Bitumen PG 58-22: Specification, Test Temperatures and Export Supply

PG 58-22 means the binder has been shown by test to meet the high-temperature criteria of AASHTO M320 at an average seven-day maximum pavement design temperature of 58 °C, and the low-temperature criteria at a minimum pavement design temperature of −22 °C. Neither half is assigned by inspection, by crude source, or by analogy with a penetration grade. Both are measured, on binder that has first been artificially aged to represent the condition it will be in when the pavement is actually at those temperatures.

Both numbers are pavement temperatures rather than air temperatures, and the gap between pavement and air runs in opposite directions at the two ends — which is the main reason this label gets misread. The 58 °C is measured near the top of the asphalt layer and averaged across the seven hottest consecutive days, so the pavement inflates it: a dark surface takes in solar radiation faster than it sheds it, and the summer figure lands a substantial margin above the air maximum that produced it. The −22 °C carries almost no such inflation. It is a surface temperature at the coldest hour of the year, when there is no solar term left to add, and it tracks the air minimum within a few degrees. The two halves of PG 58-22 are therefore not gathered from the same kind of weather. A site whose air maximum sits in the low thirties already supplies the 58 °C; a site whose pavement reaches −22 °C has had air in broadly the same region. Only one of these two numbers has to be earned outdoors, and it is the negative one. Both are quoted at a design reliability — 98 % is the usual figure in North American practice — so the grade answers the winter the site will not exceed in most years rather than the worst winter anyone remembers.

The selection driver is the low end, and that reverses the usual analysis

On the grades written for the Gulf and the equatorial belt — PG 64-10, PG 70-10 — the low-temperature half of M320 is close to a formality and the entire purchase rests on the high-temperature end and on traffic loading. PG 58-22 is the mirror image. A high-temperature grade of 58 °C is the least demanding rung on the common ladder: most straight-run vacuum residues in the paving consistency range clear G*/sin δ of 1.00 kPa at 58 °C on original binder and 2.20 kPa on RTFOT residue without any assistance at all. Binders do not fail this grade in summer. They fail it in the bending beam rheometer.

That has a direct commercial consequence which surprises buyers who read the grade ladder as a difficulty ranking. A supplier able to offer PG 64-16 comfortably may not be able to offer PG 58-22 from the same crude slate, even though both numbers in PG 58-22 are smaller. The two grades share an 80 °C span, but they ask for different things: PG 64-16 asks for six more degrees of summer, PG 58-22 asks for six more degrees of winter, and winter is the harder side to buy.

What the grade name deliberately hides

PG grades step in six-degree increments, so the label −22 covers every binder whose low continuous grade falls anywhere between −22.0 and just short of −28.0 °C. A binder continuous-grading −22.1 and a binder continuous-grading −27.6 both ship, correctly, as PG 58-22. In a winter that takes the pavement to −25 °C, only one of them is still doing its job. The grade name conceals that difference completely, which is why the continuous grade — sometimes called the true grade — belongs on the test report alongside the two-number label. On this grade in particular, the low continuous grade is the single most informative number a buyer can ask for, and it is free: the laboratory already calculated it to arrive at the label.

The rest of the sequence still has to be there

None of the above is a reason to accept a certificate that reports only the bending beam results. Rotational viscosity governs whether the binder can be pumped and will coat aggregate at the plant. Flash point governs safe storage. The RTFOT mass loss limit catches a binder that will lose volatiles and stiffen unexpectedly during mixing, which then costs low-temperature performance in service. The intermediate-temperature fatigue criterion covers the everyday pavement temperature at which load-associated damage accumulates. A grading report is a sequence, and a report missing a step in the middle has not established the ageing chain that the low-temperature results depend on.

The high-temperature tests: both run at 58 °C

Of the three temperatures on the report, only this one is read off the grade name rather than calculated from it. The high grade is the dynamic shear rheometer’s set point, so both rutting criteria for PG 58-22 are measured at 58 °C: G*/sin δ of at least 1.00 kPa on original binder, and at least 2.20 kPa on RTFOT residue — the binder rolled for 85 minutes at 163 °C under continuous air flow to AASHTO T240. The two limits differ because the material differs. The second is asked of binder that has already given up its light ends and taken its first oxidation inside the plant, and it requires that material, not the material in the tank, to keep resisting permanent deformation through the summers that follow.

