compressive-strength

Concrete strength, how to measure it

In this article, we explain the following:
Concrete strength
required concrete strength for different structural elements
Effective factors in concrete strength
Tests to measure the strength of concrete

Concrete strength is one of the most important characteristics of hardened concrete and depends on several factors. The strength of concrete is used as a measure to confirm its quality. In this article, the effective factors and concrete strength tests will be introduced.

Concrete is a material composed of water, cement and aggregate. Concrete has two states, fresh and hardened. By passing from the fresh state to the hardened state during the hydration reaction, the behavior of concrete changes. With the passage of time during this chemical reaction, the strength of concrete increases and becomes stable with the passage of time. In fact, if concrete is not exposed to destructive factors, its resistance will increase over time.

Concrete strength

Concrete strength is the ability of concrete to withstand compressive, tensile, bending and shear loads. The strength of concrete is the most important feature of hardened concrete.
The strength of concrete provides a very good view of other characteristics and is used as a criterion to evaluate the quality of concrete.

The importance of concrete strength

Concrete is the most widely used building material. The quality of concrete used in different structures depends on its properties after mixing (fresh concrete) and after finishing the curing process (hardened concrete). The resistance of concrete against different loadings is a criterion for rejecting or confirming its quality and it plays a very important role in designing and analyzing the stability of different structures.

Required strength of concrete

The required strength of concrete is the bearing capacity required to build a structure or concrete members. The required resistance is determined based on the relevant standards and based on the type of load, the amount of loading and the structure under loading. Among the Iranian national standards for determining the required strength of concrete, the following can be mentioned:

Topic 9 of National Building Regulations: Design and Implementation of Reinforced Concrete Buildings

The sixth topic of national building regulations: loads on the building

Iran Standard 2800: Design of buildings against earthquakes

The ACI 318 standard is also one of the most widely used international standards for determining the required strength of concrete. In order to better understand the strength required for different concrete structures, consider the following table. Note that these values are given as examples in the table. You can choose the resistors you need based on the available plans and instructions.

The type or location of the required compressive strength concrete structure

Lean
less than 14 MPa

Basement, foundation, corridor, yard and stairs
Between 17 and 24 MPa

The entrance to the parking lot, garage and floors
Between 20 and 28 MPa

Reinforced beams, beams, columns and walls
Between 20 and 48 MPa

Precast and prestressed concrete
Between 28 and 60 MPa

Columns in tall buildings
Between 70 and 100 MPa

According to the ACI 318 standard, structural concrete must have a minimum compressive strength of 17 MPa. Of course, according to the use of the structure, higher values may be used in the design. Achieving very high resistances requires more precision in quality control and testing of concrete. Validation of concrete strength is done using special tests.

Types of concrete strength
When discussing the strength of concrete, it is usually assumed that its compressive strength is considered. Although, according to the type of applied load, it is possible to create bending, tensile, shear and twisting stresses in concrete. Therefore, the strength of concrete can be divided into compressive, tensile, bending, shear and torsional strengths. Among these items, compressive, tensile and bending resistances are more important and are evaluated more. In the following, these items are introduced.

Compressive strength of concrete
Compressive strength of concrete is its ability to withstand compressive axial load. The compressive strength of concrete is considered its most important feature. The compressive strength of concrete is formed after concrete starts setting and increases over time. The rate of increase of compressive strength of normal concrete with type 2 cement with the passage of time is almost according to the table below. Of course, this rate changes if other types of cements are used or in other cases.

concrete age
The ratio of compressive strength at the desired age to the final compressive strength of concrete

1 day
16 percent

3 days
40 percent

7 days
65 percent

14 days
90 percent

28 days
99 percent

The compressive strength required for concrete used in different structures depends on their use. In general, for normal concrete structures and members in Iran, a compressive strength between 20 and 30 MPa is sufficient.

The compressive strength of concrete is obtained from the average results of periodical tests (in certain time intervals) on fresh concrete samples. For example, according to the relevant standard, 7-day, 28-day, 56-day or 90-day tests may be used to evaluate the compressive strength of the structure and predict its performance in the desired conditions.

Tensile strength of concrete
Tensile strength is the ability of the material to withstand tensile axial loads. Concrete is a very weak material against tensile stresses. There are usually microscopic cracks inside the concrete. When tensile loading is applied, these cracks increase in length and continue until the moment of failure. The tensile strength of concrete is about 8 to 12% (on average 10%) of its compressive strength. At high compressive strengths, this value decreases. Due to the low tensile strength of concrete, it is usually ignored in calculations.

Bending strength
Flexural strength or modulus of rupture is the ability of the material to withstand lateral loads (in the width direction). Bending and compression loads are the most common types of loads in different structures. Slabs and beams are examples of elements under bending loading. The simplest example for this is a simple two-span beam whose center is loaded. When applying load on this beam, the part below the neutral axis in the cross section of the beam is under tension and the upper part is under compression.

