Before any column or wall is built, there's a simple yet essential concrete layer placed at the bottom of the excavation. This layer is called "Blinding Concrete" or Plain Concrete (P.C) - and it's the first element cast in any building. In this article, we'll cover everything you need to know to execute it correctly.
What is Blinding Concrete?
Blinding concrete is a layer of unreinforced plain concrete cast directly on the soil at the bottom of excavation, beneath reinforced foundations (footings or raft). It's also known as:
- Blinding Concrete - Because it "blinds" and covers the soil
- Lean Concrete - Due to its relatively low cement content
- Mud Mat - Common in some countries
- Plain Concrete (P.C) - Standard technical term
💡 Important Note
Some confuse "Blinding Concrete" (plain concrete under foundations) with "Batter Board (Khanzira)" (wooden frame for setting out axes). This article covers blinding concrete only. To learn more about batter boards, click here.
Why Do We Need Blinding Concrete?
Some might ask: Why not cast reinforced foundations directly on soil? The answer is that blinding concrete serves several vital functions:
1. Providing a Level Working Surface
Natural soil is never perfectly level even after grading. Blinding concrete provides a perfectly horizontal surface that makes working easier and allows precise reinforcement placement.
2. Protecting Reinforcement Steel
Without blinding concrete, the bottom reinforcement of footings would contact the soil directly, exposing it to moisture and corrosion. Blinding concrete ensures adequate concrete cover.
3. Preventing Cement Paste Loss
When casting reinforced foundations, blinding concrete prevents the cement paste (water and cement) from seeping into the soil, maintaining concrete quality.
4. Facilitating Formwork and Reinforcement Work
The hard and level surface makes installing formwork and placing reinforcement chairs at the correct height much easier.
5. Verifying Excavation Levels
Blinding concrete helps confirm correct excavation levels, as measurements are taken immediately after casting.
Technical Specifications
| Item | Specification | Notes |
|---|---|---|
| Thickness | 10 - 15 cm | 10 cm most common for normal buildings |
| Concrete Grade | C15 (15 MPa) | Sometimes C10 for small projects |
| Mix Ratio | 1 : 3 : 6 | Cement : Sand : Gravel |
| Aggregate Size | Up to 40 mm | Can use relatively large aggregate |
| Extension Beyond Footing | 10 - 15 cm each side | To facilitate formwork installation |
✅ Site Tip
For large projects, ready-mix concrete is preferred to ensure consistent quality. For small projects, site mixing is acceptable if proper mix ratios are maintained.
Execution Steps
Verify Excavation Level
Before anything else, confirm the excavation bottom is at the correct level using a dumpy level or total station. Record readings in a report.
Clean Excavation Bottom
Remove any loose materials, wet mud, or large stones. The bottom should be clean and compact.
Compact if Necessary
If the soil is loose or has been backfilled, compact it well with a compactor before casting.
Water Spraying
Spray the soil with water just before casting to prevent it from absorbing concrete water. Don't flood it, just moisten it.
Place Side Forms
Wooden boards can be used on the sides to define casting boundaries and ensure the required thickness.
Pour Concrete
Pour and spread concrete with trowels and wooden screed. Ensure the entire area is covered with uniform thickness.
Leveling
Level the surface using a screed board. No smooth finish is required, but it must be level.
Curing
Cover the concrete with wet burlap or plastic, and spray with water for at least 3 days.
Common Mistakes and How to Avoid Them
⚠️ Mistake #1: Casting on Uncompacted Soil
Result: Uneven settlement and foundation cracks.
Solution: Ensure proper soil compaction before casting, especially in backfill areas.
⚠️ Mistake #2: Thickness Less Than Required
Result: Inadequate protection for footing reinforcement.
Solution: Use side forms and carefully control levels.
⚠️ Mistake #3: Not Extending Beyond Footing Boundaries
Result: Difficulty installing formwork.
Solution: Cast blinding concrete 10-15 cm beyond footing dimensions on all sides.
⚠️ Mistake #4: Leaving Blinding Concrete Without Curing
Result: Surface cracking and weak concrete.
Solution: Water curing for 3-7 days.
Inspections and Handover
Before starting reinforced footing work on the blinding concrete, verify:
- Level: Confirm the blinding surface level matches the design.
- Flatness: Use a level to check surface evenness.
- Thickness: Take measurements at multiple points to confirm required thickness.
- Dimensions: Ensure blinding extends sufficiently beyond footing boundaries.
- Setting: Wait until concrete has set adequately (24-48 hours) before working on it.
✅ Handover Documentation
Ensure blinding concrete acceptance is documented in an official report signed by the supervising engineer, including: date, levels, thickness, and inspection notes.
Next Element in the Series
🔜 Coming Up: Reinforced Footings
After blinding concrete has set, we move to the next phase: reinforced footings (isolated or combined). We'll cover: footing types, reinforcement detailing, casting, and common mistakes.
Quick Summary
- Blinding concrete is the first concrete element cast in a building
- Typical thickness: 10 cm, Grade: C15
- Functions: Level surface, reinforcement protection, preventing cement paste loss
- Extends 10-15 cm beyond footing boundaries on each side
- Water curing is essential for at least 3 days
🧪 Test Your Knowledge
1. What is the common thickness for blinding concrete in normal buildings?
2. Why should blinding concrete extend beyond footing boundaries?
3. What is the common concrete grade for blinding concrete?