Foam Mix a unique Asphalt Technology


Foamed bitumen was first patented by Prof. Lladis Cysani at Iowa University in the fifties initially to make roads for the US Army on Pacific islands using only volcanic ash. This patent used steam to foam the bitumen but was cumbersome. However many successful roads were built with this process including several in Quebec, the success and longevity of which prompted Soter Canada to reintroduce the process there in 1989. Cysani's patent was bought by Mobil Australia in the early sixties but they substantially modified it for use with cold water and additives. The Mobil process was licensed to contractors internationally until their patents ran out in 1993. Soter took the licence in 1989 but redeveloped the process such that their technology is now the world leader in foamed bitumen both for in situ Foamstab and the plant mixed Foamix.


The basic principle of Foamix is to expand bitumen by contact with small amounts of water under carefully controlled conditions, then mix the foamed bitumen with cold moist aggregates. The basic principle of mix design is a volume one but weight is used to simplify control. Thus bitumen at 4% by weight is equivalent to 8% by volume so by expanding the volume of bitumen by 12 times, one is mixing equal volumes of bitumen and aggregate.

The process is based on surface tension and when the bitumen foam bubbles collapse they tend to grip the fine particles tightly, forming minute droplets of binder. These activate and spread during compaction and the dispersal of the moisture to hold the coarser particles together. Low bitumen contents are effective in Foamix because the foam preferentially coats the fine particles, which then form the strong mortar that encapsulates the coarse particles. The initial grey brown appearance of the Foamix blackens after compaction and curing and the fact that coarse particles never appear coated is not of concern.


Foamed bitumen provides a uniquely strong adhesive binder system that enables the use of a wide range of new and recycled aggregates. It produces cold mixes that are often as strong after curing as hot mixes and stronger and more rapid curing than emulsion mixes. Foamix will generally be less expensive than either hot mix or emulsion mix, can use a wider range of available aggregates and cure sufficiently quickly to be open to traffic on the same day as laid.

Aggregate grading is not critical but it is better to have 100% passing a 20mm sieve and at least 5% passing the 75micron for which a simple primary crushing is generally all that is needed. Up to 15% passing 75 micron can be accommodated with an increase in bitumen content and this can be useful where quarries are trying to use surplus fines. All normal good quality aggregates can be used in Foamix but the real benefits come from its ability to use recycled aggregates such as asphalt planings, old road material, crushed concrete and mining wastes. If aggregates are not clean or contain clay then the addition of small amounts of cement or lime will usually correct the problem.

When using RAP the amount of bitumen needed is dependant on the bitumen content of the planings and the amount passing 75micron. Thus asphalt planings with 5% passing 75micron would need typically 2.5% bitumen. Clean fine aggregates with 15% passing 75micron would require approximately 6% bitumen. A typical recycled road would reuse a mix of say 50% RAP and 50% old road base and use 3.5% bitumen. It is of course best to properly design the mix as one would for hotmix and use accelerated cure in the oven before compacting sample moulds. Achievable test results are 10kN in Marshall, 30kN in Duriez and 2400 MPa in the Nottingham tester.


Foamix can be used in any pavement for roads, airports, parking and even rail track as it can be produced to give similar stiffness and performance to hot mix. Foamix was laid on the A43 in Northamptonshire in 1988 alongside equal thickness of hot mix and the foam has performed just as well as hot mix for over 12 years. Many other roads particularly in Kent using the in situ process Foamstab have also given many years of excellent service often under heavy traffic (up to 30 million standard axles).

The TRL report 386 by Milton and England, with major input by Biczysko gives up to date detail relating to design, specification, and suitable use of Foamstab in situ cold recycling in the UK. Foamix is a superior product to Foamstab in that it is impossible to accurately assess the quality of the in situ pavement and it is likely to vary in quality and thickness. With Foamix the aggregates are excavated, assessed and modified before replacing in the road. Equivalencies are being developed such as:- 150mm Foamix base =150mm hot mix base = 175mm Foamstab = 200mm emulsion base = 300mm crushed rock.

Although Foamix can be used without a wearing course for light traffic (car parks and farm roads) it is generally necessary to have some asphalt surfacing on top varying from surface dressing to 100mm of hot mix. Below are given some design suggestions, however good analysis of aggregates, subgrade, traffic and drainage are essential before starting to design a project:-

Commercial Vehicles                      FOAMIX Depth                           Surfacing
Per Day

0-2                                                   100mm                                     None or light
                                                                                                          emulsion/bit grey
2-20                                                 125mm                                     Surface Dressing
20-100                                             150mm                                     25mm hot mix
100-200                                           175mm                                     50mm hot mix


Foamix can be used as any other cold pavement material by hauling to site, spreading by paver or grader and compacting by flat, vibratory and/or pneumatic roller. The road can be opened to traffic as soon as compaction is complete and after a light emulsion/bit grit surface dressing is applied. Final surfacing should be laid within a matter of weeks depending on weather and traffic.

Foamix is moist and tacky when laid so is easy to compact to a high density but the void content achieved tends to be a little higher than with hot mix because of the moisture present. This is not a serious concern because the stiffness will still be high. After compaction further curing goes on and ultimate strength may not be reached for months even though the mix will carry the early traffic without problems. Because of this it is often difficult to take core samples with water-cooled corers, air-cooled corers of 150mm diameter are recommended.


Computerised controls on machines such as the B.G. Europa KSF 50 together with trained operators will ensure high quality material. It must be realised that a 150 mtph Foamix plant may cost only 10% of a 150 mtph hot mix plant but must be given an equivalent amount of technical attention.

The quality of foam is essential and regrettably there are some machines on the market whose constant foam quality is questionable. If the bitumen does not foam then the mix will not work. The Soter system is designed to produce high quality foam continuously but as with all equipment it must be checked regularly during operation.

The aggregates used should always be moist to ensure foam maintenance and where the Foamix is stockpiled it should always be covered to keep it from drying out or being saturated. Whilst Foamix can last 3 months in stockpile it is better to use it within 4 weeks for best quality.


Economy is achieved from having a high productivity, low cost mobile plant using low bitumen contents with a wide range of new and recycled aggregates.

Flexibility comes from the ability to mix and lay cold. Stockpile different materials on different sites and move quickly between each one. Good performance is now being recognised after years of successful service in a wide range of locations carrying from light to heavy traffic.

Environmental benefits are obtained from a non polluting process using little energy for recycling to avoid both dumping and new aggregate taxes.


Appearance is deceptive as it does not initially appear black and shiny but dull brown because of the moisture present. However it becomes black with time and eventually as stiff as hot mix.

Its simplicity and low cost can create carelessness in users or a belief in some magic properties. The process requires the same technical care and attention as any other asphalt construction.

It has been seen as a threat to the quarrying/hot mix industry however it is in fact a challenge that helps them to improve their economics and their environmental credibility.


Foamix provides a great opportunity to introduce cost effective environmentally attractive asphalt technology for the new millennium. There is now a sophisticated plant, the KSF 50, available at the leading edge of this technology that enables the user to be ahead of the competition and to participate in the rapid growth forecast in cold mix and recycling; the late Maurice Akeroyd was involved in the Mobil use of foamed bitumen since 1970 and was probably the world-leading exponent of the process. He retired in 1994 as Manager Special Products for Mobil Europe and up until his death remained interested in the success of the foam process. He also advised Soter during the development of their world leading system and did advise B. G. Europa on how best to use their Foamix plants and technology.

For more information about our range of products please call us on +44 (0)1440 821155 email us at or use our contact section.
View Our Products

Related Products