Bitumen PG 64-22: Specification, Test Temperatures and Export Supply

PG 64-22 is a binder that has passed the AASHTO M320 high-temperature criteria at an average seven-day maximum pavement design temperature of 64 °C and the low-temperature criteria at a minimum pavement design temperature of −22 °C. Both halves are rheometer results, and both are obtained on binder that has been artificially aged first, so that what is measured is the material the road will contain rather than the material that left the tank.

The problem peculiar to a default grade

Most performance grades have to be argued for. This one has to be argued about, because it is so often inherited rather than selected. PG 64-22 sits in the standard specification of a large number of North American agencies, it is what the local terminal stocks, and it is what the previous three contracts used. On most of those projects it is genuinely correct — the climate it describes is the most widely occurring one on the continent. But PG 64-22 because the standard specification says so and PG 64-22 because the site’s design temperatures say so write the same three words into a contract and carry very different exposure. Two questions separate them: what is the seven-day maximum pavement temperature at the design reliability, and what is the minimum pavement surface temperature. Both have published answers for any site in the LTPP climatic database, and neither takes long to look up.

The distinction that decides those answers is that 64 and −22 are pavement temperatures, not air temperatures. The 64 °C is taken near the top of the asphalt layer rather than in a weather screen, so a site whose summer air peaks around 35 to 38 °C reaches it without difficulty: a black surface absorbs solar radiation through the day and sheds it slowly. The −22 °C is a pavement surface temperature, and it sits much closer to air temperature than the summer figure does, because at four o’clock on a January morning there is no solar gain to add. Both are quoted at a stated reliability — 98 % is common in North American practice — so the grade answers a temperature the site will not exceed in most years, not the weather anyone remembers.

Why the grade is a temperature statement and not a consistency one

Penetration grading describes a binder at 25 °C, which is a temperature no pavement particularly cares about. Viscosity grading adds 60 °C, much closer to a hot pavement, but still says nothing whatever about winter. Performance grading tests the binder at the temperatures the road will actually reach, and in the condition it will be in when it reaches them: unaged for the tank and the pump, RTFOT-aged for the first summers after construction, PAV-aged for the middle of the pavement’s life. That is the whole idea behind M320, and it is why a certificate reporting penetration and softening point cannot satisfy a PG specification however good the numbers on it look.

Passing is not the same as passing well

The M320 envelope is six degrees wide at each end, so a pass tells you less than it appears to. A binder whose true grade is 66.8 on the high side and −24.6 on the low side and a binder at 64.3 and −22.2 both ship, correctly, as PG 64-22. The first has close to three degrees of headroom at each end and will absorb a hot spell at the plant, a stiffer-than-expected RAP fraction, or a winter that runs a little past design without leaving grade. The second has three-tenths of a degree. On a grade bought as often as this one, where the same three words cover an enormous range of material, that headroom is the difference worth pricing — and the laboratory has already calculated it, because it had to in order to arrive at the label.

64 °C: the two rutting criteria

The high-temperature grade is not derived from anything — it is the dynamic shear rheometer test temperature, so on this grade both rutting criteria are read at 64 °C. On original binder, G*/sin δ must be at least 1.00 kPa. On RTFOT residue — binder taken through 85 minutes at 163 °C in a rolling thin film oven to AASHTO T240 — the same parameter must reach at least 2.20 kPa. The two limits are not one test repeated more strictly. The first says the binder was fit when it arrived; the second says that after the plant has driven off light ends and oxidised the surface, the harder material left behind still resists permanent deformation rather than simply becoming brittle. A binder that clears 1.00 kPa easily and only just clears 2.20 kPa is telling you something about its volatility, and the RTFOT mass loss line on the same report usually confirms it.

