HMA Construction Program

HMA Construction Program
Module 8 – Compaction

Ask: Why do we compact? The last chance to make a job is during the compaction process, which must be closely controlled to achieve a quality pavement. Compaction determines the ultimate performance of the mix and pavement. Compaction is often very difficult to achieve in the field because of a number of variables. You must manage these variables and control them, in order to ensure that you achieve density. The key is to recognize the advantages and disadvantages of each type of roller per project. Setting up and maintaining the rolling pattern involves much effort on the part of the operator.

Module 8 Objectives Objective of Compaction
Engineering Properties Related to Compaction Factors Affecting Compaction Types of Rollers Considerations for Selecting Rollers

During this unit, we will look specifically at what compaction does for a 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.

Module 8 Objectives (cont’d)
Compaction Variables Roller Maintenance Roller Productivity Operating Techniques

We will look specifically at the variables facing the compaction process and the importance of keeping rollers maintained. Slow and steady is the right way to do it!

Definitions Density Compaction Pass Coverage

The density of a material is simply the weight of the material that occupies a certain volume of space. For example, an asphalt concrete mixture containing limestone aggregate might have a compacted density of 2.4 tonnes/m3; this density, or unit weight is an indication of the degree of compaction of the mix. Compaction is the process through which the asphalt mix is compressed and reduced in volume. A pass is defined as the entire roller moving over one point in the mat one time. A coverage is defined as the roller moving over the entire width of the mat one time.

Time Available for Compaction
(TAC)

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 mat cools to a temperature below °C ( °F). SMA mixes need to be compacted immediately, while hot. Little, if any, densification for SMA will occur below °C ( °F).

Importance of Compaction
Improve Mechanical Stability Improve Resistance to Permanent Deformation Reduce Moisture Penetration Improve Fatigue Resistance Reduce Low-Temperature Cracking Potential

Compaction is the single most important factor that affects the ultimate performance of a hot-mix asphalt pavement. Adequate compaction of the 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 asphalt mixture that has all the desirable mix design characteristics will perform poorly under traffic if that mix is not compacted to the proper density level.

Factors Affecting Compaction
Properties of the Materials Environmental Variables Laydown Site Conditions

Properties of the materials include the surface texture, porosity and particle shape of the aggregate and the viscosity of the liquid asphalt. Environmental variables are temperature, wind, and solar flux. Laydown site conditions include the existing road surface texture, lift thickness and profile or the sub-base stability.

Properties of the Materials
Aggregate Asphalt Cement Mix Properties

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 is graded by penetration, viscosity, or performance, and varies in hardness based on climate. Mixture properties vary based on type of mixture specified. Type of mixture specified varies directly with traffic loading. (Equivalent Single Axle Loadings, ESAL)

Dense graded 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 mixtures tend to not be dense graded but more gap or open-graded. These mixtures have a much larger proportion of coarse aggregate. Discussion will generally be held to dense graded mixtures but the discussion time will allow for thorough review of these current “state-of-the-art” mixtures also.

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 compactive effort needed to obtain density also increases. Angular particles offer more compactive resistance than rounded particles, as do harsh mixtures versus dense graded mixtures. In general, Superpave mixes will be harder to compact. High dust content mixes are also difficult to densify.

Asphalt cement is what “binds the rock together.”
The grade and amount of asphalt cement used in the mix affects the ability to densify the mix. The degree of hardening that occurs in the AC during the manufacture of the mix affects the compactibility. Modifying the asphalt increases the compactive effort needed to obtain density. Superpave binders can be similar to conventional binders. However, with some PG grades these binders will be modified. These will probably necessitate higher mixture temperatures.

Hot Mix Compaction Asphalt binder holds particles together
Provides lubrication at high temperatures Provides cohesion at in-service temperatures Prevents air and water intrusion into mat

Properly compacted and designed mixtures will ensure a durable mixture which performs long-term by not allowing air and water to pass through it. In general, a mix with too little AC may be stiff and require an increase in compactive effort, whereas a mix with too much AC may shove under the rollers.

A close-up view of a compacted HMA is shown in this slide.
The slide shows aggregate, asphalt, and air voids. The densification behind the rollers typically takes us to 8% air voids so that a properly designed mixture will consolidate to 4% air voids in the pavement, long term. A mix that is placed at a higher temperature will be easier to compact than will a mix that is lower in temperature when it is laid. The moisture content of the mix should be less than 0.5 percent by weight of mix, or 0.3 percent for high gravity mixtures.

Environmental Variables
Layer thickness Air and base temperature Mix laydown temperature Wind velocity Solar flux

Research work and field experience show that once a pavement cools to 80 oC, 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 placed on top of another existing layer of the same material. Layer thickness, air temperature, base temperature, mix temperature, wind velocity, and solar flux are the variables.

