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Heading: 1.3.1.1Aggregates for Concrete
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Hi All

Has anyone can provide me with feedback on aggregate coating -mainly fines stuck to coarser aggregates and the adverse effect on the compressive strength of concrete?

What are allowable limits for aggregate coatings on coarse aggregates ?

What test carried can be carried out to determine the extent of coatings on coarse aggregates ?

Thanking you

Regards

Vikram


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Vikram Gunnoo


Dear Vikram

If it is fines stuck to the coarse aggregate then you need not worry provided it is within the limitations provided in IS383 i.e 3% by weight 75microns passing (wet sieving).
If it is more than this pl consider the same in all-in-aggregate grading and adjust your fine content so that the fresh concrete properties remains same.

kannan
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LALGUDI SUBRAMANIAM


Vikram, There is a substantial difference between coatings which continue to adhere to the coarse aggregate and those that wash off into the mortar.
Continued adherence is likely to affect bond and to have a substantial effect on compressive (and even more so tensile) strength.
Fines which wash off, providing there is no organic impurity, will simply tend to increase water requirement. I would agree with kannan that up to 3% of non-organic fines will not make any substantial difference to water content but beyond that you are likely to see a linear relationship between % fines and water requirement. No, I may be wrong here, I am thinking 3% of fines in sand and there may be twice as much coarse aggregate in the mix as sand, so the problem may start at 1 to 2% depending on the sand/aggregate ratio.
You need to wash the aggregate by vigorous shaking in water, weighing before and after, to establish how much material was adhered and then use the washed aggregate in a trial mix to compare to unwashed.
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Ken Day


Hi Ken & kannan

Thanks for your response.

Initially i carried out trial mix using two different aggregates keeping the same mix and the initial slump the same. For the same slump the water demand for the aggregates with high fine content was greater. I carried out wet sieving of the coarse aggregates and for 6/10mm aggregates the percentage of fines passing the 75microns was 3.5% as compared with aggregates from different location (1.5%).

Eventually to achieve the same slump the water/cement ratio of the mix was increased leading to lower compressive st.

Excessive fines in aggregates also leads to higher risk of drying shrinkage.

Regards

Vikram


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Vikram Gunnoo
 
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