Hello and welcome.
Today we'll be dealing with plastic shrinkage.
Another common reason for snags on site it is called plastic shrinkage because it relates to the effect
of water being removed while the concrete is still in its plastic state.
Water can be reduced due to a few reasons.
We have bleeding that was covered in the last lecture absorption by these surroundings or just plain
evaporation in the air if you picture a big lump of concrete.
We know that there is a lot of water in there.
So a good proportion of the volume is water.
Now when we imagine this water being removed from the concrete we can see that the overall volume would
need to drop.
Meaning the concrete lump would actually get smaller.
These are exaggerated images.
It should definitely never be this bad in a lot of cases.
This is not an issue.
If I've lost a few millimeters off the edges it doesn't really make that much of a difference.
However when the concrete is restrained against something and is not able to freely contracts like in
a lump of concrete in the middle of the air then we do have issues the concrete wall cracks somewhere
to make up the space.
It has to there's no way around this.
If you've lost the volume of water then it must be made up the key is to reduce the amount of shrinkage
and make provisions for the concrete to contract in a harmless controlled way.
The first thing we should do is try to reduce the amount of shrinkage.
It's always best to do this and not just allow shrinkage because we want this water in the concrete
to help it cure and keep the hydration process going but this is easier said than done in some places
in a column for example.
It's not that big of an issue.
You just make sure it's covered using plastic or by leaving the shutters on and then the columns effectively
covered.
However when we start looking at external flows slabs it gets a bit more tricky.
We have the soil below that is trying to absorb all of our water as well as the baking sun that's trying
to evaporate all over our water as well for the ground.
We can put a damp proof membrane down a DPM which would reduce contact or just make sure the soil is
wet before casting the concrete.
No it's not perfect but it is something on top.
We need to combat the weather.
This is done by making sure the concrete is well cared.
We do this by making sure the concrete is consistently wet and we have a few options to help do this
the simplest of which is to do exactly that.
Go out a few times a day and water your concrete.
This is pretty labor intensive because you need to watch it carefully.
If the sun is baking and the wind howling it doesn't take long at all for the water to evaporate the
next level up is to build a little dam around the slab and fill with water which gives you a bit more
time without supervision.
You could create this dam with pretty much anything sand works particularly well you could also cover
the slab with sand or Hessian which holds the water for longer or even use a plastic sheet that breaks
the contact altogether with the wind and the sunlight and again all of these require maintenance and
monitoring which can be a pain.
So this is where curing compounds come in.
These days you can get a compound that is sprayed directly onto the concrete which forms in impermeable
layer that stops the water from evaporating.
They can be water or oil based and this is important.
If you go with an oil based curing compound there may be issues down the line if you need to put a screed
or paint or any other features on top of it.
The oil can sometimes break the bond between the concrete and whatever you want to put on top always
review the manufacturer's information and if unsure ask them the question it's always was being shown.
The only problem with these curing compounds is that it's not always easy to see if they have been applied
correctly which is very important and you can't afford to be in there watching every step of the construction
process.
Luckily there is a workaround for this as well.
Some of these curing compounds can actually come with a pink pigment that is brightly coloured so that
you can see exactly where it's been applied.
And even more cleverly it reacts with UV light to disappear after a few days.
Just make sure that it will actually go.
The last thing you want to have to do is explain to your client why they're brand new floors.
Lab is bright pink
before we close our plastic shrinkage.
There's one more case that I think you should be aware of and that's when cracking if you go to site
and see the concrete out in the open with a few cracks the usual first assumption is that as plastic
shrinkage cracks.
But that's not always the case.
Occasionally small cracks can be formed due to the wind running over the top of the slab and effectively
pulling the weak concrete apart at the very very top.
This is usually a much smaller deal and can be dealt with by brushing cement into the cracks to help
them close up.
They are usually only a millimeter or two below the surface so I'm not really that much of a structural
issue rarely.
So keep that in the back of your mind when on site those are the main properties of fresh concrete.
If we get all of these rights the concrete will be well on its way to becoming durable well-made concrete
next week we'll take a quick look at the properties of and.