Why is the thickness of the cover a key factor in concrete construction? Today, let’s try to analyze its technical principles and construction essentials, as well as common construction measures and knowledge of common problems and their prevention.
I. Technical principles
Reinforcing steel corrosion prevention
The highly alkaline environment of concrete (pH ≥ 12.5) forms a passivation film on the surface of the reinforcing steel to prevent oxidation. When the thickness of the protective layer is insufficient, CO₂ and Cl⁻ penetrate more quickly, leading to the destruction of the passivation film and causing corrosion expansion (a 2-4 times increase in volume), which in turn causes the concrete to crack.
When the thickness of the protective layer is ≥ the diameter of the steel bar, the bond strength is increased by 30% to 50%, ensuring effective stress transfer. If it is too thin, bond slippage is likely to occur, affecting the integrity of the structure.
Structural fire resistance
A 50mm protective layer can increase the fire resistance of beams and slabs to 2 hours (at room temperature), and delay the softening of steel bars at high temperatures (50% strength loss at 500°C).
2. Key points of construction
Thickness control
Use high-strength plastic spacers (strength ≥ C35), spacing ≤ 600mm, ≥ 4 per m² at the bottom of beams/slabs; use fixed clamps to fix the steel cage on frame columns.
Material selection
Concrete with impermeability ≥P6 is preferred, with a water-cement ratio ≤0.45 and cementitious material ≥320kg/m³.
Formwork construction
Use double-sided film-covered wooden formwork, with 3mm thick sponge adhesive strips affixed to the joints, and support spacing ≤500mm (when slab thickness ≤150mm).
Pouring process
Pour in layers with a thickness of ≤400mm, with the vibrator inserted at a distance of ≤300mm, and inserted quickly and withdrawn slowly (vibrate for 20-30 seconds at each point).
Curing management
Immediately after final setting, cover with a geotextile and spray for curing for ≥14 days (when the daily average temperature is below 5°C, use a film for curing instead).
3. Common construction measures
Beam-column joint control: Use pre-welded annular positioning ribs (Φ12mm) at a spacing of 800mm to fix the position of the main reinforcement.
Overhanging member treatment: Set Φ10mm steel stirrups below the negative moment reinforcement at a spacing of 300mm in a plum blossom pattern.
Mass concrete: Install temperature measurement points (spacing ≤5m), and activate the circulating water cooling system when the temperature difference between inside and outside exceeds 25℃.
4. Innovative processes
3D laser scanning technology: point cloud comparison is carried out before the installation of precast components to control the thickness deviation of the protective layer within ±2mm.
Self-healing concrete: incorporating microbial capsules (Bacillus subtilis), it can heal itself when the crack width is ≤0.3mm.
