Category Stone Matrix Asphalt. Theory and Practice

The following sections outline some considerations regarding the operation of rollers once the roller combination for a specific job has been determined.

10.4.2.1 General Rules

In standard conditions, rollers should follow as closely as possible behind the paver. If it is not possible for the rollers to keep up with the paver, the speed of the paver should be reduced or the number of rollers increased. The method of rolling and positions of the different rollers are thoroughly discussed in the USACE Handbook 2000, Dynapac Handbook, and German DAV Handbook (Milster et al., 2004), where detailed directives can be found.

Rolling thin SMA layers should be executed with great caution, with vibra­tion only rarely applied. The use of slightly lighter rollers instead of heavy ones is recommended.

The quantity of so-called SMA roll down (i. e., the change in thickness of a spread layer due to rolling) may be estimated as 10-15% of a layer thickness. It depends, among other things, on the design gradation curve that forms the aggre­gate skeleton.

The number of rollers, their types, and number of passes should be selected with regard to the SMA gradation, layer thickness, weather conditions, and the planned paving speed. Obviously the rollers should be in good working order, able to work at low and constant speeds.

The Minimum number of rollers to compact one layer one lane wide is two or three. Many nations’ guidelines stipulate diverse requirements for the numbers of rollers, mainly stating that one should use rollers weighing more than 9-10 tons each. If three rollers are in use, two of them should be responsible for the majority of the compaction, and one should be the finisher. Extra rollers should be added in case of

problems with reaching the design layer density. Table 10.1 shows an overall sum­mary of the usefulness of equipment for SMA placement based on German compac­tion guidelines M VA 2005.

Vibratory rollers should be carefully or, more precisely, consciously used. An addi­tional condition for using vibration is maintaining the appropriate temperature behind the paver to enable the movement of particles during rolling. Applying vibra­tion to a cool SMA mixture is a mistake that leads to the crushing of particles. Vibration is not an option when SMA is being placed in a thin layer (i. e., 20-30 mm), on a stiff base (e. g., concrete slabs or a brick or block pavement), or at too cold of a mixture. Compacting SMA with vibratory steel rollers is generally permitted, but high frequency and low amplitude vibrations are a must (Asphalt Review, December 2004).

Determining the correct type of vibratory roller to use from among the following is important when considering the use of vibration:

• Classic vibratory rollers—used when it is certain that the frequency and amplitude of vibrations do not threaten to crush particles or to squeeze mas­tic out on the layer surface.

• Vibratory rollers with other vibration techniques

• Oscillatory—marked by vibrations within a range of oscillations, com­pacting a shallower depth than classic vibratory rollers but sufficient for a wearing course

• Variable direction of vibration—features the possibility of amplitude direction control.

The use of vibration can be an acceptable method of compacting SMA layers; most producers of rollers currently have solutions to minimize the risk of crushing an SMA skeleton. Obviously using vibration does not apply to the aforementioned situations (thin layer, stiff underlayer, too cold of a mix), a priori limiting the pos­sibility of using vibratory rollers.

The following types of rollers can be used for compacting SMA mixtures:

• Static—used as basic equipment for SMA compaction. The heavy (fin­isher) and medium ones operate chiefly in the set. When compacting is executed on thin layers or layers on a stiff underlying, heavy rollers are excluded.

• Vibratory— used for compacting SMA, but only according to some rules mentioned later.

Pneumatic rollers are not typically used due to the risks of mastic sticking to the tires and dragging particles out of the rolled layer and of squeezing mastic out on the surface.

The following rules should be observed when planning for a combination of roll­ers for a work site: [63]

• An extra roller with a side-roll for layer edges should be provided, espe­cially with an increased number of construction joints or connections.

• Rollers fitted with a gritter for finishing SMA surfaces are also indispens­able (see Section 10.7.1).

