Tentative Guidelines for Lean-Cement Concrete and Lean Cement-Fly Ash Concrete as a Pavement Base or Subbase

Scope of IRC 74: Lean Cement Concrete (LCC) & Lean Cement Fly Ash Concrete (LCFAC) for Pavement Base/Subbase

• Purpose: Guidelines for design, materials, and thickness of LCC/LCFAC as base/subbase in flexible and rigid pavements.
• Mix Design:
  • Design lab mix strength = 1.25 × required 28-day field strength.
  • Trial and error method; max aggregate size ≤ 40 mm (for 10 cm layer).
• Typical Mix Proportions:

• Thickness: Minimum 10 cm for base/subbase layers.
• Load Carrying Capacity (Meyerhof equation):

[ P_u = \frac{2 f h^2}{3 (1 - \mu^2)} \times a ]

Where:

• (P_u) = ultimate load capacity (kg)

• (f) = design flexural strength (≈ 1/6 to 1/8 compressive strength)

• (h) = thickness (cm)

• (\mu) = Poisson's ratio (0.20–0.30)

• (a) = equivalent load radius (cm)

• Load Factor for Design Life:

IRC 74: Design Criteria & Thickness Design for Lean Cement Concrete (LCC) / Lean Cement Fly Ash Concrete (LCFAC) Base

Key Design Parameters:

• Subgrade CBR: 5%
• Modulus of Subgrade Reaction, k: 2.8 kg/cm³
• Minimum 28-day compressive strength (fc): 60 kg/cm²
• Traffic Growth Rate: 7.5% per annum
• Design Life: 10 years
• Traffic Intensity (T): ( 300 \times (1+0.075)^{10} = 618 ) (Class E)

Thickness Design Summary (Flexible Pavement):

• Total Thickness: 4 + 10 + 16 = 30 cm (minimum)
• Equivalent Thickness: (16 \times 1.5 + 10 \times 2 + 4 \times 2 = 52 \text{ cm}) (satisfies CBR criteria)

Design Procedure (Clause 28.0):

1. Calculate traffic intensity with growth.
2. Refer to IRC-37 CBR curves for total pavement thickness (e.g., 45 cm for CBR=5).
3. Assign thickness for surface courses (AC + BM = 14 cm).
4. Remaining thickness for LCC/LCFAC base (e.g., 16 cm).
5. Verify minimum compressive strength and compaction requirements.

Construction & Quality Control Highlights:

• Batching: Weight batching preferred; water measured by volume.
• Mixing: Power-driven mixers, 1-2 min mixing time.
• Compaction: Max 10 cm per layer (smooth roller), 15 cm (vibratory roller), 8 passes recommended.
• Curing: 48 hours wet covering + 12 days moist curing.
• Joints: Only construction joints at day-end, chamfered at 30°.
• Surface Course: Use intermediate BM

Key Specifications & Formulas for Lean Cement Concrete (LCC) & Lean Cement-Fly Ash Concrete (LCFAC) per IRC 74

1. Materials

• Cement: IS 269 (Ordinary/Low Heat Portland Cement), IS 1489 (Portland Pozzolana Cement for LCC only), IS 455 (Portland Slag Cement).
• Fly Ash: IS 3812 (Part II), lime reactivity ≥ 40 kg/cm², fineness ≥ 2800 cm²/g, carbon ≤ 12%.
• Aggregates:
  • Coarse aggregate size: 20–40 mm max.
  • Fine & coarse aggregates per IS 383, IS 3068, IS 2686.
  • Grading limits (Table 2 excerpt):

2. Mix Proportions & Strength (28-day compressive strength)

IRC 74 - Proportioning of Lean Cement Concrete / Lean Cement-Fly Ash Concrete Mixes

Key Specifications & Mix Design Criteria

• 28-day compressive strength: 40-60 kg/cm² (field strength)
• Design strength in lab: 1.25 × field strength (to allow for field variances)
• Slump: 0-12 mm (low workability for rolling compaction)
• Max aggregate size: ≤ 40 mm (for 10 cm layer thickness)
• Water content: ~9-11% by weight of total dry materials (lean cement concrete), ~10-12% for fly ash mixes

Typical Mix Proportions (by weight)

Where:

