it's common knowledge that concrete
always cracks and a lot of people say
there's nothing we can do about it to
some extent that is true however with
the correct placement right design and
right mix you can minimize the cracks to
the point that they're barely visible
i'll be going through the typical cracks
that you may see in your structure why
they may occur and how to minimize them
in your design my name is brendan your
structural engineer now let's get into
it it doesn't really matter what type of
crack you see in a concrete structure
they've all come about from a similar
action which is tensile forces have been
induced inside the concrete structure
now concrete is not great at resisting
those tensile actions so it can crack
underneath a relatively minor force so
whether it be from thermal contraction
or expansion shrinkage stress and just
the lows that are induced on the
structure they're all because of some
reason a tensile force has been induced
the first one and easiest one to
understand is when tensile forces have
been induced on the structure because of
the actions or loads that are applied to
them so this will either be flexure or
shear forces both these induce tensile
forces on the structure causing it to
crack so whether it be that tensile
action so when you're over top of
support it's trying to curve and expand
causing tensile forces into the top of
the structure or in the midpoint of a
design so the same thing the tensile
forces are on the bottom of the
structure
when you're approaching a support you
will see diagonal cracks now this is
where we're getting sheer cracking which
will typically be induced near the
center of the structure and expand out
to the outer edges but these ones the
easiest ones to understand as forces
have been induced causing a tensile
force that needs to be resisted so if
you need to really limit the cracking in
your structure and you've got some high
stresses try and use the smallest bar
possible at closer spacings will limit
the size of cracks that you'll see in
your concrete after it's built there's a
lot of other reasons why concrete cracks
beyond just the stresses that induced
inside this structure
and these are related around concrete
placement mix weather and just the plain
properties of concrete and you need to
be able to identify them to understand
how they form and how you can make sure
your design doesn't suffer from them
cracking that forms quite early on on a
concrete structure and you may actually
see it before it's actually hardened
it's called plastic shrinkage or crazing
this typically occurs on the surface of
freshly poured concrete and is due to
the evaporation of the bleed water off
the surface of the concrete meaning that
you're drying out the top form of the
concrete too quickly this will occur in
the first couple of hours of pouring the
concrete and if you catch it early
enough in fresh concrete you may be able
to re-float it and form those cracks
back up the reason why this is occurring
is the top layer is drying out it's
actually being resisted from movements
and leads to those tensile forces being
induced in the fresh concrete when it's
relatively weak so it requires
relatively small loads for these tensile
forces to crack the concrete so it's all
about slowing down that curing process
and making sure that you're not bleeding
more than the evaporation that's
occurring to the top surface of the
concrete structure the next type of
cracking that also occurs early on in
the concrete's life is plastic
settlement cracking this is when the
concrete settles around the restrained
bars and the easiest way to identify
this is when you see cracks running
along the bars you'll see a grid that
typically matches up with a
reinforcement layout so these nice long
straight cracks and typically occurs in
deeper structures and you might actually
get a bleed underneath the reinforcement
and again similar to the plastic
cracking if you catch it early on you
can make sure your form wet up properly
but you need to be careful as the cracks
typically start around the reinforcement
and you can just seal the surface but
still have the underlying crack close to
the surface above that bar white plastic
settlement cracks occur is all about the
bleeding of the concrete and the
settling as it goes down so you need to
make sure that the concrete's not
bleeding too much you've actually
compacted it so you've vibrated enough
to make sure it's settled and compacted
enough so it doesn't have that
settlement issue shrinkage cracks is
probably the most common one that you'll
see and there's a number of different
forms of why these occur
and the major feature of these you'll
see vertical cracks typically in between
restraining elements and they typically
be through the full depth of a concrete
structure to form shrinkage cracks you
need to have some form of restraint
because when the concrete is poured it's
at one extent but as the hydration
process starts to go you'll lose losing
water and chemical reactions it slowly
shrinks in
so if it's not restrained typically you
won't form shrinkage cracks however in
most structures you'll have columns
walls or some form of structure that
will be restraining it to induce those
tensile forces there's a number of
reasons why these shrinkage cracks occur
so just the chemical reaction will cause
it to shrink as the hydration process of
the cement occurs auto genius so that
hydration reaction quite often the
