display material that is less than four months old due to staining etc.
Interested to hear comments on this and cases / solutions if any.
Fantastic site by the way! We now recommend it to our design clients as a reference point.

Thanks for the extra information....have you found any method of pointing or remedial work that can be used to minimise this effect? Is there any chance of it disappearing over time?

To be honest I'm a bit fed up as the slabs weren't cheap and we paid a lot of money to have them put down properly. However I can't really call the contractor back as it appears they have done nothing wrong???

Cheers

Richard

I've not seen differential curing cause such 'neat' tone differences within flags before. All the pics sent to me by rfearn exhibit darkening along one or two edges, and it's more or less parallel to the edge, whereas the diff cure effects most commonly evidenced are like those shown in the thread....

Dark stains on riven wet cast slabs

....further down this forum. These show classic diff cure effects; tone variation across the entire flag and a lightening rather than a darkening, although this may be a consequence of the dyes used.

I'm not aware of any independent, authoritative answer to this effect. As said, there's umpteen excuses from the manufacturers, but the better companies are prepared to offer an exchange for unmarked flags, if you twist their arm enough. :)

It may be worth asking Stonemarket to send out a Tech Rep, but I don't think they'll accept any responisbility. The first port of call must be the contractor, as they supplied the materials and are the ones with a 'contract' with Stonemarket, but don't hold your breath!