Why is the upper brick wall's centre-line length shorter than the lower's?

Its outer face is set in by 0.15 m and it is thinner (0.23 m), so the centre line moves inward

Why is the top slab not counted in the M20 quantity?

It is M25 RCC, a different grade, measured separately

It is excluded because it is thin

Which check confirms the reading of the figure?

1.99 + 2 x (0.38 + 0.125) = 3.00 m

2080 Bhadra · 4+3+1 marks

Underground RCC tank: brickwork 1:4, M20 PCC, steel

Short answer

Brickwork in two steps by centre-line method:3.199 m³. M20 concrete for floor and lining wall totals1.446 m³. Steel at1% gives113.5 kg.

Drawing of an Underground RCC water tank is given below (Fig. 1). Prepare detailed estimate for the following items of works: i) Brickwork (1:4) ii) PCC for M20 RCC work iii) Steel reinforcement for M20 RCC work (assuming 1% steel) [Figure 1: plan 3000 mm × 2300 mm outside; brick wall 230 mm thick at top stepping to 380 mm at base with 150 mm offset; clear inside 1990 mm × 1290 mm; M20 RCC walls/floor 125 mm; 125 mm thick M25 RCC slab at top; section: 800 mm and 475 mm wall heights, overall 1450 mm, clear height 1150 mm; brickwork (1:4); PCC (1:3:6) base with flat brick soling. All dimensions in mm.]

Reading of Fig. 1

Outside size at base $3.00 \times 2.30$ m; clear inside $1.99 \times 1.29$ m. Wall at the base is $0.38$ m of brick plus $0.125$ m of M20 RCC, so $1.99 + 2 \times (0.38 + 0.125) = 3.00$ m and $1.29 + 2 \times 0.505 = 2.30$ m (check).
Brick wall: $0.38$ m thick for the lower $0.475$ m, stepping in by $0.15$ m on the outer face to $0.23$ m for the upper $0.80$ m (total brick height $1.275$ m = clear height $1.15$ m + floor $0.125$ m).
Inside the brick: 125 mm M20 RCC lining wall, $1.15$ m high, and 125 mm M20 RCC floor. Top slab 125 mm M25 RCC (not part of the M20 quantity).

i) Brickwork (1:4), centre-line method

Part	Centre-line length (m)	Thickness (m)	Height (m)	Volume (m³)
Lower	$2\,[(3.00-0.38)+(2.30-0.38)] = 9.08$	0.38	0.475	1.639
Upper	$2\,[(3.00-0.53)+(2.30-0.53)] = 8.48$	0.23	0.800	1.560
Total				3.199

The upper wall's outer face is set in by $0.15$ m on each side, so its centre-line rectangle is $(3.00 - 0.30 - 0.23) \times (2.30 - 0.30 - 0.23) = 2.47 \times 1.77$ m.

ii) Concrete for M20 RCC work

Member	Working	Volume (m³)
Floor	$(1.99 + 0.25)(1.29 + 0.25) \times 0.125$	0.431
Lining wall	$2\,[(1.99+0.125)+(1.29+0.125)] \times 0.125 \times 1.15 = 7.06 \times 0.125 \times 1.15$	1.015
Total M20		1.446

(For information, the M25 top slab is $2.70 \times 2.00 \times 0.125 = 0.675$ m³.)

iii) Steel in M20 RCC (1%)

$W = 0.01 \times 1.446 \times 7850 = 113.5$ kg

Answer: brickwork (1:4) = 3.199 m³; M20 concrete = 1.446 m³; steel = 113.5 kg.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Underground RCC tank: brickwork 1:4, M20 PCC, steel

Reading of Fig. 1

i) Brickwork (1:4), centre-line method

ii) Concrete for M20 RCC work

iii) Steel in M20 RCC (1%)

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