Fatigue life of asphalt concrete with rubber grains

This paper presents some results achieved in a wide research carried out in the road material laboratory of the Rome University “La Sapienza” to assess the suitability of a new hot mix asphalt (HMA), containing crumb rubber (particle size 2-5 mm), as sub-ballast in railways or base layer in road pavements. This material could be effective in the reduction of vehicular traffic vibrations; however, crumb rubber is responsible for reducing mechanical characteristics of the asphalt concrete. The aim of this step of the program here discussed is to evaluate the influence of kind and content of crumb rubber on bituminous mixes, regarding fatigue life. The experimental analysis was directed to detect the initial modulus value, above which this bituminous material will remain structurally serviceable for a considerable period. The tests were conducted on cylindrical specimens, using a servo-hydraulic loading system and operating in a loaded-controlled fashion (tension-compression), at a frequency of 10 Hz and a temperature of 20° C. The adopted failure criterion, considering an assigned constant stress amplitude, was the number of load repetition that causes a decrease in modulus to half the initial value or, depending on the progress of the test, the fracture of the specimen due to macro-cracking. The tests were performed with various stress levels, related to the stiffness of the mix. This study documents the relationship between stiffness, as characterized by dynamic (complex) modulus, and fatigue life. Four mixes, different in kind and content of rubber, were checked and compared with a traditional HMA. The interpretation of experimental results demonstrates that mixes, containing a certain kind and a weight content up to 3% of crumb rubber, show a fatigue life comparable with traditional HMA.