Asphalt compaction and rolling: density, mat temperature, and the pattern

Compaction, density, and why it decides pavement life

Asphalt compaction is the work of rolling the loose, hot mat down to a dense, tight pavement before it cools. The paver lays the mix at maybe 85 to 88 percent of its maximum density. The rollers do the rest, pressing the aggregate into a locked skeleton and squeezing out the air until the mat reaches the target. What you are chasing the whole time is density, and density is the single number that best predicts how long the pavement will last.

The reason is air voids. A loose mat has too much air in it, and air is the enemy. Voids let air and water move through the pavement, the binder oxidizes and goes brittle, and the mat ravels, cracks, and strips years before it should. Roll it to the right density and you seal those voids down to a level the mix was designed to carry. Leave it under-compacted and you have built in early failure that no amount of maintenance fully buys back.

Three things decide the density: the rollers, the pattern they run, and the temperature of the mat while they run it. Get those right and the density follows. This guide works through each one. It is a companion to the inspection and QC guide, which covers acceptance of the whole pave, and to the joints and handwork guide, which goes deep on the seam that compaction has the hardest time reaching.

Why is asphalt density so important?

Density is the most important quality number on a paving job because it controls durability, and the relationship is steep. Air voids drive how fast the pavement ages, and study after study on in-place density has tied small shortfalls to large losses in service life. The common rule the trade carries is that roughly every 1 percent of density below target costs about 10 percent of pavement life, which is why a mat that misses by 2 or 3 percent can lose a third or more of its expected service before traffic ever finds the problem.

Picture two mats off the same truck. One gets rolled to target and seals up tight. The other comes in a point or two low because the pattern was loose or the mat cooled first. From the road they look identical the day they open. Five years on, the low one is gray, raveling at the surface, and cracking, while the tight one is still black and holding. Same mix, same money, different life. Density is what separated them.

This is also why density is where the spec puts its teeth and its pay. Agencies test for it, pay for it, and dock for it, because it is the one in-place property that reliably forecasts whether the pavement they bought will reach its design life. Hit the density and most of the rest of the job tends to hold up. Miss it and the best mix in the world still fails early.

Target density and in-place air voids

Density is reported as a percentage of a reference, and air voids are the same fact stated the other way. In-place density is usually expressed as a percent of theoretical maximum density, written Gmm, which is the airless density of that mix. The air voids are whatever is left over. A mat at 93 percent of Gmm has about 7 percent air voids in place. The two numbers always add to 100, so they are the same target read from opposite ends.

For dense-graded hot mix, a common in-place target is roughly 92 to 93 percent of Gmm at minimum, which is about 7 to 8 percent air voids, and many agencies set acceptance somewhere in the 92 to 96 percent of Gmm band with around 94 to 95 percent as a frequent target. Those figures are typical, not universal. The exact target comes from the job mix formula and the project or DOT specification, so hedge every number here to the mix you are actually laying and the spec that governs the lot.

There is a floor and there is a problem on both sides. Too few percent of Gmm and the mat is open, with voids that let in air and water. Push density too far past target, usually by over-rolling, and you start crushing aggregate or driving voids so low the mix has no room to move under traffic and flushes or shoves. The window between under-compaction and over-compaction is the whole game, and the spec defines where it sits for your mix.

What temperature do you compact asphalt at?

Compaction has to happen inside a temperature window, because temperature controls whether the mat will move under the roller. Hot mix is workable while it is hot. As it cools, the binder stiffens, the mix locks up, and past a certain point the rollers cannot push the aggregate into place no matter how many passes you make. That lower limit is the cessation temperature, the point where rolling stops adding density. For many dense-graded mixes it lands somewhere around 175 to 185 degrees F, lower with some warm-mix additives, but the exact value belongs to the mix.

The top of the window matters too. A mat that is too hot can be tender and shove ahead of the drum instead of compacting, so the breakdown roller often starts a touch back from the screed rather than right against the hottest mix. Most of the density gets made in the upper part of the window, which is why the breakdown pass is the one you cannot afford to lose to a cooling mat.

Treat the window as a clock, not a thermostat. From the moment the mat hits the surface, it is cooling toward cessation, and everything about the rolling plan is a race to get the passes in before the door closes. Thin lifts, cold bases, and wind all speed that clock up. The breakdown, intermediate, and finish temperatures cited across the trade are starting points. Confirm the compaction window for your specific mix from the JMF and the producer, and roll to the mat temperature you measure, not to a number off a chart.

