At 14 December 2017 the MSc student Gianpaolo Baggio defended his master thesis on the prediction of transverse cracking of road bases constructed with cement bound reclaimed asphalt, also called cement bound RAP bases. His study was performed in cooperation with the Delft University of Technology, Boskalis Nederland BV and Kiwa KOAC BV. This type of road base, often used in heavy trafficked roads, combines the advantageous property of enhanced bearing capacity with the adverse aspect of high sensitivity to cracking. Gianpaolo investigated which factors primarily govern the occurrence of cracking. He also searched for methods for the prediction and reduction of the amount of cracks.
One grading of RAP and two levels of cement content (2% and 4% m/m) were investigated. The mechanical properties indirect tensile strength, shrinkage, thermal deformation, modulus of elasticity and relaxation were investigated for five levels of curing times, ranging from 3 days to 130 days. Most tests were performed at three temperatures in the range of 0°C to 30°C. The mechanical properties retrieved from the tests were used as input in a model for predicting the onset of cracks in a cement bound RAP base.
The model characterizes the crack pattern in terms of time of initiation of the cracks, crack spacing and crack width. The calculations were performed for a plain untreated base course and one with notches at regular spacing. The predictive model clearly demonstrates that, besides temperature and the cement content, also the month of construction of the road base has a major impact on the risk of early cracking. The mix with 4% cement is of course more sensitive for cracking than the mix with 2% cement. August appears to be an unfavourable month for starting construction even when notches or saw-cuts are applied. The effect of curing and gaining strength at a low rate unfavourably coincides with the gradual decrease of temperature in fall and winter, resulting in thermally and traffic induced stresses that may and will exceed the tensile strength of the mix. The model makes it possible to search for the optimal values for cement content, strength, curing time and installation moment to minimize the problems with cracking in cement bound RAP road bases.
For more information, please contact:
Dennis van der Ven (unit manager)
088 562 26 72