Concrete Technology

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Concrete Technology

• Reference
• Guide to concrete constructionSAA HB64 1994
• Australian Concrete Technology
• W G Ryan A Samarin

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What is concrete?

• Cement
• Water
• Aggregates Fine (0 - 5 mm)
• Coarse (gt 5mm)
• (Admixtures)
• Concrete should be
• cohesive to be transported and placed without
  segregation
• able to be compacted by means available
• satisfactory compressive strength when hardended
• Properties related to compressive strength
• Density
• Permeability
• Durability
• Resistance to abrasion
• Resistance to impact
• Resistance to sulphates

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Portland Cement

• Raw materials
• limestone
• silica, alumina in clay or shale
• ironstone

1400oC
Clinker
Grind with gypsum
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Main compounds of Portland cement

• Tricalcium silicate 3CaO.SiO2 C3S
• Dicalcium silicate 2CaO.SiO2 C2S
• Tricalcium aluminate 3CaO.Al2O3 C3A
• Tetracalcium aluminoferrite 4CaO.Al2O3.Fe2O3 C4AF
  
• C3S and C2S
• are responsible for the strength of hydrated
  cement paste
• C3A is undesirable
• is present in small quantities. Reacts very
  rapidly with water (flash set)
• gypsum retards this
• provides early strength
• prone to sulphate attack
• C4AF
• dark in color with little cementing value
• forms due to iron oxide, a useful flux during the
  burning process.

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Properties of the major constituents
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Types of cement

• General purpose Portland cement (GP)
• General purpose blended cement (GB)
• GP slag / flyash / silica fume
• High early strength cement (HE)
• early stripping of formwork, very cold weather,
  repairs
• Low heat cement (LH)
• mass concrete, hot weather
• Sulphate resisting cement (SR)
• marine construction, sulphate bearing soils
• Off white / white Portland cement
• Coloured cements
• Masonry cement
• unsuitable for concrete
• High alumina cement (HAC)
• high early strength, resistance to very high
  temperatures and sulphates
• loss of strength in warm, humid conditions

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Pozzolanic materials

• Little or no cementing properties on their own.
• Reacts with lime in the presence of water to form
  cementitious compounds.
• (a substantial quantity of lime is formed when
  cement reacts with water)
• Advantages
• cheaper that Portland cement
• slows hydration and heat evolution
• mass concrete
• improves workability
• reduces segregation and bleeding
• Pulverised fuel (fly) ash
• coal fired power stations
• reduces permeability substantially
• resistant to sulphate and chloride attack
• marine structures

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Pozzolanic materials (cont)

• Granulated blast furnace slag
• iron manufacture
• reduced strength and strength development
• resistant to chemical attack
• Silica fume
• by product of silicon manufacture
• superfine material - has to be well dispersed to
  be effective
• used with a superplasticizer
• reduction in permeability
• increase in durability
• low electrical conductivity
• greater resistance to steel corrosion
• almost complete suppression of bleeding
• high strength concrete (gt100MPa)

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Concrete aggregates

• Particle shape texture
• smooth rounded particles give maximum workability
• rough, cubital particles give optimum strength

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Concrete Aggregates

• Aggregate properties determine the water
  requirement in concrete
• Strength of concrete is inversely proportional to
  the water cement ratio
• Workability Is mainly influenced by the water
  content
• Nominal aggregate size
• larger aggregates lowers cement paste requirement
• very large aggregates lower bond area
• in RC aggregate size is controlled by the size,
  shape of member bar spacing

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Concrete aggregates

• Natural sand / gravel
• stream beds, dunes, alluvial deposits, marine
  deposits
• Crushed rock
• igneous, sedimentary, metamorphic
• Manufactured
• expanded clay shale, blast furnace slag, fly
  ash
• Sources in Adelaide
• Coarse Aggregates Fine Aggregates
• quartzite river sand
• dolomite pit sand

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Concrete aggregates

• Strength
• crushing strength of aggregates is not easy to
  determine. It is measured indirectly by
• Aggregate crushing value
• 10 fines value
• Hardness (resistance to wear)
• important in roads and surfaces subject to heavy
  traffic
• The Los Angeles value
• Specific Gravity (Particle Density)
• lt 2100 kg/m3 - light weight aggregate
• vermiculite, scoria, pumice, expanded clay and
  shales
• Normal weight concrete
• gt 3500 kg/m3 - heavy weight aggregate
• magnetite, steel punchings
• Density

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Concrete aggregates

• Particle size distribution
• Single sized
• Graded
• gt5mm - coarse aggregate
• lt5mm - fine aggregate
• 75 - 2mm - silt
• lt2mm - clay
• Impurities
• organic matter
• sugar or similar carbohydrates
• soluble salts
• clay minerals
• Properties affected by aggregates
• density
• strength
• thermal expansion
• thermal conductivity
• shrinkage and creep

