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Concrete Technology

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Vibrators should not be used to move concrete laterally. Shutter vibrators are installed externally outside of forms and impart ... – PowerPoint PPT presentation

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Title: Concrete Technology


1
Concrete Technology
  • Reference
  • Guide to concrete constructionSAA HB64 1994
  • Australian Concrete Technology
  • W G Ryan A Samarin

2
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

3
Portland Cement
  • Raw materials
  • limestone
  • silica, alumina in clay or shale
  • ironstone

1400oC
Clinker
Grind with gypsum
4
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.

5
Properties of the major constituents
6
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

7
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

8
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)

9
Concrete aggregates
  • Particle shape texture
  • smooth rounded particles give maximum workability
  • rough, cubital particles give optimum strength

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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

20
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.

21
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.

22
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.

23
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.

24
Truck Mixer

25
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.

26
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.

27
Concrete Pump

28
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.

29
Immersion vibrator

30
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.

31
EFFECT OF NOT CURING

32
Formwork
33
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.

34
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.

35
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

36
Finishing ConcretePower Trowel

37
Finishes
38
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.

39
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

40
Types of joints
41
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)

42
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

43
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

44
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.
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