chemical and hydration are put into one
as it's too hard to separate the two in
addition to the auto genius and chemical
shrinkage you also have just typical dry
and shrinkage so as the concrete is
hardening and it's losing water the
concrete volume will be reduced by the
amount of drying that's occurring so
it's a combination of the water coming
out of the system the water being sucked
out of the aggregate and also the
aggregate shrinking as well the rate of
evaporation is linked to how quickly
this concrete will shrink which is
related to humidity temperature water
cement ratio and how much of the exposed
surface of the concrete structure is
available to this environment the
majority of the drying shrinkage occurs
in the first three to four days but
after which point it will keep shrinking
over the many months the concrete
structure is placed but it's at a
logarithmic rate so it slows down
dramatically over time whether it be the
plastic cracking plastic settlement or
just drying shrinkage is a combination
of several factors whether it be the mix
the weather the design or the placement
we'll start off with the mix as this is
one of the areas where we can control
quite well and minimize the risk of
cracking in the structure whether it be
the components such as cementitious
material the aggregate any admixtures
that we've had to the design or the
water cement ratio controlling these
tightly can limit the amount of
shrinkage and cracking that we'll see
during the drying process water content
is arguably one of the biggest aspects
so when you've ever got something to
cite you should never be adhering water
to your mix and it will increase the
risk of cracking in your structure as
the drying shrinkage of the concrete
structure is greatly increased by
increasing your water cement ratio good
quality aggregates is often something
that's often overlooked in your design
so you want to make sure that your
aggregate will shrink less than the
concrete around it so the more aggro you
add to your design the better it will be
and typically the bigger aggregate size
the better it is so you want to make
sure you're maximizing the aggregate
size based on the mix as a smaller
aggregate will typically absorb more and
potentially have a greater impact on the
volume change you can also have ad
mixtures but you need to be careful that
sometimes they can change the color of
your concrete but admixtures can slow
down the rate of hydration thus limiting
the how quickly the concrete shrinks and
how much it shrinks so having an
admixture can sometimes have a
beneficial effect if you're needing to
pour in adverse conditions where there
is another aspect that can affect how
quickly the concrete shrinks as if it
shrinks fast it hasn't had time to
achieve the strengths to resist the
loads and you will see higher tensile
forces so you want to make sure you're
looking at the weather forecast and
making sure you're not pouring on days
where you can potentially see high
evaporation so typically on tas above 30
degrees if you've got low humidity or
wind now if you've got a high wind event
but it's been a cold day you can put
wind breaks up but it's all about the
airflow and how much the evaporation off
the surface correct placement and curing
is also critical to make sure the
concrete doesn't crack it doesn't really
matter how good the mix design is how
much reinforcement you got in it oh it
will definitely crack so you need to
make sure that you're compacting the
concrete properly so you're vibrating
down so it's as hard as possible you've
got to make sure that you're also curing
it for as long as possible now curing
can be done through many forms it's
typically either your wet form concrete
where you're throwing miss or hessian on
top or you can put curing compounds down
now the problem with curing compounds if
you need to have a finished surface that
is a concrete they can sometimes stain
the surface so you need to be careful
with the choices of curing compounds
that you may have good design also plays
a big role as if you need a high level
of crack control you typically need to
increase the amount of reinforcement
post tensioning so having a high pna and
also looking at design making sure that
you can try and prevent any restraining
structures so if you've got two cores
next to each other you might need to
have a permanent movement joint or
temporary movement joint so during the
drying process it's not restrained
allowing to free form and stopping those
tensile forces from occurring it's all
about locating those joints even on slab
on grounds you need to be even more
careful as the ground will somewhat
restrain it so you need to make sure
you've got joints at roughly every six
metre centers and trying to keep
everything square wherever possible or
if you've got a big change of geometry
say for example openings you want to
make sure that you've got joints in
those locations as they're the places
where the tensile actions will form and
if you're going to be cutting joints in
a concrete slab you want to make sure
you're doing it as early as possible as
you need them to form in those locations
as cracks can perform quite early so as
soon as you can cut it without chipping
it is when you need to be cutting that
concrete check out this video if you
want to learn more about the secrets of
concrete design and if you want to
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learning and i'll see you next week bye