Category Stone Matrix Asphalt. Theory and Practice

The existing pavement facilities should be prepared prior to placing the mixture of a wearing course. This preparation should result in the following:

• Appropriate, tight, and durable bonding between asphalt course and facilities (e. g., with the use of suitable polymer modified asphalt tapes [Figure 10.7])

• Protection against damage or infiltration of excess asphalt mixture (e. g., around a drain or manhole)

10.2.5 Preparing Edges of the Courses

The edges of the course should also be prepared. The selected method should ensure durable joints at the transverse and longitudinal edges. Most previous experiences have proved the poor effectiveness of traditional methods like applying hot binder,

FIGURE 10.8 PMB tape for sealing pavement joints (a) when bonding with a layer edge and (b) after application. (Photos courtesy of Slaskie Kruszywa Naturalne sp. z o. o., Poland.)

covering with emulsion, and other similar techniques. As in other cases, PMB tapes (Figure 10.8) put on manually or special PMB compounds spread mechanically (Figure 10.9) have been recommended.

The rule for SMA surface preparation is the same that as for other binder courses. The surface of the underlying course should be even, clean, and free from dirt that could disturb the appropriate bonding of the laid-down mixture with the layer underneath. Any soiling of the surface (Figure 10.6) should be removed from the pavement. Oil and fuel stains should be cleaned with an absorbent material, and their residue should be washed off. The remains of an old mixture atop an underlying layer should be removed because they may cause partial unevenness of the spread SMA mixture (see Figure 11.33). When placing an SMA course on an old pavement, one should see that patches made of mastic asphalt have been removed and a new patch of asphalt concrete has been made. Leaving the mastic asphalt in place runs the risk of bleed­ing the binder from the patch onto the surface of the SMA course. Potholes and cracks should also be filled and sealed prior to placing the next course.

10.2.4.1 Bonding Layer (Tack Coat)

Attention should be paid to the proper laydown of the tack coat beneath the SMA course. When the surface is uneven, with pockets collecting the binder of the tack coat, its excess should be completely removed, otherwise it is likely that fat spots may appear on the surface of the SMA course. The tack coat underneath the SMA is necessary, but the amount and type of required binder (bituminous emulsion or cut­back binder) should be suitably estimated, depending on the state and type of surface. After all, we do not need a tack coat that is too thick or too soft because it can become a failure (sliding) plane under the SMA. A special binder (e. g., a special bituminous emulsion) should be used for interlayer bonding. Except for special cases, typical binder emulsions made of soft binder (e. g., 160/220 [Pen@25]) are not recommended. An average amount of binder (residual) for tack coating under an SMA varies within the range of 0.1-0.3 kg/m2. For example, in Germany according to ZTV Asphalt-StB 07, the recommended quantities of a polymer modified emulsion C60BP1-S[61] (former TL PmOB Art C U 60 K) for interlayer bonding for roads with heavy or very heavy traffic depend on the type of layer beneath the SMA as follows:

• Newly constructed asphalt intermediate layer: 150-250 g/m2

• Asphalt intermediate layer with cold milling surface: 250-350 g/m2

• Very porous asphalt intermediate layer or ravelled surface: 250-350 g/m2

Note: the above amounts of bituminous emulsion are NOT residual binder (from the emulsion).

Additionally there is a requirement for the strength of the connection between lay­ers; the splitting force has to be at least 15 kN. A lack of interlayer bonding adversely affects the pavement’s service life; it can also cause some negative effects in the form of slippage under a roller (see Figure 11.32).

There are additional requirements regarding issues such as wind speed at the laydown site in many national specifications. Let us take the Polish guidelines WT2 2008, for instance, in which the maximum allowable wind speed has been set at 16 m/sec (58 km/hr). The values of wind speed at the construction site are undoubtedly a fac­tor of tremendous significance, especially when placing thin courses of SMA.

Frequent rainfall and the necessity of placing an SMA on a wet underlying layer are problems that often beset road builders. Most regulations do not allow SMA laydown on a wet surface or during rainfall (Figure 10.4). The layer of water covering the surface of the underlying layer causes a rapid drop in the SMA mixture temperature and also coun­teracts the formation of durable interlayer bonding. It has been estimated that a layer of

water about 1 mm thick causes a drop in the temperature of a 4-cm thick SMA course of about 30°C (Druschner, 2006). All in all, the thinner the SMA course being placed on a wet surface, the faster the mixture gets cold. Figure 10.5 shows the relationship between the amount of water on a surface and the drop in the mixture temperature.

Obviously a mixture should not be laid on a frozen surface or on one covered with snow or ice. But keep in mind that, when justified, the supervising engineer may approve carrying out works in unfavorable conditions, provided that all requirements with reference to the achieved properties of a course are satisfied. Typical technical regulations have stipulated that the laydown of an asphalt mixture in adverse condi­tions might be permitted as long as the following are true:

• All final requirements for the course are fulfilled.

