Cover blocks (chairs/spacers) maintain the specified clear cover between reinforcement and formwork. Choosing the right type, size, and spacing is critical — if missed, the actual cover will be wrong even if the design is perfect. Concrete cover blocks (cast on-site) are standard; plastic spacers are popular for slabs; circular cover blocks for columns.
Cover Block Types
| Type | Size | Use | Spacing | Note |
|---|
| Cement-mortar (cast on site) | 40×40×x mm or as required | All members; most common in India | — | Cast 1:3 cement:sand, dome shape, embed wire |
| Plastic snap-on (slab) | 15/20/25/30/40/50 mm cover | Slab top steel, beam bottom | — | Dome shape; quick install; check QA |
| PVC ring (column) | 20–75 mm cover (multi-position) | Column main bars | — | Wraps around bar; multiple cover sizes per piece |
| Concrete circular (column) | Dia × cover thickness | Column ring, raft | — | Cast separately; tie to bar with wire |
| Plastic chair (slab top) | 15–250 mm height | Slab top steel position | — | Holds top mat above bottom mat |
| Steel chair (custom) | Bent 10/12 mm bar | Heavy slabs, rafts | — | Site-fabricated; can rust if exposed |
| Cementitious dome (footing) | 75×75×50 mm bottom | Below footing reinforcement | — | Heavy duty for thick mats |
Recommended Cover Block Spacing
Maximum spacing in each direction; closer for heavy steel mats
| Type | Size | Use | Spacing | Note |
|---|
| Slab — bottom steel | — | Cement or plastic spacer | ≤ 750 mm c/c each direction | Or 1 m max for thin slabs |
| Slab — top steel chairs | — | Plastic / steel chair | ≤ 600 mm c/c | More if heavy mat — chairs sag |
| Beam — bottom bars | — | Cement spacer | 1 m c/c along beam | Min 2 per beam |
| Beam — side cover | — | Cement spacer | 1 m c/c on each side | — |
| Column — main bars | — | PVC ring or concrete spacer | ≤ 1 m vertically, all 4 sides | Min 1 set per 1 m height |
| Footing — bottom steel | — | Cement dome 50–75 mm | ≤ 1 m c/c each direction | Heavy steel mat needs more |
| Wall (shear wall) | — | Cement or PVC spacer | ≤ 1 m c/c each face | Both faces |
Cover Block Size by Cover Required
| Type | Size | Use | Spacing | Note |
|---|
| Cover 15 mm | 15 mm thick block | Slabs (mild exposure) | — | Internal slabs not exposed to weather |
| Cover 20 mm | 20 mm thick block | Slabs (general) | — | Most common slab cover |
| Cover 25 mm | 25 mm thick block | Beams, columns (mild) | — | — |
| Cover 30 mm | 30 mm thick block | Beams (moderate exposure) | — | External beams |
| Cover 40 mm | 40 mm thick block | Columns (moderate), footings (raft) | — | — |
| Cover 50 mm | 50 mm thick block | Footings (PCC under), severe exposure | — | — |
| Cover 75 mm | 75 mm thick block | Footings (no PCC), water-retaining | — | Bottom of footing in soil |
Cover Block Quality Requirements
| Type | Size | Use | Spacing | Note |
|---|
| Cement spacer mix | — | Min M20 strength (1:1.5:3 nominal) | — | Same or better than parent concrete |
| Cure cement spacers | — | ≥ 7 days before use | — | Hard, no crumbling |
| Plastic spacer testing | — | Compression test — must not crush under wet concrete | — | Reject brittle plastic |
| Tying wire | — | 16 SWG GI binding wire | — | Embed in cement spacer when casting |
| Reject | — | Broken, undersized, soft, oily | — | QA must check delivered batches |
Notes
• Cover blocks are the cheapest QC item with the biggest impact — never compromise
• Use the SAME cover block thickness as the design cover (don't substitute 25 mm where 30 mm is specified)
• Cement spacers must be made on-site at least 1 week in advance — air-cured properly
• Always check actual cover before pour by measuring at 4–5 random points with a steel ruler
• If cover blocks settle or move during pour, walk-down inspection is mandatory before vibration
• Plastic spacers are convenient but can leave a permanent dimple in the surface — don't use on architectural concrete
• For raft and water-retaining structures, use only cement-mortar spacers (plastic = potential leak path)
• After concrete sets, exposed cement spacers should match the parent colour — check for visible blocks at finishing
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