Mix Base °C Time for Mat to Cool to 80°C, Minutes 150, -10 120, -10 150, 10 120, 10 150, 0 120, 0 50 40 30 20 10 25 50 75 100 125 150 175 200 Mat Thickness, millimeters

A series of “cooling curves” for asphalt mixtures 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 mix to cool from the laydown temperature to a minimum compaction temperature for different compacted layer thicknesses.

Major Factors Affecting Rolling Time
allows MORE time allows LESS time Mat Thickness THICK THIN Mix Temperature HIGH LOW Base Temperature HIGH LOW

Layer thickness is the thickness of an asphalt pavement layer, and it is the most important variable in the rate of cooling of asphalt mixtures. It is very difficult to obtain the desired density on thin lifts of mix in cool weather because of the rapid loss in temperature in the mix. A portion of the heat in the asphalt 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.

Asphalt mixes are usually produced at temperatures between 130oC and 165oC, with higher temperatures being specified for modified asphalt mixtures. Depending on environmental conditions and the length of haul, the mixture can lose between 3oC and 15oC 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 50 mm and a base/air temperature of 4 oC, the time to cool to 80 oC increases from 9 min to 16 min as the placement temperature increases from 120 oC to 150 oC. For a 100 mm course and a base/air temperature of 16 oC, a change in laydown temperature from 150 oC to 120 oC reduces the time available for compaction from 36 min to 21 min. The effect of mat laydown temperature is more significant at lesser mat thicknesses and lower base temperatures.

A thin layer of 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 mix than within the mix, and can cause the surface to cool so rapidly that a crust will form and checking of the HMA may occur.

Compaction Temperature Range
(185 °F) 150 °C (300 °F)

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 mix temperature. Summarizing, a mixture placed too warm or too cold will lead to compaction problems in the field during placement. Modified AC producers typically supply end users with temperature-viscosity charts showing recommended compaction temperatures.

Laydown Site Conditions
Lift thickness versus aggregate size Lift thickness uniformity Base Conditions

Many factors–such as the relationship between lift thickness and nominal maximum aggregate size in the mix (typically 2x), 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. In Superpave, nominal max size is used, so there will be some larger material. New rule of thumb is 3 to 4x for lift thickness. It is much easier to obtain a required level of density in an asphalt layer that has a constant thickness compared with a course that varies in depth. Asphalt 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.

Types of Rollers Static Steel Wheel Pneumatic – Rubber Tired Vibratory

The type of equipment used to compact the asphalt mix obviously has a significant effect on the degree of density that can be obtained in a given number of passes of a particular roller. Three types of self-propelled compaction equipment are currently being used: static steel wheel rollers, pneumatic tire rollers, and vibratory rollers.

How Do Rollers Compact? (Contact Pressure) By applying their load
over a given area! (Contact Pressure)

Rollers compact by applying their weight over the area of the drum/tires that touch the mat (contact pressure), packing the aggregate particles together and removing most of the air voids. Each roller type applies contact pressure slightly differently, but the concept is the same. In practice, compacting isn’t so simple because of the many compaction variables involved. We know densification increases stability by recalling that if we walk across a freshly laid mat, without any roller hits, that we sink in the mat yet after even one pass of the roller we make no marks.

Static Steel Wheel Roller
Contact Pressure Operation

Mass may be increased by adding ballast to static rollers, which in turn increases the contact pressure per linear mm. Maintenance ensures not only that the equipment will work but that the finished surface will not be blemished. Poor scraper performance or an overloaded roller both cause blemishes in finished pavement surfaces.

Drawbar pull is defined as the horizontal force required to move the roller forward.
Rollers with large-diameter rolls have lower drawbar pull (rolling resistance) because they do not tend to penetrate as far into the mix as does a roller with smaller-diameter rolls. Static steel wheel rollers normally range in weight from 2.7 to 12.7 tonnes and have compression drums that vary in diameter from approximately 1 to 1.5 m. The gross weight of the roller can usually be altered by adding ballast to the roller, but this adjustment cannot be made while the roller is operating, and is not normally changed during the term of a project.

Kilograms Per Linear Millimeter (kg/mm)

Static rollers are often specified having a certain kilogram per linear millimeter of roller width or kg/mm. The kg/mm rating is the contact pressure at a contact arc of 25 mm, but the weight is not equally distributed on both drums.

Example: A 10.9 tonne roller with two, 1.3 meter wide drums. In this case, 60% of the mass is on the drive drum and 40% is on the guide drum. Calculate the kg/mm

Example problem to be worked by the students prior to displaying the next slide with the solution.

Example Solution: 1 tonne = 1,000 kg 1 meter = 1,000 mm 10.9 tonne x 1,000 kg = 10,900 kg 1 tonne 1.3 m x 1,000 mm = 1,300 mm 1 meter 60% x 10,900 kg / 1,300 mm = 5.0 kg/mm 40% x 10,900 kg / 1,300 mm = 3.4 kg/mm

The solution to the kg/mm problem.
The old standard 3-wheel roller, two large diameter drive drums and one small diameter steering drum, have very different kg/mm ratings. Between the drums, this produces an uneven density profile across the width of the entire drum. The full width of this roller is not truly available for calculating compaction production rates, and would be less efficient than a narrower roller with uniform kg/mm.

