Cutback Bitumen MC-70: Specification, Prime Coat Selection and Safe Handling

MC-70 is a medium-curing cutback bitumen — a paving-grade binder diluted with a kerosene-range petroleum distillate so that it can be sprayed or mixed at low temperature instead of at hot-mix temperatures. It is specified under ASTM D2027 and the equivalent AASHTO M82. After application the kerosene leaves the film by evaporation and by absorption into whatever it was applied to, and the bitumen left behind — the residue — stiffens back towards a working paving consistency. That residue is the binder that does the structural work. The liquid in the drum is a delivery system for it, and it is the residue tests, not the viscosity line, that tell you what you are actually buying.

The two halves of the designation carry different information, and buyers routinely misread the second one. MC names the diluent family and therefore the cure speed: kerosene, medium curing, days rather than the hours of a naphtha-cut RC grade or the weeks of a gas-oil-cut SC grade. The number 70 is the lower limit of the kinematic viscosity band at 60 °C in centistokes, measured by ASTM D2170, and the upper limit of the band is always exactly twice the lower one. MC-70 therefore runs from 70 to 140 cSt. The full grade system across all three cure families is set out on the cutback bitumen hub page.

Where MC-70 sits in the MC series

The medium-curing series runs MC-30, MC-70, MC-250, MC-800 and MC-3000. Every band spans exactly a factor of two, while the step from one grade to the next is smaller at the fluid end than higher up the series — about 2.3 times from MC-30 to MC-70, then roughly three to four times between each of the heavier grades. MC-70 sits one step up from the most fluid grade in the whole cutback system and three steps below the stiffest MC material:

• MC-30 — 30 to 60 cSt. The only 30-grade in the cutback system. In warm weather it sprays at close to ambient temperature with little or no heating.
• MC-70 — 70 to 140 cSt. Roughly twice as viscous as MC-30 at the reference temperature. Needs a modest, controlled warm-up to give an even fan from the spray bar, but nothing approaching hot-bitumen temperatures.
• MC-250 and MC-800 — 250 to 500 and 800 to 1600 cSt. These need real heat to atomise, they are not prime coat grades, and they belong to cold mix, patching and surface treatment work.

Stiffer means less solvent, not just thicker liquid

The single most useful consequence of moving from MC-30 to MC-70 is invisible in the viscosity number. A stiffer cutback is stiffer because it carries a smaller charge of diluent. ASTM D2027 requires a minimum distillation residue of 50 % by volume for MC-30 and 55 % for MC-70. Two things follow directly from those five percentage points.

First, MC-70 puts more binder on the road for every litre sprayed. At an application rate of 1.2 L/m², MC-70 leaves at least 0.66 L/m² of residual binder against at least 0.60 L/m² for MC-30 at the same rate. Put the other way round, you would need about 1.32 L/m² of MC-30 to deposit the residual binder that 1.2 L/m² of MC-70 deposits. Buyers who compare cutback offers per tonne of liquid rather than per tonne of residual binder consistently misprice this.

Second, there is less kerosene to get rid of. Every litre of diluent has to evaporate before the prime is properly cured, and in cool, humid or still conditions that evaporation is the rate-limiting step in the whole surfacing programme. A grade carrying less solvent per litre cures in a shorter window, which is why MC-70 is often the better answer late in the season even though it is the harder of the two to spray cold.

The two jobs MC-70 does

• Prime coat on open or coarse-textured granular bases. This is the dominant use and the reason the grade exists as a separate specification. Where MC-30 would over-penetrate — draining into the voids of a stony base and leaving the surface as dry and dusty as it was before — MC-70 holds a film at and just below the surface while still keying in.
• Cold plant mix, road mix and cold patching. MC-70 wets clean aggregate readily at ambient temperature and produces a mix that can be laid and compacted cold. It is the grade of choice where the mix is to be used within a short period rather than stockpiled for months.

