Viscosity grading classifies paving bitumen by how thick it is at 60 °C. Everything else in the specification follows from that choice of temperature — including the things penetration grading cannot see.
A viscosity grade is a band of laboratory results, not a brand or a quality tier. VG stands for viscosity grade; the digits are a shorthand, standing for hundreds of poise measured at 60 °C. So VG-30 names a binder of nominally 3000 poise and VG-10 one of nominally 1000. IS 73:2013, the Indian standard for paving bitumen, defines four such grades and writes each one as a two-sided band around that nominal figure.
Because viscosity increases as a binder gets stiffer, the grade number runs the same way as hardness: VG-40 is the stiffest grade in the family and VG-10 the softest. That is the opposite of penetration grading, where a higher number means a softer binder. It is the first thing a buyer moving between the two systems has to reset, and it is a genuine source of ordering errors.
The unit is the poise, the CGS unit of dynamic viscosity. One poise is 0.1 pascal-second, so the four IS 73 bands of 800–1200, 1600–2400, 2400–3600 and 3200–4800 poise are 80–120, 160–240, 240–360 and 320–480 Pa·s in SI units. Indian certificates report poise; European laboratories more often report Pa·s. The factor is ten and nothing else changes.
IS 73:2013 fixes viscosity at two temperatures, and each is answering a different engineering question.
Between the two temperatures the material changes almost beyond recognition. Convert a kinematic result to the same units as the grading figure — dynamic viscosity in poise equals kinematic viscosity in stokes multiplied by density in g/cm³ — and a binder measuring 350 cSt at 135 °C, at a density near 0.95 g/cm³, works out at roughly 3.3 poise. The same binder is somewhere between 2400 and 3600 poise at 60 °C. It is on the order of a thousand times thinner at the mixer than it is in the road. Specifying both figures is what stops a supplier satisfying one temperature at the expense of the other.
Penetration grading fixes consistency at 25 °C, and 25 °C is a bench convenience rather than an engineering condition. It is a comfortable laboratory temperature and it is nowhere near either of the two states in which a binder is actually asked to do work. A dark bituminous surface on a clear summer day absorbs solar radiation all afternoon and runs well above the shade air temperature; the binder in the top of the layer arrives somewhere around 60 °C, and that is where permanent deformation accumulates. Viscosity grading simply moved the measurement to the place where the failure happens.
The consequence is that the grading parameter is tied to the failure mode it is meant to control. Viscosity at 60 °C is a physical property measured under the conditions that produce rutting; penetration at 25 °C is an empirical index measured somewhere else on the curve, and the distance between the two points is not fixed. Take two parcels that both read 65 dmm on the needle. Nothing in that result says where either of them sits at 60 °C, because the two can lie on viscosity–temperature curves of quite different slope. The steeper of the two will have lost more stiffness by the time the pavement is hot, and it is the one that ruts first. The needle cannot see the difference. IS 73 is built around seeing it: a two-sided band at 60 °C, a floor at 135 °C and a floor on penetration at 25 °C bracket the curve at three points instead of pinning it at one.
There is a second, more practical advantage. Penetration is an index in tenths of a millimetre and cannot be used in a calculation. Viscosity in poise, Pa·s or cSt can. It feeds directly into pump and line sizing, spray-bar design, and the equiviscous rules that set mixing and compaction temperatures — which is the subject of the last section on this page, and the single most useful thing a viscosity-graded certificate gives a plant that a penetration-graded one does not.
Until the 2006 revision, IS 73 was a penetration standard. Its grades were written S-35, S-45, S-65 and S-90, with an A-series alongside them, and they are still occasionally quoted in older tender language. IS 73:2006 replaced that scheme outright with the four VG grades, and the IS 73:2013 edition carried them forward. Three pressures drove the change: summer pavement temperatures and axle loads had both risen; the specification needed to constrain temperature susceptibility rather than a single point of consistency; and a national road programme needed a grading parameter that engineers could compute with.
The approach was not new in 2006. The United States had run a viscosity-graded system for asphalt cement under ASTM D3381 — grades AC-2.5 through AC-40, graded on exactly the same measurement, absolute viscosity at 60 °C — before American practice moved on to performance grading under AASHTO M320. On the grading parameter the AC bands and the VG bands are not merely similar but numerically the same, which is set out in the cross-reference table further down this page.
