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Many researchers (4) are convinced that any failure under load, whether the loading is compressive, shear or tensile, is primarily by the tensile mode. Their conclusion is that the bond between coarse aggregate and mortar is the weakest link and thus primarily responsible for the tensile strength of concrete. Tensile strength of concrete is determined using the flexural strength test and this is then also specified in South Africa. Please remember that flexural strength depends on the type and content of fibres in the concrete, and is measured as such.

 

Just as compressive strength is affected by cement content, void content, curing and age, so is the tensile strength, but not always to the same degree. The relationship between tensile strength or flexural strength and compressive strength is complex. According to the American Concrete Pavement Association (ACPA) the flexural strength may be taken as

 

 

where ff and fc, the cube strength, are in MPa. Based on tests carried out at C&CI, K varied from 0,73 for crushed Witwatersrand quartzites to 0,48 for rounded pebble aggregate. Based on this relationship, the effect of some other properties on cube strength can be assumed to also affect flexural strength, albeit not to the same extent. Compaction is one of the important factors and it is well known that the presence of 5 % voids as a result of poor compaction may reduce compressive strength by about 30 %(4). The following equation shows the effect of voids in concrete on concrete stiffness:

 

 

where

 

Ec           modulus of elasticity of poorly compacted concrete

V             fractional volume of voids due to poorly compacted concrete

E1            modulus of elasticity of fully compacted concrete.

 

Another two important factors that affect tensile or flexural strength are aggregate properties, which include size, surface texture, shape and modulus of elasticity and age of concrete. Flexural strength is generally less effected by age than the compressive strength.

 

The manufacturing and compaction of good concrete is in vain if proper curing methods are not implemented. This is best illustrated by laboratory studies which showed that the difference between air cured (in the laboratory) and continuous moist cured samples showed the latter to have twice the strength than air cured samples after 180 days. (Fig. 12.8 from Fulton)(4).