25 °C: the fatigue criterion, and where the number comes from

The fatigue criterion is measured on PAV residue at what M320 calls the intermediate temperature — average the two grade temperatures, then add four. Written out for this grade:

• (H + L) ÷ 2 + 4
• (64 + (−22)) ÷ 2 + 4
• 42 ÷ 2 + 4
• 21 + 4 = 25 °C

At 25 °C the PAV residue must show G*·sin δ of no more than 5000 kPa. There is a coincidence here that catches people out on this grade specifically and on no other common one: 25 °C is also the standard penetration test temperature. The two have nothing to do with each other. Penetration at 25 °C is a needle sunk into unaged binder; the M320 fatigue criterion is a rheological measurement on binder that has been through RTFOT and then twenty hours in a pressure ageing vessel. Seeing 25 °C on both lines of a certificate is not corroboration, and a penetration figure at 25 °C is not a substitute for the fatigue result.

Two further points about the parameter itself. It is a product, not a quotient, unlike the rutting parameter — and a low value is what is wanted. An aged binder that is stiff and elastic at once stores strain energy rather than dissipating it as heat, and releases it eventually as a crack. The intermediate temperature stands in for the unremarkable pavement temperature at which load-associated damage quietly accumulates: neither the hottest afternoon nor the coldest night, but the ordinary day that makes up most of the year.

Do the arithmetic for every grade you buy, because the answer moves with both numbers. PG 64-16 is tested for fatigue at 28 °C, PG 58-22 at 22 °C, PG 64-28 at 22 °C. A fatigue result reported at 28 °C on a PG 64-22 certificate was measured against a different grade, whatever the header says.

−12 °C: why the cold test is warmer than the cold grade

This line generates more questions than anything else on a PG certificate, and on this grade it is asked more often than on any other simply because more PG 64-22 is bought than anything else. The bending beam rheometer is set to −12 °C, a full ten degrees above the −22 in the grade name. Nothing has been relaxed and nothing has been mistyped: the offset is written into the method on purpose.

The BBR rests a small beam of PAV-aged binder on two supports, loads it at the centre and takes both of its readings at 60 seconds. Creep stiffness S is capped at 300 MPa, which limits how much tensile stress a given amount of thermal contraction can raise in a restrained layer. The m-value — how steeply stiffness is still falling with time at that 60-second point — has a floor of 0.300, which is the requirement that the binder keep shedding that stress instead of storing it. On marginal material it is almost always the m-value that fails first, so read that line before the stiffness line.

The ten-degree offset is an application of time–temperature superposition. Bitumen is thermorheologically simple: cooling it and loading it for longer stiffen it in the same way, so the two can be traded against each other along a measurable shift factor. For paving binders the accepted trade is that 60 seconds of loading at L + 10 °C gives the same stiffness as roughly two hours of loading at L. Two hours is the physically meaningful number, because thermal cracking is not an impact event. A pavement does not brush its annual minimum and rebound; it cools through the evening, sits near the bottom for hours in the dark, and the tensile stress builds over that whole period. Two hours at −22 °C is therefore the loading condition the specification actually cares about — and testing 60 seconds at −12 °C is the practical way to measure it. It also keeps the beam deflection inside the range the instrument can resolve, since at −22 °C itself the beam barely moves and the reading would be taken at the edge of the measurement’s usefulness. A two-hour test becomes a one-minute one and answers the same question.

Two consequences for reading a certificate. BBR at −12 °C is the expected entry on a PG 64-22 report, and finding it should settle the question rather than raise one. And because the BBR temperature depends on the low grade alone, −12 °C is equally correct for PG 58-22 and PG 70-22 — a laboratory running beams at −12 °C is grading the whole −22 family at once. A report showing BBR at −22 °C has used the temperature belonging to a −32 grade and something has gone wrong in the paperwork. A report showing no BBR result at all describes a binder that has not been performance graded, whatever is stencilled on the drum.

Coastal and maritime climates are comparatively kind to a binder: the sea moderates both ends, so the annual range at the pavement surface is narrow and a grade only has to be right about one side. Continental interiors do the opposite. Away from a large body of water, land heats fast in summer and loses heat fast in winter, and the annual swing at the pavement surface opens out to something like 86 degrees. That number is not a coincidence — it is the reason the grade exists. The span in the grade name is a description of the climate before it is a description of the binder.

