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There are various near-to-surface test methods which can be used to assess strength. Pull-out, pull-off, break-off and penetration resistance tests are described in BS 1881: Part 207.
The most common form of pull-out test is the internal fracture test which was developed by BRE. In this test a wedge anchor is pulled from a pre-drilled hole. Similarly, the Capo (Cut and Pull Out) test uses an expanding insert in an under-reamed hole. The LOK test relies on inserts cast into the concrete surface. A method for determining the pull-out force is given in BS EN 12504-3, Testing concrete in structures - Determination of pull-out force.
Pull-off tests measure the force required to pull a metal block (dolly) from the concrete surface to which it has been attached using a high strength adhesive. The break-off test measures the transverse force required to break off a core drilled 70 mm into the concrete surface. Finally, the penetration resistance test (Windsor probe) is based on measuring how far a hardened pointed metal rod penetrates into a concrete surface when fired into it by a driver unit similar to a gun.
For each of the above tests, there is no unique relationship between the measured parameter and strength. Information suggesting any such relationship that might be provided with the equipment should be treated with caution as its use could lead to errors in assessment of compressive strength.
Acknowledgement: The Concrete Society
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