What are the three phases of asphalt rolling?

Rolling runs in three phases, in order, and each has a job. Breakdown rolling comes first, right behind the paver, and makes the bulk of the density while the mat is hottest. Intermediate rolling follows and adds the rest of the density, often with a pneumatic roller that kneads the surface tight. Finish rolling comes last, a static steel drum that irons out the roller marks and leaves a smooth surface before the mat cools too far to take the correction.

The sequence is not optional and the order is not arbitrary. Density is easiest to get when the mat is hottest, so the heavy work goes first, when the mix will still move. Surface finishing is the last thing you do, because there is no point making it pretty until the density underneath is already there. Run the phases out of order and you either chase density on a mat too cold to give it or you leave marks you can no longer roll out.

On a small crew one or two rollers may cover more than one phase, and on a big highway pull each phase may have its own machine working its own zone behind the paver. Either way the three jobs still happen in the same order. The rest of this guide takes them one at a time, then covers the rollers, the pattern, and the conditions that decide whether the phases land inside the window.

Breakdown rolling: the bulk of the density

Breakdown rolling is the first pass over the fresh mat and it does the heavy lifting. The mat comes off the screed already partly compacted, and the breakdown roller takes it from there up toward the target while the mix is hottest and most workable. This is where most of the density is made, so if the breakdown roller falls behind and the mat cools, the density is gone and the later phases cannot recover it.

The breakdown roller is usually a vibratory steel drum, because vibration drives density fast. The operator stays close behind the paver, holding a consistent gap so the mat is rolled in the right part of the temperature window every time. Staying close is the discipline that makes or breaks the number. A roller that wanders back to chat or fix something lets a stretch of mat cool below where it will compact, and that stretch is now a low-density streak baked into the pavement.

The breakdown operator works to the rolling pattern set on the test strip: a fixed number of passes, a set overlap, and a steady speed. The goal is to get the planned coverage and pass count in before the mat drops out of the window. On a hot day with a thick lift there is room to work. On a thin lift in wind, the breakdown roller is right on the screed's heels and there is no slack at all.

Intermediate rolling: kneading in the rest

Intermediate rolling picks up where breakdown leaves off and brings the mat the rest of the way to target density. The mat is still well above cessation but no longer at its hottest, so this phase closes the gap between what the breakdown pass achieved and the final number the spec wants. On many jobs this is the phase that decides whether the lot passes, because breakdown gets you close and intermediate gets you over the line.

The classic intermediate machine is a pneumatic rubber-tire roller. Its tires knead the surface rather than just pressing it flat, working the mix and sealing the top down in a way a steel drum does not. That kneading action tightens the mat, helps close surface voids, and can chase out a little more density that the steel drum left behind. Some crews run a second steel roller in the intermediate slot instead, depending on the mix and the spec, but the pneumatic is what a lot of agencies favor for dense-graded surface mixes.

The pneumatic also leaves the mat a little textured from the tires, which is one reason finish rolling follows it. Whether a pneumatic is required, allowed, or kept off a given mix is a spec and mix question, so confirm it for the job rather than assuming the roller and mix you ran last week apply here.

Finish rolling: smooth before it cools

Finish rolling is the last phase and its only job is the surface. By the time it runs, the density is already made, so the finish roller is not chasing the number. It is ironing out the marks the earlier rollers and the pneumatic tires left behind, leaving a tight, smooth surface that meets the smoothness spec and reads clean to the eye.

The tool is a static steel drum, run without vibration, because vibration this late can mark a cooling mat or knock density back rather than help. The operator works while the mat is still warm enough to take the correction, above the cessation temperature but cooler than the breakdown and intermediate passes. Wait too long and the marks set permanently, because a mat near cessation will hold whatever shape it has when the last roller leaves it.

Finish rolling is short and easy to shortchange when the crew is tired at the end of a pull, and it shows. The roller marks, the pneumatic texture, and the small ridges that a quick static pass would have removed are the ones the owner sees from a car at 60. The density is what makes the pavement last. The finish is what makes it look like the crew knew what they were doing.

Roller types and what each one does

Three roller types cover almost all asphalt compaction, and each earns its phase. Steel-drum rollers, run in vibratory or static mode, do most density work and all the finishing. Pneumatic rubber-tire rollers knead and seal in the intermediate phase. Combination rollers carry a steel drum on one end and tires on the other, so one machine can do more than one job on a smaller crew. The table sums up where each fits, and the spec and the mix decide which are required on a given pull.

