STONE MASTIC ASPHALT
BY MOHAMMED IMTHIYAZ M.A
DAYANANDA SAGAR COLLEGE OF ENGINEERINGDEPARTMENT OF HIGHWAY TECHNOLOGY
Accurate quantification of aggregate gradation is essential for a better understanding of its effect on the load-carrying capacity of an asphalt mixture. This is even more applicable for stone mastic asphalt (SMA), where stone-to-stone contact forms the cornerstone of its load-carrying capacity, especially against rutting. This paper presents a mix design procedure, which is based on and adapted from the Bailey method, to objectively quantify stone-to-stone contact for SMA mixtures. A total of 16 asphalt mixtures with six different aggregate gradations and three different asphalt binder contents were studied. Volumetric analysis of specimens showed that the volume of coarse aggregate in the mixtures had a significant effect on the voids in the coarse aggregate and the voids in the mineral aggregate (VMA). The coarse aggregate stone-to-stone contact was developed when the volume of coarse aggregate was in the range of 95—105% of the rodded unit weight. Asphalt binder content at 5.5% was optimal for the designed aggregate gradation of SMA mixtures. The test results indicated that the SMA mixtures having stone-to-stone contact exhibited excellent rutting characteristics. The findings also indicated that a positive correlation existed between the rutting obtained from the wheel tracking test and the deformation strain obtained from the uniaxial creep test.
Pavements and surfacings
A road consists of a pavement and a surfacing. When combined, the surfacing and the pavement are designed to carry the loads of heavy vehicles. The pavement provides the structural stability for the road and consists of layers of crushed rock and stone of selected sizes that is compacted using a limited amount of water as a binder. Figure 1 shows the typical layers in a pavement. These are (from top to bottom) are the base course of typically crushed stone or gravel, the sub-base course of selected granular material and the sub-grade which is the in-situ (native) soil. The material may be further strengthened and stabilized with cement, lime or bitumen.3 Pavements in heavily trafficked roads consist of asphalt or concrete instead of granular material in the base and sub base layers.
Typically road surfacings are either Portland cement concrete or bitumen based. The surfacing is to provide a running surface for cars and trucks and to provide a seal against water infiltrating and harming the material in the pavement.
The road user requirements of a surfacing are:
• To provide high skid resistance to assist in vehicle control• To limit the water spray and to provide better visibility• To provide an even riding surface and• To generate low noise levels if the road is near populated or noise sensitive areas.
The structural requirements of a surfacing are to carry the loads and to provide a seal against water infiltrating the material in the pavement. The underlying pavement will deteriorate very quickly under load if water finds its way into the pavement material. Every effort is made to keep the pavement free of water. The passage of heavy trucks on the road, forces the water into the pavement with pressure equal to their tyre pressure. This action will, in time, result in the structural failure of the pavement. This underlines the importance of keeping the pavement (under the surfacing) free of water.
There are three major types of asphalt surfacings, characterized by a mixture of bitumen and stone aggregate. These are:
• Dense Graded asphalt (DGA)• Stone Mastic Asphalt (SMA) and• Open Graded Asphalt (OGA)
Asphalt surfacings differ by the proportion of different size aggregate (crushed rock), the amount of bitumen added and the presence of other additives and material.
A common rural road surfacing treatment is a “chip seal” in which aggregate is rolled into a bitumen coating. This is not an asphalt and is not the subject of this report.
What is SMA?
Stone mastic asphalt was developed in Germany in the 1960’s; Stone Mastic asphalt (SMA) provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia and the United States as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resist permanent deformation. The stone skeleton is filled with a mastic of bitumen and filler to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre.
The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes
(see above picture). Improved binder durability is a result of higher bitumen content, a thicker bitumen film and, lower air voids content. This high bitumen content also improves of flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. There are no precise design guidelines for SMA mixes. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder.
SMA is characterized by a “stone on stone” structure. SMA uses a high proportion of larger stones or aggregate that contact each other. This skeleton of larger stones resists heavy loads by transmitting them to the pavement below. If the underlaying pavement is sufficiently strong then the SMA will resist the heavier loads effectively. (A surfacing cannot compensate for a weak pavement).
