Concrete curing question

mr_eddy

Posted the below on the screwfix forum but no reply yet so thought maybe someone on BR may have some answers...


So we have had a conservatory fitted, about 4/5 weeks ago the chaps came and dug the foundations and fitted the concrete floor. Its about 3" thick (roughly 7cm), any way the builder told me that concrete 'cures' rather than dries so rain water won't stop firming up etc. We have had some rain showers but the surface seems fine.

The chaps came back 3 days ago and fitted the rest of the conservatory so its been out of the rain now for 3 days. We have a fan heater on heating the room up for a couple of hours a day and we have the side window open for ventilation.

The concrete at the edges seems dry to me i.e. dry to the touch and as a test I put down some toilet paper with a weight on it and next day still dry - The middle is not quite dry yet.

My question is if I get to the point in the next few days when the middle bit is as dry as the edges will I be good to put our vinyl down?

My research suggests you need 5 days per 1cm to cure but given that this concrete has been down for nearly 6 weeks I assume as soon as the top is dry I am good to go.

Any builders out there that can offer advice?

Ta.

surrey_commuter

IANAB but why not shut the window and see how much condensation you get?

Ben6899

Concrete cures quicker under water, actually, so don't worry about the rain.

However, turn off the unnecessary heaters - as you correctly point out, you're not trying to dry it out - you're creating humid conditions which will retard the curing process.

Decent info:

https://www.concretenetwork.com/curing-concrete/#:~:text=When waiting for concrete to,concrete should be fully cured

Ben6899

Wait, I've just noticed it's been down for six weeks. Unless you've built a Hoover Dam replica (still curing in the centre due the mass of concrete) it'll be cured.

john80

You need to find out what moisture level your floor covering will take. Then you need to make sure your floor is below this. What you see on the surface is the tip of the iceberg in terms of moisture content. Most professional floor fitters will make this check before they lay particularly for bonded items such as tiles or karndeen. If you are just throwing some lino down it might not matter but check with the manufacturer. To give an example a basic 40-50mm floor screed for and underfloor heating application will takes anything up to 3-4 months to dry for direct application of tiles hence the number of decoupling membranes to allow earlier laying as most jobs can't wait this long.

rjsterry

john80 said:

You need to find out what moisture level your floor covering will take. Then you need to make sure your floor is below this. What you see on the surface is the tip of the iceberg in terms of moisture content. Most professional floor fitters will make this check before they lay particularly for bonded items such as tiles or karndeen. If you are just throwing some lino down it might not matter but check with the manufacturer. To give an example a basic 40-50mm floor screed for and underfloor heating application will takes anything up to 3-4 months to dry for direct application of tiles hence the number of decoupling membranes to allow earlier laying as most jobs can't wait this long.

All of this. The open windows and fan heater are probably giving a slightly flattering view of how dry it is. If you are laying the flooring yourself check the moisture content with a meter and as John says, check back with the manufacturer of the vinyl and the adhesive. If you are paying someone else to lay it make sure they check.

Ben6899

I obviously misunderstood the question and thought Mr Eddy wanted to know if the concrete will be full strength!

Good advice from John and RJST though.

mr_eddy

Thanks for all the input, sounds like another few days or week won't be a bad about so will give it another 7 days in the meantime I will look for a moisture meter on Amazon. Ta

darkhairedlord

If your not sure then just use some drugs or Matt's for now.

thistle_

It'll be fine.
If there's a noticeable amount of moisture coming out of the concrete after 6 weeks you've got big problems.
The big boy contractors tell us they need to be putting the flooring down 2 weeks after it's poured.

ballysmate

Not a curing question but certainly a concrete question.
Eight years ago we had part of our garage converted to a utility room. This entailed building a wall partition and back filling with concrete to raise the utility room floor.
Years later we had what looked like candy floss/ cotton wool appear on the original garage floor and now appears to be spreading and the surface breaking up. The loose bits, when swept up are like flour and dry as a bone. The original floor has been down 42 years.





Any ideas?

Ben6899

Sounds like efflorescence, from the description. Left behind when water dries on the surface of concrete, masonry, stone etc

Could the utility room be damp?

ballysmate

No Ben, there is no evidence of damp. The utility room is built on top of the existing garage floor so there is the original concrete base plus new concrete to a depth of 2 housebricks. Everything is bone dry.
My plan was just to sweep it up, put down levelling compound and floor paint, providing nothing sinister was afoot.

Ben6899

How odd.

Building works aren't my forte - RJST and John seem to be clued up - so I can't think what exactly might be going on.

It does sounds like efflorescence though, I stick by that.

ballysmate

Ben6899 said:

How odd.

Building works aren't my forte - RJST and John seem to be clued up - so I can't think what exactly might be going on.

It does sounds like efflorescence though, I stick by that.

Cheers Ben.

tonybanham

Mr-eddy,
Back in the dim and distance past I used to be a concrete technician working in the ready mixed industry and have a City & Guilds in Concrete Technology.
Water is an important component in a concrete mix as it allows the hydration of the hydroxides in the cement. These then form crystals which lock the aggregate together and give the concrete its strength. This is a cheminal process (curing) not drying out as the common mis-conception. Indeed the comment above is true concrete will cure underwater, you need a special mix design for this purpose to prevent washout of the cement.
Water is also important in that it provides the workablity of the concrete, how runny or how stiff (slump). Too much water does not allow the crystals to form in the correct way and can severly weaken the concrete. The balance is have just enough water to to get full hydration but not to much to weaken the mix. One solution is you can use additives such as plasticisers, these reduce the amount of water required to keep a low water:cement ratio but allow for good workabity making the concrete easier to place and compact. Super palsticisers are used in flooring concrete allowing to the mix flow easlily so the floors can be installed quickly, big sheds and warehouses have this type of floor.

