concrete, esp in hot weather

[saveasteading]

A word about concrete slabs, especially in hot weather.

 

Concrete is a highly sophisticated product, and with proper handling it produces effectively rock to the shape you want.

 

Many things can go wrong, but they can all be controlled.

 

The principle is that we are gluing together bits of rock, back into the form we want, using cement.

 

There should always be a proven formula for the mix. 

For a bridge or multi-storey it is very much more precise and controlled than for a garden path, but it is always worth attention.

 

The main thing that most public don't realise is the difference between cement and concrete. Cement is ONLY the dust that goes in the mix to make mortar or concrete.

 

The biggest important thing that many in construction do not know is that concrete is strong through curing, not drying. 

Do not assume that your groundworker knows all, or much, of this.

 

For a strong concrete mix there are large stones, with all the gaps between filled with small stones, and the further gaps filled with sand.

Every surface is coated with the slurry of cement and water that has been carefully mixed in. There are no pockets of cement, it is all aggregate to aggregate contact.

 

A chemical reaction then happens that makes the cement stick everything together,  and water is used in this reaction, and taken out of the mix as a permanent part of the matrix chemistry..

Any additional water will simply sit in the mix, until it evaporates, leaving lighter concrete and miniscule gaps.

 

It is essential that there is the right amount of water.

a) enough to allow the chemistry to occur.

b) not too much that it spoils the mix and eventually leaves voids in it.

 

The chemistry takes many days, and concrete gets harder for at least a month, if properly looked after.

For this to happen, the laid concrete must not be allowed to dry out, so it must be covered with plastic or wet hessian.

Once the surface is hard, water can be sprayed over it.

 

Don't use too much water, and don't allow the concrete to dry out through sun or wind.

If it is very hot or windy, postpone it.

(Frost is another matter).

 

Why do supposedly expert groundworkers have problems with cracking?

Mostly because they add extra water, by hand or by asking the delivery driver*. This makes it much easier for them to handle.

Secondly because they don't know the difference between drying and curing, and want it to go hard so they can go home.

Thirdly incorrect use of steel mesh (which is for cracking control not strength), dpm, and joint preparation.

 

(* A readymix company will allow water to be added, but will record this on the delivery chit, and thus it is not their problem.

They may also takes samples and make cubes for testing, and store  them in a tank of water for ultimate strength, for 7 and 28 days.)

 

All concrete cracks. How and how much it cracks is the skill.

Enough for now, do ask for clarification or more information.

 

 

[Oz07]

There is a certain volumetric outfit near me that always tick the added water on site box even if you dont. I sussed them on the last job and told them I wasn't signing. Its just so if ever there is a quality issue down the line they don't cop the blame. Arse covering. 

[saveasteading]

1 minute ago, Oz07 said:

tick the added water on site box even if you don't

Good advice. I never thought to look. I sometimes wonder how precise the mix is anyway, although I like the principle for small pours.

[Oz07]

They were good 10 years ago when they had transparent pricing and quoted you a mini mix rate per cube and an over 4m3 rate per cube. They've all gotten greedy now and want to quote you based on exact amount. You struggle to nail down what your paying per m3 if you're not sure how much your having which kind of defeats the object in a way.

[saveasteading]

I only ever used them for small pours or multiple small pours of bases in boxes, so the volume was known. always seemed fair.

What I didn't like was that there were no accounts and they wanted a deposit, which was difficult to get back.

Now I see that some of the big companies do it, as an option. have you ever tried them?

 

 

 

[markharro]

On 25/07/2021 at 12:11, saveasteading said:

The chemistry takes many days, and concrete gets harder for at least a month, if properly looked after.

For this to happen, the laid concrete must not be allowed to dry out, so it must be covered with plastic or wet hessian.

Once the surface is hard, water can be sprayed over it.

Hi @saveasteadinginterested to read this. We want to do all we can to ensure our newly poured slabs cure as well as possible. There were poured 2 days apart on Mon and Wed. I have now covered the earlier pour with polythene and intent to cover the Wed pour probably after tomorrow's rain. Is this a good plan? Should I be periodically removing the sheeting to spray over some water? We have just over 10 days until our timber kit arrives so keen to spend that time trying to tend the slabs optimally. After that the sheets around the perimeter at least will have to come off to allow the external walls to be fitted etc

[saveasteading]

OK happy to expand:

I'd like you to have the perfect slab.

 

The procedure in changeable weather is to 

1. hope that the surface hardens before any rain comes or it will pock mark with the rain.

2. at whatever stage you decide, balancing rain marks and not scuffing the surface while laying polythene or hessian, lay the cover.

This initial 'Set' can be 2 hours or 8 hours. depends on temperature mostly. 

3. It should not need additional water.

4. Have a check, next day. Polythene will likely have a coating underneath of condensation. If so, leave it alone.  

5. it is wet enough , so leave it covered for a few days, a week if you can. If you want to wet it further it can do no harm.

 

BUT. if you are using hessian that keeps the wind off but allows it to evaporate so give it  a spray with hose or buckets. It can't be too wet, and it won't absorb water unless it is a good thing.

 

After a week, most of the chemical reaction has completed. A large proportion of the water has combined chemically with the cement and is now dry as well as  hard.

The remaining water can be allowed to evaporate.

 

The chemical reaction creates heat, which you are also keeping in. 

Cold is very bad for concrete until set. I think the rule is min 3degC, but even then it will be slow to cure.

 

In hot or windy weather the above changes a little , but not your problem at the moment.

 

If extra water is added to the mix at pouring, this eventually evaporates and leaves air gaps in its place, too small to see, but weakening the concrete.

Add 50 litres to 6m3 and that is 50/6000 and that doesn't sound a lot. But the majority of the concrete is stone, and so this is adding a high proportion of air to the vital cement paste.

 

95% of groundworkers want to add water for their convenience, and will try to avoid taking instruction not to add any.

Refer to the concrete supplier if necessary.

Nobody dies, as it isn't a bridge or a dam, but you will get lots of cracking and that may come through surface finishes.

 

Concrete does not harden through drying but by chemical reaction. Only you and I and a few others know this. Most groundworkers think they know better than us.

Concrete always shrinks when hardening. If done skilfully there are millions of invisible cracks. If done badly there are ugly wide cracks.

 

If you have to remove areas of polythene for other works, then do so, but try to put them back asap.

 

 

If I've missed or confused anything do ask again.

[Dave Jones]

Visited Normandy couple years ago and visited many of the fortification made with slave labour. I'd dare say they had better conc then we do now. Some of the fortifications had 1000's of cube all hand done and was pass BCO today.

 

Some even had little plaques on them with the name of the local builder and the cubeage.

 

 

[Oz07]

4 hours ago, Dave Jones said:

Visited Normandy couple years ago and visited many of the fortification made with slave labour. I'd dare say they had better conc then we do now. Some of the fortifications had 1000's of cube all hand done and was pass BCO today.

 

Some even had little plaques on them with the name of the local builder and the cubeage.

 

 

How can it be better with all the technology nowadays. Have they engineered the materials to minimum acceptable spec?

[Oz07]

Ive never covered a slab with polythene before but am open to ideas. Does it give a much better end product?

[saveasteading]

1 minute ago, Oz07 said:

Does it give a much better end product?

Yes. It isn't "an idea", it is normal professional practice.

There are sprays as an option for big slabs. Or keep wetting it.

[Oz07]

1 hour ago, saveasteading said:

Yes. It isn't "an idea", it is normal professional practice.

There are sprays as an option for big slabs. Or keep wetting it.

Yes tbf they usually spray any ive had done. Suppose thats why ive never seeen the polythene