The bituminous mastic is intended to hold the aggregate in place and to inhibit the ingress of moisture into the pavement and to provide durability. The mastic consists of bitumen and fine aggregate particles; it may also include a polymer modified bitumen and filler material to increase the mastic’s strength. Fibres may also be added to stabilize the bitumen and to prevent the binder segregating from the aggregate during transport and placement.
It is important that the aggregate material consist of only the larger stones (in the structure) and fines to provide an effective mastic. The intermediate size aggregates are not included, as these keep the larger aggregate apart and reduce the strength of the SMA. See Figure 2.
Other surfacings include dense graded asphalts and open graded asphalts. These other surfacings have a different aggregate grading given by the percentage of material that will pass through sieves of given sizes, as shown in Figure 3.7 Table 1 presents the same information and lists the typical proportions of aggregate or stone of a particular size. This Table demonstrates the differences in the gradings for the different surfacings. The dense graded asphalt has about equal proportions of aggregate in each of the four sizes. Open graded asphalt (OGA) has a higher proportion of the larger stones (compared with DGA) and a smaller percentage of small stones and fine particles (material less than 1.18 mm). SMA typically has a higher proportion of the larger stonesand fine particles but relatively few stones of the intermediate sizes.
SMA also has more bitumen than other asphalts. It has about 6.5 per cent bitumen compared to approximately 5 percent bitumen for DGA. The higher content of bitumen increases its flexibility but can also create problems if it comes to the surface through the compaction process. The bitumen acts as a lubricant and this can cause the aggregate to separate from the mastic during aggressive compaction.
If an SMA mix is designed with too little bitumen, then the percentage of air voids will be too great and water will infiltrate the surface and perhaps the underlying areas. Water in the asphalt can also break the bond between the stones and the bitumen and allow the bitumen to unravel.
The specifications, the mixing, the transport, the placement and compaction of SMA are critical to achieving the desired result.
SMA is mixed and placed in the same plant as that used with conventional hot mix. In batch plants, the fibre additive is added direct to the pugmill using individually wrapped press packs or bulk dispensing equipment. Mixing times may be extended ensure that fibre is homogeneously distributed throughout the mix and temperatures controlled in order to avoid overheating or damage to the fibre. In drum plants, particular care must be taken to ensure that both the additional filler content and fibre additive are incorporated into the mixture without excessive losses through the dust extraction
system. Filler systems that add filler directly into the drum rather than aggregate feed are preferred. Pelletised fibres may be added through systems designed for addition of recycled materials, but a more effective means is addition through a special delivery line that is combined with the bitumen delivery, so that the fibre is captured by bitumen at the point of addition to the mixture.
The primary difference in placing SMA, compared to DGA is in compaction procedures. Multi-tyred rollers are not used due to the possible working of binder-rich material to the surface of the asphalt and consequent flushing and pick-up. Trafficking of the newly placed asphalt while still warm may have the same effect and it is generally preferable for surfaces to cool below about 40°C before opening to traffic. The preferred method of compaction is to use heavy, non-vibrating, steel-wheeled rollers. If these are not available, vibrating rollers may be used but vibration should be kept to a minimum to avoid fracture of coarse aggregate particles, or drawing of binder to the surface of the mix. The use of polymer modified binder may decreases mix workability and necessitate increased compactive effort to achieve high standards of compacted density. Achieving high standards of compacted density and low field air voids has been identified as an important factor in the performance of all SMA work. SMA is normally placed with a minimum layer thickness of 2.5 to 3 times the nominal maximum aggregate particle size. Greater layer thicknesses assist in achieving appropriate standards of compacted density.
Aggregates used in SMA must be of high quality – well shaped, resistant to crushing and of suitable polish resistance.
Binders used in SMA include:
Class 320 bitumen - used in many general applications. Multigrade binder - used to provide enhanced performance at higher traffic
levels. Polymer modified binder - increasingly used in heavy traffic conditions to provide
additional resistance to flushing and rutting. Cellulose fibre is most commonly used in SMA work in Australia. Other fibre types, including glass fibre, rockwool, polyester, and even natural wool, have all been found to be suitable but cellulose fibre is generally the most cost-effective. Fibre content is generally 0.3% (by mass) of the total mix.
