The fundamental principle behind pull out testing with LOK-test and CAPO test 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.
For hardened concrete, an expandable steel ring is used instead. This ring expands to fit a specially drilled hole and routed recessin the concrete. The first method, shown in figure 1 using the cast steel disc is called LOK test. The second method shown in fig.2 using expandable ring is called CAPO test (i.e. Cut and Pull out Test). The diameter of both the disc and ring is 25mm. the distance to the concrete surface is also 25mm. the inner diameter of the counter-pressure is 55mm.
Fig.1: LOK Test Insert
Fig.2: CAPO test testing principle
The relationship between the pullout force Fu in kN and compressive strength Fc in MPa is given in fig.3.
Fig.3: 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.
Fig.4: 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.
As the insert is pulled out, a roughly cone shaped fragment of the concrete is extracted. The compressive strength which is considered as an indicator of quality is obtained from the calibration curves, prepared based on laboratory and field tests conducted on concrete cubes and pull out samples cast with various grades of concrete. In the test assembly, the embedment depth and insert head diameter have to be equal with inner diameter of the reaction ring could vary between 2 to 2.4 times the diameter of the insert head. The apex of the conic frustum defined by the insert-head diameter and the inside diameter of the reaction ring can vary between 54 degree and 70 degree. The compressive strength can be considered as proportional to the ultimate pullout force. The reliability of the test is reported as good.
Since the embedment depths of the commercially available metallic inserts is of the order of 25 to 30mm, the test results cover a small portion of the near surface concrete located adjacent to the fractured surface and below the reaction ring. Thus due to the inherent heterogeneity of concrete, typical average within batch coefficient of variation of such pullout tests has been found to be in the range of 7 to 10% which is better than that of standard cube/cylinder compression test. It is superior to rebound hammer and Windsor probe test because of greater depth of concrete volume tested. This is not affected by type of cement and aggregate characteristics. 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.
Pull out tests are used to:
- Determine in-situ compressive strength of the concrete
- Ascertain the strength of concrete for carrying out post tensioning 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.
Here item No.(i) only are of relevance for residual strength assessment of old and distressed structures. It can be also used for testing repaired concrete sections. These tests are divided under the following two categories:
a) Embedding an insert into the fresh concrete while casting and then pulling out with a jack (LOK test – LOK meaning ‘punch’) after hardening after a specified period.
b) Insert fixed into a hole drilled into the hardened concrete and then pulling out with a jack (CAPO test- cut and pull out test). This second method offers greater flexibility for conducting in-situ tests on hardened concrete of existing structures and is explained above in detail.
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.