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Water

• Water fit for drinking is acceptable.
• Sea water
• no adverse effect on strength and durability.
• Surface dampness, efflorescence, staining.
• Increases risk of corrosion of steel
• Sea water is not recommended for reinforced
  concrete
• Water containing acid or organic substances
  should be avoided.
• If there is any doubt on water quality carry out
• chemical analysis
• trail mixes

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Admixtures

• proprietary mixture of chemicals
• added to concrete to alter or improve the
  properties of fresh or hardened concrete.
• Should be used if
• properties cannot be made by varying the
  composition of basic material
• produce desired effects more economically
• Unlikely to make a poor concrete better
• Not a substitute for good concrete practice
• Required dose must be carefully determined and
  administered

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Water reducers / plasticisers

• Normal water reducing admixtures
• increases the dispersion of cement particles and
  a reduction in viscosity
• ? workability for same w/c ratio
• ? strength without adding cement
• ? cement
• Superplasticisers
• flowing concrete
• no change in composition
• no reduction in strength
• heavy reinforcement / inaccessible
• rapid placement
• flowing characteristics last for only a short
  time (30 mins)
• added at site immediately before placing
• high strength concrete
• reduce w/c ratio
• maintain the same workability

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Accelerating agents

• Increases setting and early strength development
• CaCl2 is the most effective
• use in
• winter conditions
• emergency repair work
• early removal of formwork
• increased drying shrinkage
• reduced resistance to sulphate attack
• risk of corrosion of steel
• not permitted in RC
• Chloride free accelerators are available for RC
• more expensive
• less effective

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Retarders

• Slow down the setting and hardening times
• prevents the formation of cold joints
• compensates for time lost in transit
• Useful in
• large concrete pours
• sliding formwork
• hot weather concreting
• Retarding water-reducing admixtures are gaining
  popularity

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Air entraining admixtures

• Foaming agents, gas producing chemicals
• introduces millions of tiny, stable bubbles of
  uniform size that are uniformly distributed
  throughout the mix (usually about 5 of the
  volume).
• Improves properties of fresh concrete
• workability, cohesion
• reduces segregation and bleeding .
• Properties of hardened concrete
• For every 1 of air there is a 4 loss in
  strength.
• Minimised by the reduction in water content
• Improves durability
• frost, de-icing salts

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Handling and storage of materials

• Cement
• Bulk cement
• Store in weather tight, properly ventilated
  structures to prevent absorption of moisture.
• Bag cement
• stack on pallets to permit circulation of air.
• for long periods the stacks should be no higher
  than 7 sacks.

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Handling and storage of materials

• Aggregates
• Stockpiles should be maintained such that
• contamination, segregation and variation in
  grading is minimized.
• Uniform and stable moisture contents are
  maintained in aggregates.
• Coarse Aggregates
• Minimise segregation by separating the material
  into size fractions and batching them separately.
• Handling operations should not significantly
  increase the undersize material prior to their
  use in concrete.

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Handling and storage of materials

• Fine Aggregates
• Variations in grading during the production of
  concrete should be minimal.
• Material finer than 0.075mm should be controlled
  especially if it contains clay, shale, soft or
  light particles.

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Concrete mixing

• Stationary mixers
• Stationary mixers should be equipped with timing
  interlocks to prevent under or over mixing.
• Add cement with aggregates at the point when
  about 10 of the aggregates are in the mixer.
• Add water first and let it continue to flow while
  other ingredients are entering the mixer.
• Add liquid admixtures with the water.
• Maintain uniformity in batches either by visual
  observation or by consistency meters that measure
  the amperage draw of the motor drivers.

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Truck Mixer

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Transporting concrete

• Concrete is usually transported to its point of
  placement in agitating or non agitating trucks
• Concrete should be transported without
  significantly altering its desired properties
  such as slump, w/c ratio, air content and
  homogeneity.
• Loss of workability during warm weather may be
  minimized by
• expediting delivery and placement
• adding crushed ice to mixing water or cooling the
  water by refrigeration.
• injecting liquid nitrogen into the mixer or
  agitator truck
• using a retarding admixture.

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Placing concrete

• Methods of placing
• Buckets, hoppers, manual or motor propelled
  buggies, conveyor belts, pumps, tremie, paving
  equipment
• keep formwork, reinforcement and subgrades cool
• provide shade
• spray forms with water prior to placing concrete
• In very hot areas pour concrete during the night
• Concrete should be placed so that it is kept
  plastic and free of cold joints.
• Each concrete layer should be placed while the
  underlying layer is still responsive to
  vibration.

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Concrete Pump

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Compacting concrete

• Compaction expels entrapped air and packs the
  aggregates to increase the density of fresh
  concrete
• Immersion (poker) vibrators are the most
  efficient since they compact concrete directly.
• Vibrators should not be used to move concrete
  laterally.
• Shutter vibrators are installed externally
  outside of forms and impart oscillation to the
  forms and are suitable for small members or
  narrow heavily reinforced members.
• Surface vibrators are used in the construction of
  floors and road slabs and other thin sections
  with large surface areas.

