Lec#09&10.cracks,its terms,why concrete cracks.bzu

Bahauddin Zakariya University,Lahore Campus
Lecture#09
MATERIAL TESTING
REPAIR
&
MAINTENANCE
Cracks in Concrete
Prepared by:
Engr. Shamsher Sadiq(Lecturer)
10/11/2014 ershrajput@gmail.com 1

Cracks in Concrete
Definition of Crack
Terminology of Cracks in Concrete
Why Concrete Cracks???

Cracks in Concrete
Crack is a “fissure or discontinuity of pavement surface not necessarily extending
through the entire thickness of the pavements[ASTM E 1778 ].
A crack is highly invisible breakage in a structural member that may or may not
extend through the whole body of the member.It can be assessed through visual
inspection , magnifying glass , or immersion in warm water or apply moisture at the
surface.

Cracks in Concrete
Micro cracks are defined as follow:
Sub Micro Cracks:
These are very fine surface cracks ,which are not visible to
naked eyes. These cracks can be detected using scanning electron microscope at a
magnification of at least 1250.
Micro Cracks:
These are fine microscopic cracks with concrete that have a
maximum aperture of 0.1 mm. They can be seen only through microscope and are
not visible to naked eye.

Crack Terminology
Cracks can occur in a various configurations , size and shapes.
Classifications can be according to:
Surface appearance
Dimensions
Severity
Structural nature

Crack Terminology based on surface
appearance
Different terms associated with visual inspection of cracks that have characteristic cracking patterns , are enlisted
below:
Small , medium or large size cracks
 Crazing
Map
Random
Horizontal
Vertical
Traverse
Longitudinal
Diagonal
 Isolated
 Hairline

appearance
Freeze-thaw damage
Block cracking
Popouts
Pitting
Peeling
Scaling
Spalling
Delamination
Dusting

appearance
Small Cracks:
upto 3mm wide at surface
Large Cracks:
are more than 6mm wide at surface
Medium size Cracks:
greater than 3mm and less than 5mm

appearance
Crazing:
development of close network of fine hair cracks on surface of plaster , cement
paste , mortar , or concrete caused by shrinkage of surface layer.
Crazing cracks are sometimes also known as map or pattern
It’s a cosmetic problem.
These are nonstructural and most visibly on a nearly dry surface.
Less than 0.3mm wide generally not more than 2-2.5mm deep and typically from hexagonal
shaped area no more than about 40-50mm across.
Observed more quickly and densely on steel trowelled surface.
Early age and are readily visible in wet concrete

appearance
Crazing will occur if:
Mix is wet
No proper curing
Cement is sprinkled on surface to dry up the bleed water.
There is carbonation of surface
There is rapid surface drying usually caused by high air temperature , hot sun , dry
winds , or combination
Bleed water is present coz of finished too early.
No evaporation-retarding agent has been used placement of concrete in hot dry or
windy conditions.

appearance
Crazing can be avoided????

appearance
Horizontal cracks:
Excessive lateral loads.

Horizontal Cracks in Basement Walls
(As you are sitting in a basement class)
Horizontal cracks sometimes manifest on basement walls or perimeter foundations
doubling as retaining walls. Horizontal cracks in these places are usually symptoms
of serious structural instabilities.
• The presence of a cold joint is a common cause of horizontal crack propagation in
basement walls. A cold joint is formed when the basement wall concrete is poured
in multiple stages during construction.
• Ex: 6 feet of basement wall concrete is poured and allowed to dry, then another 3
feet of concrete is poured over it. In this case, a cold joint forms between the two
blocks of concrete (the 6-feet block and the 3-feet block), and this becomes a weak
point from which horizontal cracks may originate.
• Cold joints are not exclusive to basement walls. Concrete perimeter foundations, if
poured in multiple stages, will also form a cold joint.

Continued….
• Frost damage
• This could cause horizontal cracks in houses that are located in areas with frost.
When standing surface/subsurface water freezes, it exerts considerable pressure on
the upper portion of the basement wall, and this could cause it to crack.
• This situation can be prevented by ensuring that water from the rain gutters or
lawn doesn’t pool near the perimeter of the house.
• Unusual loading
• Horizontal cracks in the upper portion of the basement walls may also be caused
by unusual loading from a nearby vehicle or equipment. When a heavy vehicle
drives by or is parked close to the basement wall, its weight exerts pressure on the
soil. The soil, in turn, exerts pressure on the basement wall, and this could cause it
to crack

appearance
Vertical cracks:
These are usually caused by differential settlement of the house foundations. In
other words, a part of the foundation has settled or is still settling at a level lower
than the rest of the foundation. This causes cracks to appear at and propagate from
the point of maximum stress.(without jointing)
Excessive vertical loadings
It can be controlled by providing proper designed joint system.

Continued…

appearance
Traverse Cracking:
Right angles to long direction of concrete member.
Cracks are perpendicular to direction of traffic in pavements.

Continued…

appearance
Longitudinal cracks:
Parallel to length of member
In pavement parallel to direction of traffic

Continued…

appearance
Diagonal cracks:
Structurally related …
Inclined at 45 degree to N.A i.e. in flexural member…
Corner of windows and doors.

