Rc-70 Cutback Explained
Last updated: 2026-10-18
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RC-70 cutback is a medium-curing, solvent-thinned bitumen (typically 80/100 penetration base stock) blended with 20–35% kerosene-type diluent, giving viscosity of roughly 70 seconds by ASTM D2170 kinematic test at 60°C. It's used for prime coats, tack coats, and cold-applied patching, cures in 4–12 hours depending on temperature, and is priced around $380–$520 per metric ton FOB in 2024–2025 markets. RC-70 is not the same as MC-70 (medium-curing) or SC-70 (slow-curing), and it must never be confused with penetration-graded or viscosity-graded paving bitumen.
What RC-70 Cutback Actually Is: Composition and Specs
RC-70 falls under the "rapid-curing" (RC) family of cutback asphalts. The "70" refers to kinematic viscosity of approximately 70 seconds (mm²/s) at 60°C per ASTM D2170. The "rapid-curing" designation means the solvent is a naphtha or light kerosene fraction with a high evaporation rate — the material sets quickly after application compared with medium-curing (MC) and slow-curing (SC) grades.
Typical RC-70 specification range (ASTM D2028 / D2026 reference):
| Property | Typical Value | Test Method |
|---|---|---|
| Kinematic viscosity @ 60°C | 70 ± 15 mm²/s | ASTM D2170 |
| Flash point (Tag open cup) | ≥ 27°C | ASTM D1310 |
| Distillation to 225°C | ≤ 10% by volume | ASTM D402 |
| Distillation to 260°C | 15–45% by volume | ASTM D402 |
| Distillation to 316°C | 55–85% by volume | ASTM D402 |
| Residue from distillation | ≥ 55% by mass | ASTM D402 |
| Penetration of residue @ 25°C | 80–120 dmm | ASTM D5 |
| Solubility in trichloroethylene | ≥ 99% | ASTM D2042 |
The residue after solvent evaporation is essentially an 80/100 or 120/150 penetration bitumen, which is what gives RC-70 its binding performance. The solvent content typically runs 28–35% by mass for a 70-second grade — meaning roughly one-third of the tonnage you buy evaporates during curing.
| Property | RC-70 | MC-70 | SC-70 |
|---|---|---|---|
| Solvent type | Naphtha / light kerosene | Kerosene | Gas oil / heavy distillate |
| Cure time (25°C) | 4–8 hours | 12–24 hours | 24–72 hours |
| Flash point | Low (≥27°C) | Medium (≥38°C) | Higher (≥60°C) |
| Best use | Prime/tack coat, rapid reopening | Base stabilization, dust control | Slow-cure priming, cold mix |
| Relative price index | 100 | 92–96 | 85–90 |
| Volatile loss after 24h | 85–95% | 60–75% | 30–45% |
If a project needs traffic back on the surface the same day, RC-70 is the correct grade. If the schedule allows overnight curing, MC-70 is usually cheaper and safer to handle.
How RC-70 Cures: The Numbers Behind the Timeline
RC-70 curing is purely evaporative — there is no chemical reaction. The naphtha fraction flashes off first, followed by the kerosene-range components. Cure time is driven almost entirely by ambient temperature, wind, and application rate.
Cure time by temperature (application rate: 1.0 L/m², moderate wind):
| Ambient Temp | Surface Dry (tack) | Full Cure (no pickup) | Solvent Remaining @ 24h |
|---|---|---|---|
| 5°C | 6–10 hours | 24–36 hours | 15–25% |
| 15°C | 3–5 hours | 10–14 hours | 8–12% |
| 25°C | 2–4 hours | 6–8 hours | 3–5% |
| 35°C | 1–2 hours | 4–6 hours | <3% |
| 45°C+ | 45–90 minutes | 2–4 hours | <1% |
Practical rules of thumb:
- Below 10°C, RC-70 cure time roughly doubles for every 5°C drop — do not use it for same-day reopening in cold weather.
- Application rate above 1.5 L/m² traps solvent and can extend full cure by 50–100%.
- Rain within 2 hours of application will likely wash off 30–60% of the binder; re-application is required.
- Residual solvent above 5% at the time of hot mix overlay can cause "bleeding" or delamination, so allow a minimum 24-hour window before paving under cool conditions.
