Casting Concrete Slabs at Temperatures Above 40°C: A Comprehensive Guide
✍️ By Rehan A. Qamar, Civil Engineer
Contents
- 1. Introduction
- 2. Understanding High Temperature Effects on Concrete
- 3. Why 40°C is a Critical Threshold
- 4. Thermal Properties of Steel and Aluminum Shuttering
- 5. Primary Issues Faced During Slab Casting in Heat
- 6. Methods to Avoid Shrinkage Cracks
- 7. The Role of Formwork Temperature in Concrete Cracking
- 8. Special Precautions with Aluminum Formwork
- 9. Internal Curing: A Game Changer
- 10. Water-Cement Ratio and Slump Management
- 11. Curing Strategies in Hot Climates
- 12. Practical Site Measures and Checklists
- 13. Case Studies
- 14. Recommended BIS/ACI/IS Standards
- 15. Conclusion
1. Introduction
When temperatures soar above 40°C, casting concrete slabs becomes a high-stakes operation. Extreme heat accelerates moisture loss, causes plastic shrinkage cracks, and severely affects the hydration process. This guide offers practical, research-backed methods to ensure slab casting remains safe and crack-free under extreme weather conditions.
2. Understanding High Temperature Effects on Concrete
- Accelerated setting time
- Increased evaporation
- Risk of plastic and thermal shrinkage
- Reduced long-term strength if improperly cured
3. Why 40°C is a Critical Threshold
According to IS 7861 and ACI 305R, temperatures above 40°C require special precautions. High wind speeds and low humidity further aggravate the risk of cracking.
4. Thermal Properties of Steel and Aluminum Shuttering
| Material | Thermal Conductivity (W/m·K) | Comment |
|---|---|---|
| Concrete | 1.5 | Low |
| Steel | 50 | High |
| Aluminum | 205 | Very High |
5. Primary Issues Faced During Slab Casting in Heat
- Rapid moisture loss
- Plastic shrinkage and thermal cracks
- Hot shuttering accelerating surface drying
- Reduced workability and finishing difficulties
6. Methods to Avoid Shrinkage Cracks
a. Surface Evaporation Control
- Evaporation retardants (aliphatic alcohols)
- Covering with wet burlap or plastic sheets
- Minimizing delays after placing
b. Retarders and Admixtures
- Use of set-retarding admixtures (IS 9103)
- Superplasticizers to maintain slump
c. Timing of Concreting
Cast between 2 AM and 6 AM. Avoid daytime heat.
d. Continuous Mist Spraying
Install misting lines and spray regularly before and after pour.
e. Use of Polypropylene Fibers
0.9 kg/m³ dosage helps reduce plastic shrinkage and microcracks.
7. The Role of Formwork Temperature in Concrete Cracking
- Formwork can exceed 70°C
- Heat transfer can initiate internal cracking
Mitigation: Water-cool shuttering before pour, shade with white cloth, and avoid direct sunlight.
8. Special Precautions with Aluminum Formwork
- High thermal conductivity leads to rapid heating
- Spray shutters every 30 min before casting
- Apply shutter oil just before pour
9. Internal Curing: A Game Changer
Use saturated lightweight aggregates or hydrogels to retain moisture within the concrete mass.
10. Water-Cement Ratio and Slump Management
- Maintain w/c between 0.40 and 0.45
- Slump target: 100–125 mm
- No re-tempering of concrete at site
11. Curing Strategies in Hot Climates
| Method | Application Timing |
|---|---|
| Ponding | After 6–8 hrs of placement |
| Misting | Before, during, and after casting |
| Wet Covering | Immediately after finishing |
| Curing Compounds | Sprayed on surface post-setting |
12. Practical Site Measures and Checklists
Before Casting:
- Mist shuttering
- Cover with jute or white cloth
- Shift casting time to early morning
During Casting:
- Continuous pour, no delays
- Immediate finishing and curing
After Casting:
- Apply wet burlap or mist
- Protect from wind and direct sun
13. Case Studies
📍 Nagpur Slab Casting at 44°C
Problem: Shrinkage cracks occurred
Fix: Night casting + misting
Result: No cracks on second pour
📍 Dubai High-Rise, 43°C, Aluminum Shuttering
Issue: Surface hot and concrete hardened fast
Fix: Reflective tarps + misting
Result: 6°C drop in formwork temperature
14. Recommended Standards and Codes
- IS 7861 (Part I) – Hot Weather Concreting
- IS 456:2000 – General Concrete Code
- IS 9103 – Admixtures
- ACI 305R – Hot Weather Concreting
- BS EN 13670 – Execution of Concrete Structures
15. Conclusion
Hot weather concreting requires proactive planning, precision, and disciplined curing. With proper use of admixtures, early morning casting, and surface protection techniques like misting, crack-free slabs are not only possible but achievable—no matter how high the mercury rises.