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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