OK now that we have a good grasp on what good concrete consists of.
Let's move on to the properties of fresh concrete.
Although the concrete only stays in its fresh state for a few hours these few hours are immensely important
and set the stage for the overall quality of concrete achieved in this section will run through the
main properties of fresh concrete and what they mean practically on site.
There are two main ways of getting concrete to a site even makes it there on site or have it delivered
pretty mixed.
Either way it's very important that the consistency of the concrete is suitable for a particular use
which takes us right into our first property.
The consistency the consistency is best described as a concrete sloppiness or fluidity and it is of
interest to us because of two very important reasons.
The first of which is practicality on site.
If every batch of concrete that arrived on site was of a different consistency than the poor siting
would have to readjust their placing throughout the day and it would be absolute chaos.
The second is more long term.
By measuring the consistency you can very quickly tell if more or less water than designed for has been
added.
It is also worth remembering that the consistency that you may get at the packing plant may not be the
same as when you get on site as the concrete travels to the site.
The hydration process will begin and the concrete will start to harden so let's see who's been paying
attention quickly with an impromptu quiz.
Can you name one the mineral that is added to the cement to slow the hydration process down and to what
you would add to the concrete mix to slow the hydration process down beyond the normal rate.
I'll give you a few seconds to pause the video and think about it and then we'll have a look at the
answers for number one.
The mineral added to the cement to slow the hydration prices down is gypsum.
This was added to the cement clinker after being milled and forward.
Number two how to slow the process down even further was to add a retarding chemical admixture to the
concrete mix which would then slow the heat of hydration down beyond the normal rate getting back to
the concrete consistency.
Let's look at how exactly we test this now.
The test needs to be reliable easy to reproduce and suitable for site conditions.
The most common test which has all three of these requirements is the slump test shown here.
The tests are very easy to follow and can easily be completed onsite in a short amount of time which
is perfect for when the concrete needs to be checked quickly while waiting to be cast on site.
We don't want us sitting in the truck for any longer than it has to you start the test by cleaning the
test equipment and get a get into place on a flat surface.
You'll notice that there are two little steel pieces on either side of the car.
Although strictly not needed.
They make the test much easier.
As you can stand on these to hold the test in place with your feet now with your feet on either side
of the test holding it down you start by filling it with concrete every third you feel needs to be toned
with 25 strokes using the steel rod what we're effectively doing here is reproducing the vibrations
that will take place while casting on site after the third round of this.
You'll want the cone to be completely full If it overflows it but that doesn't really matter you'll
then take the tapping Rod and slice the top off and rollers for long.
So you've got a nice flat surface on top of the current.
Now for the tricky bit.
Lifting the mold your initial urge is to lift it quickly but actually you need to lift it slowly and
precisely.
You want the cone to go directly up and not touch or knock the sides once off turn the cone upside down
next to the concrete on the slum test kit.
I was using the foot pedals were perfectly in line with the bottom of the coat so we just measure from
ease.
But strictly speaking in a correct way to do this is to measure from the bottom of the rod to the top
of the concrete and it's not a ratio.
There's no need to whip the cone off and measure the concrete before it deforms the defamation is actually
what we want to measure
what we're looking for is a test case similar to that.
If a completely collapses or she is off then you need to start again.
And if it keeps happening you may have a problem with your concrete mix the slump range is usually specified
in the mix design.
So checking it on site will only be a matter of making sure it fits with what is being specified.
But when it is you specifying the slum.
A good rule of thumb is that compaction by a vibrator can handle a lower slump of between 50 to 120
millimetres and compaction by hand.
These are higher slump of 100 to 150 millimetres for the site team to be able to compact properly.
I'm sure it goes without saying but the test is only suited for normal concrete.
If you try with self compacting concrete well they're hilarious.
The results would be of no use to us.
In the next lecture we'll take a look at the workability of the concrete.
See you then.