Asphalt segregation: causes, prevention, and detection

What asphalt segregation is, and why the spots fail first

Asphalt segregation is the non-uniform distribution of the mix across the mat. A properly made batch is a designed blend of coarse stone, fine aggregate, and binder at a set temperature. Segregation breaks that blend apart, so instead of the same mix everywhere, you get patches that are short on fine material and binder, or patches that went down cold. The mat looks like one pavement and behaves like several.

The reason it matters is density. A segregated spot does not compact like the rest of the mat. A coarse, lean pocket has too much stone and not enough fine material and binder to fill the gaps, so it holds high air voids no matter how you roll it. A cold spot is too stiff to move under the roller and locks in low density. Either way you end up with a localized area of high voids, and high voids is where water gets in, the binder oxidizes, and the surface ravels.

That is why the scattered coarse patch or the cold streak behind the paver becomes the first pothole on the job. The road around it can be sound and the segregated spot still fails, because failure starts at the weakest area, not the average one. You control segregation by how the mix is handled from the plant to the screed, and you catch it by reading the mat, watching the infrared, and pulling density at the suspect spots. This guide is a companion to the paving inspection and QC guide and to the compaction and density guide, which go deeper on acceptance and on rolling the mat to target.

What segregation costs you in pavement life

Segregation is a durability problem before it is anything else. The mix design sets a target air void level the pavement was built to carry, usually in the range of a few percent after compaction, with the exact number coming from the mix and the spec. A segregated area misses that target locally, and the miss is what shortens the life of the whole section.

A coarse, high-void spot lets air and water move through the pavement. The binder hardens and goes brittle, the stone at the surface loses its grip, and the area ravels into a rough, open texture that throws loose aggregate. From there it cracks and then it potholes, often years before the surrounding mat shows any distress. A fine-rich spot is the opposite problem. It runs low on voids and high on binder, so it is prone to rutting and flushing under traffic.

The money side follows the durability side. A segregated lot can lose density acceptance and take a pay deduction, and a visibly segregated surface can draw a corrective-action order or a removal at the contractor's cost. Worse is the warranty exposure, because the spot that fails early is the one the owner remembers. None of that shows up the day you pave. It shows up after the crew is long gone and someone else owns the road.

What are the two types of asphalt segregation?

The two types of asphalt segregation are physical segregation and thermal segregation, and they are different problems with different fixes. Physical segregation, also called aggregate or gradation segregation, is a separation of particle sizes. The coarse stone parts from the fine material so some areas of the mat are stone-rich and others are sand-rich. Thermal segregation, also called temperature segregation, is a separation of temperature. Cooler mix forms cold spots in an otherwise hot mat.

They overlap on the jobsite because the same handling that drops cold material often drops coarse material with it, but they are not the same defect. Physical segregation is locked in by the time the mix reaches the mat, because it is a gradation problem you cannot roll out. Thermal segregation is about heat, so it shows up as low density in the cold spots and a re-blend ahead of the paver can fix it before it lands.

Knowing which one you are looking at decides what you do. A coarse, open texture in the mat points at physical segregation and sends you back up the chain to the stockpile, the truck loading, and the paver feed. A density differential at a spot that looked fine on texture, with no coarse pocket, points at thermal segregation and sends you to the truck exchange, the wait time, and the case for a material transfer vehicle.

Physical segregation: when the coarse stone separates out

Physical segregation is gravity sorting the mix by particle size. Whenever loose hot mix is dropped, pushed, or rolled, the coarse stone tends to move differently than the fine material. The big particles roll and slide to the outside and the bottom of a pile, and the fine material stays toward the center. Every drop point in the operation is a chance for that sorting to happen: the stockpile, the truck, the silo, and the paver.

What you see in the mat is a patch or a streak with too much exposed stone and not enough mortar around it. The texture reads open and rocky next to the tight, uniform surface of the rest of the mat. Those coarse-rich areas carry high air voids and low binder content, which is exactly the recipe for moisture damage, raveling, and early cracking. The fine-rich areas that the coarse left behind run the other way, low on voids and high on binder, and they tend toward rutting and flushing.

The part that makes physical segregation unforgiving is that it cannot be compacted out. You can roll a cold spot back toward density if you reach it in time, but you cannot add the missing fine material and binder to a stone pocket with a roller. By the time a gradation-segregated spot is in the mat, the choice is to live with it or take it out. That is why physical segregation is fought at the handling stage, not at the screed.