In general, most regulations and guidelines do not permit manually placing SMA mixtures; however, there are places where there is no other option but to place the mixture manually (e. g., small and irregular pieces of a roadway) (Figure 10.10). In such cases one should remember not to scatter the mixture with shovels but to carry and lay it down. Compaction should be carried out imme­diately after laying and alignment. Unfortunately such surfaces will differ in structure from adjacent areas spread mechanically, and usually they will be more porous and permeable.

10.2 COMPACTION

Bearing in mind the unique philosophy of designing an SMA aggregate skeleton (see Chapters 2 and 6), SMA compaction may be regarded as the process of form­ing a structure made up of appropriately arranged and interlocked grains filled with mastic. While rolling, the mixture volume is reduced by the tighter arrangement of particles and a decrease in the content of air voids. The binder makes compacting easier. It plays the role of a lubricant, easing the movement of aggregate at that stage of an SMA’s performance.

10.4.1

Types and Number of Rollers

The fact remains that SMA is a difficult mixture to compact, therefore the appropri­ate selection of both the number and type of rollers is of real significance.

It is worth remembering the following when mechanically spreading a layer of SMA mixture:

• Manually scattering mixture over the mechanically placed mixture is not permitted.

• Allowing any vehicles other than rollers on the hot mixture, before it is finally compacted and its temperature drops below the expected level, is not permitted.

• The end of a working lot (the transverse joint) should be finished by cutting; suitable joint bonding ought to be secured prior to laying the next lot—for example, by applying a PMB tape or a special compound (Figures 10.8 and 10.9).

• When executing the SMA layer in separate lanes (i. e., each traffic lane separately), the edge of the first layer should not be cut vertically but with a falling gradient, e. g., of 3:1 (height: width).

• Longitudinal edges should be cut while the mixture is hot. A special cutting wheel installed on a roller is user friendly; it is even more convenient with a metal pusher for the cut mixture located next to the cutting wheel, which eliminates manual removal operations.

• The utmost attention should be paid to the execution of an appropriate and tight longitudinal joint between two SMA layers; it is difficult due to the high content of chippings in mixtures; experiences have proved that the PMB compounds, laid down mechanically, are very effective.

The appropriate passage of material through the paver plays a key role in the proper spreading of a mixture. After starting in the hopper, the mixture is moved by slat con­veyors (with flow gates in older equipment) to augers and then under a screed. During each of these stages the following significant parameters affect the final result:

• The hopper should be fitted with independently lifting wings, and its shape should eliminate places from which the mixture does not slide to the slat conveyor. Such “dead areas” or “cool corners” create accumulations of cool mixture and cause other problems (see Chapter 11). For the same reasons, the insulation of wings is desirable.

• Care should be taken so that the mixture does not adhere to the walls of the hopper where it cools off fairly quickly. These effects can be seen in various forms of segregation (see Chapter 11). It is perhaps worth dedicating one of the paving crew to systematically throw the cooled remains of mixture down in to the middle of the hopper, particularly when work is done on cool and windy days.

• Completely emptying the hopper of mixture should not be permitted. Newly delivered material should be added to the hopper when it is still filled to about 20% of capacity.

• Augers are intended to divide the mixture across the width of the screed plate; the quality of the layer’s surface depends, among other things, on appropriate adjustments to the augers. The amount of mixture supplied to the augers should be constant; it can be controlled by setting the slat conveyors’ speed and allowing an adequate opening of the flow gates (if applicable).

• The distribution of mixture at the middle of the paver screed plate has a sig­nificant influence on the segregation of an SMA mixture. In some machines, there is a feeding screw without a large chain transmission on the axis of the paver; rather the feeding screw is propelled from outside by hydraulic engines and intersecting axis gears.

• The screed plate must be fitted with a heating system. A number of solu­tions are available and include power supplies, heating fuels, and gas burn­ers. A properly heated screed enables the appropriate travel of the mixture without dragging and pulling out particles.

• Screeds are fitted with vibration systems and rammers. The frequency and amplitude of vibrations and rammers’ strokes should be compatibly matched.

