Concrete is known to be a very versatile and reliable material, but
some construction errors and construction negligence can lead to
the development of defects in a concrete structure. These defects in
concrete structures can be due to poor construction practices, poor
quality control or due to poor structural design and detailing.
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Common types of defects in concrete structures are honeycombing,
form failure or misalignment of formwork, dimensional errors, rock
pockets and finishing errors.
1. Honeycomb and Rock Pockets
Honeycomb and rock pockets appear on the concrete surface where
voids are left due to the failure of cement mortar to fill spaces
around and among coarse aggregates.
Causes of honeycomb and rock pockets involve poor quality control
during mixing; transporting; or laying of concrete, under or over-
compaction of concrete, insufficient spacing between bars, and low
cement content or improper mix design.
Honeycomb and rock pockets may reduce durability because they
expose the reinforcement to the environment which may reduce the
strength of the concrete sections.
If these defects are minor, they can be repaired by using cement
mortar grout just after the removal of the formwork. If the repair
work is delayed for more than 24 hours, epoxy bonded concrete
replacement should be used.
Fig. 1: Honeycomb
2. Defects due to Poor Formwork
Installation
Formwork installation errors include misalignment, movement, loss
of support, failure of forms that can lead to cracking and structural
failure.
Settlement cracks develop due to concrete settlement caused by
the loss of support during construction. Inadequate formwork
support and premature removal of formwork are the major causes of
loss of support during construction.
Defects due to formwork placement mistakes can be repaired with
surface grinding to maintain the verticality of the structure if the
error is minor. In case of major error, the concrete member shall be
repaired by removing the concrete in the defective area and then
reconstructing that portion of the structural member using suitable
methods.
Fig. 2: Defects in Concrete due to Formwork Movement
3. Defects due to Concrete Dimensional
Errors
Dimensional errors in concrete structures occur either due to the
poor centering of a structural member or due to deviation from the
specifications. In that case, the structural member can be used if it
is acceptable for the intended purpose of the structure or can be
reconstructed if it doesn't suffice.
4. Defects due to Finishing Errors
Finishing errors in concrete structures can involve over-finishing of
the concrete surface or addition of more water or cement to the
surface during finishing of the concrete. This results in the porous
surface which makes the concrete permeable resulting in less
durable concrete.
Poor finishing of concrete results in the spalling of concrete from
surface early in their service life. Repair of spalling involves removal
of defective concrete and replacement with epoxy bonded concrete.
5. Shrinkage Cracks
The formation of shrinkage cracks in concrete structures is due to
the evaporation of water from the concrete mixture. The severity of
this issue is based on the amount of water in concrete (as water
quantity increases, the number of shrinkage cracks increases),
weather conditions, and curing regime.
This problem can be tackled by considering suitable curing regime
and adding a suitable amount of water to the concrete mixture.
Fig. 3: Shrinkage Cracks
6. Defects due to Poor Reinforcement
Placement
Errors during reinforcement installation could lead to serious
concrete deterioration. For instance, inadequate chair bars and
insufficient tying of reinforcement would cause rebar movement
which may lead to inadequate concrete cover and reduction in effect
depth of the concrete section. As a result, the durability of the
concrete structure is compromised and the structure would be
vulnerable to chemical attacks.
Fig. 4: Reduction of Concrete Cover due to Reinforcement Movement
7. Bugholes
Bugholes or surface voids are small regular or irregular cavities
formed due to the entrapment of air bubbles in the surface during
placement and consolidation. They commonly occur in vertical cast-
in-place concrete like walls and columns.
Both the number and size of bugholes vary and depend on form-
facing material and condition, release-agent type and application
thickness, concrete mix characteristics, and placement and
consolidation practices.
Bugholes are considered as defects if their width and depth exceed
3.81cm and 1.27cm respectively.
What are the defects of cracks?
Classified by the Position of the Crack
Generally, there are three kinds of crack defects within structures: through crack,
surface crack, and embedded crack (Figure 45.2).
