Codes referenced
Concrete shall be discharged from the truck-mixer within 2 hours of loading. Longer period allowed with retarders / cool / humid / chilled conditions.
Time of loading starts from adding water to the dry mix, or adding cement to the wet aggregate — whichever applies.
If discharge from the producer's vehicle is delayed at site for lack of purchaser preparedness, responsibility passes after 30 minutes.
Concrete shall be discharged within 90 min of batching unless the producer documents that a longer time does not adversely affect the concrete.
Cycle time decomposition
A truck cycle has five segments. The first three (load, travel, return) are plant-controllable; the last two (wait, discharge) are largely site-controlled. Two numbers are non-negotiable:
| Segment | Typical duration | Controllable by | Variability |
|---|---|---|---|
| Loading (charge + mix ≥60 rev) | 8–12 min | Plant | Low |
| Travel to site (10 km urban) | 25–45 min | Dispatcher / route | Medium |
| Wait at site | 0–60 min | Site supervisor | Very high |
| Discharge (chute or pump) | 10–15 min | Site / pump crew | Low |
| Return (empty) | 20–35 min | Driver | Low–medium |
| Total | 60–165 min (typical 90–110) | — | Driven by wait |
Reference: ConcreteInfo RMC operations experience; IS 4926:2003 §5.2.2.1 (discharge limit), §6.2.1 (30-min site-prep delay); ACI 304R Guide for Measuring, Mixing, Transporting, and Placing Concrete.
IS 4926 Cl 5.2.2.1 & Cl 5.2.2.2 — the 2-hour discharge clock
These two short clauses set the absolute outer limit on a transit mixer's useful life. They tie plant operations to the dispatch clock:
- "Concrete shall be discharged from the truck-mixer within 2 h of the time of loading. However, a longer period may be permitted if retarding admixtures are used or in cool humid weather or when chilled concrete is produced."
- "The time of loading shall start from adding the mixing water to the dry mix of cement and aggregate or of adding the cement to the wet aggregate whichever is applicable."
Source: IS 4926:2003 §5.2.2.1 and §5.2.2.2. Verified against the BIS reprint.
Fleet sizing formula & worked example
The arithmetic is small but the input numbers matter. The base formula:
Trucks needed = ⌈ (Pour rate × Cycle time) ÷ Truck capacity ⌉ + standby
A standby of one truck is added to absorb breakdowns, traffic and queueing variance.
Pour rate 40 m³/h, 90-min cycle, 7 m³ trucks
Typical urban mid-rise slab pour
Plugging in: (40 × 1.5) ÷ 7 = 8.57 → round up to 9 trucks, plus 1 standby → 10 trucks. At 7 m³ per trip and 7 trips per truck per day, this fleet can deliver ~490 m³/day, which is enough for an 8-hour pour at 40 m³/h with margin.
| Pour rate | Cycle time | Truck capacity | Calculated trucks | With 1 standby |
|---|---|---|---|---|
| 30 m³/h | 60 min (highway) | 7 m³ | 4.3 → 5 | 6 |
| 40 m³/h | 90 min (urban) | 7 m³ | 8.6 → 9 | 10 |
| 50 m³/h | 90 min (urban) | 7 m³ | 10.7 → 11 | 12 |
| 50 m³/h | 120 min (bad traffic) | 7 m³ | 14.3 → 15 | 16 |
| 80 m³/h | 75 min (highway) | 9 m³ | 11.1 → 12 | 13 |
Reference: ACI 304R; ConcreteInfo fleet-model templates. All cycles assumed inclusive of 15-min wait at site.
Plant utilisation rule of thumb — 70–75 %
A plant's nameplate capacity is the theoretical maximum at zero downtime. In practice:
| Plant nameplate | Realistic sustained (70–75 %) | Notes |
|---|---|---|
| 30 m³/h | 21–23 m³/h | Compact plant, single mixer |
| 60 m³/h | 42–45 m³/h | Mid-size commercial plant |
| 90 m³/h | 63–68 m³/h | Twin-mixer or large single |
| 120 m³/h | 84–90 m³/h | Twin-mixer highway project |
The lost 25–30 % comes from aggregate bin refill, cement silo refill pauses, moisture probe drift, truck queuing, mixer truck arrival pattern, and QA sampling. A plant claiming sustained output at >80 % of nameplate over a week is either over-specified or under-recorded.
