Contd. from Part-II….
* Some statistics reveal that the strength of hardened concrete in a project site structure is expected to be only within 55 to 80% of the cube strength results for the same concrete due to practical reasons like difference in manner of placing, compaction, curing etc. So, supposing concrete of M40 grade has been used in a structure and of course, the cube test results in laboratory are fulfilling the strength criteria for M40 grade. Yet, the actual strength the same concrete poured in the structure is expected to achieve a compressive strength of about 55 to 80% (depending on quality standards) of the average compressive strength shown by cubes. These kinds of probable shortcomings are factored in while designing by inducing factor of safety etc.
* Concrete mixes with higher w/c ratio demands aggregates with coarser grading & those with lower w/c ratio needs finer grading.
* Some of the key parameters influencing workability of concrete mix are water content, w/c ratio, fine aggregates to coarse aggregates ratio, grading of aggregates, max. size of aggregates etc. Of these, the first one influences workability the most.
* Some of the factors influencing strength are: w/c ratio (very important), cement characteristics, characteristics & proportion of aggregates, degree of compaction, quality of curing etc.
* For a particular w/c ratio, there always is an optimum cement content that results in the highest 28 day strength for a concrete mix. Increasing cement content beyond that would only be a waste as it may not increase, and in fact may result in reduction of strength.
* The finer the cement, the more the compressive strength of concrete usually is.
* Compressive strength of concrete normally decreases with increase in aggregates to cement ratio by mass.
* When compacting concrete by vibrators care should be taken to complete the compaction process while the concrete is still fresh & sufficiently workable. Delaying compaction, especially when the vibrator needle keeps touching the reinforcement steel inside concrete, damages the bonding between concrete & the surfaces of the rebars, besides resulting in improper compaction. Care also should be taken to avoid overuse of vibrators or overdoing compaction as it may result in bleeding, segregation etc. which affect the quality adversely.