On a cold-driven grade there is a second reason to read the RTFOT lines carefully, and it has nothing to do with rutting. That oven pass is the first link in the ageing chain the low-temperature criteria depend on. A volatile or over-cut binder that loses mass in the RTFOT arrives at the pressure ageing vessel already harder than it should be, and the property it will have lost by the time it reaches the beam is relaxation capacity. The mass loss line and the RTFOT DSR line sit next to each other on the certificate for a reason; on PG 58-22 they are early evidence about a result four tests further down the page.

The intermediate temperature: 22 °C for this grade

The fatigue criterion is run on PAV residue at the intermediate temperature, which M320 defines as the mean of the two grade temperatures plus four degrees. For PG 58-22:

• (H + L) ÷ 2 + 4
• (58 + (−22)) ÷ 2 + 4
• 36 ÷ 2 + 4
• 18 + 4 = 22 °C

At 22 °C the PAV residue must show G*·sin δ of no more than 5000 kPa. Watch what changes between this criterion and the rutting one: the modulus and the phase angle are multiplied here rather than divided, and the limit is a ceiling, so the good result is the small number. The reasoning is that an aged binder which is stiff and strongly elastic at the same time banks the energy of each wheel pass instead of turning it into heat, and hands it back later as a crack.

On this grade the 22 °C figure carries a physical meaning it does not really have on the hot-climate grades. In the northern continental and northern European climates PG 58-22 is written for, roughly 22 °C is the pavement temperature of an ordinary spring or autumn day — which is to say the condition under which the great majority of the year’s traffic actually crosses the road. Neither of the two headline temperatures describes that. The fatigue criterion is the only line in M320 aimed at the unremarkable middle of the year, and it is independent of the winter question this grade is bought to answer: a binder can relax perfectly well at −12 °C and still sit over the 5000 kPa ceiling because it was over-processed before it ever reached the PAV.

Because the intermediate temperature depends on both grade numbers, it moves in three-degree steps as either end changes. PG 58-16 is tested for fatigue at 25 °C, PG 58-28 at 19 °C, PG 52-22 at 19 °C, PG 64-22 at 25 °C. Note also that 22 °C is the intermediate temperature for PG 64-28 and PG 52-16 as well, since all three grades share the same mean. A certificate reporting G*·sin δ at 25 °C has graded a different binder to a different grade, whatever the header says.

The low-temperature test: −12 °C for a −22 grade

This is the line that generates more questions than any other on a PG certificate, and the answer is worth understanding rather than memorising. The bending beam rheometer test for PG 58-22 is run at −12 °C — ten degrees warmer than the −22 °C in the grade name. That is correct, deliberate, and not a relaxation of the requirement.

The BBR loads a small beam of PAV-aged binder in three-point bending and reads two values at the 60-second mark. Creep stiffness S must not exceed 300 MPa: the binder must not be so rigid that thermal contraction generates more stress than the mix can carry. The m-value — the slope of the log stiffness against log time curve at 60 seconds — must be at least 0.300: the binder must still be able to shed that stress rather than hold it.

The ten-degree offset is not a concession, it is a substitution — and the substitution is exact enough to specify against. Start from the field event rather than from the laboratory. A winter night does not deliver its minimum as an impact: the surface cools through the evening, settles near the bottom in the small hours and holds there while tensile stress accumulates. The loading condition the criterion is actually about is therefore something like two hours of sustained load at −22 °C, not sixty seconds of it. Two hours per beam is unusable as a routine acceptance test, and at −22 °C the beam deflects so little that the reading would be taken at the edge of what the instrument can resolve. Time–temperature superposition is the way out. Bitumen is thermorheologically simple: within its working range, holding a load for longer and cooling the material further stiffen it in the same way, so the two variables trade against each other along a shift factor that is measured rather than assumed. For paving binders the established trade is ten degrees for roughly two orders of magnitude in time — 60 seconds of loading at L + 10 °C returns the stiffness of approximately two hours of loading at L. So the beam is held at −12 °C and read at the one-minute mark. The number printed on the certificate was measured at −12 °C; the behaviour it certifies is the binder’s at −22 °C over a realistic winter night.