Where RC-70 Is Used: Applications and Application Rates
RC-70 is overwhelmingly a prime coat and tack coat material, with secondary use in cold patching and dust suppression on haul roads.
| Application | Typical Rate | Coverage per Tonne | Notes |
|---|---|---|---|
| Prime coat (granular base) | 0.9–1.5 L/m² | 700–1,100 m² | Penetrate 5–10 mm into base |
| Tack coat (asphalt-to-asphalt) | 0.25–0.50 L/m² | 2,000–4,000 m² | Dilute 1:1 with kerosene for spray |
| Cold patch binder | 5–7% by mass of aggregate | — | Stockpile life 3–6 months |
| Dust suppression (haul road) | 0.5–1.0 L/m² | 1,000–2,000 m² | Reapply every 2–4 weeks |
| Crack sealing (cold) | 0.3–0.6 kg/m of crack | — | Follow with sand blotting |
Tonnage math for a typical project: A 10,000 m² granular base at 1.2 L/m² prime coat requires 12,000 L, or about 12 tonnes of RC-70 at a density of roughly 1.0 kg/L. At $450/tonne FOB, the prime coat material cost is ≈$5,400, or $0.54/m².
| Region Type | Preferred Grade | Reason |
|---|---|---|
| Hot desert (35°C+) | RC-70 or RC-250 | Fast cure, minimal solvent retention |
| Temperate (10–30°C) | RC-70 | Balanced cure, workable spray window |
| Cold (<10°C) | MC-70 or emulsified | RC-70 cures too slowly, high solvent trap risk |
| Tropical humid | RC-70 with 10% dilution | Humidity slows evaporation 20–30% |
RC-70 Pricing, Supply, and Quality Checks
RC-70 pricing tracks two inputs: the base bitumen (usually 60–65% of cost) and the naphtha diluent (30–35% of cost). Because naphtha is more volatile in price than bitumen, RC-70 quotes can swing ±15% within a quarter.
Indicative pricing (FOB, 2024–2025):
| Market | Price Range (USD/tonne) | Packaging |
|---|---|---|
| Middle East (UAE, Oman) | $380–$450 | Bulk, 180 kg drums |
| Southeast Asia | $420–$490 | Bulk, flexitank |
| West Africa | $480–$540 | Drums only (freight-heavy) |
| Europe | $500–$580 | Bulk, IBC |
Quality checks buyers should run on every shipment:
- Viscosity @ 60°C — reject anything outside 55–85 mm²/s; high viscosity signals excessive solvent loss or wrong base stock.
- Distillation curve — a flat curve or more than 10% distilling below 225°C indicates contaminated or over-light solvent (fire risk).
- Residue penetration — must land in 80–120 dmm; out-of-range residue means the base bitumen was substituted.
- Flash point — anything below 27°C is a transport hazard and likely non-compliant with IATA/IMDG handling rules.
- Water content — should be ≤0.5%; higher water causes foaming during spray application.
Storage: RC-70 stores for 6–12 months in sealed drums at ambient temperature. Open bulk tanks lose 2–4% of solvent per month through evaporation, which raises viscosity — recirculate and re-test before use if stored beyond 90 days.
FAQ: RC-70 Cutback
What is the difference between RC-70 and RC-250?
RC-70 has a kinematic viscosity of ~70 seconds at 60°C, while RC-250 is roughly 3.5× thicker at ~250 seconds. RC-70 is used for prime and tack coats; RC-250 is used for chip seal binders and heavier cold applications where a thicker film is needed.
How long does RC-70 take to cure before paving?
At 25°C, RC-70 reaches full cure in 6–8 hours and is safe for hot mix overlay after 12–24 hours. Below 15°C, allow 24–36 hours, and verify residual solvent is under 5% before paving to avoid bleeding.
Can RC-70 be diluted with diesel or kerosene?
Yes — kerosene dilution at 1:1 is standard for tack coat spray application, and up to 20% diesel is sometimes used in cold patching. Never exceed 30% dilution, as it drops residue content below the 55% minimum and weakens the binder.
Is RC-70 hazardous for transport?
Yes — with a flash point as low as 27°C, RC-70 is classified as a flammable liquid (UN 1999 or similar, Class 3) for sea and road freight. It requires proper placarding, and many markets require a flash point ≥38°C for unrestricted road transport, so check local rules.
How many square meters does one tonne of RC-70 cover?
At prime coat rate of 1.2 L/m², one tonne (≈1,000 L) covers about 830 m². At tack coat rate of 0.3 L/m², the same tonne covers roughly 3,300 m² — always calculate by application rate, not by habit.
Ready to price your RC-70 order? Use the 6Tar cost calculator to get landed cost per tonne, per square meter, and per project — factoring in FOB price, freight, drum vs bulk packaging, and application rate. Enter your tonnage and destination, and get a delivered-cost estimate in under a minute.