The IS 11309:1985 standard establishes a uniform procedure for performing pull-out tests on anchor bars and rock bolts utilized in rock mechanics and civil construction. It outlines the methodology to determine the anchorage strength and bonding between reinforcement bars, grout, and the surrounding rock, providing a trustworthy evaluation of rock bolt effectiveness on-site. This code is indispensable for professionals assessing the load capacity and structural soundness of mechanical and grouted anchors.
Overview
The IS 11309:1985 standard establishes a uniform procedure for performing pull-out tests on anchor bars and rock bolts utilized in rock mechanics and civil construction. It outlines the methodology to determine the anchorage strength and bonding between reinforcement bars, grout, and the surrounding rock, providing a trustworthy evaluation of rock bolt effectiveness on-site. This code is indispensable for professionals assessing the load capacity and structural soundness of mechanical and grouted anchors.
Audience
Contents
Structure
IS 11309 defines the methodology for evaluating the bonding and anchorage capacity of rock bolts, inclusive of mechanical anchorage types. The standard provides formulas for bond strength calculation where failure occurs between the bar and grout, using ( \tau = \frac{P}{2 \pi R L} ), where (P) is the failure load, (R) is the bar radius, and (L) is the embedded length. It also specifies procedures to obtain ultimate anchorage capacity and mandates rounding results in accordance with IS 2-1960.
This section clarifies terms such as bond strength ((\tau)), bar radius ((R)), and anchorage capacity ((P)). It reiterates the key bond strength formula applicable when failure occurs at the bar-grout interface. It highlights the requirement for rounding test results as per IS 2-1960 and the experimental determination of anchorage capacity.
Describes the pull-out test that measures the bond between anchor bars and surrounding concrete or rock. It explains the use of a central hole jack to apply axial loads until failure, with bond stress calculated by ( \tau = \frac{P}{\pi d L} ), where (d) is bar diameter and (L) embedded length. Typical bar sizes and load ranges are provided, emphasizing adherence to rounding rules and the test's role in validating anchorage integrity.
Details the loading sequence including preloading up to 5 kN to remove slack, ensuring grout curing per IS 6066-1985 before testing, and recording load versus displacement on an arithmetic scale. The maximum load applied by the jack corresponds to anchorage capacity. It underscores controlled load increments and plotting for performance evaluation.
Presents the formula ( r = \frac{W}{2 \pi R L} ) to calculate bond strength where (W) is the load, (R) the radius of the grouted hole, and (L) the embedded length. Specifies that if failure occurs between bar and grout, (R) represents bar radius. The section also discusses testing limitations, such as not exceeding 110% of design load, and assumptions of uniform borehole diameter and grout density.
Explains bond strength calculations for steel-to-grout interfaces, using the same fundamental formula with the radius adjusted based on failure location. It includes testing parameters such as maximum load limits, the use of a 50-tonne central-hole jack with slow loading rates, and requirements for data recording and plotting load-extension curves to identify failure modes.
Defines anchorage capacity as the peak pull force recorded during testing and reiterates the bond strength formula when failure occurs between bar and grout. It provides a summary of parameters, including units and descriptions, and illustrates the decision process for evaluating failure modes and calculating bond strength accordingly.
Outlines the necessary information to report after pull-out testing: rock mass description, grout mix and curing details, anchor type and dimensions, borehole specifications, bond strength, failure mode, and the number of tests performed. It also specifies the rounding off of values per IS 2-1960 and emphasizes clarity and completeness for compliance.
Frequently Asked
IS 11309 (1985) covers pull-out testing methods applicable to both fully grouted rock bolts, anchored with cement-sand or resin grout along their length, and ungrouted mechanically anchored rock bolts including types such as lot wedge and expansion shell anchors. This standard ensures assessment of anchorage strength across common grouted and mechanical anchorage systems used in rock reinforcement.
The pull-out test uses a central-hole jack featuring a 60 mm diameter hole through its 50-tonne capacity ram. The jack is mounted around the anchor bar, with two dial gauges placed diagonally to monitor displacement with 0.01 mm precision. Loading is applied uniformly at 250 kg/min via a hydraulic pump to prevent sudden jolts. Dial readings are taken every 1 tonne increment, and the jack's ram includes low-friction seals to ensure smooth, repeatable calibration and operation.
Before testing, the borehole must be completely filled with cement-sand slurry grout under pressure using a grouting machine to ensure full column coverage around the anchor bar. The grout should be allowed to cure thoroughly as specified in IS 6066-1985, typically between 7 to 28 days depending on conditions. Testing should only commence once curing is complete, with grout mix and curing details documented in the test report.
Bond strength ( r ) between grout and rock is calculated using the formula ( r = \frac{W}{2 \pi R L} ), where (W) is the load at failure, (R) the radius of the grouted hole, and (L) the embedded length, all in consistent units. If failure occurs between the bar and grout, (R) is taken as the bar radius instead. This calculation yields the average bond strength in tonnes/cm² and depends on the failure interface identified during testing.
To avoid damage during pull-out tests, a hydraulic jack with 50-tonne capacity and a 60 mm central hole should be used. Load application must be slow and uniform at approximately 250 kg/min to prevent jerks. Sudden loading is to be avoided until bolt movement begins. Dial gauge readings should be recorded at each 1 tonne increment, and grout curing must be completed per IS 6066-1985 before testing. Monitoring load versus extension curves helps detect abnormal behavior early and ensure safe test execution.
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