The ingredients, which are added to transit mixer at small amounts during or before the production with the purpose of imporving the characteristics of concrete, are called admixtures. Admixtures can be classified into two types as chemical and mineral admixtures.
1-Chemical Admixtures
Specific types of chemical admixtures are listed below.
a) Water reducers (Plasticizer)
They ensure that the same consistency or processability can be achieved with less water. As the amount of water used in fresh concrete is reduced, the strength of concrete increases. They are classified as normal and super in proportion to the water they reduce.
b) Retarders
They retards the setting of fresh concrete. They are useful to concrete transported in long distances or hot weather placements.
c) Set accelerators
Unlike the retarders, these admixtures shortens the time for the setting of concrete. In some applications, they are used to solidify the concrete in early stripping and cold weather placements before frost.
f) Waterproof admixtures
They entrain the air at limited rate, however, watertightness of placed concrete depends on the good application of proper placement technique. In some concretes, more than one admixture can be used. However, it should be confirmed that these admixtures don’t have negative effects on each other. Chemical admixtures have become indispensable to concrete in construction industry due to its effects mentioned above.
2) Mineral admixtures
Ingredients, which are stored in silos after being powderized, such as slag, fly ash, silica fume and fine dust etc, are called “Mineral admixture”. Mineral admixtures do not have a binding effect by themselves like cement, but when used tohether, they function in a similar way to cement, thus provides the cement saving. Meral admixtures are also used in high strength concrete production.
In TS EN 206, chemical admixtures are described as “the ingredients which are added to concrete in smaller percentage than the cement mass during the mixing process with the purpose of imporving some characteristics of fresh or hardened concrete”. But, it does not contain inert or puzzolonic mineral admixtures(like silica fume). But these ingredients can be found in chemical admixture.
Today, it is widely accepted that chemical admixtures contribute to the durability and costs of concrete structures. The features that chemical admixtures improve can be stated as the facilities provided in concrete processability and compacting, reduction of the transparency of hardened concrete and increase in freeze-thaw resistance.
Features of chemical admixtures are determined in standard series for Concrete, Mortar and Grout, TS EN 934-Chemical Admixturew.
Part 2- Concrete Admixtures
Part 3- Mortar Admixtures
Part 4- Grout Admixtures
Part 5- Shorcrete Admixtures
This publication only including Part 2 concrete admixtures.
Use of chemical admxitures is determined by EN 206.
TS EN 934-2 includes the admixtures in following types:
– Water reducer/plasticizer admixtures
– Strong Water reducer/plasticizer admixtures
– Water retaining admixture
– Air entraining admixture
– Set accelerator admixture
– Retarder admixture
– Watertightness admixture
– Retarder/water reducer/plasticizer admixtures
– Retarder/strong water reducer/super plasticizer admixtures
– Set accelerator/water reducer/plasticizer admixtures
Other types of admixtures in use don’t include TS EN 934-2. But if the conformity with TS EN 206-1 is provided, they can be used.
d) Anti-freeze
It makes the water freezing harder and prevents the deterioration in strength of cement as a result of frost. Amount of these admixtures in concrete can be adjusted according to the air temperature.
b) Air entraining admixtures
They increase the frost resistance, tightness and processability of concrete by forming uniform and very small sized air bubbles in concrete.
- – Corrosion preventive
- – Shrinking preventive
- – Submerged concrete admixtures
- – Precast concrete admixtures
Features of Concrete Admixtures
Apart from special concretes like high strength concretes, maximum dosage is limited by TS EN 2006 in article 5.2.7. as 50g/kg in accordance with cement mass. Chemical admixture with dosage less than 2g/kg should be added by being mixed with water.
Performance of concrete admixtures are controlled by TS EN 934-2 according to main functions for single function admixtures, main and secondary functions for double function admixtures. These performance conditions are given in Table 1.
In addition to these general performance conditions, there ar4e general conditions applied to all chemical admixtures. These are;
• effect on setting time
• effect on pressure strength
• effect on air content of concrete
• water soluble chloride amount
• Na2O equivalent alkali amount
Performance of chemical admixtures is determined according to reference concrete. Test mixture(with chemical admixtures) is compared with control mixture(without admixture). Chloride and alkali contents are measured and declared. Testing procedures for admixtures are determined in Testing Methods for Concrete, Mortar and Grout, TS EN 480-Chemical admixtures
Maximum chloride content allowed in concrete is determined in TS EN 206-1 and contribution of chemical admixtures to these amounts can be calculated. TS EN 934-2 limits the maximum chloride conctent to 0.1% or the declaration of producer.
Maximum alkali concrent should be declared by producer for each chemical admixture in compliance with TS EN 934-2.
| Chemical admixture type
|
Performance condition
|
Value in TS EN 934-2
|
| Water reducer / plasticizer admixtures
|
Uniform water reduction in consistency
|
reduction ³%5
|
| Strong water reducer / super plasticizer admixtures
|
Uniform water reduction in consistency
Uniform consistency increase in water/cement ratio |
reduction ³%12
slump increase ³120mm
|
| Water retaining admixtures
|
Reduction in desorption
|
reduction ³%50
|
| Watertightness admixtures
|
Reduction in capillary dissolving
|
Mass reduction ³%50
|
| Air entraining admixtures
|
Airtightness features of hardened concrete
|
Gap factor £0.200mm
|
| Set accelerator admixtures
|
Reduction in initial setting | reduction ³%40 at 5°C
|
| Hardeninc accelerator admixtures
|
Pressure strength on 1. day
Pressure strength on 2. day
|
Increase ³%20 at 20°C
Increase ³%30 at 5°C
|
| Retarder admixtures
|
Increase in initial setting and final set times | setting increase ³90 minutes
final set increase £360 minutes
|
| Retarder / water reducer / plasticizer admixtures
|
Uniform water reduction in consistency
Increase in initial setting and final set times
|
reduction ³%5
setting increase ³90 minutes
final set increase £360 minutes
|
| Redarter / strong water reducer / super plasticizer admixtures
|
Uniform water reduction in consistency
Uniform consistency increase in water/cement ratio
Increase in initial setting and final set times
|
reduction ³%12
slump increase ³120mm
setting increase ³90 minutes
final set increase £360 minutes
|
| Set accelerator / water reducer / plasticizer admixtures | Uniform water reduction in consistency
Reduction in initial setting time
|
reduction ³%5
reduction in ³30 minutes and at 20°C
and at ³%40 5 °C
|
Description of Concrete Admixtures
Designed Concrete
Admixtures are used in normally designed concrete. Concrete producer should be free to select the most appropriate admixture to meet the determined concrete features. Type of the admixture used in ready mixed concrete is stated in dispatch note.
Admixtures in designed concrete are commonly used to improve the features below:
pressure strength
• consistency
• density
• strength formation
• hardening retarding
• watertightness
• other features( e.g. accelerated hardening, early high strength formation)
Prescribed concrete;
In TS EN 934-2 Part 3.1.3., it is stated that “use of recommended amounts will not mean that conformity with this standard will cover all range. In order to find the amount necessary for reaching the desired result, ingredients to be used in worksite should be tested”. Concrete producer can have these test results before determining an admixture type and dosage for a specific concrete.
Specification preparer should take the issue below into consideration;
- Same types of admixtures from different producers can provide the expected effect at various dosages.
- Effects of admixtures differ in terms of cement, mineral admixtures and aggregated used in the concrete.
Determined dosage should be in the recommended dosage range with specified types and brands. This information is provided by admixture producer.
When a combination of admixtures is used, compatibility and performance of admixtures should be controlled.