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What is pull out test for anchor bolts?
November 10,2022

What is pull out test for anchor bolts?


1. The difference between anchoring force and pulling force

The anchoring force is an indicator of the anchoring strength of the anchor during use. The greater the anchoring force, the more reliable the anchoring effect and the better the anchoring effect. Its size is tested by the pull-out force test, and the maximum pull-out force measured on the same anchor rod is the anchoring force.

The pull-out force is an index tested by the anchor rod manufacturer during the production process. The pull-out force is the resultant force of overcoming the deformation resistance of the metal and the friction between the metal and the mold wall during the drawing process. The pull-out force should be less than the anchoring force.


 2. What is the purpose of the bolt pullout test?

Determine the anchorability of the surrounding rock of the roadway, evaluate the performance of the bolt, resin, surrounding rock anchoring system and the anchoring force of the bolt.


 3. Detection of the anchoring force of the bolt

(1) Bolt anchoring force detection is done by bolt pullout test in the well;

(2) The sampling rate of bolt anchoring force testing shall not be lower than 3%, and every 300 top and side bolts shall be sampled in one group (a total of 9 bolts) for testing. If there are less than 300 bolts, 300 bolts shall be used;

(3) When the anchoring force of the anchor bolt is not lower than 90% of the design anchoring force, it is qualified. If one of them is lower than the design anchoring force, it should be re-sampled and inspected; if the anchoring force of the re-sampled inspection is not lower than the designed anchoring force, it is qualified, and if there is still one unqualified, the construction quality of the anchor rod is determined to be unqualified. Relevant personnel must be organized to study the causes of unqualified bolt construction quality and take corresponding measures.


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4. The hazards of unqualified

There is a risk of causing fragmentation and roof fall.


 5. Some precautions in the anchor bolt pull out test

Anchor bolts are widely used in highway engineering slope protection and tunnel construction due to their simple process and low cost. As a means of judging the quality of the bolt, the bolt pullout test can evaluate the performance of the bolt anchorage system and the anchoring force of the bolt. In order to ensure that the pull-out test is carried out smoothly and the results will not affect the progress of the project, according to the past project construction experience, the following details need to be paid attention to:

 1. The bolt is constructed according to the design and specifications, paying special attention to the length and fullness of grouting;

 2. The exposed length of the anchor rod in the pull-out test is controlled between 40 and 45 cm;

 3. In order to make the instrument have a good focus point during the pull-out test, during the construction of the slope anchor, it is necessary to make a concrete base with a side length of 40cm and a thickness of 10cm for the anchor bolt reserved for the test (the center of the base is left 5-6cm round hole, so that the anchor rod can pass through), the base is perpendicular to the anchor rod; tunnels and slopes (when the rock formation is good) can use steel plates instead of concrete bases



4. Before the test, the on-site technician needs to provide the pile number, anchor type, quantity, specification, and design pull-out force;

5. The number of anchor rods is 300 for the test group (one group is 3), and less than 300 are counted as a group;

6. The site must reserve the corresponding number of bolts according to the sampling frequency and requirements. Especially in tunnels, there are often no exposed bolts, which makes the test impossible;

7. Judgment of bolt pullout force: the average pullout force of 3 bolts in the same group must be greater than or equal to the design value, and the minimum value must be greater than 90% of the design value;

 As most of the anchor pullouts are tested by a third party, as long as the construction and control are carried out according to the above precautions, the anchor pullout test can be successfully completed, laying a good foundation for project quality and measurement.


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