Docstoc
EXCLUSIVE OFFER FOR DOCSTOC USERS
Try the all-new QuickBooks Online for FREE.  No credit card required.

Cracks in Buildings

Document Sample
Cracks in Buildings Powered By Docstoc
					CRACKS IN BUILDING




    CONTENTS
       INTRODUCTION
       CRACKS IN BUILDING
       LOOPHOLES OF CRACKS
       CAUSES OF CRACKS
       PREVENTIONS OF CRACKS
       REPAIR OF CRACK
       CONCLUSION




           INTRODUCTION
Cracks result in when applied forces are greater than those which
the building or its part can withstand. These forces may have
emerged externally to the building or internally within the building
or have been developed in the materials of the building as a result of
Chemical changes. There may be a single force or a combination of
forces having a single cause or several causes.




This is a residential building at San Francisco. Major cracks have
developed on the external facade of the building. The cracks
running across the entire facade of the building are diagonally
directed and diagonal cracks are generally structural cracks. They
occur due to structural problems in the buildings. Structural cracks
could be due to major or minor problem in the structural design.
Intensity of structural problem is determined on the basis of the
intensity of cracks developed.

       LOOPHOLES OF CRACKS
      1.   BUILDING BECOME UNSAFE
      2.   IT SPOIL THE APPERANCE OF BUILDING
      3.   DAMPNESS IN THE BUILDING
      4.   REDUCE IN MARKET VALUE
      5.   INCREASE IN MAINTAINENCE COST
Most cracks occur due to drying of construction water.

      These are generally superficial
      Do not affect the serviceability of the building
      Can be easily repaired

If cracks are more significant, repair work may be repeated periodically.

Structural significance of the cracks tends to be exaggerated by a natural
reaction of the owner.

Many cracks do not have any effect on the stability of structure, because
of “high factor of safety”.

Cracking has adverse effects on appearance. Sometimes several cracks
go unnoticed for years which affect the performance of the structure.

Rain penetration through a crack depends on:

  1.   Construction of a building
  2.   Degree of exposure
  3.   Capillarity of fine cracks
  4.   Wind driven rain penetration through wide cracks
CAUSES OF CRACKS
    CAUSES OF CRACKS

   Structural cracks
       Due to incorrect design
       Faulty construction
       Overloading


   Non Structural cracks
       Moisture changes
       Thermal movement
       Elastic deformation
       Chemical reaction
       Foundation movement
       Settlement of soil
       Vegetation
             CHANGE IN MOISTURE
Most building materials expend when they absorb moisture from
atmosphere and shrink when they are dry. Cement made materials
shrink due to drying up of the moisture used in their construction.
The factors causing shrinkage in cement concrete and cement
mortar are 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.

       Quantity of Cement

   As a general rule, the richer the mix is, the greater the
   shrinkage/drying will be.

       Un-graded Aggregate

   Aggregate can cause shrinkage also. If un-graded and fine
   material/aggregate is used in cement concrete and cement mortar
   which requires more water and can cause greater shrinkage.



       Curing
   After laying cement concrete mix, the hardening of cement
   takes place, causes reduction in moisture and creates shrinkage.
   This causes cracks in concrete work.
    Excessive Fine materials
Fine materials take more surface area and require more water
for mix. The use of excessive fine materials i.e. silts; clay and
dust in aggregate create more shrinkage.

   Temperature:

Concrete made in hot weather needs more water for same
workability hence results in more shrinkage
         Thermal Movement
All materials expand on heat and contract on cool. Thermal
movement in components of structure creates cracks due to
tensile of shear stresses. It is one of the most potent causes of
cracking in buildings and needs attention.
              Elastic Deformation
When the walls are unevenly loaded, due to variation in stresses in
different parts of wall the cracks are formed in walls. When two
materials having wide different elastic properties are built together
under the effect of load, different shear stresses in these materials
create cracks at the junction. Dead and live loads cause elastic
deformation in structural components of a building.
                Chemical Reaction
Chemical reactions in building materials increase their volume and
internal stress causes cracks. The components of structure also
weaken due to chemical reactions. Some common instances of
chemical reactions are following.

   Sulphate attack on cement products
   Carbonation in cement based materials
   Corrosion of reinforcement in concrete
   Alkali aggregate reaction

    Certain chemical reaction in building materials result is
    appreciable change in volume of resulting products and internal
    stresses are set up which may result in outward thrust and
    formation of cracks.

    Soluble sulphate reacts with tricalcuium aluminates in cement
    and hydraulic lime and form products which occupy larger
    volume and ends in developing cracks. An example of cracking
    of a floor due to coming in contact of the sub base made of brick
    khoa with heavy sulphate content and water can be seen in fig
       Foundation Movement and
           Settlement of Soil

Shear cracks occur in buildings when there is large differential
settlement of foundation due to any of following causes.