Thermal segregation: cold spots in a hot mat

Thermal segregation is temperature difference across the mat as it is placed. The mix leaves the plant uniform, but it does not stay uniform on the way to the screed. Mix against the cold steel of the truck bed and along the top crust of the load gives up heat, while the core of the load stays hot. When that cooler material runs through the paver without being re-blended, it lands as a cold spot in a hotter mat.

Cold mix does not compact. Asphalt has a temperature window where the binder is soft enough for the aggregate to move under the roller, and below that window the mix is too stiff to densify. So a cold spot rolls out to a lower density and higher voids than the mix around it, even though the gradation is fine. The result looks the same in service as a coarse pocket: high voids, then water, then raveling and cracking. The compaction and density guide covers that temperature window and the roller pattern in detail.

The damage tracks the size of the temperature difference. Industry and several state studies use temperature differentials as the measure, with moderate segregation often described in the range of roughly 25°F differential and severe segregation above roughly 50°F, and density losses showing up clearly once differentials get large. Treat those break points as the kind of numbers the spec sets, not fixed law. The thermal profile limit that controls your job comes from the mix and the agency specification.

Where segregation shows up in the mat

Segregation lands in patterns, and the pattern tells you the cause. Read the mat behind the paver and the location points back up the chain to where it started.

A repeating defect that shows up once per truckload is cyclic segregation, and the truck exchange is the prime suspect. A single segregated line down the middle of the lane is centerline segregation, which comes from the paver feed: a worn or low auger, missing or damaged baffle plates, an overloaded feed system, or a screed problem. Coarse or cold material concentrated at the edges and outer wings of the mat points at the hopper wings and the auger reach. A bad patch at the end of each load is end-of-load segregation, tied to the bottom of the truck and the conveyor running dry.

Thermal segregation has its own signature. Cold spots tend to land in regular intervals tied to the truck cycle, with one at the start of each new load after a wait, and along the longitudinal edges where the mat has cooled. The infrared bar makes that pattern obvious because it maps temperature foot by foot, which is why a thermal profile catches a cyclic cold spot that the eye walks right past.

Causes at the plant and stockpile

Segregation can be built into the mix before a truck ever leaves the plant, and it starts in the stockpile. Coning a stockpile, dumping aggregate into a single growing pile, sends the coarse particles rolling down the outside while the fines stay in the cone. Load out of that pile unevenly and the cold feed sends a varying gradation into the dryer. The fix is in the handling: build stockpiles in layers, do not cone single-size piles, and keep the loader from digging segregated material out of one face.

The hot side has its own drop points. A poorly adjusted batcher or a tall drop into the silo can sort the mix by size on the way down, which is its own section below. The plant should be producing a mix that meets the job mix formula gradation consistently, load to load, because a mix that varies at the plant looks like segregation in the mat even when the handling downstream is clean.

All of these are hedged to the operation and the spec. The exact stockpile practice, silo drop control, and gradation tolerance come from the producer's quality plan and the agency specification, and a plant under a tight DOT spec runs differently than a small commercial plant. The principle holds across both: every place loose material is dropped or pulled is a place gradation can separate, so handle it to keep the blend together.

Causes in truck loading: the single-drop trap

Loading a truck in one big drop is one of the most common ways to segregate a load, and it is one of the easiest to fix. When mix falls into the bed in a single pile, the coarse stone rolls down the sides of the cone and collects at the front, the back, and the bottom, while the fine material stays in the center. The driver hauls a load that is already sorted, and it comes out of the truck sorted, so the segregation shows up as a cyclic pattern once per load in the mat.

The fix is multiple drops. Load the truck in at least two or three separate drops placed at different points in the bed instead of one pile. A common practice is the three-drop method: load against the front bulkhead first, then against the tailgate, then fill the middle. The rule of thumb many crews use is three drops in a tri-axle and two in a tandem, with the exact pattern set by the truck and the plant. Spreading the load keeps each drop's cone small, so the coarse stone has less room to run.

This is a habit problem as much as a procedure. A loader operator in a hurry single-drops to save thirty seconds, and the cost shows up as a segregated streak behind the paver hours later that nobody connects back to the loading. If you see cyclic segregation timed to the truckload, go watch how the trucks are being loaded before you blame the paver.

Causes at the silo and batcher

The storage silo is a classic drop-segregation point. When mix is conveyed up and dropped into a silo from a height, the load falls into a cone inside the silo and the coarse stone rolls to the outside, the same physics as a stockpile or a truck. Pull from that silo and the gradation varies as the cone empties. The common control is to keep the drop centered and short, often with a batcher or gob hopper that releases the mix in batches straight down the center rather than letting it free-fall and scatter.