What 64 and −22 look like on the ground

The 64 °C summer figure does not require desert air. Interior summers running 30 to 38 °C, with long days and high sun angle, put a dark dense-graded surface comfortably into the low sixties, and the pavement holds heat into the evening because the layer beneath it is warm too. The −22 °C winter figure is the one that surprises people who have only worked in hot markets: it is reached across the interior plains and the northern half of the continental United States most winters, and it is reached by the surface first, because the surface radiates to a clear night sky while the layers beneath lag behind. The pavement therefore experiences its high and its low at the extreme top of the structure — the same 40 millimetres of asphalt, the same binder, twice a year, in opposite directions.

Why both failure modes are live on the same road

On a hot-climate grade the design case is rutting and thermal cracking is close to theoretical. On a cold-climate grade the reverse is true. PG 64-22 is one of the few widely used grades where both failure modes are genuinely in play on the same pavement, and that is what makes an ordinary-looking grade demanding. In August the binder must be stiff and elastic enough at 64 °C that heavy wheel loads do not push the mix sideways into wheel-path ruts. In January the same binder must be compliant enough at −12 °C, and must relax stress well enough, that contraction of a restrained layer does not tear it across the carriageway. There is no adjustment that improves both. Everything that helps the August problem hurts the January one.

The freeze–thaw complication the grade does not cover

Continental climates add a failure mechanism that M320 does not address at all, and it is worth stating plainly so the grade is not asked to do work it cannot. Repeated crossing of 0 °C drives water into cracks, freezes it, and levers the pavement apart; combined with de-icing salt and the traffic that follows a thaw, it produces potholes and edge break-up that no binder grade prevents. A correctly graded PG 64-22 protects against permanent deformation and against thermal cracking. It does not protect against moisture damage, stripping or freeze–thaw disintegration — those are answered by mix design, by aggregate selection, by anti-strip treatment and by drainage. Buying a colder low grade to fix a freeze–thaw problem spends money on the wrong mechanism.

Where the grade stops being the right answer

Two directions take a project out of PG 64-22. Northwards or upwards, the minimum pavement design temperature drops past −22 °C and the grade must step to −28 or −34; the m-value at −18 °C or −24 °C then becomes the binding constraint and the order usually stops being a simple unmodified one. Downwards in summer severity — cooler maritime or northern coastal sites — the 64 °C high grade is more than the climate asks for, and PG 58-22 does the same winter job with a softer, more forgiving binder. Both moves are decided by climatic data at the site, not by what the neighbouring state specifies.

86 °C: inside the neat-binder envelope, but not by much

The industry working rule is that a useful temperature interval up to roughly 92 °C can be held by an unmodified straight-run binder from a suitable crude, and that past 92 °C polymer modification is normally needed to satisfy both ends together. At 86 °C, PG 64-22 carries no implication of polymer modification and should not be priced as though it did. That single fact is most of the commercial explanation for the grade’s dominance: it covers a demanding climate at neat-binder cost, so an agency can put it across an entire network without a modification budget attached.

But 86 is six degrees from the edge, not thirty. Compare it with PG 64-10 at 74 °C or PG 58-22 at 80 °C, both of which most paving-range vacuum residues will reach without anyone thinking hard about the crude slate. At 86 °C the crude slate starts to matter, and it matters in a way that does not show up anywhere in the grade name. This grade is common and unremarkable to specify; it is not, in every refinery, unremarkable to make.

The trade that makes the span hard

The two ends of a PG grade are governed by opposing properties and there is no adjustment that improves both. Stiffness and elasticity at 64 °C come broadly with a higher asphaltene fraction and a stiffer maltene phase. Compliance at −12 °C, and above all continuing stress relaxation at −12 °C, want the reverse. Every industrial route to a higher high grade — air blowing, harder vacuum cut, heavy processing, prolonged hot storage, repeated reheating — buys degrees at the top and gives them back at the bottom, and it gives them back through the m-value rather than through stiffness. On a −10 grade that trade is usually worth making because the cold end has enormous slack. On this grade there is no slack to spend on either side, which is exactly what an 86 °C span means.

