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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.

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Page 1: 4 Chemical Admixtures

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.

Page 2: 4 Chemical Admixtures

Precautions: (1) conform to standards; (2) test using job

materials under job-site conditions; (3) ensure accurate batching

(Mindess et al 2003)

Page 3: 4 Chemical Admixtures

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)

Page 4: 4 Chemical Admixtures

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).

Page 5: 4 Chemical Admixtures

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

Page 6: 4 Chemical Admixtures

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

Page 7: 4 Chemical Admixtures

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

Page 8: 4 Chemical Admixtures

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

Page 9: 4 Chemical 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

Page 10: 4 Chemical Admixtures

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%)

Page 11: 4 Chemical Admixtures

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

Page 12: 4 Chemical Admixtures

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

Page 13: 4 Chemical Admixtures

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

Page 14: 4 Chemical Admixtures

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

Page 15: 4 Chemical Admixtures

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

Page 16: 4 Chemical Admixtures

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

Page 17: 4 Chemical Admixtures

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)

Page 18: 4 Chemical Admixtures

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

Page 19: 4 Chemical Admixtures

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

Page 20: 4 Chemical Admixtures

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

Page 21: 4 Chemical Admixtures

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

Page 22: 4 Chemical Admixtures

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

Page 23: 4 Chemical Admixtures

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

Page 24: 4 Chemical Admixtures

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

Page 25: 4 Chemical Admixtures

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

Page 26: 4 Chemical Admixtures

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

Page 27: 4 Chemical Admixtures

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

Page 28: 4 Chemical Admixtures

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

Page 29: 4 Chemical Admixtures

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

Page 30: 4 Chemical Admixtures

Examples of Admixtures – Kao Corporation Japan

Page 31: 4 Chemical Admixtures

Examples of Admixtures – Kao Corporation Japan

Page 32: 4 Chemical Admixtures

Examples of Admixtures – Kao Corporation Japan

Page 33: 4 Chemical Admixtures

Examples of Admixtures – Kao Corporation Japan

Page 34: 4 Chemical Admixtures

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)

Page 35: 4 Chemical Admixtures

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

Page 36: 4 Chemical Admixtures

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

Page 37: 4 Chemical Admixtures

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

Page 38: 4 Chemical Admixtures

Addition of Plasticiser

Source: Dodson, 1990

HR-N : high range, non-retarding admixture

(22 O C)

Page 39: 4 Chemical Admixtures

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

Page 40: 4 Chemical Admixtures

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

Page 41: 4 Chemical Admixtures

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

Page 42: 4 Chemical Admixtures

(FHWA HIF-07-004)

Retarders extend

Stage 2

Page 43: 4 Chemical Admixtures

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)

Page 44: 4 Chemical Admixtures

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)

Page 45: 4 Chemical Admixtures

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

Page 46: 4 Chemical Admixtures

(Mindess et al 2003)

Over retardation

needs protection

from loss of

moisture until

final set

Page 47: 4 Chemical Admixtures

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

Page 48: 4 Chemical Admixtures

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

Page 49: 4 Chemical Admixtures

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

Page 50: 4 Chemical Admixtures

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

Page 51: 4 Chemical Admixtures

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

Page 52: 4 Chemical Admixtures

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)

Page 53: 4 Chemical Admixtures

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

Page 54: 4 Chemical Admixtures

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

Page 55: 4 Chemical Admixtures

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

Page 56: 4 Chemical Admixtures

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

Page 57: 4 Chemical Admixtures

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

Page 58: 4 Chemical Admixtures

• 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

Page 59: 4 Chemical Admixtures

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)