A batcher works by collecting mix and dropping it as a contained mass, which keeps the particles together instead of letting them sort on the way down. Surge and storage bins that lack a batcher, or that are run nearly empty so the drop height grows, are more prone to sending segregated mix to the trucks. The drop height matters: the taller the fall, the more the coarse separates.

Set the specifics against the plant and the spec. The silo drop control, the batcher operation, and how low a silo can run before it segregates depend on the equipment and the producer's quality plan, and some agencies limit silo storage time and conditions outright. Hedge the numbers to the job, but treat any tall, uncontrolled drop of loose mix as a place segregation gets made.

Causes at the paver: empty hopper and cold gobs

The paver is the last place segregation gets made, and it makes both kinds. The biggest paver-side cause is running the hopper too low or empty between trucks. When the hopper empties, the conveyor slats run dry, and the cold, coarse material that collects at the front corners and along the wings of the hopper gets dragged into the auger chamber in a slug. That slug lands as a coarse, cold gob in the mat, right at the truck change.

Folding the hopper wings makes it worse. The wings are the angled sides of the hopper, and material in them sits longest and coolest and carries the coarse stone that rolled to the edges. Dumping the wings, especially onto an empty or nearly empty conveyor, sends that cold, coarse material straight through the paver as a concentrated gob. Crews fold the wings to clean out the hopper between loads, and that habit is one of the surest ways to put a segregated patch in the mat at every truck exchange.

The other paver causes are feed problems. Running the augers and conveyors faster than the mix can keep up, or starving the auger so it runs low and intermittent, sorts the mix as it moves out toward the screed. A worn auger, missing reverse paddles, or damaged baffle plates show up as the centerline streak. The cure for nearly all of it is steady, full feed, which the next two sections cover.

The hopper wings: keep them up, keep it full

The single most repeated rule on the paver is do not fold the hopper wings to dump them between loads, and do not let the hopper run empty. Both put the coldest, coarsest material in the hopper through the paver in a concentrated slug. The wing material has been sitting against cold steel, losing heat and collecting the stone that rolled to the sides, so it is exactly the material you do not want feeding the screed.

The way to run it is to keep a consistent head of material in the hopper, commonly at least a third full and never run dry, so the conveyor is always pulling from a deep, mixed supply rather than scraping the corners. When the wings hold material, leave it there and let it feed naturally with the rest of the load. If you must fold the wings, never fold them onto empty conveyor slats, because that is what fires the cold gob through. Time any wing handling to when there is mix on the slats to carry it through blended.

This is blunt for a reason. A crew that keeps the hopper full and the wings up eliminates a whole class of cyclic segregation for free, with no extra equipment. A crew that empties the hopper and folds the wings at every truck change builds a segregated spot into the mat on a schedule, and then spends the density argument explaining it. Keep it full. Keep the wings up.

What is a material transfer vehicle, and why it fixes segregation

A material transfer vehicle, or MTV, is a machine that runs between the trucks and the paver, taking the mix from the dump trucks, holding it in a surge hopper, and re-blending it before feeding it into the paver. The re-blending is the point. Most MTVs use an auger, commonly a triple-pitch design, that pulls material from across the whole hopper and remixes it, folding the cool crust and the coarse edges back into the hot, well-graded core. The mix that enters the paver comes out uniform in both gradation and temperature.

That remixing makes the MTV the single biggest fix for thermal segregation and for the truck-borne part of physical segregation. It directly undoes the sorting and cooling that happened in the truck, so the cyclic cold spot at each truck change and the coarse load just disappear into the blend. The surge capacity is a second benefit: the paver can run continuously without stopping for trucks, and a paver that never stops avoids the cold joints and density dips that come from a paver that keeps halting.

An MTV is not free and not always required, so hedge it to the job. On a segregation-prone mix, a long haul that cools the load, a high-profile surface lift, or a project where the agency spec calls for one, the MTV earns its place and is often the difference between passing the thermal profile and chasing it. On a short haul of a forgiving base mix, good handling may carry the day without one. When the mix is sensitive or the spec is tight, the MTV is the tool that re-blends the temperature and the gradation that handling alone cannot.

