High strength concrete

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HIGH STRENGTH CONCRETE(HSC)
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INTRODUCTION

• High-strength concrete (HSC) provides a high
  level of structural performance, especially in
  strength and durability, compared to traditional,
  normal-strength concrete (NSC).
• Previously employed in bridges, offshore
  structures and infrastructure projects, HSC has
  seen increased use in high-rise buildings,
  especially for columns.
• The higher compressive strength of HSC allows
  for the use of smaller-diameter columns, which
  increases the amount of usable space in a
  building.

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• ACI defines a high-strength concrete as concrete
  that has a specified compressive strength for
  design of 6,000 psi (41 MPa) or greater.

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HSC Materials

• Aggregates
• 9.5 - 12.5 mm (3/8 - 1/2 in.) nominal maximum
  size gives optimum strength.
• Combining single sizes for required grading
  allows for closer control and reduced variability
  in concrete.
• For 70 MPa and greater, the FM of the sand should
  be 2.8 3.2. (lower may give lower strengths and
  sticky mixes).

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• Supplementary Cementing Materials
• Fly ash, silica fume, or slag often mandatory.
• Dosage rate 5 to 20 or higher by mass
  of cementing material

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Admixtures

• Use of water reducers, retarders, HRWRs, or
  superplasticizers mandatory in high-strength
  concrete.
• Air-entraining admixtures not necessary or
  desirable in protected high-strength concrete.
• Air is mandatory, where durability in a
  freeze-thaw environment is required (i.e..
  bridges, piers, parking structures).
• Recent studies
• w/cm 0.30air required.
• w/cm lt 0.25no air needed.

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MICROSTRUCTURE

• From the general principles behind the design of
  high-strength concrete mixtures, it is apparent
  that high strengths are made possible by reducing
  porosity, inhomogeneity, and microcracks in the
  hydrated cement paste and the transition zone.
• The utilization of fine pozzolanic materials in
  high-strength concrete leads to a reduction of
  the size of the crystalline compounds,
  particularly, calcium hydroxide.

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• Consequently, there is a reduction of the
  thickness of the interfacial transition zone in
  high-strength concrete.
• The densification of the interfacial transition
  zone allows for efficient load transfer between
  the cement mortar and the coarse aggregate,
  contributing to the strength of the concrete.
• For very high-strength concrete where the matrix
  is extremely dense, a weak aggregate may become
  the weak link in concrete strength.

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SPECIAL METHODS OF MAKING HSC

• Seeding
• Revibration
• High speed slurry mixing
• Use of admixtures
• Sulphur impregnation
• Inhibition of cracks.

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MERITS OF HSC

• Reduced weight.
• Small support elements(architectural
  considerations).
• More load carrying capacity.
• Reduces the total amount of material.
• Economical(reduces the overall cost of the
  structure).

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• High-strength concrete columns can hold more
  weight and therefore be made slimmer than regular
  strength concrete columns, which allows for more
  useable space, especially in the lower floors of
  buildings.

• High Strength Concrete are also used in other
  engineering strucures like bridges.

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DEMERITS OF HSC

• Lower shear strength.
• Lower modulus of elasticity.
• Increased quality control is needed.
• Careful material selection is necessary.
•  Since serviceabilty conditions such as
  deflection can control design, so increased
  capacity may not be fully utilized.

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REFERENCES

• P.K. Mehta and P.J.M. Monteiro,
• Concrete Microstructure, Properties, and
  Materials.
• A.M.NEVILLE J.J.BROOKS, Concrete
  Technology,Second Edition.
• M.S.SHETTY, Concrete Technology Theory and
  Practice.
• www.concretebasics.org/articlesinfo/hpchsc.php