I was asked to take a look at some high quality wet-cast flags laid to a patio somewhere in Britain yesterday. A strange marking/discolouration has appeared on them since they were laid last November (2006) and although the manufacturer think they know what the core problem might be, there are a number of anomalies that defy explanation.
For example, the effect appears only on flags laid to this one job. There is no indication that flags manufactured at the same time and laid to other jobs, in other parts of the country are similarly affected. Which automatically makes you think it must be summat the contractor did while laying them, but try as I might, I can't find fault with either the bedding or the jointing. And just to further confuse the issue, some of the flags that are affected have been used a copings on a wall rather than as patio paving, so we can't blame bedding or jointing.
Here's a few pics....
This is a coping stone: it was a damp day, so you have to ignore the wet/damp areas and concentrate on the dark discolouration, which looks like lichen. There's a close up of the above image....
...I know it looks like cement staining, as though some eejit has spilled neat cement powder onto the surface, but we know from other examples on site that this is NOT the cause. In fact, we're fairly sure it's a carbonate deposit that has been brought up to the surface by some means, but we're not sure.
Here's a picture of a flagstone, albeit wet, which shows the problem affecting the edges of the flag, as though it was 'picture framing', but notice how it has only affected the very corner of the lower left flagstone, while it varies in width on the uppermost flag and isn't present at all on the lower right flag...
.... on other flags, the problem doesn't affect the edges at all but only appears in the centre of the flag, while some flags are literally covered completely and others haven't a mark on them.
If anyone has any experience of a similar problem, or can help us identify the cause of the problem in any way, I'd really like to hear from you. I'm happy to email hi-res full-size photos to anyone that can help, and I have a small number of other photos, but the wet state of the surface really doesn't help.
Any thoughts? Email or PM me if you prefer.
This "item" will also be posted to the News section of the website, to see if I can elicit any help from those not brave enough to enter the Brew Cabin.
have they got animals p!ssing on the slabs?
certainly looks odd
has the client used any cleaning agents or were the slabs stored in an unusual spot? say in a car workshop?
??? LLL
Tony, rather than say - this is the problem, - a few comments for you to consider. The lichen effect, considering the period of time, is it possible that black algae might be present? Alternatively, the pigments made by the chinese sometimes have a carbon content to increase the blackness, this leaches out over about a year. If the manufacturer used a two colour mix (light/dark black pigment) it could explain the irregular patching phenomenon. dependent on the water content, when vibrating the slab, in a looser mix the corners get a more watery consistency of mix, the water/colour absorbency is greater and this could influence the frame effect. Also, depending on how well/consistently the release agent is applied to the mould affects the finish. A change in the type of release agent could also affect the casting. Bottom line is I think you might have a combination of issues that are influencing the pavers.
Will keep watching this topic
regards
One other possibility is that this mix was at the end of a batch, at the end of the day, and just didnt make the grade - different consistency within the mix, and not picked up by the operative - sounds dumb but I've seen it before.
Looking at this from a cleaning angle, has someone used an acid to try and clean these and stained the paving? From the photos, it also looks like it could be a grease or oil based stain. It does not look like Black Lichen to me, to big an area. Would really need to see the paving dry.
LOOKS LIKE SOMETHING HAS CAUSED THE SLAB TO LEECH THROUGH CANT THINK OF ANYTHING THAT WOULD CAUSE THAT, UNLESS THEY PUT SOMETHING IN THE JOINTING MIX, WATERPROOFER OR SEALER?:(
general builder, maintenance engineer, gas and plumbing installations, extensions etc
The photos shopw flags that have NOT been treated with an acid-based cleaner. That has been done elsewhere on the site, in an attempt to remove the stain, with little, if any effect.
It's not oil or grease or algae/lichen, despite its appearance. It's a hard, very dark grey, inorganic deposit that can be scraped off with some effort.
seems to be chemical reaction then. Must be summat in the bedding mix... are there any other sites nearby which have not got this? What about local industry? Have seen stuff like that near industrial sites, particularly where metal grinding and other such processes are abundant how about getting the scraped stuff analysed....are these slabs real or are they manufactured ? If manufactured could be a poor graded mix? Let us know what you find, its a corker:)
general builder, maintenance engineer, gas and plumbing installations, extensions etc
Matt: The bedding is plain grit sand with cement and, according to the contractor, nothing has been added. I tend to accept that as the staining is not present on every flag.
Nor does the location have any involvement as the same flags were laid by the same contractor to the property next door and elsewhere on the cul-de-sac and they are all fine, according to the manufacturer.
Having the affected flags tested is quite expensive, more than the size of the job warrants at this point in time, but it may be something to consider if we can't find an answer. As for the mix quality, I would doubt very, very much that this was a compromised mix. First of all, more than one batch of concrete would be needed to manufacture the flags used on this job, and secondly, the manufacturer has decades worth of experience and has never seen anything like this before. Their quality control is exemplary.
Roger: AFAIK, the acid used to test clean some parts of the affected flags is a standard HCl-based patio cleaner. It has removed some hazing and affected the flag colour slightly, but has had no impact on the stain itself.
If the other sites are as close as you say, its unlikely to be an external issue, and so seems to just be down to the slabs themselves. Did the slabs for the other jobs get delivered at the same time/same batch? It does seem to be a composite problem. Maybe an odd batch of mix and just slipped through the net,or maybe someone tried toclean up a messy job and hasnt admitted to it? Going to be a pig to sort out m8. intrigued to say the least. When you scrape off the lose stuff is the slab ok underneath? Does it come back after?
general builder, maintenance engineer, gas and plumbing installations, extensions etc
So, a little bit of Sherlock. 1)The slabs/house next door, no other slabs affected. Not the base lay.Not the mix, not the Quality control. Long term storage leading to algae/water/rust staining - no, some evidence should be typical for next door. Conclusion, the problem must come from an external addition to the slab on these premises. Whats been added - a) the grout, b) some kind of aftertreatment/ spill/ influence not common to next door neighbour. Vegitative matter? seems unlikely tho possible, carbonacious surface points to reaction between iron oxides and chemical treatment, but does not explain frame effect. So, Unlikely unless we have compound errors. Could whatever is in the grout be causing the frame effect? When grouting did the guy laying make a mess of the grouting and have to clean extensively? Is the owner a amateur beer/wine /something other maker?
Tony McC wrote:Roger: AFAIK, the acid used to test clean some parts of the affected flags is a standard HCl-based patio cleaner. It has removed some hazing and affected the flag colour slightly, but has had no impact on the stain itself.
Tony,
I would try a Hydrofluoric acid based cleaner, you could try this company, www.tensid.com they will send out samples, Hydrofluoric is not nice to work with, you really need to PPE up for this, it can be a killer in the wrong hands. I use this acid, so if you want to you can give me a call or email and I can explain how to use it safely.
