Pneumatic-Tiered Roller (PTR) : Could be an ideal addition to asphalt paving fleet

A hot-mix asphalt mixture may have all the desired mix characteristics and properties when designed in the laboratory. That same mix, however, may perform poorly under traffic if that mix is not compacted to the proper level of density on the roadway. A mix that may have only marginal properties in the laboratory will often outperform a mix with more desirable properties if the marginal mix is adequately and properly compacted.

It has often been said that the degree of compaction of a hot-mix asphalt mixture is the single most important factor that affects the ultimate performance of the pavement under traffic. Compacting an hot-mix asphalt mixture to an air void content of 6% or less generally increases the fatigue life, decreases the amount of permanent deformation or rutting, reduces the amount of oxidation or aging, decreases moisture damage or stripping, increases strength and internal stability, and may decrease slightly the amount of low-temperature cracking that may occur in the mix.

The usage of steel drum static road roller and steel double drum tandem vibratory compactor is common to compress the road surface in order to achieve the density faster and for preparing the smooth road surface. But to achieve a more solid and excellent road paving job, Pneumatic-Tiered Roller (PTR) is a perfect roller tool which is regularly deployed by road contractors to perfect their paving job. The adoption of pneumatic-tiered roller particularly in the compaction of hot asphalt surface is very necessary to avoid untimely road damages, pot holes, road breakage and other road failures.

The pneumatic-tired rollers (also known as rubber tired roller) are now widely used for compaction of hot asphalt surfaces. They have rubber tires instead of steel tires or drums and generally feature two tandem axles, with three or four tires on the front axle and four or five tires on the rear; they are aligned so the rear tires cover the spaces left between the tracks of the front tires. The tires are mounted in pairs that can oscillate, or singly with spring action, so tires can move down into soft spots that would be bridged by a steel drum. The rubber tires add to their downward pressure a kneading effect, as material is pressed toward spaces between the tires. Pneumatic-tired rollers can be ballasted to adjust the weight. Depending on size and type, the weight may vary from 10 to 35 tons. However, more important than gross weight is the weight per wheel for the material being compacted.

Pneumatic-tired rollers may be equipped with 15-,17-, 20-, or 24-inch tires. Air pressure in the tires may vary for different types of material, such as 50 to 60 psi to finish asphalt and 100 psi to compact a granular sub-base. The tires must be inflated to nearly equal pressure with variation not exceeding 5 psi to apply uniform pressure during rolling. The tires may be flat, diagonal or radial. Compaction technology also depends on tire geometry. Flat tires have a wide base and it is suitable for surface sealing on bituminous work. The radial pattern enables lesser rolling resistance, lesser heat generation, and tread stiffness ensures uniform distribution of load during compaction.

Compaction and kneading action of pneumatic-tired roller provide a more uniform degree of compaction and high density than steel wheel rollers. The tires provide better static penetration into the materials that relocates the particles and increase the density.

They provide a tighter, denser surface thus decreasing permeability of the layer. PTR compact the mixture without causing checking (hairline surface cracks) and they help to remove any checking that is caused with steel wheeled rollers. With PTR there is little bridging effect between the tires; therefore, they seek out soft spots which may exist in the fill. For this reason, they are sometimes referred to as “proof” rollers.

Pneumatic roller eliminates the voids of air and space in the asphalt hot mix that would otherwise cause potholes to appear. Pneumatic rollers use static pressure in both vertical and horizontal directions to reduce air voids in material which creates compaction. This is often referred to as kneading or manipulations and helps bring compaction to materials differently than traditional static rollers or vibratory rollers. The roller kneads the asphalt gently at the surface, driving larger particles lower and bringing fine sands to the surfaces. With the fine sands at the top, the asphalt is sealed better, decreasing the chances of water getting into it when it rains.

These machines are also proving to be an asset when obtaining compaction along longitudinal joints and around manhole covers as they knead materials into these tight areas while removing air voids. They are also ideal for applications where vibration should be avoided such as next to buildings, on bridges and thin lift asphalt overlays. Pneumatic rollers offer versatility not available with a steel drum roller. The weight of the machines and air pressure of the tires can be adjusted to meet the specific pressure desired on a particular job.

Pneumatic rollers offer a different type of compaction and that's more of a bottom up style compaction as opposed to a top down. The way a vibratory roller works is they go over the mat and then build the compaction from the top down. When pneumatic rollers get on the mat, they knead the material and they'll build compaction from the bottom up so it gives a more uniform compaction when used in combination with everything else.

The main issue related to pneumatic rollers is asphalt pickup (the asphalt sticking to the tires).  Pneumatic tires and liquid asphalt have an affinity for each other due to their respective chemical structures. The rubber in the tires and the petroleum in the asphalt form a chemical bond that causes asphalt to attach to the tire. A chemical release agent can be used to minimize asphalt pickup. However, tires near mat temperature will not pick up an appreciable amount of asphalt. Asphalt binder tends to stick to cold pneumatic tires but not to hot pneumatic tires so it is better to run the roller on the hot mat. Insulating the tire area with rubber matting or plywood helps maintain the tires near mat temperature while rolling.

Pneumatic-tired rollers are used in an intermediate roller position in an asphalt paving train. This position works well because a double drum roller has already completed the breakdown phase and the material is still hot enough to keep tires warm and prevent material pickup.