What is pullout tests on hardened concrete?
Pull out testing is based on the principle that the test equipment, which is designed with a specific geometry, will yield results in the form of pull-out forces that closely correspond to the compressive strength of concrete. This correlation is achieved by measuring the force needed to pull a steel disc or ring, which is embedded in fresh concrete, against a circular counter pressure that is placed on the concrete surface in a concentric manner with the disc or ring.
Types of Pull Out Tests:
There are two types of pull out tests based on the placement of disc/ring in fresh concrete, namely the LOK test and the CAPO test (Cut and Pull out Test).
LOK Test:
The LOK-TEST system is a widely used method for determining the in-place strength of freshly cast concrete in newly constructed structures. It follows the pullout test procedure as outlined in recognized standards such as ASTM C900, BS 1881:207, or EN 12504-3, and is considered a reliable means of obtaining accurate estimates of concrete strength.
LOK Test
A steel disc with a diameter of 25 mm and a depth of 25 mm is being pulled centrally against a counter pressure ring with a diameter of 55 mm. The force required to pull out the insert, denoted as F, is measured. The concrete in the strut between the disc and the counter pressure ring is under compressive load, and as a result, the pullout force F is directly related to the compressive strength.
LOK Test Process. H indicated the highest pullout force.
CAPO test (Cut and Pull out Test)
The CAPO-TEST is a method that allows for pullout tests to be conducted on existing structures without the requirement of preinstalled inserts. This test system, similar to the LOK-TEST system, enables accurate estimation of compressive strength on-site. Procedures for performing post-installed pullout tests, including the CAPO-TEST, can be found in ASTM C900 and EN 12504-3 standards.
Cut and Pull out Test
When selecting a location for a CAPO-TEST, it is important to ensure that there are no reinforcing bars within the failure region. To prepare the surface at the test location, a planing tool is used to grind it. Next, an 18.4 mm hole is drilled perpendicularly to the surface using a diamond-studded core bit. A recess or slot is then routed in the hole to a diameter of 25 mm and a depth of 25 mm.
A split ring is expanded in the recess, and a pull machine is used to pull it out, reacting against a 55 mm diameter counter pressure ring. Similar to the LOKTEST, the concrete in the strut between the expanded ring and the counter pressure ring is in compression. Therefore, the ultimate pullout force (F) is directly related to the compressive strength of the concrete.
CAPO Test on Concrete Slab
Relationship between the pullout force and compressive strength:
The pullout force (Fu) in kilonewtons (kN) is dependent on the compressive strength (Fc) in megapascals (MPa). The relationship between these two variables is described below.
Typical Pull out Force Calibration Chart
The compressive strength of in-situ concrete can be reliably determined by measuring the pull-out force of a cast-in disc or expanded ring, using the relationship shown in Figure 4. This method provides a high degree of confidence in accurately estimating the compressive strength of the concrete.
Pull off force compressive strength relationship
The pullout test is a simple and effective method for measuring the static strength property of concrete, as it produces a well-defined result. The equipment required for the test is easy to assemble and operate. The test measures the ultimate pullout force, which is proportional to the compressive strength of the concrete. It is considered reliable, with good results reported. One of the advantages of the pullout test over other methods such as rebound hammer and Windsor probe test is that it provides a greater depth of concrete volume tested, making it superior in terms of accuracy. However, it should be noted that this test is not recommended for aggregates larger than 38mm. Another limitation of the test is that special care must be taken during the placement of inserts to minimize air voids below the disc, and pre-planning is required for its usage.
Uses:
The in-situ compressive strength of concrete is a critical parameter that needs to be determined accurately before carrying out post-tensioning operations. This strength assessment helps ensure that the concrete is strong enough to withstand the additional loads imposed by the post-tensioning process without experiencing excessive deformation or failure. The in-situ compressive strength of the concrete can be determined using various testing methods, such as cylinder testing, core testing, or rebound hammer testing, depending on the requirements of the project and the availability of resources.
Once the strength of the concrete is ascertained, it is used as a basis for determining the appropriate time for the removal of forms and shores. The in-situ strength of the concrete provides valuable information on the maturity of the concrete, which is an indicator of its overall strength development. This information is crucial in determining when the forms and shores can be safely removed without causing any damage to the structure. It helps prevent premature removal, which can lead to structural failure or excessive deformation, or delayed removal, which can result in unnecessary delays in the construction schedule.
The in-situ strength of the concrete also plays a significant role in deciding when to terminate the curing process. Curing is a critical step in the concrete construction process that involves providing the right conditions for the concrete to gain sufficient strength and durability. By monitoring the in-situ strength of the structure, the appropriate time for terminating the curing process can be determined. This helps prevent over-curing or under-curing of the concrete, which can affect its long-term performance and durability.
Furthermore, the in-situ strength of the structure can also be used for testing repaired concrete sections. After concrete repairs, it is crucial to verify the strength of the repaired sections to ensure that they have regained their original strength and integrity. By conducting in-situ strength testing, the effectiveness of the repairs can be assessed, and any additional measures or adjustments can be made if necessary to ensure the structural integrity of the repaired sections.
In conclusion, the determination of the in-situ compressive strength of concrete is a critical factor in various construction processes, including post-tensioning operations, formwork removal, curing termination, and testing of repaired concrete sections. Accurate assessment of the in-situ strength of the structure helps ensure safe and efficient construction practices, preventing structural failures, and ensuring the long-term durability of the concrete structure.
Post Test Process:
After conducting the fracture test on the concrete, the resulting holes on the surface are cleared of dust using a blower. Once cleaned, the holes are primed with epoxy glue to prepare them for repair. Immediately after priming, a polymer-modified mortar is used to fill the holes, and the surface is then smoothed out for a finished result.