RCC Beam with Excess Deflection, Flexural Cracks and a Late Load Addition
A G+4 office block in Bhopal uses RCC beam–slab floors to M25/Fe500. One interior beam is 300 mm wide × 600 mm deep with an 8.0 m clear span; it carries a 150 mm slab (tributary width 4.0 m) and a 230 mm brick partition along part of its length. Six months after casting, a joint inspection records 26 mm midspan deflection by level survey against a design prediction of 18 mm, and three flexural cracks 0.25–0.30 mm wide rising about 200 mm up the web at midspan. Cover-meter readings, bar spacing and cube results (27.5 MPa) all match the drawings, and no construction overloading is documented. The contractor proposes pressure-grouting the cracks with epoxy and handing over, arguing that 26 mm is below span/250. Separately, the architect now wants a heavier floor finish adding 1.2 kN/m² on all floors, with the remaining two floors cast using the same beam section as approved.
1. Is the measured deflection acceptable? State the governing serviceability limit, not the contractor's limit, and justify it.
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The contractor is quoting the wrong criterion. Span/250 (= 8000/250 = 32 mm) is the general limit for a beam that carries no deflection-sensitive elements. This beam supports a 230 mm brick partition, and for such members the deflection limit is the lesser of span/350 and 20 mm.
- span/350 = 8000/350 = 22.9 mm
- Absolute limit = 20 mm → 20 mm governs
Measured 26 mm already exceeds 20 mm at six months, before the full creep and shrinkage component has developed; long-term deflection is typically 1.5–2.0 times the short-term value under sustained load. So the answer is not acceptable, and grouting does not address it.
The partition itself is at real risk of cracking, and the differential deflection between this beam and the adjacent parallel beam will open a crack at the partition soffit. The deflection should be recomputed with an effective moment of inertia based on the measured cube strength, actual sustained load (finish 1.5, partition, slab, beam self-weight) and the cracking observed, to confirm the 26 mm is consistent with analysis rather than a construction defect.
2. Are the cracks flexural, shear, or material distress? What must be done before any epoxy grouting is permitted?
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Geometry first. Effective depth ≈ 600 − 40 − 10 − 10 ≈ 540 mm; for a singly reinforced section the neutral axis is of the order of 0.3d ≈ 160–200 mm. Cracks rising 200 mm from the soffit and stopping near the neutral axis, vertical and concentrated at midspan, are classical flexural cracks, not shear (shear cracks would be inclined at 30–45° near the supports) and not map-cracking from unsound cement.
Width matters: 0.25–0.30 mm sits at the IS 456 surface-crack limit of 0.3 mm for mild exposure but exceeds 0.2 mm for moderate exposure - the more likely classification for an office soffit. It is therefore marginal-to-excessive, and the crack spacing is consistent with the flexural steel that is actually provided.
Before grouting:
- Confirm the cracks are dormant - re-measure widths over 4–8 weeks with a demec gauge or crack monitor.
- Check for corrosion staining and cover to the bottom bars.
- Verify the deflection cause (props removed early, curing, actual sustained load, partition weight).
- Do not accept grouting as a handover remedy; it restores stiffness slightly but does not correct a deflection that already exceeds the limit.
3. Should the additional 1.2 kN/m² floor finish be permitted with the same beam section? Support your answer with numbers.
Read the worked answer
Estimate the existing service load on the beam per metre: slab 0.15 × 25 × 4.0 = 15.0 kN/m; finish 1.5 × 4.0 = 6.0; live load 4.0 × 4.0 = 16.0; partition ≈ 4.0; beam self-weight 0.3 × 0.6 × 25 = 4.5. Total ≈ 45.5 kN/m, of which the sustained part is about 29.5 kN/m.
The extra finish adds 1.2 × 4.0 = 4.8 kN/m to the sustained load - a 16% increase in sustained load and an 11% increase in total load. Flexure is not the constraint: ΔM = 4.8 × 8²/8 = 38.4 kN·m against Mu,lim ≈ 0.138 × 25 × 300 × 540²/10⁶ ≈ 302 kN·m.
Deflection is the constraint. Deflection under sustained load scales roughly with that load, so 26 mm × 1.16 ≈ 30 mm, plus the remaining creep - about 50% above the 20 mm limit, with correspondingly wider cracks and greater risk to the partitions.
Decision: refuse. Ask for a lighter screed (75 mm instead of 100 mm saves ≈0.5 kN/m²), and for the two uncast floors either increase the depth or provide a designed compression/bottom plate with a fresh serviceability check - not an unanalysed repeat of the same section under higher load.