Foundation Concrete: Which Cement, Which Mix and How to Protect It
Foundations are buried, hard to inspect and expensive to repair. Getting cement type, mix and protection right the first time is essential.
For most foundations, PPC or PSC is preferred because their lower heat of hydration and denser concrete resist sulphates and moisture in soil better than plain OPC; SRC is used where soil tests show high sulphates. Use a lean PCC bed (typically M10 or M15) under footings, and RCC of at least M20 — M25 or higher where the soil or groundwater is aggressive — with the cover and grade your engineer specifies, good compaction and at least 10 days of curing.
What makes foundations different?
Foundations sit in soil and groundwater that may contain sulphates, chlorides and moisture year-round. They are massive, so heat of hydration matters, and once backfilled they are almost impossible to inspect or repair.
Choosing the cement
| Condition | Preferred cement |
|---|---|
| Normal soil, no aggressive chemicals | PPC, PSC or OPC 43 |
| Thick rafts / large footings | PPC or PSC (lower heat) |
| Moderate sulphates | PPC or PSC with dense concrete |
| High sulphates (by soil test) | SRC or as designed — see SRC guide |
| Coastal / saline soil | PSC — see coastal construction |
Choosing the mix
- PCC bed under footings: typically M10 (1:3:6) or M15 (1:2:4), 75–150 mm thick, to give a clean, level base and protect reinforcement from soil.
- RCC footings, rafts and plinth beams: at least M20, and M25 or more in moderate or worse exposure, as per IS 456 and your drawing. See concrete grades.
Construction tips
- Excavate to firm strata and remove loose soil. Get the engineer to inspect the founding level.
- Dewater — never place concrete in standing water without approval.
- Lay the PCC bed and let it set before fixing steel.
- Cover: Use cover blocks so bottom steel has the specified cover (often larger for foundations in contact with soil).
- Compact thoroughly — honeycombs in footings let water reach steel.
- Low water-cement ratio — see water-cement ratio.
- Cure for at least 10 days (blended cements) before backfilling, or as specified.
- Backfill in layers with proper compaction; avoid damaging fresh concrete.
Waterproofing foundations and plinth
- Apply bituminous or other waterproof coating to foundation surfaces in contact with soil where specified.
- Provide a damp-proof course (DPC) at plinth level to stop rising damp.
- Ensure anti-termite treatment before PCC and around foundations.
How much cement for foundations?
Foundations and plinth often take a quarter or more of a house's cement. See cement for a 1000 sq ft house.
Key takeaways
- PPC or PSC suits most foundations; SRC where sulphates are high.
- PCC bed of M10/M15, RCC of at least M20 (M25+ in aggressive soils).
- Compact well, keep cover, cure 10+ days and waterproof.
Frequently asked questions
Which cement is best for foundations?
PPC or PSC is generally preferred for its lower heat and better resistance to sulphates and moisture. SRC is used where soil tests show high sulphate levels.
What grade of concrete is used for footings?
At least M20 for RCC footings, and M25 or higher where soil or groundwater is aggressive. PCC under footings is usually M10 or M15.
What is the thickness of PCC under a footing?
Typically 75 to 150 mm, as shown in the foundation drawing.
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This guide is general information for planning and buying. Quantities, grades and methods for your structure must follow your approved drawings, your engineer’s specification and the applicable Indian Standards.
