4 chemical admixtures
DESCRIPTION
4 Chemical AdmixturesTRANSCRIPT
Chemical Admixtures
• Definition (ASTM C 125): a material other than water,
aggregate, hydraulic cement, and fiber reinforcement that is
used as an ingredient of concrete or mortar and is added to
the batch immediately before or during its mixing.
• Classification
– Plasticizing admixtures ( ASTM C 494 and C 1017) – are
water-soluble polymers designed to enhance workability
or to reduce water requirements for a desired workability
– Set-controlling admixtures (ASTM C 494) - are added to
control setting and to induce early hardening
– Air-entraining agents (ASTM C 260) – are added primarily
to improve resistance to freezing/thawing cycling
– Miscellaneous admixtures
• E.g. Viscosity modifiers, corrosion inhibitors, shrinkage
reducing admixtures, etc.
Precautions: (1) conform to standards; (2) test using job
materials under job-site conditions; (3) ensure accurate batching
(Mindess et al 2003)
Water-Reducing admixtures (plasticizers)
• Increase slump without increase water and cement contents
• to facilitate difficult placements
• Achieve desired slump with less water (lower w/c)
• improve strength, impermeability, and durability
• Achieve desired slump with lower cement content without
changing w/c
• Economic reason: reduce cost
• Technical reason: reduce heat
• Classification of water reducers
(Mindess et al 2003)
Type
• Type A* – water reducing admixture (WRA)
• Type B - retarding
• Type C - accelerating
• Type D – water reducing and retarding
• Type E - water reducing and accelerating
• Type F – high range water reducing (superplasticizer, SP)
• Type G – high range water reducing and retarding
* In general, Type A WRA
• the rate of aluminate hydration (risk of flash set)
• the rate of C3S hydration (slowing strength gain).
Constituent Materials for Concrete – Admixtures
APPLICATIONS OF PLASTICISING ADMIXTURE
CONTROL MIX
w/c = x
fcu = y
Slump = z
MIX A – Plasticising
w/c = x
fcu = y
Slump > z
MIX B – Water-reducing
w/c < x
fcu > y
Slump = z
MIX C – Cement saving
w/c = x
fcu = y
Slump = z
(+WRA) (+WRA), (–Water)
(+WRA)
(–Water)
(–Cement)
Increase workability by
simple addition of
admixture
Reduce water content with
addition of admixture
Reduce water content and cement content with addition of admixture
Constituent Materials for Concrete – Admixtures
Efficiency of Admixtures
Varied by concentration (solid content) and between brands
(nominal water-reduction admixtures: 1G-10%, 2G-20%, 3G-30%)
Effectiveness varies with different cements:
(a) cement fineness
(b) equivalent alkali content (%Na2O + 0.658%K2O)
(c) C3A content
(Effectiveness reduced when any one of above factors in the cement increases)
Effectiveness lowered by increase in concrete temperature
Specification for Admixtures – EN 934-1 to EN 934-6
Euro Approach
CHEMICAL ADMIXTURES
Testing based on Standard Mortar and
Standard Concrete
(NOT ACTUAL CONCRETE ON SITE)
It is likely that admixtures will be
produced under certification
Constituent Materials for Concrete – Admixtures
Admixtures Standards – Specification
BS EN 934-1: 2008, Admixtures for concrete, mortar and grout – Part 1: Common requirements BS EN 934-2: 2001, Admixtures for concrete, mortar and grout – Part 2:
Concrete admixtures – Definitions, requirements, conformity, marking and labelling SS EN 934-2: 2008 (National Foreword provides guidance on testing for
Singapore)* BS EN 934-3: 2003, Admixtures for concrete, mortar and grout – Part 3:
Admixtures for masonry mortar – Definitions, requirements, conformity, marking and labelling BS EN 934-4: 2001, Admixtures for concrete, mortar and grout – Part 4:
Admixtures for grout for prestressing tendons – Definitions, requirements, conformity, marking and labelling SS EN 934-4: 2008 (National Foreword provides guidance on testing for
Singapore)* BS EN 934-5: 2001, Admixtures for concrete, mortar and grout – Part 5:
Admixtures for sprayed concrete – Definitions, requirements, conformity, marking and labelling BS EN 934-6: 2001, Admixtures for concrete, mortar and grout – Part 6:
Sampling, conformity control, and evaluation of conformity (Amendment 1, March 2006)
* Temperature for testing at 27O C (in place of 20 OC in EN for temperate climate)
REPLACING BS 5075: Parts 1 to 3 on Admixtures
Constituent Materials for Concrete – Admixtures
Admixtures Standards – Testing BS EN 480-1:2006, Admixtures for concrete, mortar and grout – Test methods – Part 1: Reference concrete and reference mortar for testing
BS EN 480-2:2006, Admixtures for concrete, mortar and grout – Test methods – Part 2: Determination of setting time