• Good bonding of structural and wearing courses is secured.

• Good performance of construction joints is guaranteed.

The limitation of minimum temperatures of SMA laydown has been commonly understood and adopted; however, it is quite a different matter when laying down the SMA mixture on an underlying layer that has an excessive temperature, as in the following examples:

• On a recently placed intermediate course that has not yet cooled

• While hot recycling in situ (or in place) with the simultaneous placement of a new wearing course, so-called hot remix plus

• During the execution of the so-called Kompaktasphalt—the laydown and compaction of two courses (intermediate and wearing) at the same time with one passage of a paver (see Section 10.5).

As a general rule (see Section 10.4), rollers compacting an SMA layer operate directly behind the paver. This practice is correct in typical circumstances since the mixture cools quickly. But when the underlying layer is warm, such a method may create some problems, with the most frequent one resulting in the binder bleeding under the rollers and creating fat spots.

The following comments and observations deserve mention:

• The rolling operation cannot be started until the spread mixture has reached the optimum start temperature related to the viscosity of the applied binder; therefore placing the mixture on an underlying course that is still warm— with temperature exceeding 50-70°C (e. g., on a course made several hours earlier)—should be accompanied by ongoing temperature control.

• Because of the warm underlying course, the SMA does not cool as fast, consequently giving more time for compacting and enabling a greater dis­tance between the rollers and the paver.

• In cases of fat spots being squeezed out by the rollers, rolling should be stopped until the mixture temperature in the course drops to a point where rolling does not overcompact the mat; the time (or temperature) at which roll­ing can resume should be determined experimentally for specific binders.

Unfavorable air temperatures (too low) and an excessively warm underlying course cause various problems. Having an excessively warm underlying layer cre­ates more favorable laydown conditions because this allows for more time to work with the mix.

Due to the resistance to compaction provided by the coarse aggregate skeleton, SMA mixtures can cause problems during laydown. Thus in almost all countries that have adopted SMA, some additional requirements on conditions at the work site must be observed, including the appropriate temperature, wind velocity, and the absence of rain.

10.2.1 Minimum Temperature of Air and Underlying Layer

To ensure that weather conditions are acceptable during laydown, the air tempera­ture may be checked often. In many countries, however, air temperature is not as important as the temperature of the course on which the hot mixture is going to be placed. Required laydown conditions frequently hinge on the thickness of the constructed layer, because thick layers cool more slowly, and on the type of binder used, whether modified or unmodified.

Two types of weather conditions may be distinguished in specifications: condi­tions at the time of laying and conditions within 24 hours prior to starting work. Average requirements for the minimum air temperature at the time of placing an SMA mixture range from 5°C to 10°C as exemplified by the following:

• German Dav documents stipulate an air temperature of not less than +5°C (Milster et al., 2004)

• The Czech standard CSN 736121 requires +5°C.

• The Polish guidelines WT2 2008 specify +10°C for courses less than 3 cm thick and +5°C for courses greater than or equal to 3 cm thick.

• The Australian NAS AAPA 2004 calls for +5°C, with the limitation that laying courses thinner than 4 cm or containing a modified binder should be avoided at such low temperatures.

In general, the temperature requirements are becoming stricter (increased) in cases where the thickness of a course is less than 40 mm; for that thickness, a minimum requirement of +10°C is commonly specified. In Germany’s ZTV Asphalt-StB 07, the weather limitations for SMA laying are as follows:

• When the course thickness is less than 3 cm, the minimum air temperature is +10°C, and the minimum sublayer temperature is +5°C.

• When the course thickness is more than 3 cm, the minimum air tempera­ture is 5°C.

The average requirements for the specified underlying layer’s temperatures are similar and are also within a range of 5-10°C. A higher value is often used for mixtures with modified binder. Another document, the U. S. Federal Aviation Administration Advisory Circular 150/5370-10B on civil airfields, has stipulated the following minimum temperatures of the underlying course during laydown, which depend on the thickness of the asphalt course being placed:

• For a course less than 2.5 cm thick, the underlying layer temperature must not be lower than 10°C.

• For a course 2.5-7.5 cm thick, the underlying layer temperature must not be lower than 7°C.

• For a course greater than 7.5 cm thick, the underlying layer temperature must not be lower than 4°C

Slightly lower values than those just cited have been adopted when specifying temperatures during the day directly preceding the start of laying down the hot mix­ture. For example, according to the Czech standard CSN 73 6121, the air temperature must be no less than 3°C (5°C for a thin course), and according to the Polish guide­lines WT2 2008, it must be at least 0°C (5°C for a thin course).

