Rural Highway Pavement Repair in Central Queensland, Australia

Case Study 2: Rural Highway Pavement Repair in Central Queensland, Australia

Scenario

Emergency repair of a 2.3 km stretch of regional highway (State Route A6) damaged by flood-induced erosion and thermal cracking. Site is remote — 180 km from nearest batching plant — requiring mobile on-site mixing using locally available materials: river-washed sand (low fines, high uniformity) and granite coarse aggregate (14 mm max size). Ambient temperatures range from 12°C to 42°C daily; rapid turnaround (<72 h from pour to opening) is critical for regional freight movement. Design requires early-strength development (≥20 MPa at 7 days) and abrasion resistance, but 28-day target strength is set conservatively at 32 MPa to account for variable compaction in field conditions.

Given Data

• Target compressive strength: 32 MPa
• Water–cement ratio: 0.42 (optimized for early strength and low permeability)
• Fine aggregate content: 580 kg/m³ (reduced due to excellent particle packing with angular granite)
• Coarse aggregate content: 1280 kg/m³ (increased to maximize stiffness and reduce shrinkage)
• Density of concrete: 2420 kg/m³ (verified via pycnometer test on local aggregate blend)

Calculation

Using the Concrete Mix Ratio Estimator:

1. Cement content = (density − fine_aggregate_content − coarse_aggregate_content) / (1 + water_cement_ratio)
   = (2420 − 580 − 1280) / (1 + 0.42) = 560 / 1.42 ≈ 394.4 kg/m³
2. Water content = cement_content × water_cement_ratio = 394.4 × 0.42 ≈ 165.6 kg/m³
3. Mix ratio = Cement : Fine Aggregate : Coarse Aggregate : Water
   = 394.4 : 580.0 : 1280.0 : 165.6 → normalized to cement = 1:
   = 1 : (580/394.4) : (1280/394.4) : (165.6/394.4) ≈ 1 : 1.47 : 3.25 : 0.42

Result and Decision

The estimator output (394.4 kg/m³ cement, 165.6 kg/m³ water) aligned closely with field trials using Type GP cement + 2% calcium chloride accelerator. Compressive strength reached 21.3 MPa at 24 h and 34.1 MPa at 28 days — exceeding requirements. The high coarse aggregate fraction improved skid resistance and reduced plastic shrinkage cracking. Final mix was locked in with strict moisture correction for sand surface moisture (2.1%) measured hourly.

Lesson

In remote or time-sensitive projects, the estimator’s ability to rapidly translate aggregate gradation and density data into actionable cement/water values enables confident, code-compliant decisions — even when laboratory support is unavailable on-site.