• To ensure quality of the placed layer, a constant paver speed should be maintained. Stoppages should be avoided.

Nowadays, mechanical placement is the only reliable method of executing an SMA layer. Requirements for pavers can very rarely be found in the specifications.

Basically, selection, setting, and operation of a paver are the responsibilities of the paving contractor. Most road-engineering companies, after gaining experience with SMA pavements, work out their own procedures for spreading, compacting, and achieving the required parameters. The following information might help to establish or improve such procedures.

Almost all publications on SMA underline the necessity of carefully observing the temperature of the mixture during placement and rolling. The expected range of mixture temperature is determined in different ways; it chiefly depends on the kind of binder, but such factors as the layer thickness and weather conditions are impor­tant, too. However, the most important factor is the temperature of the mixture deliv­ered to the construction site and the temperature at the end of effective compaction, below which further rolling becomes ineffective and even harmful.

Minimum temperatures for mixture supplied to a work site according to the European standard EN 13108-5 (which applies only to selected [unmodified] bitu­mens after EN 12591) are as follows:

• 160°C for paving grade bitumen 40/60

• 150°C for paving grade bitumen 50/70

• 140°C for paving grade bitumen 70/100

In the German DAV SMA handbook (Druschner and Schafer, 2000), the afore­mentioned temperatures are presented in a more general way; the suggested tem­perature of an SMA mixture in a paver hopper should not be lower than 150°C. The same rule is presented by Bellin (1997).

Different temperatures at the end of the compaction time have been assumed in various publications, from 80-100°C for ordinary binder, and from 120-138°C for modified ones. The U. S. NAPA SMA Guidelines QIS 122 stipulates no rolling when the temperature of a layer drops below 116°C. A temperature of about 100°C has been stated in German documents as the point at which to stop rolling. The mini­mum temperature at the end of the compacting time may be roughly calculated by adding 50°C to the Ring and Ball (R&B) softening point of the binder used in the mix (Daines, 1985; Read and Whiteoak, 2003).

Other relevant points include the following:

• Problems related to a mix temperature that is either too low or too high are elaborated on in Chapter 11. Additional comments may be found in Section 10.4.2.5, which deals with rolling time.

• Optimum compaction temperatures are related to the viscosity of the added binder. That implies the significance of not only the lower temperature limit of rolling but also the initial temperature of rolling (already described while discussing SMA laydown on a hot underlying layer). A mixture that is too hot also causes problems at placement.

• Remember that spreading mixtures with substantial temperature differ­ences (e. g., from a truck with a hot mixture alternated with another truck with a cool mixture) cause changes in the resistance offered by such mix­tures at spreading.

• Appearing here and there in a layer being placed, pieces of a cool mixture may cause the development of an increased content of air voids and hence decrease the pavement’s lifespan (Pierce et al., 2002).

All these remarks concerning temperatures do not apply to cases that involve the use of special additives for lowering mixture temperatures that create the so-called warm mixes.

Corrections to mixture problems occurring during delivery of a mixture may be pos­sible up until the moment of placing the SMA. The moment the SMA layer appears behind the screed plate of a paver, the chances of improving the quality diminish to a minimum. After that time only compaction is possible; errors made during the design and manufacturing stages can no longer be corrected.

The vital elements of spreading an SMA layer include the following: [62]

10.3.1
Layer Thickness

An SMA layer thickness should not be less than three times the maximum aggregate size in the mixture, and in principle, not greater than four times (higher ratios allow better compactability). An appropriately selected layer thickness with regard to gra­dation enables suitable compaction of the layer (reaching the expected compaction factor). For details on selecting SMA gradation, please refer to Chapter 6.

The layer thickness exerts a considerable influence on the speed of cooling of the layer, eventually involving temperature problems of various types (see Chapter 11). To put it briefly, the thinner the SMA layer, the more difficult the compaction. In addition, a thin layer cools off fast, magnifying compaction problems.