End-of-load segregation and the truck exchange

The truck exchange is where cyclic segregation is born, and end-of-load is the worst of it. The bottom of every truck holds the coldest, most sorted material, the stuff that sat against the bed and collected the coarse stone. When that last of the load runs out and the hopper empties before the next truck dumps, the conveyor scrapes the cold corners and sends a gob through right at the changeover.

Managing the exchange is mostly about never letting the hopper get low. Bring the next truck in and dump before the hopper drops below the working level, so the new hot mix mixes with what is left instead of the paver running on fumes. Keep the paver speed steady through the change rather than surging to catch up after a gap. A consistent, unbroken feed turns the truck change from a defect generator into a non-event.

An MTV removes the problem at the source, because its surge hopper carries the paver through the gap between trucks and its auger re-blends the cold bottom of each load into the rest. Without an MTV, the exchange is a discipline problem: load the trucks in multiple drops so the bottom is not as segregated to begin with, time the trucks so the hopper never empties, and keep the wings up so the cold material is not dumped through when the slats run dry.

Paver operation: a steady, full head of material

Almost everything the paver does to prevent segregation comes down to a steady, full head of material moving uniformly out to the screed. The augers should run nearly continuously, kept at a consistent depth of mix that covers the auger shaft, roughly at the centerline of the auger, rather than cycling on and off as the level swings. When the auger runs low and then surges full, it sorts the mix and feeds the screed unevenly, which shows up as the centerline streak and a wandering mat.

Match the conveyor and auger speed to the paving speed so the feed keeps up without running ahead. Running the conveyors and augers too fast outpaces the mix and pulls the coarse material out toward the ends, which is a common cause of edge segregation. A steady paving speed matters as much as the feed setting, because a paver that lurches and stops cools the mat and disrupts the head of material every time it moves.

The hardware has to be right too. Baffle plates, sometimes called flow gates or kickback paddles, keep the mix from sorting as it moves outward, and worn or missing reverse auger paddles or baffles let the coarse stone collect at the center, which is the textbook source of centerline segregation. The paving inspection and QC guide covers what the inspector watches behind the paver. The short version for the operator is to keep the feed steady, the augers buried, and the speed constant.

Detecting segregation by reading the mat

The first detection tool is your eyes, walking the mat right behind the paver where there is still time to react. Physical segregation reads as texture. A gradation-segregated spot looks open and rocky, with coarse stone exposed and not enough mortar around it, set against the tight, uniform surface of the good mat. Once you have seen a few, the coarse patches and streaks jump out, and the pattern they make tells you where they came from.

Thermal segregation is harder to see and that is the trap. A cold spot can look almost identical to the mat around it because the gradation is fine, so the eye walks past it. Sometimes you catch a duller, slightly different sheen on the cooler material, but you cannot count on the eye for thermal segregation. That is what the infrared is for.

Reading the mat is also about timing. Catch a segregated streak in the first few truckloads and you can fix the cause before it runs through the whole shift, whether that means changing how the trucks are loaded, keeping the hopper fuller, or stopping the wing folding. Catch it at the density cores three days later and the only conversation left is corrective action. Walk behind the paver, every shift, and watch for the texture change.

How do you detect thermal segregation?

You detect thermal segregation by measuring mat temperature as it is placed, because temperature difference is the defect itself. The eye does not see it reliably, so the field uses infrared. A handheld thermal camera or an infrared thermometer spot-checks the mat and finds cold areas, and the dedicated tool is an infrared bar mounted across the back of the paver that scans the full width continuously as the mat goes down.

An IR bar system carries a row of infrared sensors across the paver and logs temperature foot by foot, building a color-coded thermal profile of the entire mat. The profile turns thermal segregation from something you stumble on into a map you can read in real time: the cold spots stand out, the cyclic pattern at the truck changes is obvious, and the operator can react before another load goes down wrong. The profile also becomes a record, which the documentation section gets to.

The spec is what turns a temperature difference into a pass or fail. Many agencies define a thermal segregation limit as a temperature differential, with moderate and severe break points often near 25°F and 50°F differentials, and some specs tie a thermal profile requirement directly to the IR data and call for corrective action or coring where it is exceeded. Those numbers vary by agency and by mix, so hedge them to the project. Confirm the differential limit and the profile requirement against the DOT or project specification and the AHJ before you set the threshold.

Detecting segregation with cores and the density gauge

Density is where segregation gets proven. A segregated spot does not reach the same density as the surrounding mat, so the way to confirm it is to test the suspect area and compare it to the good mat nearby. The measure is the density differential, the gap between the segregated spot and the surrounding pavement, and a meaningful gap is the hard evidence that a texture change or a cold spot actually hurt the pavement.

