17 - Concrete Admixtures - Memphis Handouts/17 - Concrete...CIVL 3137 2 Admixtures admixture (n.)...
Transcript of 17 - Concrete Admixtures - Memphis Handouts/17 - Concrete...CIVL 3137 2 Admixtures admixture (n.)...
Admixtures
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Admixtures
admixture (n.) any material other thanwater, aggregates, hydraulic cement andfiber reinforcement, used as an ingredientof concrete or mortar, and added to thebatch immediately before or during mixing.
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Types of Admixtures
1. Air-entraining admixturesadded to improve freeze-thaw durability
2. Chemical admixtureswater-soluble compounds used to improve theproperties of the fresh concrete
3. Mineral admixturesfinely divided solids usually added to improvethe strength of the hardened concrete
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Air Entrainment Goals
tiny air bubbles, uniform in size
uniform dispersion in cement paste
air content = 9% of mortar volume
one billion bubbles per cubic yard
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Air Entrainment
hydrophilic group(s)
hydrophobic component
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Air Entrainment
Repulsion between negative surface charges prevents coalescence
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Air Entrainment
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Air Entrainment
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Chemical Admixtures
set acceleratorsset retarders
stabilizerswater reducers
Chemical Admixtures
• Type A – water reducing• Type B – retarding• Type C – accelerating• Type D – water reducing and retarding• Type E – water reducing and accelerating• Type F – high-range water reducing (HRWR)• Type G – HRWR and retarding
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Set Accelerators
Typically used in cold weather to reduce settingand curing times; also used to speed removal offormwork
Salts (calcium chloride, sodium chloride) arecheap set accelerators but can corrode rebarand reduce resistance to sulfate attack
Nitrates and nitrites are less effective and moreexpensive but are also non-corrosive
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Set Retarders
Typically used in hot weather to slow setting andcuring so concrete can be finished; also used inmass pours to eliminate cold joints
Sugars, starches, and cellulose derivatives areabsorbed onto the surface of cement particles todelay hydration of the calcium silicates
As little as 0.05% by mass will delay setting forfour hours; 1% prevents setting completely
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Stabilizers
Lets leftover concrete be returned to plant andreused the next day
Forms protective barrier around cement grains toprevent hydration of both calcium silicates andcalcium aluminates for up to 72 hours
Activator dissolves the protective barrier and letssetting proceed in a normal manner with no harmto the concrete
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Water Reducers
low-range (5-10%)
mid-range (10-15%)
high-range (15-30%)“superplasticizers”
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Water Reducers
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Superplasticizers
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Slump Loss
withsuper
without super
Slu
mp
-in
Supplementary Cementitious Materials
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Mineral Admixtures
Natural Pozzalans
volcanic ash
diatomaceous earth
burnt clays
Artificial Pozzalans
fly ash
silica fume
blast furnace slag
burnt rice hulls
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Pozzalans
pozzalan (n.) siliceous materials which bythemselves have little or no cementitiousvalue but will, in finely divided form and inthe presence of moisture, chemically reactwith calcium hydroxide (CH) at ordinarytemperatures to form compounds (CSH)that have cementing properties.
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Hydration Chemistry
CS H CSH CH heat
calcium silicate
water
calcium silicate hydrate gel
calcium hydroxide crystals
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True Pozzalans
silicateshigh specific surface
usually sphericalusually glassy
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Uses of Pozzalans
Save money by replacing expensive cement
Lower heat of hydration due to slow strength gain
Increase sulfate resistance if low in alumina
Improve workability due to spherical shape
Increase strength by converting CH to CSH
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Effects on Strength
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Fly Ash
Tiny spheres of glassy silica and alumina that are electrostatically precipitated from exhaust
gases given off by coal-fired power plants
Anthracite = Class F fly ashLignite = Class C fly ash
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Blast Furnaces
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Blast Furnace Slag
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Silica Fume
Tiny spheres of glassy silica electrostatically precipitated from exhaust gases given off by
electric arc furnaces used to make silicon
Carbon-free = white silica fumeCarbonaceous = black silica fume
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Silica Fume
0
4000
8000
12000
16000
0 10 20 30 40 50 60Age (days)
Stre
ngth
(ps
i)
705 lb/yd3 cement102 lb/yd3 silica fume
705 lb/yd3 cement
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