REPAIRS AND REHABILITATION OF STRUCTURES

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REPAIRS AND REHABILITATION OF STRUCTURES UNIT
III MATERIALS AND TECHNIQUES FOR REPAIR
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SPECIAL TYPES OF CONCRETE -with
out-of-the-ordinary properties or those produced
by unusual techniques.
Light Transparent Concrete
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STRUCTURAL LIGHTWEIGHT CONCRETE Structural
lightweight concrete is similar to normal- weight
concrete except that -it has a lower
density. -it is made with lightweight
aggregates or -it is made with a combination
of lightweight and normal- weight aggregates.
Lightweight concrete
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The term "sand lightweight - made with coarse
lightweight aggregate and natural sand.
Structural lightweight concrete has -air-dry
density in the range of 1350 to 1850
kg/m3 -28-day compressive strength in excess of
17 MPa
sand sandwich board light weight concrete
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-for comparison, normal-weight concrete
containing regular sand, gravel, or crushed stone
has a dry density in the range of 2080 to 2480
kg/m3 Structural lightweight concrete is used
primarily to reduce the dead-load weight in
concrete members, such as floors in high-rise
buildings.
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• Structural Lightweight Aggregates
• -usually classified according to their
  production process because various processes
  produce aggregates with somewhat different
  properties, which includes
• Rotary kiln expanded clays, shales, and slates

Expanded clay
Expanded slate
Chipped Shale
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• Sintering grate expanded shale and slates
• Pelletized or extruded fly ash
• Expanded slag

Fly ash Pellet
Expanded slag
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• Structural lightweight aggregates can also be
  produced by processing other types of material,
  such as naturally occurring pumice and scoria.

Pumice
scoria
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• Structural lightweight aggregates have densities
  significantly lower than normal- weight
  aggregates, ranging from 560 to 1120 kg/m3,
  compared to 1200 to 1760 kg/m3 for normal-weight
  aggregates.
• These aggregates may absorb 5 to 20 water by
  weight of dry material.
• To control the uniformity of structural
  lightweight concrete mixtures, the aggregates are
  pre wetted (but not saturated) prior to batching.

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HIGH-DENSITY CONCRETE High-density (heavyweight)
concrete has a density of up to about 6400 kg/m3.
Heavyweight concrete used for radiation
shielding As a shielding material, heavyweight
concrete protects against the harmful effects of
X-rays, gamma rays, and neutron radiation.
Heavyweight concrete to protect shield places
with greater risk of radiation.
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Selection of concrete for radiation shielding is
based on space requirements and on the type and
intensity of radiation. Where space requirements
are not important, normal-weight concrete will
generally produce the most economical shield
Where space is limited, heavyweight concrete
will allow for reductions in shield thickness
without sacrificing shielding effectiveness.
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• Properties of High-Density Concrete
• Both the freshly mixed and hardened states can
  meet job conditions and shielding requirements by
  proper selection of materials and mixture
  proportions.
• Except density, the physical properties are
  similar to normal-weight concrete.
• Strength is a function of water-cement ratio
  thus, for any particular set of materials,
  strengths comparable to those of normal-weight
  concretes can be achieved.
• radiation shield has special requirements, trial
  mixtures should be made with job materials and
  under job conditions to determine suitable
  mixture proportions.

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EXPANSIVE CEMENT Concrete made with ordinary
Portland cement shrinks due to loss of free
water. Concrete also shrinks continuously for
long time. This is known as drying shrinkage.
Cement used for -grouting anchor bolts or
-grouting machine foundations or
-grouting the prestress concrete ducts, if
shrinks, There has been a search for such type
of cement which will not shrink while hardening
and thereafter. As a matter of fact, a slight
expansion with time will prove to be advantageous
for grouting purpose. This type of cement which
suffers no overall change in volume on drying is
known as expansive cement.
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Cement of this type has been developed by using
an expanding agent and a stabilizer very
carefully. Proper material and controlled
proportioning are necessary in order to obtain
the desired expansion. One type of expansive
cement is known as shrinkage compensating cement.
This cement when used in concrete, with
restrained expansion, induces compressive
stresses which approximately offset the tensile
stress induced by shrinkage. Another similar
type of cement is known as self-stressing cement.
This cement when used in concrete induces
significant compressive stresses after the drying
shrinkage has occurred. Fibre content 0.5 to
2.5 by volume of mix
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• POLYMER CONCRETE
• Continuous research by technologists to
  understand, improve and develop the properties of
  concrete has resulted in a new type of concrete
  known as polymer concrete.
• This concrete is porous in nature and this
  porosity due to air-voids, water voids or due to
  the inherent porosity of gel structure itself.
• The development of concrete-polymer composite
  material is directed at producing a new material
  by combining the ancient technology of cement
  concrete with the modern technology of polymer
  chemistry.

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• TYPES OF POLYMER CONCRETE
• Four types of polymer concrete materials are
  being developed,
• Polymer Impregnated Concrete(PIC)
• Polymer Cement Concrete(PCC)
• Polymer Concrete(PC)
• Partially Impregnated and Surface coated polymer
  concrete

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• POLYMER IMPREGNATED CONCRETE (PIC)
• The monomers used in this type are,
• Methylmethacrylate(MMA)
• Styrene
• Acrylonitrile
• t-butyl styrene
• Other thermoplastic monomers
• POLYMER CEMENT CONCRETE (PCC)
• The monomers that are used in PCC are,
• Polyster-styrene
• Epoxy-styrene
• Furans
• Vinylidene chloride

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• APPLICATIONS OF POLYMER IMPREGNATED CONCRETE
• The following are the application of the PIC,
• Prefabricated structural elements
• Prestressed concrete
• Marine works
• Desalination plants
• Nuclear power plants  
• Sewage works-pipe and disposal works  
• Ferrocement products  
• For water proofing of structure  
• Industrial applications

