Asphalt longitudinal joint density and construction: the seam that fails first

Longitudinal joint density, and why the seam fails first

Longitudinal joint density is the in-place density of the seam where two adjacent paving passes meet, running with traffic down the road. It is almost always the lowest-density, most permeable line on the whole job. The field of the mat gets rolled tight between confined edges, but the seam sits over the spot where the first pass had a free, unsupported edge when it was laid, and that edge never reached the density the rest of the mat did.

That low density is why the joint is where the pavement gives out first. Water and air move through the open seam, the binder oxidizes and goes brittle, and the joint ravels and cracks from the line out while the mat on either side is still sound. Drive any old road and the failure is down the lane line and along the edges, not in the middle of the mat. The seam tells on the crew years before anything else does.

Building a dense, bonded longitudinal joint is what makes the seam last as long as the mat around it, and it takes three things working together: the right method for the joint, tack or adhesive on the face so the new mat bonds to the old, and rolling the joint on purpose instead of letting the field pattern leave it lean. This guide goes deep on the density at that seam. The general rolling, the temperature window, and density acceptance live in the compaction and rolling guide, and the joint geometry, the transverse joint, and the handwork live in the joints and handwork guide. Here the subject is narrower and one thing only: getting density into the longitudinal joint and keeping it there.

Why does the longitudinal joint fail first?

The longitudinal joint fails first because it is the hardest place on the job to reach density, and low density is where every asphalt failure starts. In the field of the mat the roller works material confined on both sides, so the aggregate has nowhere to go but down and together. At the joint the first pass had an open edge with nothing beside it, so under the roller that edge spread sideways instead of compacting down, and it ended up lean.

Lean means high air voids, and at the seam those voids connect into channels. Once they connect, water and air move through the joint instead of running off it. The binder strips and oxidizes along the line, the surface ravels, the raveled groove holds more water, and the cycle feeds itself through every freeze and thaw. Research on in-place density ties this directly to service life: a 1 percent gain in density at the joint has been estimated to improve fatigue performance by roughly 8 to 44 percent and rutting resistance by 7 to 66 percent, which is the same fact stated as a benefit instead of a loss.

A well-built joint runs only a point or two below the surrounding mat. A bad one runs five to ten points low, and that is the joint you watch open into a crack in two or three winters. The failure was built in the day the seam was placed, not the day it showed.

The joint density differential

The number that defines the problem is the differential, the gap between the density of the joint and the density of the adjoining mat. Because the seam sits over an edge that was unconfined, it will read lower than the mat almost every time, and the question the spec asks is how much lower is acceptable. Past research and field experience put a well-built joint at about 2 percent below the mat density, and that 2 percent figure is what many agencies write as the limit on the differential.

Stated the other way, agencies set a floor on the joint itself. A common form is a minimum joint density around 91 percent of theoretical maximum density, Gmm, or a maximum allowed difference between interior mat density and joint density of about 3 lb/ft3, which works out close to the same 2 percent in most mixes. Below that line the spec writes a notice of non-compliance or a pay reduction. The two forms, a differential limit and a minimum percent, are two ways of fencing the same gap.

The numbers cluster but the spec governs the call. The exact differential, the minimum percent, and whether the joint is paid separately from the mat all come from the job mix formula and the project or DOT specification. Hedge every density figure here to the mix you are laying and the spec on the contract. What does not change is the direction of the problem: the joint runs lean, and the whole job is holding that gap small enough that the seam lasts as long as the mat.

The unconfined cold edge: the root cause

Every method in this guide is fighting one thing: the unconfined cold edge of the first pass. When the first lane is laid, its outside edge has nothing beside it. Under the roller that free edge wants to spread sideways and roll down rounded instead of compacting flat, so it comes out lean and a little low in shape. That is the edge the second pass will later butt against, and it is the reason the seam starts behind before anyone touches it.

