Date post: | 03-Apr-2018 |
Category: |
Documents |
Upload: | karam-jaradat |
View: | 232 times |
Download: | 0 times |
of 21
7/28/2019 Admixtures 8
1/21
Admixtures
Additive: Substances which are added at the cement
manufacturing stage (pozzolan, gypsum, etc.
Admixtures: Ingredients in concrete other than Portlandcement, water, and aggregate that are added to the
mixture immediately before or during mixing
7/28/2019 Admixtures 8
2/21
Accelerators
Accelerate the hardening or the development of early
strength of concrete.
Has little effect on setting time.
Most common type is calcium chloride (CaCl2).
Accelerators are used for early form removal, cold
weather and urgent repair.
CaCl2 may be added to different types of Portland
cements.
7/28/2019 Admixtures 8
3/21
Dosage: 1 to 2 percent of cement weight (dissolved in
mixing water).
CaCl2 acts as catalyst in the hydration of C3S and C2S.
Side Effects:
1. Reduces the resistance of cement to sulfate attack.
2. Increase the risk of alkali-aggregate reaction.
3. Increase shrinkage and creep of concrete.4. Increase the potential of steel corrosion in reinforced
and prestressed concrete. ( ASTM and BS limit the used
of CaCl2 in concrete).
7/28/2019 Admixtures 8
4/21
Set-accelerators (quick-setting)
Reduce the setting time.
Used in shotcreting and for urgent repair.
Examples: sodium carbonate(washing soda), potassium
carbonate, aluminum chloride.
Side effect: reduce strength.
7/28/2019 Admixtures 8
5/21
7/28/2019 Admixtures 8
6/21
o The increased retardation occurs especially with cements
which have a high C3A content because, once some C3A hashydrated, it does not absorbs the admixtures; it is therefore
available for reaction with calcium silicates.
o The use of retarding admixtures reduces the early strength
but later the rate of strength development is higher, so that
the long-term strength is not much different.
o Retarders tend to increase the plastic shrinkage because
the plastic stage is extended, but drying shrinkage isunaffected.
7/28/2019 Admixtures 8
7/21
Water-reducers (plasticizers):
These admixtures are used for three purposes:
1. Achieving higher strength by decreasing the w/c ratio.
2. Decreasing the cement content as to reduce the heat of
hydration in mass concrete, while maintaining workability
and strength.
3. Increasing the workability so as to ease placing of concretein inaccessible locations, pumping, tremie.
7/28/2019 Admixtures 8
8/21
Classification of Water reducer according to ASTM C494 (see
table 8.2) as follows:
Type A: water-reducing only
Type D: water-reducing and retarder
Type E: water-reducing and accelerator
Type F: high-range water reducing (superplasticizer)
Type G: high-range water reducing and retarder
7/28/2019 Admixtures 8
9/21
How water-reducer works?
The principle active components of water reducing admixtures
are surface-active agents.
The surface-active agents are absorbed on the cement particles,
giving them negative charge, which leads to repulsion betweenthe particles and results in stabilizing their dispersion; air
bubbles are also repelled and cannot attach to the cement
particles.
Lubricate the mix so that the workability is increased
7/28/2019 Admixtures 8
10/21
Water-reducer basic materials:
Hydroxylated carboxylic acid
Lignosulphonic acid
The reduction in water content for these materials rangefrom 5 to 15%, depending on cement content, aggregate
type, pozzolan and air-entraining agent if present.
Typically the dosage rate of Type A water-reducers rangefrom 130 to 390 mL per 100 kg of cementitious materials.
Higher dosages may result in excessive retardation of the
concrete setting time.
7/28/2019 Admixtures 8
11/21
Other Advantages:
1. The dispersing ability of these materials results in a greater
surface area of cement exposed to hydration, for this reason
there is an increase at early strength compared with a
control mix (without water-reducer) with same w/c ratio.
2. Long-term strength may also be improved because of a more
uniform distribution of the dispersed cement throughout the
concrete.
Side effects: possible bleeding (Hydroxylated carboxylic acid
type)
7/28/2019 Admixtures 8
12/21
7/28/2019 Admixtures 8
13/21
Superplasticizer are usually used to
1. Produce flowing concrete in situations where placing in
inaccessible locations such as:
Thin sections
Congested reinforcing steel
Tremie Pumped concrete
1. Produce very high strength concrete (strength as high
as 100 MPa), using normal workability but a very low
w/c ratio.
A water reduction of 25 to 35% can be obtained
7/28/2019 Admixtures 8
14/21
7/28/2019 Admixtures 8
15/21
The improved workability produced by superplasticizer is of
short duration and thus there is a high rate of slump loss; after30 to 90 min the workability return to normal. Due to this
slump loss, these admixtures are often added to the concrete at
the jobsite.
Dosage:
1 to 2% of cement weight, for example using 2% raises the
slump from 30mm to 200 mm, or reducing the w/c ratiofrom 0.60 to 0.45 for same workability
7/28/2019 Admixtures 8
16/21
7/28/2019 Admixtures 8
17/21
Tests have shown that some plasticized concrete bleedmore than do control concretes of equal w/c ratio. But
bleed significantly less than do control concretes of equal
high slump.
Superplasticizers do not significantly affect the setting of
concrete except in the case of cement with a very low C3A
content when there may be excessive retardation.
7/28/2019 Admixtures 8
18/21
AIR-ENTRAINING ADMIXTURES
Air-entraining admixtures are used to purposely introduce
and stabilize microscopic air bubbles in concrete.
Air entrainment will dramatically improve the durability of
concrete exposed to cycles of freezing and thawing.
Entrained air can be produced in concrete by use of an air-
entraining cement, by introduction of an air-entraining
admixture, or by a combination of both methods.
An air-entraining cement is a Portland cement with
an air-entraining addition interground with the clinker
during manufacture..
7/28/2019 Admixtures 8
19/21
7/28/2019 Admixtures 8
20/21
Other admixtures and additives
1- Mineral additives/admixtures
Pozzolanic materials: fly ash, silica fume, blast furnace
slag..etc.
Inert materials: hydrated lime, limestone dust as workability
aids for grouts and masonry mortar, Coloring pigments are
also considered as inert admixtures.
2- Gas forming admixtures: such as powdered zinc or
aluminum. Used to produce aerated concrete (gas concrete)
used as lightweight concrete for insulation purposes.
7/28/2019 Admixtures 8
21/21
3- Bonding Admixtures: polymer emulsions (latexes) used
to improve the bond of fresh concrete to old concrete.
4- Water-repellent admixtures: used to reduce the
concrete permeability (water tanks)
5- Corrosion inhibitors (calcium nitrite, sodium nitrite,
6- Shrinkage reducers (propylene glycol )
7- Alkali-silica reactivity inhibitors(barium salts)
8- Colouring admixtures (coloured pigments: blue, red
.etc.)