for Concrete Pipe
Transcript of for Concrete Pipe
2/12/2015
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Chemical Admixtures for
Concrete Pipe
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Concrete Admixtures
Learning Objectives Provide an introduction of chemical admixtures
for use in concrete Discuss definitions, uses, benefits, and
cautionary measures
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3 Introduction
Definition A material other than water, aggregates,
hydraulic cement, and fiber reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing
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Introduction
• A modern wetcast concrete is more than a mixture of cement, aggregate and water
• ADMIXTURES and MINERAL COMPONENTS are becoming as ESSENTIAL as PORTLAND CEMENT when making modern concrete
Today Concrete without mineral components or admixtures is
found in museums
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Introduction
• Major reasons for using admixtures are: To reduce the cost of concrete construction
To achieve certain properties in concrete more effectively than by other means
To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse and varying weather conditions
To overcome certain emergencies during concreting operations
• Keep in mind that no admixture of any type or amount can be considered a substitute for good concrete practice
Introduction Admixtures can be used to modify various fresh and
hardened concrete properties:
Fresh state Hardened state
decrease water content
increase workability
reduce segregation
reduce the rate of slump
loss
improve pumpability
improve placeability &
finishability
modify the rate of bleeding
retard or accelerate
setting time
improve freeze / thaw resistance
improve impact & abrasion resistance
inhibit expansion due to ASR
inhibit corrosion
reduce shrinkage cracking
reduce permeability
produce colored concrete
produce cellular concrete
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Chemical Admixture Types
• Water-reducing admixtures (plasticizers) • Retarding admixtures • Accelerators • Corrosion inhibitors • Air-entraining admixtures • Lubricants or surfactants
Water Reducers
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9 Water Reducers and High Range Water Reducers (HRWR / Superplasticizers)
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Water Reducing Admixtures
• Overview Can be used to reduce water content, improve
slump or both Three groups: low-, medium- and high-range
• How it works Reducing flocculation and improves water
efficiency
• Effects on Concrete Increased strength Increased slump and workability
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11 Water Reducer and High Range Water Reducer
Definition (ACI 116) Admixtures that either increase slump of freshly-
mixed mortar or concrete without increasing water content OR maintain slump with a reduced amount of water, the effect being due to factors other than air entrainment
12 Water Reducer and High Range Water Reducer
How they work
Without Admixture With Admixture
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13 Water Reducer and High Range Water Reducer
How they work
Without Admixture With Admixture
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How Conventional Type A Water Reducers Work
Cement grains naturally cluster together to form flocs, which trap water inside them
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Water Cement +
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15 Floc Busting Action of Water Reducers
1. Water Reducer coats the cement grains
2. Water Reducer imparts negative charge to cement grains
3. Like charges repel cement grains away from each other
Freed Water
Cement -
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The Result: • Increased Water Cut (Lower W/C Ratio): Lower
water/cement ratio means improved product strength and quality, “insurance policy”
• Improved Workability: Concrete that is easier to place
and finish, with no sacrifice in quality
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Cement flocs are broken up and water is evenly dispersed
+ admix =
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No Water Reducer (Materials are clinging
together)
Water Reducer (disperses materials for better flowability)
Effect of Water Reducer
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Steric (physical) Repulsion Force
+ + + + +
Cement Cement
COMB POLYMERS Cement Dispersion by Steric Repulsion
Hooking units, Comb Backbone
Comb Teeth
Comb polymer
Water
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Cement Cement
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How Superplasticizers Work- video
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Why Water Reduction?
• Provides dispersion of cement particles • Strength reduces water demand while maintaining
slump (workability) thus increases concrete strength
• Benefit lower concrete permeability can reduce concrete costs with less
cementitious material if used correctly
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• Normal water reducer TYPE A (decreases the water requirements by about 5 – 10%) Ca or Na salts of lignosulfonic acid salts of hydroxycarboxylic acids Carbohydrate Gluconates
• Mid-range water-reducing (decreases the water requirements by about 6 – 12%) …. no ASTM Complex aqueous solution of lignosulfonates with
accelerating admixtures Polycarboxylate ether with set and strength enhancing
ingredients
• Superplasticizer TYPE F&G (decreases the water requirements by about 12 - 30%) Sulfonated melamine formaldehyde Sulfonated naphtalene formaldehyde Polycarboxylates
Classification
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• The use of water reducer / superplasticizer:
Increases slump
Improves flow
Improves placeability
Improves pumpability
Improves finishability
Improves formed surfaces
Can have an effect on air content and setting
Effects on Properties of Fresh Concrete
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Slump Flow Test - SCC
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Effects on Properties of Hardened Concrete Due to Lower w/cm Ratio
When using water reducer / HRWR
Compressive strength increases
Permeability decreases
• Chloride resistance increases
• Frost resistance improves
• Increases sulfate resistance
• Increases resistance to abrasion
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Retarding & Set Stabilizing
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How it works Decreases the rate of cement hydration (C3S) Admixture absorbs into calcium hydroxide
atoms and temporarily inhibit their growth into larger crystals
Effects on Concrete Delays Initial set Extends workability time
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Accelerators
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Accelerators
• How they work Increases rate of cement hydration (C3S)
• Why accelerate concrete? Shorten the setting time Quicker early strength Reduce bleeding Earlier finishing Improved initial protection against freezing Earlier use of structure / piece Reduction of protection time to achieve a
given quality
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Accelerating Admixtures
• 2 Classes of Accelerating Admixtures: Set Accelerator Strength Accelerator (Early Age)
• Calcium Chloride is a well known accelerator, but BEWARE, it should not be used in reinforced concrete
• Several non-chloride, non-corrosive accelerators are available, but generally most are not as effective as calcium chloride
30 Calcium Chloride
• Very effective & economical accelerator • Cautions when using chloride-bearing
admixture DO NOT use in ANY reinforced concrete high potential to cause corrosion calcium chloride should not exceed 2% in non
reinforced concrete calcium chloride should not exceed 1% when
concrete contains uncoated aluminum conduit can cause discoloration issues (dark and light
gray spots, especially on hard trowled finishes)
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Time of Set Control Retarding vs Accelerating
Always a compromise
Corrosion Inhibitors
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Corrosion Inhibitors • How it works Passive film enhances the protection of
reinforcing steel from corrosion in the concrete Generally, corrosion inhibitors are not needed
to protect steel reinforcing, due to the passivating effect of the high pH in the concrete.
