Construction materials: Concrete

Constituents

  • fine aggregate (sand) <5mm
  • coarse aggregate (stone) >5mm
  • cement
  • water

cement              fine aggregate

    +            +               +

water.              coarse aggregate

- By volume:

  • aggregates 60-75%
    • fine is 0.3-0.8* volume of coarse
  • paste 25-40%
    • water is 0.5* volume of cement
  • Air 1-2%
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Portland cements

  • basic ingredient in cement
  • concrete form when water is added to the portland cement
  • common way to maufacture portland cement:
    • dry method
    • 1st: quarry principal raw materials (limestone, clay, etc)
    • 2nd: crush the rock in several stages
    • 3rd: combine with iron ore and fly ash then ground, mix and feed into cement kiln
    • 4th: heat kiln to 2700 degrees F driving off unnecessary materials as gases forming clinker
    • 5th: clinker is cooled, ground and mixed with small amounts of gypsum and limestone
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Chemical admixtures

  • DEFINITION: natural or manufactured chemicals added to concrete used to give concrete special properties
  • Common types include:
    • air entraining: produces microscopic air bubbles to improve resistance to damage caused by freezing and thawing and the effects of the chemicals in de-icer
    • water reducers: lower water content in plastic concrete and increase strength; obtain higher slump without adding water
    • retarders: delay initial setting time of concrete often in hot weather
    • accelerators: reduce initial setting time of concrete and give it a higher early strength
    • high range water reducers: superplacticisers that reduce water content by 12-25%; used to increase strength and reduce permeability by reducing water content
    • corrosion inhibitors: prevent corrosion of reinforcement steel
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Aggregates

  • inert granular materials
  • need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay
  • 2 categories:
    • fine: sand or crushed stone
    • coarse: mostly gravel qith some crushed stone
  • gravel and sand are dug from a pit, river, lake or seabed
  • crushed aggregte is from crushed quarry rock, boulders, cobbles, large gravel etc
  • after harvesting, it is crushed, screened and washed
  • selection of aggreagtes is important and certain characteristics are expected
    • grading
    • durability
    • particle shape and surface texture
    • abrasion and skid resistance
    • unit weights and voids
    • absorption and surface moisture
  • particle shape and surface texture is more influential in fresh concrete than hardened concrete: smoother, rounded compact aggregate requires less water
  • abrasion and skid resistance of aggregate is important if the concrete will be used in heavy duty floors or pavements
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Concrete mix design

  • the process consists of selecting the correct proportions of cement, fine and coarse aggregates and water to produce concrete that meets the specified requirements:
    • workability/ consistence of fresh concrete
    • compressive strength at a specified age
    • durability
      • by specifying minimum cement content or max water/ cement ratio
  • the process of arriving at the right combination of cement, aggregates and admixturs is not easy because it involves the art of balancing various conflicting requirements
  • steps involved in concrete mix design
    • decide on mean strength required
    • water/cement ratio obtained from chart
    • workability is chosen from tables- gives water content
    • cement/ aggregate ration togive the required workabilty is chosen from the tables
    • allowance for free water
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Quality control

  • quality control takes place on the site but before the placement
  • slump is a measure of shear resistance of fresh concrete flowing under its own weight. 
    • a slump cone is used and the hight the concrete has sunk once the cone has been removed is measured.
    • the optimum range is between 25-100mm
  • Vebe time is another measurement often used for stiff mixes and it measures mobility.
    • a cone is firstly slumped
    • then the concrete is vibrated flat with the time measured
    • the optimum range is between 15- 30 seconds
  • neither of these tests measure stability
    • there are major criticisms involved
    • they are both single point tests
    • the results depend upon the apparatus as well as the operator
    • neither of the tests can cover all of the workability ranges
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Properties of fresh concrete

  • fresh concrete should be
    • easily mixed
    • easily transported and placed
    • easily compacted and finished
  • many things affect the workability of fresh concrete:
    • time: slump decreases the longer concrete is left due to setting
    • water content: internal shear resistance decreases as water/ concrete ratio increases; and the maximum cohesion occurs when water/ concrete ratio is 0.5
    • aggregate: shape, quantity and grading will influence mobility; optimum mobility occurs at ~ 45:55 coarse:fine aggregate
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Properties of fresh concrete 2

