Plain Cement Concrete (PCC) in Foundations: Ratio, Grades and Site Procedure

Plain cement concrete (PCC) is concrete made of cement, sand, coarse aggregate and water with no steel reinforcement. In foundation work it is laid as a thin bed, usually 75 to 150 mm thick, under footings and floors, and the usual PCC ratio is 1:4:8 (about M7.5) or 1:3:6 (about M10) by volume. Richer 1:2:4 (about M15) is used where the plain concrete itself carries load or faces moderate exposure.

What is plain cement concrete?

Plain cement concrete is concrete used without any reinforcing bars. Its tensile strength is only about one tenth of its compressive strength, so it is used only where it is squeezed, not bent or pulled. On Indian sites it is also called lean concrete, levelling course, blinding or mud mat. PCC in construction shows up as the bed under footings, rafts and plinth beams, the base course under floors, mass fill in trenches, and sills and copings.

Why PCC work goes under a foundation

PCC work in foundation construction is not counted in the structural design; the footing is designed as if it sat on the soil. The bed is there for construction quality:

• Level working surface. A PCC bed gives a flat, true surface at founding level, so the footing thickness and cover come out as drawn.
• Clean base for steel fixing. Bars and cover blocks sit on hard concrete instead of mud, and the footing outline can be marked on it.
• Protects the fresh footing concrete. Without a bed, dry soil sucks water and paste out of the bottom of the footing and loose soil mixes in, weakening the layer that holds the main steel.
• Separates reinforcement from the soil. It keeps bars out of the ground and away from soil moisture and salts. It is not a substitute for cover: IS 456:2000 clause 26.4.2.2 still asks for a minimum nominal cover of 50 mm to the main steel in footings.

PCC ratio: common nominal mixes and grades

On site, PCC is almost always a nominal mix batched by volume, written as cement : sand : coarse aggregate. The grades below are the ones conventionally paired with each ratio in Indian practice. They are targets, not guarantees, since a volume-batched nominal mix is not designed or tested for strength.

Coarse aggregate is normally 20 mm or 40 mm nominal size; 40 mm suits thick beds and mass fill.

What IS 456:2000 actually says about PCC

Many notes claim IS 456 gives M7.5 = 1:4:8. It does not. Three clauses matter.

Table 9: nominal mix proportions by mass, not volume

Clause 9.3 allows nominal mix concrete for M20 or lower. Table 9 gives, for each grade, the maximum total mass of dry aggregate (fine plus coarse) per 50 kg of cement and the maximum water per 50 kg of cement. It does not print any 1:x:y volume ratio.

The fine to coarse aggregate proportion is generally 1:2 by mass, with the sand share reduced as the sand gets finer and the coarse aggregate gets larger.

The site volume ratios roughly match these masses. One 50 kg bag is about 0.0347 m³ (50 ÷ 1440), so 1:4:8 means 0.139 m³ of sand and 0.278 m³ of aggregate per bag. At assumed loose bulk densities of 1550 and 1450 kg/m³, that is about 215 + 403 = 618 kg, just inside the 625 kg limit for M7.5. Real bulk densities vary with moisture and grading, so treat the match as approximate.

Table 5: minimum grade of plain concrete by exposure

So lean M5 to M10 beds fit only where the plain concrete is in mild exposure or is treated as a non-structural levelling layer. IS 456 Table 3 puts concrete buried in non-aggressive soil in moderate exposure, which is why many consultants specify M10 or M15 beds on wet or saline ground. The project drawings decide.

Clause 13.5.1: curing period

Exposed surfaces must be kept continuously damp for at least 7 days with ordinary Portland cement, and at least 10 days where mineral admixtures or blended cements (PPC, PSC) are used. In dry and hot weather the minimum is 10 days, and for blended cements the code recommends extending to 14 days.

How thick is PCC under a footing?

IS 456 does not fix a thickness for a levelling bed. Typical site practice:

• Under footings and rafts: 75 to 100 mm, commonly 100 mm, projecting about 100 to 150 mm beyond the footing on each side so the formwork has somewhere to sit.
• Under ground-floor flooring: 75 to 150 mm over compacted fill.
• Soft or wet ground: 150 mm or more, sometimes over brick or rubble soling.

