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. The principal uses of LOK-TEST are:
- Determining whether in-place concrete strength is sufficient for early application of loads, such as due to formwork removal, application of prestressing.
- Determining whether the in-place strength is sufficient for terminating curing and thermal protection.
- Evaluating the quality of the critical cover layer protecting the reinforcement in the finished structure.
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.
Loading is performed either to a required force, in which case the test is nondestructive, or to the peak-load, which results in a slightly raised, 55-mm diameter circular crack on the surface.
The disc is cast into concrete either by attaching it to formwork before placing concrete or by inserting it manually into the fresh concrete. Various LOK-TEST inserts are available.
Correlation and Accuracy of Estimated Strength
LOK-TEST provides an accurate estimate of in-place strength because the peak pullout force has a well-defined correlation to compressive strength measured using standard cylinders or cubes. More than 30 years of correlation experience from all over the world indicates close agreement, suggesting that one general correlation is applicable for all normal density concrete mixtures, as shown below. A different correlation, however, has been found for concrete made with lightweight (low density) aggregate.
LOK-TEST Inserts and Ordering Numbers
LOK-TEST inserts are supplied in four different configurations and in two strength classes: (normal strength) 0 to 50 kN and (high strength) 0 to 100 kN pullout force.
- L-40 Control inserts for nailing to wooden formwork. The formwork has to be removed before testing.
- L-42 Early stripping inserts, with a steel plate for attachment to a removable plug through a porthole in the formwork, for use if testing has to be performed before the formwork is removed.
- L-45 Disc and stem, only, for replacement of used inserts
- L-49 Floating inserts for insertion into the top surface of newly cast concrete