Concrete Mix Design

[gkgyedu]

I am trying to prepare trial mixes to achieve a specified strength class with some given aggregates and CEM1 and I have a couple of questions
1) Is it necessary to weigh the aggregates oven dry (if so, how come the aggregates for batching are not weighed oven dry)?
2) Can I go ahead and perform the mix design without performing a test for absorption on the fine aggregates (the local labs are not equipped to do absorption on fine aggregates)

I also would like to have the reference to the BS EN codes for concrete mix design.

Help would be much appreciated
Thanks

 

[Ron]

Aggregates sbhould be weighed in a saturated, surface dry (SSD) condition, not oven dried.

Absorption is a simple test. If you have a scale and an oven you can do it. If the laboratory doesn't have at least this minimal equipment, don't use them. In the absence of absorption data, most fine aggregates from a siliceous origin will have an absorption of about 1 percent.

 

[gkgyedu]

Thanks Ron But do you have the references to the codes I mentioned?

 

[Ron]

Sorry, no. I'm in the US and do not use those codes.

Further, I should have given a more complete response to the original post.

A concrete mix design is done under the premise that both the coarse and fine aggregates are in a saturated, surface-dry condition at the time of batching. SSD is a very transient condition which is why we need to know the absorption of the aggregates and their actual moisture content at the time of batching. This allows us to adjust the amount of water that we add to the mix to bring the aggregates back to the SSD condition and maintain the designed water-cement ratio. For example, if the actual moisture content is below the absorption we have to add an amount of water to the mix to compensate for the deficient moisture. Conversely, if the actual moisture content is above the absorption value we reduce the amount of added water so that we don't compromise the water-cement ratio and reduce the strength of the concrete.

 

[gkgyedu]

Thanks Very much Ron, This is what I want to know, but can you describe to me how to conduct the absorption test on fine aggregates and also how to determine the SSD condition of fine aggregates. what we do here with coarse aggregates is that we clean each on with a rag to be sure there's no moisture on the surface but I cant see how we can do that with fine aggregates.

 

[Ron]

gk....I'm not familiar with the BS EN methods, but in the US we use ASTM standards (ASTM C128). For fine aggregate, towels are not used as they are with coarse aggregate. A fan may be used for drying the fine aggregate. You have to keep the sample moving around to remove the surface water without drying the aggregate internally. The fine aggregate testing employs a picnometer and a special mold (like a slump cone) for checking the SSD condition of the fine aggregate.

I believe the equivalent BS is 812; but you should check that.

 

[podosci]

sorry to deviate abit but i have a Question.
Whats the correct mix ratio to acchiev 25Kn/m3 concrete strength?

 

[Ron]

Podosci....please start a new thread for your question and make your question more specific. As it is, your question is too general to answer with any reasonable degree of certainty.

 

[BigH]

To paraphrase BS 812 Part 2 - see attached

 

[robyengIT]

here attached the complete list of Standards ruling in Europe for concrete and aggregates (i.e. EN Standards).
As far as I know there is no standard for mix design but only literature and practice, as it is obvious

 

[Gammyleg]

Just one other point to mention, the water/cement ratio is calculated on the amount of free water in the mix and does not include the absorbed water in the sand and aggregate.
As mentioned previously there is no EN standard for mix design as such, the closest you will get is BS-EN 8500.