IS 73:2013 grades the binder in its original, unaged state. The viscosity figure that names the grade is measured on the material as supplied, not on a laboratory-aged residue. Ageing is controlled separately, through the RTFOT lines at the bottom of the specification. This is worth knowing because the older American AR system — AR-1000 through AR-16000 — graded on the viscosity of the aged residue instead, so an AR number and a VG number are not the same kind of quantity even though both are viscosities at 60 °C.
Viscosity grading under IS 73:2013 is the national system in India, and it travels with Indian engineering: tenders in neighbouring South Asian markets, and projects across Africa and Southeast Asia that are designed, financed or contracted from India. Buyers in those markets are frequently handed a specification naming VG-30 or VG-40 while their local supply base quotes 60/70 and 80/100. That mismatch is the most common commercial problem on this product family, and it is usually a certificate problem rather than a material problem — the cross-reference table below and the note beneath it explain why.
Both figures on a viscosity grade certificate come from capillary viscometry, but they are produced by different apparatus under different driving forces and they are not interchangeable. Knowing which is which is what lets a buyer read a certificate properly.
The bitumen is heated only until it pours freely — never to the point of fuming — and stirred to release entrained air, because a trapped bubble in a capillary invalidates the run. The same conditioned sample can serve both tests, but each is charged into its own viscometer and brought to its own bath temperature. Charging is done hot, so the binder is fluid enough to fill the bulb to the mark without leaving voids.
The charged vacuum-capillary viscometer is equilibrated in a bath held at 60 °C. At that temperature paving bitumen will not flow through a capillary under gravity in any reasonable time, so a partial vacuum — 300 mm Hg in ASTM D2171 — is applied to the outlet arm to draw it through. The time taken for the leading meniscus to travel between successive timing marks is recorded and multiplied by the viscometer's calibration factor. The result is dynamic, or absolute, viscosity in poise. This is the number that names the grade.
A second viscometer is charged and equilibrated in a bath at 135 °C. No vacuum is needed: at 135 °C the binder is fluid and flows through the capillary under its own head. Flow time between the timing marks, multiplied by the calibration factor, gives kinematic viscosity in centistokes — equivalently mm²/s. The tube size is chosen so that flow time is long enough for precision, conventionally not less than 60 seconds, which is why laboratories keep a range of capillaries rather than one.
Absolute and kinematic viscosity are related through density: dynamic viscosity in poise equals kinematic viscosity in stokes multiplied by density in g/cm³, and one stokes is 100 centistokes. Converted onto a common basis, the two measured points fix two positions on the binder's viscosity–temperature curve, and the line joining them is its temperature susceptibility. That line is not decoration — it is what the mixing and compaction temperatures are read off, and it is why IS 73:2013 insists on both figures rather than just the one the grade is named after.
This is the whole standard in one view. The test methods are identical across the four columns; only the limits move. Each Indian method is shown with the international method that corresponds to it, because export certificates are written against both.