Contact Pressure

Effective weight or contact pressure, in terms of kiloPascals, kPa, of contact area, is the key variable for this type of equipment and is dependent on the depth of the penetration of the rolls into the mix. The greater the depth of penetration, the greater the contact area and thus the less the contact pressure.

Roller Contact Pressure
Roller Contact Pressure at Penetration Depths (in kPa) Static Roller(depth, mm) 10.9 tonnes(with ballast)

Roller contact pressure increases as the mix’s internal strength increases during compaction.
As a typical 10.9-tonne roller makes its first pass, the contact pressure may range from kPa as the drum sinks in 12.5 to 19.0 mm, depending on the initial stiffness of the mix. As additional passes are made, the mix becomes stiffer as the aggregate is packed together and the air voids are reduced. The drum eventually “walks out” of the mix and the contact pressure becomes extremely high. At 1.6 mm penetration, the contact pressure is 910 kPa. (A 266% increase over the original pressure)!

Frictional force turns trailing drum
Travel Non-powered drum Powered Drum R W W R Frictional force turns trailing drum

Once the size and weight of a static steel wheel roller is selected, the variables under the control of the roller operator are the speed of the roller, the position of the roller on the mat in relation to the paver, operation with the drive wheel toward the paver, and the number of passes made with the roller.

Pneumatic Tired Rollers
Wheel load Tire design Inflation pressure Contact area

The tire pressure used depends in part on the number of plies used in the tires.
If the mix is tender, a lower tire pressure will displace the mix less than will a higher pressure in the tire. For a stiff mix, a higher tire pressure can be used, because the mix will be stable enough to support the weight of the roller without the mix shoving laterally under the tires. Tire pressure is normally kept constant for a particular project, but the level selected should be dependent on the properties of the mix being compacted and the position of the roller on the mat.

Most pneumatic rollers are operated in the intermediate roller position, behind a vibratory or static steel wheel breakdown roller and in front of a static steel wheel finish roller. These rollers are sometimes used for initial rolling of the mix as well as occasionally for finish rolling. Pneumatic tires tend to pick up some modified mixes. The higher binder contents in SMA make them especially susceptible to pick up by pneumatic tires. Pneumatic tired rollers are therefore not used on SMA.

Tire Inflation Pressure Versus Ground Contact Pressure

For this type of roller, the compactive effort applied to the mix is a function of the wheel load of the machine, the tire pressure, the tire design, and the depth of penetration of the tires into the mix. All of the tires on the roller should be the same size, ply, and tire pressure. The area of each tire footprint and the wheel load of the roller are the primary factors affecting the effectiveness of a rubber tire roller. The greater the contact pressure between the tire and the mix, the greater the compactive effort applied by the roller.

Inflation Pressure and Ground Contact Pressure at Various Wheel Loads and Ply Ratings
Example Ply Wheel Tire Contact Ground Rating Load Pressure Area Contact Pressure kg kPa mm kPa A , , , B , , 14 1, , C , , 14 1, ,

A: Changing wheel loading.
Increasing the wheel load from 567 kg per tire to 1270 kg per tire, while keeping the inflation pressure constant, will increase the contact pressure on the tire from 538 kPa to 634 kPa, an 18% increase from the original. B: Changing inflation pressure. Increasing the inflation pressure from 241 kPa to 896 kPa, while maintaining a constant load of 1043 kg per tire, will increase the contact pressure from 386 kPa to 607 kPa, a 57% increase from the original. C: Changing ply rating. Increasing the ply rating from a 10 to a 14 ply, and inflating them to the maximum inflation pressure will increase the contact pressure from 503 kPa to 634 kPa, a 26% rise.

The tires on a pneumatic roller will often pick up the mix when an oversanded surface course mix or a mix with some particular additives is being compacted. Many times attempts are made to eliminate this pickup problem by spraying water or a release agent on the tires during the rolling process. A better solution is to allow the tires on the roller to reach the same temperature as the mix being compacted without adding water or release agent to the tires.

Get the tires hot by running the pneumatic roller on existing pavement first and then by slowly getting them hot on the new pavement. The use of ‘skirts’ not only helps in getting the tires hot but also ensures that they will stay hot all day by limiting the winds cooling effects.