• n = ratio of specific gravities of fly ash to sand = 0.827
• p = proportion of sand replaced by fly ash (typically 0.5)

Fly Ash Mix Proportioning Formula:

[ \text{Mix} = 1 : (1-p)x : p x n : y ]

• (x, y) = sand and coarse aggregate in lean cement concrete
• Adjust water content: (w + 1%) by weight of total aggregate

Aggregate Grading Limits (by weight % passing)

Key Specifications for Batching & Mixing Equipment (IRC 74)

Batching:

• Materials for lean cement concrete/lean cement-fly ash concrete must be batched by weight.
• Volume batching allowed only if unavoidable.
• Water measured by volume using calibrated containers.
• Adjust mix proportions for moisture in fly ash and aggregates.

Mixing:

• Use power-driven concrete mixers of adequate capacity (IRC: 43-1972).
• Add water first, then cement & fly ash, finally aggregates.
• Mixing time: 1-2 minutes to ensure uniformity.
• Avoid overloading the mixer.

Quality Control:

• Refer Table 5 for minimum test frequencies (e.g., cement quality per IS:269/455/1489 once initially and per consignment).
• Strength tests on cubes at 7 and 28 days (IS:516), one test per 50 m³.

Typical Mix Proportions (by weight)

Note: n = ratio of specific gravities of fly ash to sand (≈0.827).

Batching & Mixing Workflow

flowchart TD
    A[Measure Water] --> B[Add Water to Mixer]
    B --> C[Add Cement & Fly Ash]
    C --> D[Add Aggregates]
    D --> E[Mix for 1-2 minutes]
    E --> F[Transport & Place Concrete]

Additional Notes:

• Use weigh batching for accuracy.
• Mixing sequence ensures good dispersion.
• Compaction with smooth/vibratory rollers follows mixing (8-10 tonne rollers).
• Maintain curing for at least 14 days after compaction.

For detailed equipment specs, refer IRC: 43-1972 and Table 5 of IRC 74.

Key Specifications & Design for Subgrade/Subbase (IRC 74):

1. Subgrade Preparation:

• Compaction: Achieve a minimum CBR of 5% for the compacted soil subgrade.
• Moisture Conditioning: Ensure optimum moisture content for maximum dry density.

2. Subbase/Base Course Thickness (Flexible Pavement):

• Design thickness over compacted subgrade (CBR=5) is calculated as:

  [ \text{Total Thickness} = 16 \times 1.5 + 28 = 52 \text{ cm} ]

3. Typical Pavement Layer Thicknesses:

4. Material Specifications:

• Lean Cement / Lean Cement Fly Ash Concrete:
  • Minimum 28-day compressive strength: 60 kg/cm²
  • Used as base or subbase for improved strength and durability.

Summary:

• Prepare subgrade to CBR ≥ 5.
• Use lean cement/fly ash concrete base of 16 cm thickness.
• Overlay with bituminous macadam and asphaltic concrete as per design.

flowchart TD
    A[Compacted Subgrade (CBR=5)] --> B[Lean Cement/Fly Ash Concrete Base (16 cm)]
    B --> C[Bituminous Macadam (10 cm)]
    C --> D[Asphaltic Concrete (4 cm)]

This layered design ensures structural adequacy and longevity of flexible pavements as per IRC 74 guidelines.

IRC 74: Construction Procedures for Lean Cement & Lean Cement-Fly Ash Concrete

Key Mix Design Formulas

• For lean cement concrete:
  Aggregate/Cement ratio ≈ 12-18
  Water content ≈ 9-11% by weight of dry materials

• For lean cement-fly ash concrete:
  Replace 50% sand by fly ash volume:
  [ \text{Mix} = 1 : (1-p) x : p x n : y ] where:

  • (p = 0.5) (sand replacement fraction)
  • (n = 0.827) (specific gravity ratio fly ash/sand)
  • (x,y) = sand, coarse aggregate proportions
    Water content ≈ ((w + 1)%) (trial adjusted)

Typical Mix Proportions (by weight)

Quality Control Tests (Min. Frequency)

Construction Highlights

• Batching: By weight; volume only if unavoidable.
• Mixing: Power-driven mixers; add water → cement/fly ash → aggregates.
• Compaction:
  • Smooth wheel rollers: 8-10

Surface Course Considerations - IRC 74 Key Points

1. Reflection Cracking Prevention (Clause 8.1):

• Lean cement concrete (LCC) or lean cement-fly ash concrete (LCFA) base is semi-rigid and prone to transverse cracks due to thermal/drying shrinkage.
• Thin bituminous wearing courses (e.g., surface dressing, premix) directly over LCC/LCFA may reflect cracks.
• Solution: Provide an intermediate layer of water bound macadam (WBM) or bituminous macadam (BM) before the wearing course.
• Minimum combined thickness of intermediate + wearing course: ≥ 10 cm.