Equipment and methods
Stone Mastic Asphalt does not require specialized site equipment, but the compactors are utilized differently to normal mixes. Vibratory compactors are used in static mode on SMA to avoid the possibility of fracturing the coarse aggregate. Pneumatic-tired compactors are not used; the kneading action of a wheeled machine is not effective
once stone on stone contact has been achieved and the rubber wheels tend to pick up material.
During Aggregates and Road building’s Harnden & King were using their Barber-Greene BG-260B 200 Series paver. SMA must be placed in a relatively narrow temperature range of 145-155°C and at Martin Rd this meant material leaving the asphalt plant at a maximum temperature of 165°C degrees.
Compaction was completed by two dual steel drum rollers; a10 tonne Dynapac CC42 for breakdown, followed by a Ferguson 8 tonne roller. Both machines were doing four passes and were achieving densities of about 90 per cent and 93 per cent.
SMA’s relatively rapid cooling rate means it must be compacted immediately after placing, so the breakdown compactor makes each pass as close as possible to the paver screed. SMA also has a sticky nature and compactor drums are lubricated with liquid soap to ensure no pick up occurs.
At Brock Rd., the rolling pattern was adjusted by the addition of a second Dynapac CC42 with each machine making four passes, followed by the Ferguson with three passes. Overall production rates achieved were about 80 tonnes/ h at Martin Rd. and 110 tonnes /h at Brock Rd..
SMA provides a textured, durable, and rut resistant wearing course. The surface texture characteristics of SMA are similar to Open graded asphalt
(OGA) so that the noise generated by traffic is lower than that on DGA but equal to or slightly higher than OGA.
SMA can be produced and compacted with the same plant and equipment available for normal hot mix, using the above procedure modifications.
SMA may be used at intersections and other high traffic stress situations where OGA is unsuitable.
SMA surfacings may provide reduced reflection cracking from underlying cracked pavements due to the flexible mastic.
The durability of SMA should be equal, or greater than, DGA and significantly greater than OGA.
Increased material cost associated with higher binder and filler contents, and fibre additive.
Increased mixing time and time taken to add extra filler, may result in reduced productivity.
Possible delays in opening to traffic as the SMA mix should be cooled to 40°C to prevent flushing of the binder to the surface (bleeding).
Initial skid resistance (lack of Friction) may be low until the thick binder film is worn off the top of the surface by traffic. In critical situations, a small, clean grit, may need to be applied before opening to traffic.
Where SMA should or should not be used?
SMA is not considered to be the only surfacing alternative and respondents considered that the pavement selection guidelines should be consulted. Generally, SMA is ideal for highways as its strength makes it more resistant to rutting. It is particularly useful when there has been some cracking of a previous (lower) surfacing. SMA with its higher bitumen content is a more flexible surfacing and this allows it to accommodate more movement. As a result SMA decreases the tendency of cracking in the lower pavement layers reflecting (or propagating) through and affecting the SMA surface. Other surfacings like DGA do not have this capability. The ability to reduce cracking provides for a longer pavement life. SMA is not suitable for small areas or for areas where the asphalt laying plant has restricted access. SMA is very stiff and less workable than other asphalts. This stiffness makes it harder to compact particularly if modified binders havebeen used. The addition the use of modified binders decreases the time available for compaction. Construction crews, plant and supervision must develop skills for the effective placement and compaction of SMA. If these skills are not available then SMA should not be used. Because SMA is difficult to compact by hand, it is not suitable for small or restricted areas.
SMA is constructed with the significant stone on stone contact providing considerable compressive strength. It is also constructed with a textured surface where the stones on the upper surface stand proud of the mastic. This means that the stones are less resistant to sideway (shear) forces on the surface caused by trucks making tight turns. In these cases the tyre – surface forces can cause the “proud” stones to “roll out” and leave the binder behind. SMA is often not recommended at smaller roundabouts and DGA may be more appropriate. It is emphasized that any surfacing should not be considered to be a “universal” solution for all conditions and local knowledge and experience should be used to select the most appropriate mix.
