Although concrete is a basic building material, how it is mixed, poured, and cured can affect how strong it is. Accurately determining the strength of concrete is essential to guaranteeing the longevity and safety of structures. This strength is measured using a variety of techniques, each having pros and cons of its own.
One popular technique is to test concrete cubes or cylinders in a laboratory. Prior to being put through compression tests, these samples are removed from the construction site and cured under carefully monitored circumstances. Although it takes time and tools, this method provides a clear picture of how the concrete behaves under pressure.
Using non-destructive testing methods, such as ultrasonic pulse velocity tests or rebound hammer tests, is an additional strategy. These techniques are helpful for determining the concrete strength of already-existing buildings or structures because they enable rapid evaluations without endangering the structure.
Every technique has a place, based on the particular requirements of a given project. Engineers and builders can guarantee that concrete structures fulfill safety regulations and function as planned for the duration of their lives by comprehending and selecting the appropriate technique.
The many faces of concrete strength
Concrete strength is a very broad concept.
Concrete strength comes in various forms:
- Design strength — allows full load on concrete of the selected grade. By default, such value should be for the product after standard testing of the sample at 28 days of age with natural aging.
- Normalized strength is determined by regulatory documents and standards.
- Required strength — symbolizes the minimum value that is allowed under the designed loads. Identified in construction laboratories.
- Actual strength — strength that is recognized directly during testing. It is precisely this that is the release strength — not less than 70% of the design strength.
- Stripping strength — the value of this characteristic shows when it is possible to strip samples or products without deformations.
Strength is defined as cubic compression strength in the commonly used sense.
However, among concrete workers in particular, they always make it clear which quality attribute they are referring to:
Let’s examine each of them in more detail on its own.
Compressive strength
Concrete’s cubic strength has historically served as the foundation for marking concrete. Its values are determined by subjecting cubic samples to a press test at the age of 28 days, with rib sizes of 150×150 mm. This number is accepted as the industry standard for figuring out how resistant concrete is to axial compression.
Other sizes of samples are acceptable. The obtained data are different based on the scale change.
Further calculations are provided in this instance to equalize the obtained values up to the cubic level. The process is very straightforward: the values are multiplied by the scale factor C, whose value is given in GOST 10180 – 2012.
Prismmatic strength (RB), although a standard test of cubic samples is conducted in all large plants, is the primary strength for compressed concrete elements. Compared to testing standard samples with a 150 mm edge (R), it displays lower values. It’s interesting to note that strength decreases as the height (h) to prism base area (a) ratio increases.
The strength value becomes comparatively stable at h/a=4. Consequently, the temporary resistance to axial compression with an aspect ratio of h/a=4 is used to compute the prismatic strength.
Why is only the cubic strength used if the prismatic strength more closely represents the primary attributes of concrete samples? Such an ambiguous question has a very straightforward answer.
Take note! Numerous factors influence a concrete sample’s strength, but the primary ones are the raw materials themselves, the composition chosen, and the curing conditions. However, inadequate compaction can also cause the sample to exhibit "poor" strength. And regrettably, this is not unusual.
In laboratory conditions, it is physically challenging to create a well-compacted sample of rigid concrete if there is no such issue with more mobile mixtures. Because of this, it is common to regard the cube strength as the primary in order to prevent human error from distorting the obtained values. Prismmatic strength is utilized in the design of reinforced concrete structures, though.
Bending tensile strength
Any concrete’s primary function, regardless of brand, is to support compressive loads. Its strength lies precisely in this. Thus, a property like concrete’s tensile strength under bending is hardly ever employed in "construction, industrial use." These indicators are useful for design-related tasks.
Since it can be challenging to generate the required loads for a sample, testing the tensile strength of a concrete mixture under bending is therefore a relatively uncommon procedure in any construction laboratory. As a result, these traits are more deliberate. Designers employ numbers and values that have historically been obtained from design schools.
Transfer strength
The transfer strength of concrete is a real concept. Construction sites do not use this terminology, and foremen are not always aware of "what it is and how to use it." This definition, which is solely production-related, indicates the concrete’s strength at the point of compression when the reinforcement’s stress is transferred to the concrete.
It is impossible to manufacture any prestressed product qualitatively without this crucial feature. The design documentation and other technical documents for the produced reinforced concrete product standardize this value. It typically receives a minimum of 70% of the design strength.