Vibratory amplitude, frequency, and speed

A vibratory drum has two knobs that change how hard it hits: amplitude, the height of the drum's movement, and frequency, how many times per minute it impacts the mat. Roller speed is the third lever, because it sets the spacing between impacts. Run too fast and the impacts spread out, leaving a washboard pattern and uneven density. The rule the operators carry is to keep speed steady and slow enough that the impacts stay close together, commonly in the range of 10 to 12 impacts per foot, but confirm the target against the roller manual and the spec.

Match amplitude to the lift. A thick lift can take high amplitude, which drives energy deep and compacts the full depth. A thin lift cannot. Run high amplitude on a thin lift and the drum hits the hard base underneath, bounces, and instead of compacting it crushes aggregate at the surface and can actually decompact the mat, a problem sometimes called double-jump where the drum loses contact between impacts. On thin lifts you drop to low amplitude, or turn vibration off and roll static, depending on the machine and the mix.

Over-vibrating a thin mat is one of the quieter ways to lose density while looking like you are making it. The roller is working hard, the mat looks rolled, and the cores come back low because the energy went into breaking stone instead of seating it. Set amplitude to the lift thickness and the nominal maximum aggregate size, follow the roller manufacturer's guidance, and verify the settings on the test strip rather than carrying yesterday's setup onto a thinner mat.

What is a rolling pattern?

A rolling pattern is the fixed routine each roller runs to hit density: how many passes it makes, where it overlaps the last pass, how far behind the paver it stays, and at what speed. It is the plan that turns rolling from random coverage into a repeatable operation that puts the same compactive effort over every square foot of mat. Set the pattern, then hold it, pass after pass, all day. The pattern is the difference between a mat that tests uniform and one that passes in some spots and fails in others.

The pattern gets established at the start of the job on a test strip, where you find the pass count that brings the mat to target density and then lock it in. Once it is set, the operators run it the same way every cycle. The temptation on a long pull is to drift, to skip a pass when the crew is moving fast or add coverage when something looks low. Both break the uniformity the pattern exists to protect, and uniformity is what the density spec is really measuring.

Hold the pattern, but do not run it blind. Conditions change. A cooling afternoon, a thinner lift over a soft spot, a stretch in the wind, all of these can pull density off even with the pattern held. When the mat or the conditions change enough, you go back and re-check the pattern on a new test strip rather than assuming the morning's pattern still fits the afternoon's mat. The pattern is fixed until the conditions tell you it is not.

The test strip that sets the pattern

The test strip, or control strip, is where the rolling pattern is born. At the start of the job you place a length of mat and roll it while measuring density after each pass, watching the number climb. You keep adding passes until the density stops rising, where another pass brings no real change, sometimes set as less than about 1 lb/ft3 of gain. That pass count, with the overlap and speed that produced it, becomes the pattern the crew runs for the rest of the work under those conditions.

The test strip also proves the rest of the operation before you commit to a day of paving. It confirms the mix will reach target density at the laydown temperature, the lift thickness, and the roller setup you planned. If the strip cannot hit the number, you find out on a short stretch you can deal with, not a mile of placed mat. Better to learn the mix needs another roller or a higher laydown temperature on the strip than at the pay meeting.

How formal the test strip has to be depends on the spec. Some agencies require a control strip with documented density readings and a signed-off pattern before production paving starts. Others leave it to the contractor's QC. Either way, building one is good practice, because a pattern set by measurement beats a pattern set by habit every time. Confirm the control-strip requirement against the project specification.

Roller speed, overlap, and steady motion

Coverage is what the pattern is really protecting, and it comes from overlap. Each pass overlaps the last by a set amount so no strip of mat gets missed and no strip gets double the effort. A common practice is to overlap each pass by a few inches up to several inches, and to figure the number of passes from the paved width divided by the drum width, but the exact overlap belongs to the pattern you set on the strip. Miss the overlap and you leave low-density seams between roller paths that no later pass fixes.

Speed has to stay steady, and not just for impact spacing on a vibratory drum. A roller that surges and slows puts uneven effort down the mat, dense where it dawdled and light where it sped up. Steady speed at a steady gap behind the paver is what makes the density uniform, which is the whole point.

