Surface Hardness
The hardness of concrete is a measure of its rigidity and strength. It depends on several factors, which will be discussed in this article. The amount of aggregates, water to cement ratio, curing time, environmental conditions, and agitation during the mixing process are some of the factors that can impact the hardness of concrete.
Concrete Hardness
Concrete hardness is a physical property that measures the rigidity of the material, and it is typically determined by surface testing rather than calculation. While knowing the hardness of concrete is not always necessary, it can be useful in determining the in-situ compressive strength of the material. This is because hardness and compressive strength are often correlated. However, knowledge of the compressive strength is generally considered more important than knowledge of the hardness of concrete.
What influences concrete hardness?
As previously mentioned, concrete hardness is a measure of its rigidity, and it can be affected by various factors related to the materials within the concrete sample. Some of the factors that can impact concrete hardness are:
Concrete grade
The grade of the concrete is directly related to its hardness, with higher grade concrete being harder than lower grade concrete. Additionally, the hardness of concrete is directly related to its density, which increases as the grade of the concrete increases.
Curing
Curing is an important treatment for concrete that can enhance its properties. Adequate moisture in the concrete allows for the completion of the hydration process, which results in higher surface hardness and increased strength.
Aggregate
Coarse aggregates make up the highest volume of concrete, and their hardness can impact the hardness of the concrete.
Compaction
Proper compaction of the concrete can increase its density and strength, which in turn increases its hardness. Therefore, better compaction leads to higher concrete hardness.
Surface Hardness Test Method
Preparing Test Surface
Ensure the testing surface has a diameter of at least 150 mm.
If the surface is heavily textured or contains loose mortar, grind the surface until it is flat. Note that results from prepared surfaces may not be comparable to unprepared surfaces.
Remove any free moisture or water from the surface prior to testing.
Do not test frozen concrete; wait until it has thawed.
Avoid direct testing over steel reinforcement when the cover thickness is less than 20 mm. Use a rebar locator to avoid testing on shallow depth reinforcement.
Rebound Hammer Direction
Hold the instrument firmly and ensure the plunger is perpendicular to the test surface.
Record the orientation of the instrument with respect to horizontal to the nearest 45 degree increment. Use a positive angle if the instrument points upwards and a negative angle if it points downwards.
Taking Measurements
Push the instrument towards the test surface until the hammer impacts.
Maintain pressure on the instrument and depress the side button to lock the plunger in its retracted position.
Record the rebound number to the nearest whole number.
Take 10 readings from each test area.
Ensure the distances between impact points are at least 25 mm and at least 50 mm from the edges of the member.
If the impact crushes or breaks through a near surface air void, disregard the reading and take another one.
Calculation of Rebound Number:
Calculate the average of the 10 readings.
Discard any readings that differ from the average by more than 6 units.
Calculate the average of the remaining readings and report it as the average rebound number.
If more than 2 readings differ from the average by 6 units, discard the entire set of readings and determine rebound numbers at 10 new locations within the test area.
The range of ten readings should not exceed 12.
Note: The rebound number is a measure of surface hardness and is affected by factors such as surface texture, moisture content, and temperature. Therefore, it is important to ensure that test conditions are consistent to obtain accurate results.
Application of rebound hardness
Rebound hammers are widely used for measuring the hardness of concrete by employing the rebound principle. However, researchers have explored the method for assessing various properties of concrete. Some of the applications of rebound hammers are to assess the in-place uniformity of concrete, to delineate regions of poor quality or deteriorated concrete, and to estimate the strength of concrete.
Using the rebound hammer to estimate the strength of concrete remains a controversial topic among researchers. While some manufacturers provide calibration curves to estimate strength using rebound number, research has shown little theoretical relationship between concrete strength and rebound number. However, if a relationship between concrete strength and rebound number is established for a given concrete, the rebound number can be used for in-place evaluation of strength.
Two widely used approaches for establishing a relationship between concrete strength and rebound number are the ACI 228.1R and EN 13791 methods. The ACI 228.1R method requires a minimum of 12 concrete cores from 6 or more locations with different rebound numbers. The EN 13791 method requires only 9 cores from 9 different locations. To obtain accurate results, it is crucial to take test specimens from locations with similar surface texture and exposure conditions.
While the rebound hammer provides a quick and inexpensive method for nondestructive testing of concrete, the test results can be influenced by various parameters, including the smoothness of the test surface, size, shape, and rigidity of specimens, age of test specimens, surface and internal moisture conditions of concrete, type of coarse aggregate, type of cement, type of mold, and carbonation of the concrete surface.
It is essential to recognize the limitations of the rebound hammer test method and use it to determine the uniformity of concrete in structures, compare one concrete against another, and reduce the number of core samples. Rebound hammers should not be regarded as a substitute for standard compression tests.
In conclusion, rebound hammers developed by Schmidt provide a cost-effective and quick method for nondestructive testing of concrete. However, the limitations of the test method must be recognized and considered while interpreting the results.