SMA and OGA have been developed to provide an effective surface texture. This is a prime safety requirement and helps to maintain skid resistance at the higher speeds. The texture is also useful in decreasing the water depth on the surface. These qualities make for safer roads. Skid resistance is a function of the micro texture (or the roughness of the individual pieces of exposed aggregate) and the macro texture (developed from the arrangement of the aggregate on the surface). These textures are shown diagrammatically in Figure 4 from the Austroads “Guidelines for the Management of Road Surface Skid Resistance”. Both qualities are required to produce effective skid resistance at highway speeds.
Summary and Recommendations - The use of SMA
The national surfacing guides have indicated that SMA has a number of advantages over other asphalt pavement types. In particular, the advantages of SMA include its durability and long life qualities as well as its texture to provide skid resistance at higher speeds with lower water spray and noise characteristics.
There is a strong imperative for road authorities, including Main Roads, to develop and provide pavements and surfacings with lower whole-of-life costs. SMA has been recognized to provide an average texture depth to assist in providing skid resistance. SMA is considered to be a suitable pavement for higher speed roads and most intersections. It is most useful on the higher-use roads as its greater durability means less maintenance.
SMA has been, and is still, used by a number of authorities around Australia and the world. Authorities are continuing to develop SMA as a durable asphalt with large average texture depth.
There is a need to develop an alternative asphalt surfacing that is durable and with an appropriate texture and skid resistance. Overseas, SMA has been this alternative surfacing.
SMA is complex and dependent on the properties of the constituent natural materials that, over time, come from different sources. SMA requires continued research and development to maintain qualities of durability as well as texture and skid resistance.
The modification and development of SMA requires studies of the performance of small specimens in the laboratory under controlled tests and also studies of the surfacing on the road. These field trials are important to understand the performance of the SMA surfacings and its associated material properties. Important aspects of these field trials are:
1. That they are undertaken at sites similar to proposed installations
2. That there is a benchmark surfacing installed at the same time on the same section of road and 3. That the performance of the road sections be monitored closely.
Crash studies to assess the performance of a surfacing
In a recent report by Road Systems and Engineering (RS&E), the crash rate at sites before SMA was installed was compared with the crash rate afterwards. The conclusions drawn were that
The average annual crash rate before SMA was installed was 40.6 crashes per km for every 100 million vehicles; after the installation of SMA the annual crash rate was virtually unchanged to 40.7 crashes per km for every 100 million
vehicles. This compared with an annual crash rate across the state of 44 crashes per km for every 100 million vehicles. Motorists are less likely to crash on roads surfaced with SMA than all other roads.
The average annual fatal crash rate before SMA was installed was 0.77 fatal crashes per km for every 100 million vehicles; after the installation of SMA the annual fatal crash rate was less at 0.47 crashes per km for every 100 million vehicles. This compared with an annual fatal crash rate across the state of 0.77 fatal crashes per km for every 100 million vehicles. Motorists are significantly less likely to have a fatal crash on roads surfaced with SMA than all other roads.
This report has addressed these major elements through interviews with a number of respondents and through an investigation of all sites where the Department of Main Roads used SMA.
It is concluded that SMA is an appropriate surfacing to use. Main Roads must continue to develop surfacings, which are durable and have a longer life and lower whole-of-life cost. These surfacings must also have appropriate skid resistance, through an appropriate surface with both microtexture and macro texture.
It is concluded that the process of introducing SMA into the Main Roads could have been better. A number of recommendations are directed towards improving procedures for introducing innovation in the Department of Main Roads.
The use of SMA does not show any systemic safety issues. There are however institutional issues that influence the effective use. Recommendations in this report indicate how these issues can be addressed. A few sites where SMA has been used should have the crash experience analyzed in more detail to determine means of crash rate reductions.
“Civil engineering can be summarized as the task of providing safe and effective infrastructure to the community.”