The blunt rule is no sudden stops and starts on a hot mat. A roller that brakes hard or takes off fast shoves the mix into a ridge or a crescent that you then have to roll back out, if it can be rolled out at all. When a roller has to reverse, it eases the direction change onto cooler, already-compacted mat where possible, not into the tender mix right behind the screed. Smooth, steady, and consistent beats fast every time on asphalt.

Compacting the longitudinal joint

The longitudinal joint is the lowest-density, first-to-fail spot on most pavements, and how you roll it decides whether it holds. It is the seam between adjacent passes or lanes, and it is hard to compact because the edge of the first mat had nothing to push against when it was laid, so it sits low and open. When the next lane goes against it, the joint is the place where density is hardest to reach, which is exactly why joints ravel and crack from the seam out before the field of the mat shows any wear.

Rolling the joint right is its own technique. Where the new mat meets a confined, already-compacted edge, the roller overlaps the joint by a few inches onto the hot side and pinches the seam tight, with most operators needing only 3 to 4 inches of overlap onto the cold mat to lock it. The exact method depends on whether the joint is hot against hot, as in echelon paving, or hot against a cold cut edge, and on what the spec calls for. The joints and handwork guide covers the joint geometry, the notched wedge, and joint density testing in detail; here the point is narrower. The joint needs deliberate roller attention, not just whatever the field pattern happens to give it.

Many agencies now test joint density separately and pay on it, because the joint is where the pavement quietly loses years. Treat the joint as its own density target, roll it on purpose, and check it. A field mat at target with a starved joint is still a pavement that fails early along every seam.

Edge compaction and confinement

Edges share the joint's problem. An unconfined edge, the outside edge of a mat with nothing beside it, wants to spread sideways under the roller instead of compacting down, so it ends up low in density and rounded off in shape. That open edge is a path for water to get under the mat and a weak line where the pavement breaks down first.

The fix is to confine and compact the edge deliberately. Where the design allows, the edge gets supported, by an adjacent lane, a curb, or backfill, so it has something to push against. The roller is positioned to compact the edge rather than leaving it hanging off the drum, sometimes running a few inches over the edge and sometimes held just in from it depending on the detail and whether the edge will be confined later. Some crews use an edge-restraining device or a beveled edge where the spec calls for it.

On a job where the edge stays exposed, like the outside of a widening or a shoulder, edge density is worth the same attention as the joint. It is one more low-density line where failure starts, and it is cheap to roll right and expensive to fix once traffic has worked it loose.

How is asphalt density measured and accepted?

In-place density gets measured three ways, and most jobs use more than one. A nuclear density gauge reads density fast by sending radiation through the mat and measuring what comes back, which lets the crew check the pattern in near real time. A non-nuclear gauge does a similar quick read using electrical properties, without the radioactive source and the licensing that comes with it. Cores, cut from the finished mat and tested in the lab, are the most accurate measure and the usual basis for acceptance.

The gauges and the cores work together. The crew runs a gauge during paving to confirm the pattern is holding and to catch a low spot before the next truck dumps. Acceptance, the measurement the agency pays on, usually rests on cores or on a gauge correlated to cores, because the lab core is the number that holds up in a dispute. A gauge that drifts out of correlation will tell you the mat is fine while the cores say otherwise, so the gauge gets checked against cores, not trusted on its own.

How acceptance is scored varies by agency. Many DOTs use a percent-within-limits, or PWL, approach, where a lot is paid on the share of its tests that fall inside the spec band rather than pass or fail on a single reading, with pay factors that reward a tight, uniform lot and dock a marginal one. The acceptance method, the test frequency, and the pay schedule all live in the project or DOT specification, so confirm them for the job before the first test point.

Monitoring mat temperature behind the paver

Mat temperature is the variable the crew actually controls in the field, so it gets measured, not guessed. A handheld infrared gun reads the surface fast and lets the roller operator and the inspector watch the mat as it cools across the pull. A probe thermometer pushed into the mat reads the temperature through the depth, which matters because a thin surface can read cooler than the mix underneath that the roller is really working.

There are two temperatures worth knowing. The temperature behind the paver tells you where the breakdown roller is starting, near the top of the window, and the temperature at the back of the rolling train tells you whether the finish pass is happening before cessation. Watching both tells you whether the rolling train is keeping pace with the cooling mat or falling behind it. When the back of the train drifts toward cessation, the rollers need to tighten up the gap or the density at the tail end is at risk.

