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 :

Determine in-situ compressive strength of the concrete

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

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

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

It can be also used for testing repaired concrete sections.