English Version of a publication in the German magazine BITUMEN issue 1/2003

Influence of the surface energy status of aggregates on the performance of asphalt hot mix

Otto HARDERS
Ingo NOESLER

Elf Bitumen Germany, Technical Service Bitumen

ABSTRACT

Aggregate selection for asphalt production is mostly governed by economic factors. This includes transportation costs, availability, capacity and seasonal effects. The storage time of the aggregates after crushing can vary remarkably. The surface energy of freshly broken minerals is changing over the time due to reorganisation of polar component and absorption of water and dust at its surface. This process takes some months to stabilise. The knowledge about the actual status of this energy is most important for application of bitumen emulsions as it has an influence on the breaking time and absorption. It can also have an influence on the performance of asphalt hot mix formulations. A work program included the investigation into the influence of different sources of minerals and granular sizes as well as the general behaviour of asphalt compositions. The interaction with different sources of bituminous binder was another topic of the investigation. The different surface activities were addressed by the methylene blue (MB) test and the performance of asphalt compositions was evaluated with a rolling wheel rut tester at 50°C under water.

1. INTRODUCTION

Due to economic reasons mineral aggregates in asphalt road construction are normally chosen from nearby quarries. After crushing of the mineral aggregates to the demanded fractions they are delivered to asphalt hot mix plants. At the mixing plant the mineral aggregates are stored temporarily for different periods in deposits until they are used for the production, the surfaces of mineral aggregates have differently strong electrical energy due to the dipole character of the respective petrography. Because of this a directed adsorption on other molecules with dipole characteristics is exerted. With mineral aggregates which are mainly composed of quarz (SiO2) the dipole characteristic is far more distinct than with mineral aggregates that are mainly composed of calcium carbonate (CaCO3). Therefore water with its high dipole moment is adsorbed to a higher extent by mineral aggregates with a high silicate content than by basic mineral aggregates. The binder bitumen has a dipole moment of nearly zero and consequently adheres better to mineral aggregates, that are mainly composed of calcium carbonate [BECKER, 1953].The polarity, i.e. the distinctiveness of dipole moments of surfaces of mineral aggregates is not only determined by the respective petrography but also by the different surface activity of the mineral aggregates resulting from the different storage times at the mixture manufacturers of the mineral aggregates after the crushing in the quarry. It is proven that the surface activity of mineral aggregates is most distinctive directly after the crushing. After about 120 days (about 4 months) after the crushing of the mineral aggregates a minimum value of the surface activity is reached [JOSTEIN, 2000]. As the polarity i. e. the surface activity of surfaces of mineral aggregates are set in a basic relation to the adhesion [WAGNER, 1971], the conclusion is that different storage times of mineral aggregates after the crushing in the quarry influence the adhesion considerably and consequently also exert an influence on the deformation behaviour of rolled asphalt hot mix layers. Also in SCHMALZ, ET AL [1997] it is pointed out that the affinity of the surface of mineral aggregates shortly after the crushing reaches the highest extent and decreases after a longer storage period of the mineral aggregates on an open deposit. Furthermore the effect of contamination appears by which the free valences at the surface of the mineral aggregates are saturated. The free valences that are present at the beginning at the aggregate surfaces are saturated during storage on open deposits by airborne particles as well as by ions solved in rainwater and therefore are no longer available for a following adhesion with bitumen [cf. NOESLER, 2000]. Objective of the study is the quantification of the influence of the above mentioned different surface activities of the mineral aggregates on the deformation behaviour of asphalt mixtures. With this aim in view two sorts of mineral aggregates of different petrography (Norit und Oolith) with a storage time of about 120 days after crushing were tested in the Hamburger wheel tracking device with regard to deformation behaviour. The different surface activities are simultaneously quantified with a methylene blue (MB)-test value. The ensuing test results are to contribute to the guarantee of the asphalt quality.

