Aggregates Week 5 General Occupy 70-75% of the total volume

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Aggregates

• Week 5

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General

• Occupy 70-75 of the total volume of concrete
• Defined as an inert mineral filler for OPC BS
  882

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Sizes

• Coarse aggregate e.g. Gravel 4.76mm or more
  (5mm)
• Fine aggregate e.g. sand less than 4.76mm
• Silt varies from 0.02mm 0.06mm
• Clay much finer than 0.02mm

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Classification

• A) Petrological BS 812
• B) Density
• C) Shape texture

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A) Petrological

• 1.) Igneous - Granites, basalts dolerites,
  gabbros porphyries Hard, tough, dense
  Excellent aggregates
• 2.) Sedimentary - Sandstones - When hard dense
  suitable - Siliceous better than calcareous which
  are liable to acid attack
• Limestones - Sedimentary rocks chiefly
  composed of calcium carbonate Harder, denser
  types are suitable

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Contd

• Shales - Poor aggregates weak, soft, laminated
  absorptive
• 3.) Metamorphic - Variable character marbles
  quartzites usually massive, dense adequately
  tough strong
• Schists slates are often thinly laminated
  therefore unsuitable

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B) Density

• 1.) Heavy SGgt4.0
• i) Magnetic, natural iron ore (Fe3O4)
• ii) Barytes (BaSO4)

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Contd

• 2.) Normal SG 2.5-3.0
• i) Sands gravels division is arbitrarily on
  size sands pass 4.76 mm BS sieve
• ii) Granites basalts hard tough
• iii) Sandstone hard dense types
• iv) Limestone hard dense types
• v) Broken bricks must be free of plaster low
  sulphur content
• vi) Air cooled blast furnace slag

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Contd

• 3.) Lightweight - SGlt2.5
• i) Sintered PFA
• ii) Foamed slag
• iii) Vermiculite
• iv) Expanded polystyrene
• v) Pumice
• vi) Wood waste

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C) Shape Texture

• Particle shape classified to BS 812 1975
• Terms used
• Well rounded, rounded, sub-rounded, sub-angular,
  angular

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Contd

• Surface texture classified to BS 812 1975
• Relates to the degree of polish or dullness,
  smoothness or roughness of particles surfaces
• Depends on hardness, grain size and pore
  character
• Note BS 812 BS 882 have been replaced by BS
  EN 12620

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Bond of Aggregate

• Partly due to interlocking
• Partly due to physical chemical characteristics
  of the aggregate
• Other factors include the cement paste

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Aggregate Properties

• Ideally should be chemically inert, durable, hard
  tough
• Should be capable of being compacted to the
  appropriate density provide good bonding with
  the cement paste

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A) Chemical Properties

• Soluble salts sulphates - cause problems with
  hydrated cement or chloride which accelerate
  hydration increase risk of corrosion of steel
  reinforcement.
• Reactive aggregates ASR concrete cancer
  not common now but has been a problem in the past

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Alkali-Silica Reaction
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Contd

• Clay silt defined as materials passing a 75
  micron sieve harmful to concrete in substantial
  amounts increase specific surface hence water
  requirements
• Weak (unsound particles) lower the strength of
  the concrete

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B) Physical Properties

• Strength
• Porosity
• Thermal
• Bonding
• Moisture
• Density - relative (SSD) solid (x1000)
• - bulk (packing air)

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Calculation of Relative Density

• Relative Density (SSD)
• mass of sample of SSD aggregate
• Volume of water displaced SSD sample x 1000
• Where, SSD saturated surface dry all
  accessible pores are full of water, but the
  aggregate surface is dry
  

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Bulk Density

• Stockpiles of loosely packed aggregates contain
  large volumes of air trapped between particles,
  usually many times that of the volume of air
  present within the particles Bulking
• Coarse aggregates 30-50 of total space
  occupied
• Fine aggregate approx 20 - varies with
  moisture content

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Batching of Aggregates

• Bulking of aggregates produce uncertainty in the
  solid content of aggregates batched by volume
• Batching by weight (mass) is therefore preferred,
  hence most concrete mixes are batched by weight

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Aggregate Durability

• Soundness degradation through interstitial
  crystallisation
• Organic matter act as retarders
• Mica Muscovite or Biotite
• Chalk affects strength (freeze/thaw)
• Shell strength

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Contd

• Sulphates limited to 4 overall
• Chlorides corrosion (lt0.06)
• Metallic Pyrites Lead, Zinc, Iron impurities

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Sand Gravel

• Sources pits or dredging from river or sea bed
  screening washing
• Marine aggregates
• 1985 - gt20M Tonnes/year
• 1995 - gt300M Tonnes /year
• Goodwin Sands (off Kent coast) 8000 Tonnes/day
  use for Channel Tunnel concrete Suction hopper
  dredgers with leading trailing pipes

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Ship Dredging

• Up to1968 800 tonnes/day
• 1970s 2500 10000 Tonnes/day

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Problems with Marine Aggregates

• Liable to contain impurities of shell salt
• Southern, North Sea English Channel granites,
  sandstones or limestones generally
  unconsolidated sedimentary deposits
• Up to 1968 50 of aggregates came from sand
  gravel deposits
• Emphasis has now shifted to hard rock sources

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Grading of Aggregates

• Refers to the size distribution of the
  aggregate
• Sieve analysis BS 410 1976
• Grading limits BS 882 1983
• Note The Euro-standard BS EN 12620
• now covers testing specification for
• aggregates

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Graded Aggregate

• Uniform Graded equal weights of each particle
  size present
• Gap graded several consecutive particle sizes
  missing
• Single graded consist predominantly of a single
  particle size

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Fine Coarse Aggregates

• Fine Aggregate - lt4.76mm (5mm)
• Coarse Aggregate - gt4.76mm
• Grading
• Fine Coarse
• C-Coarse 40, 20, 14mm down
• M-Medium
• F-Fine

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Aggregate Uses

• Concrete
• Mortar
• Renders
• Screeds
• Unbound pavements

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Contd

• Bituminous Materials
• Railway ballast
• Filter media - biological