What MC-70 is not

• Not a tack coat material. A bond coat between pavement layers wants a binder that stays at the surface and sets quickly. That is RC territory, and in modern practice it is emulsion territory. MC-70 will track under tyres for days.
• Not a drop-in replacement for MC-30. On a tight, fine-graded, well-compacted base MC-70 will sit on top as a skin that never keys in. The grades are not interchangeable in either direction.
• Not the same as RC-70 or SC-70. The three share a viscosity band and nothing else. They differ in diluent, cure speed, flash point and application.
• Not a hot binder. Nothing about handling penetration grade bitumen at 160 °C transfers to a liquid that flashes at 38 °C.

The number that makes MC-70 different

The flash point is the temperature at which a liquid gives off enough vapour to form an ignitable mixture with the air above it. For bitumen 60/70 that temperature is at least 250 °C by ASTM D92, roughly 90 °C above the hottest sensible handling temperature — a wide, comfortable margin. For MC-70 the flash point is set by the kerosene, not by the bitumen, and ASTM D2027 sets a Tag open cup minimum of just 38 °C to ASTM D3143.

Compare that with how the material is used. MC-30 usually sprays at 30 to 60 °C and in hot weather barely needs heating at all. The stiffer grades — MC-250 and above — carry a higher Tag open cup minimum of 66 °C, and it is worth being precise about what that higher number buys, because it is routinely oversold. It protects the drum in the yard and the container on the quay, where a lighter grade's vapour space goes flammable on weather alone. It buys nothing at the spray bar, because those grades are applied at 75 °C and above and are therefore also worked above their own flash point. MC-70 gets the worst of both ends: it is stiff enough to need warming to 50–80 °C for an even spray fan, and it flashes at 38 °C, at the very bottom of that window, so it has neither MC-30's option of spraying cold nor MC-250's margin in storage. There is no version of the common site rule hold the tank at least 25 °C below the flash point that can be satisfied while spraying MC-70 properly.

The consequence is not that MC-70 cannot be used. It is that the distributor tank, the drum and every vessel holding warm MC-70 must be treated as containing a flammable atmosphere at all times, rather than as something that becomes hazardous only if it is overheated. A sealed drum standing in the sun on a tropical quay reaches 38 °C on its own, before anyone has connected a heater.

Heating: indirect only, never an open flame

• Indirect heat only. A hot oil jacket, a steam coil or a hot water bath are the only acceptable methods. Nothing that burns, glows or arcs may touch a vessel holding MC-70 — no burner tube under a drum, no torch on a seized chime, no immersion element dropped into a part-empty drum, no improvised heat trace. Every warming habit carried over from drummed paving bitumen has to be unlearned before the first MC-70 bung comes out.
• Keep the coil drowned. Product must cover the entire heating surface before heat is applied and stay over it as the tank draws down. A coil firing through an exposed surface film boils kerosene straight off it and puts hot metal into a vapour space that is already inside its flammable range at 50 °C.
• Two independent temperature controls. The control thermostat is one; a high-temperature cut-out on a separate sensor and a separate circuit is the other. Prove both before the season, because an untested cut-out is a label rather than a safeguard.
• Set the limit from the batch, not from the standard. The 38 °C in ASTM D2027 is a floor the grade must clear, not a description of your cargo. Take the measured Tag open cup value off the Certificate of Analysis for the batch in the tank, chalk it on the tank, and set the thermostat against that. Batches vary, and the one testing closest to the minimum is the one that will arrive on the day nobody checks.
• Heat the day's quantity and no more. A tank held warm across a week loses its lightest kerosene fractions to the vapour space, and the viscosity of what remains climbs quietly out of the 70 to 140 cSt band with nothing appearing on any delivery note. The load sprayed on Friday is not the load that was tested on Monday.
• Vents open, discharging somewhere safe. A warming cutback tank produces solvent vapour continuously. A restricted or plugged vent makes that a pressure hazard and a fire hazard in the same moment, and the discharge point has to be clear of walkways, ignition sources and the operator's breathing zone.

The mistake that starts tank fires

MC-70 and hot bitumen must never meet inside a vessel — not MC-70 tipped into hot binder, not hot binder run onto MC-70, and not the warm residue of one left in equipment about to receive the other. Paving bitumen at 150 °C sits far above the boiling range of the kerosene diluent. The solvent charge does not warm through; it flashes, effectively all of it, in seconds, throwing hot liquid out of the hatch and laying a vapour cloud directly over whatever ignition source happens to be nearest. Tank and kettle fires following exactly that sequence have killed people. The rule extends to sequencing on site: a distributor, storage tank, transfer line, pump or drum heater that has just carried hot binder is cooled and cleaned before it sees MC-70, and a vessel that is merely empty still counts as hot.