DEFECTS IN CONCRETE STRUCTURES
Cold joints
A cold joint is the point of connection where old and new concrete meet. It commonly
occurs during construction when one batch of concrete has begun to set before the
next batch is added. The old and the new concrete do not intermix and a so-called
“cold joint” is formed as a result, creating a weakness in the concrete and a possible
passageway for water intrusion.
Cold joint – Where old and new concrete do not intermix.
Service lines and z-bar penetrations
Defects in concrete structures are commonly caused by penetrations through the
concrete made for service lines or created by z-bars that are used for creating
formwork ties or temporary tie-downs into the concrete.
Once the caps and bolts are removed, the z-bar holes are inserted with cement-
based plugs or are patched with sand and cement mix. Pipe penetrations are also
sealed with a cement-based plug or a patch from the interior side of the building
during the original construction. The plug does not seal the gaps throughout the
whole wall or slab and the bond to the concrete usually fails over time. This pathway
allows water to find its way into the gaps and seeps through the concrete around the
pipe penetration.
When a hole for the service pipe line is drilled through the concrete, the coring
process can cause damage to the concrete structure by creating cracks. The core
hole can often provide a direct path for water to pass through the concrete structure.
Pipe penetrations through concrete slab.
Bug holes / blow holes / bony areas / porous concrete
In these cases, there is too much air in the concrete, usually caused by improper
vibration, non-permeable formwork and mix-design, for example insufficient fines to
fill the voids between the aggregate.
Whether they are considered defects or not depends on the width and depth of the
holes or gaps.
Consolidation related surface issues in concrete.
Construction joints
A construction joint is a break or gap between two slabs of concrete, where two
successive slabs are joined together in a concrete structure. Construction joints are
placed in the concrete at the time of placement with the help of dividing barriers of
steel or plastic. Despite being a potential weak link that may cause serviceability
problems, construction joints are in many situations a necessary requirement where
there are multiple concrete placements.
Problems with construction joints arise frequently due to insufficient attention to
design planning and their location. that may cause serviceability problems and lack
of durability.
Control / contraction joints
Control joints are pre-planned and installed to prevent concrete cracking due to
shrinkage during curing. A control joint is saw-cut into the curing concrete when the
concrete is just hard enough, usually within 6-12 hours after the concrete has been
poured. The timing depends on the concrete mix and the surrounding environment.
The cuts should be made at a predetermined spacing, depth and pattern to meet
structural engineering specifications and only after the concrete has obtained
sufficient strength, but before internal cracking begins.
Cutting too early causes ravelling (pulling the aggregate out of position) and creates
a weakened edge along the cut.
Cutting it too late results in uncontrolled cracking as shrinkage cracking has already
begun during the hardening process of the concrete.
If the joint is cut too deep, the interlocking aggregate may not be sufficient to
transfer loads and if the saw cut is shallow, then uncontrolled cracking may occur.
Crack formed outside control joints.
Expansion joints
An expansion joint is used in concrete to allow the concrete to absorb predicted
movement by expanding or contracting with daily temperature variations. Lack of
expansion joints may lead to uncontrolled cracking.
Deteriorating joint fillers and sealants leave the sidewall of the joint unprotected. If
left untreated, joint spalling as well as water and other material ingress can result in
decreased service life of the concrete structure.
1) Plastic shrinkage concrete cracks:
Concrete cracks form during construction due to plastic shrinkage on the
freshly laid concrete’s surface before it has set. Initial shrinkage of the top
layer occurs when the concrete surface loses water faster than the
bleeding process. Concrete cracks appear on the surface because the
concrete is still in a flexible state and cannot withstand any tension. When
the water evaporates, it leaves enormous gaps between the solid
particles. The concrete becomes weaker and more likely to crack as a
result of these empty areas. “Plastic shrinkage cracking” is a term for this
sort of cracking that occurs regularly. Shrinkage does not occur if there is
a continuous layer of water on the surface.
Slabs frequently develop plastic shrinkage cracks. The concrete slab’s
surface becomes extremely stiff as it dries quickly. As a result, it won’t
flow and won’t be able to withstand the tensile stresses caused by
restricted shrinkage. Cracks appear on the slab’s surface as a result of
this.
