Alberto Clemente hails from Sunny Spain and for some reason, probably linked to my technical incompetence, he's not able to post directly to The Brew Cabin, so he's asked me to post this on his behalf....
Hi Tony,
first of all, congratulations for your website, it's the best that I have found on the topic. I'm pretty new in the paving world, just started to work for a small, familiar manufacturer and it has really allowed me to catch up. Here is something I hope you can help me with:
We have received some reports about dark stains on randomly located concrete slabs in urban floors, and I'm trying to get to the cause. However, I suspect there are different types of stains, with different origins. I'll try to describe both of them.
In this case, the problem is located next to a water font. There is a blackish shade on every slab inside the water splash radius, including the red concrete bikeway (picture 1). This makes me think there is a superficial-nature stain, probably from fungi or algae, or due to a more intense walking in that area. I have cleaned the surface with a metallic brush and the stain kind of fades away, and also bleach seems to partially take it away.
On the other hand, other type of stains have appeared a little further from the font, downhill, which suggests draining water above or beneath the slabs is involved (picture 2). These stains have a rather different look, with watermarks similar to those in the efflorescence cases, so I would think of either a pigment migration ascending through the slab due to capillarity and surface evaporation. I've checked that black and red pigments have a much bigger migration capabilities due to their smaller particle size, so in some cases these pigments can be expected to appear on the surface. Also, some impurities (iron?) present in the mortar or sand bed, or in the back of the slab may be involved.
In some other places (pictures 3 and 4), similarly looking stains have appeared, with no water source to be seen (although moisture could very well be hidden underneath). Also I’ve noticed these stains seem to be more common in areas covered under a roof (pictures 5 and 6), maybe meaning that the evaporation rate is important as those areas are more shadowy and would dry more slowly.
I’ve seen these types of stains in a lot of places, in slabs made by different manufactures, but surprisingly haven’t read a lot about them on the net. Have you ever seen this problems before? Can you give me a hint on was going on? Also, if you can point me any further reading on this issues I would also appreciate it (no problem with chemistry jargon).
Thanks a lot for your time. As I said, best website on the topic. Keep the good work up!
Hi Alberto
welcome
no chemistry expert am I but this looks like god old efflorescence and some reflective staining
I guess the slabs have a high cement content and were laid on a cerement bound bed ?
the constant splashing of water either rain or from the fountain has caused this
IMHO
thanks for the post Tony, I have finally fixed the issue with the logging.
LLL, thanks for your answer. There is some efflorescence, I agree, although it looks to me that there is something else going on. I think the stains are too dark for that, especially those in pictures 3 and 4. I also thought about reflecting staining, but the slabs are supposed to have been laid on a full mortar bed. The cement used in the laying, I don’t know. Regarding the high cement content in the slabs, I haven’t thought about that. Can you develop it a little bit more? Given the case, what would you think its happening?
I said in my first post that I'm facing stains with different origins, and I bring a couple of pictures that point that way. These are slabs severily staind by the flux of water, above or underneath them. I guess its a pretty common thing but, are there any ideas on why it happens?
Hi Alberto
On this particular image the staining appears to start immediately below the weep hole (assuming that’s what it is) and flows away from that point. Could that be a flower bed behind the (retaining?) wall, if so could the contamination be emanating from there in the form of a diluted fertiliser?
Craig
Hi Craig, thanks for your reply. You're right there is a grass bed and a tree there, so the fertilizer could be a cause, although I don't know by what process. The stains are superficial, but are not dragged soil, and also appear from joints between slabs out of the water flow, probably because there is water arising there. This is a link to a street view of the place, just to let you whatch the whole picture.
I have been minding the floor for a month (something I didn't do before) and I have noticed this kind of water stains in many places, but I can't figure out why they happen...
Hi again, I know I’m being persistent but I’m afraid I’m not being very clear here =). I’ll try to illustrate my thoughts with pictures. I’m looking for the origin of dark stains on slabs,but I have found what look like completely different types. However, all of them seem to be related with moisture or water:
1) In some cases dark frames appear near the joints, and stay even when the slab and the joint material (I guess) are dry. https://imageshack.com/i/po7tQBZYp
2) In others, looks like rain water has dragged material form the bed or joint, staining the slabs downstream permanently. https://imageshack.com/i/pmPSfbLQp
3) Other stains appear on the surface, generally in the center of the slab, even though they were laid on a full mortar bed. As LLL pointed, I agree in the first link there are efflorescence on top of a moisture stain, and that probably gives the impresion of the stains being darker. I wonder how that moisture stain is produced, and if could be other things in play, like iron content or ferrite reaction with CO2 to form iron oxide. https://imageshack.com/i/pmnKTbj0p https://imageshack.com/i/po2GPEhMp
So, in my opinion water is in the middle of all this. What does water do? Is it the cause, or just a vehicle for whatever is causing the stains? What is the cause of moisture preferential retaining on full mortar laid slabs, like in this picture? https://imageshack.com/i/poCysog8j
Why do sometimes remain dark after drying, like in the first link?
The more I look, the more of this “defects� I find wherever I go, regardless the manufacturer. Any more ideas? Any thoughts on any of the points would be of much help.
...and another one. Maybe this one is more illustrative.
This is a pile of all-concrete big slabs, stacked in the open for several years. http://imageshack.com/a/img924/3589/C2RGIA.png
To my eye, they have a diferential curing problem because of the stacking and a classical efflorescence, but there is also a darker watermark even the slabs are dry. Any ideas on what is that dark boundary? Cause looks like it has been draged by moisture evaporation...
I think you have a variety of causes at play here.
Picture 1 looks like iron staining caused by water splashing onto the MH cover and grating
Pictures 2, 3 and 4:that klooks like what we call 'Black Efflorescence;, a chemical reaction that takes place in some concrete products under certain conditions. I've never heard two arguments that agree with the precise cause of Black Eff, Some say it's a result of certain minerals in the clay used during cement manufacture; others say it's poor air flow during the curing phase; a popular belief is that it's linked to local groundwater chemistry; and I've even read suggestions that it's caused by bedding contamination.
Personally, I think it's most often a combination of summat in the cement (possibly the clay minerals) combined with summat happening locally, probably summat in the groundwater or local environment, but that's little more than an experienced guess as I'm not a materials chemist.
I've seen Black Eff affect products from all manufacturers, from the big mega-multinationals to the one-bloke-and-a-vibrating-table. It seems to be no respecter of reputation, and it doesn't seem to have a 'fix'. In bad examples, it's just easier to swap out affected paving.
Pictures 5 and 6: I'm assuming these are the same area of paving. I think this is partly a dirt/grime issue complicated by differential curing. The roof area will have afforded additional protection to the flags (I'm also assuming they were still fairly 'green' when originally laid) and this has impacted on the curing/weathering.
When this sort of thing happens and strats to cauise major concern, the usual answer is to lift some of the affected flags and pack them off to the lab for testing. While this can sometimes indicate higher-than-usual levels of this or that mineral (the aluminium minerals are often implicated) it doesn;t expolain how it all came about or how it can be avoided in the future.
It's a fascinating problem, and one I've discussed with dozens of people at scores of manufacturers for decades, and on four different continets, but no-one seems to have a definitive answer. 'Tis a mystery!
thanks Tony, that has been of great help. If I get to perform some SEM-EDX analyisis of the stains, I'll let you know what elements are causing it. However, you're right, that won't explain the mechanism by it self...
If you happen to have some reading about this black efflorescence at hand, ill be happy to learn more about it.
