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What is pullout tests on hardened concrete?

 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  (Cut and Pull out 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.

LOK Test

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.

LOK Test Process. H indicated the highest pullout force.

  • CAPO test  (Cut and Pull out Test)

The CAPO-TEST permits performing pullout tests on existing structures without the need of preinstalled 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.

Cut and Pull out Test

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.

CAPO Test on Concrete Slab

Relationship between the pullout force and compressive strength:

The relationship between the pullout force Fu in kN and compressive strength Fc in MPa is given below,

Typical Pull out Force Calibration Chart

By measuring the pull-out force of a cast-in disc or expanded ring, the compressive strength of in-situ concrete can be determined from the relationship in fig.4 to a great degree of confidence.

Pull off force compressive strength relationship

The pullout test produces a well defined in the concrete and measure a static strength property of concrete. The equipment is simple to assemble and operate.

The compressive strength can be considered as proportional to the ultimate pullout force. The reliability of the test is reported as good. It is superior to rebound hammer and Windsor probe test because of greater depth of concrete volume tested.  However this test is not recommended for aggregates beyond size of 38mm.

The major limitation of this test is that it requires special care at the time of placement of inserts to minimize air void below the disc besides a pre-planned usage.

Uses:

  1. Determine in-situ compressive strength of the concrete
  2. Ascertain the strength of concrete for carrying out post tensioning 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.

Post Test Process:

After the concrete has fractured by this test, the holes left in the surface are first cleaned of the dust by a blower. It is then primed with epoxy glue and the hole is filled with a polymer-modified mortar immediately thereafter and the surface is smoothened.

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