Damage like this needs to be avoided.
Although the density of the pavement is probably being achieved, the finished surface is mottled and will most likely have surface blemishes which may hold water. The key is to get the tires hot and keep them hot by keeping the roller moving. Modified asphalt - more of a problem

This is a typical amount of surface pickup when using the pneumatic tired roller properly.
Once the size of the pneumatic roller and the pressure to be used are selected, the only variables that can be controlled easily by the operator are the rolling speed, the location of the roller with respect to the paver, and the number of roller passes over each point in the pavement surface. If the compactive effort applied by the pneumatic roller is not adequate, the operator should alter the wheel load on the tires and/or change the inflation pressure in the tire. Pneumatic rollers are ideal for uneven courses.

Vibratory Rollers Amplitude Frequency Impact Spacing Operation

Amplitude is defined as the weight of the drum divided by the eccentric moment of the rotating weight and is a function of the weight of the drum and the location of the eccentrics. The frequency of vibration is the number of cycles that the eccentrics rotate per minute. The spacing of the impacts of the applied force is a function of the frequency of the vibration and the travel speed of the roller. The operator is in control of more variables when using a vibratory roller and should be well educated in the selection and interaction of these variables.

Vibratory rollers come in a variety of configurations.
Roller speed, location on the layer being compacted, number of passes made, nominal amplitude, and vibratory impact can all be varied.

Single-drum vibratory rollers are manufactured with both a rigid frame and an articulated frame.
Double-drum vibratory rollers come in rigid-frame, single-articulated-frame, and double-articulated-frame models. Can be operated in any one of three modes: static (vibrators off); one drum vibrating, one static; and with both drums vibrating.

Vibratory rollers have two types of compactive forces that are applied to the hot-mix asphalt: static weight, caused by the weight of the rolls and frame; and dynamic (impact) force, produced by a rotating eccentric weight located inside the drum(s), which rotates about the shaft inside the drum. The eccentric weight rotates about the shaft inside the drum, and a dynamic force is produced. Changing the eccentric moment arm or adjusting the eccentric mass has a directly proportional effect on the dynamic force.

Up Amplitude (A) Movement Time Down Time (T) For Full Cycle Frequency, f = the number of hertz (cycles/s)--a single cycle is one full rotation of the eccentric weight. Frequency = 1/T Amplitude, A = the maximum deviation from position at rest -- one-half the total movement.

The elements of comparison for the dynamic component of a vibratory roller are the magnitude of the centrifugal force, its vibrating frequency, the nominal amplitude, and the ratio of the vibrating and non-vibrating masses acting on the drum.

AMPLITUDE

Normal values of nominal amplitude range from 0.25 to 1 mm.
Some rollers can operate at only one fixed amplitude, while others have “high” and “low” amplitude positions. Typically as the layer thickness increases, it is often advantageous to increase the nominal amplitude applied to the asphalt mix. Unless ‘high’ amplitude is needed to achieve a particular density level, the vibratory roller should be operated in ‘low’ amplitude.

FREQUENCY Low Frequency High Frequency Impact Spacing
DIRECTION OF TRAVEL

The frequency of vibration is the number of complete cycles that the eccentrics rotate per minute.
The faster the rotation of the eccentrics, the greater the frequency of vibration. Some vibratory rollers can operate at only one frequency or have a very limited selection of frequencies. Other vibratory rollers can alter the frequency of the applied load between 1600 and 3000 vibrations per minute. Frequencies below 2000 vibrations per minute are not normally acceptable to compact asphalt mixtures.

Typical Data for Vibratory Tandem Rollers
Vibratory Oper. Drum Drum Static Dynamic VPM Nom. Steel Tandem Wt. Diam. Width Drum Drum Amp. tonne kg m m kg/mm kg/mm mm , , , , > , ,

A review of the chart gives us an opportunity to familiarize ourselves and the students to the metric values used for analyzing vibratory rollers.

The spacing of the impacts of the applied force is a function of the frequency of the vibration and the travel speed of the roller. A smaller impact spacing and thus a greater number of impacts per meter, is usually preferred. A recent survey of major roller manufacturers recommended the following impact spacings. The impact spacing should be in the range of 30 to 36 impacts per meter, or 25 to 30 mm between impacts, to ensure the highest efficiency of the vibratory rollers and reduce the possibility of leaving ripples in the finished pavement. Proper impact spacing and amplitude are the keys to successful pavement compaction.

Easy Does It! Humorous slide showing that the roller operator needs patience when stopping and starting, to move in different directions, to achieve acceptable compaction.

Stick With Your Pattern!
Humorous slide showing that the operator needs patience in staying with their rolling pattern for the entire day to ensure proper compaction of the pavement. The roller operator is in control of more variables when using a vibratory roller and thus should be well educated in the proper selection and interaction of the variables. In addition to roller speed, location on the layer being compacted, and number of passes made, both the nominal amplitude and the frequency of the vibratory impact can be varied.