Continued…

Load-displacement behavior of beam
• The complexities involved in obtaining a progressive-failure solution to a reinforced concrete problem
can be illustrated by considering the simple beams shown in Fig. 7.1a and b, subjected to a simple
centerpoint load. The crack patterns shown are those obtained experimentally in two identical beams
which failed in shear, one without and one with stirrups (Scordelis, 1972). The load-displacement
response of the beams under increasing load can be roughly divided into three stages:
• The uncracked elastic stage. At low loads the beam behaves essentially like an uncracked elastic
member.
• The crack-propagation stage. Vertical flexural cracks then occur at midspan, resulting in a redistribution
of stress and causing some bond slip.
• The plastic stage. If shear and diagonal tension are not critical, the beam eventually fails by yielding of
the longitudinal tensile steel reinforcement or by crushing of the concrete in the compression zone. If
shear and diagonal tension are critical, the formation of a significant diagonal-tension crack activates
resistance to vertical shear by dowel action in the main longitudinal reinforcement, subsequent
aggregate interlock along the diagonal crack, and resistance in vertical stirrups, if any exist. This leads to
the final shear-compression failure of concrete under a combined state of stress.

Load-displacement behavior of beam
Experimental crack patterns of beams failing in shear: ( a) beam without stirrups
and ( b) beam with stirrups. (Scordelis. 1972.)

appearance
Diagonal cracks:
When RCC frame moves/deflects due to loads, temp,variation etc. it causes diagonal
cracks in walls which are located parallel to the movement

Continued…

appearance
Hair line cracks:
less than 0.1mm in width
Hardly visible due to narrow width.
Not disturb the structural stability
No need to repair but can be grouted with OPC

appearance
Freeze thaw damages:
•
Freeze/thaw damage occurs in concrete when the water molecules in concrete freeze and expand
beyond the volume constraints of the concrete.When the >91% of the pores of concrete are filled
with water, the concrete is known to be saturated. When these water molecules freeze, they expand
by 9%, and because there is no room for their increased volume, the concrete distresses. The
freeze can cause the bonds in cement around the aggregate to break and the concrete can crack in
those places. Thus, the higher the water/cement ratio in the concrete, the higher risk it is for
freeze/thaw damage. As the seasons pass, concrete goes through the process of freezing and
thawing, wearing out over time. As winters come and go from year to year, the concrete
deteriorates.
• Certain measures can be taken to prevent freeze/thaw damage to pavements and any concrete
structure. Prevention measures include adding deicing chemicals to the concrete during the winter
in order to decrease the freezing point of precipitation as it falls onto the pavement as well as using
high strength, air-entrained concrete. By reducing the freezing point of the precipitation, chemicals
such as sodium chloride, calcium chloride, magnesium chloride, and potassium chloride in high
concentrations work to reduce the exposure of the pavement to freeze/thaw cycles

Continued….

appearance
Popouts
Usually, the culprits are porous, absorptive, moisture-susceptible aggregates within
the mix. If these aggregates become saturated by water ingress, they can expand
with great force and pop-out the cement matrix covering them. Inspection will find
the responsible aggregate at the bottom of the popped hole.
Small portions of concrete surface break away due to internal pressure , which
leaves a shallow , typically conical depression such as flint gravels and igneous
rocks that are generally resistant to frost have been observed to cause popouts.
Its neither related with concrete quality nor the method of manufacturing but
generally due to frost.
Popouts occurs at the largest pieces of aggregate.

Continued….

appearance
Pitting
It is the development of relatively small localized cavities in surface caused by
phenomena such as: corrosion or cavitation , or in concrete localized disintegration
such as popout.

Continued….

appearance
Peeling
Peeling is a process in which thin flakes of mortar are broken away from a

Continued….

appearance
Scaling

Continued….

appearance
Spalling

Continued….

appearance
Delamination

Continued….

Why Concrete Cracks
Concrete cracks when its tensile strength is unable to withstand the forces to which the
concrete is subjected to.
As soon as the concrete is cast it is affected with micro/macro level cracks,which develop
and expand due to complexity:
Loads
Temperature differentials
Moisture loss
These micro/macro cracks may propagate under the increasing compressive stress(not
always compulsory).
Also due to drying shrinkage, volume changes may occur , which cause the internal cracks in
mortar coarse aggregate boundary.
This may occur prior to any load application.

Why Concrete Cracks
Further propagation of these micro cracks depends upon:
Rate of loading
Intensity of loading
Type of loading
Age of concrete
Type of concrete
Type of mix
Environment and general weather condition
Type of service to which the structure is subjected

Factors influencing Cracking process of Concrete
The basic factors include the followings(ACI 303 R-91):
Applied loads
Restraint of drying shrinkage
Creep
Thermal Stresses
Foundation Settlement

Crack due to choice of wrong materials
construction defeciencies,lack of post construction
care of structure
Effect of cement on cracking behavior of concrete
More than required water in a mix
Inadequate curing
Concrete of poor quality
Improper or inadequate jointing practice
Non-uniform loading condition
Excessive loading on structure
Use of reactive aggregates

Lec#09&10.cracks,its terms,why concrete cracks.bzu

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Lec#09&10.cracks,its terms,why concrete cracks.bzu

Similar to Lec#09&10.cracks,its terms,why concrete cracks.bzu (20)

Recently uploaded

Recently uploaded (20)

Lec#09&10.cracks,its terms,why concrete cracks.bzu