Category Stone Matrix Asphalt. Theory and Practice

Despite the great impact of the stabilizer on SMA quality, few countries have devel­oped testing methods and criteria for the practicality of stabilizers. Some assess­ment methods for the application of stabilizers in various countries are presented in Chapter 5.

Practice proves that a reliable stabilizer-effectiveness assessment can be carried out when the same SMA mixture (the same JMF*) is tested with different stabiliz­ers. Comparison of test results from tests on different SMA mixtures is unreliable. The results of one test will not be comparable with the those of the others because the results for each mixture depend on the SMA composition, such as the type of aggregate and binder contents.

A direct method of checking the effectiveness of stabilization (i. e., absorption capacity) is the oil-absorption method (see Chapter 5).

The procedure for the quantitative selection of a stabilizer in a mix is simple, although it may be a little laborious. One of the draindown testing methods described in Chapter 8 may be adopted here. A series of SMA samples of the same composition should be prepared, but with different quantities of stabilizer. If the procedure is carried out with a proven product (e. g., cellulose fibers), testing may be limited to 0.2%, 0.3%, and 0.4% (m/m). If it is a product that has not yet been used, the testing series ought to cover a somewhat wider range based on the expected behavior (e. g., 0.1%, 0.2%, 0.3%,

0. 4%, and 0.5% [m/m]). Draindown should be determined for each stabilizer content, and the results should be plotted as shown in Figure 4.10. An important remark—the test temperature should be as close as possible to the real production temperature of the mix in the asphalt plant (see Chapter 8). The amount of stabilizer required for complying with the contract specifications may then be read off the diagram.

FIGuRE 4.10 Example relationship between draindown and stabilizer content used for the selection of an effective content of stabilizer in an SMA mix.

The optimal content of a stabilizer is the one that produces less than 0.3% by mass draindown material (recommended value is less than or equal to 0.2% m/m), unless otherwise specified.

The second way to counteract the draindown phenomena is to use a binder viscos­ity promoter. The most common agents used to be polymers, either plastomers or elastomers. Test results show, however, that the efficiency of polymers is not as good as that of fibers. Currently in many countries, regardless of the type of applied binder (modified or paving grade), fiber stabilizers are required. Binder modified with special materials may be considered, except for polymers directly metered into a pugmill at an asphalt plant. Polymer stabilizers also have a higher viscosity over the preferred range of production temperatures. This is when the use of a classic stabilizer is indispensable because, as a number of failed attempts have shown, modified binder itself does not protect an SMA mix from segregation.

The application of viscosity promoters acting at temperatures of 100-160°C may also increase problems with compaction on site because an SMA mix is hard to compact, even without an additive that stiffens the binder and makes compacting the mix even more difficult. This fact should not be forgotten when choosing a stabilizer to use.

Loose fibers are packed in self-shrinkable plastic bags. They are thrown into a mixer intact and the film melts into the binder during mixing. Granulated cellulose fibers may be bagged like other fibers (Figure 4.7) or packed in big-bags (Figure 4.8). Both loose and granulated fibers can be supplied in autotankers (Figure 4.9).

4.2.1.2 Summary

The following is a comparison of loose or pelletized fibers and granulated fibers:

• Loose or pelletized fibers, packed in bags of a fixed weight, are easy to throw manually into a pugmill at a batch asphalt plant, though the method is problematic for big contracts, high-output plants, and plants without semiautomatic metering. Loose fibers have their staunch fans, claiming that this type of fiber can be better distributed in a mix and that a lesser amount of them (compared with granulated products) is needed to achieve a similar effect. These fibers do not present any problems during laboratory testing, and they require no extra treatment or equipment. Their disadvantages lie in water absorption, which requires covered storage, and the necessity of manual operations at batching.

• Granulated fibers usually contain the same fibers as loose fibers, except that they have been coated with a binder (or other material), enabling production of granulated products that are 3-4 mm in diameter and 3-5 mm long. The binder coating makes them more water resistant (i. e., less moisture-sensitive). Another advantage of granulated fibers is the option of automatic dosage into the mixer at an asphalt plant, usually through a screw-pneumatic feeder. Granulated fibers do require a suitable tem­perature and mechanical mixing to release them and allow them to be dispersed properly in the SMA during production, so granulated products must be added at the right moment and mixed properly.