2. Rigid Pavement Case (Clause 8.2):

• When LCC/LCFA is used as subbase under rigid pavement, the cement concrete wearing course can be laid directly without an intermediate layer due to higher rigidity preventing crack reflection.

Equivalency Factors for Design (Clause 1.50)

Typical Flexible Pavement Section Example

Summary Diagram

graph TD
    A[Asphaltic Concrete (4 cm)] --> B[Bituminous Macadam (10 cm)]
    B --> C[Lean Cement / Fly Ash Concrete Base (16 cm)]
    C --> D[Compacted Soil Subgrade (CBR=5)]

Additional Notes

• Compaction: Max layer thickness compacted per pass:
  • Smooth wheel roller: 10 cm
  • Vibratory roller: 15 cm
• Curing: 48 hours wet covering + 12 days moist

Quality Control for Lean Cement Concrete / Lean Cement-Fly Ash Concrete (IRC 74)

Key Tests & Frequency (Table 5):

Important Specifications:

• Batching: By weight (volume batching only if unavoidable).
• Mixing: Power-driven mixers; add water first, then cement/fly ash, aggregates last; mixing time 1-2 min.
• Compaction: 8 passes by smooth wheel roller (8-10 tonnes) or vibratory roller; max layer thickness 10 cm (smooth) / 15 cm (vibratory).
• Curing: 48 hrs with wet gunny bags, then 12+ days with wet sand or frequent watering.
• Transportation & Placement: Avoid segregation; surcharge 20-25% of layer thickness.
• Construction Joints: Chamfer edges at ~30° angle.

Mix Proportioning for Lean Cement-Fly Ash Concrete:

[ \text{Mix} = 1 : (1-p) \times x : p \times x \times n : y ]

• (p \approx 0.5) (proportion of sand replaced by fly ash by weight)
• (n = \frac{\text{Specific Gravity of Fly Ash}}{\text{Specific Gravity of Sand}} \approx 0.827)
• Adjust water content (w% + 1

Illustrative Thickness Design for Lean Cement/Fly Ash Concrete Base (IRC 74)

Key Design Parameters:

• Subgrade CBR = 5%
• Design wheel load: As per traffic (e.g., 300 veh/day)
• Equivalency factor for lean cement concrete (LCC) = 1.5
• Minimum 28-day compressive strength = 60 kg/cm²

Design Thickness Calculation (Example):

Given bituminous layers + LCC base thickness:

Total thickness = 4 + 10 + 16 = 30 cm

Using equivalency factor for LCC (1.5):

[ \text{Equivalent thickness} = 16 \times 1.5 + 10 + 4 = 24 + 10 + 4 = 38 \text{ cm} ]

This satisfies the CBR-based design requirement.

Specifications Summary:

• Mixing & Batching: Weight batching preferred; water measured by volume.
• Compaction:
  • Max thickness per layer: 10 cm (smooth roller), 15 cm (vibratory roller)
  • 8 roller passes recommended
  • Start rolling from edges inward
• Curing:
  • 48 hours wet gunny bags/hessian
  • 12 days moist curing (wet sand/watering)
• Joints: Only construction joints at day-end with 30° chamfer.
• Surface Course:
  • Use intermediate bituminous/macadam layer ≥10 cm before wearing course to prevent reflection cracks.

Quality Control (Table 5 Summary):

flowchart TD
    A[Subgrade (CBR=5%)] --> B[Lean Cement/Fly Ash Concrete Base (16 cm)]
    B --> C[Bituminous Macadam (10 cm)]
    C --> D[Asphaltic Concrete (4 cm)]
    D --> E[