When you say "more laitance" ?? Laitance forms during the first few hours as it compacts and sets. Once its done its done. You dont get more appearing afterwards. Drying rate is a bit faster than 1mm per day so depends on depth and of course the drying conditions. At 15mm its about 14 days at 50mm it's about 28 days but sanding typically anytime after about 7 days. In terms of drying time it's best not to rely in time but to do a test.
Interesting as I've been grinding gypsum floors for a while and in the early days I noticed that if ground (not this particular one) off within 10 days there has been more laitance appear so I always book in 28 days after it has been poured and not had an issue since. Hence why I'm intrigued about this TS-15.
You sure that’s Laitance and not site monkeys messing the surface. It’s a shame building sites don’t cover the new screeds. Sent from my iPhone using Tapatalk
TS-15 is no different to any of the other gypsol screed formats in the requirements for preparation. There are some different versions in terms f low laitance, laitance free and standard laitance but in reality they should all be treated the same way in terms of prep for coverings. Laitance is fine particulates in the sand and the binder which come to th esurface of the screed along with bleed water during the compaction phase which happens in the first couple of hours. Once it is compacted and the bleed water evaporates thats it. No more laitance can appear. You may have seen something else e.g. efflorecence or something or you may have not removed it all in the first place but i can categorically assure you it is not new laitance. That would be impossible. I have loads of debates on site with floor layers and adhesive reps on this subject so its perhaps good to talk about it here. Often laitance ets the blame when its nothing to do with things. A good example was a site in London I visitted recently which had been properly sanded and all loose laitance removed (as far as we could tell). This was done about 14 days after instalation and extended to several thousands of square meters. Unfortunately, a few weeks after sandnig the decorators using their spray paint had left a layer of a fine powdery, almost flaky substance over most of the screed. The floor layer rightly queried this but wrongly queried it as laitance. It was paint. The effect is the same in that it still needed resanding but thats beside the point. This happens a lot on contract sites, not so much on domestics. Your target of 28 days is fine as it is more likely you will catch and remove all the other cr@p that the site leaves on the screed and which needs remving just as much. I always say sanding and prep should be done just before floor coverings are to be applied. Im never quite sure where the 7 days thing came from. Its a bit of a myth that the laitance must be removed after 7 days. It doesnt really matter when its removed as long as its removed. Notice I said Loose laitance as well. (awaits the fall out) it is not necessary to remove material from the sreed surface if it is fully and completely bonded to it. its easier to say remove all laitance than it is to say remove loose firable laitance as this can be seen as subjective. however it is only necessary to remove loose laitance along with any other contamination likely to affect adhesion. All too often we see people grinding several mm off the top of the screed. Most of the time its just not necessary. it does depend on the primer used but unfortunatley it also depends heavily on the knowledge of the primer supplier which is aften flawed. I blame the university of google for that... The only time it becomes necessary to remove several mm is when there is actually a defect with the screed e.g segreation due to over watering when you get a thick chalky powder on the top of the screed. this is not laitance and needs to be trated slightly differently. questions on a postcard please...
I always thought you were supposed to remove the surface, to reveal some aggregate. Basically, the soft shit. That said, and as I have said on here many times, most new build sites get absolutely no grinding work completed and we have installed 1000`s of sq of lvt over them, with just a coat of 121 primer and green bag, or similar. So, is it true that grinding improves drying time, or not ?
No. I'm afraid that's another enduring myth. We used to say so but it had never been tested so was an assumption. We tested Gypsol in 2016 and found that laitance made no difference to the drying times. The report in in the public domain so if you want a copy ... its only about 26 pages or so What we do see though is people use surface impedance meters to measure moisture. Not only are these no where near accurate enough they are also affected by the surface characteristics. Laitance generally gives a higher reading giving the impression that it's taking longer to dry. We dont recommend these meters for anything other than a rough indicative survey.
Does that mean protimeters etc or similar ? Whats the test should be used ? Hygrometer ? I have heard a carbide bomb test, but I have to admit I don`t know what that is I am surprised opening up the surface does not improve drying times, but there you go.
Weird that as we did a job last year where we tested a slab and got 90-93 RH , the customer got it grinded ( well sanded ) 2 months later it was still high 80s. I then did a 2mm grind and a month later it was below. So very surprised if it don’t help it dry. Both tested with hydro boxes though. That’s the way f ball and ardex say to test. what way do you test the floors Alan.
I've got ocd, so nothing is ever right for me. As said early days it appeared that it was still giving off laitance had to grind it again in couple of areas but from then on I always booked in further jobs 28 days and never had the issue again. Was it me? was it the floor? it wasn't contaminates so not sure! just decided from then on to leave it 28 days regardless.
Grinding subfloors is ridiculous. There’s no need for it. Put the right screed down and you won’t need to grind it or wait ages for it to dry Sent from my iPhone using Tapatalk
There are 3 tests which are appropriate, hygrometer (I dont like hygrohoods and prefer direct read hygrometers), carbide bomb which us quickest and oven dried sample. These are the only tests recognised in British standards.
Grinding subfloors is necessary if theres something wrong with the subfloor. Grinding and sanding are very different. However ALL screeds should be subjected to mechanical preparation. (I await the "I've never sanded a screed and never had an issue" brigade to which I would answer my advice is based of best practice, British standards, national trade association advice and not on what you might be lucky to get away with. Mechanical prep is not about laitance. As for the right screed... for me of course its one of 11 different formats of Gypsol ( note...shameless plug)
Best practice is fine. But, yes, in the real world, no-one would sand/grind a sand & cement screed. For the most part, I find its about the BREEM & load rating.
So are we saying that this TS-15 can be laid on day one and 7-10 days you can sand/grind it and it will be dry as well?
