Gettu Lecture

7/29/2019 Gettu Lecture

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CHEMICAL ADMIXTURES FORSTRUCTURAL CONCRETE

Ravindra Gettu

UNIVERSITAT POLITCNICA DE CATALUNYA

Barcelona, SPAIN


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Chemical Admixtures

Chemical admixtures are now common and, in

many cases, essential components of high-quality

concrete.

About 90-95% of the concrete produced in manycountries incorporates some type of admixture.

Admixtures have led to the development ofseveral high performance concretes; e.g., High-Strength Concrete and Self-Compacting

Concrete.


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Correct Use of Admixtures

It is important to know the effect of the admixtureon concrete properties, both those for which it has

been designed and those in which it can interfere,

as well its secondary effects.

Know the specifications and recommendations ofthe supplier.


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Factors that Affect the Action of theChemical Admixture

Dosage and procedures

Characteristics of the cement and aggregates Environmental conditions

The dosage of the admixturesshould be controlled rigorously.


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Types of Chemical Admixtures

Water-reducing agents and Superplasticizers Reduce the amount of water needed for increasing the

workability or yield high workability with any change inthe water content.

Air-entraining agents Incorporate upto 8% of air in the concrete.

Accelerators Increase the rate of hardening or early-age strength

development.


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Types of Chemical Admixtures

Special purpose admixtures:

Shrinkage-reducing admixtures

Alkali-aggregate expansion-reducing admixtures

Corrosion inhibitors Viscosity-modifying or Antiwashout agents

Fungicides


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Air-Entraining Agents

Incorporate a controlled quantity of airthat isuniformly distributed in the concrete as microscopic

bubbles. The quantity of air incorporated not only depends

on the type and dosage of the admixture but alsoon several other factors:

Composition and fineness of the cement

Type and proportions of the aggregates Temperature

Mixing and compaction processes Interaction with other admixtures


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Air-Entraining Agents

Applications:

Reduction of the damage produced in theconcrete by freeze-thaw.

Reduction of bleeding and improvement of theuniformity of the concrete, as well as the

workability and consistency.

Lowering the density of concrete.


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Accelerators

Act as catalysts of the hydration reactions of the

cement, reducing the setting time and,consequently, the curing period.

Applications: Cold-weather concreting

High early-age strengths


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Accelerators

Disadvantages:

Accelerators based CaCl2 tend to increase thecorrosion of the reinforcement

Reduce the long-term strength

Increase autogenous shrinkage

Reduce the effectiveness of air-entraining agents

Reduce the resistance against sulphate attack Can cause stains on the concrete surface


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Retardors

In general, act by covering the cement particlesand, thereby, retarding the hydration processes.

Primary application is in hot-weather concreting.

Also used in mass concrete since they permit thecontrol of the temperature rise of the concrete,

reducing the possibility of thermal cracking.


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Viscosity-Modifying Agents

Enhance the cohesion of the concrete.

Minimize the accumulation ofbleed water.

Formulation:

Water-soluble synthetic or natural organic polymerswith high molecular weight

Emulsions of several organic materials


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Applications of Viscosity-ModifyingAgents

Underwater concrete

Facilitates sufficient mobility of the concrete underwater with little loss of cement.

Self-compacting concrete

Leads to high flowability with no segregation.

Grouting

Eliminates the migration of water from the grout due tothe differential pressure.

Helps maintain the cement particles in suspension

once injection ceases.


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SUPERPLASTICIZERS

Mechanisms of action,dosage and use


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History of Superplasticizers

1930: Use of a napthalene for dispersing coloring agentin concrete (USA).

1940s: Lignosulphonates began to be used. 1960s: Naphthalene/Melamine based superplasticizers

introduced:

To reduce the w/c - Japan (Hattori): -naphthalenesulphonate

To improve workability without increasing the w/c -Germany (Einesburger): melamine sulphonate

Present: Synthesis of new and more efficient copolymer

formulations.