Then the edge cools. By the time the second lane goes down, hours or a day later, the first pass edge is cold, stiff, and set, and it has no give left. The new hot mat has to bond to that cold vertical face and still drive density across it, and cold asphalt does not want to bond or move. The two problems stack: the edge was lean when it was placed, and it is cold and hard when the new mat meets it.

This is why the cheapest density at the joint is bought on the first pass, hours before the joint even exists. Confine and compact that edge while it is still hot, with an edge restraint, a notched-wedge shoe, or a deliberate edge roll, and you have raised the density of the seam before the second lane is ordered. Leave the edge to spread and cool on its own and you are chasing density across a lean, cold face for the rest of the job, which is a fight you usually lose by a few points.

What is the difference between a hot joint and a cold joint?

A hot joint is made hot against hot, with both sides of the seam still workable when the roller passes over it. A cold joint is the second lane paved against a first lane that has already cooled and set. The difference decides how much density you can get, and it is the single biggest factor in how the joint performs.

At a hot joint there is no cold, stiff edge to fight. The two mix faces are both hot, they knit together under the drum, and the unconfined-edge problem mostly disappears because the edge gets compacted while it can still move. A true hot joint can come out with no visible seam and density at the line as high as the mat around it. That is the best joint there is, and echelon paving is how you make one.

At a cold joint the first pass edge is the lean, cold face described above, and the new mat has to bond to it and densify across it from one side only. This is the normal case on overlays and lane-at-a-time work under traffic, and it is where joints go bad. Almost everything else in this guide, the tack, the notched wedge, the overlap, the rolling sequence, the joint adhesive and membrane, is a way to make a cold joint behave more like a hot one. When you cannot pave hot against hot, you are managing a cold joint, and you manage it on purpose.

What is echelon paving?

Echelon paving is two pavers running side by side, one staggered slightly behind the other, laying adjacent lanes at nearly the same time so the longitudinal joint is made hot against hot. It is the method that builds the strongest joint, and where the work allows it, it is what you want.

It works because the rollers behind the two pavers can pass directly over the joint while both sides are still hot. Neither edge has cooled, so the seam compacts as one piece of mat instead of a cold face and a hot face meeting at a line. The second mat laps the still-hot first mat by about an inch, the roller works the seam from the hot side with mix that has not set, and the lean-edge problem never gets a chance to form. Agencies that have pushed joint density hard lean on echelon paving for exactly this reason.

The catch is logistics, which is why you do not see it everywhere. Echelon needs two pavers, the plant output to feed both at once, and enough closed width to run them together, so it fits new construction and wide reconstruction far better than a single-lane overlay under live traffic. When the job will not give you the width or the second paver, you fall back to a cold joint and the methods that make one hold. Echelon is the goal; the rest of the guide is what you do when you cannot reach it.

What is a notched-wedge joint?

A notched-wedge joint is a tapered longitudinal joint shaped on the first pass by a device on the screed, instead of a square vertical edge. A shoe or restrictor forms a gentle ramp from the top of the mat down to the base, with a small vertical notch at the top and another at the bottom. The taper typically runs about 12 in wide, and the notch depth is usually set to at least one nominal maximum aggregate size of the mix, so a 3/4 in mix gets a notch on the order of 3/4 in. The dimensions and the lift thickness that triggers the method come from the agency spec, so confirm them.

The wedge buys density two ways. The slope confines the edge of the first pass so it cannot spread and run lean under the roller, which raises the density of that edge before the joint is ever made. Then the second pass laps up the ramp, and because the face is sloped instead of a square butt, the new mat seats against more surface and the roller can drive the seam tighter. Studies on the notched wedge generally find it holds higher joint density than a plain butt joint, mostly from that edge confinement.

The notches matter as much as the taper. The top notch keeps the wedge from feathering to a thin ragged edge that ravels on its own and gives the next pass a clean line to match. The bottom notch keeps the toe of the wedge from being a zero-thickness feather that crumbles under traffic. Pull the wedge too flat and you have a long feather that gets chewed off; make it too steep and you are back to a butt joint with extra steps. Set the shoe to the spec geometry and check it, do not eyeball it.