Urban vs highway — the geography factor
Travel segment is the second-largest contributor to cycle time, and it changes with route type:
| Route type | 10 km each way | 25 km each way | Speed assumption |
|---|---|---|---|
| Dense urban (Mumbai, Pune old city) | 45 min | 90–110 min | 15 km/h avg |
| Suburban arterial | 25 min | 50–60 min | 25–30 km/h avg |
| Highway / expressway | 10–12 min | 22–28 min | 60–80 km/h avg |
A 40 m³/h pour with 7 m³ trucks takes 9 trucks on a 90-min urban cycle, 6 trucks on a 60-min highway cycle. The fleet needed to deliver the same pour halves as soon as the highway opens. This is why dispatchers care about the route map more than the pour schedule.
Why wait time at site is the worst problem
In the cycle-time breakdown above, three of the five segments are reasonably predictable from past data. The fourth — wait time at site — is where 60 % of all delivery-failure complaints originate. Causes include:
- Formwork or rebar inspection delayed — pour slot missed because the engineer is on another floor.
- Pump not ready — boom not positioned, pipeline not primed, prime-line grout already set.
- Previous truck still discharging — chute stuck, pump blocked, downstream placement team short-handed.
- Pour paused for cube result — site insists on a 7-day cube from an earlier load before pouring further.
- Site access blocked — barricade up for crane lift, chhath puja road closure, waterlogged access ramp.
Every minute of waiting is two losses: a minute the next truck cannot be loaded, and a minute closer to the 2-hour discharge clock of IS 4926 Cl 5.2.2.1. IS 4926 Cl 6.2.1 makes the site responsible after a 30-minute delay caused by lack of purchaser preparedness — but the load may already be lost by then. The plant's only direct control is the 30-minute call-ahead, the agreed pour-window in the PO, and a printed site-readiness checklist sent to the site supervisor the day before.
Multi-site batching strategies
When two sites are pulling from the same plant, the dispatcher must avoid both being short at the same moment. Three standard responses:
Site A receives 06:00–10:00, Site B receives 10:30–14:30, with a buffer pour at each end. Plants with single batch panel often default to this.
Trucks are colour-coded or assigned; a fixed ratio of trucks runs to each site. Requires discipline, but prevents the loudest site from starving the quiet one.
The faster pour rate gets the first wave; once its pour rate is met, the slower site gets the remainder of the fleet. Use only when both sites accept variable start times.
Common dispatch failures
Dispatcher computed fleet on 60-min cycle. Average cycle over the month is 95 min. Result: chronic 30-min waits, late deliveries, rejected loads. The fix: use median cycle from the last 30 days, not the best-day cycle.
One truck breaks down. Pour falls 1/10th behind immediately. Crew overruns into evening. The fix: one standby is non-negotiable above 8 trucks.
Driver waits 75 min at site. Truck was loaded 90 min ago. Pour is still 5 min away. The fix: dispatch software shows clock per truck; driver calls when 90 min is reached even if waiting.
60 m³/h plant + 90-min cycle + 7 m³ truck = (60 × 1.5)/7 = 12.9 trucks for 60 m³/h pour. But the plant never actually delivers 60 m³/h. Real fleet needs to match realistic sustained output, not the brochure.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how the cycle-time model resolves into a decision: hire another truck, or change the route, the pour window or the plant's stated output.
Questionnaire — asked at the dispatch desk, before the day's fleet is committed
- What is the median cycle time from the last 30 days? Not the best day, not the brochure figure. Sizing on the best-day cycle is how a plant ends up with chronic waits.
- What pour rate is being committed, in m³/h, and over what window? The rate and the window are what the truck count has to serve.
- What truck capacity is on the order? 7 m³ and 9 m³ fleets behave very differently against the same pour.
- Is the plan built on the plant's nameplate, or on its sustained output? Realistic sustained output is 70–75% of nameplate. Anything above 80% sustained over a week is over-claimed.
- Does the fleet include one standby truck? One standby is non-negotiable above 8 trucks.