Roller Maintenance Water Systems Hydraulic Systems Mechanical Systems
Vibratory Systems Rolls, Tires, Pads, Scrapers

The water system is designed to keep the mix from sticking to the drums or tires, and is typically the highest maintenance item on a roller. Most rollers operate extensively by hydraulics and the importance of keeping these well maintained is vital. Mechanical systems need to be properly maintained or the roller will become uselessly defective. Vibratory systems make the vibratory roller unique, and need to be maintained well, or the roller could retire before its time. Rolls, tires, and pads should be inspected for wear or damage and need to be replaced periodically.

This slide shows a well maintained and operating water discharge system on a steel wheeled roller.
A uniform, solid stream of water ensures uniform wetting and uniform pavement smoothness. Spray systems that clog will cause asphalt to pick up onto the steel or rubber tired rollers.

It is a good idea to have spare pads, especially with pneumatic rollers, because they wear down.
Scrapers on rubber tired rollers are as important as those on steel rollers.

Scrapers also wear down and need replacing.
Scrapers should be made of softer material than the drums or tires or they may gouge the surface.

Whether gravity feed or pressure feed, only enough water should be used to keep the drum wet.
Any more water is wasteful and will require the roller to be out-of-service more frequently while re-watering. For pressure fed systems without a gravity flow backup, keeping an additional water pump available is a good idea.

The roller maintenance diagram shows where to find key parts of the roller as well as showing an owners recommended frequency to lubricate, change and check parts on the roller.

The operators control panel on a vibratory roller is shown here.
For vibratory rollers vibration frequency, vibration amplitude, and direction of travel are all under control of the operator. Each of the factors has an effect on the level of density achieved under the compactive effort applied to the mix.

A vibrating reed-type tachometer is an excellent tool to verify the performance of a vibratory roller system. For under $300, it is an inexpensive investment, especially when working with difficult to compact mixes. Other types are available and some rollers have a tachometer mounted directly on the roller.

A checklist for roller maintenance can be found in the Participant Manual (Form 8-1).

Compaction Variables Roller Speed Number of Coverages Rolling Zone
Rolling Pattern

The primary compaction variables for all types of rollers that can be controlled during the rolling process are: (a) roller speed, (b) number of roller passes, (c) rolling zone, and (d) roller pattern. Each of these factors has an effect on the level of density achieved under the compactive effort applied to the mix.

The faster a roller passes over a particular point in the new asphalt surface, the less time the weight of the roller “dwells” on that point, meaning that less compactive effort is applied to the mixture. As roller speed increases, the density achieved with each roller pass decreases. The roller speed selected is dependent on a combination of factors: productivity, layer thickness, and the position of the equipment in the roller “train.”

Typical Range of Roller Speeds (km/hour)
Type of Roller Breakdown Intermediate Finish Static Steel Wheel to to to 8 Pneumatic to to to 11.3 Vibratory to to

Compactive effort is significantly improved at slower roller speeds.
Roller speed will also be governed by the lateral displacement or tenderness of the asphalt mixture. If the mixture moves excessively under the rollers, the speed of the compaction equipment should be reduced. In addition, for vibratory compactors, roller speed also affects the impact spacing. Roller speed is usually established by the speed of the paver. Speeds change substantially with an increase to the thickness of loose mixture being placed.

One Coverage How many passes of the roller are needed to cover the width of the mat one time ?

To obtain the target air void content and uniform density in an asphalt mixture, it is necessary to roll over each point in the pavement mat a certain number of times. The actual number of passes depends on many variables such as the type of compaction equipment used. The capabilities of each type of roller vary, however, with mat thickness, mix temperature, mix design and environmental conditions. In addition, the number of passes required depends on the position of the rollers in the roller train. A test strip aids us in this decision process.

Rolling Zone (conventional HMA)
Paver Rolling Zone (conventional HMA)

Compaction must be achieved while the viscosity of the asphalt cement in the mix and the stiffness of the mix is low enough to allow reorientation of the aggregate particles under the action of the rollers. The rule of thumb commonly used is that the proper level of air void should be obtained before the mix cools at laydown to 80 °C. Initial compaction should occur directly behind the paver in order to reach the required density quickest. If the stability of the asphalt mixture is high enough, breakdown rolling can be carried out very close to the paver, while the mat temperature is still high. Superpave has shrunk the distance between the roller and the paver to as little as 1 to 5 m.

Rollers are “busy” most of the time on a paving project.
The question is whether they operate correctly and effectively. Generally, compaction is applied, but not necessarily in the right place. Numerous compaction studies have shown that the middle of the width of the paver pass typically receives more compactive effort than the edges of the pavement. This is unfortunate, because traffic uses the wheelpath areas and travels near the edge of the pavement more often than in the center of a lane.

Operating Techniques Transportation Intermediate Rolling
Test Strip Construction Establishing Roller Patterns Breakdown Rolling Transverse Joints Longitudinal Joints Intermediate Rolling Finish Rolling Re-watering Concluding Operations

Equipment must be transported safely and in accordance with manufacturer’s recommendations.
Some states do not allow steel drum rollers to travel on bridge decks. In this case, adequate transportation must be in place to move rollers as the project requires.

