Chemical Admixtures •
•
Definition (ASTM C 125): a material other than water, aggregate, hydraulic 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) – 1017) – are 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) – 260) – are are added primarily to improve resistance to freezing/thawing cycling Miscellaneous admixtures •
E.g. Viscosity modifiers, corrosion inhibitors, shrinkage reducing admixtures, etc.
(Mindess et al 2003)
Precautions: (1) conform to standards; (2) test using job materials under job-site conditions; (3) ensure accurate batching
(Mindess et al 2003)
Precautions: (1) conform to standards; (2) test using job materials under job-site conditions; (3) ensure accurate batching
Water-Reducing admixtures (plasticizers) •
Increase slump without increase water and cement contents •
•
Achieve desired slump with less water (lower w/c) w /c) •
•
•
to facilitate difficult placements 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* – A* – water water reducing admixture (WRA)
•
Type B - retarding
•
Type C - accelerating
•
Type D – D – water water reducing and retarding
•
Type E - water reducing and accelerating
•
Type F – F – high high range water reducing (superplasticizer, SP)
•
Type G – G – high high range water reducing and retarding
* In general, Type A WRA •
the rate of aluminate hydration (risk of flash set)
•
the rate of C 3S hydration (slowing strength gain).
Constituent Materials for Concrete – Concrete – Admixtures
APPLICATIONS OF PLASTICISING ADMIXTURE Increase workability by simple addition of admixture
Reduce water content with addition of admixture
(+WRA)
MIX A – A – Plasticising Plasticising w/c =x f cu =y Slump > z
CONTROL MIX w/c =x (+WRA), ( –Water) –Water) f cu =y Slump = z (+WRA) ( –Water) –Water) ( –Cement) –Cement)
MIX C – C – Cement Cement saving w/c =x f cu =y Slump = z
MIX B – B – Water-reducing Water-reducing w/c < x f cu > y Slump = z
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 27 O 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 admixtu res 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 consistence
Equal consistence
Equal w/c ratio
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
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 w ater (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 Reference concrete
a When
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
testing at equal w/c ratio requirements for consistence apply only to control mix
b Alternatives c Control d For
Cement content c – kg/m3
a
to be chosen before starting test
mix only. Resulting cement content of test mix may change due to effects of admixture added
high range water reducing/superplasticizing admixture consistence of test mix shall be not less than
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)
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
Vicat Penetration Standard Mortar
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, SiO 2, Al2O3, Fe2O3, MgO, SO3, K2O, Na2O, P2O5, TiO2 main and minor in quantity, but influence is not always proportional to mass fraction, e.g. SO 3, 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
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
Mechanism of Water Reduction Cemen particle
•
Electrostatic repulsion negatively charged organic molecules adsorb primarily at the solid-water interface
–
Water reducing admixture molecules
-Molecules of WRA interact to neutralize surface charges on cement particles and cause all surface to carry uniform charges of like sign Trapped water
Cement particles
Particles repel each other, remain dispersed
–
Mechanism of Water Reduction (cont’d) Cement particles
Water reducing admixture molecules
Freed water •
Steric Repulsion Some of the newer admixtures use negatively charged polymers
–
The bulky nature of the adsorbed molecules can cause additional steric repulsion
–
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
Example
–
Adding a second dose
w/c=0.45
–
Delaying the initial addition
Slump 50mm at 150 min
Low slump loss
W/C = 0.45
Addition of Plasticiser Target slump at time of placing: 50 mm 2.5 hours after addition of water at plant
Target :50 mm
Mindess, 2003
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)
Addition of Plasticiser
(22 O C) HR-N : high range, non-retarding admixture
Source: Dodson, 1990
Effect on the Properties of Fresh Concrete •
Bleeding and air entrainment –
Bleeding •
–
Air entrainment •
•
Overdose of superplasticizer should be avoided
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 –
–
•
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 –
–
•
When WRA is used to lower water requirement, strength
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
Retarders extend Stage 2
(FHWA HIF-07-004)
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 •
–
Inorganic retarders •
•
able to strongly adsorb onto the nuclei of CH and inhibit their growth into large crystals more complex, can form coating around C 3S 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
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
Over retardation needs protection from loss of moisture until final set
(Mindess et al 2003)
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 (
•
L
) , 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)
•
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) •
hobic
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
–
(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
1
Segregated portion SR
2
Compressive strength
3
Air content in fresh concrete
Reference concrete
Test Method Requirements EN 12350-11 Control mix ≥ 15 % and SR ≤ 30 % Test mix SR ≤ 70 % of the value obtained with the control mix
prEN 480-15
EN 13390-3
At 28 days: test mix ≥ 80 % of control mix
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
Corrosion inhibitors •
Admixtures that reduce corrosion of steel reinforcement –
Do not inhibit the corrosion reactions completely
–
But reduce the rate of corrosion (Mindess et al. 2003) - Ability to accept electrons - effective only at sufficiently high concentration
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Anodic inhibitors are the most widely use •
Calcium nitrite, also used as accelerating admixture
•
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
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drying shrinkage
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contact angle between fibers and pore solution
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Improve bonding between fibers and cement paste matrix