BS EN 480-4:2005, Admixtures for concrete, mortar and grout – Test methods – Part 4: Determination of bleeding of concrete
BS EN 480-5:2005, Admixtures for concrete, mortar and grout – Test methods – Part 5: Determination of capillary absorption
BS EN 480-6:2005, Admixtures for concrete, mortar and grout – Test methods – Part 6: Infrared analysis
BS EN 480-8:1997, Admixtures for concrete, mortar and grout – Test methods – Part 8: Determination of conventional dry material content
BS EN 480-10:1997, Admixtures for concrete, mortar and grout – Test methods – Part 10: Determination of the water soluble chloride content
BS EN 480-11:2005, Admixtures for concrete, mortar and grout – Test methods – Part 11: Determination of air void characteristics in hardened concrete
BS EN 480-12:2006, Admixtures for concrete, mortar and grout – Test methods – Part 12: Determination of the alkali content of admixtures
BS EN 480-13:2002, Admixtures for concrete, mortar and grout – Test methods – Part 13: Reference masonry mortar for testing mortar admixtures
BS EN 480-14:2006, Admixtures for concrete, mortar and grout – Test methods – Part 14: Determination of the effect on corrosion susceptibility of reinforcing steel by potentiostatic electro-chemical test
National Forward to SS EN 943- series indicates test temperature for Singapore
Constituent Materials for Concrete – Admixtures
EN 934-1:2008, Admixtures for concrete, mortar and grout – Part 1: Common requirements
Table 1 – General requirements (as in Table 1 of EN 943-2:2001 with more information on corrosion behaviour and
additional requirement on property – silicon dioxide SiO2 content) Clause 5 Corrosion behaviour (test method, EN 480-14)
5.1 No testing for corrosion behaviour is required for admixtures containing only substances on the approved list A.1 and declared list A.2 (see EN 934-1) Admixtures containing substances on the declared list A.2 shall have the names of the substances declared on the label
5.2 Test requirement When tested in accordance with EN 480-14 the calculated current density of each of the three test mix specimens shall not exceed 10 A/cm2 at any time between 1h and 24 h. In addition, there shall be a similar trend in the progression of the current density vs. time curves for the control mix and the test mix Silicon dioxide SiO2 content (test method, EN 196-2) Manufacturer’s stated values and characteristics shall be provided in writing to the user upon request. The silicon dioxide content is only required when silica is a constituent intended to exceed 5% by mass of the admixture. This requirement does not apply to natural sand Requirement: Not above manufacturer’s stated maximum value in % by mass (0%)
Constituent Materials for Concrete – Admixtures
Chemical Admixtures BS EN 934-2: 2001 (Amendment A1:2004, A2:2005)
Part 2: Admixtures for concrete, mortar and grout – Part 2: Concrete admixtures –
Definitions, requirements, conformity, marking and labelling Compliance dosage
dosage of an admixture, expressed in % by mass of cement, stated by manufacturer which will meet the requirements of this standard (within recommended range of dosage)
Recommended range of dosage
dosages between limits expressed in % by mass of cement which the manufacturer recommends for the product based on experience on site
Reference concrete and mortar
concrete and mortar as specified in EN 480-1 for testing admixtures for conformity with this standard (Note: not materials and mix on site)
Primary function
a single function of a multifunction admixture designated by the manufacturer
Secondary function
a function of a multifunction admixture which is additional to the primary function, e.g. set retarding and water reducing
Constituent Materials for Concrete – Admixtures
Requirements for Specific Types of Admixtures
Water reducing/plasticizing admixtures Table 2
High range water reducing/plasticizing admixtures Tables 3.1 and 3.2
Water retaining admixtures Table 4
Air entraining admixtures Table 5
Set accelerating admixtures Table 6
Hardening accelerating admixtures Table 7
High range water reducing/plasticizing admixtures Tables 3.1 and 3.2
Set retarding admixtures Table 8
Water resisting admixtures Table 9
Set retarding/water reducing/plasticizing admixtures Table 10
Set retarding/high range water reducing/
superplasticizing admixtures Tables 11.1 and 11.2
Set accelerating/water reducing/plasticizing admixtures Table 12
CONFORMITY CONTROL
Requirements for conformity control are given in EB 934-2001. The frequency of testing in connection with factory production control is given in Table 13.