Test Strip Construction
Simulating Actual Conditions Establishing Roller Patterns Calculating Effective Roller Speed

The actual rolling pattern to be used to compact the mix on a paving project should be determined at the start of the project through the construction of a roller test strip. This strip should be located at a convenient point where the pavement layer placed will remain as part of the final pavement structure. The mix should be representative of the material to be produced for the project; generally the plant should produce mix for a short period of time before mix is made for the compaction test section. The thickness of the layer should be the same as that used for the rest of the project and 150 m long.

Due consideration should be given to the selection of rollers that are to be employed to densify the pavement layer. The combination of rollers used on a previous project might not be the most cost-efficient or effective for the variables involved in the present job. Although vibratory rollers are usually used for breakdown rolling and pneumatic tire rollers for intermediate rolling, a greater degree of density with a lesser number of total roller passes might be obtained in some mixes. Determination of the “optimum” combination is key.

Establishing Roller Pattern
Selecting compaction equipment Width of paving Width of roller Number of coverages needed Nuclear gauge

Different mixes may require considerably different levels of compactive effort and thus different compaction equipment and rolling procedures. An asphalt mix containing large aggregate, for example, may need different types of rollers to achieve a required level of density than an asphalt concrete mix made with smaller size coarse aggregate.

Roller Types by Application (tonnes)
Breakdown Rolling Static (3 Wheel Pneumatic Vibratory (Static) and Tandem) 7.3 to mm rim to 10 7.3 to mm rim > 11.3 9.1 to to 7.3

For each roller employed on a project, the mat width can be divided by the width of the compaction rolls to determine the number of passes needed to cover each transverse point in the surface. If a large pneumatic tire roller is used in the breakdown position, it is generally difficult to determine the degree of density obtained in the mat through the use of a nuclear gauge because of the rough texture of the mat after the first several passes of the pneumatic roller.

Intermediate Rolling Static (3 Wheel Pneumatic Vibratory (Static) and Tandem) 7.3 to mm rim to 7.3 9.1 to mm rim to 10 607 mm rim > 11.3

Often, the roller used for “breakdown” and “intermediate” rolling is the same one.
Multiple passes are needed to completely compact the transverse width of the lane being paved as well as over each point in the pavement surface to assure attainment of the required level of density.

Finish Rolling Static (3 Wheel Pneumatic Vibratory (Static) and Tandem) 7.3 to mm rim to 7.3 7.3 to mm rim to 10 9.1 to > 11.3

This slide shows the finish roller types.

Standard paving width of 3. 1, 3. 7, 4. 3 and 4
Standard paving width of 3.1, 3.7, 4.3 and 4.9 meters can affect the efficiency of certain rollers. Ideally, the roller that can cover the width of paving with the fewest number of passes will be the most efficient. Other smaller width rollers are routinely used, but the number of total passes will increase.

Drum Width vs. Lane Width

Graph showing number of passes for one coverage for various pavement and drum widths.

One Roller Coverage 1 CROWN 2 3 6 5 4 2-lanes @ 3.7 meters Drum Width

If the width of the roller drum is 2
If the width of the roller drum is 2.1 m, only two passes of the roller are needed to cover the 3.7 meter wide lane, including a 150 mm overhang at each edge of pavement. A roller that is 1.8 m wide cannot cover the complete 3.7 meter wide lane in only two passes. Three passes of the 1.8 m wide roller would be necessary to properly compact the lane. If the roller had drums that were 1.5 m wide, three passes of the roller would be required, similar to the roller with the 1.8 m wide drums. A 1.4 m drum needs four passes for one coverage. For thick lifts, first pass should be about 1 ft in from edge to prevent shoving.

How Many Repeat Coverages to Assure Density?

Ask the question on the slide.
The number of roller passes needed over each point on the pavement surface is a function of a large number of variables, the most important of which is the level of density required in the pavement layer. If a certain percentage of laboratory density or theoretical maximum density is needed, the higher the required percentage, the more compactive effort that will have to be applied to the pavement layer.

The most common method for monitoring changes in density with roller passes is through the use of a nuclear density gauge, which transmits gamma rays into the mix and measures the amount of radiation reflected back to the device in a given time. The count data that are obtained can be related to the relative density of the layer. Nuclear gauge readings should be taken after each pass of each roller, and the rate of increase in density after each pass determined. When no appreciable increase in density is obtained with additional roller passes, the maximum relative density for that mix has been obtained.

High Passing Density Density Low 1 2 3 4 5 6 Roller Passes

When a pneumatic tire roller is used in the compaction process, particularly in the breakdown mode, it is often very difficult to obtain an accurate density reading with a nuclear gauge. One or more passes with a steel wheel roller may be necessary before a valid nuclear gauge reading can be obtained. The density value determined with the nuclear gauge is relative and is generally not the same as the density value obtained from cores cut from the pavement.