The corollary is equally firm: do not attempt to make or adjust MC-70 on site. Thinning a stiff load with kerosene, or letting someone tip solvent into a bitumen tank to hit a viscosity target, is how people are killed. Cutback blending is a refinery or terminal operation carried out in purpose-built equipment with the base binder held at a controlled temperature, closed transfer and vapour control. Buy the grade the specification names.

Static, hot work and confined spaces

• Fill from the bottom, and start slow. A stream of MC-70 falling through its own vapour space accumulates static charge above a liquid whose vapour is ignitable at ambient temperature. Use a bottom-entry connection or a dip pipe reaching below the surface, and hold the initial flow rate down until the inlet is submerged.
• Bond first, then flow. Tank, distributor, drum, pump and hose are bonded together and earthed before any transfer begins, and continuity is measured rather than eyeballed. Corroded clamps and painted contact faces are the usual reason a bonding cable is doing nothing at all.
• No hot work on anything that has held MC-70. Welding, cutting, grinding or drilling a drum or tank has killed people who reasoned that an emptied, drained, odourless container was safe. It is not: the vapour left behind is the explosive part. The sequence is cleaning, gas-freeing, then a gas test read by someone competent to interpret it, and only then a permit.
• Kerosene vapour sinks. Heavier than air, it settles into bunds, pits, sumps, trenches and tank floors and can sit there at ignitable concentration long after the air at head height reads clear. Any MC-70 tank is a confined space carrying a flammable and potentially oxygen-deficient atmosphere until a gas test proves otherwise.
• Assume hydrogen sulphide in the ullage. It builds up in the vapour space of warm bitumen and cutback tanks even when the product itself carries almost none, and at the concentrations that matter it destroys the sense of smell that would otherwise warn you. Nobody gauges, dips or samples over an open hatch on a warm tank without personal monitoring.

Water: what the 0.2 % limit is really protecting

The 0.2 vol % maximum water content by ASTM D95 is a quality line and a safety line, but the safety argument for MC-70 is narrower than the one properly made for the heavier grades, and overstating it teaches crews to ignore it. The whole 50 to 80 °C spray window for MC-70 lies under 100 °C, so free water in a correctly operated MC-70 tank never reaches the temperature at which it would flash to steam. What it does is spoil the work: it interferes with wetting and penetration into the base, and it froths and surges through a warm tank, giving an erratic fan and streaked coverage.

The hazard becomes serious the moment temperature control fails or the material meets something hotter than it should. Water trapped under binder that reaches 100 °C expands roughly seventeen hundred fold as it turns to steam, and the result is a boil-over that ejects hot flammable liquid from the vessel — which is one more reason an overheated MC-70 tank, a runaway heating coil and a vessel shared with hot bitumen are treated as gravely as they are in this section. The precautions cost nothing either way: check drums and tank bottoms for standing water before charging, drain and dry transfer lines at the start of the season, keep bungs tight and drums off wet ground, and keep open vessels covered in wet weather.

If it catches fire

• The media that work are foam and dry powder rated for flammable liquids, with carbon dioxide acceptable on a small contained fire such as a drum or a pump housing. Position them where a fire would actually start — beside the distributor, beside the tank, beside the drum decanting point. An extinguisher hanging in the site office is inventory, not protection.
• A water jet into burning cutback makes the fire bigger. The jet drives water beneath the burning surface, it flashes there, and the expanding steam throws burning liquid across a wide radius. Water has exactly one legitimate role at a cutback fire: fog applied from a distance to cool drums, tanks and structures not yet involved. It never goes into the burning liquid.

