The fundamental principle behind pull out testing is that the test equipment designed to a specific geometry will produce results (pull-out forces) that closely correlate to the compressive strength of concrete. This correlation is achieved by measuring the force required to pull a steel disc or ring, embedded in fresh concrete, against a circular counter pressure placed on the concrete surface concentric with the disc/ring.

Types of Pull Out Tests:

Depending upon the placement of disc/ring in he fresh concrete, pull out test can be divided into 2 types,

1) LOK test

2) CAPO test

LOK Test:

The LOK-TEST system is used to obtain a reliable estimate of the in-place strength of concrete in newly cast structures in accordance with the pullout test method described in ASTM C900, BS 1881:207, or EN 12504-3.A steel disc, 25 mm in diameter at a depth of 25 mm, is pulled centrally against a 55 mm diameter counter pressure ring bearing on the surface. The force F required to pullout the insert is measured. The concrete in the strut between the disc and the counter pressure ring is subjected to a compressive load. Therefore the pullout force F is related directly to the compressive strength

CAPO test

The CAPO-TEST permits performing pullout tests on existing structures without the need of pre-install inserts. CAPO-TEST provides a pullout test system similar to the LOK-TEST system for accurate on-site estimates of compressive strength. Procedures for performing post-installed pullout tests, such as CAPO-TEST, are included in ASTM C900 and EN 12504-3.When selecting the location for a CAPO-TEST, ensure that reinforcing bars are not within the failure region. The surface at the test location is ground using a planing tool and a 18.4 mm hole is made perpendicular to the surface using a diamond-studded core bit. A recess (slot) is routed in the hole to a diameter of 25 mm and at a depth of 25 mm.A split ring is expanded in the recess and pulled out using a pull machine reacting against a 55 mm diameter counter pressure ring. As in the LOKTEST, the concrete in the strut between the expanded ring and the counter pressure ring is in compression. Hence, the ultimate pullout force F is related directly to compressive strength.

Uses:

1) Determine in-situ compressive strength of the concrete

2) Ascertain the strength of concrete for carrying out post tension operations.

3) Determine the time of removal of forms and shores based on actual in-situ strength of the structure.

4) Terminate curing based on in-situ strength of the structure.

5) It can be also used for testing repaired concrete sections.

The pull-off test

The pull-off test, also called stud pull test is a near-to-surface method in which an adhesive connection is made between a stud and the carrier (or object to be tested) by using a glue, possibly an epoxy or polyester resin, that is stronger than the bond that needs to be tested. The force required to pull the stud from the surface, together with the carrier, is measured.

Simple mechanical hand-operated loading equipment has been developed for this purpose. When higher accuracy is required, tests can be performed with more advanced equipment called a bond tester. A bond tester provides more control and possibly automation. Applying the glue automatically and curing with UV light is the next step in automation. This methodology can also be used to measure direct tensile strength or/and the bond strength between two different layers.