Only a few of the above Tables are used to illustrate the approach for conformity
High Range Water Reducing/Superplasticizing Admixtures
Table 3.1
At equal consistence
Reference Concrete I
Control mix:
Slump: 70 10 mm (60 – 80)
Test Mix:
Slump: 12% (60 > 80)
(70 > 90)
(80 >100)
Table 3.2
At equal w/c ratio
Reference Concrete IV
Control mix:
Slump: 30 10 mm (20 – 80)
Test Mix:
Slump: 120 mm
At 30 min after addition:
Slump: not less than initial
value of control mix
Set Retarding/High Range Water Reducing/Superplasticizing Admixtures
Table 11.1
At equal consistence
Reference Concrete I
Control mix:
Slump: 70 10 mm (60 – 80)
Test Mix:
Slump: 12% (60 > 80)
(70 > 90)
(80 >100)
Setting time:
Reference Mortar
(Vicat apparatus)
Table 11.2
At equal w/c ratio
Reference Concrete IV
Control mix:
Slump: 30 10 mm (20 – 80)
Test Mix:
Slump: 120 mm
At 30 min after addition:
Slump: not less than initial
value of control mix
EN 934 – 2: Tables 2, 3.1, 3.2, 4, 5 and 6 (also Tables 7 to 12)
Equal consistence
Equal w/c ratio
Equal consistence
Equal consistence
Equal consistence
Constituent Materials for Concrete – Admixtures
Testing of Admixtures
EN 480-1:2006, Admixtures for concrete, mortar and grout – Test Methods –
Part 1: Reference concrete and reference mortar for testing
This standard specifies the constituent materials, the composition and
the mixing method to produce reference concrete and reference mortar
for testing the efficacy and the compatibility of admixtures in accordance
with the series of EN 934
Cement: CEM I cement of strength class 42,5 or 52,5 conformity to EN 197-1
C3A content – 7% to 11% by mass
Specific surface – 3 200 cm2/g to 4 000 cm2/g (Blaine fineness)
(Na2O eqv not specified, but recommend for its value to be reported)
Aggregate for reference concrete: natural normal weight aggregate
conforming to EN 12620 with water absorption less than 2% by mass and
size fractions within limits given in Table 1 – Aggregate for reference
concrete
Aperture size (mm) 31,5 16,0 8,0 4,0 2,0 1,0 0,5 0,25 0,125
% by mass passing 100 75 – 95 45 – 70 35 – 50 25 – 40 20 – 35 10 – 25 4 – 12 1 – 8
The variation in the quantity passing each sieve of the chosen grading for both
mixes (control and test mix) shall not exceed 2% by mass
Constituent Materials for Concrete – Admixtures
Testing of Admixtures
EN 480-1:2006, Admixtures for concrete, mortar and grout – Test Methods –
Part 1: Reference concrete and reference mortar for testing
Aggregate for reference mortar: standard sand according to EN 196-1
Mixing water: water according to EN 1008
Distilled or de-ionised water may be used in special cases
Not allowed to use wash water from concrete production
Reference mortar – comparative tests
Performance of admixtures is determined by comparing the reference
mortar containing an admixture (test mix) with the reference mortar
without an admixture (control mix)
Standard mortar conforming to EN 196-1 used as reference mortar:
one part of cement : three parts of standard sand : one half part water
(1 : 3 : 1½ by mass – with admixture added to test mix)
Except when testing water resisting admixtures at equal w/c ratio, for test
mixes the water added shall be sufficient to give equal consistence to
that of the control mix (Clause 7, Production of reference mortar)
Constituent Materials for Concrete – Admixtures
Testing of Admixtures
EN 480-1:2006, Admixtures for concrete, mortar and grout – Test Methods –
Part 1: Reference concrete and reference mortar for testing
Reference concrete – comparative tests
Performance of admixtures is determined by comparing the reference
concrete containing an admixture (test mix) with the reference concrete
made without the an admixture (control mix) but otherwise with the same
aggregate/cement ratio and constituent materials from the same delivery.