Vapour exposure, skin and first aid

• Breathing it. Because MC-70 is sprayed warm and atomised, it puts far more kerosene vapour into the air per litre than an open drum ever will. Headache, dizziness, nausea and the impaired judgement that comes with central nervous system depression arrive at concentrations that are entirely ordinary around a spray bar in still air, inside a shed, or in a tank compound with no cross-draught. Put the crew upwind, extract mechanically for enclosed work, and treat respiratory protection as the answer only where ventilation genuinely cannot fix the exposure.
• Swallowing it. The danger is aspiration rather than poisoning: the light fraction is fluid enough to run into the lungs, and it does most of its damage on the way back up. Vomiting must not be induced. The casualty goes for medical care immediately, with the Safety Data Sheet in hand.
• Skin and clothing. Kerosene strips the natural oils out of skin and produces dermatitis with repeated contact, and a soaked overall holds it against the body for hours. Nitrile gloves rather than latex; gauntlet cuffs outside the sleeve so a splash runs off the arm instead of down into the glove; contaminated clothing off and away rather than worn for the rest of the shift. Washing binder off skin with solvent, diesel or kerosene turns a nuisance into an injury.
• Burns. Binder at 70 °C inflicts a thermal burn and the solvent in it adds a chemical insult on top. Flood the area with clean cold water at once, keep it there for at least twenty minutes, and go on cooling on the way to hospital. Adhered bitumen stays where it is. Once cool it acts as a sterile covering; peeling it or dissolving it away takes skin with it and opens the burn to infection. Whether and how it is removed is a burns unit's decision, not a first aider's.

Dangerous goods, packing and storage

MC-70 ships as dangerous goods, normally declared as UN 1999, Tars liquid including road oils and cutback bitumens, Class 3 flammable liquid. One point of precision is worth carrying into the booking, because it is regularly muddled. The 38 °C in ASTM D2027 is a Tag open cup figure, whereas transport regulations define a Class 3 flammable liquid and assign its packing group from a closed cup flash point, which reads lower than the open cup value on the same material. For MC-70 that settles the question in one direction only: an open cup result at or near 38 °C guarantees a closed cup result far below the 60 °C threshold, so MC-70 is always a regulated flammable liquid and there is no exemption to argue for. Packing group remains a measurement: closed cup below 23 °C gives packing group II, 23 to 60 °C gives packing group III, and MC-70 normally falls in group III. Take the declared classification from the Safety Data Sheet and the batch data, not from this page.

• Specify new steel drums with sound closures and correct Class 3 labelling. A leaking cutback drum is a fire hazard, not a housekeeping problem, and reconditioned drums are the usual source of both leaks and contamination disputes.
• A dangerous goods declaration and an IMDG-compliant container packing certificate travel with the shipment, and a current Safety Data Sheet is required by the carrier, the terminal and increasingly by destination customs.
• Some carriers and ports restrict or surcharge Class 3 stowage, and routings can differ from those for ordinary bitumen cargo. Confirm acceptance before fixing the shipment — see bitumen logistics and shipping.
• Drums stand upright with bungs tight, under a roof, shaded and ventilated, with no ignition source and no hot bitumen operation anywhere near them. Unshaded drums in a tropical compound reach 38 °C on their own, and a cutback stack sharing a yard with an open kettle is an incident waiting for a windy day.

Stated in one sentence: base texture sets the starting grade, ambient temperature moves it one step, and your ability to heat the distributor decides whether you can act on that. Everything below is an expansion of that rule.

What a prime coat is trying to achieve

A prime coat is not a bond coat and it is not a seal. It is sprayed onto a compacted granular base to do four things at once: bind the loose fines at the surface so the base does not ravel under construction traffic, waterproof the base against rain in the interval before surfacing, plug the surface voids so the asphalt layer above does not lose binder into the base, and leave a slightly tacky film that the asphalt can key into. To do all four, the binder has to travel into the base — commonly specified as 3 to 10 mm of visible penetration — while still leaving something behind at the surface.

Both failure modes come from getting that balance wrong. If the binder goes in too far, you have over-penetration: the material disappears, the surface remains dry and dusty, nothing is bound and no film is left for the asphalt to key into. If it does not go in at all, you have a skin: a black film sitting on an unbound surface that stays tacky for days, picks up on construction plant tyres, and becomes a slip plane under the asphalt layer.