Notched-wedge joint

A tapered longitudinal joint with small top and bottom notches, formed by a screed shoe to confine the first pass edge and densify the seam

Joint maker / restrictor

The shoe or device on the screed edge that shapes the wedge or pinches the unconfined edge of the first pass

The vertical butt joint

The vertical butt joint is the traditional longitudinal joint: the first pass is laid with a square, near-vertical edge, and the second pass butts hot mix against that vertical face. It is the simplest joint to lay and it needs no special shoe on the screed, which is why it is still common on smaller work and where the spec does not call for a wedge.

The trade-off is density. A vertical edge gives the second mat the least surface to seat against and gives the roller the least help confining the seam, so a butt joint is the hardest of the common methods to drive to target. The square edge of the first pass also spreads under the roller more than a confined or wedged edge does, so it tends to start lean before the second lane ever arrives. On a cold butt joint with no edge treatment and no tack, the differential runs wide.

A butt joint can still make density, but it has to be built deliberately. Confine the first pass edge while it is hot, tack the cold face before the second pass, set the overlap, and roll the seam from the hot side. The butt joint is not wrong, it just gives you no margin, so the technique around it has to carry the whole load that a wedge or a hot joint would share.

Edge restraint and confining the first pass

The biggest gain in joint density comes on the first pass, by confining the unconfined edge while it is still hot. The free edge wants to spread and roll down lean, and an edge restraint stops it from spreading in the first place, which raises the density of the edge before it ever becomes a joint face. This is the cheapest density at the seam you will buy all day, because it happens on the lane that gets paved first.

There are a few ways to do it. A pinch device or edge-restraining attachment, sometimes a rubber wheel or a vertical plate that rides the edge behind the roller or the paver, presses a vertical confinement force against the edge so it compacts down instead of out. A notched-wedge shoe does the same job by shaping a confined slope rather than a free vertical edge. And the simplest version is technique: roll the unconfined edge with the drum hanging a few inches over it in a deliberate edge pass, so the edge gets driven down rather than left hanging off the drum.

Whichever method you use, the principle is the same. Density at the joint is won on the first pass edge, not on the second lane. Confine and compact that edge while it can still move, and the second lane has a dense, square or sloped face to bond to. Skip it and you are starting the joint already behind by the few points the spread edge gave away.

Tack and adhesive on the vertical face

Tack the cold joint face before you pave against it. This is the step crews skip most and pay for first. The vertical edge of a cooled lane is cold asphalt, and the new mat will not bond to it dry any more than an overlay bonds to a milled surface without tack. A dry, unbonded seam is a crack waiting to open and an open path for water from the day it is laid.

The material is an asphalt emulsion, brushed or sprayed in a light, complete coat down the exposed face, top to bottom, so the whole vertical surface is wetted. The residual tack rate is a spec number, set higher on a milled or rough face and lower on a smooth one, and on a notched-wedge joint the practice is to tack the entire wedge face, not just the vertical notch, because the second mat seats against the whole slope. Confirm the rate and the material against the project documents and let the emulsion break before the mat goes on top.

When the joint carries traffic or works hard, a hot-applied joint adhesive replaces the thin tack with a heavier, polymer-modified bond. The point at the seam stays simple regardless of which material the spec calls for: do not pave a cold lane against a dry edge. A perfectly rolled joint over an untacked face is still an unbonded seam, and it will delaminate and ravel along the line while the cores you pulled read fine.

The overlap and luting back

When the second pass goes down, it laps onto the cold mat by a small, set amount, leaving a ridge of fresh material standing proud of the seam so there is enough mix to compress down into the joint instead of starving it. On a square butt joint the lap is commonly about 1 to 1.5 in. On a notched-wedge joint, where the new mat runs up the ramp, the lap onto the cold notch is often tighter, on the order of 0.5 to 1 in, and the dimension comes from the method and the spec.

