Average concrete strength: how to determine

The durability and stability of buildings, bridges, and other structures are influenced by the strength of the concrete, which is an important consideration in construction. Knowing the average strength of concrete can help guarantee that construction projects adhere to safety regulations and function as planned over time.

Compression tests are one of the main methods used to gauge the strength of concrete. Concrete samples are taken for these tests, and they are compressed until they break. The findings provide builders and engineers vital information for their designs by clearly indicating the maximum weight that the concrete can support before failing.

Concrete strength is affected by a number of variables, such as the mixture of materials used, the curing process, and the placement environment. It is possible to produce concrete that satisfies particular strength requirements for various project types by carefully controlling these variables.

Multiple samples from the same batch are tested, and the average of their results is used to determine the average strength of the concrete. This method offers a more accurate assessment of the overall performance of the concrete by taking into account variations within a batch.

Sustaining constant concrete strength requires frequent testing and quality control. Construction workers can guarantee the longevity, dependability, and safety of their concrete structures by exercising constant vigilance and initiative.

Influence on the average compressive strength of concrete

A key factor in the design and construction of walls, foundations, and other building and structure components is compressive strength, which is expressed in MPa or kgf/cm^2.

Here, the consumer is informed by the concrete grade (M100, M200, M300, etc.) about the weighted average strength of the concrete at 28 days, expressed in kgf/cm2, and by the concrete strength class (B15, B20, B25, etc.) about the concrete’s guaranteed compressive strength (150 kgf/cm2, 20 kgf/cm2, 250 kgf/cm2, and t.p.).

The following primary factors determine the average strength of heavy concrete, as demonstrated by practice:

  • Cement activity. To prepare durable concrete, only newly manufactured material should be used.
  • Compliance of the amount of binder with the accepted proportions. Increasing the amount of cement above a certain norm leads not only to a significant increase in the cost of the product, but also to a deterioration in shrinkage, fluidity and average strength.
  • Ratio: grout-cement. Here the rule applies: the lower the grout-cement ratio, the higher the strength of the product and vice versa. The technical meaning of the rule is as follows. For workability of the mixture, when preparing concrete, a water-cement ratio of 0.5-0.9 is used depending on the brand of material. This is enough for the interaction of cement and other components. Water added above the specified ratios is “parasitic” and, by forming pores in concrete, significantly reduces its strength.
  • Strength, purity and geometry of large aggregate. Strength of concrete based on granite crushed stone is higher than the strength of concrete based on pebbles or gravel filler.
  • Quality of mixing of components and quality of compaction. When preparing concrete using a concrete mixer, vibration or turbo mixer, the strength of the final product is 20-25% higher than the strength of the product obtained by gravity mixing – manually.
  • Conditions for gaining strength and hardening. Under standard conditions (ambient temperature 18-20 degrees Celsius, ambient humidity 90-100%), the increase in strength occurs within a standard 28 days, and corresponds and corresponds to the "maximum possible". For example, the average strength of concrete in 15 hardening at a temperature of 5 ° C, at 28 days (age) corresponds to 68% of the grade strength, at a temperature of 10 ° C – 80% of the grade strength, at a temperature of 20-25 ° C – 110% of the grade strength.
  • Repeated forced vibration of the poured structure. It is carried out before the setting process using special equipment. It achieves an increase in the average strength of concrete class by an average of 15-20%.

Finding the average strength of concrete is crucial to guaranteeing the longevity and security of building endeavors. In order to determine how well samples from a batch of concrete can withstand pressure, they are tested under carefully monitored circumstances. Builders and engineers can decide whether concrete is suitable for a given structural application and of good quality by knowing the standards and procedures for testing concrete strength.

Technology for determining the average strength of concrete GOST 18105-2025

Laboratory testing at the manufacturer’s plant determines the average strength or grade of heavy concrete. GOST 18105-2010 compliant samples measuring 150 x 150 x 150 mm are formed from the concrete that is produced. Samples are filled into metal molds and left for 28 days in "normal" environmental conditions.

The samples are then compressed until they are destroyed by the working components of a laboratory press. The compression force’s magnitude is tracked. We calculate the class of concrete "B" using the following formula: B = R (1 – 1.64v), where R is the average strength of the samples and V is the strength variation coefficient equal to 13%. This is done by taking the arithmetic mean of the average strength of the concrete samples.

Within a grade, there is a 15% possibility that the control concrete samples’ average strength will be higher. An unwarranted rise in concrete consumption results from a continued increase in this indicator. The lower limit of the average strength of concrete should be between 70 and 80 percent of the material’s design grade strength, as per SNiP 3.03.01-87 "LOAD-BEARING AND ENCLOSING STRUCTURES" requirements.

Determination of the average strength of concrete by grade

Conducting laboratory tests in low-rise buildings or the field is both practically impossible and economically unfeasible. There is an approximate method that involves using a chisel and a 500–600 gram hammer to determine the average strength.

The basic technology is a visual evaluation of the material damage caused by the collision of a medium-strength hammer and a chisel:

  • After an impact of medium force, a barely visible mark remains on the surface of the concrete – the concrete corresponds to the M300-M350 grade (average strength 294-360 kgf / cm2).
  • After the impact, a clearly visible mark was formed – concrete grade M200 (196 kgf/cm2).
  • The chisel tip penetrated the body of the structure to a depth of 0.5 mm. It can be said that in front of you is concrete grade M150 (163 kgf/cm2).
  • The chisel tip penetrated the body of the material more than 10 mm – concrete grade M75-M100 (65-98 kgf/cm2).

The actual load that a specific concrete structure can sustain over an extended length of time is determined by the concrete’s average strength value. Thus, in order to maximize average strength, you should strictly follow the above-mentioned "Factors Affecting the Average Compressive Strength of Concrete" when preparing it yourself, or buy building materials from concrete plants.

Average Concrete Strength How to Determine
Compressive Strength Test Measure using a hydraulic press on a concrete sample after curing
Flexural Strength Test Test beams of concrete to check their resistance to bending
Rebound Hammer Test Use a rebound hammer to estimate surface hardness
Ultrasonic Pulse Velocity Send ultrasonic pulses through concrete and measure speed

To guarantee the longevity and safety of any construction project, it is imperative to ascertain the average strength of concrete. Through the process of testing and analyzing concrete samples, engineers and builders can acquire important knowledge about how the material behaves in different situations. Making well-informed decisions regarding the mix design, curing techniques, and overall quality control is aided by this.

Concrete strength can be tested using a variety of techniques, the most popular of which is the compressive strength test. To measure resistance, concrete samples are crushed in a controlled environment. A thorough grasp of concrete’s capabilities is also facilitated by additional methods such as non-destructive testing procedures and flexural strength tests.

Accurate determination of concrete strength requires following standardized procedures and using properly calibrated equipment. Consistency in sample preparation, curing, and testing conditions is vital to obtaining reliable results. By adhering to these practices, construction professionals can ensure that the concrete used in their projects meets the necessary strength requirements and contributes to the structural integrity of their buildings.

Long-term advantages come from knowing and keeping an eye on the strength of the concrete in addition to helping projects succeed immediately. It makes sure that buildings stay secure and operational over time, which lowers the need for expensive maintenance and repairs. Building professionals and engineers play a crucial role in developing robust and sustainable infrastructure by placing a high priority on precisely measuring the strength of concrete.

Video on the topic

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Determination of concrete strength

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Marina Petrova

Candidate of Technical Sciences and teacher of the Faculty of Construction. In my articles, I talk about the latest scientific discoveries and innovations in the field of cement and concrete technologies.

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