During this unit, we will look specifically at what compaction does for an HMA mix design and for a pavement. We will look at the variables and factors that affect the compaction process. We will look specifically at the types of rollers or compaction equipment available and how to pick the best roller for the job.
The construction environment is dangerous. The need for safety on the job needs to be emphasized.
This slide shows the typical compaction process. The paving machine will compact the HMA mix to about 85 % of G mm . Generally the first roller will be a vibratory roller – it will compact the HMA mat to about 91 % of G mm and then the pneumatic roller will compact it to about 94% of G mm . The last steel-wheeled roller is there to seal of the surface and iron out any marks that may have been left by the pneumatic roller.
The density of a material is simply the weight of the material that occupies a certain volume of space. For example, an HMA mixture containing limestone aggregate might have a compacted density of 140 pounds per cubic foot; this density, or unit weight is an indication of the degree of compaction of the HMA. Compaction is the process through which the HMA is compressed and reduced in volume. A pass is defined as the entire roller moving over one point on the HMA mat one time. If you were standing on the side of the road counting the number of time the roller goes by in each direction, that total number would equal the number of passes. A coverage is defined as the roller moving over the entire width of the HMA mat one time. Since the roller is almost always less than the width of the HMA mat, it takes multiple passes to cover the mat. It is best illustrated in the next slide.
This slide shows graphically the concept of pass and coverage.
The time available for compaction is the time (in minutes) that a particular mix is at the right temperature range for efficient compaction. Ideally, there is a TAC for breakdown, intermediate, and finish rolling. Compaction must be accomplished before the HMA mat cools to a temperature below 175-185 °F (80-85 °C).
Compaction is the single most important factor that affects the ultimate performance of a HMA pavement. Adequate compaction of the HMA mix increases the fatigue life, decreases permanent deformation (rutting), reduces oxidation or aging, decreases moisture damage, increases strength and stability, and decreases low-temperature cracking. An HMA that has all the desirable mix design characteristics will perform poorly under traffic if that HMA mix is not compacted to the proper density level.
Properties of the materials include the surface texture, porosity and particle shape of the aggregate and the viscosity of the liquid asphalt binder. Environmental variables are temperature, wind, and solar flux. Letdown site conditions include the existing road surface texture, lift thickness and profile or the sub-base stability.
Aggregates vary greatly across the nation from sedimentary to metamorphic and igneous. Aggregates may be natural deposits with little to no processing, processed aggregates such as limestone, dolomite, expanded shale, quartzite, and granite, and synthetic aggregates such as steel and blast furnace slags. Asphalt binder is graded by penetration, viscosity, or performance, and varies in hardness based on climate. HMA mixture properties vary based on type of HMA mixture specified. The type of HMA mixture specified varies directly with traffic loading. (Equivalent Single Axle Loadings, ESAL)
Dense graded HMA mixtures are the most common types of HMA pavements specified worldwide. Dense graded mixtures are comprised of coarse, medium and fine aggregate particles. Today, Superpave, Stone Matrix Asphalt (SMA) and other specialty HMA mixtures tend to not be dense graded but more gap or open-graded. These HMA mixtures have a much larger proportion of coarse aggregate.
Gap Graded or Porous Pavements, aggregate size will depend on the use / loading
Stone Matrix Asphalt or SMA
Aggregate is “the rock that carries the load.” Three properties of the coarse aggregate particles used in an asphalt mixture that can affect the ability to obtain the proper level of density are: particle shape of the aggregate, the number of fractured faces, and the surface texture. As the crushed content, nominal maximum size, and hardness of the aggregate increase, the compaction effort needed to obtain density also increases. Angular particles offer more compaction resistance than rounded particles, as do harsh mixtures versus dense graded HMA mixtures. In general, HMA mixes will be harder to compact. It is difficult to achieve density with high dust content mixes.
As the crushed content, nominal maximum size, and hardness of the aggregate increase, the compaction effort needed to obtain density also increases. Angular particles offer more compaction resistance than rounded particles, as do harsh mixtures versus dense graded mixtures. In general, HMA mixes will be harder to compact. High dust content mixes are also difficult to densify.
Asphalt binder is what “binds the rock together.” The grade and amount of Asphalt binder used in the mix affects the ability to densify the HMA. The degree of hardening that occurs in the asphalt binder during the manufacture of the HMA affects the compaction. Modifying the asphalt binder increases the compaction effort needed to obtain density. Asphalt binders can be similar to conventional asphalt binders. However, some PG grades will be modified. These will probably necessitate higher HMA mixture temperatures.