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hf

W L SUP

hw h l h

W L SUP

in waterin water + plasticizer

UP

Sedimentation of Cement in Water

after 48 hours

closer view

In water + superplasticizer

P. C. Atcin

50 gm of cement in 1 liter of water

50 gm of cement in 1 liter of water

+ 5 ml of (super)plasticizer


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Superplasticizer Action

Flocculation in the absenceof superplasticizer

Entrappedwater

Water Cement particle Water Cement particle

Effect of the superplasticizer


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Chemical Formulation

Surfactants soluble in water, with different

functional groups:

Sulphonate (SO3-)

Carboxylate (COO-),

Hydroxide (OH-) or

Phosphonate (PO3-)

Modified Lignosulphonates (MLS) Salts of naphthalene sulphonate andformaldehyde condensates (SNF)

Salts of melamine sulphonate andformaldehyde condensates (SMF)

Comb-type polymers

Hydrophilic group

Hydrophobic group


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Mechanisms of Action

Types of interaction between cement particles

and the superplasticizer

PHYSICAL

Adsorption and

generation of

repulsive forcesbetween cement

particles

CHEMICAL

Chemisorption,

formation of

admixture-Ca2+complexes and

interaction with the

hydration reactions


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MECHANISMS: Physical Interactions

--

-

-

--

-

-

-

- - -

-

-

-

-

--

1. FORMATION OF AN ADSORBED LAYER ON THE

CEMENT PARTICLES.

2. GENERATION OF REPULSIVE FORCES BETWEEN

CEMENT PARTICLES DEFLOCCULATION.

Electrostatic Repulsion(SNF & SMF)

Steric hindrance due to thelateral chains of thecomb-type polymers


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MECHANISMS: Chemical Interactions

Chemical adsorption

Change in chemical composition as a function of the

thickness of the adsorbed layer.

Formation of complexes between the

superplasticizer and calcium ions Reduces the concentration of Ca2+ in the aqueous

solution, retarding the setting of cement.

Interaction with the chemical reaction sites

Blocks reactive sites, inhibiting the chemical reactionsbetween the cement and water.

Superplasticizer Silica Fume


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Superplasticizer-Silica Fume

Interaction

Without superplasticizer, the cement + water + silica

fume system tends to coagulate, making the use of a

superplasticizer essential.

Silicafume

Silicafume

COAGULATION

REPULSIVE FORCES

DISPERSION


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Usage of Superplasticizers

Constant w/c: Increase inthe workability

Constant workability:Lower w/c

same workability

LOWER WATER CONTENT

Lower w/c

No MLS SMF SCadmixture


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Consequences of Superplasticizer Usage

Reduces placing and compaction time, leading tolower construction costs.

Facilitates the casting ofelements with complexshapes and dense reinforcement.

Improves the surface finish of the concrete elements.

Leads to superior strength and durability with lowercement contents.

A li ti h S l ti i


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Applications where a Superplasticizeris Essential

Fluid/Flowing/Pumpable concrete

Shotcrete

Self-compacting concrete

High-strength concrete

High-durability concrete

Concrete with low shrinkage and creep


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HIGH PERFORMANCE CONCRETE (HPC)

Superior mechanical properties and durability,at minimum cost.

Compact microstructure (low permeability andhigh strength).

Low water/cement ratio (w/c)

SUPERPLASTICIZER


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Determination of the optimum dosage.

Superplasticizer-cement compatibity

Superplasticizers


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Selection of the Superplasticizer

Study of the compatibility

Optimum superplasticizer dosage

Cost-benefit considerations

In several cases, this order is inverted,

resulting in costly consequences

Marsh Cone Test: Evaluation of the


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Marsh Cone Test: Evaluation of thecompatibility and dosage

15.5 cm

29 cm

6 cm

Diameter: 8 mm

800-1000 ml

200-500 ml

Ma

rshconeflow

time

% sp/c

SATURATIONPOINT

Fluid

ity

0.5 1.0 1.5 2.0 2.5 3.0 3.5

5.0

5.5

6.0

6.5

7.0

0

5

10

15

20

25

0(Pa)

% sp/c

Flow

time(s)

