Hello and welcome back.
We now have a good understanding of what concrete is and how it is made as well as its properties in
its fresh state which are vital to get into concrete to its final state of good quality hard hardened
concrete before we walk away from concrete that has been placed.
We need to check that it will perform as expected and inspect it for any defects to make sure it will
perform as expected.
We need to crush some of the concrete in a controlled environment to confirm that it actually is as
strong as it's been designed to be.
In reality it would not be very smart.
Cutting bits of your structure out to test unless you absolutely have to.
This is where test cubes come in onsite.
As we get ready to cast the concrete you'll generally see two things take place at the same time.
First you'll see the slump tests being performed.
And second you'll see someone out putting concrete into a small square molds these cubes will become
our window into the performance of the concrete.
We look into that window by crushing these cubes that 7 21 and 28 days and measuring the strength will
run through why these specific dates and a little bit.
But it's vital that these samples are taken and cubes made on site correctly I'm often sent test cube
results from cubes that were made out of batch implant miles away from the slide show that I'll give
you an indication of strength but I don't know what's happened between them taking that sample and being
cost on site.
I'll give you an example of a time that I've caught people messing with these for personal gain I showed
up unannounced to one of my sites.
Very good habit to get into and watched in the back on this team did the slum test and formed the concrete
cubes.
Their technique was a bit shoddy so I knew already that there could be a problem you know after forming
the company cubes.
They took them away and got ready for the poll but one stayed back to a hosepipe and stood there pouring
water into the mix so that it would be easier to cast on site this week in the concrete and if I hadn't
been there to deal with this there would be no evidence that it had actually happened or no sign that
it had actually happened.
Every country has slightly different ways of making the concrete cubes but it all follows the same pattern
of pouring an amount of concrete in tap a certain amount of times pour more concrete and tamp a bit
more and so on and so forth until it has been filled.
You then roll off the top leaving a nice level surface and when ready to transfer over to a warm bath
with it where it will cure it's all crushed.
Just remember to mark them correctly.
Once out of the molds all look exactly the same and you won't be able to tell them apart so why 7 21
and 28 days.
What is so significant about these numbers to understand that.
Let's start at the back with 28 days 28 days is its highest final strength used in the design of concrete
structures.
For example a twenty five NBA concrete mix must reach twenty five NBA by day 28 and going even further
than that.
The 28 days was chosen for design purposes because it's a good balance between enough time for the concrete
to reach the majority of its strength without being too long that it would hold up the progress on site
to get the strength if you ever look at this graph of the concrete strength game you can see that by
28 days the rate of strength gain has started to level off of it.
Pretty much hit our peak this 21 day test is probably the least important overall but it can affect
your progress on site.
For example if we have a slab that has been cast in cold weather it could take pretty long to get to
a point where it's able to carry the load.
In this case we would have homework and support below the beam and slab to support it until it's ready
to carry load.
Now this costs money which people don't like spending so contractors can actually use a 21 day results
to justify when the props can actually be removed and tenderly moved earlier occasion 814 de cubes will
also be taken and crushed for the exact same reason one thing to keep in mind is that you need to modify
the result to account for the cold weather the concrete test cubes are kept in a nice warm bath so they're
not actually affected in the same way that your structure out in the open will be the seven day results.
In my mind the most important as we can see on the graph around 70 percent of the strength gain has
been completed in the first seven days.
This gives us a good early indication of if we should be worried about the 28 day results coming out
if it is so low that it's clear that it would not pass at all.
It also gives the contract a more time to knock it down and try again.
I have yet to see Q results come in.
That looked good at 7 days and then completely failed at 28 days.
It's a fairly good indication and as with the twenty one day results these seven day results can actually
be used to justify the carrying capacity of the concrete to remove some of the supports.
But always remember that your deflection is a function of how early supports are removed so removing
them too early could actually result in more deflection than originally allowed for with the seven day
results.
You can also get false negatives.
You may get results that seem to be about 10 percent lower than they should be.
This could be a sign of bad concrete or it could be potentially something else something harmless in
the mixed design.
Always go over to your mixed design and take a look before taking action.
You might have mineral and chemical add mixtures in that the chemical ones are pretty obvious to see
what they do but some mineral admixture such as fly ash will actually slow the initial set of the concrete
giving a lowest seven day result.
That would be expected but trying to remember that it is slowed but not lost in most cases the concrete
will make up the strength gain by the time you get to the 28 days let's call it a day there in the next
lecture.
We'll look at the core test and the tensile strength of concrete.
See you then.