Concrete strength is measured and specified based on it compressive strength after 28 days For example a C20/25 concrete with have a minimum crushing strength of 20MPa (cylinder) or 25MPa (cube). Assuming normal OPC it gains most of this strength with the first seven days. As you do not know the final strength until 4 weeks after you have placed the concrete they are over-designed based on statistical analysis, so in reality will be a bit stronger.

The other issue is over working of the concrete, causing water to be moved to the surface (bleeding). On floor slabs over tapping can cause this, this then forms a weak layer on the top which can then fail by cracking, spalling or dusting.

In your case once the slab has been poured it should have been covered to protect it from the rain. The reason for this is to prevent this washout on the surface. Looks like it is failing by shrinkage cracking (concrete shrinks over time), spalling the surface you can see the aggregate and once it drys out you could have dusting issue as well. The top layer will come away exposing the aggregate underneath. I will leave it up to you but some form of remedial action is needed, ideally breakout the exsting floor and replace the slab.

I personally do not think it is efflorescence, this is generally found on clay brickwork but you can get build up of the free hydroxides washed out over time which can form a hard white crystaline material on the surface of the concrete.

Apologies for the long reply but hope it has helped in some way.

ballysmate

Thanks for the detailed reply.
The problem I have is not with the new concrete but with the slab that has been down 40 years.
Is it possible that moisture from the new floor has somehow leeched underneath the partitioning wall, causing the damage to the original garage floor?

Ben6899

ballysmate said:

Thanks for the detailed reply.
The problem I have is not with the new concrete but with the slab that has been down 40 years.
Is it possible that moisture from the new floor has somehow leeched underneath the partitioning wall, causing the damage to the original garage floor?

FWIW, I think it's entirely possible.

tonybanham

Apologies, getting the OP and this cases mixed up.

Without know the exact construction of the new slab and how any damp proof membranes were installed it is difficult to confirm what is happening. If you are geeting the white bloom it would appear there is moisture present, not sure of its source. This can cause some washout of the hydroxides but looking at the photo it would appear the top surface is spalling away, I suspect the mix was either to wet or was over worked.
If no moisture present you can get some repair material to put it right Sherwin Williams make one but I am sure there are others.

ballysmate

Thank you both for your help.

john80

As others have said it is either a damp problem because the original damp course has failed or a fault in the original concrete pour.

If we assume that the garage floor is below the house floor and damp proof course level you would have been better to add a new damp proof course/membrane to the whole job. But hey ho it does look like a minor problem in what is probably an unheated area.

ballysmate

Cheers John.
Much obliged to all

Pross

Garage floors don't generally have a DPC due to needing to be set level with the adjacent ground

vinilJcfodbM

I wasn't aware until recently that concrete just keeps getting harder over time. Found out the 'hard' way when i tried to break up some that must be 20 years old.

TheBigBean

My grandfather built his own house. My parents built their own extension. The worrying thing is I don't even understand the question on this thread!

tonybanham

The good thing about this is that they knew what was going on with the builds.
Back when I was a technician we supplied lots of concrete to house builders for footings and oversights. Normal mix for this was C20 at 50mm slump which is a low cost lean mix but suitable for the job. A 50mm slump concrete is very sticky so is hard work to shovel around a footing. Rather than pay a few pounds a cubic metre extra to add a plasticiser to make the concrete flow and easier to place, when my back was turned they just added loads of water. Sure this made it flow but also made the concrete very weak. I once got a sample on the QT and tested it... was not testable at 1 day, it just crumbled, 3kN after 7 days and just made 7kN at 28 days!! Bear in mind we used to to cure the cubes in temperature controlled water tanks, the actual concrete was out in the cold so would be even weaker.
That said this was over 30 years ago and times have changed so I am sure the housing developers now have much stricter on site QA.........

rjsterry

😬

Pross

Takes me back to the concrete technology element of my HNC. Mixing concrete, chucking it in a slump cone and measuring it, getting it into cubes and then testing it at 7 and 28 days, seeing who the lazy ones were who didn't follow the instructions and ended up with a cube of sand in a cement crust. Obviously every builder batches their concrete by mass and not volume in the real world on small construction sites!

rjsterry

The surface was like polished marble.

TheBigBean

tonybanham said:

The good thing about this is that they knew what was going on with the builds.
Back when I was a technician we supplied lots of concrete to house builders for footings and oversights. Normal mix for this was C20 at 50mm slump which is a low cost lean mix but suitable for the job. A 50mm slump concrete is very sticky so is hard work to shovel around a footing. Rather than pay a few pounds a cubic metre extra to add a plasticiser to make the concrete flow and easier to place, when my back was turned they just added loads of water. Sure this made it flow but also made the concrete very weak. I once got a sample on the QT and tested it... was not testable at 1 day, it just crumbled, 3kN after 7 days and just made 7kN at 28 days!! Bear in mind we used to to cure the cubes in temperature controlled water tanks, the actual concrete was out in the cold so would be even weaker.
That said this was over 30 years ago and times have changed so I am sure the housing developers now have much stricter on site QA.........

My parents used old bricks because they were cheaper, so I can't imagine they messed around with fancy plasticisers if they cost more. The foundations were never really the problem though - it appears that no one in the family had a clue about roofing, so it leaked for 20 years.

tonybanham

Maybe they got lucky, a leaky roof is easier to fix than major subsidence due to failure of the foundations! Still things were in their control unlike buying a new build house from one of the larger developers, what they tell you they do and what actually happens might be different things.
Nearly all concrete is supplied to sites big and small by ready mix companies, the exception being the major projects (Hinckley, HS2 and the like) which will have dedicated on site batching plants.