2. Surface activity of mineral aggregates

2.1 Surface Activity - Adhesion between Mineral Aggregate and Bitumen

After the notion of the theory concerning the molecular orientation the molecules of a bitumen orientate themselves, as soon as they get into contact with the surface of a mineral aggregate, towards the interface, so that the energy of the mineral aggregate surface is saturated to an optimum. The bitumen molecules orientate themselves during the wetting process in such a way that a negative mineral aggregate molecule is equalised by a positively charged part of the bitumen molecule. The charges of the mineral aggregate surfaces are neutralised. At the beginning the thickness of the bitumen layer in which the molecules orientated themselves towards the mineral aggregate surface, was determined as very thin, it only amounts to 10-12 m. The theory of multilayer adsorption explains itself by an individual orientation of the polar bitumen molecules. The polar molecules can combine (associate) to polymolecules and reversibly dissolve again (dissociate). The positive dipole goes to the negatively charged mineral surface aggregate and at the opposite negative dipole, pointing away from the mineral aggregate surface, further positive bitumen molecules settle down. Consequently several layers of bitumen molecules orientated towards the mineral aggregate surface come into being. Polymolecules dissociate e. g., when they get into contact with a more strongly charged mineral aggregate surface. The theory of interface energy is highly appreciated in the specialist literature. The adhesive behaviour between mineral aggregate and bitumen is considered as thermodynamic phenomenon in connection with the surface energy of the employed building material. This theory does not require any exceptions in view of the molecular structure of the employed building materials, but the molecular polarity of the employed building materials is taken into account. On the basis of this theory polar liquids have a stronger attractive power to a mineral aggregate surface than covalent liquids. The reason is that polar liquids can minimise their free surface energy to the thermodynamic stable state of the smallest surface energy. Because of this it can be explained that a stripping and a separation of bitumen (covalent liquid) from mineral aggregate by water (highly polar liquid) can take place (stripping effect) [NOESLER, 2000]. In the specialist literature it was proven, that mineral aggregates can only been moistened with bitumen when the surface tension of the mineral aggregates is higher than the surface tension of the bitumen. With isotropic liquids the surface energy is numerically equivalent to the surface tension. During such wetting processes a so called immersion warmth is released by the caused energy loss (D T = 5 × 10-6 K), that can be proven by sensitive thermometers. The resulting energy flux can be represented in dependence on the time.

2.2 Adhesion Failure between Mineral Aggregate and Bitumen

The building material mixture asphalt in a road construction is exposed to external stresses during its usable life cycle, caused by traffic and climate. The vehicle traffic causes a mechanically dynamic loading. This loading value alone normally is not sufficient to destroy the adhesion between mineral aggregate and bitumen. In the presence of water these stresses can cause undesired loss of adhesion. There are two kinds of failure between mineral aggregate and bitumen, the cohesion failure and the adhesion failure. There is cohesion failure, when the mineral aggregate grains are separated from each other though the adhesion between mineral aggregate and bitumen is still guaranteed. During a failure of adhesion however a partial or total fracture face is formed within the interface between mineral aggregate and bitumen. Failures of cohesion normally appear in asphalt mixtures, with which pure and dry mineral aggregates were used. Failures of adhesion however develop in the presence of water. The influence of the rainwater is mostly aggressive and mostly shows an acid reaction. Other impurities of the water can lead to a stronger reduction of its surface tension. According to thermodynamic principles the different causes for failures of adhesion are derived. Consequences are often a displacement, an infiltration or separation of the bitumen from the mineral aggregate. A failure of adhesion mostly due to a combination of several influencing factors [NOESLER, 2000]. According to the infiltration theory the adhesion between mineral aggregate and bitumen fails, but the bitumen film remains intact. This happens because moisture reaches the interface. The moisture can reach the interface because of two reasons. Either it migrates from the moist inner part of the mineral aggregates through capillaries to the interface, or it diffuses from outside through the bitumen film. The moisture can be present in liquid or gaseous form. The diffusion process towards the interface is reversible, i.e. the penetrating moisture can after the dissolution of the adhesion diffuse out (e. g. by drying processes). By these means the adhesion can be totally restored. The biggest influence during the diffusion process is exerted by the bitumen. During the process water molecules penetrate until the bitumen film is saturated. If the adhesion between mineral aggregate and bitumen destroyed, then the water molecules diffuse to the mineral aggregate surface and there electrolytes are formed. Alkali- and alkaline earth metal ions can be leached by the infinitesimal aqueous phase, so that the pH-value of the water increases. Hereby absorbed acids in bitumen can be dissociated, thus the bitumen gets a negative charge similar to the contact surface of the water. As both surfaces dispose of the same charge, they repel each other, and consequently more water is drawn into the interface. By this process the stripping of the bitumen is continued, because the water film below the bitumen film becomes thicker. It was determined that low viscous bitumen are more resistant against an infiltration of their bitumen film than high viscous bitumen. Mostly the strength of the adhesion between mineral aggregate and an infiltrated or not infiltrated bitumen is under similar stress conditions.

