Concrete Making and Testing

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Concrete Making and Testing

• Unique material
• Made specially for each job
• Handling on job affects quality

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Concrete Materials

• Cement
• Portland, rapid-hardening, white, coloured, ...
• reacts with water to form a gel - curing process
  important
• Coarse aggregate
• provides bulk
• grading and shape essential
• must be at least as strong as final concrete
• Fine aggregate
• fills spaces between
• Water
• cheapest but most critical element
• Additives
• plasticizers, accelerators retarders,
  waterproofing, ...

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Proportions

• Cement paste coats all surfaces of aggregate
• Fine fills the spaces between coarse aggregate

• Detailed grading by supplier of readymixed
  concrete
• ratio cement to total aggregate 14 to 17 by
  weight

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Weighing and Mixing

• Weigh-batching necessary for accuracy
• Mixed in factory and transported in agitator
  truck
• Site mixing uneconomical, not accurate enough

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Handling on Site

• Chute and/or barrow
• Concrete pump
• most common
• Vibrators
• to eliminate air pockets
• ensure compaction around reinforcement
• into corners
• immersion vibrators most common
• Trowelling
• shovel, trowelling machine, wood float or steel

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Stages of Setting

• Initial set
• stiffening of the concrete
• begins within about 2 hours of adding water
• Hardening
• main gaining of strength
• main gain in first week
• most of strength after 28 days - but continues
• Ageing
• slowly for years

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Curing

• Procedure for retaining moisture in concrete for
  several days
• Prolongs the chemical reaction of hydration
• Will improve compressive strength
• Reduces drying shrinkage cracking
• Improves protection of reinforcement

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Curing (cont.1)

• Keep moisture available throughout the setting
  process - for at least 3 days
• must be continuous
• Must start no longer than 3 hours after placing
• Fast and slow setting cements
• retardants if want special finishes
• Temperature affects rate
• heat speeds up process

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Curing (cont.2)
will get cracking otherwise
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Curing Methods

• Curing compounds
• applied by spray or roller
• Plastic sheeting to prevent evaporation
• Formwork for edge beams and face panels
• left in place
• Ponding of water where practicable

intermittent wetting down morning and night IS
NOT CURING
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Strength

• Water/cement ratio affects strength
• More water less strength
• Try to limit water content
• But it must be workable

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Workability

• Concrete must be fully compacted to remove air
  bubbles
• Water assists compaction
• Try to have it wet enough
• dry mix too difficult to compact
• wet mix too sloppy - weak
• Ideal combination requires experience
• Use vibration to improve workability
• Use additives to improve workability

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Measuring Workability

• The slump test
• Not perfect, but simple and quick
• Good guide to uniformity between batches

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Economy

• Cement is dearer than aggregate
• Try to limit cement content
• cement responsible for strength
• but also for most of cost
• and the shrinkage

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Shrinkage

• Concrete shrinks on setting
• loss of volume when water and cement react
  chemically
• loss of excess water
• Starts off rapidly - continues for years
• Cement paste shrinks, aggregate doesnt
• Try to limit water content
• Good curing delays shrinkage
• Reinforcement helps limit cracks

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Conflicting Requirements

• The various requirements above are
• in conflict

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Specifying and Measuring Strength

• The cylinder crushing strength (in MPa)
• Usually measured at 28 days
• Test cylinders cast on site
• Cured in lab before testing

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Testing Cylinders

• Cylinders crushed in lab at 28 days
• Some early tests at 7 days
• Drastic consequences if under-strength after 28
  days

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Testing Other Materials

• Quality control of other materials is usually
  done by manufacturers
• Homogeneous materials like steel are made to
  close tolerances of strength
• Natural materials like timber vary greatly, and
  are classified into several grades

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