Asphalt Q&A: Spray Pavers?

A spray paver combines a paving machine and a distributor in one unit. The spray bar applies the tack immediately ahead of the placement of the mix. The tack is never driven on by any of the equipment. The spray paver opens up options for type of tack and quantity of tack thats never been possible before. There are papers from TRB and AAPT that indicate there are performance benefits imparted to the mix more than from just providing a good bond. One other benefit is the fact the process is truly trackless, regardless of tack type or quantity used.

I have used a spray paver for the last 15 years, or should I say I used a spray paver until my emulsion supplier developed a product that could be applied in thick layers, break quickly and  not get picked up by either the paver or trucks.

No system is perfect but I have never had much faith with the spray paver method, the tank at 4,500 litres needs to be refilled during the day, this stops paving, and hence you get a joint, the tanker can spray ahead and be refilled on site, though usually the tanker has about double the capacity of the paver tank.

The other issue is the distance of the leading edge of the screed from the tow point, while it gives better rideability, any slight change in direction can leave a poor joint behind, it looks like a child has been driving the machine.

The position of the jets makes it more difficulty to swap out a blocked jet than with the truck, and lets not kid ourselves jets get blocked.

The last point is that if you have a local depression in the pavement and your rollers are running  a lot of water on the drums you can get flushing of the emulsion occuring in thin layers, this is then picked up by the roller drums and you have fun.

My experience is based on the third machine built by Finnpave in the UK, bought out by Finning after Cat bought Bitelli, it was designated a Bitelli 691C, the tractor was a Bitelli 681 Crawler, the spraying system was from ACMAR in France. It was not always reliable, either electrics or pneumatics causing problems regularly, though this could be down to sporadic use.

I am sure the current product offered by Vogele and others is far superior but it begs the question why invest if you already have a sprayer and paver and you can get access to polymer modified tack coat that won’t pick up on tyres, as somebody whose company did invest I would think twice before doing so again.

Lastly, the spray paver was a great talking point due to it’s size, we used to park it in the plant yard beside Head Office, customers always asked what it was and what it did, a billboard would have been a cheaper marketing tool.

Spray pavers have been it existence for a long time with a lot of history in Europe. The paver enabled thin overlays to perform where they were not able to previously due to the heavy application of a polymer modified tack that created a better bond to the existing surface. Spray pavers have improved over time and new suppliers have entered the market. In recent years, emulsions and hot applied binders have been developed that can address the pick up and tracking issues with varying degrees of success. The one thing that is just starting to be researched and understood is how to test a composite asphalt system The composite being the asphalt mixture plus the tack coat material. As the tack coat application rates increase, they can have an impact on the overall overlay performance. The most current research I’m aware of is at the University of Illinois and the University of Florida. Both universities have developed test methods for evaluating the combination of tack plus mixture. Bringing the conversation back to spray pavers, the equipment can apply almost any emulsion providing the viscosity is within limitations. With polymer technology, we have an opportunity to significantly improve our current overlay performance through enhanced tack coat materials. The non-tracking materials are going to be more limited in the range of properties they can have and still be trackless. I believe spray pavers will be a game changer in the future of asphalt paving.

Yes, existing pavers can be modified that is how the first ones were built, but as the engineer operating the third Bitelli system built I would recommend against it.

You have to extend the screeed arms significantly to allow space for the spraybar, the geometry must be perfect or else the screed will not ride perfectly.

The tank and spraybar are simple plumbing exercises but then you need an onboard generator for heating the tank.

If you are seriously considering buildingone, get one from a producer on a demonstration and develop a list of what you would have to do, after this you will probably shy away from it. Our machine had three interlinked spraybars, one at the front between the tracks, and two at the back of the tracks to both cover the area where the tracks had been and out either side towards the screed extensions.

If you must build one using your own machine, it might be worth asking a manufacturer if they would consider it, or if they would sell you the complete kit to convert.

Again as in my previous comment I would use a fast breaking PMB emulsion.

Regarding the comment from Exline above, I have done joint research with our National Roads Authority and our Bitumen Supplier on the effect of composite action. Our old system was if the HMA had been left for longer than three days it was to recieve a tack coat, if less than three days none was required. Our trial looked at standard tack coat and PMB tack coat, we had a control section with no tack coat, we had two other sections, one with standard emulsion the other with PMB emulsion.  We applied Base, tack coat, base, tack coat, binder. We tested the material with a FWD, we found a 20% increase in pavement stiffness using the emulsion, we found no difference between the straight emulsion and PMB emulsion.

20% increase in stiffness equates to pushing the pavement into a category of infinte life for our traffic loading, we would only need surface course replacement every few years (15-20).

Can existing pavers be modified to accept the spray system and tank?

Current paver manufacturers have not shown much interest in designing a spray retrofit kit for their pavers. The market for spray pavers is small compared to total pavers being supplied to the marketplace and the cost for development will likely be high.

In regard to Mr. Fitzsimmons comment on stuctural stiffness improvement from the use of tack on each pavement layer, I certainly agree. The inability to see a difference between an unmodified tack and a polymer modified tack could be for several potential reasons. One, the FWD may not be sensitive enough to pick up the difference from just tack coat type change. Second, the application rates applied, polymer type and concentration may be a factor.

The testing I’ve seen reported or been involved in are more focused on the composite of the tack plus the overlay. In these cases, as tack rate is changed and tack type is changed, the response is different for standard tack coat materials versus polymer modified materials. The tests have been focused on bond and cracking characteristics. This is related to spray pavers by the fact the equipment can shoot non-typical materials at higher than standard rates. It can open new ways to improve pavement performance at a relatively low cost. It may also solve some performance related problems of overlays that we have tried to address through the mixtures that maybe a function of poor bond.