When and why should you consider using adhesion additives in asphalt pavement? Manufacturer AkzoNobel’s additives expert Dr David Needham answers crucial questions.

TP: Why consider using additives at all?

David Needham: Asphalt in the UK is an excellent product but it is always worth asking if it can be improved. In most cases asphalt has performed above and beyond expectations that were placed upon it decades ago but as budgets are cut and surfacing is required to last longer, performance enhancing adhesion additives have a lot to offer.

TP: What do they do?

David Needham: Normally the two main components of asphalt – bitumen and aggregate – bond together naturally. But there are occasions when the two lack affinity and adhesion promoters have been used for almost 50 years to overcome this problem. Now adhesion promoters are increasingly used to improve the durability of asphalt even if there is no inherent adhesion problem. They come in several forms and at AkzoNobel we specialise in the cationic surfactant type.

TP: What’s that?

David Needham: Surfactants are ‘surface active’ which means they prefer to reside at a surface or interface. They are quite special molecules having one part that likes to be in an oily substance like bitumen (oleophilic) and another part that does not (oleophobic). In the case of nitrogenous, cationic surfactants the oleophobic head group carries a positive charge which means it can bond to negative sites such as those on an aggregate surface. So the adhesion effect is created from a molecule that likes to be immersed in bitumen but which is able to bond chemically to an aggregate surface.

TP: How does this improve surface durability?

David Needham: Strengthening the bond between bitumen and aggregate clearly has a beneficial effect on the durability of asphalt. The presence of cationic surfactant molecules at the interface improves the bond between bitumen and aggregate meaning that the end product is able to resist the effects of water for longer. Resisting water is the key to improving durability. Cationic surfactant adhesion promoters offer the possibility of laying asphalt that will last longer in service; leading to longer intervals between repair and replacement and better surface quality during service including fewer potholes. They are easily added at low dosage levels to bitumen before asphalt production and can be used in pothole repair asphalt to increase the performance of these systems.

TP: Surfactant additives are also used to reduce asphalt temperatures – so called Warm Mix. Can you explain more?

David Needham: Warm mix asphalt (WMA) is growing at a fast rate in many countries. There is plenty of activity including trials in the UK but no major projects have used WMA to date. Warm mix has a lot to offer in terms of ease of laying, health & safety and environmental benefits so the industry has a lot to gain. Our surfactant technology not only reduces working temperatures but improves durability at the same time.

TP: Are the additives costly?

David Needham: Well, nothing is for free. But the initial cost of adhesion additives is low when compared to the long term price of investment in asphalt and the benefits to be gained. The cost of using a cationic surfactant adhesion promoter in a road that is designed to last 15 years is paid off if the road lasts only one month longer. And the reality is that adhesion promoters can greatly extend the lifetime so the cost really does become insignificant to the client.

TP: Any research to demonstrate longevity?

David Needham: Well for one thing there is 50 years of history to consider! The long term effect of surfactant type adhesion promoters was investigated specifically in Sweden in 2008. An AkzoNobel product was used during construction of the Jonkoping airport runway in 1992 and provided a model where performance over a 16 year period could be investigated. Comparison between performance test results on materials laid in 1992 with cores taken from the asphalt runway in 2008 demonstrated that the adhesion promoter provided enhanced resistance to moisture over the 16 year period and was still going strong.

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