(air content of control mix shall not exceed 2% by volume)
Table 2 – Requirements for reference concrete a
Reference concrete Cement content c – kg/m3
Consistence at required test temperature
Slump (mm) b EN 12350-2 Flow (mm) d EN 12350-5
I 350 5 70 10 400 20
II 300 5 120 20 450 20
III 350 5 50 10 350 10
IV 350 5 30 10 350 20
a When testing at equal w/c ratio requirements for consistence apply only to control mix b Alternatives to be chosen before starting test c Control mix only. Resulting cement content of test mix may change due to effects of admixture added d For high range water reducing/superplasticizing admixture consistence of test mix shall be not less than
that of control mix with no upper limit on consistence of test mix
Constituent Materials for Concrete – Admixtures
Testing of Admixtures
EN 480-1:2006, Admixtures for concrete, mortar and grout – Test Methods – Part 1: Reference concrete and reference mortar for testing
Production of reference concrete
Cement content in accordance with Table 2 (I, II, III or IV)
Aggregates shall be used in an oven dry condition ( 150 OC)
If not oven dry, determine moisture content to correct specific gravity
In case of dispute, oven dry aggregate shall be used
Details in Clause 6.2 Mixing and testing
Testing and curing temperature: (20 2) OC to be (27 2) OC for laboratory temperature in “warm countries” (ISO)
3 cubes or cylinders for compressive strength testing – if one of the specimens varies from the average for the set by more than 10%, it shall be discarded and the average recalculated on the remaining two specimens
Test report – Clause 8: to include detailed information on aggregates, cement, control concrete and/or mortar, test concrete and/or mortar
Constituent Materials for Concrete – Admixtures
EN 480-2:2006, Admixtures for concrete, mortar and grout – Test Methods – Part 2: Determination of setting time
The setting time is determined by observing the penetration of a needle into a reference mortar until it reaches a specific value (Vicat apparatus – same needle used for determining both initial and final setting time, as for initial setting time of cement)
The reference mortar with admixture (test mix) shall have the same consistence as the reference mortar without admixture (control mix) that conforms to EN 480-1
The mixing water required (test mix) shall be determined in advance according to EN 413-2 Masonry cement – Part 2: Test methods
The time measured from completion of mixing until the time at which the distance between the needle and the base plate is 4,0 mm (penetration of 26 mm) is the initial setting time for the mortar.