   Unequal bearing pressure under different parts of the structure
   Bearing pressure being in excess of safe bearing strength of the
    soil
   Low factor of safety in the design of foundations
   Local variation in the nature of supporting soil

    Buildings on expansion clays are extremely crack prone. The soil
    movement in such clay is more appreciable upto a depth of 1.5
    to 2M and this cause swelling and shrinkage and results in crack
    in the structure. The cracks due to settlement are usually
    diagonal in shape. Crack appearing due to swelling is vertical.
 Movements of the ground

    Mining subsidence, land slips, earthquakes, moisture changes due to
    clay shrinkable soils (for example, Black cotton soil).

    Cracks occur because a part of the building is displaced from the
    rest without any change in the actual size of the material.

 Overloading

   Overloading of the ground
   Overloading of the building itself
   Overloading of the building parts result in cracks

    For example; Cracks under a floor due to overloading of slab.



 Overloading forced may be due to

1. External (excessive wind/snow loads)
2. Internal (from heavy machinery etc.)
                      Earth Quake
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. This results in cracks.



                       Vegetation
The roots of trees located in the vicinity of a wall can create cracks
in walls due to growth of roots under foundation. The cracks occur
in clay soil due to moisture contained by roots.
              Structural Cracks
Structural cracks mainly occur due to:

      a)     Defective design and defective load assumptions and
             perception of behavior of the structure.

      b)     In correct assessment of bearing capacity of
             foundation soil and soil properly.



      c)     Defective detailing of joints of components like roof
             with brick wall corner joints of walls

      d)     Defective detailing of structural detailing of steel
             reinforcement.



      e)     Lack of quality control during construction.
PREVENTIONS OF CRACKS IN BUILDING
PREVENTIONS OF CRACKS IN BUILDING
      CHANGE IN MOISTURE

1. Excessive Water

   Use minimum quantity of water required for mixing cement
   concrete or cements mortar according to water cement ratio.
   Never allow cement concrete work without mechanical mix and
   vibrator.

2. Quantity of Cement

   Do not use excessive cement in the mortar mix.

3. Un-graded Aggregate
Use largest possible aggregate and ensure good grading of
materials. The use of water according to required workability has
less shrinkage because of reduction in the porosity of hardened
concrete.

4.Curing
Proper curing should be started as soon as initial setting has taken
place and be continued for at least seven to ten days. When
hardening of concrete takes place under moist environment, the
shrinkage due to drying is comparatively less.

5.Excessive Fine materials

Do not use fine materials containing silt, clay and dust. Use coarse
sand/fine aggregate in cement concrete and cement mortar mix
which has silt and clay less than 4%. Use coarse aggregate and fine
aggregate after washing to reduce silt contents.
     Prevention of Thermal Cracks
To prevent thermal cracks expansion joints, control joints and joints
in case of change of shape and direction of wing in a structure are to
be provided
Prevention of Elastic Deformation

Create slip joints under the support of RCC slab on walls. Masonry
work on RCC slabs and beams should not be started before drying
RCC slab and beam. Provide horizontal movement joints between
the top of brick panel and RCC beam/slab.



 Prevention of Chemical Reaction

Use dense and good quality concrete i.e. richer mix of cement
concrete 1:1.5:3 to prevent cracks. Repair corrosive cement
concrete surface by ‘guniting’/ injecting technique after removing
all loose and damaged concrete and cleaning reinforcement from
all rust also.



Prevention of Foundation Movement
      and Settlement of Soil
The design of foundation must be based on sound engineering
principles and good practice.
       Prevention of Earth Quake
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.

         Prevention of Vegetation

Do not let trees grow too close to the buildings, compound walls
etc. Remove any saplings of trees as soon as possible if they start
growing in or near of walls etc.
REPAIR OF CRACKS
            REPAIR OF CRACKS
Cracks must be repaired immediately when observed otherwise
the life of structure will reduce causing structural failure. They not
only weaken the building but spoil the beauty of walls also. This
write up tells you about repair of cracks in plaster, RCC member,
Brick walls, Floor and Fine Cracks. The cracks can be prevented at
the time of construction.



                 1. Repair Cracks in Plaster

Two types of cracks are found on surface of plaster i.e.

A. Crack with hollow sound in plaster.

B. Crack with no hollow sound in plaster.

Hollow sound can be checked by striking the plaster.
A. Repair cracks giving hollow sound

  Remove plaster applying proper cut

Defective plaster should be removed and the edges should be
slightly cut in square or rectangular shape to provide a neat joint.



  Prepare the surface thoroughly

Clean the surface with wire brushes and wash it. Wet the surface
24 hours before application of cement mortar.



  Apply fresh cement mortar and finish the surface

Defective portion should be filled in with cement mortar. Keep the
surface wet at least for 3 days. When surface is dry, finish it
according to adjoining area on the wall.
B. Repair cracks if it is solid without hollow sound
Clean cracked portion of all loose materials and fill crack sealants in
cracked portion. Finally finish the surface according to adjoining
area.
         Repair Cracks on RCC Surface
  Clean cracked portion and apply guniting
The cracked portion should be widening gently and be cleaned of
all loose materials. Fill the cracked portion by pressure grouting or
‘grunting’. The material for such grouting can be epoxy or cement
(with suitable admixture).