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• SULPHUR-INFILTERATED CONCRETE
• New types of composites have been produced by the
  recently developed techniques of impregnating
  porous materials like concrete with sulphur.
  Sulphur impregnation has shown great improvement
  in strength.
• In the past, some attempts have been made to use
  sulphur as a binding material instead of cement.
• Sulphur is heated to bring it into molten
  condition to which cores and fine aggregates are
  pored and mixed together.
• On cooling, this mixture gave fairly good
  strength, exhibited acid resistance and also
  other chemical resistance, but it proved to be
  costlier than ordinary cement concrete.
• Recently, use of sulphur was made to impregnate
  lean porous concrete to improve its strength and
  other useful properties considerably.
• In this method, the quantity of sulphur used is
  also comparatively less and thereby the processes
  is made economical.
• It is reported that compressive strength of about
  100MPa could be achieved in about two days time.

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• Two procedures are adapted A after 24hours of
  moist curing, the specimen is dried in heating
  cabinet for 24hours at 1210C.
• Then the dried specimens are placed in a
  container of molten sulphur at 1210C for 3 hours.
  
• Specimens are removed from the container, wiped
  clean of sulphur and cooled to room temperature
  for 1hour and weighed to determine the weight of
  sulphur in filtrated concrete.
• In procedure B, the dried concrete specimen is
  placed in an airtight container and subjected to
  vacuum pressure of 2mm mercury for 2hours.
• After removing the vacuum, the specimens are
  soaked in the molten sulphur at atmospheric
  pressure for another half an hour.
• The specimen is taken out, wiped clean
  and cooled to room
• temperature in about 1hour.The specimen
  is weighed and the weight of
  sulphur-impregnated concrete is determined.
•  

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• FERROCEMENT CONCRETE
• Ferro-cement technique though of recent origin,
  have been extensively used in many countries,
  notably in U.K., New Zealand and China.
• There is a growing awareness of the advantages of
  this technique of construction all over the
  world.
• It is well known that conventional reinforced
  concrete members are too heavy, brittle cannot be
  satisfactorily repaired if damaged, develop
  cracks and reinforcements are
• liable to be corroded.
• Ferrocement is a relatively new material
  consisting of wire meshes and cement mortar. It
  consists of closely spaced wire meshes which are
  impregnated with rich cement
• mortar mix.
• The wire mesh is usually of 0.5 to 1.0mm dia wire
  at 10mm spacing and cement mortar is of sand
  ratio of 12 or 13 with water /cement ratio of
  0.4 to 0.45.

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• The ferrocement elements are usually of the order
  of 2 to 3cm in thickness with 2 to 3mm external
  cover to the reinforcement.
• The steel content varies between 300kg to 500kg
  per cubic meter of mortar.
• The main advantages are simplicity of its
  construction, lesser dead weight of the elements
  due to their small thickness, its high tensile
  strength, less crack widths compared to
  conventional concrete, easy reparability, non
  corrosive nature and easier mould ability to any
  required shape.
• There also saving in basic materials namely
  cement and steel. This material is more suitable
  to special structures like shells which have
  strength through forms and structures like
  roofs, silos, water tanks and pipelines.
• The development of ferrocement depends on
  suitable casting techniques for the required
  shape. Development of proper prefabrication
  techniques for ferrocement is still not a widely
  explored area and gap needs to be filled.

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• FIBRE REINFORCED CONRETE
• Plain concrete possess a very low tensile
  strength, limited ductility and little resistance
  to cracking.
• Internal micro cracks are inherently present in
  the concrete and its poor tensile strength due to
  the propagation of such micro cracks, eventually
  leading to brittle fracture of the concrete.
• In plain concrete and similar brittle materials,
  structural cracks(micro-cracks) develop even
  before loading, particularly due to drying
  shrinkage or other causes of volume change.
• When loaded, the micro cracks propagate and open
  up, and owing to the effect of stress
  concentration, additional cracks form in places
  of minor defects.
• The development of such microcracks is the main
  cause of inelastic deformations in concrete.

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• It has been recognized that the addition of
  small, closely spaced and uniformly dispersed
  fibres to concrete would act as crack arrester
  and would substantially improve its static and
  dynamic properties. This type of concrete is
  known as Fibre Reinforced Concrete.
• Fibre Reinforced Concrete can be defined as a
  composite material consisting of mixture of
  cement, mortar or concrete and discontinuous,
  discrete, uniformly dispersed suitable fibres.
• Continuous meshes, woven fabrics and long wires
  or rods are not considered to be discrete fibres

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• FIBRES USED
• Many types of fibres used in cement and concrete,
  not all of them can be effectively and
  economically used. Each type of fibre has its
  characteristics properties and limitations.
• Some of the fibres that could be used are steel
  fibres, polypropylene, nylons, asbestos, coir,
  glass and carbon.
• The fibre is often described by a convenient
  parameter called Aspect ratio. The aspect ratio
  of the fibre is the ratio of its length to its
  diameter. It ranges from 30 to 150.  

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• Steel fibres are most commonly used and the
  diameter may vary from 0.25 to 0.75mm.  
• Glass fibre is a recent introduction in making
  fibre concrete. It has tensile strength of about
  1020 to 4080N/mm²  
• Polypropylene and nylon fibres are found to be
  suitable to increase the impact strength. They
  posses high tensile strength, but their low
  modulus of elasticity and higher elongation do
  not contribute to the flexural strength.  
• Asbestos is a mineral fibre and has proved to be
  most successful of all fibres as it can be mixed
  with Portland Cement. Tensile strength would be
  from 560 to 980N/mm².  
• Carbon fibres perhaps posses very high tensile
  strength 2110 to 2815N/mm².