Another, indirect method of specifying temperature requirements is by limit­ing the permissible calendar period of SMA laydown—for example, by imposing a ban on works over a specified period of the year (e. g., between October 15 and March 15).

Finally, it is worth noting that a bituminous mixture with an unmodified binder can be laid down at a temperature slightly lower than a bituminous mixture with a modified binder.

Special self-propelled hoppers (e. g., MTVs) have been used to avoid segregation dur­ing the discharge of a mixture from the dump truck to the paver hopper and to minimize the risk of placing cool mixture (by remixing). These mobile machines operate between the dump truck and the paver to eliminate any contact between the two. MTVs are equipped with conveyors to transport the mixture to the paver hop­per. These devices eliminate jarring of the paver caused by contact with the delivery trucks or by the mass of mix dropping into the hopper. They hold a larger mass of material than a paver hopper, which helps to keep the temperature high, and allows the paver to keep moving forward. Another advantage of MTVs is the possibility of remixing the mixture constituents. This eliminates the risk of segregation and

enables the breaking of large lumps of cool mixture that may occur during trans­portation. Then the mixture is transferred to the paver hopper using a conveyor belt. MTVs are especially useful when laying mixtures susceptible to segregation and paving during cooler periods of the year (see Chapter 11, Section 11.8).

The main objective of transporting a mixture from the asphalt plant to the laydown site is to deliver the mix without changing its properties and in a state that allows for appropriate placement and compaction. Minimizing heat loss and preventing seg­regation of the mixture constituents are two of the most important issues regarding transportation. Requirements for transporting the mixture to the work site are often defined in technical specifications. Typical requirements are shown, as examples, in the following:

• Specified maximum distance between the asphalt plant and laydown site (e. g., 40 km)

• Maximum travel time from the asphalt plant to the work site (e. g., 2 hours)

Specifying the distance between the asphalt plant and the construction site can be quite a misleading operation since traveling a distance of 40 km in a rural area does not take the same amount of time as driving the same distance in a large city. Driving time is the more accurate requirement; however, some differences may be noted even in that. Mix will not cool as much after 2 hours during the sum­mer as it will after the same amount of time in the late autumn. Probably the best way of setting the requirements for transportation is to establish temperature con­ditions for the mixture. This method of specifying requirements may be found in the European standards concerning bituminous mixtures (e. g., in EN 13108-5 [the European standard on SMA]). There are maximum production temperatures speci­fied for various types of bituminous binders and a minimum supply temperature for the delivered mixture. One should meet that temperature range when planning the production and transport of a mixture to a work site, taking into account such factors as the prevailing weather conditions, and the location of the asphalt plant. This is definitely the more common solution.

When transporting hot SMA, a crust of cooler mixture is formed on its surface. The higher the degree of cooling of the mixture, the thicker the crust, and the more problems it causes. As long as the mixture is well-protected thermally (by a tarp or insulation), the layer of cooler mixture will be thin and the chance to intermix it with the rest of the material during laydown will remain high. Significant cooling of the
mixture during transportation or while waiting for discharge leads to the build-up of a pretty thick crust. Large pieces of cool, unmixed material cause a nonuniform texture in a laid-down course (for details see Chapter 11). In that case, good mixing may be provided by a buffer like a material transfer vehicle (MTV) or Shuttle Buggy (Brown, 2002) (see Section 10.1.4).

There is an analysis concerning the heat losses of an asphalt mix during site transportation in the paper by Spuziak (2002). The analysis assumes that heat losses during mixture transportation may range from 5°C/hr (with good insula­tion) up to 48°C/hr (in exposed locations). The drop in the mixture temperature depends on the following:

• Weather conditions during transportation, including the air temperature, humidity, and wind velocity

• Air streamline speed (the speed of the truck loaded with mixture)

• Time of the mixture haulage

• Mass of the load

• Shape of the truck bed and its insulation properties

Concerning weather conditions, wind velocity is a decisive factor in the rate of cooling of the mixture during transportation. The mass, volume, and shape of the hauled material is also of great significance. Any reduction in the thickness of the transported layer of mixture induces quicker cooling, hence the shape of the truck bed is also of considerable importance. Taking this into account, beds with rounded or wedged edges are ideal (Figure 10.1) (Ulmgren, 2000, Spuziak, 2002).

In cool weather, the best way to retain the mixture temperature is to use an appro­priate means of transport. Trucks with insulated beds are the best solution, then the effect of the reduction of heat waste is perfectly obvious (Figures 10.2 and 10.3). In the case of such a truck being unavailable, tight and well-fitting tarps on trucks should be adopted as a minimum. Minimizing drops in the temperatures of SMA mixtures containing polymer modified binder (PMB) is particularly important.