Reading the base

The base surface tells you which risk you face, and it takes about two minutes to assess:

• Tight, fine-graded, densely compacted base with visible fines at the surface. Few interconnected voids, low absorption. The risk here is a skin. Use MC-30.
• Crusher-run or coarse-graded base with an open, stony surface texture and little fine material showing. High absorption, large void paths. The risk here is over-penetration. Use MC-70.
• A base that has been swept hard, or that has lost fines to wind and rain since compaction. Loss of fines opens the surface, and the base now behaves like a coarse base regardless of its original grading. Reassess on the day of priming, not on the day of compaction.

A practical field check: drop a small amount of clean water on the swept surface. If it stands as a bead or a film for more than a few seconds, absorption is low and MC-30 is indicated. If it disappears almost immediately, the base is thirsty and MC-70 will hold its position better. The water test is an indicator and not a substitute for a trial patch, but it will keep you from ordering the wrong grade.

Ambient temperature moves the decision

Viscosity falls steeply with temperature, and so does the base's ability to hold the binder near the surface. That gives two distinct temperature effects, and they pull in opposite directions.

Hot conditions, above roughly 35 °C ambient with a base hot to the touch. The binder in the film thins further, the base takes it more readily, and MC-30 that would have been correct at 25 °C now over-penetrates. On a grade it can also run, giving a prime that is heavy in the low points and dry on the crown. In hot climates a base that reads as marginal should be primed with MC-70.

Cool conditions, roughly 10 to 20 °C. Here the case for MC-70 is different and less obvious. It is not about penetration — it is about cure. Every litre of kerosene has to evaporate before the prime is properly cured, and evaporation slows sharply as temperature falls. Because MC-70 carries a smaller diluent charge than MC-30 — a minimum 55 % residue against a minimum 50 % — there is less solvent to remove and the prime reaches a surfaceable condition in a shorter window. On a project where the surfacing crew is waiting, that difference is worth more than a slightly easier spray.

The catch is sprayability. Cold MC-70 does not atomise; it leaves streaks, ropes and bare strips from the bar. To use MC-70 in cool weather you need the distributor tank, the pump, the hoses and the spray bar all pre-warmed, and the material held at the upper end of its 50 to 80 °C window. If you cannot heat reliably and indirectly, MC-30 is the honest choice in cool weather, and you accept the longer cure.

Application rate is not a constant

Published prime coat rates for MC grades typically run in the region of 0.8 to 1.5 litres per square metre, and within that band MC-30 on a tight base sits towards the lower end while MC-70 on an open base sits towards the upper end. Treat those figures as a starting point for a trial and nothing more. The correct rate is the largest quantity the base will absorb in 24 hours without leaving free binder standing on the surface, and it varies with grading, fines content, degree of compaction, moisture and the weather on the day.

Two rules survive every site. First, if free binder is still visible after 24 hours, the rate was too high — apply a light sand blotter so traffic can run, and reduce the rate on the next section. Second, if the surface looks dry and dusty after a full application, do not simply increase the rate on the same grade; that is the signature of over-penetration, and the answer is a stiffer grade rather than more of a thin one.

Cure time before surfacing

Allow 24 to 48 hours for an MC-70 prime before the asphalt layer goes on, and longer in cool, humid or still conditions. Surfacing over an uncured prime traps solvent under an impermeable layer, and the result is a soft, unstable interface that shows up as shoving and slippage cracking months later. The prime is ready when the surface is firm, no longer tacky enough to lift on a boot, and shows no free binder.

When neither grade is the right answer

• The base is damp, or rain is expected inside the cure window. Water in the surface blocks penetration for either grade. Postpone.
• Cutback is restricted at the destination. Many jurisdictions limit medium-curing grades to specific seasons on air quality grounds, and ban rapid-curing grades outright. Confirm before importing — a cargo that cannot legally be sprayed is a total loss.
• The site cannot store or heat a Class 3 flammable liquid safely. A bitumen emulsion prime carries no solvent, no meaningful fire risk and no dangerous goods paperwork. Purpose-made penetrating emulsion primes have narrowed the technical gap considerably, though they have not closed it on the tightest bases.
• The base itself is out of specification. A prime cannot rescue a base that is under-compacted, segregated or short of fines. Correct the base.