Method of conducting a pull-out test on anchor bars and rock bolts

IS 11309: Scope & Key Specifications

• Scope:
  IS 11309 covers the methods for testing and determining the bond strength and anchorage capacity of rock bolts, including mechanical anchorages.

• Key Formula for Bond Strength (Clause 6.1):
  When failure occurs between the bar and grout, bond strength ( \tau ) is calculated using:
  [ \tau = \frac{P}{2 \pi R L} ]
  Where:

  • ( P ) = Anchorage capacity (load at failure)
  • ( R ) = Radius of the bar
  • ( L ) = Embedded length of the bar in grout

• Anchorage Capacity (Clause 7):
  Defines test procedures to determine ( P ), the ultimate anchorage capacity of mechanical anchorages.

• Rounding Off Results:
  As per IS 2-1960, final test values must be rounded off according to standard rounding rules.

Summary Table:

flowchart TD
    A[Rock Bolt] --> B[Grout]
    B --> C[Surrounding Rock]
    P[Load P] -->|Applied Load| A
    subgraph Bond Interface
        A -- Bond Strength τ --> B
    end

This standard ensures reliable evaluation of rock bolt anchorage for safe design in rock engineering.

IS 11309 - Definitions & Key Formulas

IS 11309 covers mechanical anchorages for rock bolts. Key definitions and formulas include:

Definitions

• Bond Strength (τ): Stress at the interface between bar and grout.
• Radius of Bar (R): Used in bond strength calculations.
• Anchorage Capacity (P): Ultimate load capacity of mechanical anchorages.

Key Formula (Clause 6.1)

If failure occurs between bar and grout, bond strength is calculated by:

[ \tau = \frac{P}{2 \pi R L} ]

Where:

• ( \tau ) = bond strength (stress)
• ( P ) = applied load (anchorage capacity)
• ( R ) = radius of the bar
• ( L ) = embedded length of the bar in grout

Additional Notes

• Results must be rounded per IS 2-1960 rounding rules.
• Anchorage capacity ( P ) is determined experimentally as per Clause 7.

flowchart TD
    A[Applied Load P] --> B[Bond Stress τ]
    B --> C[Interface Bar-Grout]
    C --> D[Radius R]
    C --> E[Embedded Length L]
    B --> F[Calculate τ = P/(2πRL)]

This formula helps assess the bond strength and anchorage capacity crucial for rock bolt design.

IS 11309: Anchor Bar Pull-Out Test — Key Points

Description (Clause 3)

The Anchor Bar Pull-Out Test evaluates the bond strength between the anchor bar and surrounding concrete or rock.

Test Setup:

• Central Hole Jack (Clause 2.1): Special jack with a central hole through the ram, allowing force application directly on the anchor bar.
• The bar is pulled axially using the jack until failure or a specified load.

Key Parameters & Formulas:

• Pull-out Load, P (kN): Maximum load applied to the anchor bar.
• Bond Stress, τ (MPa):

[ \tau = \frac{P}{\pi \times d \times L} ]

Where:

• ( d ) = diameter of anchor bar (mm)
• ( L ) = embedded length of the anchor bar (mm)

Typical Specifications:

Notes:

• Rounding off numerical values should follow IS code rules.
• Test results help verify anchorage adequacy and quality of installation.

flowchart LR
    A[Anchor Bar Embedded] --> B[Central Hole Jack Applies Load]
    B --> C[Measure Pull-Out Load P]
    C --> D[Calculate Bond Stress τ = P/(π*d*L)]
    D --> E[Evaluate Anchorage Quality]

This concise summary aligns with IS 11309 for conducting anchor bar pull-out tests.

IS 11309: Application of Loading for Rock Bolt Testing

Key Specifications and Procedures:

• Initial Load (Clause 4.3):
  Apply an initial load ≤ 5 kN to remove slack before testing.

• Curing Period (Clause 3.5):
  Ensure grout curing as per IS 6066-1985 before loading.

• Load vs. Movement (Clause 5.1):
  Record and plot load (kN) vs. bar movement on an arithmetic scale to assess behavior.

• Anchorage Capacity (Clause 7.1):
  Maximum pull by the ram (in tonnes) = actual anchorage capacity.

Typical Load Application Steps:

flowchart TD
    A[Start Test] --> B[Check grout curing (IS 6066)]
    B --> C[Apply initial load ≤ 5 kN]
    C --> D[Increase load gradually]
    D --> E[Record load vs. movement]
    E --> F[Determine max pull = anchorage capacity]
    F --> G[Report results]

Notes:

• Use arithmetic plotting for clarity in load-movement response.
• Maximum pull load defines the rock bolt anchorage strength.
• Follow curing and loading strictly for test validity.

