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Pull Out Test for Anchor Bolts: Understanding the Basics
December 10,2022

The difference between anchoring force and pulling force

Anchoring force is the strength indicator of the anchor rod during use. The greater the anchoring force, the more reliable the anchor rod’s action and the better the anchoring effect. Its size is tested by a tensile test, and the maximum tensile force obtained from the same anchor rod is the anchoring force.

Pulling force is an indicator tested by the anchor rod manufacturer during the production process. The pulling force is the combined force that overcomes the deformation resistance of the metal and the frictional force between the metal and the mold wall during the pulling process. The pulling force should be less than the anchoring force.

What is the purpose of anchor bolt pull out test?

To determine the anchoring ability of the surrounding rock in the tunnel, evaluate the performance of the anchor rod, resin, and surrounding rock anchoring system, and determine the anchoring force of the anchor rod.

Detection of pull test for anchor bolts

(1) The detection of anchor rod anchoring force is completed by anchor rod tensile testing underground.

(2) The sampling rate for the detection of anchor rod anchoring force is not less than 3%. One group (9 rods) of top and bottom anchor rods is sampled for every 300 rods. When the number of rods is less than 300, it is based on 300 rods.

(3) The anchoring force of the anchor rod should not be less than 90% of the design anchoring force to be qualified. If one of them is lower than the design anchoring force, it should be re-sampled and tested. If the re-sampled anchoring force is not lower than the design anchoring force, it is qualified. If there is still one unqualified, the construction quality of the anchor rod is judged as unqualified. Relevant personnel must be organized to study the reasons for the unqualified construction quality of the anchor rod and take corresponding measures.


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 rods are currently widely used in highway slope protection and tunnel construction due to their simple process and low cost. As a means of judging the quality of anchor rods, anchor rod tensile testing plays a role in evaluating the performance of the anchor rod anchoring system and the anchoring force of the anchor rod. In order to ensure the smooth progress of the tensile test and the results do not affect the project schedule, based on past project construction experience, the following details need to be noted:

1.The anchor rod should be constructed according to the design and specifications, with special attention to the length and grouting fullness.

2.The exposed length of the anchor rod for tensile testing should be controlled between 40-45cm.

3.In order to ensure a good point of force for anchor pull test equipment during the tensile test, a concrete base with a side length of 40cm and a thickness of 10cm (with a 5-6cm round hole in the middle for the anchor rod to pass through) should be prepared for the anchor rod reserved for testing during slope anchor rod construction. The base should be perpendicular to the anchor rod. For tunnels and slopes (when the rock layer is good), steel plates can be used instead of concrete bases.

Pull Out Test for Anchor Bolts.webp

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

5.The anchor rod tensile test is carried out in groups of three (one group for 300 rods), and when there are less than 300 rods, it is calculated as one group.

6.The corresponding number of anchor rods must be reserved on-site according to the sampling frequency and requirements, especially for tunnels, where there are often no exposed anchor rods, which can lead to the inability to conduct the test.

7.The judgment of the anchor rod tensile force: the average tensile force of the three anchor rods 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 anchor rod tensile testing is mostly carried out by third-party testing, as long as the above precautions are taken during construction and control, the anchor rod tensile test can be completed smoothly, laying a good foundation for project quality and measurement.



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