Concrete Aggregates

Care should be taken in chosing the aggregates for a good concrete mix. Some of the below listed recommendations are,

Size of Aggregates :

* The bigger the size of the aggregate, the less is the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required for lubricating the surface to reduce internal friction. 
* For a given quantity of water and paste, bigger size of aggregates will give higher workability.
The above, of course will be true within certain limits.

Shape of Aggregates :

* Angular, elongated or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical shaped aggregates.
* Contribution to better workability of rounded aggregate will come from the fact that for the given volume or weight, it will have less surface area and less voids, than angular or flaky aggregate.
* Not only that, being round in shape, the frictional resistance is also greatly reduced. This explains the reason why river sand and gravel provide greater workability to concrete, than crushed sand and aggregate.

Surface Texture of Aggregates :

* The influence of surface texture on workability is again due to the fact that the total surface
area of rough textured aggregates is more than the surface area of smooth rounded aggregates of same volume.
* Rough textured aggregates will show poor workability and smooth or glassy textured aggregates will give better workability.

Grading of Aggregates :

* This is one of the factors, which will have maximum influence on workability.
* A well graded aggregate is the one, which has least amount of voids in a given volumeOther factors being constant, when the total voids are less, excess paste is available to give better lubricating effect .
* With excess amount of paste, the mixture becomes cohesive and fatty, which prevents segregation of particles.
* Aggregate particles will slide fast each other, with the least amount of compacting efforts.
* The better the grading, the less is the void content and higher the workability. The above is
true for the given amount of paste.

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Concrete Admixtures

Types of Concrete Admixturesare,

1. Plasticizers
2. Superplasticizers
3. Retarders and Retarding Plasticizers
4. Accelerators and Accelerating Plasticizers
5. Air-entraining Admixtures
6. Pozzolanic or Mineral Admixtures
7. Damp-proofing and Waterproofing Admixtures
8. Gas forming Admixtures
9. Air-detraining Admixtures
10. Alkali-aggregate Expansion Inhibiting Admixtures
11. Workability Admixtures
12. Grouting Admixtures
13. Corrosion Inhibiting Admixtures
14. Bonding Admixtures
15. Fungicidal, Germicidal, Insecticidal Admixtures
16. Colouring Admixtures.

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How is Water Added to Concrete Mix in Mixing Process?

We all know the amount of water is very important to have perfect hydration of cement and water/cement ratio is one of the most important factors to controlstrength of concretewithout influencing economy of mix. So the amount of water added to mix should be accurate and no loss of water is expected to have soundconcrete mix.

Graduate bucket is the way in which water is added to concrete in liters. But now-a-days use of graduate bucket is becoming limited as it has possibility to water spillage. Now either vertical tank or horizontal tank is used to measure water. After each batch they are filled up. With this facilities water addition can be controlled to any desire quantities. In many cases, water supply includes water meters. With the help of water meter exact amount of water can be supplied to mixer. 

The latest batching plants have microprocessor controlled sophisticated automatic arrangements to weigh batching. This provides not only exact amount of every constituents of concrete but also provides desire moisture content in aggregate. Sensitive sensor automatically measure moisture content and whenever it find inaccurate moisture content it takes corrective measures to deduct water in excess of desire amount. In many countries of the world this modern batching plants are working for last decade.

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