Why crude source is a technical question, not a commercial one

The tendency worth knowing is that waxy and strongly paraffinic residues often reach 64 °C without difficulty and then fail the m-value at −12 °C. Below the cloud point, dissolved paraffin crystallises into a network through the binder; that network gives the material an elastic skeleton that resists flow. Creep stiffness may still come in under 300 MPa, so the S line passes cleanly — but relaxation collapses and the m-value falls short. A PG 64-22 candidate that passes stiffness and fails the m-value is, more often than not, reporting a wax problem, and no amount of blending adjustment creates relaxation capacity the base material does not have. More asphaltic residues generally relax better in the cold and may instead need help holding 64 °C at the top, which is a solvable problem in a way the first one is not.

The practical consequence for a buyer is that two compliant PG 64-22 cargoes can come from entirely different feedstocks and will not behave identically in ageing, in workability or in fatigue. That is permitted variation inside one label rather than a fault in either cargo — but it is a reason to grade the batch you are being sold instead of accepting a grade report from an earlier production run. A PG grading report is batch-specific and does not transfer.

Traffic moves the grade even when the climate does not

Superpave practice adjusts the high-temperature grade for loading as well as for climate: one step of six degrees for slow transient loads, two steps for standing loads, with a further step considered where traffic volumes are very high. Applied here, PG 64-22 becomes PG 70-22 for a slow climbing lane or a heavily loaded arterial and PG 76-22 for a bus stop, a container yard, a toll plaza or a signalised approach. The low grade never moves with traffic, because thermal cracking is not caused by traffic.

This is where PG 64-22’s comfortable 86 °C position disappears, and it disappears faster than it does from the −16 grades. A single bump takes the span to 92 °C and a double bump to 98 °C — the first sits on the modification boundary and the second is beyond it. Starting from PG 64-16, by contrast, a one-step bump lands at 86 °C and stays comfortably neat. So a PG 64-22 project with any crawling or standing traffic is much closer to a modified-binder order than its unbumped span suggests. Resolve the traffic adjustment before the grade goes into the contract, because it changes the product, the price basis and often the supplier.

Reading working temperatures off the right curve

The 3 Pa·s ceiling at 135 °C belongs to the plant rather than to the pavement — it exists so that the binder can be pumped through a line and will wet aggregate, and it says nothing about how the road will perform. Mixing and compaction temperatures are a wholly separate calculation, read from the binder’s own viscosity–temperature chart at the conventional equiviscous targets of approximately 0.17 Pa·s for mixing and 0.28 Pa·s for compaction. Take those from the viscosity data of the binder actually supplied rather than from a generic table. Two cautions specific to this grade: the equiviscous method is not valid for polymer modified binders, so if a traffic bump moves the order to PG 76-22 the supplier’s recommended range replaces the calculation entirely; and in a continental climate the compaction window is short in spring and autumn, which tempts crews into raising mix temperatures to compensate. Extra heat ages the binder, and on a grade with no low-temperature margin to spare it is the m-value that quietly pays for it.

A penetration grade cannot be converted into a PG grade

No arithmetic turns 60/70 into PG 64-22. Penetration at 25 °C says nothing about behaviour at 64 °C, nothing about behaviour at −12 °C, and nothing at all about the binder’s condition after PAV ageing — which is the state in which both the fatigue and the low-temperature criteria are judged. The high side of a 60/70 will often land somewhere near 64 °C, but that is a tendency rather than a rule, and the cold side cannot be inferred at all: two 60/70 cargoes drawn from different crudes can sit in the same penetration band and still grade several degrees apart at −12 °C. So when a project is written in PG, the document that settles it is a grading report carrying the measured DSR and BBR values at the temperatures above, tied to the tank or batch being loaded for you. Anything less is a grade nobody has measured.