Concrete is the most famous building material in the world of building due to its great properties and availability all around the world, it is consist of crushed stone in different sizes and percentages mixed together with cement and water to have concrete mix. But with the modern technology in the world of building and the increase need for improved properties of concrete to match with these modern buildings such as when casting concrete for some members in the building that have a very dense steel bars, we have to improve the workability of concrete by some additive materials called concrete admixtures. Admixtures added to concrete can modify its properties on both the fresh and hardened stages, but it is used mostly to modify the properties of fresh or plastic concrete.
Concrete admixtures have been around for many years, but in the recent years there has been an extensive development for a new admixture. It is possible that almost all concrete used these days contains some type of admixture. Admixtures can be employed to entrain air for free-thaw resistance, to accelerate or retard setting time, to control strength development, to achieve shrinkage compensation and to improve workability. Most of unwanted effects of early admixtures have been overcome. The most dramatic development has been in the area of superplasticizers or high range water reducers. These products now allow high level of water reduction without a loss in workability. Whilst early superplasticizers had a limited time over which they were effective, modern superplasticisers allow sufficient control for the setting in, for example, a large pour, until all the concrete has been placed.
When making a concrete mix design care should be taken to use the same admixture for both the mix design and the concrete to be used for the building due to the different in there ingredients for each material. So chemical admixtures are material in the form of powder or fluids that are added to concrete to give it certain properties that will not be achieved with plain concrete. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.
What types of chemical admixtures are available today?
1. Accelerators: speed up the hydration (hardening) of the concrete.
2. Retarders: slow the hydration (hardening) of the concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
3. Air-entertainers: add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increase the concrete’s durability.
4. Plasticizers: (water-reducing admixtures) increase the workability of plastic of fresh concrete, allowing it is placed more easily with less consolidating effort.
5. Superplasticizers: (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when use to significantly increase workability. Alternatively; plasticizers can be used to reduce the water content of a concrete (and have been called water reducer due to this application) while maintaining workability. This improves its strength and durability characteristics.
6. Pigments: can be used to change the color of concrete, for aesthetics. Mainly they are ferrous oxides.
7. Corrosion inhibitors: are used to minimize the corrosion of steel and steel bars in concrete.
8. Bonding agent: is used to create a bond between old and new concrete.
9. Pumping aids: improve pumpability, thicken the paste and reduce dewatering of the paste.
Finally the different types of admixtures will result to the differentiation of concrete types.
This article written by Taha Mesleh working as an experience civil engineer in the field of building and contracting , if you want to read similar articles visit:construction-and-architecture.plogspot.com
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