The time measured from completion of mixing until the time after which the needle no longer penetrates 2,5 mm into the specimen is the final setting time for the mortar
Note: The values so determined have no relationship with those determined based on ASTM C 403 or BS 5038 using wet sieved mortar from a concrete
SETTING TIME TEST – EN 480-2 vs SS 320 (BS 5075)
Vicat Penetration
Standard Mortar
Setting Time Test (ASTM C 403, BS 5075, SS 320)
Standard Penetration Needle
Wet-sieved Mortar from Concrete
Time to 3.5 MPa – potential cold joint formation
A more rational method for determining
potential cold joint time needs to be
developed for testing fresh concrete both
in the laboratory as well as on site
Constituent Materials for Concrete – Admixtures
Chemical Admixtures
Performance based testing using standard mortar and standard concrete for conformity assessment only (TEST RESULTS NOT DIRECTLY APPLICABLE TO SITE REQUIREMENTS – PERFORMANCE TESTING OF DESIGNED CONCRETE NEEDED)
For application, initial testing for required performance, e.g. dosage to provide specified consistence of designed concrete for a project
Test certificates show results not necessarily the same way as for current test methods
Setting time test based on penetration of Vicat needle on standard mortar (standard EN sand) mainly to assess delay in setting with retarding admixture
Current practice on setting time (penetration of standard rod on wet-sieved mortar from concrete) may need to be retained for potential cold joint formation time (new test method directly on concrete to be developed and correlated to time beyond which cold joint formation can be detected)
Factory production control and certification are likely to be adopted by major manufacturers, locally or from overseas
Constituent Materials for Concrete – Admixtures
Efficiency of Admixtures
Varied by concentration (solid content) and between brands
(nominal water-reduction admixtures: 1G-10%, 2G-20%, 3G-30%)
Effectiveness varies with different cements:
(a) cement fineness
(b) equivalent alkali content (%Na2O + 0.658%K2O)
(c) C3A content
(Effectiveness reduced when any one of above factors in the cement increases)
Effectiveness lowered by increase in concrete temperature
Constituent Materials for Concrete – Admixtures
Admixture Testing
Conformity tests for admixtures are based on standard mortar and
standard concretes under prescribed conditions
The performance is verified against stated values in BS 934 series of
admixture specifications for different applications
For site concretes, the constituent materials for concrete and their
proportions adopted are in general not those in the standard
mortar or standard concretes for admixture testing
Initial testing is necessary to established the required dosage for
intended application, particularly when more than one type of
admixtures are added
Concrete – A Simple but Complex Composite Material Concrete – Simple Material
A composite material made from cement, water, fine aggregate and
coarse aggregate plus one or more chemical admixtures
Concrete – Complex Composite Material
(e.g. two phase material = particle in matrix)
Composite Dispersed (particle) phase Matrix phase
Concrete Coarse aggregates Mortar
Mortar Fine aggregates Cement paste
Cement paste Unhydrated cement grains Hydrated cement paste
Cement – chemically consisting of many oxides, CaO, SiO2, Al2O3, Fe2O3, MgO,
SO3, K2O, Na2O, P2O5, TiO2 main and minor in quantity, but influence is not
always proportional to mass fraction, e.g. SO3, K2O, Na2O,
Potential compounds – tricalcium silicate, di-calcium silicate, tricalcium
aluminate and tetracalcium aluminoferrite
Hydration products cover a range of silicates of slightly different compositions
and crystal structures plus calcium hydroxide
Physically, particles of unhydrated cement of different sizes and aggregates of
different shapes and sizes as well as surface textures
Concrete – A Simple but Complex Composite Material
Behaviour of Concrete
In the design of concrete structures formulae are based on properties of
a homogeneous material and constitutive equations of a continuous
medium (similar to steel or other metallic materials)
Concrete of the same composition may develop different strength and
other properties due to processes of handling (from mixer to finished
structure), compaction, curing and exposure conditions
Much higher variability within its mass due to the above factors in
addition to local distribution of dispersed phase and degree of cement
