Chapter 2
Asphalt: an overview
2.1 Definition
Asphalt is defined by the American Society for Testing and Materials (ASTM) as “a dark brown to black cementitious material in which the predominating constituents are bitumens which occur in nature or are obtained in petroleum processing”. About the 85% of the asphalt refined from petroleum (the source of nearly al the asphalt today) is used as a paving material [20], commonly called asphalt cement. Another major application of bitumens is in waterproofing membranes. In the majority of cases, asphalt used to pave roads is made from the residuum that remains after the refineries remove the distillates to manufacture fuel, kerosene, lubricating oils and other commodities. Figure (2.1) shows the composition of various petroleum crude oils. It can be noted that the percentage of bitumens varies a lot.
2.2 Chemical composition
Asphalts are complex mixtures of organic compounds belonging mainly to the families of aliphatic, aromatic and naphthenic hydrocarbons; they also contain small quantities of organic acids, bases and heterocyclic components where nitrogen, oxygen, sulphur and some metal atoms can be found. Considering that an exact determination of the real asphalt composition is not possible, usually the chemical nature is identified by dividing the molecules into different groups. Before analyzing the chemical structure of each group, it must be pointed out that the extremely complex nature of asphalt is strongly varying with the source. Figure (2.2), [21], shows an elemental analysis of four representative petroleum asphalts.
Figure (2.2) – Elemental analysis of various petroleum asphalts.
Many attempts have been done in order to classify different molecular groups composing the bitumen on the basis of subsequent separation by different means. Unfortunately, different separation strategies lead to different fractions (composed themselves by mixtures) with different properties, which has led to confusing and contradicting predictions of behaviour. The two most frequently used fractionation methods are the chromatographic method of Corbett [22], and the precipitation method of Rostler [23]. The former method uses differential absorption and desorption, while the latter uses sulphuric acid of varying strength. Despite there are few differences between the fractions obtained using one model or another, four main categories can be recognized: saturates, aromatics,
Asphaltene molecules are formed by a core of saturated, condensed aromatic rings and an aliphatic periphery. Heteroatoms have been found in the rings. Asphaltentes provide elasticity and strength to the asphalt binder. Molecular weight estimations vary in a really broad range essentially because of the tendency of asphaltene molecules to aggregate together or also with resins. For example molecular weight up to 300000 was given by centrifugal method, while estimation with vapour pressure osmometry led to a range between 1000 and 5000 [24].
Resins are aromatic groups with high polarity. Structure is similar to asphaltenes but with a higher hydrogen/carbon ratio. This is presumably caused by a lower aromatization of the rings. By chromatography, the molecular weight range is of about 800 to 2000 [25]. Resins can also have heteroatoms. Resin fractions are exceedingly adhesive materials and are the dispersing agents or peptizers for the asphaltenes (see later).
Saturates and aromatics are the fraction with the lowest molecular weight, ranging from 300 to 2000 [26]. The hydrogen-to-carbon ratio of the oil fraction is much higher than those of the asphaltenes and of the resins, since they’re predominantly composed by naphtenic and aromatic rings linked with a high number of aliphatic chains, plus paraffin waxes in various amounts. Oils act as a dispersion medium for the asphaltenes.
2.3 Structure
It is generally accepted that asphalt is a substance with colloidal characteristics in which asphaltenes particles are covered by a stabilizing phase of polar resins creating micelles which are homogeneously dispersed in the oily fraction. The origin of this structure has to be found in the insolubility of asphaltenes in the oil fraction; this explains the fundamental role of the resins for the stability of the whole system. The micellar structure is represented in Figure (2.3).
Figure (2.3) – Colloidal structure of asphalt.
