What is the Pull-out test?
The pull-out test is an on-site method for evaluating the compressive strength of concrete structures based on the resistance to the pull out of embedded discs from a defined distance below the surface.
How does the Pull-out test work?
The test consists of pulling out an embedded insert (small metal disc with a rod fixed centrally on one side) by applying a tensile force through a manually calibrated hydraulic jack until concrete failure.
The ultimate failure force is empirically correlated to the in-situ concrete strength with calibration curves proposed by the manufacturers or implemented in laboratory with similar concrete materials.
What is the applicability of Pull-out testing?
Deterioration process | Defects | Control of repairs |
All deterioration processes inducing loss of concrete strength | All defects associated with loss of concrete strength, namely scaling, eroded concrete | Concrete replacement in-situ quality control |
How do I carry out a Pull-out test?
A steel disc 25mm in diameter is cast below the concrete surface at a depth of 25mm.
Once concrete has hardened and test values are desired this insert is pulled from the centre against a 55m counter pressure ring bearing on the surface.
Increasing force is applied until the inserted disc breaks free from the concrete and this maximum force is recorded by the test device.
As the concrete in the strut between the disc and the counterpressure ring is subjected to compressive load, the pull-out force is directly related to the compressive strength. A schematic of the test set up is shown below.
What equipment and expertise are required for Pull-out testing?
Due to the popularity of this test type many commercial systems are available from brands such as Hydrajaws, Impact test equipment or Big Ben. They are composed of a pull tester with a force measurement capability and a range of discs for embedding.
What are the advantages of Pull-out testing?
Simple and cost-effective.
Provides a direct correlation to the compressive strength.
Reliable and easy to use test method.
Superior to rebound hammer as the larger volume of concrete assessed creates a more representative assessment of overall concrete strength.
Wide application as calibration (conversion curve) is not affected by concrete mix variation.
What are the disadvantages of Pull-out testing?
Test setup does not replicate real-life conditions perfectly: the concrete is compressed while the bar is under tension, which is not typically the case in structures.
The test can exhibit greater resistance to splitting due to friction at the bearing end
Test precision can be affected by trapped air from improper disc placement
Embedded length can influence results due to “cone failure” or local cracking
Max size and type of aggregate can affect test results reducing ability to benchmark between different projects
How accurate is Pull-out testing?
An accuracy of +/- 3 N/mm2 for aggregates with 16 mm maximum size is possible according some manufactures.
What are the limits of Pull-out testing?
Pull out tests are partially destructive and induce local damage to a small area of the concrete
Discs need to be cast in before the element is cast otherwise a CAPO test should be specified.
This test is not suitable for concretes with aggregates greater than 38mm.
The evaluation of concrete compressive strength using this technique cannot replace the mechanical characterisation using a compression-testing machine but can provide and estimation with a reasonable accuracy.
Ancillary information
Maturity of test: >10 years
Qualification & interpretation : Inspector
Service disruption: No
Preliminary works: Yes
Time consumption Low (< one hour)
Cost Medium
Access to element 1 face
Guides and further information