| Property | Test method | Unit | VG-10 | VG-20 | VG-30 | VG-40 |
|---|---|---|---|---|---|---|
| Absolute viscosity at 60 °C | IS 1206 (Part 2) / ASTM D2171 | poise | 800–1200 | 1600–2400 | 2400–3600 | 3200–4800 |
| Absolute viscosity at 60 °C, SI equivalent | IS 1206 (Part 2) / ASTM D2171 | Pa·s | 80–120 | 160–240 | 240–360 | 320–480 |
| Nominal viscosity the grade name refers to | — | poise | 1000 | 2000 | 3000 | 4000 |
| Kinematic viscosity at 135 °C, min | IS 1206 (Part 3) / ASTM D2170 | cSt | 250 | 300 | 350 | 400 |
| Penetration at 25 °C, 100 g, 5 s, min | IS 1203 / ASTM D5 / EN 1426 | dmm (0.1 mm) | 80 | 60 | 45 | 35 |
| Softening point, ring & ball, min | IS 1205 / ASTM D36 / EN 1427 | °C | 40 | 45 | 47 | 50 |
| Flash point, Cleveland open cup, min | IS 1448 (P:69) / ASTM D92 | °C | 220 | 220 | 220 | 220 |
| Solubility in trichloroethylene, min | IS 1216 / ASTM D2042 / EN 12592 | wt % | 99.0 | 99.0 | 99.0 | 99.0 |
| Viscosity ratio at 60 °C on RTFOT residue, max | IS 9382 / ASTM D2872, then IS 1206 (Part 2) | ratio | 4.0 | 4.0 | 4.0 | 4.0 |
| Ductility at 25 °C, 5 cm/min, on RTFOT residue, min | IS 1208 / ASTM D113 | cm | 75 | 50 | 40 | 25 |
| Normal field of use (not part of the standard) | — | — | Spray and seal work; base binder for emulsion and cutback plants; severe winter regions | Lighter traffic and cool or hill climates; a middle grade with no direct penetration counterpart | Dense-graded highway mixes in hot climates carrying heavy commercial traffic | Locations where loads are slow, standing or channelised rather than routes |
Every line closes off a specific failure mode or a specific way of gaming the grading parameter. Knowing which line does what tells you what to query when a certificate comes back marginal — and what to leave alone.
This is the grade itself, and it is the only property in the table with both a floor and a ceiling. Below the floor the binder is softer than the grade at pavement service temperature and will deform more readily under heavy or channelised traffic in summer. Above the ceiling it is stiffer than the grade: better against rutting, but harder to compact at the tail of the roller window and less forgiving in cold weather. Because it is two-sided, it is the line a marginal batch fails first, and the line worth reading as an actual measured figure rather than as a range copied down the certificate column.
The minimum climbs in even 50 cSt steps across the family: 250, 300, 350, 400. That regularity is the clearest signal in the standard that the two viscosity lines are meant to be read as a pair rather than cleared as separate hurdles.
It reads oddly as a minimum until you notice what it is guarding. Grading at one temperature invites a binder that hits the target there and nowhere else, and the way that happens is a steep viscosity–temperature curve: the steeper the curve, the further the material has fallen by the time the sample reaches 135 °C. Setting a floor at the hot end is therefore a limit on slope expressed as a limit on a reading. Compare how the two ends of the family scale and the standard's priorities become visible. From VG-10 to VG-40 the bottom of the 60 °C band goes up fourfold, 800 to 3200 poise, while the 135 °C floor rises only from 250 to 400 cSt. In proportional terms the hot-end requirement is therefore loosest on the hardest grade: VG-40 is allowed a steeper viscosity–temperature curve than VG-10 is. That is deliberate rather than sloppy. VG-40 is written for hot places and standing loads, where the cold end of the curve is the smaller risk; VG-10 is written for cold regions and sprayed work, where it is the larger one.
This is where buyers arriving from penetration grading are most often caught out. IS 73:2013 sets only a minimum — 80, 60, 45 and 35 dmm for VG-10 to VG-40 — and no upper limit at any grade. A fully compliant VG-30 batch may well report penetration above 70 dmm. That is not a defect and it is not a claim, unless a ceiling was negotiated into the sales contract as a separate commercial line.
The floor is not a hardness classification. It is a temperature-susceptibility guard. A binder that measures 3000 poise at 60 °C but only 35 dmm at 25 °C has a far steeper viscosity–temperature curve than one measuring 3000 poise and 55 dmm, and it is much more likely to crack in service. Setting a penetration minimum stops a refiner reaching the viscosity band with an over-blown, brittle product. Because the guard is scaled to the grade, the minimum falls as the grade number rises — a stiffer binder is legitimately expected to give a lower penetration, so requiring 80 dmm of VG-40 would be nonsense.
Ring and ball to IS 1205 / ASTM D36, floors of 40, 45, 47 and 50 °C, no ceiling anywhere. Look at how little that ladder moves. The viscosity band quadruples between VG-10 and VG-40 while the softening point floor rises by ten degrees in total, and by only two degrees between VG-20 and VG-30. There is a reason for that. Ring and ball is an equiviscous measurement in disguise — a bitumen softens when it reaches roughly the same consistency regardless of grade — so the temperature at which it does so varies far less across the family than the 60 °C viscosity does. In a four-grade comparison it is therefore the weakest of the consistency lines for telling one grade from another, and it earns its place as a cheap sanity check on the rest of the certificate rather than as a discriminator.
