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How to measure the hardness of concrete and why it is important

Hardness is seen as a “property” of objects that is difficult to physically alter when subjected to different forms of deformation.

It is common knowledge that diamond is the hardest material, but why is this?

To measure the hardness of something you need a scale and instruments that allow it. The first scale was devised by German geologist Friedrich Mohs.

Mohs worked on cataloguing and developing a scale to evaluate the hardness of various minerals. In this way, he was able to determine that diamond is the hardest material.

Cured concrete is a relatively hard material, as its hardness varies between 3 and 7 Mohs. Although this measurement does not indicate anything relevant to concrete, it bears a remarkable relationship to its strength.

Source: redbubble.com – Designed by DedEye

What is hardness

Hardness is not a true property of materials, since it depends on certain properties of a material, such as ductility, resistance, rigidity, elasticity, viscosity, deformation, among others. Rather, it is a property that is attributed to any object capable of resisting change when it is suffers abrasion or scratching.

Objects such as wood, which can be easily scratched, have a lower hardness compared to steel or granite, since it is difficult to scratch them.

How to measure the hardness of a mineral

There are numerous tests to measure the hardness of any material, such as the Knoop test, the Rosiwal test, the Shore test and the Roswell test (in its two variants). In addition, each of the tests has its own scale.

However, none of the above-mentioned alternatives are used to measure the hardness of cured concrete, as they are specialized industrial mineralogy techniques. Non-destructive and easy-to-apply methods are required to analyse concrete on site, such as:

Rebound test

This non-destructive test consists of evaluating the impact resistance of a concrete structural element. To perform it, an instrument called Schmidt’s hammer, also known as a sclerometer, is used.

Source: https://www.proceq.com/compare/schmidt-hammers/

The sclerometer contains a weight, which is tensioned by a powerful spring to impact the concrete surface, producing a rebound that is measured by the instrument. The unit of measurement is psi (pound per square inch).

Currently, digital sclerometers are used, which are capable to store and analyze large amounts of data, creating tables with maximum and minimum values.

It must be mentioned that this method is not completely reliable, as the measurements may vary by:

  • The angle of inclination of the sclerometer in respect to the surface of the concrete. There must be perpendicularity to obtain exact values.
  • Presence of dust and coatings on the concrete, as these damp the rebound.
  • Aggregate and concrete reinforcing steel can increase the value of the actual hardness.

Scratch test

Also known as hardness scale measurement, it is a very simple and economical method, which is why it is very common.

Basically, it consists of scratching the concrete surface with a series of 4 pencils, each one with a standardized point and calibrated according to the Mohs scale. The pencils are numbered from 2 to 9.

It is necessary to clarify that the Mohs scale starts at 1 and ends at 10, where talc represents the minimum value and diamond represents the maximum value.

To start the test, select the pencil with a lower caliber, either #2 or #3. By pressing the tip of the pencil against the concrete, draw a line about 5 centimeters long.

Finally, the pencil that has caused a significant scratch will determine the hardness of the concrete according to the Mohs scale.

Why it is important

The measurement of the hardness of concrete is important because it allows to know its resistance without employing destructive tests. Although the compression test is the conventional and most accurate method, it is destructive and costly.

Thanks to the rebound test it is possible to obtain approximate values of the compressive strength of cured concrete. The sclerometer produces readings in psi, which is a unit of stress (pressure), the nature of which is similar to compression.

Source: https://www.ibertest.es/products/ensayo-de-compresion/

As mentioned above, the rebound test is unreliable. However, with due consideration, acceptable values can be obtained, which are close to the actual value of the concrete strength.

As a complement, the scratch test allows to verify the resistance of a concrete floor to abrasion.

Improve hardness on concrete floors

In areas of industrial activity, it is essential that the concrete floor is maintained in good condition despite the traffic of heavy machinery.

In addition, it must have a maximum resistance to face any abrasion to which it is exposed, such as the transport of loads.

Proper maintenance and treatment of the concrete floor helps to increase its hardness and improve the surface for all types of industrial activities.

A BECOSAN® treatment improves the resistance and hardness of the concrete floor several points on the Mohs scale.

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