Thus a correlation must be developed between the nuclear density reading and the actual unit weight of the pavement. That unit weight must be compared to the maximum theoretical unit weight of the mix in order to calculate the actual in-place air void content of the layer.

Thermometers Types Uses Potential problems Calibrating
Actually using one

There are several types of thermometers available but all do the same thing…they measure temperature. The more expensive ones tend to do it faster. The problem with inexpensive ($5) dial thermometers is that the calibrating adjustment is a nut on the back of the dial, which can work loose after rough work, losing the calibration. The problem with the infrared-type is they display a consistently lower temperature than a probe-type. They are also much more expensive (about $200).

Measuring temperatures is not exciting, but it can tell you a great deal about the mix and conditions you are working in, and possibly improve the work. Take enough readings to develop a comfort index with the specific conditions you are working in. Occasionally compare your temperature value with the quality assurance department on the job. Achieving density in a consistent and quick way is directly proportional to HMA mat: the colder the mat the more difficult it is to achieve compaction.

Never completely trust a single thermometer, especially if you don’t know where it’s been…run a couple quick checks. For a precision check, insert several probe thermometers into the center of a pile of hot mix (a shovel-full will do) and wait a minute–they should all read the same temperature. If you are using an infrared unit, aim it at the same pile and compare the probe measurements to the infrared; this will allow you to develop a simple correlation between surface and depth temperatures. For a quick accuracy check, place them in a pan of boiling water, you should get 212 °F (100 °C) at sea level. Note that thermometer in photo is reading in °F.

Slide showing four different temperature measuring devices.
Safety, safety, safety: Watch out when bending over to take a reading, make sure the roller operator and others know where you are at all times.

Roller Production Rate Problem

Students will work a roller production rate problem at this time.
Devote 20 minutes to the roller production problem.

Balancing Production HMA Facility Paving Trucking Compaction

Although fairly simple to calculate on paper, balancing production rates in real life is much more difficult, due to the number of complicating factors that arise daily. Trucks tend to bunch up and arrive all at once, the paver speed can vary, and rollers do need to be re-watered. The best way to deal with this is to buffer the operation with a slightly higher roller production rate, enabling you to deal with these situations as they arrive. Setting up a paving operation with too low a compaction production rate is not recommended.

Breakdown Rolling Determine the rolling zone by: Experience Estimating

Breakdown rolling is the first interaction between the roller and the HMA mat and can be a friendly or a troublesome meeting. Most contractors use steel drums. Some use pneumatic tired to breakdown the mix, increase the mat density, and establish the mat smoothness. Rollers should stop and start slowly on uncompacted mix and angle the drum when stopping to reverse. The majority of the compaction is obtained during breakdown rolling, so it is important to keep this roller moving as much as possible. When the roller does stop, it is best practice to park on a cold mat. Otherwise, you may leave marks.

As discussed, the rolling zone is that area of the mat which is at the right temperature range to be most efficiently compacted. This will vary by mix type and other compaction variables already discussed. In addition, each roller in the rolling train will have its own rolling zone, whether it be in the breakdown, intermediate, or finish rolling stage. A commonly accepted rolling zone is approximately 75 meters for normal paving operations. The estimated length depends on many factors and should be checked during production; it is the length where uniform density can be maintained.

If you are monitoring compaction, one of the most important tools you should have at your side is a thermometer, preferably several. In addition, you should know how and when to use it, and finally you should use it! Knowing your pavement temperature at placement and the time it takes to cool to the point where no more passes increase density is crucial especially at certain times of the year. The following section discusses a method used to determine your rolling zone through knowing temperatures.

Calculating Your Rolling Zone
Effective Compaction (C-Rate) Production Rate equals 8.5 meters per minute. TAC from Environmental Variables chart equals 10 minutes for 50 mm thick mat with mix temperature of 121 oC and base temperature of 10 oC. C-Rate times TAC = 8.5 mpm x 10 minutes = 85 meters

This example should be discussed thoroughly with the students and the instructor may want to work through additional problems similar to this one. The rolling zone will be determined from the last problem performed, using an Effective Compaction Production Rate of 8.5 meters (28 ft) per minute. In our problem we assume a 50 mm (2 in) thickness, mixture temperature measured in the field equal to 121oC (250 oF) and a base temperature of 10 oC (50 oF). Utilize the Environmental charts discussed earlier in Chapter 8 to determine the Time Available for Compaction (TAC). Remember to remind the students that this is only an estimated working zone for compaction.

Compacting Transverse Joints

Good practice is to make an initial breakdown pass and then check the joint with a straightedge.
In case the new mat is too high, the mix can still be reworked and then recompacted. It is important to understand that the first pass will still leave the mat slightly high, but experience will determine if it is too high. The goal is to produce a transverse joint that rides as smooth as the rest of the mat. The two possible methods to roll a transverse joint are Transverse Rolling and Longitudinal Rolling. There is no right way to roll a transverse joint; use the method that works efficiently and consistently.

