Urban Residential Foundation Slab in Mumbai, India

Case Study 1: Urban Residential Foundation Slab in Mumbai, India

Scenario

A 12-storey reinforced concrete residential building is under construction in Mumbai’s high-humidity coastal zone. The foundation slab must resist sulfate-rich groundwater (exposure class XS1 per IS 456:2000) and accommodate tight site logistics — limited on-site storage and no access for large batching plants. Local sand has moderate silt content (<5%), and crushed basalt coarse aggregate (20 mm nominal size) is sourced within 40 km. The project mandates minimum 25 MPa strength at 28 days but requires reduced permeability; hence a lower water–cement ratio is prioritized despite higher cement cost.

Given Data

• Target compressive strength: 25 MPa
• Water–cement ratio: 0.45 (selected to limit chloride ingress and improve durability)
• Fine aggregate content: 650 kg/m³ (adjusted upward to compensate for coarser local sand grading)
• Coarse aggregate content: 1150 kg/m³ (reduced slightly to maintain workability with lower w/c)
• Density of concrete: 2380 kg/m³ (measured from trial batch with local aggregates)

Calculation

Using the Concrete Mix Ratio Estimator:

1. Cement content = (density − fine_aggregate_content − coarse_aggregate_content) / (1 + water_cement_ratio)
   = (2380 − 650 − 1150) / (1 + 0.45) = 580 / 1.45 ≈ 400.0 kg/m³
2. Water content = cement_content × water_cement_ratio = 400.0 × 0.45 = 180.0 kg/m³
3. Mix ratio = Cement : Fine Aggregate : Coarse Aggregate : Water
   = 400.0 : 650.0 : 1150.0 : 180.0 → normalized to cement = 1:
   = 1 : (650/400) : (1150/400) : (180/400) = 1 : 1.63 : 2.88 : 0.45

Result and Decision

The estimator yielded a cement content of 400.0 kg/m³ — satisfying IS 456 minimum (320 kg/m³ for XS1) and providing adequate paste volume for durability. A trial mix confirmed slump of 75 mm (acceptable for pump-fed slab placement) and 28-day strength of 28.4 MPa. The team adopted this mix, specifying Portland Pozzolana Cement (PPC) Type I to further enhance sulfate resistance without altering the ratio.

Lesson

Durability-driven constraints (e.g., exposure class) often necessitate deviating from standard w/c ratios — but the estimator quantifies the resulting cement and water demands transparently, preventing under-dosing that compromises long-term performance.