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Immersion vibrator

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Curing concrete

• Curing prevents the loss of moisture from
  concrete until the hydration of cement takes
  place.
• Methods of curing
• Leave formwork in place
• Cover with plastic sheeting
• Apply a curing compound
• apply soon after the sheen has disappeared while
  the concrete is still damp
• not effective if application is delayed
• Ponding water
• Sprinkling or fog curing
• Wet coverings
• High early strengths can be obtained by steam
  curing.

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EFFECT OF NOT CURING

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Formwork
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Formwork

• The cost of formwork can be 1/3 to 2/3 of the
  total cost of a concrete structure.
• Requirements of formwork
• tolerances should be within specified limits.
• sufficiently strong to carry construction loads
  and side pressures from fresh concrete
• rigid enough to prevent movement during
  placement.
• Joints should be tight to prevent leakage of
  mortar.
• Should provide the desired surface finish.
• Designed to be simply erected and dismantled
  specially if it is to be re-used.
• Formwork should not be stripped before concrete
  has developed adequate strength.
• Vertical forms can generally be stripped before
  beam or soffit forms.

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Concrete handling

• Concrete is usually transported to its point of
  placement in agitating or non agitating trucks
• Concrete should be transported without
  significantly altering its desired properties
  such as slump, w/c ratio, air content and
  homogeneity.
• Loss of workability during warm weather may be
  minimized by
• expediting delivery and placement
• adding crushed ice to mixing water or cooling the
  water by refrigeration.
• injecting liquid nitrogen into the mixer or
  agitator truck
• using a retarding admixture.

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Finishes

• It is usually economical for the structural
  material to provide the surface finish.
• Surface finish depends on materials used
• sawn timber
• new boards are sealed with form oil to to prevent
  absorption
• plastic coated plywood
• fibreglass
• used for difficult shapes and profiles
• gives a very smooth mirror finish
• High quality surface finishes are required if the
  surface is to be bush hammered or sandblasted for
  architectural reasons.
• Poor quality formwork causes
• Honeycombing, blowholes, misalignment, plastic
  cracking

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Finishing ConcretePower Trowel

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Finishes
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Joints in concrete

• Construction joints
• made such that new concrete is bonded with
  hardened concrete and concrete appears to be
  monolithic and homogenous across the joint.
• Usually located where shear forces are a minimum.
  In beams and slabs this is usually in the middle
  third of the span.
• Whenever possible positioned at locations of
  movement joints or where a plane of weakness is
  required.
• Joint surface needs to be prepared before new
  concrete is placed.
• wire brushing and removal of loose material
• scabbling with high pressure water or sand
  blasting to expose the coarse aggregate.
• In liquid retaining structures, water stops are
  usually used to seal a vertical construction
  joint.

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Joints in Concrete

• Some rules
• Need isolation joints around columns
• Diamond shape corners to match up with the slab
  joints
• Avoid re-entrant angles
• Joint spacing 30x thickness (4.5 m max)
• Rectangular panels
• max length 1½ x short side

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Types of joints
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Properties of hardened concrete

• Strength
• depends on w/c ratio
• w/c ? 0.25 for full hydration of cement
• strong in compression, weak in tension
• Compressive strength
• 150f x 300 or 100f x200 cylinders
• 150 cubes (1.25 cylinder strength)
• Tensile strength
• 0.1 x compressive strength
• cannot be measured directly
• Indirect tensile strength
• Flexural tensile strength (usually higher)
• Curing
• moist curing for a long period gives the best
  strength
• 75 of final strength in 28 days
• strength at early ages can be increased by
  raising the temperature (steam curing)

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Properties of hardened concrete

• Elasticity
• used for computing deflections
• Ec w1.5 0.043?fc
• n 0.1 - 0.3
• Shrinkage
• increases with w/c ratio, cement content
• Creep
• is an important factor in prestressed concrete
• creep can be reduced by
• ? strength concrete
• ? volume of cement paste
• larger aggregates
• Thermal properties
• slight gain in strength up to 100C
• reduction in mortar strength above 400 C

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Properties of hardened concrete

• Durability
• specifications focus on the following
• Corrosion of reinforcement
• increases with chloride content
• abrasion of floors and pavements
• abrasion resistance increases with strength
• degradation by freezing and thawing
• degradation by wetting and drying
• precipitation of dissolved chemicals
• creation of thermal gradients
• greater availability of oxygen
• improved by
• higher concrete strength
• longer curing time
• increasing the cover to reinforcement
• air entrainment

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Properties of hardened concrete

• Permeability
• Important in water retaining structures.
• Water penetration can lead to
• corrosion of reinforcement
• spalling of concrete as the water expands as it
  freezes to form ice.