CT Sub-Base Construction & Quality Control Manual

Cement-Treated Sub-Base (CTSB) Construction Methodology

Standard Quality Assurance & Execution Guidelines for Flexible and Semi-Rigid Pavements

1. Overview of CTSB Construction

Cement-Treated Sub-Base (CTSB) is a structural layer used in modern pavement engineering to provide a high-stiffness foundation above the prepared subgrade. By blending aggregate materials with a specified proportion of hydraulic binder (ordinary Portland cement) and water, CTSB enhances load distribution, mitigates moisture intrusion, and reduces overall pavement thickness requirements.

flowchart TD %% Material Sourcing A["Identification Of\nQuarries By\nContractor"] B["Collection Of\nCertificates About\nQuarry Source"] C["Collection Of Coarse,\nFine Aggregates From\nQuarry and Binding\nMaterial (Cement)"] D["Test on Coarse Aggregate Sample:\nGradation, Liquid Limit, Plastic Index,\nPlasticity Modulus, Plastic Product,\nUniformity Coefficient, Water Absorption,\nSoundness, Elastic Modulus etc."] E["Test on Blended Sample:\nSieve Analysis, LL, PI"] F["Check For MDD"] %% Subgrade & Approval G{Is Subgrade\nLayer Completed?} H["Construction Of\nShoulder For\nLateral Confinement\nWith Approved Material\nAfter Testing"] I["If all Materials are OK, then allow\nthem for Construction By\nMixing Materials in The Plant\nand/or directly at Site"] P["Rectify Subgrade\nBefore Proceeding"] %% Construction J["Spreading of CT Subbase at\nMoisture between OMC to 2%\nabove OMC Material Over\nPrepared Subgrade"] K["Spread Uniformly In\nThe Desired Layer\nThickness With Paver/\nMotor Grader"] L["Remove/\nScarify"] M["Rolling/Compaction within 2\nhours of its mixing"] N["Checking With\nTemplates or\nSurvey Levels"] O["Test Field Compaction /\nStrength (UCS)"] %% Connections A --> B --> C --> D D --> E --> F F --> I G -->|Yes| H G -->|No| P P --> G H --> J --> K --> L --> M --> N --> O O -->|Fail| L I --> J %% Styling classDef start fill:#e6f7ff,stroke:#1890ff,stroke-width:2px; classDef process fill:#fff7e6,stroke:#fa8c16,stroke-width:1.5px; classDef decision fill:#ffffe6,stroke:#cc9900,stroke-width:2px; classDef test fill:#f6ffed,stroke:#52c41a,stroke-width:1.5px; classDef fail fill:#ffe6e6,stroke:#d4380d,stroke-width:2px; class A,B,C start; class D,E,F,H,I,J,K,L,M,N process; class G decision; class O test;

2. Sequential Execution Guidelines

Phase 1: Material Sourcing & Laboratory Testing

The contractor identifies approved aggregate quarries and collects source validation certificates. Samples of coarse aggregates, fine aggregates, and cement undergo rigorous laboratory testing to determine suitability:

  • Individual Aggregate Quality: Gradation, Liquid Limit (LL), Plasticity Index (PI), Plasticity Modulus, Water Absorption, Soundness, and Elastic Modulus.
  • Blended Mix Checks: Sieve analysis on combined gradations to ensure adherence to specified envelope boundaries.
  • Moisture-Density Relationship: Heavy compaction tests (Modified Proctor) to establish Maximum Dry Density (MDD) and Optimum Moisture Content (OMC).

Phase 2: Subgrade Inspection & Lateral Confinement

CTSB placement cannot commence until the underlying subgrade is inspected, compacted to required levels, and formally signed off.

Subgrade Verification Protocol: If subgrade elevation, slope, or field density checks fail, the section must be scarified, re-compacted, or rectified beforeCTS placement. Once approved, earthen shoulders must be constructed simultaneously to provide lateral confinement for compaction equipment.

Phase 3: Mixing, Laying & Time-Critical Compaction

Approved materials are proportioned in a central Pugmill batching plant (or via approved in-situ reclaimer mixing). Water content must be controlled between OMC to (OMC + 2%) to account for evaporation losses during transport and spreading.

Critical Initial Setting Window: Compaction and surface finishing must be completed within 2 hours of cement hydration initiation (water addition at the plant). Delayed compaction destroys hydration bonds, severely reducing final Unconfined Compressive Strength (UCS).

Phase 4: Level Checks & Strength Validation

Post-compaction geometry is verified using auto-levels or cross-section templates to guarantee surface smoothness and grade compliance. In-situ field density (sand replacement/nuclear gauge) and 7-day Unconfined Compressive Strength (UCS) core/cube tests are executed. If field checks fail density or strength criteria, the layer must be scarified, removed, and replaced.

3. Quality Control & Material Testing Standards

Quality Check Parameter Target Specification / Boundary Sampling & Testing Frequency
Aggregate Gradation Conforming to specified sub-base grading envelope 1 test per 500 m³ of aggregate
Atterberg Limits (LL / PI) Plasticity Index (PI) < 6% on fine fraction 1 test per 1,000 m³ of aggregate
Mixing Moisture Content OMC to (OMC + 2.0%) Periodic checks during daily production
Compaction Window Completed within ≤ 120 minutes of mixing Continuous monitoring per batch load
Field Density (MDD) ≥ 98% of Laboratory Modified Proctor Density 1 test set per 1,000 m² of compacted area
7-Day UCS Strength As per design specifications (typically 1.5 to 3.0 MPa) 1 set of 6 cubes per 250 m³ of mixed material
Curing Requirement Moist curing / Bituminous emulsion membrane for 7 days Immediate application post-compaction