Two practical consequences follow. First, on a Certificate of Analysis for PG 58-22, BBR at −12 °C is the expected entry and seeing it should reassure you rather than worry you. Second, the BBR temperature depends only on the low grade, so −12 °C is also the correct test temperature for PG 64-22 and PG 70-22. A laboratory running the beam at −12 °C is grading the whole −22 family. If a report shows BBR at −22 °C, the beam was run at L rather than at L + 10, and the result belongs to no standard grade at all — low grades step in sixes, so the neighbouring beam temperatures are −18 °C for a −28 grade and −24 °C for a −34. That is a paperwork error rather than a lenient one: the binder was tested colder, and therefore harder, than the method asks. If there is no BBR result at all, the binder has not been performance graded, whatever the label on the tank says.

When a pavement cools, the asphalt layer tries to contract. It cannot. It is bonded to the layer below it and it is continuous along the length of the road, so the contraction is restrained and the restraint appears as tensile stress in the mix. That stress builds as the temperature falls, and it builds faster than the temperature drop alone would suggest, because the binder is stiffening at the same time: each additional degree of cooling produces both more contraction and a stiffer material to resist it.

Only two things relieve that stress. The first is viscoelastic relaxation — the binder flowing, very slowly, so that the stress bleeds away instead of accumulating. The second is the tensile strength of the mix, which is what is left to resist the stress that did not relax. When the accumulated thermal stress exceeds the tensile strength, the layer tears. The result is a transverse crack running across the full width of the carriageway, roughly perpendicular to the direction of travel, appearing without any vehicle having passed over it.

Why the m-value, not the stiffness, usually governs

The two BBR criteria control the two halves of that story, and they are not interchangeable.

• Creep stiffness S, maximum 300 MPa — controls how much stress a given amount of contraction generates. A stiffer binder converts the same thermal strain into more stress.
• m-value, minimum 0.300 — controls how quickly the stiffness falls away under sustained load, and therefore how fast the accumulated stress relaxes. A high m-value means the stiffness curve is steep and the binder keeps flowing; a low m-value means the binder has effectively become a solid that holds whatever stress you put into it.

On PAV-aged binder the m-value is the criterion that fails more often, and it fails for a structural reason. Oxidative ageing builds larger, more associated molecular structures. Those structures raise stiffness somewhat, but they attack relaxation capacity much harder, because relaxation depends on the binder’s ability to rearrange internally under load. An aged binder can therefore still pass the 300 MPa stiffness limit comfortably while its m-value has collapsed to 0.28. That binder will crack, and the stiffness line on its certificate will look perfectly healthy. If you read only one number in the low-temperature half of a PG 58-22 report, read the m-value.

Two distinct cracking mechanisms, one specification

The dramatic version is single-event low-temperature cracking: one exceptionally cold night in which the thermal stress crosses the mix strength in a single cooling cycle, and cracks appear across a whole section at once, often widely spaced at first. This is what the −22 °C design temperature and its reliability level are chosen against.

The quieter and more common version is thermal fatigue: repeated cooling cycles to temperatures well short of the extreme, each one taking the mix part of the way to failure and none of them getting there alone. Over several winters, microdamage accumulates and cracks appear anyway. Thermal fatigue is governed by relaxation rather than by peak stress, which is another reason the m-value carries more weight than the label suggests, and why a binder with adequate stiffness but poor relaxation fails a pavement that never saw a record winter.

What it costs on the road

A transverse thermal crack is not a cosmetic defect. It is an open path to the base. Water enters, saturates the unbound layers, and in a freeze-thaw climate is pumped up and down through the crack by traffic, stripping fines from the base and undermining the crack edges. The crack widens, the edges spall, and secondary cracks branch from it. Regularly spaced transverse cracks that start twenty or thirty metres apart migrate over successive winters to spacings of a few metres, and eventually interconnect into block cracking across the whole surface. From that point the pavement is losing structural capacity, not just ride quality, and the remedy is an overlay rather than crack sealing. The recurring annual cost of routing and sealing transverse cracks is, in most cold-region agencies, a larger maintenance line than anything caused by rutting.