The amount matters in both directions. Too little overlap and there is not enough material to fill the seam, so the joint compacts lean. Too much and you trap a thick, fat strip of material that never compacts right and stands as a bump. Either way you have built a low-density line, just for opposite reasons. The overlap rides on the screed setup and a raker watching the seam to keep it even, because a wandering overlap, fat in one spot and starved in the next, gives you a joint that is dense in patches and lean in patches.

Some crews bump the excess back with a lute, knocking the small ridge off the cold side onto the hot mat so the roller compresses it into the seam, rather than rolling it flat over the cold lane. Others roll the overlap in place. Both are defensible, and the spec or the crew's tested pattern decides which. What loses every time is no overlap at all, a cold lane jammed dry and flush against a cold edge, rolled flat, and called a joint.

How do you get density at the longitudinal joint?

You get density at the longitudinal joint by stacking the methods, because no single one carries the seam on a cold joint. Confine the first pass edge while it is hot. Tack the cold face. Set the overlap so the seam has enough material to compress. Roll the joint from the hot side. Where the spec or the risk calls for it, add a notched wedge, a joint adhesive, or a void-reducing membrane. Each step closes part of the gap, and together they bring the seam within a point or two of the mat.

The order in time is the part crews miss. Most of the density is won before the joint is even made, on the first pass edge, and lost there if the edge spreads and cools unconfined. By the time the second lane is down and the joint exists, you are working with whatever density that edge gave you, plus whatever the overlap and the hot-side rolling can add across a cold face. That is why the strongest move is always the cheapest and earliest one: do not let the first pass edge run lean.

If you only do two things, confine the edge and roll from the hot side. Echelon paving makes both happen at once by keeping the whole seam hot. When you cannot run echelon, the rest of the methods are how you build a cold joint that performs like a hot one. The general rolling pattern, the pass count, and the temperature window behind all of this are in the compaction and rolling guide; the point at the joint is that the seam gets its own deliberate attention, not whatever the field pattern leaves at the edge.

Rolling the joint

Rolling the joint right is what turns a built seam into a dense one, and it is its own technique, separate from rolling the field of the mat. The seam needs deliberate roller passes aimed at the joint, not just the coverage the field pattern happens to leave at the edge. A mat rolled to target with a starved joint is still a pavement that fails along every seam.

The principle is to work the seam while the hot side can still move, and to pinch the new material down into the joint rather than ironing the cold edge flat. Where the new mat meets a confined, already-compacted lane, the roller overlaps the seam and presses the fresh mix into it from the hot side. The general density behind this, the breakdown, intermediate, and finish phases, the pattern set on a test strip, and the gauge-versus-core check, are all covered in the compaction and rolling guide and apply at the joint the same as anywhere.

What changes at the joint is the aim and the sequence. The roller is positioned with the seam in mind on a set first and second pass, and the timing is tight because the joint is the last place the hot mat will give up density before it cools. Roll the joint cold, or from the wrong side, and you flatten the lean edge without ever closing the voids under it. The next section lays out the hot-side sequence that makes the difference.

Rolling from the hot side and the pinch method

The modern practice is to roll the joint from the hot side, not the cold side. On the first pass, the breakdown roller works the hot lane with the drum held back from the seam, commonly about 6 in inside the hot mat, so it compacts the bulk of the hot lane while it is hottest. Then on the next pass the roller moves over and overlaps onto the cold side by about 6 in, pinching the seam from the hot mat that can still move. The exact offset is a pattern-and-spec detail, so confirm it, but the sequence is hot lane first, then pinch the seam.

Hot-side rolling beats the old cold-side pinch for a simple reason. Rolling from the hot side keeps the drum on hot mat in vibratory mode for the whole pass, so it is making density the entire time, and the seam gets pinched while the hot mix is still workable. Pinching from the cold side parks the roller on cold, set mat where vibration does nothing, and lets the hot lane lose temperature while the roller fusses with the seam. Same joint, worse result.

