What is a pull-out test and when is it required
Pull-out testing for in-situ concrete strength explained: what it does, how it works, and when it is the right test for your project.
Pull-out testing is one of the most useful in-situ tests available for concrete. It produces a quantitative, defensible estimate of compressive strength on hardened concrete — usually within a working day of attendance — without taking the structure out of service. It is widely used in the UK for verification, dispute work, and on existing structures where the original mix design is not known.
What it actually is
Pull-out testing measures the force required to pull a small steel insert out of the concrete. That force correlates with the compressive strength of the concrete at the same location, against a calibration curve that has been established over decades of laboratory and site work.
Two related methods are used. The Lok-test is used on new concrete: a metal insert is placed in the formwork before pouring, and the test is carried out a few days later when the concrete has cured. The Capo-test is used on existing concrete: a hole is drilled, an insert is mechanically expanded inside, and the same kind of pull-out force is measured.
The result is a force in kilonewtons. That force is converted, using the manufacturer’s calibration, to an estimated cube strength.
Why it works
The geometry of the test is what makes it defensible. The insert is pulled out against a counter-pressure ring of fixed diameter. The concrete failure mode is a small cone-shaped piece pulled out from the surface, and the force at failure is governed by the compressive strength of the concrete in the failure cone.
Decades of correlation studies — running pull-out tests against companion cube and core results — have produced calibrated relationships between pull-out force and compressive strength. The standard governing the method (BS EN 12504-3) sets out how the test is to be carried out and how the results are to be interpreted.
When it is required
Pull-out testing is the right tool whenever you need a quantitative estimate of in-situ strength without taking core samples — or in addition to coring, to give wider coverage at lower disruption.
On existing structures. Where the original mix design is unknown, the structure has been altered, or there is a question over compliance, Capo-testing gives a defensible strength estimate for the engineer’s calculation.
On new construction. Lok-testing in the formwork allows the contractor to verify strength at critical points — for example, before formwork striking — without sending samples to a lab.
For dispute resolution. When two parties disagree about the strength of an installed pour, a properly designed pull-out test programme produces a defensible answer that holds up to scrutiny.
As part of a wider assessment. On any non-trivial structural assessment, pull-out testing is one of a battery of tests. It pairs naturally with rebound hammer (cheap, fast, broad coverage) and core extraction (definitive, expensive, narrow coverage). The three together produce a calibrated picture that no one of them produces alone.
What the deliverable looks like
A defensible pull-out test deliverable includes:
- A method statement and risk assessment supplied before attendance.
- Calibration record for the testing apparatus.
- A record of every test location, with photographs.
- The raw force readings.
- The estimated cube strength derived from the calibration.
- Statistical analysis across the population of tests, including characteristic strength estimate where the population supports it.
- A short narrative interpretation.
- A surveyor sign-off.
For dispute work, expect the surveyor to use a slightly more conservative interpretation and to triangulate against any cube or core data that exists for the same pour.
What pull-out testing is not
Pull-out testing is not a replacement for laboratory testing of cores, where the brief calls for a definitive strength figure. It is an in-situ estimate against a calibration. The accuracy is good — typically within a few MPa of a companion core — but for the most rigorous engineering work the brief usually scopes both methods.
It is also not a structural assessment in itself. It tells you something about the strength of the concrete; it does not tell you anything about the reinforcement, the geometry, or the load. A pull-out programme is one input into the engineer’s wider assessment, not the answer on its own.
How to commission a pull-out programme
Briefing a pull-out programme well takes three things:
- A clear statement of the question. (“Is the concrete in this slab compliant with C32/40?” is a different brief from “What is the actual strength here for an assessment calculation?”)
- A representative sampling plan. Random distribution across the element gives a defensible result; cherry-picking does not.
- An honest read of access constraints. Capo-testing leaves a small visible mark, and the location matters.
A surveyor on the quote call should be able to walk you through the choice between Lok and Capo, the sampling density required for the question, and the calibration provenance. If they cannot, find another surveyor.