Ways for measuring the Strength of Concrete in Construction Sectors

In this article, we will review the potential options and practical solutions for on-site evaluation of concrete strength. Strength of Concrete is by far the most important property of concrete. It represents the mechanical properties of concrete.

On-site evaluation of the strength of concrete is the main challenge in the condition assessment of existing infrastructure. Owners and managers of such facilities prefer non-destructive methods to avoid further damage to an already struggling structure.

1- Compression Test On Concrete Cores

Coring and testing for the strength of concrete is the first and most reliable solution. In this case, the concrete core is taken from the existing structure. The core needs cutting (sawing) and surface preparation. The core is tested for compressive strength.

Pros:

1) This is the most reliable method to estimate the compressive strength. The technique is relatively fast.

Cons:

1) It is destructive. Not only it damages concrete integrity, but it might also affect reinforcing bars in RC structures. Rebar locating tools are needed to avoid this problem.

2) Selecting test locations can be difficult. Choosing the best site of cores is relatively subjective.

3) The locations of cores need to be repaired.

4) Coring is not an option for owners of important structures, especially when there are concerns about further damaging the structure.

2- Pull Out Test

The concept behind this method of testing the strength of concrete is that the tensile force required to pull a metal disk, together with a layer of concrete, from the surface to which it is attached, is related to the compressive strength of the concrete. The pull-out test is generally used for early diagnosis of the strength of concrete problems. However, it can be used to evaluate the strength of concrete in existing structures. Pull out testing involves attaching a small piece of equipment to the exterior bolt, nut, screw, or fixing. This is then pulled to the designated stress load level to determine how strong and secure the fixing is.

Pros:

1) Relatively easy to use.

2) If the relationship to strength is established, the method can deliver robust test results.

Cons:

1) It involves crushing and damaging concrete

3- Rebound Hammer

The methods based on the rebound principle consist of measuring the rebound of a spring-driven hammer mass after its impact with concrete. The test has been widely used, since its introduction in 1948. The main reason behind its popularity is its simplicity and convenience of use for testing the strength of concrete. Rebound hammer is used to evaluate the surface hardness.

Pros:

1) It is easy to use for most field applications.

2) The test can be used to study the uniformity of concrete

Cons:

1) The method is very subjective

2) surface condition, presence of rebar, presence of sub-surface voids can affect the test results

On-Site Evaluation of Concrete Strength – Rebound hammer.

4- Ultrasonic Pulse Velocity

Ultrasonic Pulse Velocity (UPV) is an effective method for quality control of concrete materials and detecting damages in structural components. The UPV methods have been traditionally used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. The test procedure has been standardized as “Standard Test Method for Pulse Velocity Through Concrete” (ASTM C 597, 2016). The concept behind the technology of strength of concrete measures the travel time of acoustic waves in a medium and correlating them to the elastic properties and density of the material. Travel time of ultrasonic waves reflects the internal condition of the test area. Some researchers have tried to develop a relationship between strength and wave speed.

Pros:

1) UPV can be used to detect other sub-surface deficiencies

Cons:

1) The method is affected by the presence of rebar, voids, and cracks.

2) There are not enough results for assessing the reliability of the method in the field.

5- Combined Methods

Combined methods involve a combination of NDT methods for predicting the on-site strength of concrete. Several researchers have studied the combination of UPV and Rebound hammer. Mixed techniques often deliver more comprehensive results. The improvement of the accuracy of the strength prediction according is achieved by the use of correction factors taking into account the influence of cement type, cement content, petrologic aggregate type, fine aggregate fraction, and maximum aggregate size. The accuracy of the combination of rebound hammer and ultrasonic pulse velocity results in improved efficiency in estimating the compressive strength of concrete.

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