What is Admixtures in Concrete ?
Reasons for using Admixtures
- Reduce construction cost.
- Achieve more workability.
- Ensure quality of concrete during stages of mixing, transporting, placing and curing in adverse conditions.
- Overcome certain emergencies during concreting.
- Improve or modify some or several properties of concrete
Technical term to use of admixtures:
- Additional influences Properties admixture may have.
- Physical properties of the material
- Chemical properties of the material such as chlorides, sulphates etc.
- Concentration of active ingredient.
- pH value of admixture.
- Potential occupational hazards for users.
- Conditions for storage and good life of admixture.
- Instructions for preparation of admixture and procedures for introducing it into the concrete mix.
- Recommended dosage under identified conditions, max permissible dosage, and effects of over-dosage.
2. Follow recommended dosage, but run relevant tests to confirm effects with the same materials as will be used in the field.
3. Ensure reliable procedures are established for accurate batching of the admixture, especially with chemical admixtures which may have dosages below 0.1% by weight of cement.
Be aware of the effects the admixture can have on other concrete properties as most affect several concrete properties.
Classification of Admixture
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: Those admixtures that don’t fall under the above categories.
Types of Chemical admixtures
The different types of admixtures are as follows:
1. Accelerating admixtures,
2. Retarding admixtures,
3. Water-reducing admixtures,
4. Air-entraining admixturer
5. Superplasticizing admixtures.
Accelerating admixture are used for increases the rate of hydration of a hydraulic cement, shortens the time of set, or increases the rate of hardening or strength development.
Porpose of use :
1. These are used to increase the rate of strength gain of the concrete.
2. They are used to speed construction permitting earlier removal of formwork,
3. Earlier finishing of concrete surfaces.
4. For Earlier load carrying capacity.
5. These also include admixtures for quick-setting applications, in a few minutes (like shotcreting, plugging leaks and emergency repairs).
6. They can also be beneficial for cold weather concreting.
Often, instead of using a special cement, it is possible to change some of the properties of the more commonly used cements by incorporating a suitable additive or an admixture. In other cases, such incorporation is the sole means of achieving the desired effect. A great number of proprietary products are available: their desirable effects are described by the manufacturers but some other effects may not be known, so that a cautious approach, including performance tests, is sensible. It should be noted that the terms ‘additive’ and ‘admixture’ are often used synonymously, though, strictly speaking, additive refers to a substance which is added at the cement manufacturing stage, while admixture implies addition at the mixing stage.
These are admixtures which accelerate the hardening or the development of early strength of concrete: the admixture need not have any specified effect on the setting (or stiffening) time. However, in practice, the setting time is reduced, this adversely affects strength but makes urgent repair work possible. Other examples of set-accelerating admixtures are: aluminium chloride, potassium carbonate, sodium fluoride, sodium aluminate. and ferric salts.
This type of admixture using for delays the setting of cement concrete, and hence of mixtures, such as mortar or concrete containing cement.
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.
Delaying the introduction of the retarders until the concrete has been mixed (up to about 10 min) can enhance its performance.
This is because some of the hydration reactions have already occurred and this allows the more of the retarder to react with the C3S.
- Effects on Concrete Properties
- Delay the set of the concrete.
- Because some are water reducers, they may increase the amount of entrained air.
- Increase slump.
- They may increase the rate of slump loss though the set has been retarded thus decreasing the time available for placing
An admixture which either increases workability of freshly mixed mortar or concrete without increasing water content or maintains workability with a reduced amount of water.
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
- Improve durability.
Alternately it may be used to maintain the same w/c ratio but increase workability for difficult placement.
These admixtures arc used for following purposes:
- To achieve a higher strength by decreasing the water/cement ratio at the same workability as an admixture-free mix.
- To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.
- To increase the workability so as to case placing in inaccessible locations.
There are used primarily to improve freeze-thaw durability. Air entrainment refers to the introduction of large quantities of tiny air bubbles in the concrete matrix. The main reason for air entrainment is to improve the durability of the concrete to freezethaw degradation. The Air-Void System As unreacted water freezes it expands 9 % by volume on phase change. This internal volume expansion causes internal stresses in the matrix. It can generate cracks in the concrete, which may allow water to infiltrate and the process can get progressively worse. It can lead to significant degradation of the concrete.