The discipline is timing and consistency. The joint is the tail end of the hot mat's workable life, so the roller has to get to it before the seam cools below where it will compact. Run the same hot-side sequence on every cycle, hold the offsets, and do not let the joint become the spot the roller reaches last and coldest. A joint rolled hot and pinched from the hot side is how a cold joint still comes within a couple of points of the mat.

Joint adhesive and the void-reducing membrane

When geometry and rolling alone will not close the seam, materials can win density that technique cannot. Joint adhesive is a hot-applied, polymer-modified asphalt run down the vertical face of the cold lane before the next mat goes on, gluing the new lane to the old edge with a heavy, flexible bond instead of a thin tack. It seals the face and gives the seam something that flexes under load and temperature rather than cracking.

The void-reducing asphalt membrane, sold as VRAM or a longitudinal joint sealant, works differently and is the newer approach. It is a highly polymer-modified binder placed hot, around 300 F, in a band on the existing or milled surface directly under where the joint will fall, before paving. As the mat goes down and compacts, the membrane migrates up into the bottom of the seam and fills a large share of the air voids, commonly cited around 50 to 70 percent, cutting the permeability at the one line where water always finds the pavement. It is a way to buy density at the joint through the material when the construction cannot get there alone.

Neither one replaces building the joint right, and neither covers for an untacked face or a lean unconfined edge. They are insurance on the seam that is hardest to compact, and the spec decides which one, if any, the project pays for. Treat them as a supplement to method and rolling, not a substitute, because a membrane under a joint that was never confined or pinched still leaves a seam that underperforms the mat.

Joint adhesive

A hot-applied polymer-modified asphalt run on the cold joint face to bond the new lane to the old edge

VRAM / longitudinal joint sealant

A void-reducing membrane placed hot under the joint before paving; it migrates up into the seam to fill voids and cut permeability

The joint density spec and the incentive

Many agencies now hold the longitudinal joint to its own density requirement, separate from the mat, because everyone in the trade knows the seam runs lean and a mat-only spec lets a bad joint hide inside a passing average. Roughly half of state highway agencies have adopted some form of joint density spec, and the count has grown as the research tying density to service life has piled up.

The spec takes one of two common shapes. The first is a minimum joint density as a percent of theoretical maximum, often around 91 percent of Gmm, below which the agency writes a pay reduction or a notice of non-compliance. The second is a differential limit, a maximum allowed difference between the mat density and the joint density, commonly about 2 percent or roughly 3 lb/ft3. Some specs pair the two. Either way the agency is putting money on the seam, with a pay reduction below the line and, on some specs, an incentive for a joint that holds tight to the mat.

The reason the spec exists is the same as the reason it pays: the joint is where the pavement the agency bought actually fails, so that is where they put the teeth. The exact minimum, the differential, the lot size, and the pay schedule all live in the project or DOT specification, and they vary by agency and by mix. Confirm them for the job before the first joint goes down, because building to last year's spec on this year's contract is how a passing joint still gets a pay cut.

How is joint density measured?

Joint density is measured the same ways as mat density, with cores and gauges, but taken at the seam instead of in the field of the mat, and that location is the whole point. A core cut right at the joint and tested in the lab against the mix Gmm is the most accurate measure and the usual basis for acceptance, because the lab core is the number that holds up in a dispute. Many specs cut a set number of joint cores per lot, taken at the seam.

Density gauges read faster and let the crew check the joint as it is built, but the nuclear gauge has a real limitation at the seam. Most agencies do not allow a nuclear reading too close to the joint, commonly not within about 1 ft of the edge, and a typical practice is to set the source rod 8 to 12 in from the mid-vertical edge of the mat. A non-nuclear gauge can read closer in some cases, but cores still settle acceptance. On a notched-wedge joint, specs often fix the core or reading location a set distance from the upper notch, so the measurement is taken at a repeatable spot rather than wherever is convenient.

