Hello and welcome.
Today we'll be looking at settlement and bleeding within our concrete similar to water and oil.
The cement and aggregates have differing densities to water and would sink to the bottom if given enough
time if the aggregates begin to sink.
There needs to be a displacement of water to maintain equilibrium this upward movement of water is what
is known as bleeding this process takes place while the concrete is setting.
After the initial set is complete it's no longer an issue.
When it comes to what influence is bleeding it's pretty much exactly what we would expect the most influential
factor is the relative density of the aggregate.
So if we have particularly dense and heavy aggregate it's easy to see that it's more likely to sink
to the bottom.
The same applies for the amount of water in the mix.
If there is a lot of water then there's likely to be a lot of bleed as well as the water keeps trying
to get out above the aggregate if there is less water then there's less bleeding but then we have other
considerations if you're deficient in fine materials or are using admixture as this could negatively
affect the bleeding.
So you need to keep an eye on that but some add mixtures can actually help here both chemical and mineral
on the chemical side adding an air and training agent will help the workability and bleeding and on
the mineral side you could use one of these cement extenders to help here condense silica fume is a
good one for example for bleeding if you're battling to remember which one condensed it like a fume
is that it was the one that needed an electric shock to be transported now it's not often that the condenser
like a fume is the most practical way to deal with bleeding in particular if that's the only issue you
have but it may be worth looking at if you're using it for its added benefits as well when it comes
to other mineral admixture such as ground granulated blast furnace like coal fly ash although they may
increase the amount of fine material they both slow this testing of the concrete This then gives the
concrete more time to bleed so actually a bad thing I wouldn't recommend going that route if you're
only concerned with reducing bleeding
you may be asking why we even care so what if a little bit of water leaves the concrete it makes it
stronger doesn't it.
And yes I have heard that on site before now it is true that water moves up and out so it is possible
that the lower down concrete could benefit from a slight increase in strength.
But what about the concrete on top.
You can get a slurry of sorts forming across the top of the concrete where there's bleed water has mixed
with the concrete and created a concrete with a much higher water cement ratio.
This is particularly bad when the element isn't wearing surface such as a floor slab because this is
where it will actually see its highest loads the water doesn't always make it to the top either.
It often gets trapped behind stones or even reinforcement bars once the concrete sets the water usually
permeate through the concrete leaving a gap.
And now this gap is a weak spot that affects the durability of the concrete as a whole So in summary
the best way to deal with bleeding early on is to look at the mixed design and make sure you have enough
fine materials and don't have a particularly high water cement ratio.
This is usually enough in most cases to avoid bleeding if you still have a mix that is prone to bleeding
or you can see signs of it once the concrete starts to set on site.
The best thing you can do is to reapply break the concrete just before it sets this effectively we mixes
everything at a stage when there's not actually enough time for the concrete to bleed.
This leads nicely onto plastic shrinkage which will cover in the next lecture.
See you then.