hydration (with depth from external faces)
Unlike metals at ambient conditions, deformation of concrete increases
with time of sustained loading (known as creep)
Beyond a critical level of tensile strain (even within design service
conditions), micro-cracking develops at particle/matrix interface
(interfacial transition zone, ITZ)
The apparent post linear behaviour is contributed by a series of crack
formation, crack arrest and eventually crack propagation until cracks
are linked resulting in final fracture – due to its complex composite
structure
Mechanism of Water Reduction
• Electrostatic repulsion
– negatively charged
organic molecules
adsorb primarily at the
solid-water interface
-Molecules of WRA
interact to neutralize
surface charges on
cement particles and
cause all surface to
carry uniform charges of
like sign
– Particles repel each
other, remain dispersed
Cement particles
Water
reducing
admixture molecules
Cement particles Trapped
water
Mechanism of Water Reduction (cont’d)
• Steric Repulsion
–Some of the newer admixtures use negatively charged polymers
–The bulky nature of the adsorbed molecules can cause additional
steric repulsion
Cement particles
Water
reducing
admixture molecules
Freed
water
Examples of Admixtures – Kao Corporation Japan
Chemical Admixtures:
1st Generation : Air entraining
1930’s
2nd Generation: Water reducing
1940’s (1G plasticiser)
3rd Generation: High range water reducing (superplasticizer)
1960’s (2G plasticiser)
4th Generation: Slump retensive high range water reducing
1990’s (3G plasticiser)
Co-polymers:
Engineered admixtures with specific functions
Examples of Admixtures – Kao Corporation Japan
Examples of Admixtures – Kao Corporation Japan
Examples of Admixtures – Kao Corporation Japan
Examples of Admixtures – Kao Corporation Japan
Composition
• Regular WRA
– Lignosulphonates
– Hydro-carboxylic acids and their salts
– Hydroxylated polymers derived from hydrolyzed starch
• High-range WRA (superplasticizers)
– Sulphonated naphthalene-formaldehyde condensates
– Sulphonated melamine-formaldehyde condensates
– Modified lignosulphonates
– Carboxylated acrylic ester copolymers (polycarboxylates)
Effect on the Properties of Fresh Concrete
• Flowing concrete
– Increase slump
• Use of SP allows high slumps to be achieved without
excessive segregation and bleeding
• pumping concrete, self-leveling concrete, tremie placement
Effect on the Properties of Fresh Concrete
• Slump loss
– Adding a second dose
– Delaying the initial addition
W/C = 0.45
(Mindess et al 2003)
Example
w/c=0.45
Slump 50mm
at 150 min
Low slump loss
Addition of Plasticiser
Note:
Good slump retention plasticiser has low slump loss after 180 min (1)
Plain concrete, w/c = 0.45, initial slump 100 mm, drops to about 25 mm after 150 min (4)
With delayed addition (e.g. 75 min) achieves about 50 mm slump after 150 min (6)
Plain concrete at w/c = 0.60, initial slump 225 mm, slightly above 50 mm after 150 min (2)
With first dose at w/c = 0.45, initial slump 225 mm, drops to blow 50 mm after 150 min (3)
With second dose (after 60 min) achieves about 50 mm slump after 150 min (5)
Target :50 mm
Mindess, 2003
Target slump at time of placing:
50 mm 2.5 hours after addition
of water at plant
Addition of Plasticiser
Source: Dodson, 1990
HR-N : high range, non-retarding admixture
(22 O C)
Effect on the Properties of Fresh Concrete
• Bleeding and air entrainment
– Bleeding
• Overdose of superplasticizer should be avoided
– Air entrainment
• Regular WRA will reduce the amount of an AEA required to
attain a given air content
• Retardation of set
– Many regular or mid-range WRA will also act as retarding
agents, some SP at high dosages have retarding effect
– Beneficial in hot weather concreting so long as it does
not affect setting and strength development significantly
Effect on the Properties of Hardened Concrete
• Compressive strength
– When WRA is used to lower water requirement, strength
– in the compressive strength is up to 25% greater than would
be anticipated from the in w/c alone, attributed to more
uniform microstructure
• High-strength concrete
– SPs are used in most concrete with 28-day compressive
strengths of more than 80 MPa
– Lower w/c improve the rate of strength development at early
age, can be used for precast concrete plant
• Other properties
– Reduce permeability
– in the rate of shrinkage is often observed; after 90 days of
drying, little difference compared with control concrete