How can the strength of concrete be ascertained? Sure, it’s really easy.
For this, the transfer strength of concrete is calculated using the following straightforward formula:
- Rbp = 0.7B,
- Where: Rbp — transfer strength;
- B — design strength;
- 0.7 — constant coefficient.
Take note! Removing the product from stress is advised if the value obtained during testing matches the calculated value. If not, the decision to prolong the product’s prestressing time is made at the technologist’s or the laboratory head’s discretion.
Devices and equipment for determining the strength of concrete
Concrete strength can now be determined using a variety of techniques.
The necessary devices are utilized in accordance with them:
- Press — standard equipment of any construction laboratory. It can have different operating principles, but the most reliable and popular is hydraulic. There are many models and types of such equipment. Some can only test concrete samples: cubes for compression, and tension of cement beams. Others expand the scope of their use to testing large-sized blocks, bricks and other materials. You can use it to determine the strength of concrete in just a couple of minutes, you just need to know how to work with it and record the necessary values.
- Devices for determining the strength of concrete using a non-destructive method have gained unprecedented popularity today. A sclerometer can be used to check the strength of concrete structures during inspections in construction sites and in buildings that have long been commissioned. No need to cut cubes out of solid wood. Everything is done much easier. At the same time, the price of such devices is quite high – depending on the type and functions that the device for determining the strength of concrete using a non-destructive method has. You can test the required structure yourself, without the help of specialists. You only need to clearly follow all the parameters that the instruction for use. How to use a sclerometer, you can watch in more detail in the video in this article.
- Another device designed to identify the main characteristics is a hammer for determining the strength of concrete. Before the widespread use of glass meters, the standard Kashkarov hammer was constantly used at construction sites and in laboratories. Conducting a test using the elastic rebound method is quite difficult. Such a method for determining the strength of concrete requires certain skills and knowledge. 229690-88 GOST for determining the strength of concrete by non-destructive methods will help you navigate in this area. But it is best to master the elastic rebound in practice – this way there is a better chance of learning how to correctly conduct such testing.
| Method | Description |
| Compressive Strength Test | Measures how much pressure concrete can withstand before breaking, typically done using a hydraulic press. |
| Flexural Strength Test | Determines how well concrete resists bending forces, often used for concrete beams. |
| Split Cylinder Test | Assesses tensile strength by applying pressure to the sides of a cylindrical sample until it splits. |
| Ultrasonic Pulse Velocity Test | Uses sound waves to check the density and uniformity of concrete, which helps estimate its strength. |
| Rebound Hammer Test | Measures surface hardness with a spring-loaded hammer to estimate concrete strength. |
| Core Sampling | Involves drilling out cores from the concrete structure and testing them in a lab to determine strength. |
Determining the strength of concrete is crucial to guaranteeing the longevity and security of construction endeavors. Every strength measurement technique has a unique set of benefits and drawbacks. While more conventional methods, such as compression tests, offer a precise standard, more recent approaches offer more speed and flexibility. Selecting the best approach depends on the particular requirements of the project and the available circumstances.
When it comes to accuracy and dependability, conventional techniques like the compression test continue to be the best. These tests are widely recognized in the industry and offer a precise indicator of the load-bearing capacity of concrete. They do, however, need careful sample preparation and curing, which can take time.
Conversely, non-destructive testing techniques like rebound hammer tests and ultrasonic pulse velocity provide a rapid means of estimating concrete strength without causing structural damage. These techniques are useful for evaluating in-situ concrete and spotting possible problems in existing structures, even though they might not be as accurate as compression tests.
In the end, the method of choice is determined by the particulars of the project, such as the required level of accuracy, the state of the concrete, and the available resources. Comprehending the advantages and disadvantages of every technique aids in guaranteeing that the selected strategy efficiently satisfies the project’s requirements and enhances the construction’s overall safety and efficiency.
Concrete’s strength must be determined in order to guarantee its performance and durability during construction projects. Diverse techniques, ranging from basic tests and visual inspections to more sophisticated methods, offer distinct perspectives on the strength and quality of concrete. Comprehending these techniques, along with their benefits and drawbacks, aids engineers and builders in making well-informed decisions and guaranteeing the longevity and safety of structures.