FIGURE 10.1 Location of cooled spots of a mixture on a cross section of a truck bed (a) and (b). Truck bed shape prevents cold corners (c) and (d). (From Spuziak, W., Proceedings of the 6th International Conference Durable and Safe Road Pavements, Kielce, Poland, 2000. With permission.)

10.1.3 Discharging

The suitable discharge of a mixture from a truck to the paver hopper helps to avoid some problems. The dump truck with the mixture should pull in directly in front of the paver to allow for the following:

• The truck to be positioned exactly on its motion axis, enabling the push roll­ers on the paver to contact both rear wheels of the truck.

• The truck can avoid bumping into the paver; bumping the paver can cause a bump in the pavement.

As a general rule, the paver should approach the truck and not the other way round (then the paver can push the dump truck without bumping).

Once the SMA has been produced at a hot mix plant, it must be transported to the job site where it will be placed and compacted. This chapter describes issues that need to be considered during this stage of the construction process.

10.1 TRANSPORT OF THE MIXTURE TO A LAYDOWN SITE

While it may seem that hauling SMA to the job site is straightforward, there are things that can go wrong that can affect the rest of the construction process and ultimately the performance of the pavement. This section will describe potential problems and how to prevent them.

10.1.1 Loading a Truck

The hot SMA mixture, at first stored in a silo, must be transported to the work site. The way the mixture is loaded into the bed of the truck affects its subsequent quality. The following rules should be observed to prevent segregation of the mix:

• The discharge into the bed of the truck should be performed in defined drops; generally the first drop should be placed in the front part of the load bed, the second one toward the rear, and the rest evenly distributed in the middle (Hensley, undated; Roberts et al., 1996).

• The addition of small amounts of mixture to reach the full weight limit of a loaded truck may bring about the separation of larger particles of the aggre­gate from the mixture (segregation) and should be avoided.

• When using trucks with large beds, the truck should be loaded in four or more separate drops, overlapping the individual dropped piles to help remix the material.

Detailed instructions for loading mixtures out of silos may be found in the USACE Handbook (2000).

The draindown effect (i. e., binder or mastic draindown followed by its collection on the bottom) may happen under the following circumstances (Ulmgren, 2000): [60]

Also, remember to suitably prepare the truck bed prior to loading the mixture. The bed should be clean and free from dirt (see requirements according to EN 13108-21). Sideboards and bottoms of the beds of the delivery trucks should be covered with a special release agent (antiadhesive fluid) to avoid adhesion; diesel oil and other fluxing agents that may degrade the bituminous binder are not permitted to be used in this capacity. The truck bed should be free of major dents in which substantial amounts of release agents could gather. The boards should be evenly coated, and any excess release agent that may collect in deformations of the bed should be removed prior to loading the truck.

Today almost all asphalt plants are computer controlled. It is worthwhile mentioning the necessity for caution during SMA production. This also applies to these modern asphalt plants. Usually such machines give personnel a feeling of confidence and a sense of complete control over the production process. However, many instances have proved that an excess of trust leads to problems (some of which are described in Chapter 11).

Particular attention should be paid to checking weight batchers, thermometers, and proportioning meters. All data produced by the computer system should be peri­odically checked. A good example of the type of problems that may occur is the clog­ging of batching devices for granulated stabilizers between the balance and the chute

Stabilizer

FIGURE 9.11 The SMA asphalt mixture with the stabilizer after extraction and before aggregate screening. (Photo courtesy of Krzysztof BlaZejowski.)

gate. The actual amount of stabilizer weighed out by the machine can be checked by disconnecting the chute pipe and measuring the amount of material batched during a production cycle (Figure 9.10). Another typical method is performing an extrac­tion and checking for the presence of the stabilizer in the mixture (Figure 9.11). Unfortunately this method does not allow for the determination of the exact amounts of fibers added to each batch.

Finally, a brief note about extraction. While performing an extraction of an SMA mixture, the rules for the correct preparation of the sample should be observed (hav­ing the stabilizer in mind as the mixture’s constituent—see Figure 9.11). The stabi­lizer fibers should be removed prior to starting the aggregate screening.

Issues concerning procedures for production of bituminous mixtures under EN 13108-21 and specified tolerances are discussed in Chapter 14.

9.5.2 Other Examples of Production Control

Some brief examples from around the world include

• In some countries, the accuracy of stabilizer dosage is ±10% of its mass as determined in a laboratory formula.

• If binder viscosity is the basis for establishing the mixing temperature of a binder and aggregate, some specifications may quote a permissible devia­tion from that temperature (e. g., ±10°C).

• In most countries, the permissible deviation of a mixture gradation and the content of binder depends on the number of tested samples.

• Some regulations provide clauses permitting a situation in which the grada­tion curve of a produced mix, considering given deviations, may deviate outside the area of the gradation envelope (between the so-called upper and lower limit curves).