Two tools do the work. A nuclear or non-nuclear density gauge gives a quick reading at the suspect spot and a reference spot, so a crew can check a streak the same shift it appears. Cores are the referee: cut the segregated area and a control area, run the densities, and the difference is undeniable. The compaction and density guide covers how the gauge is correlated and how cores are taken and run, so the readings hold up.

Set the acceptance against the spec. Some agencies define segregation by a density differential threshold, others by air voids or by the relationship to the surrounding mat, and the exact number and method come from the project or DOT specification, not a universal figure. The principle is steady: a real segregated spot shows lower density than the mat around it, and a density check at the suspect location is how you turn a suspicion into a defect you can act on or a worry you can set aside.

Spec limits: what the agency actually enforces

Segregation is judged against the specification, and the specification can come at it from more than one direction. A thermal profile spec sets a temperature differential limit on the mat as placed, often read from an IR bar or thermal camera, with break points for moderate and severe segregation and corrective action tied to them. A density spec catches the same defect on the back end, because a segregated spot fails the density or air-void acceptance for its lot. Some agencies also carry a direct segregation specification that defines the defect by texture, gradation, or a density differential and spells out the remedy.

The trend in recent specs is toward measuring segregation rather than judging it by eye, with infrared thermal profiling and density data doing the work. That is good for the contractor who runs a clean operation, because the data backs the work, and hard on the one who does not, because the cold spots are on the record. Which combination of these applies to your job is set by the agency.

Treat every number in this guide as the kind the spec sets, not as fixed law. The temperature differentials, the density differentials, the air-void targets, and the segregation remedies all vary by agency, by mix, and by code cycle. Confirm the limits and the acceptance method against the mix design, the DOT or project specification, and the AHJ before you hold the crew or the lot to any of them.

What a segregated spot turns into

A segregated spot follows a predictable path to failure, and it is short. The high voids let water and air into the pavement, the binder oxidizes and goes brittle, and the surface starts to ravel as the stone loses its grip. Raveling becomes a rough, open patch that throws loose aggregate, then it cracks, and then it potholes. The localized failure is the signature of segregation: a sound road with a bad spot that fails on its own schedule, years ahead of the mat around it.

Fine-rich segregation fails the other way. Low voids and high binder make those areas prone to rutting under wheel paths and to flushing, where binder works to the surface and the pavement goes slick. Both ends of the gradation problem cost pavement life, just through different distresses.

The contractual consequences land before the pavement ever potholes. A segregated lot can lose density acceptance and take a pay deduction, a thermal profile that exceeds the spec limit can trigger corrective action, and a visibly segregated surface can be ordered out and replaced at the contractor's cost. On a warranty job, the early failures are the contractor's to fix. The cheapest place to deal with all of it is behind the paver, the day it is paved, not at the pay meeting or the warranty callback.

Preventing segregation: the short list that works

Prevention is mostly handling, and a handful of habits remove most of the risk. Handle the aggregate and the mix to keep the blend together at every drop point: build stockpiles in layers instead of cones, keep silo drops centered and short with a batcher, and do not let any loose mix free-fall far enough to sort. The principle is the same everywhere material moves, so handle and load to avoid segregation.

On the trucks and the paver, the rules are specific. Load trucks in multiple drops, not one pile, so the load is not sorted before it leaves the plant. Keep the paver hopper full, never run it empty, and do not fold the wings onto empty slats, so the cold coarse material never feeds through as a gob. Run a steady, full head of material with the augers buried and the speed constant. And on a sensitive mix or a long haul, put an MTV in the train to re-blend temperature and gradation before they reach the paver.

Watching is the other half. Walk the mat behind the paver every shift, run the IR bar or a thermal camera so the cold spots show up on the profile while there is still time to react, and pull density at any suspect spot to confirm or clear it. Keep the hopper full and the wings up, load in multiple drops, use an MTV on the mixes that need it, and watch the infrared. That is the program.

Repairing a segregated area: remove or leave it

Once segregation is in the mat, the question is whether the area comes out or stays, and the answer turns on how bad it is and what the spec allows. A thermal cold spot caught very early, while the mix is still in its compaction window, can sometimes be brought toward density with immediate rolling, but that is the exception. Most segregation is found after the fact, and a gradation-segregated spot cannot be compacted out at all, because the missing fine material and binder are not there to add.

