for concrete pipe

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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


How they work

Without Admixture With Admixture

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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

+

+

+

+ +

+

+

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

+

+ +

+ + +

+

+

+

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

+ + + + +

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

www.concrete-pipe.org

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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

V

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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 10 20 30 40 50 60

Number of F & T cycles

Cu

mu

lati

ve m

ass

loss

(kg

/m2)

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

+ -

+ -

+ - Cement

+ -

+ -

+ - Cement

+

-

+

- +

-

+ -

+

- +

- Aggregate

+

-

+

- +

- Aggregate

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!


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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