A number of factors affect the relation between two strengths. These are
The relation between the flexure strength and compressive strength depends on the type of coarse aggregate used, except in high strength concrete, because the properties of aggregate, especially its shape and surface texture, affect the ultimate strength in compression very much less than the strength in tension or cracking load in compression. The behavior was confirmed by Knab. In experimental concrete, entirely smooth coarse aggregate led to lower compressive strength, typically by 10 percent, than when roughened. It seems that the properties of fine aggregate also influence the ft/fc´ ratio. The ratio is furthermore affected by the grading of aggregate. This is probably due to the different magnitude of the wall effect in beams and in compression specimens: there surface/volume ratios are dissimilar so that different quantities of mortar are required for full compaction.
age is also a factor in the relation between ft and fc´: beyond about one month, the tensile strength increases more slowly than the compressive strength. So the ft/fc´decreases with time. This in aggrement with the general tendency of the ratio to decrease with an increase in fc´.
The tensile strength of concrete is more sensitive to inadequate curing than the compressive strength, possibly because the effect of non-uniform shrinkage of flexure test beams are very serious. Thus air-cured concrete has a lower ft/fc´ ratio than concrete cured in water and tested wet.
Air-entrainment affects the ft/fc´ ratio because the pressure of air lowers the compressive strength of concrete more than the tensile strength, particularly in the case of rich and strong mixes. The influence of incomplete compaction is similar to that of entrained air.
e) Light-weight concrete
Light-weight concrete conforms broadly to the pattern of the relation between the ft and fc´ for ordinary concrete. At very low strength (300psi) the ratio ft/fc´ can be as high as 0.3, but at higher strengths it is the same as ordinary concrete. However, drying reduces the ratio by some 20% so that in the design of light-weight concrete a reduced value of ft/fc´ is used.
f) Method of Test
As stated above, the tensile strengths of concrete measured by different tests, produce results of varying value. Incidentally, the value of the compressive strength is also not unique but is affected by the shape of the test specimen. So the numerical value of the ratio of the tensile strength to the compressive strength is not the same. For these reasons, in expressing the ratio of the tensile to compressive strengths, the test method must be explicitly stated. If the value of flexural strength is of interest, a factor relating the splitting strength to flexural strength needs to be applied.