Comparison with yield shear stressesobtained with a viscometer

Bingham yield stress (Pa)

Marsh cone flow time (s)

Cement I 52.5R

w/c=0.33Superplasticizer SD1

Practical Significance of the


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Practical Significance of theSaturation Point

50

90

130

170

210

5 min

60 min

2.82.42.01.61.20.80.4

Superplasticizer dosage (% sp/c)

Marshcone

flow

time,s

w/c = 0.35

T = 22C

0.0

Saturation Point

Cement/Superplasticizer


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Cement/SuperplasticizerCompatibility

60

80

100

120

140

160

180

200

60 min

5 min

60 min

5 min

0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

w/c = 0.35T = 23 C

Cement A

Cement BMarshconeflow

time,s

Superplasticizer dosage (% sp/c)


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Selection of Superplasticizer

( ) (timecost/kgs.r.

sp/c%=CBR

COST-BENEFIT RATIO

( ) 1s5euros/kg3s.r.0.3sp/c%0.25

CBR ==

( ) 26.3s7euro/kg1s.r.0.4sp/c%1.5CBR ==

Tim

e

(s)

w/c = 0.33


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Paste-Mortar-Concrete Comparison

In general, there isgood correlation

between thebehaviour of paste,mortar andconcrete.

Sand with a highcoefficient ofabsorption canincrease thesuperplasticizer

demand.

exudacin

Factors that Affect the Saturation


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Factors that Affect the SaturationPoint

Type of cement

Water/cement ratio

Presence of mineral admixtures

Mixing sequence (better to separate theincorporation of water and superplasticizer by

at least 1 minute of mixing)

Temperature

Effect of Temperature on the Fluidity an


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Effect of Temperature on the Fluidity anSaturation Point

Fluidity increases with increase in temperature.

The saturation point is unaffected by temperature variation

Saturation point = 1% sp/c

c = I 52.5 Rsp = SNw/c = 0.33

0.0 1.0 2.0 3.0 4.0

% sp/c

8

12

16

20

Marsh

Coneflowt

ime(s)

5 C

35 C25 C

45 C

15 C

0.0 1.0 2.0 3.0% sp/c

0

2

4

6

8

10

12

14

Marsh

Coneflowt

ime(s)

Saturation point = 0.3% sp/c

c = I 52.5 Rsp = SCw/c = 0.33

5 C

35 C

25 C

45 C

15 C

Effect of Temperature on the Loss of


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Effect of Temperature on the Loss ofFluidity

Loss of fluidity in the paste is lower for polycarboxylatbased superplasticizers.

There is no clear trend with respect to temperature.

c = I 52.5 Rsp = SNw/c = 0.33sp/c = 1%

0 5 15 30 45 60 75 90

Time (min)

5

10

15

20

Mars

hConeflow

time(s) 35C

5C

15C

25C

45C

c = I 52.5 Rsp = SCw/c = 0.33sp/c = 0.3%

0 5 15 30 45 60 75 90

Time (min)

4

8

12

16

MarshConeflow

time(s)

5C

15C

25C

35C

45C

Effect of Temperature on the Water


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Effect of Temperature on the WaterDemand of Cement

The water demand ofcement increases wit

an increase intemperature.

This demand decreasdue to incorporation o

superplasticizer untilthe saturation point.

c = I 52.5 Rsp = SN

0.0 1.0 2.0 3.0 4.0

% sp/c

0.18

0.20

0.22

0.24

0.26

0.28

0.30

aer

emanw

c

5 C

15 C

25 C

35 C

45 C

Mechanisms that Control the Fluidit


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Mechanisms that Control the Fluiditof Pastes at High Temperatures

There are two competing mechanisms:

The increase in fluidity due to thelowering of the viscosity.

Increase in the water demand ofcement, which tends to decrease thefluidity.


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Shrinkage Reducing Admixtures

Effect on Shrinkage, Creep andOther Properties


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Shrinkage

hours days weeks months yearsTim

Plastic

Thermal(contraction)

Autogenous

Drying

Carbonation

Sh i k M h i


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Shrinkage Mechanisms

Plastic shrinkage: Due to the loss of water in the plasticstate due to evaporation.

