There are a very large number of bituminous mixtures, which vary according to density, bitumen content, bitumen grade, aggregate size and aggregate grading. Previously in the UK these mixtures used to be classified into two groups, namely ‘asphalts’ and ‘macadams’. The move to European standardisation has meant that bituminous mixtures are now classified into seven material specifications as described in the BS EN 13108 series. Of these seven only five have any relevance to the UK and of these five only two have major relevance.
Under BS EN 13108, bituminous mixtures are classified according to the grading of the aggregate (mixture type), their upper sieve size (maximum nominal aggregate size), the intended use of the material and the binder used in the mixture. Table 1 summarises the designation of the bituminous mixtures.
Asphalt concretes (previously termed dense macadams) are characterised by relatively low binder content and a continuously graded aggregate. They rely on the packing and interlock of the aggregate particles for their strength and stiffness. The binder coats the aggregate, and acts as a lubricant when hot and an adhesive and waterproofer when cold. The grade of binder used is softer than for hot rolled asphalts, being 20/30 to 160/220 pen inclusively.
Because of their lower binder content, asphalt concretes are cheaper than hot rolled asphalts (HRA) and stone mastic asphalts (SMA). In general asphalt concretes have a higher void content than HRA and SMA and are therefore more permeable and less durable. Asphalt concrete is used for surface courses, binder courses and bases and is specified in BS EN 13108 Part 1. A summary of asphalt concrete material options for the UK is listed in Table 2.
Hot Rolled Asphalts
Hot rolled asphalts (previously termed ‘asphalts’) are dense materials that are characterised by their high bitumen content and high filler/fines content. They derive their strength and stiffness from a dense stiff mortar of bitumen, filler and fines. The coarse aggregate content is relatively low so the overall particle size distribution is gap-graded. The material transmits load through the mortar continuum. This mortar, being rich in bitumen, is expensive and the coarse aggregate serves to increase the volume of the mortar with a relatively cheap material, thereby reducing the overall cost. The binder used for hot rolled asphalt will normally be between 30/45 and 100/150 penetration grade bitumen.
Hot rolled asphalts are used in surface courses, binder courses and bases, and are specified in BS EN 13108 Part 4. A summary of hot rolled asphalt material options for the UK is listed in Table 3. Surface course mixtures may be either type F, incorporating fine sand, or type C, incorporating crushed rock or slag fines that are more coarsely graded.
Table 4 shows the grading specification for the group of preferred hot rolled asphalt mixtures. It can be seen that each mixture is designated according to the coarse aggregate content and its nominal size. Thus a 50/14 mixture has 50% coarse aggregate with a nominal size of 14 mm. Freshly laid surface course hot rolled asphalt presents a smooth surface with coarse aggregate particles submerged within the mortar.
In order to provide a skid-resistant surface, coated chippings are rolled into the surface, which adds to the cost. Hot rolled asphalts have a very low permeability, and are capable of transmitting high stresses whilst providing some ductility. They are therefore very durable and normally used where traffic loads are high or durability is important.
Porous asphalt is a bituminous material designed to provide a large volume (at least 20%) of interconnected air voids so that water can drain through the material and run off within the thickness of the layer. This requires the underlying binder course to be impermeable. It is used exclusively for surface courses and can be laid in more than one layer.
The very high content of interconnected voids not only allows the passage of water but also allows the movement of air, thereby providing noise reducing characteristics. Porous asphalts are specified in BS EN 13108 Part 7. The aggregate grading consists predominantly of coarse aggregate: about 75% retained on the 2 mm sieve.
Fine aggregate fractions are added to enhance the cohesion and stiffness of the mixture but in sufficiently small quantity so as not to interfere with the interlock of the coarse particles, and to leave enough voids to provide a pervious structure.
Because of its porous nature, the material is vulnerable to ageing through oxidation of the bitumen. To counteract this, the bitumen content must be sufficient to provide a thick coating on the coarse aggregate particles. The advantages of porous asphalt are that it minimises spray in wet weather, reduces surface noise, improves skidding resistance, and offers lower rolling resistance than dense mixtures.
However it is weaker than denser mixtures and in the UK has limited relevance and dwindling use owing to the increased use of SMA and thin surfacings on highways (see BRE Special Digest 1).
Stone Mastic Asphalt
Stone mastic asphalt (SMA) is specified in BS EN 13108 Part 5. SMA has a coarse aggregate skeleton but the voids are filled with a mortar of fines, filler and bitumen. It thus resembles hot rolled asphalt, particularly the high-stone-content mixtures, but it may best be considered as having a coarse aggregate structure similar to that of porous asphalt but with the voids filled.
SMA differs from hot rolled asphalt in that the quantity of mortar is just sufficient to fill the voids in the coarse aggregate structure. It therefore provides high stiffness owing to the interlock of the coarse aggregate particles, and good durability because of a low void content. Stone mastic asphalts are used in surface courses and binder courses, and a summary of SMA material options for the UK is listed in Table 5.
Recipe and Designed Mixtures
The majority of bituminous mixtures are recipe mixtures. In other words, the mixtures are put together according to prescribed proportions laid down in the appropriate European standard. These mixture proportions have been derived through experience in use and, provided the separate ingredients meet their specifications, the mixture will provide the required performance in most situations.
This approach is consistent with the empirical method for the structural design of roads that predominated until relatively recently. Thus the design chart used to determine the thickness of the base layer requires that the base material has the same mixture proportions as the materials in the roads from which the design chart was originally established.