The formation of ice in the pore spaces generates pressure on any remaining unfrozen water. Introducing a large quantity of air bubbles provides a place for this water to move in to relieving the internal pressure. What is desired is to generate very many small air bubbles well distributed throughout the matrix rather than a smaller number of larger bubbles.It’s been determined that the optimum air content for frost protection is about 9% by volume of the mortar fraction.
With respect to the concrete volume, the air content should be in the range of 4-8% by volume. The concrete normally has entrained air, the admixture increases the total volume of the air voids by 3-4% of the concrete volume. Total air content is only a part of the formula for frost resistance. The nature of the entrained air is equally important. The critical parameter of the air-entrained pasteis the spacing factor (max distance from any point in the paste to the edge of a void). It should not exceed 0.2 mm; the smaller the spacing factor the more durable the concrete. The air bubbles themselves should be in the range of 0.05 – 1.25 mm in diameter.
Air Entraining Materials What is needed is an agent that causes the water to foam into a very small matrix of very small bubbles.
The admixtures are of the same family as household detergents, but these do not generate small enough bubbles and are not stable enough. Air entraining agents contain surface-active agents or surfactants. These lower the water surface tension so bubbles can form, and stabilize the bubbles once they are formed. Increasing the admixture dosage will increase air content, decrease bubble size, and decrease spacing factor. Thus decreasing the total strength of the concrete. Effect of Air on Other Concrete Properties
- Increase workability and cohesiveness of fresh concrete.
- Considerable reduction in bleeding and segregation.
- Decreased strength (10-20% for most air entrained concrete)
- Increased durability
- If a lower w/c ratio is used to account for the increased slump, some of the strength reduction will be offset.
- In addition, the lower w/c ratio that can be used and the better compaction
- characteristics results in more impermeable concrete and a better overall resistance to
- aggressive agents (i.e. sulfates).
These are linear polymers containing sulfonic acid groups. Two major commercial formulations
- Sulfonated melamine-formaldehyde condensate; and
- Naphthalene sulfonate-formaldehyde condensate
Effect on Other Concrete Properties
- Improved workability of fresh concrete (flowing concrete with use of superplasticizers, SP)
- Some types may increase bleeding (hydroxycarboxylic acids).
- They tend to increase air entrainment (so less air entraining admixture can be used) Tend to retard set times.
- Rate of slump loss increases with normal-range water reducers about same for superplasticizers.
- Increased compressive strength due to ability to reduce w/c ratio and better dispersion of cement in paste
- Increased durability due to lower w/c ratio.
- SP Rapid strength gain without increased heat generation.
- SP used for high strength concrete.
- These are a more recent and more effective type of water-reducing admixtures known in the US as high range water reducers and called. There exists also a high range water-reducing and set-retarding. The dosage levels are usually higher than with conventional water-reducers, and the possible undesirable side effects are considerably reduced. For example, because they do not markedly lower the surface tension of water, superplasticizers do not entrain a significant amount of air. Superplasticizers are used to produce flowing concrete in situations where placing in inaccessible locations, in floor or pavement slabs or where very rapid placing is required. A second use of superplasticizers is in the production of very-high strength concrete, using normal workability but a very low water/cement ratio. resulting flowing concrete is cohesive and not subject to excessive bleeding or segregation, particularly
if very angular, flaky or elongated coarse aggregates are avoided and the fine aggregate content is increased by 4 to 5 per cent. It should be remembered, when designing formwork. that flowing concrete can exert full hydrostatic pressure. When the aim is to achieve high strength at a given workability, the use of a superplasticizer can result in a water reduction of 25 to 35 per cent (compared with about one-half that value for conventional water-reducing admixtures).
Test of Chemical Admixture
- Dry Material Content
- Ash content
- Relative density of liquid admixture
- PH value of admixture
Type of Mineral Admixtures
- Silica Fume
Read more :
- Test of Concrete
- Dry content test of admixture