24 May 2014
Note: This article has been published by Taylor & Francis in the Road Materials and Pavement Design journal, and can be downloaded at the following link: www.tandfonline.com/doi/abs/10.1080/14680629.2012.743669#.VLTVQnvzVm7
Hydrated lime has been known as an additive for asphalt mixtures from their very beginning. It experienced a strong interest during the 1970s in the USA, when moisture damage and frost became some of the most pressing pavement failure modes of the time.
Given its extensive use in the past 40 years, hydrated lime has been seen to be more than a moisture damage additive: It reduces chemical ageing of the bitumen and stiffens the mastic more than normal mineral filler above room temperature. All these properties impact durability and hydrated lime is now seen has an additive to increase asphalt mixture durability.
This article is a literature review on the fundamentals of hydrated lime effect. The reasons why it is so effective lie in the strong interactions between aggregate and bitumen, and a combination of 4 effects, two on the aggregate and two on the bitumen.
Hydrated lime modifies the surface properties of aggregate, allowing for the development of a surface composition and roughness more favourable to bitumen adhesion. Then, hydrated lime can treat the existing clayey particles adhering to the aggregate surface, inhibiting their detrimental effect on the mixture. Also, hydrated lime reacts chemically with the acids of the bitumen, which in turns slows down the age hardening kinetics and neutralizes the effect of the “bad” adhesion promoters originally present inside the bitumen, enhancing the moisture resistance of the mixture. Finally, the high porosity of hydrated lime explains its stiffening effect above room temperature.