Investigation of Low Temperature Properties of Asphalt Mixture Containing Recycled Asphalt Materials

Abstract

L’uso di sempre maggiori quantita’ di Reclaimed Asphalt Pavement (RAP) nella costruzione di pavimentazioni stradali in conglomerato bituminoso e’ diventata una delle maggiori priorita’ per i suoi benefici economici e ambientali. Di recente, altri materiali quali Recycled Asphalt Shingles (RAS) hanno trovato stesso campo di applicazione. Tuttavia pochi sono gli studi eseguiti sull’impiego di RAP e RAS alle basse temperature. Questa tesi si occupa di studiare l’uso di tre materiali di riciclo, RAP, Tear off Scrap Shingles (TOSS) and Manufacturer Waste Scrap Shingles (MWSS) nelle miscele di conglomerato bituminoso alle basse temperature, tramite prove sperimentali, analisi statistica e modellazione. La parte sperimentale prevede prove di three-point bending creep con il Bending Beam Rheometer (BBR) su 17 miscele di conglomerato bituminoso. L’effetto di RAP, TOSS e MWSS su creep stiffness, m-value, thermal stress e critical temperature e’ stato valutato tramite analisi statistica, evidenziando come TOSS e MWSS influenzino le prestazioni del conglomerato bituminoso solo per specifici contenuti di RAP. Nella parte teorica, i provini di conglomerato bituminoso sono stati analizzati sulla base di digital image analysis, modelli micromeccanici e analogici e con simulazioni agli elementi finiti. La frazione volumica e la distribuzione granulometrica sono state stimate a aprtire dalle binary images attraverso digital processing. La frazione volumica e la distribuzione granulometrica media degli aggregati dei 17 conglomerati bituminosi analizzati sono risultate molto simili nonsotante siano stati impiegati differenti quantita’ di materiale riciclato per il loro confezionamento. La struttura interna delle miscele di conglomerato bituminoso e’ stata analizzata tramite la valutazione della spatial correlation functions dei provini di BBR. L’assenza di ampie fluttuazioni delle correlation functions ha indicato che la microstruttura delle differenti miscele non e’ influenzata dalla presenza di RAP, TOSS e MWSS. Due modelli, uno micromeccanico e uno analogico, sono stati usati per la backcalculation della creep stiffness del legante bituminoso per un numero ridotto di miscele e i risultati successivamente confrontati con la creep stiffness ottenuta sperimentalmente dal legante estratto. Questo ha permesso di notare come i riusltati della backcalculation diano valori di creep stiffness molto piu’ elevati rispetto a quelli del legante estratto. Infine i due modelli, micromeccanico e analogico, sono stati validati tramite simulazioni bidimensionali agli elementi finiti.

The use of increased proportions of Reclaimed Asphalt Pavement (RAP) in the construction of asphalt pavements has become a top priority due to its economical and environmental benefits. Moreover other materials as Recycled Asphalt Shingles (RAS) have recently found their applicability in the same field. However in spite of a significant number of studies on the use of RAP and RAS, little was done to investigate the behavior at low temperature of pavement containing these two types of materials. In this thesis the effect of using three recycled materials RAP, Tear off Scrap Shingles (TOSS) and Manufacturer Waste Scrap Shingles (MWSS), on the asphalt mixture behavior at low temperature is investigated based on statistical analysis and modeling of an extensive set of experiments. The experimental part consisted of three-point bending creep tests performed on BBR (Bending Beam Rheometer) beams (6.25 × 12.5 × 100 mm) obtained from 17 different asphalt mixtures. Statistical analysis of the effect of RAP, TOSS and MWSS on creep stiffness, m-value, thermal stress and critical temperature was performed showing that TOSS and MWSS affect asphalt mixture performance only for specific amount of RAP. In the theoretical part, asphalt mixtures specimens were analyzed based on digital image analysis, micromechanical and analogical models and finite element simulations. The volumetric fractions and particle size distributions of the different asphalt mixtures were estimated from their binary images after digital processing. The volumetric fraction and the average size distribution of aggregates for the 17 asphalt mixtures investigated were found to be very similar despite of the varying amount of recycled material contained. Detailed information on the internal structure of asphalt mixture was investigated by estimating the spatial correlation functions of the beam specimens. No large fluctuation of the functions were detected meaning that the microstructure of the asphalt mixtures was not affected by the presence of RAP, TOSS and MWSS. Micromechanical and analogical models were used to backcalculate the creep stiffness of the binder for a limited number of mixtures and the prediction was compared to the creep stiffness obtained from the corresponding extracted binders. A noticeable difference was detected with the prediction having much higher values. Finally, two-dimensional finite element simulations of three point bending were used to validate the micromechanical an analogical model used.