The quantity of micelles (and so the possible number of interactions between them) is strongly varying with the relative proportions of the different fractions and with temperature. When the temperature is risen, the resin layer in the micelles responds to temperature by releasing some oil fraction increasing the volume fraction of maltene phase. When the temperature is dropped, the resin layer absorbs some oil fraction from maltene phase, increasing the size and volume fraction of micelles. Consequently at considerably high temperatures the degree of dispersion is so high that asphalt behaves like a purely Newtonian liquid. At low temperatures instead, micellar structure is favoured, and a consequent delayed elasticity is conferred to the binder [25]. The colloidal model was examined by a committee of researchers in the Strategic Highway Research Program [27]. According to SHRP model (which questioned the existence of separate physical phases in asphalts) polar, aromatic molecules, which tend to form associations, representing what is there called the “dispersed moiety”, are dispersed in a bulk “solvent moiety” that consists of nonpolar, aliphatic molecules. The structures formed by molecular association processes were referred to as microstructures. The polar molecular associations help to create three-dimensional intermolecular structures. The bonding (hydrogen bonding, Van Der Waals forces etc.) that produced these associations are really weak and easily destroyed by heat and stress: this explains the viscoelastic nature of asphalt. Time is also affecting the structure of asphalt. On a short time scale, time dependent properties will be carefully
Asphaltenes
Resins
Maltenes
cement, which becomes more brittle and subject to cracking. In practice, a considerable amount of oxidative hardening occurs before the asphalt is placed, during the high temperature mixing with aggregates. Since a normal road pavement is made by only 5%wt of asphalt, asphalt cement is distributed in form of very thin layers, offering a high exchange area to the oxidation process. As quite obvious, the hardening process essentially affects the lightest fractions of the bitumen.
2.4 Polymer Modified Asphalts (PMAs)
Considering both road application and “in life” service, asphalt can be subjected to temperatures ranging from many degrees below zero to about 190-200°C. The glass transition region of asphalt usually starts at temperatures of about -20°C, and the Newtonian region may start at temperatures of about 70°C. In relation to the road application, the different viscoelastic behaviour of asphalt leads to different defects in the paved surface. At usual summer temperatures of pavement surface (60°C), under traffic load, asphalt is not able to maintain the original shape of the pavement, thus leading to permanent deformation known as rutting. In low temperatures, the asphalt gets brittle and tends to crack, because the stiffer structure is unable to relax the internal stresses originating from traffic load. Both of these distresses are not avoidable for neat asphalt, resulting in a shorter pavement lifetime. To overcome these problems various modification of the neat asphalt are studied. For the sake of brevity, we will here discuss the polymer modification only. The idea of such modification, is essentially to improve the mechanical properties of the binder, which are supposed to become similar to the ones of the modifying polymer. This is usually obtained by adding from 2 to 8%wt of polymer on the total mass of the PMA. The modification has to be done with respect to some requirements. First of all, of course, the modification has to be cost effective (reduced maintenance costs has to be taken in account). Secondly, a good compatibility with neat asphalt is required, especially in case of road paving, where high temperature storage specifications [28] define temperature and period of storage during which the polymer should not separate from the binder. Moreover, the increase in viscosity shouldn’t be very high when the asphalt is in its molten state, in order to allow the use of the existing paving
2.4.1 Modifiers
There are three main categories of polymers that have been used for the modification of asphalt binders. The most widely used (about 75%), are thermoplastic elastomers (TPE), followed by plastomers (15%) and reactive polymers (10%) [29]. Plastomers and reactive polymers confer a high rigidity to the binder and strongly reduce deformations under load. Examples of plastomers are polyethylene (PE), polypropylene (APP) and ethylene-vinyl acetate (EVA). These polymers are often used for asphalt modification in application such as roofing or other waterproofing uses [30]. The limited use in road paving is motivated by the very high incompatibility of asphalt with saturated, non-polar aliphatic chains. Reactive ethylene terpolymers are commercially available and can be found with different content of glycidylmethacrylate (GMA) content and ester groups: epoxy rings can easily react, during a curing period at the storage temperature, with carboxylic functional groups present in asphaltenes, thus forming an ester link. This bond will prevent separation improving storage stability. The effect of the thermoplastic elastomers is essentially to confer elasticity to the asphalt binder, in order to allow the binder to recover past deformations. Examples are Styrene-Butadiene-Styrene (SBS), Styrene-Isoprene-Styrene (SIS) and Styrene-Butadiene (SB). To describe the properties of this class of compounds, we will refer to the most widely used one, the SBS. SBS has a two phase morphology consisting of glassy microdomains made of styrene connected with the polybutadiene segments. At regular operating temperatures, styrene blocks are below their glass transition temperature, while the polybutadiene segments are in the rubbery state and the material exhibits a physical crosslinking of styrene domains into a three-dimensional network, shown in Figure (2.4)
When mixed with asphalt SBS is swollen, mainly by the oily fraction of asphalt, but maintains its microstructure and therefore is able to confer elastomeric properties to the whole material without losing processability at high temperatures. This of course causes an alteration of the colloidal equilibrium of the asphalt, because of the removal of part of the maltenic fraction, but the global effect is that of an increased elasticity. The solubility of SBS is only partial, which leads to another problem: the macroscopic separation from asphalt during high temperature storage. The problem is of course more accentuated as the percentage of polymer increases; if the percentage of maltenes absorbed by the polymer is really high, the micellar system can in fact lose stability and tend to segregate in a dark, asphaltene rich phase. Crucial for a good balance between solubility and good mechanical properties of the final PMA, is the modification process, which has to be carefully controlled in all its operating conditions. Low shear stress in the mixer is required in order to keep the “memory” of the original structure of the modifier; on the other hand, good mixing of the two phases is essential to prevent possible separations.