The trap is a gauge that drifts out of correlation with the cores, telling you the joint is fine while the lab says it is not. The gauge gets checked against cores, not trusted on its own, especially at the seam where the geometry already makes the reading harder. Confirm the coring rate, the gauge offset, and the exact location against the project spec, because where the core comes from changes the number as much as how the joint was built.

Offsetting the joint from the lane line

Where the longitudinal joint lands is a planning decision, and the rule is to keep the seam out of the wheel path. The joint is the weakest line on the pavement, and the wheel path carries the heaviest, most repeated loading, so putting the two together is asking for early failure. On a normal road the joint belongs on the lane line or the centerline, where tires straddle it rather than track on it.

The seam should also line up with the final lane markings rather than fall a few inches off, so traffic is channelized away from the joint once the road is striped. A joint that lands in the middle of a travel lane gets pounded by every tire in that lane, and a lean seam under that loading opens fast. Planning the pass widths so the joints fall on the lines is cheap at layout and expensive to fix once the mat is down.

This is a layout call made before paving starts, not a problem you fix at the roller. Set the paving widths and the starting line so the longitudinal joints land where the wheels will not, and the seam carries the lighter load it was always going to be the weakest at handling.

Staggering joints between lifts

Stagger the longitudinal joints between lifts so they do not stack. The seam in the surface course should not sit directly over the seam in the lift below it, because two lean lines on top of each other make a full-depth weak plane that water runs straight down. Offset them so each lift's joint lands over solid mat in the lift beneath, not over another joint.

The common offset is on the order of 6 in between successive lifts, enough that the seams do not coincide, though the exact figure comes from the spec. The principle holds regardless of the number: do not line the joints up through the pavement section. A staggered set of joints means any single point in the mat has at most one lean seam above or below it, not a column of them.

The same logic applies to transverse joints between lifts, offset by a few feet so the day-joints do not stack into one bump. Plan the stagger at the start, because once the lower lift is down its joint location is fixed, and the only way to stagger the surface joint is to set its line deliberately off the one below. Confirm the offset against the project documents and lay the joints so they never coincide through the section.

The edge drop-off and the safety edge

While the longitudinal joint is the seam between two passes, the outside edge of the work has the same density problem and one more on top of it. The unconfined outside edge runs lean for the same reason the joint does, and when one pass sits higher than the next or higher than the shoulder, the vertical drop-off between them is a wheel trap that can catch a tire and pull a vehicle off the line.

The safety edge addresses both at once. A device on the screed shapes the unconfined edge into a compacted taper, commonly around a 30-degree slope, so a tire that drifts off can climb back on instead of grabbing a square edge. Because the device compacts the edge as it shapes it, the taper solves the density of the free edge and the drop-off together. Whether it is required is an agency call, but on a high-speed unconfined edge it is good practice regardless of who is watching. The general edge compaction is covered in the compaction guide; here it is the drop-off between adjacent passes worth flagging.

How the joint fails in the field

Almost every longitudinal joint failure traces back to the same short list, and they all end at the same place: a lean seam that let water in. The root is usually upstream of the crack you see.

A cold, unconfined, low-density edge is the most common, where the first pass edge spread and cooled with no confinement and the seam started behind. No tack on the vertical face leaves the new lane unbonded to the old, so the joint delaminates and ravels along the line even if the density reads fine. The wrong overlap, too little or too much, starves the seam or traps a fat strip that never compacts. Rolling the joint in the wrong sequence, cold or from the cold side, flattens the edge without closing the voids. Stacked lift joints make a full-depth weak plane that opens through the whole section. And not checking joint density at all lets every one of these hide until traffic finds it. Each shows up months or years later as raveling and cracking down the seam, long after the crew that built it is gone.

What to document

The record on the joint is what defends the work when a seam ravels or a core comes back light, and it is thinner than it should be on most jobs because the joint gets less attention than the mat. Capture the method, the treatment on the face, the density at the seam, and the rolling, tied to station or area, so the joint can be reconstructed instead of argued about. The table is a working minimum; the project or DOT spec sets the required frequency and form. Holding the joint method, tack, density, and location together in a record like FieldOS keeps the as-built tied to the seam it covers.