The point about ageing that matters commercially

All of this gets worse with time, and it gets worse asymmetrically. A binder’s low-temperature grade is not a fixed property of the product — it is a property of the product in a particular state of ageing. The M320 requirement is written against PAV residue precisely so that the grade reflects a mid-life condition rather than the tank condition. But binders that start with a marginal m-value have nothing left to give, and every avoidable heat cycle between the refinery and the road takes a little more. Prolonged hot storage, repeated reheating of drums, an overheated tank coil and an over-long haul at temperature all age the binder in exactly the direction that costs the m-value. On a −10 or −16 grade that hardly matters. On a −22 grade it is the difference between a road that cracks in year twelve and one that cracks in year four.

80 °C is well inside unmodified territory

The working rule across the industry is that a useful temperature interval of up to about 92 °C is achievable with an unmodified straight-run binder from a suitable crude, and that beyond roughly 92 °C polymer modification is normally required to hold both ends at once. At 80 °C, PG 58-22 carries no implication of polymer modification. It sits two full grade steps inside that boundary, and an agency can specify it across an entire network without a modification budget. If an offer of PG 58-22 arrives priced as a modified product, ask what the modification is for, because the span does not call for it.

Why the span still understates the problem

Span is a good first filter and a poor second one, because it treats six degrees of summer and six degrees of winter as equivalent purchases. They are not. Reaching 58 °C on the high side is close to automatic for a paving-range vacuum residue. Holding m ≥ 0.300 at −12 °C on PAV residue is a genuine constraint that a large share of the world’s crude slates cannot meet, and no amount of blending adjustment creates relaxation capacity that the base material does not have. PG 64-16 and PG 58-22 both show an 80 °C span; they are not equally easy orders, and a supplier quoting confidently on one may have nothing to offer on the other.

Wax is the specific enemy of this grade

One crude characteristic decides more PG 58-22 offers than anything else, and it appears nowhere in the grade name. Paraffin dissolved in a binder stays dissolved only above its cloud point; below it the wax comes out of solution and crystallises into a network threaded through the maltene phase. That network behaves like a skeleton — it will carry a load, but it will not flow. At the bending beam this produces a result that looks self-contradictory until the mechanism is understood. Stiffness is not the problem: a waxy binder can sit well under 300 MPa and the S line passes without comment. What the skeleton removes is the material’s ability to rearrange internally under load, and that is exactly what the m-value measures. Pass on S, fail on m, at −12 °C is the signature of a waxy source more often than it is anything else — and it cannot be corrected downstream, because relaxation capacity is a property of the base material rather than something a blend can add.

This is also why European practice carries a specification line that the ASTM system does not. EN 12591 paving bitumen specifications include a paraffin wax content limit, determined by distillation to EN 12606-1 and commonly set at a maximum of 2.2 % by mass. It exists for exactly this reason. If your project sits in a European or CIS specification environment and the low-temperature end matters, that line is worth carrying into the contract alongside the PG requirement, because it constrains the mechanism rather than only the symptom.

Processing pushes a binder the wrong way for this grade

Every route to a stiffer, higher-grading binder costs low-temperature performance. Air blowing, heavy processing, prolonged hot storage and repeated reheating all raise the high-temperature grade and lower the m-value at the same time. On a −10 grade that trade is often worth making. On a −22 grade there is nothing to trade: the high end is already satisfied and the low end is the binding constraint, so every degree gained at the top is a straight loss. Be specific with a supplier that the material must be straight-run paving binder graded on the low end, not a reblend that hits 58 °C easily and was never tested cold.