2.4.2 SEBS-modified asphalts
We have noticed various properties of SBS and pointed out how those properties made it the most widely used elastomer for asphalt modification designated for road paving. The presence of the double bond in the butadienic segment has a serious consequence. This double bond represents an environmental problem, since it causes SBS to be prone to ageing (like all unsaturated rubbers), and this limits the possibility of recycling the end-of-life road pavement. This is why increasing attention is being paid to SEBS, which is obtained by the simple hydrogenation of SBS. The chemical structure of SEBS is shown in Figure (2.5).
However, if on one side, the double bond saturation can solve the ageing problem; on the other side, it strongly reduces the polymer polarity thus lowering the compatibility with asphalts, which is the main reason why the use of SEBS as a modifier for road paving binders is still rather limited. Nevertheless, SEBS remains an interesting material, especially for the production of impermeable membranes where compatibility requirements are less stringent, because the PMA is rapidly cooled just after the mixing procedure and the shear-induced metastable morphology is frozen in. Of course, another disadvantage of SEBS is its cost, which is almost twice [31] of the cost of SBS.
2.5 Road paving: the problem of rutting
In Pavement Engineering, rutting is the result of deformation in one or more of the asphalt pavement layers. At one extreme, the deformation occurs in the uppermost layer, this is the case of surface rutting; while on the other extreme, the main component of deformation could come from the subgrade and this is identified as structural deformation [32]. The surface rutting occurs mainly due to the defects of asphalt concrete mix design and construction. In this context binder properties play a major role and accumulated strain is one of the main considerations. It is believed that accumulated strain in asphalt binder, as a consequence of traffic, is mainly responsible for the rutting of asphalt pavements. There were attempts to formulate a specification parameter that can describe the affinity of a binder to the increase of deformation under the periodic loading. For example, the currently used specification in North America [9] uses the parameter 1/J" ( = 10 rad/s) as the specification for high temperatures. Since the loss compliance J" measures the energy dissipated per cycle of sinusoidal deformations [33] it was assumed that a larger value of J" will lead to greater deformation in the binder. Consequently the pavement with such a binder will be more prone to rutting. This parameter, which was found to work well for unmodified asphalts, does not give the correct predictions for polymer-modified asphalts
In attempt to capture the gradual accumulation of strain in asphalt binders (conventional as well as modified) a new test is now extensively studied. This test uses a sequence of shear creep and recovery experiments and is usually referred to as the dynamic creep. The dynamic creep test consists of a number of “cycles” of one second creep followed by nine seconds of recovery. The number of cycles was fixed to 100 by Bahia et al. [34]. In this experiment, the stress level is fixed to 300Pa per each cycle, and the accumulated strain after 100 cycles is recorded. Also, the analysis of the 50th and the 51th cycles is done by means of fitting with the Burgers model, in order to obtain the so called “viscous modulus”, GV. Lately, (Bahia, 2005) a Multiple Stress Creep Recovery test, (MSCR) was purposed. In this case, 10 cycles are performed at a stress level of 100Pa, and 10 other cycles immediately follow, at the stress level of 3200 Pa. The accumulated and recovered strain per each cycle are recorded, and the average of recovered strain at 3200Pa is calculated.
In Europe, researchers suggested using the zero shear viscosity as a high temperature specification parameter [37]. The advantage of using the zero-shear viscosity lies in its sensitivity towards high molecular weight additives to the asphalt binder and has been shown to rank the performance of unmodified as well as modified binders adequately [37,38]. The disadvantage of using zero-shear viscosity is the difficulty encountered in measuring it accurately. The procedure to obtain a reliable value of viscosity can be a very time consuming process [39].