   Apply epoxy and finish the surface
Epoxy has more strength and adhesion than cement and can go
into very thin cracks as fine as 0.1 mm. The surface should be
finished according to adjacent area on the wall.



    Repair Cracks on face of plaster
When two components having different elastic properties such as
RCC slab and brick work abating each other, the crack on face of
plaster is formed between these components due to their thermal
expansion.


  Remove plaster and old material
The plaster should be removed from cracked portion. The joints
should be widened at least 20 mm to 25 mm in thickness. Clean old
material by washing and brushing.

  Apply cut and prepare the surface
Apply slight cut on edges in straight line to provide neat joint.
Make the surface damp 24 hours before application of new plaster.
  Apply cement mortar
Cement mortar should be applied on defective portion. Make 20
mm wide and 25 mm thick groove at the junction of two
materials/components.

   Do curing and finishing
Keep the surface wet at least for 3 days. When surface is dry, finish
it according to adjacent area on wall.
    Repair Cracks in Masonry Walls
Two types of cracks are seen in masonry walls i.e.

A. Crack in mortar joints of walls.

B. Crack in bricks/Diagonal Cracks

A. Repair cracks in mortar joints of walls

  Remove mortar

The mortar should be removed from cracked joint with the help of
iron tools at least up to 25 mm depth. All loose material should be
cleaned from cracked portion.

  Wet the surface and apply cement mortar

Cracked portion must be kept moist at least for 24 hours before
doing repair. Fill in the joints with cement mortar of coarse sand in
1:4.

  Do curing and finishing

After filling the joints with cement mortar, keep the repaired
surface wet at least for 3 days. After drying the surface, finish the
repaired area according to the adjoining area.
B. Repair cracks in bricks/diagonal cracks

  Remove broken bricks

Before doing any repair of cracks, make sure that the cracks are
stabilized and are not widening further. Remove broken bricks by
scratching cement mortar from the joints gently.

  Prepare the surface

All loose material should be cleaned from the portion by brushing
and washing. Wet the surface 24 hours before application of new
plaster.

  Fix new bricks thoroughly

Lay fresh cement mortar with coarse sand in 1:4 and fix new bricks.
Fill all joints around bricks with mortar tightly with the help of iron
tools.

  Do curing and finishing

The repaired surface should be kept moist up to 7 days. When
surface is dry, it should be finished according to adjoining area.
           Repair Cracks in Floor
  Remove loose material and apply mortar
Create V shape groove in cracked portion and remove all loose
material by washing and brushing. Fill in the groove with polymer
modified mortar (for wide cracks) or with polymer modified
cement grout (for thin cracks).

   Remove cracked panel or material
If there are many cracks in the floor, remove cracked panels and
old materials.

  Prepare the surface and lay new panel
Clean the panel by washing and brushing. Prepare floor material
according to the existing floor and lay new panel.
    Repair Hair Cracks in plaster
  Broaden the crack gently
Widen the crack about ¼ inches with chisel and remove all loose
plaster from the cracked portion.

  Apply vinyl spackle compound
Use vinyl spackle compound to fill the cracked portion. Press the
vinyl spackle into the crack and let it dry. Sand or sponge the
surface smooth.

  Seal the patch and finish the surface
Seal the patch with proper priming coat and finish it with proper
shade according to the adjoining area.

  Repair Fine cracks

Apply surface water proof coating on the entire area having very
fine cracks (especially on roof) where filling /sealing of each
individual crack is not possible.
                          CONCLUSION
    Modern structures are comparatively tall and slender, have thin walls are
    designed for higher stresses and are built at a fast pace. These structures are
    therefore, more prone to cracks as compared with old structures, which used
    to be low, had thick walls, were lightly stressed and were built at a slow pace.
    Moreover moisture can easily reach the inside of the modern buildings due to
    the usage of thin walls. Thus measures for control of cracks in buildings
    assume much greater importance than ever before.

    Cracks result in applied forces greater than those which the building or its part
    can withstand. These forces may have emerged externally to the building or
    internally within the building or have been developed in the materials of the
    building as a result of Chemical changes. There may be a single force or a
    combination of forces having a single cause or several causes.

    Most cracks occur due to drying of construction water.

   These are generally superficial
   Do not affect the serviceability of the building
   Can be easily repaired

      If cracks are more significant, repair work may be repeated periodically.
      Structural significance of the cracks tends to be exaggerated by a natural
       reaction of the owner.
      Many cracks do not have any effect on the stability of structure, because
       of “high factor of safety”.
      Cracking has adverse effects on appearance. Sometimes several cracks go
       unnoticed for years which affect the performance of the structure.

    Cracks must be repaired immediately when observed otherwise the life of
    structure will reduce causing structural failure. They not only weaken the
    building but spoil the beauty of walls also. This write up tells you about repair
    of cracks in plaster, RCC member, Brick walls, Floor and Fine Cracks.

    The cracks can be prevented at the time of construction

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:979
posted:12/9/2011
language:Latin
pages:33