What is Pull-off testing of concrete?
Pull-off testing of concrete determines the tensile strength of concrete surfaces and the bond strength of coatings or repair materials. It involves adhering a metal disc to the concrete and measuring the force required to detach it, providing insights into the concrete's integrity and the effectiveness of surface treatments.
This test is crucial for evaluating the durability and safety of concrete structures and the success of repairs made to them, guiding maintenance decisions, and ensuring adherence to quality standards.
How does the Pull-off test work?
The test consists of pulling a small metal disc, which has been glued to the surface of the concrete, until concrete failure.
The ultimate failure force is empirically correlated to the on-site concrete strength with a calibration curve proposed by the manufacturers or implemented based on laboratory trials.
When the test is used for repair systems applied to the concrete surface it can give the tensile bond strength of repair materials to the substrate concrete and provide quantification of the repair success.
What is the Pull-off test used for?
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 | Adhesion quality control |
How do I carry out a Pull-off testing?
The methods of testing are employed dependant on the composition of the material to be tested.
For homogenous concrete where an assessment of overall compressive and flexural strength is required a metal disc is attached directly to the concrete surface via epoxy resin and pulled free using the test device. The force required to do so is then recorded.
For concretes expected to have varied performance or composed of multiple layers such as patch repairs, partial coring to below the repair level is used to isolate the test area and directly measure interface bonding.
What equipment and expertise are required for Pull-off testing?
Due to the high popularity and relative simplicity of Pull off tests and devices many commercial systems are available to fulfil a wide range of applications.
Examples include the Limpet device from Amphora consulting which can provide on site assessment concrete strength, the bond strength of patch repairs and the progressive deterioration of concrete and other building materials due to their interactions with the exposure environment.
What are the advantages of Pull-off testing?
Cheap, simple and quick to perform.
Leaves only a 50mm defect on the concrete surface.
Consistently accurate and reliable results can be achieved by an unskilled operator.
An inspection of the fracture surface allows unsatisfactory failures to be instantly discounted and gives useful information on the quality of the concrete.
Does not require planning in advance of placing the concrete.
Provides a check on safety in relation to when shuttering can be stripped, shoring can be removed or prestress can be applied, and can be used to check in situ strength if cubes do not achieve the design strength.
Devices are light and easy to handle making them perfect for onsite tests in challenging locations.
Pull off strength can be converted to compressive strength using an empirical correlation chart like shown below.
What are the disadvantages of Pull-off testing?
Adhesive curing time limits how quickly the test can be carried out
Partially destructive test method which creates minor, local concrete damage
How accurate are Pull-off tests?
Pull-off testing in concrete is a well-established method for determining the tensile strength of concrete surfaces, the bond strength of concrete repair, and overlay materials. Following standardised procedures such as those from BS 1881: Part 207 or ASTM C 1583, ensures adherence to specific quality controls that are vital for consistency and reliability.
While the pull-off test is standardized and can provide reliable data, the accuracy of the results also depends on the condition of the concrete, the quality of the bond between the test disc and the concrete, and the precision with which the test is performed.
Test accuracy can be significantly enhanced by using automated equipment (Limpet) which are easy to use and precisely record parameters such as the maximum load applied, bond strength calculations, and graphical load rate recording. This equipment ensures that the test is performed consistently and in accordance with the applicable standards, eliminating operator variations.
Researchers such as Ramos et al (2012) encountered a notable range in variation coefficients in pull out test results, spanning from lower bound of 40% to a substantial 100%. While there is a lack of consensus on acceptable coefficients of variation, 40% is typically acceptable for in-situ tests if at least 5 are carried out.
Completing a large number of pull off tests significantly reduces confidence intervals and can reliably establish a typical value for an entire building or building element. Average results and coefficients of variation from one their investigation is presented below.
What are the limits of Pull-off tests?
Pull off tests induce local damage to concrete because it is a partially destructive test.
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
References and further information: