Hello and welcome back.
Today is all about the workability of concrete.
At first glance you may think that consistency and workability are the same thing that is understandable
but they're actually pretty different the consistency is easily measured.
But the workability takes a bit more experience and gut feel the workability relates to how easily the
science team is able to work with the concrete emotion both a concrete mix and a mortar mix.
We could design both to have the exact same consistency which would result in the exact same second
test results.
But in this case of extremes we can clearly see that the mortar is going to be much more workable and
easier to handle.
So how do we test for this.
Well we can't really.
It's more of an experience and gut feeling situation.
However the slum test does give a good starting point.
And designing to the right slum test will usually get you 80 percent of the way.
But after that the next best thing to do with a new mixed design is to do a trial badge by creating
a trial badge and compacting it.
You can assess how easily it was to handle and what the final product looked like.
For example you might have bits of stone sticking out of the top of the finished concrete and unless
you specifically want this it would be a sign that the stone content is too high and your mix is not
quite right.
Ideally you'd like the stone to read about three millimeters below the surface anymore and you may have
too little style.
There are loads of factors that influence the workability but in different degrees going from the least
influential.
Let's look at a few of these factors.
The first of which is the stone size.
Like in the previous example this would be the first thing that you would look at when workability is
not quite right.
When changing the mix design it's best to change it slowly.
Don't dump in 20 percent more stone and expect it to change slightly it will be quite a dramatic change.
The next thing to look at is the sand.
The proportion of very fine sand to be precise.
The more fine sir and the more sticky the mix will be which is good.
Up to a point if there is not enough fine material then the mix will battle to hold itself together
and it just won't be cohesive enough.
We also have a similar situation with the amount of cement used the more cement used the more cohesive
and sticky the concrete will be OK.
Time for a quick pop quiz question.
It's a tricky one.
So pay attention to these specific words I use which chemical admixture would be the best choice to
increase the workability and cohesion of a concrete mix without affecting the water requirements much
I'll give you a few seconds to pause and think about it OK.
So the answer is air in training admixture.
This was the admixture that allows small stable air bubbles to form in the fresh concrete.
It increases the workability and cohesion properties and is most often used to deal with the freeze
thaw effect you may have been tempted to say a place the size of it which is almost correct but it does
also have an effect on the water requirement.
By the way if you would like me to go into more detail on mechanical add mixtures or anything else really
let me know.
On the Facebook group and I'll get it on my to do list in the next lecture we'll take a closer look
at settlement and bleeding which can be a real problem for concrete reinforced concrete in particular.
See you then.