Five of the eight properties in IS 73:2013 move with the grade. Three do not, and two of those three are here: the Cleveland open cup floor is 220 °C and the solubility floor is 99.0 % for every grade from VG-10 to VG-40, because neither is describing pavement performance. (The third is the RTFOT viscosity ratio, capped at 4.0 throughout.) One describes safety and one describes authenticity, and neither requirement relaxes because a binder happens to be soft.
The 220 °C figure deserves attention whenever a viscosity grade is offered into a contract originally drafted around penetration grades, since those routinely carry a 250 °C flash point clause. Both documents can be satisfied by one cargo, or neither can, and the gap between them is 30 °C of margin nobody thinks about until a laboratory reports 235 °C. It is a drafting problem with a one-line fix before signature and no fix at all after loading.
The solubility floor (IS 1216 / ASTM D2042 / EN 12592) asks a question none of the other seven lines ask: is the sample entirely soluble organic binder, or has something insoluble been carried along with it? Filler, catalyst fines and insoluble cracked residue all move the consistency readings in ways that can be tuned to land inside a grade. None of them dissolves. One per cent is a narrow allowance, and it is deliberately narrow — this is the one line in IS 73:2013 that is checking what the material is rather than how it behaves, and it costs almost nothing to run, so its absence from a certificate is a finding rather than an oversight.
The VG-30 band runs to 3600 poise and the VG-40 band starts at 3200. Between 3200 and 3600 poise a single viscosity measurement does not identify the grade at all. What separates them in that region is the rest of the column: kinematic viscosity minimum 350 against 400 cSt, penetration minimum 45 against 35 dmm, softening point minimum 47 against 50 °C, and post-RTFOT ductility minimum 40 against 25 cm. This is a real commercial issue, not a curiosity — a certificate reporting 3400 poise proves nothing on its own, and it should be assessed line by line against the grade actually ordered.
Two things. First, nothing in IS 73:2013 describes low-temperature cracking behaviour directly; the penetration floor is a proxy and no more. If low-temperature performance is the governing risk on your project, that is an argument for looking at performance grading, where the low pavement design temperature is measured explicitly. Second, the standard says nothing about how the binder was produced. A straight-run vacuum residue and a semi-blown or blended product can both land inside the same viscosity band, and the RTFOT lines discussed next are the closest the standard comes to distinguishing them.
Every grade in IS 73:2013 is checked for how much it hardens through the asphalt plant, and the check is the rolling thin film oven test — not the thin film oven test used on penetration-grade export sheets. The conditions below are fixed by the method; only the two acceptance limits change between grades.
| Item | VG-10 | VG-20 | VG-30 | VG-40 | Test method |
|---|---|---|---|---|---|
| Ageing test required by IS 73:2013 | RTFOT | RTFOT | RTFOT | RTFOT | IS 9382 / ASTM D2872 |
| Oven temperature | 163 °C | 163 °C | 163 °C | 163 °C | ASTM D2872 |
| Conditioning time | 85 min | 85 min | 85 min | 85 min | ASTM D2872 |
| Sample charged per glass container | 35 g | 35 g | 35 g | 35 g | ASTM D2872 |
| Carriage rotation | 15 rev/min | 15 rev/min | 15 rev/min | 15 rev/min | ASTM D2872 |
| Air flow into each container | 4000 mL/min | 4000 mL/min | 4000 mL/min | 4000 mL/min | ASTM D2872 |
| Viscosity ratio at 60 °C, max | 4.0 | 4.0 | 4.0 | 4.0 | IS 1206 (Part 2) / ASTM D2171 on the residue |
| Ductility at 25 °C, 5 cm/min, on the residue, cm, min | 75 | 50 | 40 | 25 | IS 1208 / ASTM D113 |
The grade is selected from climate, traffic loading and the type of work — in that order. The table follows the way the grades are used in Indian road practice, which is the practice most tenders naming VG grades are drafted against.