• Effects on Concrete May accelerate initial set May improve early age strength
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Corrosion Inhibitors • Control Corrosion of Steel
Reinforcement • Dosage dependent on anticipated
chloride level
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Air Entraining
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• Overview Create stable system of microscopic air
bubbles
• How it works Microscopic air bubbles gives water a place to
expand during freezing conditions
• Effects on Concrete Increased resistance to freeze-thaw damage Improved workability of fresh concrete Every 1% air entraining potentially reduces the
concrete strength by up to 5-10%
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Air-Entraining Admixtures Definition
Are used to produce concrete that is resistant to the effects of freezing and thawing and to improve workability
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Polished section of air-entrained concrete as seen through a
microscopic
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Air-Entraining Admixtures
Significance of freezing and thawing
The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of de-icing chemicals used for snow and ice removal
Contrary to fresh concrete which can be protected, we can’t avoid the exposure of mature concrete to alternating freezing and thawing
Due to freezing and thawing, hardened concrete can suffer both internal as well as surface damage. Internal damage can lead to loss of strength and structural integrity.
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Mechanism of Frost Damage
• Hydraulic pressures Caused by the 9% expansion of water upon freezing Growing ice crystals displace unfrozen water If a capillary is above critical saturation (91.7% filled with
water) hydraulic pressures results as freezing progresses At low water contents, no hydraulic pressure should exist
Critical saturation begins at 91.7%
Water expands 9% on freezing
Water is forced ahead of the advancing freezing front
Internal hydrostatic pressures can disrupt the concrete
40 Effect of Entrained Air
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Number of F & T cycles
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loss
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2% air
4% air
6% air
Surface Damage
ASTM C 672: Samples are monitored visually and for loss of mass over a period of 50 or more cycles of freezing and thawing in presence of salts
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41 Mechanism of Air Entrainment
Air
Hydrophobic end is attracted to air within bubbles
Surface tension reduced – stable bubbles
Charges around bubbles lead to repulsive forces between bubbles (prevent coalescence)
Bubbles adhere to cement & aggregate particles – cohesion of mix improved
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- Aggregate
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AEA’s do not add air to the concrete…they make the air that is already there…STABLE
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Un-wanted air in Concrete (Wetcast)
• How much air should be in concrete (no admixtures)? Less than 3%
• How much air should be in concrete with no AEA admixture, only WR or other admixture?
Less than 3% If you have MORE than 3% AIR in concrete with NO admixtures… it’s a cement issue
If you have MORE than 3% AIR in concrete with just WR (no AEA) … it’s an admixture issue or a cement issue
The above assumes that the quality of the water is OK, bad water can cause air issues (too much-too little)
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Entraining Air
Many factors effect the development of
air entrainment.
Mixing time Materials
Sequencing Temperature
Mixing Action Equipment
Dosage Rates Admixtures
*Almost everything!
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Lubricants and Surfactants
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Lubricants & Surfactants (Drycast)
• How it works Decreases surface friction and therefore
aids in stripping forms Increased reaction to vibration Improved moisture retention
• Effects on Concrete Can improve appearance Improved water tolerance Increase in surface paste
46 Dry Cast admixes Productivity improvement
Reduction in cracking Improved surface swipe
Sharper joints / reduced repair
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ASTM Specs
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ASTM C 494
ASTM C 260
ASTM D 98
ASTM C 869
ASTM C 1141
ASTM C 1017
ASTM C 937
ASTM C 979
Chemical Admixtures
Air-entraining Admixtures
Calcium Chloride
Foaming Agents
Admixture for Shotcrete
Flowing Concrete
Grout Fluidifier
Pigments
Current Admixture Standards
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QUESTIONS?
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Thank You
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