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Both strength and stiffness of concrete are of importance in pavement design. The correct way to determine stiffness is through appropriate testing at the correct stress level. Under comparable conditions and for loads less than 50 % of the ultimate strength the static moduli of elasticity from testing the concrete in tension, flexure and compression are approximately the same. Fulton(4) proposes the following equation to derive elastic modulus:

 

 

where

 

E       =      elastic modulus of concrete at 28 days

f        =      concrete characteristic (cube) strength at 28 days (MPa)

a       =      a coefficient

K       =      an aggregate stiffness factor related to the elastic modulus of the aggregate and its volume concentration.

 

Based on the above information and taking into account that the 28 day flexural strength of concrete normally used in road pavements is between 3,7 and 5,5 MPa, the following equation may be used to determine the relationship between the flexural strength and the elastic modulus after more than one year:

 

 

where

 

C     constant that varies from 8 800 for 3.7 MPa flexural strength to 6 700 for 5.5 MPa flexural strength,

f     flexural strength of concrete (MPa).

 

In the above equation it is assumed that properties are determined at 28 days and that the gain with time in cube strength is more than that of flexural strength. The above equation, however, does not take poorly compacted or poorly cured concrete into account, neither are secondary effects, such as alkali-aggregate reaction, plastic shrinkage cracking, or other environmental effects, accounted for in this equation. Designers must consider increasing the variation around the mean value if a possibility exists that these factors may play a role.

 

In the design of continuously reinforced concrete (CR), the tensile strength is important in determining crack spacing.

 

Some researchers have however found indirect tensile and direct tensile strengths to be very similar.

 

The European Concrete Committee has suggested that the indirect tensile strength is 60 % of flexural strength for aggregates used in Europe. The South African experience is that this figure varies from 65 % to 80 %.