Influences of Cyclic Loadings on the Properties of Pavement Concrete: Fid-Bau Portal

Influences of Cyclic Loadings on the Properties of Pavement Concrete

Breitenbuecher, R. / Kunz, S.

For the longtime behavior of concrete pavements fatigue plays a significant role. Fatigue is mainly influenced by continuously varying flexural stresses, which are caused by cyclic traffic loads and by variable restraint stresses. However, currently such cyclic effects are only considered in the pavement design by inclusion of a general impact factor without any detailed knowledge of the real material behavior. For fatigue the ratio between stress σ and strength f is considerably important (Kessler-Kramer 2000). Hence, various load scenarios should be considered in appropriate tests. In addition to short-term cyclic traffic loads long-term cycles of restraint stresses due to daily and seasonal temperature changes should be taken into account. In extensive lab tests the behavior of typical pavement concrete under flexural cyclic loads was investigated considering various ratios of σmin/σmax and σmax/ffl. Based on these configurations concrete beams were cyclically loaded up to ten million load cycles using a frequency of about 4.5 Hz to accelerate the tests. It was proven that at the stress levels σmax/ffl of 40%, 50%, and 70% degradation processes in the concrete microstructure occur long before the actual fatigue failure takes place. This degradation process is accompanied by the formation of microcracks. These effects can be proven on the basis of ultrasonic runtime measurements and the dynamic E-modulus calculated from these values. Within the tested load cycles the dynamic E-modulus decreased by about 10 to 15%. The residual flexural strength and the stiffness (E-modulus), however, were affected to a lower degree. Besides these effects on the mechanical properties the transport of moisture and harmful ingredients (e.g. alkalis) into the concrete microstructure is likewise promoted by such microcracking.