When is it "hot weather"?
Concrete is sensitive to all four of these — temperature alone isn't enough. A 25°C morning in Pune with low humidity and a 25 km/h dry wind can dry fresh concrete faster than a 40°C still afternoon. Treat each as a separate trigger.
ACI 305R — International Guidance (Evaporation Thresholds)
Note: ACI 305R is the American Concrete Institute guide — treat it as international guidance, not a controlling Indian code. Where it converges with IS 456 / IS 7861, either source works; where it diverges, follow the Indian code and project specification.
ACI 305R's evaporation chart indicates the risk of plastic-shrinkage cracking based on air temperature, concrete temperature, relative humidity and wind speed. The categories below are typical ranges from the chart; read the chart itself for the exact envelope used in your project.
| Evaporation (kg/m²/hr) | Concrete temp | Typical action |
|---|---|---|
| < 0.5 | ≤ 40°C | Normal precautions |
| 0.5 – 1.0 | 40 – 45°C | Cooling / windbreaks recommended |
| > 1.0 | > 45°C | Special precautions — fog mist, ice, or postpone |
Reference: ACI 305R — Guide to Hot Weather Concreting (international guidance, not a controlling IS code).
Time-of-day placement
The cheapest hot-weather concreting strategy is to pour when it isn't hot. For most of India, that means the night-early morning window. Here's a practical schedule for summer pours:
Ambient still moderate. Concrete temperatures manageable. Recommended slot.
Peak heat. Concrete temp may exceed 40°C. Avoid major pours.
Heat breaking. Cool aggregates / chilled water still needed.
Lowest ambient; most stable conditions. Plan illumination & logistics for night pours.
Mix & material modifications
- · Shade stockpiles, sprinkle with cool water
- · Use chilled washed aggregate when feasible
- · Saturated surface-dry (SSD) condition — not oven-dry
- · Chill to 5°C or use ice (part of the mix water)
- · Below-grade tanks keep water cooler than overhead tanks
- · Pumping chilled water is often the most effective single step
- · PPC / PSC generate less heat of hydration than OPC
- · Fly ash / slag reduce peak temperature rise
- · Use lower-heat cement in mass pours
- · Retarders — extend workability in transit
- · Plasticizers — reduce water demand
- · Avoid accelerators (raise heat of hydration)
Curing — IS 456:2000 Cl 13.5.1
Hot weather doesn't change the chemistry — it accelerates water loss. Extend curing, not shorten it.
| Exposure / Cement | Minimum curing |
|---|---|
| Normal exposure · OPC | 7 days |
| Blended cement (PPC / PSC / slag) | 10 days |
| Severe / extreme exposure · PPC or PSC | 14 days (common practice) |
| Mass / hot-weather pours, project-specified | Per project specification |
Reference: IS 456:2000 Cl 13.5.1 (7 days for OPC, 10 days for other cements under normal exposure). IS 456:2000 Table 5 sets minimum cement content / max w-c ratio by exposure. The 14-day row is common Indian practice for severe/exposure conditions; always follow the project specification.
Questionnaire — Site Engineer Self-Check
A short set of questions to answer before you call the truck. These are working prompts — not a checklist you can file in place of a method statement or pour card.
- What is the forecast ambient at placement time, and what is the overnight low expected within 24 hours of the pour? Any reading above 40°C, or expected to cross it, triggers IS 456:2000 Cl 14.1 controls.
- What is the concrete temperature at the point of placement, not at the plant? Record at first load and at every shift change. Plant readings are not placement readings.
- What is the relative humidity and wind speed at the pour location, and how have they trended over the last hour? 25°C with low humidity and a 25 km/h dry wind dries concrete faster than a 40°C still afternoon.
- What is the project specification's placement temperature cap? IS 7861 (Part 1) defines 40°C as the hot-weather threshold; many specifications require 32–35°C, and mass pours commonly target 27°C.
- How is the concrete being cooled — chilled water, ice, chilled aggregate, or none? Chilled water is the single most effective lever; record the supply temperature.
- What is the planned pour window, and what is the contingency if the morning slot is missed? Pours scheduled for 10:00 AM–4:00 PM in summer need an explicit go/no-go rule.
- Has the curing method been chosen for the actual exposure condition, not just the cement type? Severe/exposure conditions extend curing beyond the Cl 13.5.1 minimums.
Checklist — Records to Verify
Items to confirm and file with the pour record. The list is not exhaustive; adapt it to the project's QA plan and IS 456:2000 Cl 13 requirements.
- Concrete temperature reading at the form for every load; record truck number, time of test, and instrument ID.
- Ambient temperature, RH, and wind speed at the pour location, logged at the start of placement and at each break.
- Evaporation rate calculation using the ACI 305R chart (or equivalent calculator), with the value recorded against the readings above.
