1. Difference Between Anchorage Force and Pullout Force
Anchorage force is the anchoring strength index of an anchor bolt during use. The greater the anchorage force, the more reliable the anchor bolt’s performance and the better the anchoring effect. Its value is determined through pullout tests, where the maximum pullout resistance measured for a single anchor bolt is considered the anchorage force.
Pullout force, on the other hand, is a parameter tested during the production process by the manufacturer. It represents the combined force required to overcome the metal’s deformation resistance and the friction between the metal and the mold wall during the pulling process. The pullout force should always be less than the anchorage force.
2. Purpose of Anchor Bolt Pullout Tests
To determine the anchoring capacity of the surrounding rock in roadways and to evaluate the performance of the anchoring system, including the anchor bolt, resin, and surrounding rock, as well as the anchorage force of the anchor bolt.
3. Testing of Anchor Bolt Anchorage Force
1. Underground Testing :
The anchorage force of anchor bolts is tested using pullout tests conducted underground.
2. Sampling Rate :
The sampling rate for anchorage force testing should not be less than 3%. For every 300 roof or wall anchor bolts, one set of samples (a total of 9 bolts) is tested. If fewer than 300 bolts are present, sampling is still based on 300 bolts.
3. Acceptance Criteria :
The anchorage force is considered acceptable if it is no less than 90% of the design anchorage force. If any bolt’s anchorage force is below the design value, additional samples must be tested. If all retested bolts meet the design value, the result is acceptable. If even one bolt fails, the anchor bolt construction quality is deemed unacceptable. In such cases, the relevant personnel must investigate the cause of the quality issue and implement appropriate corrective measures.
4. Risks of Nonconformance
Nonconforming anchor bolts pose risks of wall collapse and roof failure.
5. Key Considerations for Anchor Bolt Pullout Tests
Anchor bolts, with their simplicity, low cost, and economic advantages, are widely used in slope protection for highway projects and tunnel construction. Pullout tests, as a method of assessing anchor bolt quality, are essential for evaluating the performance of the anchoring system and the anchorage force of the bolts. To ensure the smooth progress of pullout tests and prevent delays in construction, the following points, derived from past project experience, should be noted:
1. Construction Compliance :
Anchor bolts must be installed according to design specifications and standards, paying special attention to their length and the fullness of grouting.
2. Exposed Length Control :
The exposed length of the anchor bolt for pullout testing should be controlled within 40–45 cm.
3. Preparation of Testing Base :
– For slope anchor bolts, ensure the test anchor bolts have a concrete base with dimensions of 40 cm (side length) by 10 cm (thickness). The base should include a central circular hole (5–6 cm in diameter) for the anchor bolt to pass through, and the base should be perpendicular to the anchor bolt.
– In tunnels or slopes with stable rock formations, a steel plate may be used as an alternative to the concrete base.
4. Key Information to Provide Before Testing
Before conducting the test, the on-site technician must supply the pile number, anchor bolt type, quantity, specifications, and the designed pullout force.
5. Group Testing Criteria
Pullout tests are conducted in groups of three anchor bolts for every 300 installed. If fewer than 300 anchor bolts are present, one group (three bolts) is tested.
6. Reserve Anchor Bolts for Testing
It is essential to reserve the required number of anchor bolts based on sampling frequency and specifications, particularly in tunnels. A lack of exposed anchor bolts often prevents testing from being conducted.
7. Pullout Force Evaluation
For a group of three anchor bolts:
– The average pullout force must be equal to or greater than the design value.
– The minimum pullout force must be at least 90% of the design value.
Since pullout tests are usually conducted by third-party inspectors, adhering to the above considerations during construction and control ensures successful completion of the tests. This establishes a solid foundation for engineering quality and measurement.
