03_Admixtures

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Concrete Admixtures

Learning Objectives

Provide an introduction of chemicaladmixtures for use in concrete

Discuss definitions, uses, benefits, and

cautionary measures


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3Introduction

Definition (ACI 116 R-00)A material other than water, aggregates,

hydraulic cement, and fibre reinforcement,

used as an ingredient of a cementitiousmixture to modify its freshly mixed, setting, orhardened properties and that is added to thebatch before or during its mixing


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Introduction

A modern concrete is more than amixture of cement, aggregate andwater

ADMIXTURES and MINERALCOMPONENTS are becoming as

ESSENTIAL as PORTLANDCEMENT when making modernconcrete

Today Concretewithout mineralcomponents or

admixtures is foundin 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 moreeffectively than by other means

To maintain the quality of concrete during thestages of mixing, transporting, placing, andcuring in adverse weather conditions

To overcome certain emergencies duringconcreting operations

It should be borne in mind that noadmixture of any type or amount can be

considered a substitute for good concretepractice


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Introduction

Admixtures can be used to modifyvarious fresh and hardened concreteproperties:

Fresh state Hardened state

decrease water content

increase workability

reduce segregation

reduce the rate of slumploss

improve pumpability

improve placeability &finishability

modify the rate of bleeding

retard or acceleratesetting time

improve frost resistance

improve impact & abrasionresistance

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 and densifiers Self Consolidating Concrete (SCC)

ASTM Specification


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Water Reducers and

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

Changes surface charges of particles, reducingflocculation and water adsorption

Effects on Concrete

Increased strength

Increased slump and workability


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10Water Reducer andSuperplasticizers

Definition (ACI 116)Admixtures that either increase slump of freshly-

mixed mortar or concrete without increasingwater content OR maintain slump with areduced amount of water, the effect being due

to factors other than air entrainment

W t R d d


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11Water Reducer andSuperplasticizers

How they work

Without admixtureWithout admixture With admixtureWith admixture


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No Water Reducer(Materials are clinging

together)

Water Reducer(disperses materialsfor better flowability)

Effect of Water Reducer


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Why Water Reduction?

Provides dispersion of cement Strength

reduces water demand while maintaining slump(workability) thus increases concrete strength

Benefit

lower concrete permability Can reduce concrete costs with less cementitious

material if used correctly


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Normal water reducer (decreases the waterrequirements by about 5 10%)

Ca or Na salts of lignosulfonic acid

salts of hydroxycarboxylic acids

Carbohydrate

Mid-range water-reducing (decreases the waterrequirements by about 6 12%) . no ASTM

Complex aqueous solution of lignosulfonates withaccelerating admixtures

Polycarboxylate ether with set and strength enhancingingredients

Superplasticizer (decreases the water requirements byabout 12 - 30%)

Sulfonated melamine formaldehyde

Sulfonated naphtalene formaldehyde

Polycarboxylates

Classification

Effects on Properties of Fresh


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The use of water reducer /superplasticer:

Increases slump

Improves flow

Improves placing

Improves pumpability

Improves finishability

Improves formed surfaces

Effect on air content and setting

Effects on Properties of FreshConcrete

16Eff t P ti f


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16Effects on Properties ofHardened Concrete

When using water reducer /superplasticizer

Compressive strength increases

Permeability decreases

Chloride resistance increases

Frost resistance improves

Increases sulfate resistance

Increases resistance to abrasion

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Retarding Admixtures

How it works

Decreases the rate of cement hydration(C3S)

Admixture absorbs into calcium

hydroxide atoms and temporarily inhibittheir growth into larger crystals

Effects on Concrete

Delays Initial set

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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 protection against freezing / thawing

Earlier use of structure

Reduction of protection time to achieve agiven quality

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Calcium Chloride

Very effective & economical accelerator

Cautions when using chloride-bearingadmixture

do not use in prestressed concrete

high potential to cause corrosion calcium chloride should not exceed 2% in non

reinforced concrete

calcium chloride should not exceed 1% whenconcrete contains uncoated aluminum conduit

can cause discoloration issues (dark and light

gray spots, especially on hard trowledfinishes)

Time of Set Control Retarding vs


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Time of Set Control Retarding vsAccelerating

Always acompromise

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Corrosion Inhibitors

How it works Passive film protects reinforcing steel

from corrosion in the high pH

environment of concrete Chloride screening

Effects on Concrete

May accelerate initial set

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C i I hibi


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Corrosion Inhibitors

Control Corrosion of SteelReinforcement

Dosage dependent onanticipated chloride level

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Air Entraining Admixtures

Overview

Create stable system of microscopic airbubbles

How it works

Microscopic air bubbles gives water a place toexpand during freezing conditions

Effects on Concrete

Increased resistance to freeze-thaw damage Improved workability of fresh concrete


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Self-Consolidating Concrete

How it works

Superplasticizers make more fluid Viscosity Modifiers prevent aggregate and paste

segregation during flow

Effects on Concrete Increased strength

Increased slump and workability

Better appearance

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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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03_Admixtures

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