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Note 26 Level 1
18
Technical
Technical Guidance Note
TheStructuralEngineer
May 2013
Cracking in concrete
Introduction
In order for concrete to crack, there has to be a tensile stress present, be it
applied or induced. Concrete is actually designed to crack, as it is assumed
that the steel reinforcement within it resists all of the tension stress within
the element. The sight of cracks does cause concern however, and an
understanding of what causes them is important when inspecting new
works and/or existing structures.
This Technical Guidance Note describes the causes of cracking in
concrete. It does not extend to the numerical analysis of cracked concrete
elements as this is beyond the scope of a Level 1 guidance note.
Plastic
settlement
Plastic
shrinkage
Spalling
Plastic
shrinkage
Whatever the symptoms of cracking, the
ultimate cause is always the same: the
tensile strength of the concrete has been
exceeded due to either an induced action
or an applied one. If you are in any doubt
as to why cracks are forming in a concrete
element, always remember this
fundamental point.
Non-structural cracking can easily be
misidentified as structural in nature and in
some extreme cases this can lead to the
condemning of perfectly sound structures.
It is therefore important to recognise that
these types of crack are defects to the
surface of the concrete, not signs that the
element is about to collapse.
In most cases, non-structural cracks can
be prevented by employing good practice
with regards to design, construction and
TSE17_18-20.indd 18
W Further reading
W Web resources
Figure 1
Different types of crack found in concrete structures
Crazing
Non-structural cracking
W Cracking
in concrete
W
Cracking in concrete
Cracks in concrete can be divided into
three categories: non-structural, thermal
and structural. Non-structural refers to
cracking in concrete that has occurred due
to effects that do not have a significant
impact on the integrity of the structure.
Thermal cracks are those that come about
either during the curing process (soon after
the concrete element has been formed) or
from movement of the structure in response
to varying temperatures. Structural cracks
are those that appear due to defects in
the structure and are considered to be a
failure of some sort. Figure 1 is a section
of a concrete frame that has various crack
patterns on it, all of which will be described
in more detail in this guidance note.
ICON
LEGEND
"Cracks in
concrete can be
divided into three
categories: nonstructural, thermal
and structural"
especially detailing of reinforcement within
concrete elements. Where no reinforcement
is present, careful management of the
placing and curing of concrete is required in
order to prevent cracking.
The formation of cracks within concrete
elements can be considered from two
perspectives. The first is the appreciation
of the tensile strength of the concrete
element and the extent of its restraint. If it
is unrestrained then it is free to expand and
contract without hindrance from any other
agent. If on the other hand the concrete
is restrained, which is usually the case, a
tensile stress develops around the point
of restraint, which leads to the concrete
cracking. It is important to note that
concrete changes over time. This change in
material property affects the elastic modulus
of the material and can have an impact
on how it responds to the surrounding
environmental conditions it is exposed to
during its lifetime.
The second perspective is to consider
the tensile strain capacity of concrete.
This changes over time, because when
the concrete is setting, its tensile capacity
is very high as it is in a liquid phase and
deforms quite readily. As it hardens,
deformation becomes more difficult and in
turn its tension strain capacity decreases.
Over time, however, tension strain capacity
does increase soon after the concrete
has set, as it absorbs moisture and thus
becomes more amenable to deformation.
The increase is not enough to prevent it
from cracking, hence it being more prone
to do so in the earlier part of a concrete
element’s existence.
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19
Either way, no crack that is formed in this
manner can be considered to be structural.
They are occurring due to the properties
of the material and the environment within
which it is placed. They can however lead to
water ingress that can have an impact on the
structure by causing the concrete to spall
(and the steel reinforcement to corrode).
Grading of cracks
As previously explained; concrete is
designed to crack. Indeed it is preferable
for it to do so in order for the reinforcement
within it to be efficiently designed. The
question arising, however, is what is
acceptable in terms of cracking? Typically
this comes down to a combination of both
durability and aesthetics. Table NA.4 of the
UK National Annex to BS EN 1992-1-1 states
that for in situ concrete elements exposed
to most environments, the maximum crack
width that can be permitted to develop
is 0.3mm. BS EN 1992-3 concerns water
retaining structures and their limits on crack
widths are reduced to a range between 0.05
and 0.2mm.
There is an issue with setting these limits on
crack widths as they only concern the visible
crack that is at the surface of the concrete.
There is no recognition given to the nature
of the crack underneath the surface, which
can widen and spur off into more cracks that
are not visible. Without carrying out intrusive
investigations, however, it is not practical to
determine the exact form and depth of
a crack.
Water ingress and corrosion
Corrosion of reinforcement within concrete
causes the spalling of concrete as the steel
expands and degrades. To prevent this
from occurring, certain environmental and
chemical conditions need to be present
within the concrete. Therefore, it is important
that:
• the level of chloride within it is not
exceeded beyond a critical level
• the reinforcement is placed appropriately
and surrounded by good quality concrete
that is not carbonated
• there are no stray electrical currents
present
The limit of 0.3mm described in BS EN 19921-1 has been deemed to be acceptable in
terms of preventing moisture ingress having
an impact on the environmental conditions
of the reinforcement. Water ingress can
obviously impact on the reinforcement,
therefore maintaining the limit of crack
width will most likely reduce the risk of
this occurring.
