Cracks in Concrete

Cracks In Concrete
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Cracks in concrete
• A building component develops cracks whenever the
stress in the components exceeds its strength. Stress
in the building component could be caused by
externally applied forces, such as dead, live, wind or
seismic loads, foundation settlement etc. or it could
be induced internally due to thermal movements,
moisture changes, elastic deformation, chemical
action etc.
Cracks in concrete(continue)
 Cracks in buildings could be broadly classified as structural
and non-structural cracks.
1. Structural Cracks : These occur due to incorrect design,
faulty construction or overloading and these may endanger
the safety of a building. e.g. Extensive cracking of an RCC
beam.
2. Non structural Cracks: These are mostly due to internally
induced stresses in buildings materials and do not endanger
safety of a building but may look unsightly, or may create an
impression of faulty work or may give a feeling of instability.
• In some situations due to penetration of moisture through
them non structural cracks may spoil the internal finishes thus
adding to the cost of maintenance, or corrode the
reinforcement, thereby adversely affecting the stability of the
Structure in long run. e.g. Vertical crack in a long compound
wall due to shrinkage or thermal movement.
 Cracks may appreciably vary in width from very thin hair crack
barely visible to naked eye to gaping crack. Depending upon the
crack width cracks are classified as:
(a) Thin Crack - less than 1 mm in width,

(b) Medium Crack - 1 to 2 mm in width,
(c) Wide Crack - more than 2 mm in width.
(d) Crazing - Occurrence of closely spaced fine cracks at the
surface of a material is called crazing.
• The commonly used building material namely
masonry, concrete, mortar etc. are weak in tension
and shear. Therefore the stresses of even small
magnitude causing tension and shear stresses can
lead to cracking.
Principal Causes Of Cracks
• The principle mechanism causing non-structural
cracks in the building are :
i. Moisture Change
ii. Thermal Movement
iii. Elastic Deformation
iv. Creep
v. Chemical Reaction
vi. Foundation Movement & Settlement Of Soil
vii. Growth Of Vegetation
Principal Causes Of Cracks (continue)
1. Moisture change:-
Most of the building materials (e.g. Concrete, mortar,
burnt clay brick, timber, plywood etc.,) are porous in their structure in
the form of inter-molecular space, and they expands on absorbing
moisture from atmosphere and shrinks on drying. These movements are
reversible.
Apart from reversible movement certain materials
undergo some irreversible movement due to initial moisture changes
after their manufacture or construction. The incidences of irreversible
movement in materials are shrinkage of cement and lime based
materials on initial dryin.
• The extent of initial shrinkage in cement concrete
and cement mortar depends on a nos. of factors
namely :
a) Cement content.
b) Water content.
c) Maximum size, grading & quality of aggregate.
d) Curing
e) Temperature
f) humidity
2. Thermal movement :-
All materials more or less expands on heating and
contracts on cooling. When this movement is restraint, internal stresses
are set-up in the component, and may cause cracks due to tensile or shear
stress. Thermal movement is one of the most potent causes of cracking in
buildings and calls for careful consideration. The extent of thermal
movement depends upon :
Co-efficient of thermal expansion – Expansion of cement mortar &
concrete is almost twice of the bricks and brick work. Movement in
brickwork in vertical direction is 50% more than in horizontal direction.
3. Elastic deformation :-
Structural components of a building undergo elastic

deformation due to dead and the super imposed live loads, in
accordance with hook law. The amount of deformation
depends upon elastic modulus, magnitude of loading and the
dimension of the component. This elastic deformation under
certain circumstances causes cracking in the building as under
:
 When walls are unevenly loaded with wide variations in stress
in different parts, excessive shear stress is developed which
causes cracking in walls.
 When a beam or slab of large span undergoes excessive
deflection and there is not much vertical load above the
supports (as in the case of roof slab), ends of beam /slab
curl up causing cracks in supporting masonry.
 When two materials, having widely different elastic
properties, are built side by side, under the effect of load,
shear stresses are set up at the interface of the two materials,
resulting in cracks at the junction. Such a situation is
commonly encountered in the construction of RCC framed
structure and brick masonry panel (external) and partition
(internal) walls.
3. Creep :-
Normally used building material such as
concrete, brickwork, mortar, timber etc. when
subjected to sustained load not only under go
instantaneous elastic deformation but also under go
a gradual and slow time dependent deformation
known as creep or plastic strain.
 In concrete, the extent of creep depends on :
1. Water & cement content.
2. Water cement ratio.
3. Temperature and humidity.
4. Use of admixture and puzzolonas.
5. Age/strength of concrete at the time of loading.
6. Size and shape of the component.
4. Movement due to chemical reaction:-
Certain chemical reactions in building materials result in
appreciable increase in volume of materials, due to which internal stresses
are setup which may results in outward thrust and formation of cracks.
The material involve in reaction also become weak in strength. The
common instances of chemical reactions are :
• Sulphate attack
• Carbonation in cement based materials
• Corrosion of reinforcement in concrete and brickwork
• Alkali-aggregate reaction
4. Foundation movement and settlement of soil:
shear cracks in buildings occurs when there is large differential
settlement of foundation due to one of the following cause.
(a) Unequal bearing pressure under different parts of the structure.
(b) Bearing pressure being in excess of safe bearing strength of the soil.
(c) Low factor of safety in the design of foundations.
(d) Local variations in the nature of supporting soil, which remained
undetected and could not be taken care of in the foundation design at the
time of construction.
(e) Foundation resting in active zone on expensive soil
4. Growth of vegetation :
Roots of a tree generally spread
horizontally on all sides to the extent of height of the
tree above the ground and when the trees are
located in the vicinity of a wall, they can cause cracks
in walls due to expensive action of roots growing
under the foundation.

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Cracks In Concrete

(a) Thin Crack - less than 1 mm in width,

Structural components of a building undergo elastic

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