PCC work in foundation: step-by-step site procedure

1. Excavate to founding level. Trim the last 50 to 75 mm by hand so the bucket does not loosen the bottom. Remove loose soil, roots and standing water.
2. Check levels and bearing. Confirm the level against the drawing. Report soft pockets to the engineer instead of backfilling them with loose soil.
3. Compact the base. Ram or plate-compact the lightly wetted bottom. On weak ground, lay brick or stone soling if specified. Apply anti-termite treatment to the soil at this stage if specified (IS 6313 Part 2).
4. Fix shuttering or level pegs to the PCC outline, with pegs set at the finished top of PCC.
5. Batch and mix. Measure cement by whole bags and sand and aggregate in a measuring box (about 35 litres matches one bag). Allow for bulking of damp sand, and add only enough water for a workable, non-sloppy mix. Extra water to ease spreading is the most common reason lean beds end up far below grade.
6. Place and compact. Place within about 30 minutes of mixing, spread and tamp or screed-vibrate to the pegs. Leave the surface slightly rough.
7. Cure. Keep damp with wet hessian or ponding, per clause 13.5.1 above. A dry, dusty bed draws water out of the footing concrete cast on it.

Worked example: materials for PCC 1:4:8

Problem. Find the cement, sand and coarse aggregate needed for 1 m³ of PCC 1:4:8, then for the bed under one footing of 1.5 m x 1.5 m with a 150 mm projection on each side and 100 mm thickness.

Step 1, dry volume. Loose dry materials shrink in volume once mixed, because sand fills the voids in the aggregate and cement fills the voids in the sand. Site practice multiplies the wet volume by a dry volume factor of about 1.54 (1.52 to 1.57 are also used). This is a site convention, not a code value.

Dry volume = 1 m³ x 1.54 = 1.54 m³

Step 2, sum of ratio parts. 1 + 4 + 8 = 13

Step 3, cement.

• Volume = 1.54 x 1/13 = 0.1185 m³
• Mass = 0.1185 m³ x 1440 kg/m³ = 170.6 kg
• Bags = 170.6 ÷ 50 = 3.41 bags (buy 3.5 per m³)

Step 4, sand. 1.54 x 4/13 = 0.474 m³ (x 35.31 = 16.7 cft)

Step 5, coarse aggregate. 1.54 x 8/13 = 0.948 m³ (x 35.31 = 33.5 cft)

Step 6, water limit. Table 9 caps water for M7.5 at 45 litres per 50 kg of cement, so the maximum is 3.41 x 45 = 153.5 litres per m³. Use less if the mix is already workable.

Step 7, one footing. PCC size = (1.5 + 2 x 0.15) x (1.5 + 2 x 0.15) x 0.10 = 1.8 x 1.8 x 0.10 = 0.324 m³

• Cement = 0.324 x 170.6 = 55.3 kg = 1.11 bags
• Sand = 0.324 x 0.474 = 0.154 m³
• Aggregate = 0.324 x 0.948 = 0.307 m³

Twelve such footings need 12 x 0.324 = 3.89 m³ of PCC and about 13.3 bags of cement, plus 2 to 5 per cent for wastage.

PCC vs RCC

If a member bends, cantilevers or spans, it must be RCC. PCC is only for beds, fills and members in compression on firm support.

The code is available from the Bureau of Indian Standards (IS 456 Concrete), and open-access research on concrete is published in the International Journal of Concrete Structures and Materials. If you are studying for a diploma in civil engineering, the quantity method above is the one used in estimating and costing papers.

FAQs

What is the ratio of PCC under a footing?

The most common PCC ratio under footings is 1:4:8 (cement : sand : coarse aggregate by volume), taken as about M7.5. 1:3:6 (about M10) is used where the specification or wetter ground calls for a richer bed, and 1:5:10 (about M5) for lean fill on firm, dry ground.

Does IS 456 specify 1:4:8 for M7.5?

No. IS 456:2000 Table 9 gives nominal mixes by mass: the maximum total dry aggregate per 50 kg of cement (625 kg for M7.5, 480 kg for M10, 330 kg for M15) and the maximum water. Volume ratios such as 1:4:8 and 1:3:6 are site conventions that roughly match those masses.

What is the thickness of PCC in foundation?

IS 456 does not fix it. Site practice is 75 to 100 mm under footings and rafts, commonly 100 mm, projecting about 100 to 150 mm beyond the footing on each side. Under floors it is 75 to 150 mm, and more on soft ground.

How many bags of cement are needed for 1 m³ of PCC 1:4:8?

About 3.41 bags. Dry volume = 1 x 1.54 = 1.54 m³; cement = 1.54/13 = 0.1185 m³; mass = 0.1185 x 1440 = 170.6 kg; bags = 170.6/50 = 3.41. The same method gives 0.474 m³ of sand and 0.948 m³ of 20 or 40 mm aggregate.

How many days should PCC be cured?

IS 456:2000 clause 13.5.1 requires at least 7 days of moist curing for ordinary Portland cement and at least 10 days for blended cements such as PPC or in dry, hot weather.

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