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Cracking due to settlement
Cracking of concrete due to settlement
concerns how the concrete is placed, and
not the movement of the structure as a
whole due to the deflection of a support. It
is known as ‘plastic cracking’ and occurs
soon after the concrete has been poured. As
the concrete begins to set, it is vibrated into
place and it is at this point plastic cracking
can appear. As the concrete is vibrated the
water within it rises to the surface while
the heavier elements, such as the cement
and aggregates, settle towards the bottom.
This process is known as ‘bleeding’ and it
influences the likelihood of plastic cracks
forming. With a combination of elements of
the concrete being restrained and a high
amount of bleeding, plastic cracks occur in
the surface of the concrete.
To overcome this, admixtures can be
installed within the concrete to reduce the
amount of bleeding. This can be done in
combination with designing and detailing
the concrete element to reduce the amount
of restraint to the concrete as it sets. It is
also possible to eliminate plastic cracks by
re-vibrating the concrete 2-3 hours after
it has been formed, as it is still relatively
pliable at this stage.
Shrinkage cracking
A variant of the plastic crack is the
shrinkage crack. These typically occur
over large slab pours and walls, which can
appear within 2-3 hours of the concrete
setting. If the concrete is unprotected
while it cures it can dry too quickly, due
to wind passing over it if it is exposed.
This can result in ‘crazing’, which is a
form of cracking on the outer surface of
the concrete that consists of cracks that
when linked together form small polygonal
shapes. Provided the finish to the concrete
is appropriately troweled and/or floated,
these cracks can be graded out. However,
there are instances where on the surface
no cracks can be detected but instead they
have formed underneath a layer of mortar
that has been created by the troweling/
floating action.
Various methods have been adopted to
prevent such cracks from developing. They
are primarily focussed on reducing the rate
at which the concrete sets. The faster it
sets, the stiffer it becomes and thus it has a
reduced capacity for tension stress. Again,
it always returns to the key point of crack
propagation in concrete, which is dependent
upon the tensile capacity of the material.
This occurs due to the water evaporating
from the concrete. Reducing the rate of
"Whatever
the symptoms
of cracking,
the ultimate
cause is always
the same"
evaporation is key to the prevention of
shrinkage cracking. One way to address this
is to apply a compound to the surface of
the concrete within a few hours of it being
placed. This lessens the rate at which water
escapes the concrete and thus significantly
reduces the likelihood of cracks forming.
A more traditional method is to cover the
surface of the concrete with hessian or
polythene sheeting in order to prevent the
bleed water from evaporating too quickly,
thus allowing the concrete to set in a more
controlled manner.
Thermal movement
Concrete structures should be allowed
to move in response to the varying
temperatures they are exposed to during
their design life. If they are not, then cracks
can appear as the structure expands
and contracts, leading to tension being
generated in the concrete. This is more
likely to occur in lower temperatures, as
that is when concrete shrinks. Where the
concrete is restrained it tends to crack as
the tension within the concrete exceeds
its capacity. Smaller structures, such as
those with a footprint dimension of no
S
Figure 2
Contraction joints in concrete
Free contraction joint
Tied partial contraction joint
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Note 26 Level 1
20
Technical
Technical Guidance Note
TheStructuralEngineer
May 2013
more than 50m, are less prone to thermal
effects. Larger concrete structures
however, do require require either additional
reinforcement within them or the inclusion
of movement joints. These are joints which
split the structure into separate structures.
The integrity of each of these structures
needs to be maintained. The amount of
movement the structure is likely to undergo
during its lifetime must be allowed for within
the movement joint.
One way to address cracking as the
concrete sets is to introduce construction
joints i.e. a straight crack. If no continuity
of reinforcement is present, then a crack
"Concrete is
designed to
crack. Indeed it is
preferable for it to
do so"
can be generated as the concrete shrinks
towards the steel reinforcement. By
creating a construction joint, the crack
does not appear and hence the integrity
of the structure is maintained. Figure 2
shows two examples of induced contraction
joints, which are a form of construction
joint that produce straight line cracks in a
controlled manner.
Glossary and
further reading
Bussell M. N. and Cather R. (1996) Design
of construction joints in concrete structures
(R146) London: CIRIA
Bamforth P. B. and Price W. F. (1994)
Concreting Deep Lifts and Large Concrete
Pours (R135) London: CIRIA
The Concrete Society (2010) TR22 Nonstructural cracks in concrete Camberley,
Surrey: The Concrete Society
The Concrete Society (2008) TR67 Movement,
restraint and cracking in concrete structures
Camberley, Surrey: The Concrete Society
Bleeding – Water that rises to the surface of
concrete after it has been vibrated.
Eurocode 0.
Web resources
Plastic cracking – Cracks that appear in
the concrete 2-3 hours after setting.
Spalling – The breaking and cracking of
concrete due to internal pressure within the
concrete element. This typically occurs due
to corrosion of steel reinforcement.
The Concrete Centre:
www.concretecentre.com
The Concrete Society:
www.concrete.org.uk
Further Reading
Bamforth P. B. (2007) Early-age thermal crack
control in concrete (C660) London: CIRIA
The Institution of Structural Engineers library:
www.istructe.org/resources-centre/library
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