Rural Bridge Replacement in Assam Floodplain

Case Study 2: Rural Bridge Replacement in Assam Floodplain

Scenario A 24-m single-span reinforced concrete slab bridge is being constructed in Cachar District, Assam — a monsoon-prone floodplain with seasonal access restrictions, locally sourced aggregates of variable grading, and limited lab infrastructure. The bridge must withstand aggressive sulfate exposure from alluvial soil (SO₄²⁻ > 1500 mg/L) and cyclic wet-dry conditions. Structural requirement: M25 concrete (fck = 25 MPa) for substructure and abutments. Constraints include reliance on local sand (fineness modulus = 2.1, moisture = 4.2%), river gravel (max size 20 mm, absorption = 1.8%), and need for low-cost, field-adjustable mix design.

Given Data

• Target Strength (fck): 25 MPa
• Water-Cement Ratio (w/c): 0.42
• Cement Content (C): 320 kg/m³ (Type II Portland Pozzolana Cement, confirming sulfate resistance per IS 269)
• Fine Aggregate Content (S): 580 kg/m³ (as-received, including surface moisture)
• Coarse Aggregate Content (G): 960 kg/m³ (as-received, including absorption)
• Admixture Percentage: 0.0% (no admixture used due to supply chain unreliability and cost sensitivity)

Calculation Using the Concrete Mix Design Calculator:

• Water Content (W) = w/c × Cement Content = 0.42 × 320 = 134.4 kg/m³
• Total Volume = sum of absolute volumes (corrected for aggregate moisture and absorption):
  • Cement volume = 320 ÷ 3000 = 0.1067 m³ (PPC density ≈ 3000 kg/m³)
  • Water volume = 134.4 ÷ 1000 = 0.1344 m³
  • Fine aggregate dry mass = 580 ÷ (1 + 0.042) = 556.6 kg → volume = 556.6 ÷ 2600 = 0.2141 m³
  • Coarse aggregate dry mass = 960 ÷ (1 + 0.018) = 943.0 kg → volume = 943.0 ÷ 2650 = 0.3559 m³
  • Sum = 0.1067 + 0.1344 + 0.2141 + 0.3559 = 0.8111 m³ → air content ~2% assumed → total volume = 0.8111 ÷ 0.98 ≈ 0.83 m³ (tool rounds to two decimals)
• Concrete Mix Proportions = [Cement: 320.0, Fine Aggregate: 580.0, Coarse Aggregate: 960.0] kg/m³ (as batched — i.e., including moisture)

Result and Decision Field trials revealed that the calculated 134.4 kg/m³ water yielded stiff, non-workable concrete (slump < 20 mm) due to unaccounted surface moisture in sand. Adjusted batch water was reduced to 112.0 kg/m³ (−16.7%) after measuring actual aggregate moisture on-site. Final 28-day strength averaged 27.1 MPa (acceptable), and accelerated sulfate resistance test (ASTM C1012) showed <1.5% expansion at 180 days. The mix was standardized with mandatory moisture correction logbook entries for each delivery.

Lesson Published mix inputs assume oven-dry aggregates — in remote or resource-constrained sites, always measure and record real-time aggregate moisture before batching, and treat calculator outputs as starting points, not final prescriptions. Field verification trumps theoretical volumetric assumptions.