• Another significant issue that should be considered is the susceptibility of fibers to water. It is little wonder that a stabilizer absorbs water if it is meant to absorb binder. A wet stabilizer may harm the mixture considerably (see Chapter 11). Loose fibers or pellets are more sensitive to moisture than the coated granulated products.

• Various combinations of fiber blends have been appearing here and there (e. g., cellulose-mineral and cellulose-polymer). Some of them reveal cer­tain additional features, such as building up the mastic and enhancing its shear strength.

• The amount of fibers in a mix are determined using Schellenberg’s method or other methods (see Section 4.5 and Chapter 8), normally at a level of

0. 3-0.4% (m/m).

Granulated products (Figure 4.6a and b) are produced by coating fibers with a binder or other binder-soluble agents. A binder coating enables control of the forces on fibers during granulation and separation of individual fibers, which is necessary for distributing them evenly in an SMA mix.

The granulated form of these products makes dosage at production easy. The fiber granules are supplied in big-bags or autotankers and are stored in the bags or silos. Automatic loading with screw-pneumatic feeders is also possible.

Special attention should be paid during the preparation of laboratory samples. Due to compression of the granules during production, a higher shear force is needed to distribute the fibers; therefore one should use a mechanical mixer. If samples must be mixed by hand, it is recommended to warm the granulated product in an oven to a temperature above the softening point of the coating agent. Otherwise the stabilizer will not work effectively in laboratory samples, possibly leading to unrealistically high-binder draindown.

Granulated products are the best options for SMA production at medium and large-capacity asphalt plants because of the ease of handling.

4.2.1.1 Granulated Cellulose Fibers with Additives

Several mixtures of cellulose fibers and various modifiers are also available. These types of compounds have been produced with two goals in mind—binder stabiliza­tion and a change in the specific characteristics of a mix (e. g., workability or dura­bility). These compositions enable the simultaneous feeding of mixtures with fibers and additive. The effectiveness of such products should be tested in the laboratory to confirm the final properties of the asphalt mixture.

Loose fibers may be pressed into pellets to keep the fibers together without bind­ing agents. Their shape makes metering of the SMA mix during production easier. Pelletized fibers are formed into different shapes; one example is presented in Figure 4.5. Automatic loading with pneumatic feeders is also possible and easy. The pellets are usually supplied in large packages (big-bags) and stored in silos. All other features of loose fibers remain unchanged.

When performing tests of SMA mixtures in a laboratory, pelletized fibers may be treated without any special preparations. A mechanical mixer is usually not required;

however, in the case of very dense pellets, it is better to use one to ensure that the pellets are broken up and thoroughly mixed.

Loose cellulose threads (Figure 4.4) have the longest history of application. One of their advantages is that they become effective immediately after mixing them with aggregate and binder. They should be protected from moisture as they are highly hydrophilic and easily absorb water. Wet fibers are not able to absorb the binder and therefore are not effective.

Loose cellulose fibers are supplied in bags of thermo-shrinkable plastic. The mass of fiber per bag can be prearranged with the manufacturer and should be tailored to the batch volume of a given batch from an asphalt-mixing plant. An automatic

metering process for SMA production in an asphalt-mixing plant has been possible for some time now; loose fibers are delivered by autotankers and stored in a silo. During mix production, they are blown into the pugmill via an automatic system. The same pneumatic metering system can be used in drum-mixing plants.

When adding loose fibers, it is worthwhile to remember the following:

• The time at which the fibers are batched into the mixer is very important; bags should be emptied into the mixer during the dry mixing time, prior to binder loading.

• An excessive increase in the dry (binder-free) mixing time may lead to grinding of the fibers into dust by the aggregate, limiting the effectiveness of the stabilizing action.

• Loose fibers are very sensitive to moisture and therefore should be properly protected and stored; wet fibers lose their absorbing power.

When testing SMA mixtures in a laboratory, loose fibers may be treated without any special precautions. A mechanical mixer is not required.

Binder-absorbing additives are the most popular SMA stabilizing agents. The fol­lowing properties are required of a stabilizing material:

• Adequate binder absorbing power—this is the most significant property

• Ability to act without weakening the mixture—the stabilizer must not cre­ate glide planes and lessen the grain-interlocking strength.

Stabilizers of this type occur in various forms related to the following kinds of raw materials: [18]

• Glass—in the form of threads (like fiberglass wool)

• Others—for example, leather waste products (leather dusts)

The crucial difference among stabilizers is their absorbing power. To date, the most effective of the binder-absorbing stabilizers are cellulose fibers. They have a very high-binder absorption, which results in holding the binder firmly in position.