- What is the route type? For 10 km, a dense urban run is 45 min, a suburban arterial 25 min, a highway 10–12 min. The route decides the fleet.
- Who confirms site readiness, and is there a call-ahead 30 minutes before arrival? That is the only direct control you have over the wait.
- Is the 2-hour clock tracked live per truck, from water addition onwards? IS 4926:2003 Cl 5.2.2.1 with Cl 5.2.2.2, and the driver's own call at 90 minutes even while waiting.
Inspector's checklist — what to verify on site
- Fleet size computed and written down with the arithmetic showing: pour rate × cycle time ÷ truck capacity, rounded up, plus one standby.
- Cycle time built from the five segments, not from a single round number: loading 8–12 min, travel 25–45 min on a 10 km urban run, wait 0–60 min, discharge 10–15 min, return 20–35 min.
- The standby truck rostered and physically available, not a name on a list at the plant office.
- Dispatch screen showing elapsed time per truck from the moment of loading, with the driver instructed to call at 90 minutes even if the truck is still waiting at site.
- Site-readiness checklist sent to the site supervisor the day before the pour, with the agreed pour window in the purchase order and a 30-minute call-ahead built into the run sheet.
- Multi-site days worked on one of the three standard splits (time-window, plant-share, pour-rate priority), recorded so nobody improvises at 14:00 when both sites are short.
- Refused and returned loads logged against the delivery, so the cycle time in the model keeps matching what actually happened on the ground.
- Site-caused delays over 30 minutes recorded, because responsibility passes to the purchaser after that point under IS 4926:2003 Cl 6.2.1, and only a record will prove it.
What happens if… three worked examples
What happens. The formula gives (40 × 1.5) ÷ 7 = 60 ÷ 7 = 8.57, so 9 trucks after rounding up, plus 1 standby, so 10 trucks. At 7 m³ per trip and 7 trips per truck per day that fleet can deliver about 490 m³/day (10 × 7 × 7), against an 8-hour pour of 40 × 8 = 320 m³. The fleet has margin.
Likely outcome. The pour is served without a truck queue, provided the wait at site stays inside the 15 min the calculation assumed. Note where the assumption came from: the median cycle of the last 30 days. Run the same arithmetic on a best-day 60-minute cycle and you come out with 7 trucks against the 10 you actually need, which is how a plant gets chronic 30-minute waits, late deliveries and rejected loads.
What happens. On a 60-minute cycle, (40 × 1.0) ÷ 7 = 5.71, so 6 trucks after rounding up, plus 1 standby, so 7. The calculated count for the same 40 m³/h pour falls from 9 on the urban cycle to 6 on the highway cycle, on route choice alone, before anybody buys a vehicle. The travel segment is the second-largest term in the cycle, and it is the one you can change.
Likely outcome. Route selection beats truck purchase, every time. And if the wait at site comes down by 15 minutes, that saves roughly one more truck, which is why dispatchers care about the route map more than the pour schedule.
What happens. On nameplate, (60 × 1.5) ÷ 7 = 90 ÷ 7 = 12.86, so 13 trucks. But 70–75% of 60 m³/h is 42–45 m³/h of realistic sustained output, and at 42 m³/h the count is (42 × 1.5) ÷ 7 = 63 ÷ 7 = 9, so 9 trucks plus 1 standby, so 10.
Likely outcome. The honest number is 10, not 13. Sizing on nameplate buys three trucks that stand idle while the plant loses 25–30% of its nameplate to bin refills, silo pauses, probe drift, queueing and QA sampling. A plant claiming sustained output above 80% of nameplate over a week is either over-specified or under-recorded.
Frequently Asked Questions
What is the maximum transit time for RMC concrete per IS 4926?
How many trucks are needed for a 50 m³/h pour?
Why is wait time at site the worst problem for truck cycle time?
Related articles
RMC Rate Calculator — Cost Significance
Why cycle-time slip directly inflates the delivered m³ cost — the rate-per-km lens.
RMC Plant Quality Management
Dispatch is one of three QA pillars — sampling, batch records, and dispatch all feed the audit trail.
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For dispatch-software setup, fleet sizing templates, or dispatcher training, contact amit@concreteinfo.in.
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