Fig 1: Shrinkage Cracks in Concrete
Source: The Constructor
2) Plastic settlement cracks:
In a reinforced structure, plastic settling cracks form in freshly placed/laid
cement concrete. Plastic settlement fractures appear on the surface
before the concrete has hardened, when there is a lot of bleeding. The
downward sedimentation of the solids is also hampered by some type of
obstacle (e.g., reinforcement bars). Fresh concrete tends to settle or
recede when poured in deep formwork such as a wall or column. Short
horizontal fractures will result if this settlement is slowed by obstructions
like as steel bars or big aggregates. Furthermore, these impediments
break the back of the concrete above them. Plastic settling cracks are a
type of subsidence caused by a decrease in volume.
Fig 2: Plastic Settlement Cracks
Source: ReseachGate
3) Expansion concrete cracks:
Concrete slabs expand outwards when they are exposed to heat. Slabs
crack when they don’t have enough room to expand. Concrete expands
and pushes against anything it comes into contact with (a brick wall or
adjacent slab for example). The increasing force can cause concrete to
crack when neither can flex.
Expansion joints are used to separate static surfaces. Expansion joints,
which are often formed of a compressible material such as asphalt,
rubber, or wood, must act as shock absorbers to ease the stress that
expansion places on concrete and avoid cracking.
Fig 3: Expansion Concrete Crack
4) Heaving concrete cracks:
Heaving cracks are another type of temperature-related crack. Exposure
to extremely cold temperatures causes the slab to condensate, resulting
in heaving cracks. The slab expands back to its original shape when the
temperature returns to normal. Heaving cracks are frequently formed as a
result of this shift in shape.
Fig 4: Heaving Concrete Crack
5) Concrete cracks due to overloading the slab:
Concrete is an extremely robust building material, yet it has limitations.
Putting too much weight on top of a concrete slab might cause it to crack.
Overloading of the concrete slab itself is uncommon in residential
concrete slabs. Excessive overload on the ground beneath the slab is
more prone to develop.
Excessive weight on the slab after heavy rain or snowmelt, when the
ground below is soft and wet, can press the concrete down and cause
cracks. This type of cracking is more likely to appear in driveways where
large recreational vehicles or dumpsters are parked.
6) Concrete cracks due to premature drying:
Cracks can form when a concrete slab (or its top layer) loses moisture
quickly. When the top layer of the slab naturally loses moisture, crazing
cracks emerge, mimicking a spider’s web. When the top layer is cured for
pattern embedding, crusting fractures appear. Although both of these
varieties may appear unpleasant, they are mostly harmless to the slab’s
structural integrity.
Fig 5: Cracks due to premature drying
7) Cracks due to chemical reaction:
Concrete cracking can be caused by harmful chemical reactions. These
reactions could be caused by the materials used in the construction of the
concrete or by objects that come into touch with it after it has hardened.
Concrete may crack over time as a result of expanding interactions
between active silica aggregate and alkalis coming from cement
hydration, admixtures, or other external sources for e.g., curing water,
groundwater, and alkaline solutions stored or used in the finished
structure.
8) Cracks due to corrosion of reinforcement:
Corrosion of steel results in the formation of iron oxides and hydroxides,
which have a much larger volume than the original metallic iron. High
radial bursting stresses around reinforcing bars result in local radial cracks
as a result of the increase in volume. Splitting cracks can spread along the
bar, causing longitudinal cracks or concrete spalling. Delamination, a well-
known problem in bridge decks, can also occur when a broad crack forms
at a plane of bars parallel to the concrete surface. Minor splitting cracks
provide easy access for oxygen, moisture, and chlorides, accelerating
corrosion and cracking.
Fig 6: Cracks due to corrosion in reinforcement
9) Cracks due to errors in designing and detailing:
Poorly specified reentrant corners in walls, precast members, and slabs,
inappropriate reinforcement selection and/or detailing, and restraint of
members susceptible to volume changes due to temperature fluctuations
and moisture, lack of adequate contraction joints, and improper design of
foundations, resulting in differential movement within the structure are all
design and detailing errors that can lead to unacceptable cracking.