- Chilled-mix confirmation — water temperature, aggregate temperature, ice quantity (if used), and time of introduction.
- Placement time window — start time, end time, and any interruption longer than 30 minutes.
- Curing method, start time, and duration — water curing, membrane, or wet covering; per IS 456:2000 Cl 13.5.1 minimums adjusted for exposure.
- Cube register update — sample the same load, label the moulds, and link back to the truck number for traceability.
What Happens If…
Three short cases with technically qualified outcomes. They are illustrative, not prescriptive — treat each as a starting point for the engineer's review.
Case 1 — Concrete at placement reads 41°C, ambient 35°C
The 40°C threshold of IS 7861 (Part 1) — referenced by IS 456 Cl 14.1 — has been crossed by 1°C. The placement is technically non-compliant with the controlling code. Real-world tolerance is rarely 1°C; the engineer's call is whether to (a) hold the truck and re-chill water, (b) accept the load with a documented concession and a low w/c or SCM adjustment, or (c) reject the load. The decision and the placement temperature record go into the pour dossier. Project specification may impose a stricter cap.
Case 2 — Ambient 38°C, RH 35%, wind 18 km/h, concrete at 33°C
Concrete placement is under the 40°C control limit, but on the ACI 305R chart the evaporation rate often crosses 1.0 kg/m²/hr under these conditions. Plastic-shrinkage cracking is the dominant risk. Typical responses: stand up windbreaks, apply fog mist between placing and finishing, use an evaporation retarder, and start curing immediately after finishing. The pour can proceed, but the curing plan is the load-bearing control.
Case 3 — Mass raft pour, ambient 30°C, peak core temperature forecast 78°C
IS 456:2000 Cl 14.1 is met at placement, but the heat of hydration can drive core temperatures well above 70°C in mass pours. The risk shifts to thermal cracking and delayed ettringite formation (DEF) at sustained internal temperatures above ~70°C. Controls are mix-level (low-heat cement, SCM replacement, low cement content from IS 456 Table 5) and operational (pre-cooled aggregates, post-cooling pipes, temperature monitoring). The 40°C cap on placement is not the dominant control here.
Run the ACI 305R Evaporation Calculator
Plug in your ambient temperature, concrete temperature, relative humidity, and wind speed. The calculator applies the modified Menzel/Uno equation and returns the evaporation rate in kg/m²/hr — with the same 1.0 kg/m²/hr danger threshold ACI 305R uses. Treat the result as international guidance, not as a substitute for IS 7861 (Part 1) referenced by IS 456:2000 Cl 14.1.
Open Evaporation Rate CalculatorFrequently Asked Questions
Straight answers pulled from the code references and guidance above — read the relevant section for full context.
IS 456:2000 Cl 14.1 governs hot weather concreting by directing you to IS 7861 (Part 1), which defines hot weather concreting as placement temperature exceeding 40°C. Treat 40°C as a control limit, not an ideal target — many project specifications require lower placement temperatures (commonly 32–35°C), and some large/mass pours are limited to 27°C to control heat of hydration.
The figure is not a comfort number — it's the threshold above which cement hydration accelerates, slump loss steepens, plastic-shrinkage risk rises, and 28-day strength variation grows. Many project specifications require lower placement temperatures (32–35°C), and mass pours are commonly limited to 27°C, so the pour should be planned well below the boundary rather than up to it.
IS 7861 (Part 1):1975 defines hot weather concreting as any operation at atmospheric temperatures above 40°C, or where the concrete at placement is expected to exceed 40°C (Clauses 2.1, 6.1, 7.1 & 7.4). Aggregate, water and cement shall be kept at the lowest practical temperatures to hold the concrete below 40°C at placement.
ACI 305R's evaporation chart treats evaporation greater than 1.0 kg/m²/hr as the danger threshold for plastic-shrinkage cracking. Under 0.5 kg/m²/hr calls for normal precautions, 0.5–1.0 kg/m²/hr calls for cooling or windbreaks, and above 1.0 kg/m²/hr calls for special precautions — fog mist, ice, or postpone.
Typical steps: chill mixing water to 5°C or use ice as part of the mix water; shade aggregate stockpiles and sprinkle with cool water; use chilled washed aggregate kept saturated surface-dry; use PPC/PSC or fly ash/slag to lower heat of hydration; use retarders to extend workability; and schedule the pour in the early morning (6:00 AM–10:00 AM) or night window (7:00 PM–6:00 AM) rather than the 10:00 AM–4:00 PM peak-heat period.
IS 456:2000 Cl 13.5.1 sets a minimum curing period of 7 days for OPC under normal exposure and 10 days for blended cement (PPC/PSC/slag). Project specifications commonly extend this to 14 days for severe or extreme exposure conditions.
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years of experience in construction QA/QC and concrete technology. NRMCA Certified Trainer and ISO Lead Auditor.