Grade bumping does not protect the cold end

Under Superpave practice, loading moves one end of the grade and only one. Slow transient traffic lifts the high-temperature grade a step of six degrees and standing traffic lifts it two, so PG 58-22 becomes PG 64-22 for a long climb or a heavily loaded arterial and PG 70-22 for a bus stop, a container yard or a toll plaza. The low grade does not move at all, because no amount of traffic causes thermal cracking. That produces two conclusions worth stating plainly. First, you cannot buy your way out of thermal cracking with a stiffer binder — a stiffer binder makes it worse, which is the exact opposite of the intuition that serves buyers well in hot climates. Second, a traffic adjustment on this grade widens the span from 80 °C to 86 or 92 °C while the winter requirement stays put, which is normally the point at which an unmodified order becomes a modified one. Resolve the traffic adjustment before the grade goes into the contract, because it changes the product being bought.

If the project is specified to M332 instead

Some agencies now grade to AASHTO M332, which replaces the G*/sin δ limit on RTFOT residue with a multiple stress creep recovery test to AASHTO T350 and writes traffic into the grade name as a letter — PG 58S-22, PG 58H-22, PG 58V-22. On a cold-driven grade the useful point is what M332 leaves alone. The letter tightens the non-recoverable creep compliance demanded at 58 °C and does nothing else; the low-temperature half of the specification is untouched and is still verified on the bending beam at −12 °C to AASHTO T313, with the same 300 MPa and 0.300 limits. M332 changes how traffic is answered, not how winter is answered, and PG 58S-22 is the direct counterpart of the M320 PG 58-22 set out on this page. Establish which standard governs before ordering, because an M320 certificate does not contain the T350 results and they cannot be derived from it.

Reclaimed asphalt consumes the cold end first

RAP binder is heavily oxidised: high stiffness, low m-value, strongly negative ΔTc. Blending it into a mix pulls the combined binder’s low continuous grade warmer, and it does so faster than it affects the high end. Agencies typically respond by dropping the virgin binder one low-temperature step once RAP content passes a stated threshold, which turns a PG 58-22 order into a PG 58-28 order. If your mix design carries RAP, settle the virgin grade against the blended target before you order, and expect to be asked for the continuous grade rather than the label, because the blending calculation cannot be done from a label.

A penetration grade cannot be converted into a PG grade

No arithmetic turns 80/100, 85/100 or 100/150 into PG 58-22, and on this grade the gap between the two systems is wider than on any other. Look at what each measurement is. Penetration is a single depth reading taken on unaged binder at one temperature, and it describes consistency. The property that decides PG 58-22 is a rate — how fast stiffness decays under a sustained load — measured on binder that has been through an oven and then twenty hours in a pressure vessel. There is no route from a depth on virgin material to a decay rate on twice-aged material. Wax is why the gap cannot even be closed by correlation: the crystal network that destroys the m-value forms well below the penetration temperature and is completely invisible at 25 °C. Two 80/100 cargoes from different crude slates, sitting in the same penetration band, can continuous-grade several degrees apart at the cold end. Where a project is written in PG, the document that settles it is a grading report carrying measured DSR and BBR values at the temperatures set out above, tied to the tank or batch being loaded. A PG grade asserted without that report is a grade nobody has measured.

Handling, and why it matters more on a cold-climate grade

The 3 Pa·s ceiling at 135 °C answers a plant question rather than a pavement one — can this binder be moved through a line, and will it wet the aggregate. Nothing about service performance follows from it. Mixing and compaction temperatures are a separate calculation entirely, read off the supplied binder’s own viscosity–temperature chart at the conventional equiviscous targets of about 0.17 Pa·s for mixing and 0.28 Pa·s for compaction. Use the viscosity data for the binder actually being delivered rather than a generic table, and note that the equiviscous method does not apply to polymer-modified binders at all.

Two cold-destination points are worth adding, because they act directly on the property this grade is bought for. Cold ambient conditions shorten the compaction window sharply — the mat loses heat to the air and to a cold base far faster than in a warm climate — which pushes crews toward higher mixing temperatures and longer holding times to compensate. Both age the binder. And drummed or packaged material arriving into a cold market takes considerably longer to bring up to working temperature, which tempts operators into aggressive local heating. Coils must stay covered by product and heat must be applied gradually and uniformly; localised overheating carbonises binder against the metal and, short of that, quietly consumes the m-value margin you paid for. On a −22 grade, the handling regime is part of the specification in everything but name.