– With creep, the situation is less clear, different admixtures may
or the creep
Set-Retarding Admixtures (Type B in ASTM C 494)
• Applications
– Offset the effect of high temperature
– Prolong the plasticity of fresh concrete
– Eliminate cold joints
– Special effect on concrete surface with exposed aggregate
• Composition
– Lignosulphonic acids and their salts water reducing,
– Hydroxycarboxylic acids and their salts can be classified
– Sugars and their derivatives as ASTM Type D
– Phosphates and organic phosphonate salts
– Salts of amphoteric metals, such as zinc, lead, or tin
water-reducing, set-retarding admixtures
(FHWA HIF-07-004)
Retarders extend
Stage 2
Mode of Action of Set Retarding Admixtures • Effect on C3S hydration
– Slow down the rate of early hydration by extending the
length of induction period (Stage 2), the length of induction
period depends on the amount of retarders added
– Once Stage 3 begins, hydration can proceed normally again
– Organic retarders
• able to strongly adsorb onto the nuclei of CH and inhibit their
growth into large crystals
– Inorganic retarders
• more complex, can form coating around C3S particles that
severely reduce the rate of reaction
• Extended set control
– Retard concrete setting in emergencies in ready-mix trucks
(sugar, carbonated beverages)
– Special admixture to pause hydration until activated by
another admixture (e.g. spray concrete for tunnel lining)
Effect on Concrete Properties
• Retardation increases when the addition of a retarder
to concrete is delayed for a few minutes after the
addition of water
(Mindess et al 2003)
Effect on Concrete Properties
• Strength
– One-day strength reduced (for long delay in setting)
– Approach the strength of control concrete within 8 days
– Increase ultimate strength
• Drying shrinkage and creep
– Lab experiments indicate that admixtures increase the
rate of drying shrinkage and creep, but not the ultimate
values
• Because retarders are sensitive to cement
composition, admixture should be evaluated with the
cement that is to be used on the job
(Mindess et al 2003)
Over retardation
needs protection
from loss of
moisture until
final set
Set-Accelerating Admixtures
• Categories
– Admixtures that accelerate the normal processes of setting &
strength development (speed construction, for winter
concreting)
– Admixtures that provide very rapid setting not normally
associated with OPC (setting in a few minutes, used for
shotcrete, plugging leaks)
• Composition
– Soluble inorganic salts
• Accelerate setting and hardening, e.g. Calcium chloride
(increase reinforcement corrosion, should not be used in
reinforced concrete and prestress concrete), Calcium
nitrite
• Quick setting, e.g. carbonates, aluminates, and ferric salts
– Soluble organic compounds
• Calcium formate, calcium acetate
Mode of Action (Set-accelerating admixtures)
• Conventional accelerators have exactly the opposite
action that retarders have
– the rate of C3S hydration & strength development
– Induction period is shortened, the rate of hydration during
Stages 3 & 4
• In the cases with Calcium salts: increase the rate at which CH
crystals nucleate and grow
• In the cases with organic
accelerators: increase the
rate of C3S hydration
• Quick setting admixtures cause fast setting of C3A by
promoting very rapid hydration
Effects of Accelerating Admixtures on
Concrete Properties
• Cut down the time during which concrete can be handled
• Increase 1-day strength
• Later strength (≥ 28 days) are likely to be lower than the
strength of concrete without accelerating admixtures
• Specifics
– Quick setting admixtures: reduce durability of concrete if
they have adverse effect on strength
Air Entraining admixtures (AEA)
• Used to protect concrete from damage due
to repeated freezing/thawing cycles
• Entrapped air (exist in all concrete)
– Irregular shape
– Size can be up to several mm
• Entrained air voids (introduced into
concrete by using AEA)
– Spherical in shape
– Size ranging from 0.05 to 1.25 mm
• Air-entrained concrete cannot be
distinguished from non-air-entrained
concrete with the naked eyes
• In field, only total air content is controlled
Air Entraining admixtures (AEA)
• AEA , air content of concrete
• Amount of air required for durability purpose
–Mortar: ~9% by volume of mortar
–Concrete: 4-8% depending on the max aggregate size
• Spacing factor ( ) , should be < 0.2 mm
– the average distance from any point in the paste to the
edge of the nearest void
• Specific surface area ()
