If there is overdosing of admixture then there is chance of segregation,shrinkage and other many problems and company also only provides dosages as 0.5-1.5% of cement in their certificate so how can we know the exact dosage of admixture for concrete??
Dosage of admixture for concrete is very crucial parameter depending upon your requirement of fresh properties of concrete. Depending upon your workability requirement at site (Methods of transportation, pouring, compaction etc...). Selection of admixtures is also important along with dosage based on concrete grades, workability requirements etc...
To decide dosage of admixtures based on requirement, trials need to be done or marsh cone test can be useful for that...
Admixture percentage depends on the compatibility of admixture with the cement and therefore some trial may be made for workability on slump, Vebe tests to ensure that there is no segregation and bleeding of concrete. This will ensure right doses of admixtures in normal concrete. However, if you are making self compacting concrete then there are different test to ensure compatibility and doses of admixtures.
Imo, you need to make initial trials for your particular mix. The effectiveness of the admixture depends on many factors, ranging from the w/c value, through the type and class of cement and the grain size of the aggregate. There is no one answer.
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