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The stiffness, strength and shrinkage of the concrete slab depend on the properties of the cement and aggregate as well as the water and cement content (4). Since failures in a road pavement slab primarily occur as a result of tensile stress exceeding tensile strength, strength in this mode is of interest. Therefore flexural strength is one of the most critical parameters and careful consideration of values being used in design is recommended. Although an average flexural strength of more than 5 MPa can be obtained in the laboratory, variation on site needs to be considered, especially the minimum expected values. Also see the section on flexural strength.

 

The ratio of concrete slab stiffness to concrete flexural strength (E1/f) may vary between 7 000 and 10 400 depending on the type of aggregate used in the mix. The following is a guide for crushed aggregate (for more details, consult Fulton’s (4)):

 

 

In case of the ultra-thin continuously reinforced (UT) design, the factor E1/f may be lowered by as much as 20 % because of closer crack spacing.

 

The ratio of indirect tensile strength to flexural strength (ITS/f) depends on the aggregate type and curing of the concrete. Laboratory manufactured and cured samples using 19 mm stone, have a ratio that varies from 0.76 to 0.86 but the ratio for samples cored from the pavement itself are quite different as shown in the table below (9). A default value of 0.75 is recommended for normally cured pavements.