• Bubble frequency (n)
L
Note: freezing cycle may lead to
spalling of concrete
Air-Entraining Materials
• Containing surface-active agents
that will concentrate on air-water
interface, lower the surface tension
to promote the formation of stable
bubbles (foams)
– Molecules which at one end
have chemical groups that tend to
dissolve in water [hydrophilic –
(water-loving)], while the rest of
the molecules is repelled by water
[hydrophobic (water-hating)]
– The molecules tends to align at
the air-water interface with their
hydrophilic groups in the water
and the hydrophobic portion in air
hobic
(Mindess et al 2003)
Factors Affecting Air Entrainment
• Air entraining potential of an AEA depends on the
concrete materials used and their proportions
• An air-entraining admixture should always be tested
under field condition because the amount of entrained
air it provides can be affected by a variety of factors
• Typical dosage
– 0.005 – 0.05% of active ingredient by mass of cement
– The dosage of the AEA can be adjusted to bring the air
content to within the recommended limits
Effect on Concrete Properties
• Fresh concrete
– Improve workability
• tiny bubbles act as low-friction, elastic fine aggregate that
reduce interaction between large aggregate particles
– Improve cohesiveness, reduce bleeding and segregation
• Hardened concrete
– Detrimental on concrete strength in general
• Rule of thumb: each 1% entrained air will lower strength
by 5%
• However, air entrainment has much less effect or may not
affect the strength of lean concrete (cement < 300 kg/m3)
– There is an optimum amount of air for good durability
• Too high air content generally indicate large bubble size,
will drastically reduce strength and reduce concrete
resistance to stress from freezing
Viscosity modifiers
• Increase the cohesiveness of fresh concrete
• Reduce segregation and bleeding
• Applications
– Concrete placed under water (antiwashout admixtures)
– Concrete or grout placed by pumping (pumping aids)
– Self-compacting concrete
• Materials commonly used
– Polyethylene oxides
– Cellulose ethers
– Natural gums
– Polyvinyl alcohol
BS EN 934-2: 2009 + A1: 2012
(SS EN 934: 2008 to be updated)
• Viscosity modifying admixture
Admixture incorporated in concrete to limit segregation by
improving cohesion
Table 13 – Specific requirements for viscosity modifying admixture
No Property Reference
concrete
Test Method Requirements
1 Segregated
portion SR
prEN 480-15
EN 12350-11 Control mix ≥ 15 % and SR ≤ 30 %
Test mix SR ≤ 70 % of the value
obtained with the control mix
2 Compressive
strength
EN 13390-3 At 28 days: test mix ≥ 80 % of
control mix
3 Air content
in fresh
concrete
EN 12350-7 Test mix ≤ 2 % (by volume) above
control mix unless stated
otherwise by the manufacturer
prEN 480-15, Admixtures for concrete, mortar and grout – Test methods – Part 15:
Reference concrete and method of testing viscosity modifying admixtures
EN 12350-11, Testing fresh concrete – Part 11: Self-compacting concrete – Sieve
segregation test
Corrosion inhibitors
• Admixtures that reduce corrosion of steel reinforcement
– Do not inhibit the corrosion reactions completely
– But reduce the rate of corrosion
– Anodic inhibitors are the most widely use
• Calcium nitrite, also used as accelerating admixture
(Mindess et al. 2003)
- Ability to accept
electrons
- effective only at
sufficiently high
concentration
• Alkali-aggregate reaction inhibiting admixtures
(Lithium and barium salts)
– Convert alkali-silica gels to insoluble lithium or barium
salts
– Reduce the expansion associated with the alkali-silica
reaction
• Shrinkage reducing admixtures
– capillary tension that develops within the concrete pores
as it dries
– drying shrinkage
– contact angle between fibers and pore solution
– Improve bonding between fibers and cement paste matrix
Comparison of ASTM and BS-EN Standards
ASTM BS-EN
Chemical
admixtures
C 260 – Spec for
AEA for concrete
934-1: 2008- common requirements
(SS EN 934-1: 2008)
934-2: concrete admixtures
(including AEA, P, SP, accelerating,
retarding admixture + water
retaining & water resisting
admixture)
(SS EN 534-2: 2008)*
BS EN 934-2: 2009 + A1: 2012
934-3: admixture for masonry
mortars
934-4: admixture for grout for
prestressing tendons
934-5: admixture for sprayed
concrete
* To be updated
C 494 – Spec for
Chemical admixtures
for concrete, Type A,
B, C, D, E, F, G
C 1017 – Spec for
chemical admixtures
for use in producing
flowing concrete
(slump >190 mm)