Question

3. Define minimum strength. Discuss why minimum strength is considered more critical than mean strength. What steps are taken during concrete mix design to meet minimum strength requirements? (10 points)

Answer 1

Minimum strength -   Minimum strength refers to the characteristic compressive strenth i.e. strength of concrete mix below which not more than 5% of the sample's compressive stregth fall. eg. in concrete of grade 30Mpa, 30Mpa is the minimum strength of concrete.

Why minimum strength is considered more criticle than mean strength -   in concrete mix design minimum strength of concrete i.e. compressive strength of concrete is more criticle than mean strength because the value of mean strength in concrete mix design is kept always heigher than the minimum strength . The value of mean strength is obtained from local codal provisions , mean strength depends upon the standard devation of minimum strength of concrete and minimum strength itself.

Therefore in concrete mix, if minimum strength is not achieved , mean strength can never be achieved . That's why minimum strength is more criticle than mean strength.

STEPS TO BE TAKEN IN CONCRETE MIX DESIGN TO MEET MINIMUM STRENGTH REQUIREMENT- Following steps can be taken to achieve minimum strentgth requierement- 1. Water to cement ratio should be kept proper. 2. Cement content should be kept proper . 3. Aggregate proportioning should be kept proper. 4. Placing of concrete should be done in a proper manner. 5. While placing of concrete , compaction of concrete should be done properly so that minimum air voids should remain in concrete and maximum strength can be achieved. 6. Use of flaky, elongated as well as rounded aggregates should be avoided as these reduces the strength of concrete. 7. Ratio of coarse to fine aggreagete should be kept proper as it affects water demand of concrete which further affects the minimum strength of the concrete. 8. Curing of concrete should be done properly because the most important factor for the minumim strength of the concrete is the curing . 9. Rate of loading should be kept heighr on the structure because higher rate of loading leads to higher strength of concrete as there is less time for creep. 10. grading of coarse aggreagate as well as fine aggregate should be done properly as it affects the strength of concrete significantly.