2.3 Testing Method for Determination of Surface Activity (Methylene Blue Adsorption)

The dyestaff methylene blue (3.9-bis Dimethylamino phenazothioniumchlorid C16H18N3ClS) is a powder. To obtain a methylene blue solution with desired concentration, commercial methylene blue powders were dissolved in distilled water. The MB powder used possesses a high solubility in water at 20 °C of 50 g/L and a thermal disintegration starting from 180 °C. This high solubility in water is based on a high polarity and on the molecule range of the MB - powder, which is many times greater than the water molecule H2O. It possesses the ability to accumulate on the mineral surfaces. In the literature different concentrations of methylene blue are used for the MB-test. The concentrations range from 1 mg/L over 4.5 mg/L to 10 mg/L. During these investigations a concentration of 10mg/L like in BRENNAN [1997] was used. The test was performed according to the test method of the International Slurry Surfacing Association [ISSA, 1989]. For the test the sieved fraction 0/2 (unwashed) of the mineral aggregates was used. 200 ± 0.1 g of the above mentioned mineral aggregates 0/2 were put into a 1000 ml beaker and 200 ± 0.1 g distilled water was added. The solution was stirred with a magnetic stirrer for two minutes followed by 5 minutes waiting time. Next step was a gradual addition of 2 ml MB-solution with a pipette and 2 minutes stirring. After each addition of the MB-solution and the following stirring one drop of the water phase was placed on a filter paper. When the spot on the filter showed a gloriole (halo) – effect, this indicated that all mineral surfaces of the 0/2 mm fraction had been covered with the MB. Excess of MB shows the halo – effect. The amount of MB-solution (i.e. the Methylen Blue Value) measured in ± 0.5 ml was stated. The amount of MB-solution versus the time of storing of the aggregates after crushing in the quarry was discussed in section 6.

3. Asphalt Hot Mix – Asphalt binder 0/16

For the examination of the deformation behaviour of compacted asphalt hot mix formulations aggregates and bituminous binders as shown in section 3.1 and 3.2 were used. The asphalt hot mix formulation was an asphalt binder course 0/16 based on the German specification ZTV Asphalt-StB 01. The gravimetric composition of the asphalt hot mix is listed in table 1 and the particle sizes distribution is shown in figure 1.

Table 1. Mixture Composition of the Asphalt Hot Mix – Asphaltbinder 0/16

Figure 1. Grading Curve in Accordance to the German Specification ZTV Asphalt-StB 01

3.1 Characterization of used Mineral Aggregates

As the mineralogical constitution of the used minerals is of great importance for the surface activity and the adhesive behaviour between aggregate surface and bitumen, the petrographic details are given in the
table 2.

Table 2. Petrography of used Mineral Aggregates

3.2 Characterization of used Bitumen Grades

The two bitumen grades (PmB 45 and standard bitumen 50 / 70) which were used are characterised by their material nominal values in the attached table 3. To warranty the comparability of the test results the used bitumen was divided into several sample parts. By this means it became ensured, that for each preparation of the different asphalt hot mix the origin bitumen product was used i.e. each used bitumen sample became thus warmed up only once.

Table 3. Analysis of used Bitumen

4. Long-term storage of the mineral aggregate

To study the effect of aged and unaged aggregates on the performance of asphalt hot mix formulations the selected aggregates fractions from 0/2 to 11/16 of Norit and Oolith were stored in open air for 120 days. One half of the materials have been stored as raw material on a wooden support exposed to the weather (rain, sun and wind). The other part after immediately after it was brought from the quarries was dried and sieved and thereafter used for the production of test specimens. These were Marshall specimens and also asphalt specimens (cf. figure 1) for the rutting test.To measure the surface activity of the aggregate small samples from the crushed sand fraction 0/2 were taken from inside of the stockpiles. After extraction of the test samples the pile was covered again with the removed surface layer. The other fractions (2/5, 5/8, 8/11 and 11/16) were not touched during the storage time.

5. Hamburger Wheel Tracking Test

The Hamburg wheel tracking test is used to measure in the laboratory the resistance against deformation and stripping of the binders on compacted asphalt samples. Asphalt plates can be tested dry or under water with loaded steel or rubber coated wheels. The test conditions are shown in the documentation (TP A StB). The test is mainly used for extended qualification tests and as a tool for development of improved mix formulations or development of improved bituminous binders. Great experience is based on asphalt binder mix formulations measured at 50°C under water with a steel wheel. A principal drawing of the test arrangement is shown in figure 2. For our investigation we took the conditions with the greatest amount of experience.