IS 11309: Bond Strength Between Grout and Rock

Key Formula for Bond Strength (Clause 5.2)

[ r = \frac{W}{2 \pi R L} ]

Where:

• r = Bond strength (tonnes/cm²)
• W = Load taken by the bar (tonnes)
• R = Radius of grouted hole (cm)
• L = Length of embedded bar (cm)

Special Cases (Clause 6.1)

• If failure occurs between bar and grout, use the same formula but R = radius of bar (not hole).

Testing Limits (Clause 4.9)

• Bolts should not be tested beyond 10% over design load to avoid damage.

Assumptions (Clause 1.1)

• Borehole diameter and grout density are uniform along the embedded length.

Summary Table

flowchart LR
    A[Pull-out Test] --> B{Failure Location?}
    B -->|Between Grout & Rock| C[Use R = Radius of hole]
    B -->|Between Bar & Grout| D[Use R = Radius of bar]
    C --> E[Calculate r = W / (2πRL)]
    D --> E

This formula and approach ensure reliable evaluation of bond strength critical for rock bolt anchorage design.

IS 11309: Bond Strength Between Steel and Grout

Key Formulas:

1. Bond Strength (r) when failure is between grout and rock:

[ r = \frac{W}{2 \pi R L} ]

• W = Load taken by the bar (tonnes)
• R = Radius of grouted hole (cm)
• L = Length of embedded bar (cm)

2. Bond Strength (g) when failure is between steel bar and grout:

Use the same formula as above, but R = radius of the steel bar.

Specifications & Testing:

• Maximum test load: Do not exceed 110% of design load (Clause 4.9).
• Pull-out test setup: Use a 50-tonne capacity central-hole jack with slow uniform loading at 250 kg/min to avoid jerks (Clause 4.2).
• Data recording: Record dial gauge readings every 1 tonne load increment.
• Plot: Load vs. bolt extension curve to identify failure mode (Fig. 2).

Summary Table:

flowchart LR
    A[Load W applied] --> B[Bond interface]
    B --> C{Failure Mode}
    C -->|Grout-Rock| D[Use R = hole radius]
    C -->|Steel-Grout| E[Use R = bar radius]
    D --> F[Calculate r = W/(2πRL)]
    E --> F

This concise approach ensures correct bond strength evaluation per IS 11309.

IS 11309: Anchorage Capacity of Mechanical Anchorages of Rock Bolts

Key Points from Clause 7 (Determination of Anchorage Capacity)

• Anchorage Capacity (P): Defined as the maximum pull force exerted by the ram during testing, measured in tonnes.

• Test Method: Pull-out test as per IS method is used to determine this capacity.

Clause 6.1: Bond Strength (Failure between bar and grout)

• Use the formula for bond strength considering R = radius of the bar.

Typical Formula for Bond Strength (if failure is between bar and grout):

[ \tau = \frac{P}{2 \pi R L} ]

where:

Summary Table of Parameters

flowchart LR
    A[Apply Pull-out Load P] --> B[Mechanical Anchorage]
    B --> C{Failure Mode?}
    C -->|Between Bar & Grout| D[Calculate Bond Strength \tau = P / (2πRL)]
    C -->|Other Modes| E[Use Relevant IS 11309 Clause]

Note: For detailed test procedures and reporting, refer to the IS standard method for conducting pull-out tests on anchor bars and rock bolts.

IS 11309 — Reporting of Test Results (Clause 8.1)

When reporting pull-out test results on anchor bars and rock bolts, include:

• Lithological description of the rock mass.
• Grout details: mix design, curing period, dates of grouting and testing.
• Bolt/anchor type used.
• Bolt dimensions: length (L) and diameter (d).
• Hole dimensions: length and diameter.
• Bond strength and mode of failure (e.g., tensile failure of bolt or bond failure).
• Number of tests conducted.

Additional Notes:

• Final values must be rounded as per IS 2-1960 (Rules for rounding off numerical values).
• Actual anchorage capacity = Maximum pull force by ram (in tonnes).

Summary Table for Report Content

flowchart TD
    A[Test Report] --> B[Lithological Description]
    A --> C[Grout Details]
    A --> D[Bolt Type & Dimensions]
    A --> E[Hole Dimensions]
    A --> F[Bond Strength & Failure Mode]
    A --> G[Number of Tests]

This ensures clarity, repeatability, and compliance with IS 11309.