If you’ve ever been responsible for quality control of new construction or assessing the condition of existing infrastructure, you know that on-site evaluation of concrete strength can be a major challenge. Most stakeholders will prefer non-destructive and non-intrusive methods of testing in order to avoid further damage to a structure that may already be struggling or require concrete shoring. These methods can mean less intervention, shorter periods of downtime, and savings on the project overall. However, the strength of concrete remains critical and must be evaluated thoroughly. The following list includes both conventional test methods for this factor as well as non-destructive alternatives for the evaluation of on-site concrete strength.
Compression Test on Concrete Core
Often considered a cost-effective and reliable solution, extracting concrete samples and testing for compressive strength is a common choice among many construction workers. In some cases, codes and guidelines will list this as the only approved method for evaluating concrete strength, requiring that a core is taken from the existing structure. One reason this may be preferred is it is a relatively fast method
This core will need to be cut or sawed, as well as to undergo surface preparation before the core is tested for compressive strength. However, there are a number of considerations to make with this technique, such as where to take the cores from, how many to take, and how to handle them properly.
While this is a reliable test, it is also more destructive than many others and may dimmish the concrete’s integrity. The location of the cores will ultimately need to be repaired, and this may not be an option when structures are important and cannot undergo any further damage.
The Pull-Out Test
This test relies on the tensile force required to pull a metal disk and a layer of concrete from the surface it is attached to, in order to determine the compressive strength of the concrete. The pull-out test is typically used for early diagnosis of strength problems and is relatively easy compared to other methods while providing robust results.
The pull-out test is performed by attaching a small piece of equipment to the exterior bolt, nut, screw, or fixing. It is then pulled to the designated stress load level, determining how strong and secure the fixing is. In many cases, this involves crushing or damaging part of the concrete and may necessitate future concrete shoring.
Rebound Hammer for Concrete Strength
The rebound principle consists of measuring the rebound of a spring-driven hammer mass after its impact with concrete, known as the rebound hammer. This test has been very popular in recent decades because it is simple and convenient in most field applications, especially when seeking the surface hardness of concrete.
Some do not prefer this method because it can be subjective, and surface hardness is not always directly correlated to the strength of concrete, though there is an association shown. It is also important to note that surface condition, the presence of rebar, and the presence of sub-surface voids may impact results.
Ultrasonic Pulse Velocity
UPV, or Ultrasonic Pulse Velocity, has been used as an effective method of quality control in concrete materials, detecting damage in structural components and sub-surface deficiencies. This method is most often used for homogenous materials such as metals or welded connections, but recent advancements have made it available for concrete materials as well. UPV works by measuring the time travel of acoustic waves in a medium and correlating them to the elasticity and density of a material, reflecting the internal condition of test areas.
UPV results can be negatively impacted by the presence of rebar, voids, and cracks in a surface.
Penetration Resistance Test
In this test, a specialty device drives a small pin or probe into the surface of the concrete being tested. The force used to penetrate the surface and the depth of the hole can be correlated to the strength of the existing concrete.
This method is relatively easy to use, especially directly on-site. However, data can be impacted by surface conditions as well as the type of form and aggregates used. This test will also require pre-calibration with multiple concrete samples in order to determine accurate strength measurements.
Combined NDT Methods
Some workers choose to combine non-destructive methods in order to cross-reference results without damaging structures in place. A popular combination is UPV and the Rebound Hammer method, which can deliver comprehensive results and prevent problems that may require concrete shoring or repair in the future.