Admixtures

Admixtures

In concrete, a substance other than active and inert matter, added in small quantities to the mix to alter its properties.

Admixtures are additions to the mix used to achieve certain goals.

General Groupings of admixtures

Chemical Admixtures: There are water soluble compounds added primarily to control setting and early hardening of fresh concrete or to reduce the water requirements.

§  Air-entraining agents: These are used primarily to improve freeze-thaw durability. Air entraining agents entrains microscopic air bubbles in the concrete: when the hardened concrete freezes, the frozen water inside the concrete expands into these air bubbles instead of damaging the concrete. For example Vinsol resin and Dadex

Mineral admixtures: There are finely divided solids to improve workability, durability, or provide additional cementing properties. (i.e slags, silica fume, fly ash, and pozzolans).

Miscellaneous admixtures: The admixtures that don't fall under the above categories.

Chemical Admixtures

􀁸Type A: Water-reducing (WR)

􀁸Type B: Set retarding (SR)

􀁸Type C: Set accelerating (SA),

􀁸Type D: WR + SR

􀁸Type E: WR + SA

􀁸Type F: High-range water-reducing (HRWR)

􀁸Type G: HRWR + SR

Water-Reducing Admixtures

These admixtures lower the water required to attain a given

slump, thus lowering the w/c ratio. This will:

§  Improve the strength

§  Improve the water tightness (permeability)

§  Improve durability.

Alternately it may be used to maintain the same w/c ratio but increase workability for difficult placement.

Typical reductions in water requirements are 5-10%

There are admixtures called "superplasticizers" or "high range water reducers" which can reduce water contents by 15-30%.

The water reducers induce the electronegative charges on the fine cement particles allowing them to disperse more readily in the water. (similar to the use of Calgon in hydrometer tests). This reduces the tendency for flocculation of the cement particles in the paste.

Composition

Three General Categories

1. Salts and derivatives of lingo sulfonates.

2. Salts and derivatives of hydroxyl carboxylic acids.

3. Polymeric materials.

Accelerators:are added to concrete to reduce setting time of the concrete and to accelerate early strength. The amount of reduction in setting time varies depending on the amount of accelerator used.

§  These are used to increase the rate of strength gain of

the concrete.

§  They are used to speed construction permitting earlier removal of formwork, earlier finishing of surfaces, or earlier load carrying capacity.

§  These also include admixtures for quick-setting applications, in a few minutes (like shotcreting, plugging leaks and emergency repairs).

§  They can also be beneficial for cold-weather concreting.

Composition

There are 2 general Groups:

1. soluble inorganic salts (CaCl₂, carbonates, aluminates, fluorides, and ferric salts)

2. soluble organic compounds (triethanolamine, calcium formate, calcium acetate)

Calcium chloride is the most popular choice due to low

cost and high rate of acceleration for a given dosage.

Retarding Admixtures

Generaly used for:

§  Offset effects of high temperature which can

decrease setting time.

§  Avoid complications when unavoidable delays may

occur between mixing and placing.

§  Resist cracking of recently poured concrete due to

form deflection during successive pours.

The retarders slow the rate of early hydration of C₃S

by extending the length of the dormant period. They

also tend to retard the hydration of C₃A phases

Composition

1. Salts and derivatives of lignosulfonates.

2. Salts and derivatives of hydroxycarboxylic acids.

3. Sugars and their derivatives (a bag of sugar mixed in a truck of concrete can stop the set in case of emergency!).

4. Inorganic salts.

Note 1&2 are also water reducers.

Effects on Concrete Properties

1. Delay the set of the concrete.

2. Because some are water reducers, they may increase the amount of entrained air.

3. Increase slump.

4. They may increase the rate of slump loss though the set has been retarded thus decreasing the time available for placing.