A
crack is a complete or incomplete separation of concrete into two or more parts
produced by breaking or fracturing. Concrete structure has been started
applying since the mid-19th century, because of the low quality of cement and
at that time the development of concrete structure was slow. Until the end of
the 19th century, concrete structure was getting faster development with the
development of production, experimental work, computational theory and
improvement of construction technique and now it has become one of the most
widely used building materials in the modern construction.
Cracks are one kind of universal problem of concrete
construction as it affects the building artistic and it also destroys the
wall’s integrity, affects the structure safety even reduce the durability of
structure. Cracks develop due to deterioration of concrete or corrosion or
reinforcement bars due to poor construction or inappropriate selection of
constituent material and by temperature and shrinkage effects.
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| Cracked Concrete |
Cracks
can be divided into two types:
·
Structural Cracks: These
cracks occur due to incorrect design, faulty construction or overloading and
these may endanger the safety of a building.
·
Non-Structural Cracks: They
are due to internal forces developed in materials due to moisture variations,
temperature variation, crazing, effects of gases, liquids etc.
Some of the causes of cracks & its preventive measures
in concrete structures
Elastic Deformation
It occurs when a material strains under
stress. When two materials (having different elastic properties) built together
under the effect of load then different shear stresses in these materials
create cracks at the junction. Dead and live loads are the main cause of
elastic deformation in any structural components of a building.
Preventive Measures: Create slip joints
under the support of concrete slab on walls. Provide horizontal movement joints
between the top of brick panel and reinforced cement concrete beam/slab.
Thermal Movement
Most materials expand when they are
heated, and contract when they are cooled. The expansion and contraction with
changes in temperature occur regardless of the structure’s cross-sectional
area. It is one of the most potent causes of cracking in buildings which need
attention.
Preventive Measures: Joints should be constructed like
construction joints, expansion joints, control joints and slip joints. The
joints should be planned at the time of design and be constructed carefully.
Chemical Reaction
Chemical reactions may occur due to the materials used to make the
concrete or materials that come into contact with the concrete after it has
hardened. Concrete may crack with time as the result of slowly developing
expansive reactions between aggregate containing active silica and alkalis
derived from cement hydration, admixtures or external sources.
Preventive
Measures: If sulphate content in soil exceeds 0.2 percent or in ground water
exceed 300 ppm, use very dense concrete and either increase richness of mix to
1:1/5:3 and to prevent cracking due to corrosion in reinforcement it is
desirable to specify concrete of richer mix for thin sections in exposed
locations.
Shrinkage
Most of the building materials expand when they absorb moisture from atmosphere
and shrink when they are dry. Shrinkage can be of plastic or dry. The factors
causing shrinkage in cement concrete and cement mortar and their prevention measures are
as following.
Excessive Water: The quantity of water used in the mortar mix can cause
shrinkage. Vibrated concrete has less quantity of water and lesser shrinkage
than manually compacted concrete.
Preventive
Measures: Use minimum quantity of water required for mixing cement concrete or
cements mortar according to water cement ratio. Cement concrete is never
allowed to work without mechanical mix and vibrator.
Quantity of Cement: As a general rule, the richer the mix is, the greater the
shrinkage/drying will be.
Preventive
Measures: Do not use excessive cement in the mortar mix.
Foundation
Movement and Settlement of Soil Shear cracks in buildings occur when there is
large differential settlement of foundation and it may be either due to the
following reasons:
- Unequal bearing pressure under different parts of the structure
- Bearing pressure on soil being in excess of safe bearing strength of the soil
- Low factor of safety in the design of foundation
- Local variation in the nature of supporting soil
Earthquake
Crack may occur due to sudden shift in lower layer of the earth. The voids in
the earth might have suddenly collapsed and be filled with soil from the above.
Many geological events can trigger earth movements but is continuous movement.
Preventive
Measures: Construct the foundation of buildings on firm ground while doing
construction. Tie up the building with connecting beams at foundation level,
door level and roof level.
Vegetation
Fast growing trees in the area around the
walls can sometimes cause cracks in walls due to expansive action of roots
growing under the foundation. The cracks occur in clay soil due to moisture
contained by roots.
Preventive Measure: Do not grow trees too close to the
building. Remove any saplings of trees as soon as possible if they start
growing in or near of walls.
Poor Construction Practices
There is a general lack of good construction practices
either due to ignorance, carelessness, greed or negligence. For a healthy
building it is absolutely necessary for the construction agency and the owner
to ensure good quality materials selection and good construction practices.
Preventive
Measure: Proper monitoring and use of good quality of materials is required at
the time of construction
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