The blood and stomarch pills is happening to my slab!

[MikeSharp01]

Arrived at site this morning to find some weird happenings on my slab, see attached, I have no idea what this is all about but they all start on the edge of the DPC strip under the sole plate and were surrounded by salt deposits until I swept them to see what was going on. If I lift the DPC is does not continue under it!

 

Any ideas?

 

Wonder if I should lift the sole plate and let the concrete cure some more. DPC was put down 10 days after pouring and slab was kept wet for first 5 days.

 

 

[Ian]

Unlikely that it's anything to worry about. It looks a bit unsightly but it looks from the photo as if it is simply some efflorescence coming to the surface of the concrete  as it dries.

explanation link:

http://www.concreteconstruction.net/how-to/efflorescence-causes-and-solutions_o

 

Ian

[MikeSharp01]

Thanks @Ian great link, I will get some acid and see if I can sort it.

[Ian]

@MikeSharp01 

I'd try brushing it off for a while and see how you get on. There's a good chance you won't need acid.

 

It depends on how much time you have but if it's still happening when you are ready to apply floor finishes on top of the slab then it will need dealing with properly as adhesive etc won't stick to it.

Edited April 3, 2017 by Ian

[MikeSharp01]

Thanks again but this is supposed to be the finished surface there are no finishes. This is the garden room slab and has been designed to test the processes for the main house. I need to avoid problems with the main slab. Will keep brushing. 

[Ian]

@MikeSharp01

it helps if you can get the slab covered by the roof and out of the rain as soon as possible after the concrete is poured. Often it's slabs that are exposed to a lot of wetting and drying which experience problems with efflorescence.

[MikeSharp01]

Ah it was probably my fault as I could only wet it and cover it on a daily basis. So it was drying out. I will have to make a cover for the main slab and keep it misted. 

[Ian]

1 hour ago, MikeSharp01 said:

Ah it was probably my fault as I could only wet it and cover it on a daily basis. So it was drying out. I will have to make a cover for the main slab and keep it misted. 

@MikeSharp01

it depends on the time of year that you pour the slab. Here in the U.K. you would not normally wet a slab like that by misting with water unless it was a very hot summer. I've never seen that done here in the UK (maybe it's advice from the USA?) Here in the U.K. normally they are shaded from direct sun and rain and if it's summer the usual method to slow down evaporation of water from the concrete and aid proper curing is to cover it with a layer of polythene as soon as you are able to after the slab is poured.

Edited April 3, 2017 by Ian

[ToughButterCup]

6 hours ago, Ian said:

[...]

Here in the U.K. normally they are shaded from direct sun and rain and if it's summer the usual method to slow down evaporation of water from the concrete and aid proper curing is to cover it with a layer of polythene as soon as you are able to after the slab is poured.

 

Thank you. Pouring tomorrow.

Ian

 

[MikeSharp01]

7 hours ago, Ian said:

Here in the U.K. normally they are shaded from direct sun and rain and if it's summer the usual method to slow down evaporation of water from the concrete and aid proper curing is to cover it with a layer of polythene as soon as you are able to after the slab is poured.

Thanks @Ian I think I just thought through the need to keep the evaporation rate down so keep it wet. I did cover it for the first three days and wetted it under the cover, but was worried that the tarpaulin would damage the surface. It does get direct sun and the last few weeks have been decidedly warm for the time of year on the Kent coast. This was a learning exercise and I have learned a lot in readiness for the main pour. I have it flat with +/-2mm (actually -2, +1.5) and the vast majority of the power floated surface is very good. I just need to take more care of the slab on the main pour. Every day is a school day after all!

[Dudda]

As it's localised and wetter I think it's extra moisture or rain running along the DPC and falling onto the slab at these areas. The additional extra moisture and efflorescence in that area of the slab compared to the rest would explain it and also explain why it doesn't run under the DPC. The key to stopping it would be to ensure that any moisture or rain that builds up on the DPC and flows along it like a gutter drains out and not into or on the slab.

[Ian]

2 hours ago, MikeSharp01 said:

but was worried that the tarpaulin would damage the surface.

@MikeSharp01

As a possible alternative there are liquid chemical (spray on) curing compounds that are available however I have no personal experience of those.

 

2 hours ago, MikeSharp01 said:

@Ian I have it flat with +/-2mm (actually -2, +1.5) and the vast majority of the power floated surface is very good. I just need to take more care of the slab on the main pour. Every day is a school day after all!

 

That's an excellent result Mike!

 

2 hours ago, MikeSharp01 said:

@IanEvery day is a school day after all!

 

Mike - you mentioned that your concrete slab will not have an applied finish. I think the biggest single risk with the larger house slab that you will be doing next is cracking which would then be visible as the slab surface will not covered later by carpet or tiles. If an engineer has designed the slab for you then they should be able to advise on that. Have you got reinforcement mesh in the slab and does it have UFH?

[jack]

One thing to consider is that concrete is very sensitive to mixing and components. We had terrible spalling on the surface of our slab.  While the weather (unexpected frost and rain squalls during and after the pour) didn't help, I'm convinced that the onsite mixer lorries didn't help.  At various points, the MBC guys were shouting out for less water in the mix.  