Roll to the mat temperature you measure, not to the clock or the chart. The breakdown, intermediate, and finish temperature ranges that get quoted are starting points; the real window is set by the mix and confirmed by what the gun and the probe read on the day. Cold base, wind, and a thin lift all pull the readings down faster than a calm warm day, so the same mix can give you a generous window one afternoon and a tight one the next.

The tender zone, where the mat shoves

Some mixes go through a tender zone, a temperature range where the mat becomes unstable under the roller and shoves, cracks, or checks instead of compacting. It often shows up in a middle band, roughly the 180 to 240 degree F range depending on the mix, where the mix is too cool to flow cleanly but still too hot to hold its shape under the drum. Roll hard inside that zone and you push a wave of mix ahead of the roller and leave hairline checking across the mat.

Tenderness usually traces back to the mix, things like a high natural sand content, excess moisture, or a binder that stays soft through that band, so the real fix lives upstream at the plant. In the field, the move is to recognize the zone and work around it. That can mean getting most of the density done with the breakdown roller above the tender zone, easing off or staying off the mat while it passes through the unstable band, then coming back with the finish roller once it has cooled below it.

The tell is visual and immediate. The mat moves ahead of the drum, a crescent or a wave builds, or fine cracks open behind the roller. When you see it, you do not roll harder. You back off, let the mat move out of the zone, and adjust the pattern around it. The exact temperature band where a given mix gets tender is a mix property, so confirm it for the JMF rather than assuming a fixed range.

Cold weather shrinks the window

Cold cuts the compaction window short. A cold base, cold air, and wind all pull heat out of the mat fast, so the mix reaches cessation temperature sooner and the rollers have less time to make density before the door closes. The same mat that gives you several minutes to work on a warm calm day might give you a fraction of that on a cold windy one, and the breakdown roller has to be right on the screed to use what little window there is.

Wind matters more than people expect, because it strips heat off the surface faster than still air at the same temperature. So does a thin lift, which has less mass to hold heat. Put a thin lift on a cold base in wind and the mat can drop below cessation almost as fast as it is laid. The defenses are a higher laydown temperature where the mix allows, a tighter rolling train, and sometimes a hard stop on paving below a temperature the spec sets. Cold-weather paving limits and any minimum surface temperature are spec and mix questions, so confirm them for the job before you commit a load to a cold morning.

Lift thickness, aggregate size, and held heat

Lift thickness changes how long you have to compact, because a thick lift holds heat and a thin one loses it fast. More material means more mass and a slower cool, so a thick mat stays in the window longer and is more forgiving to roll. A thin lift gives up its heat to the base and the air quickly, which is why thin overlays are the ones that catch crews short on the breakdown pass.

Thickness is also tied to aggregate size, and the ratio matters for compaction. The lift has to be thick enough relative to the nominal maximum aggregate size for the stones to move and reorient under the roller rather than lock up against each other. The common rule is a lift thickness of at least three to four times the nominal maximum aggregate size for dense-graded mixes, though the exact ratio belongs to the mix and the spec. Lay a lift too thin for its stone and you cannot get density no matter the pattern, because the aggregate has nowhere to go.

On the thick end, a lift placed too deep for the rollers to compact through its full depth leaves the bottom open even when the surface tests fine, which is why agencies cap maximum lift thickness too. Match the lift to the mix and the spec, and remember that a thin lift is a short clock. Plan the rolling train to be tighter and faster on thin work than on a thick base course.

Thermal segregation and cold spots

Thermal segregation is uneven temperature in the mat as it is placed, and it quietly wrecks density. A cold spot, a patch of mat that came out of the truck or the hopper cooler than the rest, hits the rollers already below where it will compact well. The pattern that gets the hot mat to target leaves the cold spot low, so you end up with a low-density patch surrounded by good mat, and that patch is the one that ravels and fails first.

Cold spots usually come from the way the mix is handled, end-of-truck loads that sat and cooled, mix that hung up in the hopper wings, or a material transfer issue, more than from the rolling itself. The rollers cannot fix a cold spot, because by the time the mat is down the heat is already gone. The fix lives in handling the mix to keep its temperature uniform before it reaches the screed.

