Hello and welcome back.
Today we'll look at the difference between the concrete course cubes and cylinders as well as the tensile
stress of concrete.
You may be wondering why there are so many shapes of concrete being tested and what even is the difference
between a core and a cylinder.
Is a fair question let's start with the core test which is probably the outlier of the three the core
test is what you do when you need to know the strength of an existing concrete structure.
It involves a specialist contractor going to the site was a core drill to remove a sample of about one
hundred and two hundred diamonds.
Personally I have never seen anything smaller than 75 diamond to produce any reliable results.
But the technology is always improving so this could change in the future.
This core sample is then taken back to the lab where it is crushed in a controlled environment and give
you the actual in situ strength of the structure you're most likely going to counter this method when
assessing instructor's ability to carry additional load.
The difference between the cube and cylinder test is a bit more arbitrary.
Simply put the cube test is a square bit of concrete that is crushed.
To give you a design strength and the cylinder is a cylindrical bit of concrete that is crushed to give
you a design strength.
We talked about the 28 day strength in the last lecture that is used in the design.
Well this is where it comes in when specifying the concrete strength.
There are actually three ways to specify the exact same grade of concrete for example concrete specified
as thirty five forty five or thirty five forty five could all be referring to the same thing.
Annoying isn't it you do need to be careful what testing medium your design strengths were placed in
the UK to avoid confusion we would specify the concrete as a C thirty five forty five And the reason
for this comes down to the change from the old British Standard over to the new Europe coat which used
different formats the first number 35 comes from the Europe code and is based on the cylinder strength.
This is what is used for design purposes in the UK.
The second number forty five is the equivalent cubed strength which some other countries use for their
design.
If you're wondering how countries use different strengths for the same concrete it comes down to countries
specific factors in the design.
In the end they all pretty much evened out once you apply these.
The reason for having the second number in the UK is that only cubes are made on site for testing.
So we need to know what results constitute a pass or fail and an easy way to remember which is which
between the cube and the cylinder is to remember that a cylinder of a certain diameter is smaller than
a square with the same dimensions.
So that would be a lower value.
This is just for remembering it's not actually how it works it's worked out based on millimeter squared.
So your area and the core strength is roughly 80 percent of the cube strength
the tensile strength of concrete is its ability to resist being pulled apart and in most designs we
completely ignore it because of how weak it is compared to the compressive strength to get an idea of
what the tensile strength of a given grade will be.
Divide a point 10 the tensile strength is often only about 10 percent of the compressive strength if
you'd like to get a bit more accurate you could take the cylinder strength to the power of zero point
seven eight three multiplied by zero point one eight five which is based on empirical testing.
It's worth checking your local design codes for this as most countries have slightly different ways
of working this out accurately and some are much simpler than others.
That closes aren't the properties of hardened concrete by this stage.
If you've followed everything we've discussed you love quality concrete in place ready to do its job
for the next 10 50 or even 100 years.
Take care and keep learning.