Autogenous shrinkage: Chemical shrinkage (lower volumeof hydrates than cement and water) +Autodessication(reduction in the pore water due to hydration).

Thermal contraction (or thermal shrinkage): Due to thedecrease in temperature after setting.

Drying shrinkage: Due to the loss of water to theenvironment in the hardened state.

Carbonation shrinkage: Volume reduction due to the

reaction of hydrated cement paste with CO2 in the presenceof moisture.

W f R d i Sh i k


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Ways of Reducing Shrinkage

Reduction of the water content (by usingsuperplasticizers).

Reduction of the cement content (by optimizing thepaste volume, using complementary materials).

Utilization ofspecial cements and expansive

agents.

Utilization ofshrinkage-reducing admixtures.


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Shrinkage Reducing Admixture (SRA)

First used in Japan, in the 1980s.

Results in the literature:

Reduces shrinkage by 35-60%.

Reduces restrained shrinkage cracking. Reduces permeability and macro-pore volume in

the cement paste. Increases the fluidity (plasticizing effect).

Slightly reduces the compressive and tensile

strengths, and the modulus of elasticity.

M h i f A ti


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Mechanism of Action

SRA reduces the surface tension of theevaporable water in the pores.

Leads to lowercapillar stresses during drying.Cement particle

Water

St d f SRA i C t


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Study of SRAs in Concrete

Evaluation of time-dependent behaviour:

Plastic shrinkage

Drying shrinkage Autogenous shrinkage

Basic creep Drying creep

Influence of the SRA on other properties: Workability

Early-age temperature rise

Mechanical properties


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Plastic shrinkage

Occurs in the fresh concrete, principally due

to high evaporation rates.

Factors:- Environment (temperature, humidity and windvelocity)

- Concrete composition- Boundary conditions (geometry and restraints)


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Plastic Shrinkage Cracking

When the bleed water does not compensate thewater loss due to evaporation, shrinkage occurs.

When plastic shrinkage is restrained, surface

cracking occurs.

Elements and structures with high surface/volumeratios, such as pavements, tunnel linings andbridge decks, are prone to cracking.

Pl i Sh i k T


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Plastic Shrinkage Tests

Normal strength concrete (35 MPa, w/c = 0.45):

Fresh concrete specimens subjected to a

temperature of 47C, relative humidity of 26%

and a wind velocity of 26 km/hr; evaporation

rate = 1.5 kg/m2/hr.

High strength concrete (70 MPa, w/c = 0.35):

Fresh concrete specimens subjected to a

temperature of 37C, relative humidity of 31%

and a wind velocity of 25 km/hr; evaporationrate = 0.6 kg/m2/hr.

Pl ti h i k T t fi ti


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Plastic shrinkage: Test configuratio

Panel

Prisms

Evaporation pan

EnvironmentSensors

Plastic Shrinkage Tests: Prism


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Plastic Shrinkage Tests: Prismspecimen

Stress riser, 106 mm high

Anchor bolts,5 mm diameter

Displacement sensor

Concrete prism,150x142x600 mm

Plastic sheet

Insulation


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Time

Horizontaldisplacem

ents

C

rackwidth

A

B

C

D E F

A-B: No shrinkage or some expansion

B-C: Increasing displacement at a

decreasing rate

C-D: Displacement remains constant

E: Air flow is stopped after four hour

E-F: Displacement unaffected by cooli

Displacement-Time Curves

Ad i t St di d


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Admixture Density

kg/ltSuperplasticizer D

Superplasticizer G

SRA E

SRA SSRA R

Solids

%1.15 44.4

Type

Naphthalene based (Non-surfactant)Polycarboxylate (Ethoxylatednon-ionic surfactant)

Wax based (Ethoxylated non-ionicsurfactant)

Glycol based (Non-ionic surfactant)

Glycol based (Non-ionic surfactant)

1.06 21.6

3.70.90

0.95 26.9

0.94 39.8

Admixtures Studied


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lastic Shrinkage Test Results