Fiber-free stabilizers have a wide range of binder absorption powers, so any new product should be tested in the laboratory every time (see Chapter 8). One should also remember that stabilizers may have substantially different densities, which are directly translated into their volumes in a mixture. High-density mineral fibers need a higher addition rate at batching, usually on the order of 0.4-0.6% (m/m). The same is true of polypropylene and glass fibers.

FIGURE 4.2 An example of 6-mm long polypropylene fibers: (a) macroscopic view, (b) the same fibers in microscope image, x 100 magnification. (Photo [a] courtesy of Krzysztof BlaZejowski and photo [b] courtesy of Jan B. Krol, Warsaw University of Technology.)

Furthermore, if a stabilizer does not work efficiently enough, increasing its quan­tity may improve the draindown performance. However, larger amounts of stabi­lizers may cause unexpected troubles, such as decreasing mixture workability. For example, plastic fibers (Figure 4.3) are marked by a lower absorption power. Using a high quantity of them (say, about two times more than cellulose fibers) will produce a substantial growth of stiffness and low workability. Small wonder—6 kg of such fibers per 1 metric ton of an SMA translates into a substantial volume; fibers are clearly visible in the mix as shown in Figure 4.3.

The most popular stabilizers—namely, products of cellulose—occur in the fol­lowing forms:

• Loose fibers—in the form of irregularly shaped cellulose threads

• Pellets—granulated products without a binding agent

• Granulated products—cylinder-shaped granules that consist of threads coated with bituminous binder or another agent (e. g., wax or plastic)

In the 1960s—during the beginning stages of SMA manufacture—the need arose to incorporate stabilizing agents to prevent binder draindown. Such additives are called stabilizers; they stabilize or keep the binder in place. Because of these stablizers, an increase in the binder film thickness on the aggregate is possible.

The two main techniques of reducing binder draindown are as follows: [17]

Each of these methods will be discussed later in this chapter, but let us start with a definition of a stabilizer. A stabilizer or drainage inhibitor is an additive to an SMA asphalt mixture put in to prevent binder (or mastic) from draining-off. Stabilizers may be made up of various materials, including both binder absorbers (e. g., fibers) and viscosity boosters (e. g., polymers).

In addition to helping to retain binder on the aggregate, stabilizers may also improve other properties of the binder itself, the mastic, or the mixture (see Section 4.2.1.4). There are examples of such tests available in the literature—for an example, see Behbahani et al. (2009).

But are stabilizers really necessary for SMA? Research done in the United States during the 1990s revealed a 70 times higher binder draindown in a mix without any stabilizer compared with the same mix containing 0.3% of cellulose fibers (Brown and Mallick, 1994).

The author’s experience at the beginning of 1990s showed that even with using mod­ified binders, fat spots often occurred. Then trials testing modified binders with half of the typical amount of stabilizer were conducted, and fat spots also occurred. Now, independent of binder type, at least a minimal amount of stabilizer is commonly used. So the answer to the question, should be stabilizer used or not? is evident; however, one has to note that some compositions of SMA are less susceptible to draindown.

Stone mastic asphalt (SMA) mixtures require a high content of binder, which results in thick binder films on the aggregate grains. To avoid the draindown effect, stabi­lizing additives (drainage inhibitors) are indispensable in most cases. This chapter describes the types of stabilizers and methods of testing them.

4.1 THE DRAINDOWN EFFECT

Have you ever seen fat spots on an SMA surface? Or binder running out of a truck hauling a hot SMA mixture? If you have, those troubles may have been caused by a binder or mastic draindown.

An SMA asphalt mixture has an intentional binder surplus. The specific surface of the mineral mixture is too small in relation to the designed binder volume. Under normal conditions, that binder is not bonded with the mineral mixture grains and does not remain on the grains’ surface; instead it drains-off. The draindown effect results from the separation of part (binder or mastic) of the SMA asphalt mixture. Keeping in mind that SMA has a lot of binder—in fact, SMA has a deliberate excess of binder—one should always take into account the risk of draindown.

In many countries, in the early applications of SMAs there were some cases of hot binder running out of a silo that held a hot SMA mix. Similar occurrences took place out of the backs of trucks carrying SMA to construction sites, eventually appearing as fat spots (bleeding) on the finished surfaces.

Methods to determine the amount of draindown are explained in Chapter 8 and other problems that may occur on the construction site in Chapter 11.