| Where it is going | Grade normally specified | What drives the choice | What to watch |
|---|---|---|---|
| Bituminous concrete and dense bituminous macadam on highways in hot regions | VG-30 | Summer pavement temperatures approaching 60 °C alongside heavy commercial traffic. The dense-graded mix designs used across the Indian highway programme assume this grade as their default binder. | Check the measured viscosity, not the grade name. A batch at 2450 poise and a batch at 3550 poise are both VG-30 and will not behave the same under the roller. |
| Intersections, toll plazas, bus bays, truck lay-bys, container and port yards | VG-40 | Slow-moving, standing or channelised heavy loads concentrate shear into a single wheel path. IRC and MoRTH practice treats these as heavy-duty locations calling for the stiffer grade. | VG-40 has the narrowest compaction window of the four. Confirm the paving team can hold laying and rolling temperatures before specifying it on a remote site. |
| Semi-dense bituminous concrete, overlays and periodic renewal in hot regions | VG-30 | Same climate and traffic logic as the main paving case. Using one grade across the build-up also lets a plant run a single binder tank, which is a real saving on a remote job. | Where an overlay is thin and the underlying pavement is cracked, binder grade is rarely the governing variable — the mix and the interlayer treatment are. |
| Paving in cold-climate and hill regions | VG-20, or VG-10 where winters are severe | Low winter temperatures make thermal and fatigue cracking the governing risk rather than rutting, so a softer grade is the correct answer. | VG-20 is the least traded of the four grades and is not always held in stock. Confirm availability before it goes into a tender document. |
| Surface dressing, chip seal and other sprayed applications | VG-10 | The binder has to spray evenly through a distributor bar and wet the chippings on contact. The 250 cSt floor at 135 °C is set with sprayability in mind. | Sprayed work is far more sensitive to binder temperature at the bar than to the grade choice. Temperature control governs the result. |
| Bitumen emulsion manufacture | VG-10 | An emulsion plant needs a soft base binder that will disperse cleanly through the colloid mill; a harder base raises mill temperature and hurts stability. | The emulsion standard governs the residue and the emulsion properties, not the base grade. Confirm what the emulsion specification actually requires. |
| Cutback bitumen for prime coat, MC grades | VG-10 | A cutback is made by blending a soft base binder with kerosene. Starting from a stiffer grade needs more solvent to reach the same cutback viscosity band. | Cutback grades are defined by their own kinematic viscosity bands at 60 °C, not by the base grade used to make them. |
| Airport, industrial and heavy hardstanding paving | VG-30 or VG-40, by loading | Wheel loads are high but the decisive factor is whether traffic is fast-moving or slow and channelised. Standing loads push the choice up to VG-40. | Many airport specifications call for a modified binder rather than an unmodified viscosity grade. Read the project specification before assuming a VG grade is acceptable. |
Buyers routinely have to show an engineer that what is on offer satisfies a specification written in a different system. These are the nearest counterparts, together with an honest statement of how far each comparison actually holds.
| VG grade | Absolute viscosity at 60 °C | ASTM D3381 viscosity grade | Penetration grade usually compared | How far the comparison holds |
|---|---|---|---|---|
| VG-10 | 800–1200 poise | AC-10 — nominal 1000 poise ± 20 %, i.e. 800–1200 | 85/100 under ASTM D946; 70/100 or 100/150 under EN 12591; 80/100 as traded | The D3381 band is numerically identical, not merely close, because both standards grade on absolute viscosity at 60 °C over the same window. The penetration comparison is indicative only: IS 73 sets an 80 dmm floor with no ceiling, while D946 and EN 12591 both set two-sided bands, so a soft penetration cargo does not automatically pass VG-10 and a VG-10 batch will not always sit inside one penetration band. |
| VG-20 | 1600–2400 poise | AC-20 — nominal 2000 poise ± 20 %, i.e. 1600–2400 | no clean counterpart; sits between 60/70 and 80/100 as traded | Identical to AC-20 on the grading parameter. VG-20 is the grade the penetration systems never had — it falls between two established bands instead of onto one, which is a large part of why it is the thinnest-traded of the four and why substituting a penetration grade against it is the least defensible of the four substitutions. |
| VG-30 | 2400–3600 poise | AC-30 — nominal 3000 poise ± 20 %, i.e. 2400–3600 | 60/70 | The comparison drawn most often, because the two grades occupy the same commercial slot and are frequently cut from comparable feedstock. That is an observation about supply, not a compliance argument. A 60/70 specification is closed at 25 °C and is silent at 60 °C, so it can neither confirm nor exclude the VG-30 band. |
| VG-40 | 3200–4800 poise | AC-40 — nominal 4000 poise ± 20 %, i.e. 3200–4800 | 40/50 under ASTM D946; 30/45 under EN 12591; 30/40 as traded | Identical to AC-40 on the grading parameter. The penetration comparison is loose in both directions here more than anywhere else in the table, because the VG-40 band is 1600 poise wide and the 35 dmm floor is a temperature-susceptibility guard rather than a hardness class. |
This is the practical payoff of viscosity grading. Mixing and compaction temperatures do not have to be taken from a generic table — they can be read off the binder's own measured data, batch by batch.