Recipe mixtures provide a satisfactory performance in many cases and there is some advantage in the simplified approach that recipe mixtures offer. However, there are limitations to the use of recipe mixtures that match the limitations of empirical structural design of roads. These are as follows:
- Non-specified materials cannot be used. For example, locally available sand may not meet the grading requirements of the specification but may produce a satisfactory mixture. Recipe mixtures preclude any assessment of the properties of a mixture containing that sand.
- No procedure is available to assess causes of failure.
- No procedure is available to optimise the mixture proportions. This is particularly important as far as the bitumen is concerned because this is the most expensive ingredient and has a strong bearing on the performance of mixtures, especially those which are denser.
These drawbacks have led to the development of a procedure for the design of bituminous mixtures, which has occurred in parallel with the development of analytical procedures for the structural design of roads. An analytical approach to road design enables the determination of the thickness of the road structure through an analysis of its behaviour under the applied load.
This clearly requires knowledge of certain properties of the materials and it follows that materials will have to be produced with particular characteristics. The most commonly used procedure for mixture design (BS 598: Part 107: 2004) is based upon the Marshall test, which was originally developed in the USA for designing mixtures for use on airfield runways.
The objective of the procedure is to determine an optimum binder content from a consideration of mixture strength (stability), mixture density, and mixture deformability (flow). Test samples of binder/aggregate mixtures are prepared using the materials to be used in the field.
The aggregate grading is kept constant and samples with a range of binder content are produced. The samples are prepared and compacted in a standard way into moulds 101.6 mm in diameter and 70 mm high. The state of compaction achieved is determined by measuring the bulk density and calculating the compacted aggregate density.
At low binder contents the mixture will lack workability and the densities will be correspondingly low. At high binder contents, aggregate will effectively be displaced by bitumen, and again the densities will be low. Each of these measures of density will thus produce an optimum binder content, as shown in Fig. 1a and b.
To test the strength and resistance to deformation of the material, the specimens are heated to 60°C and subjected to a compression test using special curved jaws that match the curved sides of the specimens, as shown in Fig. 2. Thus the load is applied radially. The jaws of the machine are driven together at a constant rate of 50 mm per minute until the maximum load is obtained, which is termed the ‘stability’. The deformation of the sample at this maximum load is also recorded and termed the ‘flow’.
Typical plots of stability and flow against binder content are shown in Fig. 1c and d. The stability plot gives a third optimum binder content, and the design binder content is obtained from the average of this and the optima from the two density plots. The flow at this design binder content can then be read off. Minimum values of stability and flow are specified according to the amount of traffic that the road will carry. In evaluating mixtures it is helpful to consider the Marshall quotient, Qm, which is derived from the stability and flow:
Qm = stability/flow
Thus Qm bears some resemblance to a modulus (ratio of stress to strain) and may be taken as a measure of mixture stiffness. More recently new approaches to bituminous mixture design have been proposed. The Superior Performing Asphalt Pavement (Superpave™) mixture design process was developed in the USA through the Strategic Highway Research Program (SHRP) in the early 1990s (Harrigan et al., 1994).
The Superpave™ method incorporates a pavement design methodology that takes into account the environ mental conditions that the pavement could expect to experience during its design life. The design method involves the selection of the bitumen based on a performance grading (PG) system together with volumetric measurements (density, air voids content, etc.). Asphalt mixtures on heavily trafficked roads are also subjected to permanent deformation and fatigue tests as part of the mixture design process.
Methods of Production
The process of manufacture of bituminous materials involves three stages. Firstly, the aggregate must be proportioned to give the required grading, secondly the aggregate must be dried and heated, and thirdly the correct amount of binder must be added to the aggregate and mixed to thoroughly coat the aggregate particles and produce a homogeneous material. The most common type of plant in the UK is the indirectly heated batch mixing plant. A schematic diagram of this type of plant is shown in Fig. 3.
The aggregate is blended from cold bins and passed through a rotary drier/heater. Here the moisture is driven off and the aggregate temperature raised to the prescribed mixing temperature for the type of material being produced. The aggregate is then transported by hot elevator to hot storage bins, where it is separated into fractions of specified size.
Aggregates are released into the weigh box in the desired proportions and then released into the mixer. Bitumen heated to the prescribed temperature is also introduced to the mixer, the quantity being determined using a weigh bucket or volumetrically using a flow meter. The mixing time varies up to 60 seconds but should be as short as possible in order to limit oxidation of the binder. After mixing, the material is discharged directly into a wagon.
This type of plant is very versatile, being capable of producing a wide range of different asphalt mixture types, and being able to easily adjust to a wide range of mixture specifications. A variation on this type of plant is to dry and heat the aggregate in batches before it is charged into the mixer. This eliminates the need for hot aggregate storage, but the proportioning of the cold aggregate needs to be very carefully controlled.
An alternative type of mixer is the drum mixer, which gives continuous rather than batch production. Here the cold aggregates are proportioned and conveyed directly into a drum mixer that has two zones. The first zone is where drying and heating occur, and in the second zone bitumen is introduced and mixing takes place.
The advantages of this type of plant are that the amount of dust emission is reduced, the process is simpler and, above all, the rate of production can be very high – up to 500 tonnes per hour. This is advantageous where large quantities of the same type of material are required, but it is difficult to change production to a different mixture.