  • bleeding of concrete: 
    • upward migration of water resulting in a layer of water on the concrete surface
    • water gets trapped under larger aggregate particles leading to a weaking of the bond and under reinforcement which increases risk of corrosion
    • can be minimised by increasing amount of fine aggragate, minimising the amount of water or/ and adding admixtures
  • plastic settlement cracking:
    • a direct consequence of bleeding
    • any local restraints cause the concrete to crack usually forming above the steel which can lead to problems with erosion
    • can be minimised by reducing bleeding and re-vibrating the surface after cracks start to appear while the concrete is still plastic
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Properties of fresh concrete 3

  • plastic shrinkage
    • caused by the evaporation of water from the concrete surface
    • the surface tried to shrink as water evaporates but is restrained by the mass of the concrete
    • more likely to occur where there is a large surface area compared to volume eg paving slabs
    • can be minimised by reducing the rate of water evaporation which is done by protecting the concrete from drying out as soon as is practical
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Curing process

  • There are various methods to assist curing
    • covering surfaces with water and/ or polythene sheets
    • spraying the surface with a curing membrane
    • protecting the surface from wind and sun with windbreakers and sun shields
  • the temperature of concrete rises as time after casting increases
  • the higher the cement content in the concrete the higher the temperature
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Properties of hardened concrete

Main concerns

  • shrinkage
    • water evaporation leads to contraction
    • this leads to cracking and problems with claddings
    • can be minimised by reducing water content of the mix and by good curing
  • creep
    • inelastic flow under a sustained load
    • the amount of creep is influenced by the maturity of concrete as well as the applied load/ strength ratio
    • increases deflections and shortens columns with time 
  • durability
    • resistance to abrasion
    • resistant to weathering and freeze thaw
    • aggressive chemicals
    • permeability
    • corrosion of reinforcement
      • breakdown of passive film on rebar or neutralisation by reacting with CO2
    • discoloration
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Properties of hardened concrete 2

  • to achieve good durability
    • good workmanship
    • low water/cement
    • good detailing
    • careful selection of materials
  • factors affecting strength
    • water/cement ratio
    • cement type
    • aggregate properties
    • age
    • curing conditions
  • as water cement ratio increases compressive strength decreases
  • as age of concrete increases compressive strength increases
  • moist-cured concrete has a high compressive strength which decreases as time in air during curing is increased
  • measure compressive strength using either a cube or cylinder
  • strength of cylinder = 0.8 x cube
  • strength increases as specimen size decreases
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properties of hardened concrete 3

  • measuring tensile strength directly via pull tests gives practical difficulties
  • measured indirectly using cylinder splitting test
    • used for quality control where tensile strength is important
    • fs=2P/(pi)DL
  • flexural strength 
    • fb=PL/bd^2
  • in-situ strength
    • likely to be less than standard cube/cyl strength because of differences in compaction and curing
    • structural member is under stress
    • 67% is allowed for by factors of safety
    • within-member variations: usually top is weaker than bottom by up to 50%
    • cut and test cores or us callibrated non-destructive tests
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Properties of hardened concrete 4-standard tests

ensures that the mix was properly designed and proportioned

shows statistical variablitily in the properties of the concrete

reveals problems arising due to changes in materials or conditions

ensure efficient quality control sytems

identify any potential problems with the material

results may be needed before further construction operations can be carried out

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Cement replacement materials

  • pulverised fuel ash:
    • produced in power stations that burn coal.
    • ash collected after extraction by flue gases and cooled
    • spherical shape allows reduction in amount of water needed to achieve workability
    • these blends can lower temperature rise in structural members
    • lower early age strength but long term strength is the same 
    • improves overal durability of the concrete mix
  • ground granulated blast furnace ****:
    • produced from extraction of iron from it ores
    • properties influenced by method of quenching after leaving blast furnace
    • effects on concrete properties are similar to those imparted by PFA
  • condensed silica fume;
    • prodcued in manufacture of silicon/ its alloys from burning quartz in electric arc furnaces
    • leads to increase in water content 
    • similar strengths obtained at 28 days compared to standard concrete
    • improves durability
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