Properly compacted and designed mixtures will ensure a durable mixture which performs long-term by limiting air and water to pass through it. In general, a mix with too little asphalt binder may be stiff and require an increase in compaction effort, whereas a mix with too much asphalt binder may shove under the rollers.
A close-up view of a compacted HMA is shown in this slide. The slide shows aggregate, asphalt binder, and air voids. The densification behind the rollers typically takes us to 8% air voids so that a properly designed HMA mixture will consolidate to 4% air voids in the pavement, long term. A HMA mix that is placed at a higher temperature will be easier to compact than will a HMA mix that is lower in temperature when it is laid. The moisture content of the HMA mix should be less than 0.5 percent by weight of HMA mix, or 0.3 percent for high gravity mixtures.
Research work and field experience show that once a pavement cools to about 170 F, the internal friction and cohesion of the mix increases to the point that little density gain is achievable. Six variables were found to have an effect on the rate of cooling a layer of asphalt binder placed on top of another existing layer of the same material. Layer thickness, air temperature, base temperature, HMA mix temperature, wind velocity, and solar flux are the variables.
A series of “cooling curves” for HMA illustrate the amount of time available for compaction under different combinations of the variables. The figures shown assume air temperature to be equal to the surface temperature of the base. A constant wind velocity of 10 knots and a constant degree of solar radiation is also used to generate the graphs. The curves then provide the time in minutes, for the HMA mix to cool from the laydown temperature to a minimum compaction temperature for different compacted layer thicknesses.
Layer thickness is the thickness of an HMA pavement layer, and it is the most important variable in the rate of cooling of HMA mixtures. It is very difficult to obtain the desired density on thin lifts of HMA in cool weather because of the rapid loss in temperature in the HMA mix. A portion of the heat in the HMA layer is lost to the air as well as the base as it is placed. Depending on the air and base temperature as well as moisture content, the loss of temperature could be large or small.
HMA mixes are usually produced at temperatures between 280F and 330F, with higher temperatures being specified for modified asphalt binder mixtures. Depending on environmental conditions and the length of haul, the HMA mixture can lose between 10F and 30F from the plant to the paver. As the temperature of the HMA being placed is increased, the time available for compaction is greater.
For a thickness of 2 inches (50 mm) and a base/air temperature of 40F(4 C), the time to cool to 175F ( 80C) increases from 9 min to 16 min as the placement temperature increases from 250 to 300F (120C to 150C). For a 4 inch (100 mm) layer and a base/air temperature of 60F (16C), a change in laydown temperature from 300F to 250F (150C to 120C) reduces the time available for compaction from 36 min to 21 min. The effect of HMA mat laydown temperature is more significant at lesser HMA mat thicknesses and lower base temperatures.
A thin layer of HMA mix will cool more quickly in a strong wind than when there is little or no wind. Wind has a greater effect at the surface of the HMA mix than within the HMA mix, and can cause the surface to cool so rapidly that a crust will form and checking of the HMA may occur.
The amount of radiant energy available from the sun is a function of many variables, including the position of the sun above the horizon, the distance above sea level of the project, the amount of turbidity in the air, and the degree of cloud cover. The amount of solar flux is more important in its effect on base temperature than HMA mix temperature. Summarizing, a HMA mixture placed too warm or too cold will lead to compaction problems in the field during placement. Modified asphalt binder producers typically supply end users with temperature-viscosity charts showing recommended compaction temperatures.
Many factors–such as the relationship between lift thickness and nominal maximum aggregate size in the HMA mix (typically two or three times), or the uniformity of the lift thickness–at the laydown site directly affect the ability of the compaction equipment to gain the required level of pavement density. The suggested lift thickness is 3 to 4 times the nominal max size. It is much easier to obtain a required level of density in an HMA layer that has a constant thickness compared with a course that varies in depth. HMA leveling courses that, by their very nature and purpose, are nonuniform in thickness, are often difficult to densify to a given air void content uniformly, when placed over a rutted or wavy road.
Compact effort increases due to• Crushed content• Nominal maximum size• Hardness of the aggregate• Angularity of aggregate• Less natural sand• High dust content mixes• Note: Typically, HMA mixes will be harder to compact.Construction Compaction 16
Hot Mix Compaction • Asphalt binder holds particles together – Provides lubrication at high temperatures – Provides cohesion at in-service temperatures • Limits air and water intrusion into HMA mat Construction Compaction 18