Hot-mix practice sets binder temperature by target viscosity rather than by a fixed number of degrees. The conventional targets are:
These are viscosities, not temperatures. The temperature that produces them is a property of the individual binder, and two batches of the same grade will not give the same answer.
The procedure takes about five minutes and needs nothing beyond the Certificate of Analysis.
For the mid grades this normally lands the mixing temperature somewhere in the region of 150 to 165 °C, with VG-40 sitting at the top of that region or a little above it and VG-10 below. Those are the numbers a generic handling table would have given you anyway — but the chart tells you whether this batch is an outlier, and a temperature-susceptible batch can shift the answer by several degrees.
Both errors cost money in the same place. Run the plant too hot and the binder ages further than the RTFOT limit assumes, arriving at the paver already stiffened; the mat loses density under the roller and the pavement cracks early. Run it too cold and the aggregate is incompletely coated, the mix is harsh and the roller cannot achieve target density before the mat cools out of the window. Neither failure is visible on the day. Both are visible three years later.
This is also the argument to make when a plant engineer asks why a specification insists on the 135 °C figure when the grade is named after the 60 °C one. The 60 °C number decides whether the road will rut. The 135 °C number decides whether the road can be built properly in the first place, and together they are what let the plant work from data rather than habit.
Whatever the chart says, three hard constraints override it. Do not exceed about 180 °C bulk temperature: above that, oxidation accelerates sharply and fuming becomes serious. Remember that the IS 73:2013 flash point floor is 220 °C, not the 250 °C many penetration-grade export sheets carry, so the margin between working temperature and flash point is smaller than operators used to penetration grades assume. And keep every heating coil submerged and every drum side wet with product before heat is applied: an exposed coil or a dry wall cokes the binder against the metal long before the bulk gauge registers anything unusual, which is both how a batch is lost and how a tank fire starts.
It is paving bitumen sorted into grades by how thick it is at 60 °C rather than by how far a needle sinks into it at 25 °C. IS 73:2013 defines four such grades — VG-10, VG-20, VG-30 and VG-40 — occupying bands of 800–1200, 1600–2400, 2400–3600 and 3200–4800 poise, with the measurement made under vacuum in a capillary viscometer (IS 1206 Part 2, matching ASTM D2171). Because the grade number tracks viscosity directly, stiffness rises with the number instead of falling. Seven further requirements sit around that grading figure: a minimum kinematic viscosity at 135 °C, minimums for penetration and softening point, a flash point floor, a solubility floor, and two limits applied to RTFOT residue.
Because 60 °C is roughly what the top of a bituminous layer reaches on a hot afternoon, and that is the state in which rutting develops. A measurement made at 25 °C is taken at neither the service condition nor the construction condition, so it functions as an index rather than as a property of the pavement. Two parcels reading the same penetration can lie on viscosity–temperature curves of different slope, and the steeper one will have lost more stiffness by the time the road is hot — a difference the needle cannot report. A second reason is arithmetic: poise, Pa·s and cSt are quantities a plant can calculate with, which is how mixing and compaction temperatures are derived from a batch's own certificate. Tenths of a millimetre cannot be used that way.