For a localized, severe spot, the durable repair is full-depth removal and replacement: saw or mill out the segregated area and the surrounding margin, tack the edges, and place and compact fresh mix. A surface seal or a thin treatment over a segregated area only hides the texture. It does not fix the high voids underneath, and the spot keeps deteriorating below the cover.

Whether a given area must come out is the agency's call, against the segregation, density, or thermal profile spec. Minor segregation may be accepted with a pay reduction, moderate areas may be sealed or monitored, and severe areas are removed and replaced. Hedge the threshold to the project, document the spot with location, texture, IR, and density, and let the spec and the AHJ decide the remedy. The blunt rule holds: if the spot is bad enough to fail early, it is cheaper to take it out now than to warranty it later.

What to document

Segregation is an argument you win with records, and the records are easy to keep if you keep them as you go. The thermal profile from the IR bar is the strongest single document, because it maps the mat foot by foot and timestamps the cold spots against the truck cycle. Pair it with the density data at the suspect locations and the visual notes on texture, and you have the defect described three ways.

Capture the location and station of each segregated area, the type and the suspected cause, the temperature differential or thermal profile reading, the density at the spot and at a control area, the lot it falls in, and the action taken. Photograph the texture next to good mat. If you changed the operation, loading, hopper level, speed, or adding an MTV, log when and what changed, so the improvement shows on the record. A field tool like FieldOS keeps the photo, the location, the readings, and the action together on one record instead of scattered across a phone, a notebook, and a gauge printout.

The reason to bother is the same as on any pay item. The lot that gets questioned at the pay meeting is the one where the data answers the question, and the spot that shows up as a warranty callback is the one where the record shows whether it was caught and addressed or paved over and ignored.

Common mistakes

• Folding the paver hopper wings and dumping the cold coarse material through, especially onto empty conveyor slats.
• Single-drop truck loading that sorts the load before it leaves the plant.
• Running the paver hopper empty between trucks so the conveyor scrapes the cold corners.
• Skipping a material transfer vehicle on a segregation-prone mix or a long, cooling haul.
• Ignoring the IR cold spots on the thermal profile instead of reacting while the mat is still going down.
• Stockpile coning or an uncontrolled silo drop that segregates the mix before it is ever loaded.
• Treating a temperature or density differential break point as fixed law instead of confirming it against the spec.

Field checklist

Standards and references

The mix design and the agency specification are what govern segregation on your job. The job mix formula sets the gradation and the air-void target the mat has to hit, and the DOT or project specification sets the segregation limits, the thermal profile requirement, the density acceptance, and the remedies. State DOT specifications differ on how they define and measure segregation, so the controlling document is the one the agency adopted for the contract, with any special provisions.

For the engineering behind the practice, the Asphalt Institute treats the causes and cures of segregation directly, and the NCHRP work on segregation in hot-mix asphalt pavements, commonly cited as Report 441, is the reference many specs trace back to. Pavement Interactive and AASHTO materials cover aggregate and temperature segregation, material transfer vehicles, and the handling practices that prevent them. Several state DOTs and university transportation centers have published the infrared thermal-profiling and temperature-differential research that newer specs are built on.

Cite the document that controls the point. The differentials, the density numbers, and the acceptance methods all come from the mix design, the DOT or project specification, and the AHJ, and they change by agency and by code cycle, so confirm them against the adopted edition before you hold the work to any specific limit. The constants across all of it are the practices: handle and load to avoid segregation, keep the hopper full and do not fold the wings, and use an MTV and watch the IR.

Units and terms

Segregation comes with a small vocabulary that is worth keeping straight, because the same word means different things to the plant, the paver crew, and the inspector.

Temperature is read in °F on most North American jobs and in °C on metric specs, and thermal segregation is described by a differential, the gap between the cold spot and the surrounding mat, not by an absolute temperature. Density is reported as a percent of a reference, and a density differential is the gap between the segregated spot and the good mat. Gradation is the particle-size distribution the job mix formula sets.

Segregation

Non-uniform distribution of the mix across the mat, in gradation, temperature, or both

Physical / gradation segregation

Separation of coarse from fine aggregate, leaving stone-rich or sand-rich areas

Thermal / temperature segregation

Temperature difference across the mat as placed, leaving cold spots that compact poorly

MTV (material transfer vehicle)

Machine between truck and paver that re-blends gradation and temperature before the paver

Thermal profile

A foot-by-foot temperature map of the mat, usually from an infrared bar on the paver

Density differential

The gap in density between a segregated spot and the surrounding mat

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