What the rolling crew can do is watch for it. An infrared camera or a thermal bar reads temperature across the whole mat and makes cold spots show up as dark patches well before any density gauge would find them, and the thermal pattern correlates with where the low density will land. Watch the IR, flag the cold spots, and chase the cause back upstream, because a recurring cold streak is a handling problem that will repeat on every load until something changes.

How compaction fails in the field

Most compaction failures are a handful of the same problems, and they share a root: the density target was missed inside the temperature window, somewhere the crew either did not see or did not correct.

Under-compaction leaves high air voids, the open mat that lets in air and water and ages fast. Rolling too cold is its most common cause, where the mat dropped below cessation before the passes were in and no later effort could recover it. Over-rolling or rolling in the tender zone pushes the other way, shoving and checking the mat or crushing aggregate. A loose or drifting pattern misses coverage and leaves low-density strips between roller paths. A cold longitudinal joint fails along every seam. And thermal segregation leaves cold-spot patches low in an otherwise good mat. Each one is a density problem first and shows up as cracking, raveling, or shoving second, months or years after the crew is gone.

What to document

A compaction record is what backs the lot at acceptance and what answers the question later when a stretch fails early. Capture the pattern you set, the temperatures you rolled at, and the densities you measured, tied to station and lot, so the operation can be reconstructed instead of argued about. The table is a working minimum; the project or DOT spec sets the required frequency and form.

Common mistakes

• Under-compacting and leaving high air voids, so the mat takes in air and water and ages early.
• Rolling too cold, below the cessation temperature, where no number of passes will reach the density.
• Over-rolling or rolling in the tender zone, shoving and checking the mat or crushing aggregate.
• Running an inconsistent pattern that drifts and misses coverage, leaving low-density strips between roller paths.
• Leaving a cold, low-density longitudinal joint that ravels and cracks along the seam first.
• Ignoring thermal segregation, so cold-spot patches roll to low density in an otherwise good mat.
• Running high amplitude on a thin lift, crushing aggregate and decompacting instead of compacting.
• Trusting a density gauge that has drifted out of correlation with the cores.

Field checklist

Standards and references

The target density, the air voids, and the acceptance method come from the job mix formula and the project or DOT specification, not from any single number in a guide. Many state DOTs write acceptance as a percent of Gmm with a percent-within-limits, or PWL, pay schedule, and the target band, the test frequency, and the pay factors are all spec-specific. The Asphalt Institute and the National Asphalt Pavement Association, NAPA, publish compaction guidance the trade leans on for pattern, temperature, and density practice, and AASHTO and ASTM cover the test methods for the gauges and the cores. Cite the standard that governs the point, and let the project specification override any rule of thumb when it is stricter.

Hedge the numbers the same way every time. The density targets, the temperature window, the tender-zone band, and the pass counts in this guide are typical values that vary with the mix, so confirm them against the JMF, the producer, and the spec for the work in front of you. Roller settings, amplitude, frequency, and speed come from the roller manufacturer's manual for the machine and the lift. The adopted spec and the AHJ control acceptance.

Three things hold across almost every mix and spec, and they are where to put your attention. Hit the density inside the temperature window, because density is what makes the pavement last and the window is the only time you can make it. Set the rolling pattern on a test strip and hold it, because uniformity is what the density spec is really measuring. And compact the longitudinal joint on purpose, because the joint is where a good pavement quietly fails first.

Units and terms

Compaction has its own vocabulary, and the same fact often shows up under two names across a spec, a gauge readout, and a lab report.

Density is reported as a percent of Gmm or against the lab Gmb, temperature in degrees F on most US jobs, and lift thickness in inches against the nominal maximum aggregate size, or NMAS, of the mix. Air voids and density are the same measurement read from opposite ends. Keep the terms straight and the rest of the spec reads cleaner.

Air voids

The percentage of air in the compacted mat; high voids let in air and water and shorten pavement life

Gmm

Theoretical maximum density, the airless density of the mix; in-place density is reported as a percent of it

Density

How tightly the mat is compacted, the inverse of air voids, and the strongest predictor of how long the pavement lasts

Breakdown rolling

The first rolling phase, right behind the paver, that makes the bulk of the density while the mat is hottest

Rolling pattern

The fixed pass count, overlap, speed, and gap each roller runs, set on a test strip and held to keep density uniform

Tender zone

A temperature band where the mat is unstable under the roller and shoves or checks instead of compacting

Cessation temperature

The mat temperature below which rolling no longer adds density, the bottom of the compaction window

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