Absolute — or dynamic — viscosity is measured at 60 °C in a vacuum capillary viscometer to IS 1206 (Part 2) / ASTM D2171 and reported in poise. A partial vacuum, 300 mm Hg in ASTM D2171, is needed because bitumen will not flow through a capillary under gravity at 60 °C in any reasonable time. Kinematic viscosity is measured at 135 °C by gravity flow through a calibrated capillary to IS 1206 (Part 3) / ASTM D2170 and reported in centistokes. The two are related through density: dynamic viscosity in poise equals kinematic viscosity in stokes multiplied by density in g/cm³, and one stokes is 100 centistokes. The 60 °C figure names the grade and predicts rutting; the 135 °C figure controls pumping, spraying and aggregate coating at the plant.
VG is viscosity grade, and the digits give the nominal 60 °C viscosity in hundreds of poise: 1000, 2000, 3000 and 4000 poise respectively. IS 73:2013 then allows a band about each nominal figure — 800–1200, 1600–2400, 2400–3600 and 3200–4800 poise, or 80–120, 160–240, 240–360 and 320–480 Pa·s if the contract is written in SI. Two features of that ladder catch buyers out. The bands widen as they climb, so VG-40 spans 1600 poise where VG-10 spans 400. And the top two overlap between 3200 and 3600 poise. Note also that stiffness rises with the number here, which is the reverse of penetration grading.
VG-30 for general paving in a hot climate under heavy traffic, which covers most highway bituminous concrete and dense bituminous macadam work. VG-40 where traffic is very heavy, slow-moving or channelised — intersections, toll plazas, bus bays, container and port yards. VG-20 for paving in cold-climate and hill regions, moving to VG-10 where winters are severe. VG-10 for sprayed applications and surface dressing, and as feedstock for emulsion and cutback manufacture. Where a project specification, tender document or schedule of rates already names a grade, supply that grade rather than an equivalent, and have any substitution approved in writing.
The overlap is a consequence of how the bands are constructed rather than an oversight. Each grade is a ± 20 % window about its nominal figure, so VG-30 runs 2400–3600 and VG-40 runs 3200–4800; once the windows are proportional and the nominal values are only 1000 poise apart, they necessarily meet. Practically it means a viscosity reading between 3200 and 3600 poise is grade-neutral, and the decision falls to the rest of the column — 350 against 400 cSt at 135 °C, 45 against 35 dmm penetration, 47 against 50 °C softening point, 40 against 25 cm residual ductility. Two consequences follow. A certificate reporting 3400 poise identifies nothing on its own. And a supplier quoting either grade from the same tank is not necessarily wrong, provided every other line matches the grade on the contract.
Not as a matter of course, and the obstacle is normally the paperwork rather than the barrel. The two grades occupy the same commercial slot and are often cut from comparable feedstock, but a 60/70 sale is closed on a two-sided penetration band at 25 °C and the 60 °C viscosity test is simply never run. Since a VG grade is defined by that test, the material may or may not sit inside the band and the certificate cannot say which. Making the substitution work means arranging the missing testing — absolute viscosity at 60 °C and kinematic viscosity at 135 °C to IS 1206 Parts 2 and 3, plus viscosity ratio and ductility on RTFOT residue — and agreeing at contract stage which laboratory runs it, before the cargo loads rather than after it discharges.
Yes, and that is one of the main practical reasons for viscosity grading. Convert the absolute viscosity at 60 °C into kinematic units — poise divided by density in g/cm³, multiplied by 100 — then plot that point and the measured 135 °C figure on a bitumen viscosity–temperature chart of the type standardised in ASTM D2493. The relationship is close to a straight line over the working range, so read the mixing temperature where the line crosses 170 ± 20 cSt and the compaction temperature where it crosses 280 ± 30 cSt. For the mid grades the answer usually lands in the region of 150 to 165 °C for mixing, but the point of the exercise is that it is derived from the batch in the tank rather than assumed from a table.
Tell us which of the four grades the project calls for, along with tonnage, packing, discharge port and Incoterm. If you are still deciding between two grades — or holding a specification written in penetration grades against a tender written in VG — send that document with the enquiry and it will be read against IS 73:2013 before any price is put on it.