Effective factors in concrete strength
The strength of concrete depends on various factors such as the quality and quantity of ingredients (water, cement and aggregate), the ratio of water to cement, age and curing temperature, etc. In the following, the way of the impact of important materials has been introduced.

The effect of cement on resistance
Cement is the most important factor in concrete strength. The type, quantity, proportion of chemical compounds, the way of making and the conditions of the cement storage environment affect the final strength of concrete. These items should be determined according to the standards and required usage. The ratio of cement to other materials such as water and aggregate is very important in controlling the physical and chemical properties of concrete, especially strength.

Effect of aggregate on concrete strength
About 70 to 80% of the volume of normal concrete is aggregate. Therefore, the quality of aggregate and its characteristics, including dimensions, shape, texture, particle resistance, chemical substances, and its granulation have a great impact on the strength of concrete. The most important factor in the final strength of concrete is the distribution of aggregates used in it. The use of fine-grained sand reduces voids between particles and weak areas of concrete, which increases the final strength of concrete. The ratio of coarse to fine particles in aggregate, the ratio of aggregate to cement and the moisture content of aggregates also affect the strength.

The effect of water on the strength of concrete
The quality and composition of water used in concrete affects its strength. Drinking water is the best option for making concrete with desired resistance properties. The role of water in concrete strength is not limited to quality and composition. The water-cement ratio of the concrete mix also affects the strength of the concrete. In this way, by reducing the ratio of water to cement under constant conditions, the strength of concrete increases. Of course, water must be sufficient to form concrete.

The effect of age on concrete strength
The age of concrete shows the progress of curing. This parameter is a measure to determine the relationship between temperature, age and strength of concrete. As time passes and the concrete begins to set, the hydration reaction continues at a certain rate. The age of concrete is used as one of the main criteria for resistance. Normal concrete reaches 99% of its final strength after 28 days. As a result, the older the concrete, the greater its resistance (before reaching the final strength).

Effect of temperature on concrete strength
The completion rate of the hydration reaction and concrete curing depends on the temperature. As the temperature increases, the reaction speed also increases, and in this way, with other parameters being constant, the speed of increasing concrete strength at high temperatures increases. Nevertheless, the final strength will be lower in these conditions; Because with the increase in hydration rate, the physical structure of cement paste will have less order and more porosity.

Concrete strength test
Concrete strength testing is a set of direct and indirect methods used to measure concrete’s compressive, tensile and bending strength. Concrete strength testing is performed in accordance with national or international standards and in a laboratory or on-site basis.

Compressive strength of concrete
Various tests have been defined to determine the compressive strength of concrete. These tests can be performed both in the laboratory and on-site. The standards for determining the compressive strength of concrete in the laboratory are:

Iranian standard 6048 according to ASTM C39 for cylindrical specimens
Iranian standard 3-1608 in accordance with BS EN 12390-3 for cubic and cylindrical samples
Iranian Standard 8985 in accordance with ASTM C873 for cast-in-situ cylindrical specimens

Despite the different shape of the samples (cylindrical, cubic or prismatic) and the way they are collected (sampling on site or made in the laboratory), the process of testing the compressive strength of concrete is similar to each other. The general steps of the test to measure the compressive strength of concrete are:
Measurement of sample dimensions,
Placing the sample in the testing machine and record the failure load of the specimen.

The compressive strength of the concrete sample is calculated by dividing the load at the moment of failure by the cross-sectional area under loading.

The size and shape of the tested sample has an effect on the obtained strength. In order to remove the influence of the dimensions of the sample, the strength obtained from the non-standard sample should be multiplied by the correction coefficients. The correction coefficients of compressive strengths are given in the existing standards in this field.

Tensile strength of concrete
The tensile strength of concrete is determined by three methods: direct tension, bending and diagonal compression. Since the tensile strength of concrete is very low compared to its compressive strength and its measurement is difficult, the direct method does not give accurate results. For this reason, indirect methods are more useful. One of the most widely used methods to indirectly determine the tensile strength of concrete is Iran’s standard 6047 or ASTM C496, entitled Brazilian test.

In the Brazilian test, a cylindrical sample is pressed horizontally (diagonal compression method). After determining the compressive load at the moment of failure, the value of the tensile strength is determined using the relationships provided in the existing standards.

Flexural strength of concrete
Flexural strength of concrete or modulus of rupture is determined according to Iran standard 490 or ASTM C78. In the test introduced in this standard, in addition to the bending strength of concrete, its tensile strength is also calculated indirectly; Because in homogeneous materials, bending and tensile strength are equal. Specimen specifications and equipment used for bending strength tests are different from compressive and tensile strength tests. The test to determine the flexural strength of concrete is performed as three points (applying the load to the center) and four points. The stages of testing to determine the flexural strength of concrete are:

Sample preparation
Placing the sample in the device (observing the distances on both sides and the loading axis)
load application

Finally, according to the dimensions of the sample, the distances of the supports and the loading points and the final load, the bending strength is calculated using the relationships provided in the existing standards.

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