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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, partly as a consequence of a general decrease in bitumen quality due to the petroleum crisis of 1973, when moisture damage and frost became some of the most pressing pavement failure modes of the time. Hydrated lime was observed to be the most effective additive and as a consequence, it is now specified in many States and it is estimated that 10% of the asphalt mixtures produced in the USA now hold hydrated lime.
Given its extensive use in the past 40 years in the USA, hydrated lime has been seen to be more than a moisture damage additive. As is detailed in this report, hydrated lime is known to reduce chemical ageing of the bitumen. Furthermore, it stiffens the mastic more than normal mineral filler, an effect that is only observed above room temperature. This impacts the mechanical properties of the asphalt mixture, and if strength and modulus are seen to be modified by hydrated lime addition for a little more than half of the mix formulas, it improves the rutting resistance in about 75% of the mix formulas. In all cases, most of the studies focus on hydrated lime contents of 1-1.5%, and these effects are generally more pronounced for higher hydrated lime contents. Finally, the few published studies on fatigue resistance indicate that hydrated lime improves the fatigue resistance of asphalt mixtures in 77% of the cases.
In line with the observation that hydrated lime does not exhibit a higher stiffening effect than mineral filler at low temperature, no effect on the thermal cracking resistance is reported in the literature.
The reasons why hydrated lime is so effective in asphalt mixtures lie in the strong interactions between the major components, i.e. aggregate and bitumen, and the 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 (calcium ions) and roughness (precipitates) 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. The temperature dependence and the kinetics of the stiffening effect might explain why hydrated lime is not always observed to stiffen asphalt mixtures and why it is more efficient in the high temperature region where rutting is the dominant distress.
The various ways to add hydrated lime, i.e., into the drum, as mixed filler, dry to the damp aggregate, as lime slurry, with or without marination are described. No definitive evidence demonstrates that one method is more effective than the other, and all methods are seen to allow for the beneficial effects of hydrated lime to develop. As far as fabrication control is concerned, hydrated lime can be easily dosificated inside the mixture.
Given that all the above mixture properties impact the durability of asphalt mixtures, the use of hydrated lime has a strong influence on asphalt mixtures durability. The field experience from North American State agencies estimate that hydrated lime at the usual rate of 1-1.5% in the mixture (based on dry aggregate) increases the durability of asphalt mixtures by 2 to 10 years, that is by 20 to 50%.
The European experience is not yet as developed as in the USA, but the beneficial effects of hydrated lime on asphalt mixture durability have also been largely reported. As an example, the French Northern motorway company, Sanef, currently specifies hydrated lime in the wearing courses of its network, because they observed that hydrated lime modified asphalt mixture have a 20-25% longer durability. Similar observations led the Netherlands to specify hydrated lime in porous asphalt, a type of mix that now covers 70% of the highways in the country. As a result, hydrated lime is being increasingly used in asphalt mixtures in most European countries, in particular Austria, France, the Netherlands, the United Kingdom and Switzerland.
If the benefits of hydrated lime on asphalt mixtures are clearly demonstrated with a diversity of materials (aggregate, bitumen, mixture formulas) covering the 5 continents, the European experience remains somewhat lower than the one coming from the USA. As a consequence, the effect of hydrated lime on asphalt mixtures as measured by several European standard test procedures are not described in the literature. Among those of the highest interest, ITSR and fatigue must be mentioned.
Also, the description of hydrated lime in the European standards for aggregates is not totally appropriate. First, test methods such as the delta ring and ball test can not be performed on hydrated lime, although they are required for mineral fillers. Hydrated lime being considered as a filler in the standards on asphalt mixtures, it is critical to resolve this situation. Then, the mixed filler classes appearing in the aggregate standards do not cover all existing products currently used.
Finally, some theoretical aspects remain to be understood, and in particular the temperature dependence of the stiffening effect of hydrated lime in bitumen and the modification of the aggregate surface after hydrated lime treatment.
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