NORMAL STRENGTH CONCRETE HIGH STRENGTH CONCRETE

0 60 120 180 240

Time, min

-100

0

100

200

300

400

500

HorizontalDisplacements

,microns

CG-S

CG-E

CG-0

CG-R

CD-0

0 60 120 180 240

Time, min

-100

0

100

200

300

400

500

HoizontalD

isplacemen

ts,microns

HPD-E

HPD-0

Study of Shrinkage and Creep of


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Study of Shrinkage and Creep ofConcretes with SRA

Age

St

ra

in

Curing

Instantaneousstra

in=

i

i+ Basic creep strain

i+ Drying

creep strain

Drying shrinkagestrain

Autogenousshrinkage strain

Test Details


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Test Details

Tests of 1530 cm cylinders

35 MPa concrete (w/c = 0.4), with slump = 19 cm

superplasticizers: Naphthalene (SN), Melamine(SM), Polycarboxylate (SC)

SRAs: 4 different products (3 based on glycols and

1 wax-based), dosages: 1-2%

Test conditions: Autogenous shrinkage (sealed specimens) andDrying shrinkage (specimens at 50% R.H.)

Basic creep (sealed) and Drying creep (at 50% R.H

Test Configuration


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Test Configuration

Sensors


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Sensors

P ope ties of the Conc etes


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Properties of the Concretes

42.2171.5%0.14%CSRA3-SC

43.2202.3%0.12%CSRA2-SC

39.8172%0.33%CSRA1(2%)-SN

42.8171.5%0.40%CSRA1(1.5%)-SN

45.21800.14%CREF-SC45.01700.69%CREF-SN

fc (28 das)Slump (cm)SRA/csp/cConcrete

Results: Autogenous Shrinkage


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Results: Autogenous Shrinkage

C-SN

C-SC

C-ARR1%

C-ARR2%

100 200 300 400Tiempo (das)

-0.08

-0.04

0

0.04

0.08

0.12

Deformacin(mm/m

)

Strain(mm/m

)

Time (days)

Results: Drying Shrinkage


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Results: Drying Shrinkage

0.1 1 10 100Tiempo de secado (das)

0

0.1

0.2

0.3

0.4

Deformacin

porsecado

(mm/m)

REF-SM

REF-SC

RE-SN

SRA1(1.5%)-SN

SRA1(2%)-SN

SRA2-SCSRA3-SC

SRA4-SN

DryingShrinkag

eStrain

(mm/m)

Time (days)

Results: Basic Creep


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Results: Basic Creep

0.01 0.1 1 10 100log (t-to), das

0

0.4

0.8

1.2

C

oeficientedeFluenciaBsica

C-SN

C-SC

C-ARR1%

C-ARR2%

Bas

icCreep

Coefficient

Log (Time, in days)

R lt D i C


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Results: Drying Creep

0.01 0.1 1 10 100

log (t-to), das

0

0.4

0.8

1.2

Coe

ficientedeF

luenciaporsecado

C-SN

C-SC

C-ARR1%

C-ARR2%

DryingCre

epCoeff

icient

Log (Time, in days)

Results: Summary


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Results: Summary

Considerable reduction in the drying shrinkage of

concrete (30-50%), as a function of the type and

dosage of polypropylene glycol SRA. In the case of a

wax-based SRA, the reduction is 13%.

Absence of autogenous shrinkage in concretes with

SRA.

Results: Summary


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Results: Summary

The incorporation of an SRA leads to a

significant reduction in the drying creep

(33-46% for SRA/c = 1-2%).

The SRA does not affect the basic creep.

Slight decrease in the compressive strength

(5-12%) in concretes with SRA.

Increase in workability due to the incorporation

of glycol based SRAs.

Why Use Admixtures in Concrete ?


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Why Use Admixtures in Concrete ?

To satisfy the growing demands of the society andthe construction sector.

To provide better stability under certainenvironmental conditions.

To increase the productivity/efficiency during

fabrication, transport and placing.

The capacity of traditional materials to satisy these

demands is limited

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