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Concrete Making and Testing

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Concrete Materials Cement Portland, rapid-hardening, white, coloured, ... reduce need for extra water Waterproofing additives to seal microscopic pores ... – PowerPoint PPT presentation

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Title: Concrete Making and Testing


1
Concrete Making and Testing
  • Unique material
  • Made specially for each job
  • Handling on job affects quality

1/19
2
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, ...

2/19
3
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

3/19
4
Weighing and Mixing
  • Weigh-batching necessary for accuracy
  • Mixed in factory and transported in agitator
    truck
  • Site mixing uneconomical, not accurate enough

4/19
5
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

5/19
6
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

6/19
7
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

7/19
8
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

8/19
9
Curing (cont.2)
will get cracking otherwise
9/19
10
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
10/19
11
Strength
  • Water/cement ratio affects strength
  • More water less strength
  • Try to limit water content
  • But it must be workable

11/19
12
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

12/19
13
Measuring Workability
  • The slump test
  • Not perfect, but simple and quick
  • Good guide to uniformity between batches

13/19
14
Economy
  • Cement is dearer than aggregate
  • Try to limit cement content
  • cement responsible for strength
  • but also for most of cost
  • and the shrinkage

14/19
15
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

15/19
16
Conflicting Requirements
  • The various requirements above are
  • in conflict

16/19
17
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

17/19
18
Testing Cylinders
  • Cylinders crushed in lab at 28 days
  • Some early tests at 7 days
  • Drastic consequences if under-strength after 28
    days

18/19
19
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

19/19
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