 

If I were doing it again, I'd insist on premixed concrete rather than the onsite mix, even if it were more expensive.  This would doubly be true if I were relying on the final surface being perfect.

 

You should also look into what admixtures might be used to improve curing, reduce cracking, increase hardness, etc.  I know there's a lot of thought goes into these where a polished concrete floor is being laid, so there's a chance similar considerations might apply where you're relying on the finished slab surface.  I'd talk to a concrete polishing company and see what they think (PM me if you want to know who we used).

[MikeSharp01]

12 hours ago, Ian said:

If an engineer has designed the slab for you then they should be able to advise on that. Have you got reinforcement mesh in the slab and does it have UFH?

@IanThe passive slab will be engineer designed, they will know its to be the finished surface and it will have UFH.

 

10 hours ago, jack said:

You should also look into what admixtures might be used to improve curing, reduce cracking, increase hardness, etc.  I know there's a lot of thought goes into these where a polished concrete floor is being laid, so there's a chance similar considerations might apply where you're relying on the finished slab surface.  I'd talk to a concrete polishing company and see what they think (PM me if you want to know who we used).

@jack Thanks for this, I will PM you for your details, the other half does not like polished (Terazzo style) concrete but would like us to end up with the shiny grey surface. We are leaving 6mm of headroom to allow us to put a very thin finish on if we need / want to with a sunken mat zone at the front door.

[divorcingjack]

Hi @MikeSharp01, 

 

Following with interest, as we're planning on a power floated concrete finish too for our MBC slab. They have agreed to let our contractor (farm sheds are his speciality!) work the slab after them to get the final finish. 

 

Have you asked MBC for a different mix or anything, or is it just the regular C35? 

[MikeSharp01]

We used RC25/30 concrete mix as this is used in slabs with reinforcing steel and is specified for light traffic warehouses s should be up to a normal home. It can be power floated and further polished if required after chemical densification.  The slab we poured was the garden room slab and designed to be an experiment for all aspects of the main house construction following a couple of months behind. As I have said elsewhere we learned a lot but perhaps the biggest on the day learning experience was to powerfloat in the same area sequence as the pour and over about the same period. We ended up power floating the final area too early, we were constrained by the hire of the float and although it has come out acceptably it is not as good as the rest of the surface.

 

The salts coming out happened later, after about 10 days in the form photographed above but was happening around areas of drying out from day 3. It looks to have been my own silly fault as I only covered it for the first two days and from then on I would wet it when I arrived at site and keep it wet during the day but it was often dry the next day so as @Ian has pointed out this constant wetting and drying probably caused the problem. The problem is not getting worse, no more areas have appeared and I keep those that have swept. I am hopeful all will be well.

 

[mvincentd]

@MikeSharp01 how has your powerfloat worked out?  My interest is that I have what I think is efflorescence all over mine totally obscuring the underlying colour/texture.  It was poured about 10 months ago and responds similarly today as it did a week after the pour, ie dull whiteish coating which when washed and brushed or jetsprayed reveals lovely powerfloat beneath but then dries back to the same whiteish coating.

I'm now looking to seal the floor (not grind) with a penetrating sealer to end up with a look pretty much like when the slab is wet, but less shiny hopefully.  

I'm struggling for a convincingly unified viewpoint from google so very interested in your real life experience.  I don't want to seal in the white scum appearance!  Also, its waterproof concrete.....did you achieve a penetrating sealer on waterproof? Thanks

[MikeSharp01]

Ours is just over a year old now and it is looking good. The two or three tiny imperfections on the floor described above have all but disappeared and they don't show up unless you are looking for them.  We have no white coating like you describe. Ours is usually coated in mud, as we use the space as a site hut, but when it dries I just brush it off and it looks as good as it did a few days after the pour. MOH is happy with the finish so we will do the same for the main house only this time densify / seal as you suggest to get the shinyer surface. Ours is RC25/30 so no more waterproof than that. Not sure I can be of any more help about the white stuff still emerging from your slab.

 

One thing I have noticed is how optimistic I was about timing.  "A few months behind" - my eyes how nieve was / am I.

[mvincentd]

Thanks @MikeSharp01 that's interesting....seems my white scum might be 'abnormal' then.  Oh well, just another hurdle to get over!  Bracing myself now for someone to say 'oh no, you can have an exposed powerfloated slab if it's waterproof concrete..."

Great that you have so few imperfections in yours too.....ours has some 'gnarly' areas....we happen to be perfectly happy with this look luckily.  If I were passing one thing onto people regarding powerfloats it's that it must be accepted that what you'll end up with is a bit of a lottery. 

 

Timing.....heh, all targets be they time, financial or quality seem only to serve as a record of what didnt happen.  From the moment the first spade hit the ground i'd say we've dropped an average of a week a month, but from December to March we might as well have gone skiing, I think it was worth maybe 3 dry weeks work.

All academic though Mike isn't it.....there's no turning back from these follies...keep the faith and soldier on.