Compacting Longitudinal Joints

There are three main methods to compact a longitudinal joint.
Each method can produce similar densities. The best practice is to compact a joint consistently and efficiently.

Cold Hot

The oldest method is to roll the joint from the cold side with most of the drum on the previously placed mat, overlapping approximately 150 to 300 mm. The problem is that this is the most inefficient method since most of a roller pass is wasted. Vibratory rollers should not operate in the vibratory mode on this first pass. Of the three methods, this one produces the lowest density on the joint.

One state had a method specification that required this type of rolling, and they had very poor longitudinal joint performance.

Rolling from the hot side places most of the roll on the hot mat and overlapping approximately six inches on the cold mat is the most efficient method. Vibratory rollers, in the vibratory mode, can be used in this method since the drum will not impact on the cold mat.

An additional method that can be tried is to roll from the hot side but hold the drum away from the joint about six inches, leaving a ribbon of uncompacted material. A subsequent pass will be made to compact this ribbon with an attempt to force more mix into the joint area and allow the mix to remain in contact with the joint longer prior to compacting. This method does require an additional pass and is not as efficient as rolling from the hot side.

Photo showing fractured aggregate from rolling on the cold side in the vibratory mode.
This pavement is now likely to experience premature failure because of this poor habit.

Intermediate Rolling

Intermediate rolling may or may not be required on a project.
If adequate density cannot be achieved with the breakdown roller, an intermediate roller may be needed. Intermediate rollers can typically be operated at higher speeds than a roller in a breakdown mode. Intermediate rollers should follow a roller pattern and not concentrate on running down the middle of the mat.

Some states require usage of an intermediate roller in intermediate rolling, typically a pneumatic tire for surface texture and kneading action. There is a “tender zone” in Superpave mixes between °C ( °F). Mix will shove under a steel wheel roller. Can either let it cool or use a pneumatic roller.

Finish Rolling

Finish rolling is the last step of the operation and is normally used to “iron” out any roller marks left by a breakdown or intermediate roller. This roller is typically a static steel wheel roller, or a vibratory roller operating in a static mode. Finish rolling is not the place to count on obtaining additional density. It is a cosmetic process.

Theoretically the finish rolling operations begin approximately 1/2 hour after the paving operations and should end approximately 1/2 hour after the paving operations end or before the mat temperature reaches 80 °C. Comments?

Some contractors equip the finish roller operator with an infrared thermometer so that finish rolling can occur within a range of temperatures. New equipment has infrared thermometer mounted directly on the roller.

This roller is normally well behind the paving operation and operating alone.
Traffic safety devices must remain until finish rolling is finished. The operator and any additional personnel, performing a coring or nuclear density operation for example, must be careful of stray traffic entering the site. The traveling public may not see a lone roller or technician bent over a nuclear gauge until it is too late.

Re-Watering During Paving

Rollers must re-water periodically during paving.
Maintaining a spray system that uses the minimum amount of water will reduce these stops, but not eliminate them. It is best to re-water during a temporary lull in paving, and refill a half empty tank, than to wait until production is peaking and run out of water. If this cannot be avoided, and the mix is cooling too rapidly to wait, the intermediate or finish roller should be moved to the breakdown position until the original breakdown roller is available.

Ensuring access to local fire hydrants should be taken care of through City Hall prior to the start of paving. Usually municipalities assign contractors portable meters for payment as shown here.

Concluding Operations

Rollers are the last equipment on and off the pavement.
The last section of a pavement placed must receive the same attention to detail and compactive effort as the first section. Rollers should be checked, and maintained at the end of each day. If something needs repair, repair it now.

Opening to Traffic

If the mix hasn’t cooled below a mix-specific temperature before opening to traffic, additional densification can occur in the mix, primarily in the wheel paths. It is possible that heavy traffic could over-densify the mix and cause rutting. Therefore, agreement should be reached in the pre-construction meeting as to the minimum temperature (surface or internal) before the pavement is opened to traffic. Intersections, night paving, and ramps are key areas. In some areas, a water truck has been used to cool the pavement temperature.

Module 8 Objectives Review
Objective of Compaction Engineering Properties Related to Compaction Factors Affecting Compaction Types of Rollers Considerations for Selecting Rollers

Ask for specific responses to these questions.
Use questions that will help draw out responses, such as: What do you remember? How is it related to what you know? How will you use this information in the future?

Module 8 Objectives (cont’d)
Compaction Variables Roller Maintenance Roller Productivity Operating Techniques

Discussion on good post-rolling densities:
Compaction variables include # of passes, speed, and position on the mat. Discussion on good post-rolling densities: 92 to 94% of max theoretical seems to be good for SMA, Superpave Recall that there is variability in Rice calculations 4-6% air voids depending on what are max theoretical calculations

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