Common mistakes

• Leaving a cold, unconfined, low-density edge on the first pass, so the seam starts behind before the second lane arrives.
• Paving against a dry, untacked vertical face, so the new lane never bonds to the old edge and delaminates along the line.
• Running the wrong overlap, too little so the seam starves, or too much so a fat luted strip never compacts.
• Rolling the joint cold or from the cold side, flattening the lean edge without closing the voids under it.
• Stacking the longitudinal joints lift over lift instead of staggering them, making a full-depth weak plane.
• Not checking joint density at all, so a lean seam hides inside a passing mat average until traffic finds it.
• Putting the joint in the wheel path instead of on the lane line, loading the weakest line with the heaviest traffic.

Field checklist

Standards and references

What governs the joint is the project specification and the state DOT or owner agency. The longitudinal joint density requirement, whether a minimum percent of theoretical maximum or a differential from the mat, the joint coring rate and location, the gauge offset from the edge, the notched-wedge geometry, and the tack rate all live in the agency spec, and roughly half of state agencies now carry a joint density spec. The numbers in this guide, the 2 percent differential, the roughly 91 percent minimum, the 1 to 1.5 in overlap, the 12 in wedge with a notch about one NMAS deep, and the 6 in rolling offsets, are the commonly cited ranges, not the contract values. Confirm them against the project documents and the adopted specification before building to any single figure.

The Asphalt Institute and the National Asphalt Pavement Association are the trade sources for joint best practice, and FHWA and NCAT have published longitudinal-joint tech briefs and research that lay out the notched wedge, echelon paving, the hot-versus-cold joint, hot-side rolling, and the materials approaches. AASHTO and ASTM cover the test methods behind the cores and the gauges, and the percent-of-Gmm framework and the gauge-versus-core reconciliation are covered in the compaction and rolling guide. Airfield work follows its own joint and density requirements under the FAA advisory circulars rather than the state DOT spec, and the joint limits there can be tighter. The adopted document and the AHJ control acceptance, and the section numbers and editions shift on a cycle, so verify them against the adopted version before citing a clause on a submittal.

Three things hold across almost every mix and spec, and they are where to put your attention. Build density at the joint by confining the first pass edge while it is hot, because the seam is won there before the second lane is laid. Tack the vertical face, because a perfectly rolled joint over a dry edge is still an unbonded seam. And roll the joint right from the hot side and stagger the joints between lifts, because the seam needs its own deliberate rolling and no column of lean lines through the section.

Units and terms

The joint gets described across a few names and unit systems, so the same seam reads differently across a plan set, a spec, and a density report. Joint density is a percent of Gmm, the theoretical maximum specific gravity, the same reference as the mat, often written as a minimum percent or as a differential of about 2 percent below the mat, and the differential is sometimes given in lb/ft3 instead. Overlap, notch depth, taper width, and rolling offsets are in inches. Tack is a residual rate in gallons per square yard. The lift stagger is in inches.

The terms carry the meaning. A longitudinal joint runs with traffic between two adjacent passes. A cold joint is a new lane paved against a cooled, cured edge; a hot joint is made hot against hot, as in echelon paving. Joint density is the in-place density at the seam, and the density differential is how far that density falls below the adjoining mat. The notched wedge is a tapered, notched joint shaped on the first pass to confine and densify the edge.

Longitudinal joint

The seam running with traffic between two adjacent paving passes or lanes

Cold joint

A new lane paved against a first pass edge that has already cooled and set, the normal case on overlays

Echelon paving

Two pavers running side by side and staggered, laying adjacent lanes hot against hot for the strongest joint

Notched wedge

A tapered longitudinal joint with top and bottom notches, formed on the first pass to confine the edge and raise density

Joint density

The in-place density at the seam, reported as a percent of Gmm, almost always lower than the adjoining mat

Density differential

How far the joint density falls below the adjoining mat density, commonly limited to about 2 percent

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