A flexural test also known as the modulus of rupture test or the bending test, is a standard method used to determine the strength and performance of concrete in bending or flexural conditions. This test method is critical for assessing the flexural properties of concrete. This is essential for structural applications where flexural stresses are prevalent.

During the flexural strength test, a rectangular concrete specimen is subjected to a gradually increasing load at its midpoint, causing it to bend until it fractures. This process is a key part of flexure testing and involves understanding the flexural modulus of the material. The maximum stress the sample can withstand before failure is a measure of its flexural strength.

The flexural strength of concrete provides valuable information about its ability to resist bending or flexural stresses. It is an essential parameter for structural design, as it influences the performance and durability of concrete members subjected to loadings that cause bending.

Test methods like the ASTM D790 are used for evaluating the flexural properties of plastics and other materials. The flexural stress and flexural strain experienced by the specimen provided insights into the material’s mechanical properties.

Steps to Conduct a Flexural Test

Steps to conduct a flexural test

Specimen Preparation

The specimen width and rectangular cross-section are crucial in these tests. Preparing small specimens according to ASTM standards is vital. This includes ensuring that the support span and loading points are set correctly for the three-point bending or four-point loading tests.

Test Setup

Place the cured specimen on the supports of the flexural testing machine. Ensure that the specimen is centered and aligned correctly. The loading points are typically positioned at a specific distance from each other, such as 135 mm (5.3 in), depending on the testing standard being followed.

Adjustment and Leveling

Make necessary adjustments to ensure the specimen is level and horizontal. This step ensures that the applied load is distributed uniformly along the length of the sample during the test.

Loading Arrangement

The loading span, span length, and loading nose are adjusted to apply the applied force effectively. The test speed and strain rate are controlled to simulate various loading conditions.

Preload Application

Apply a slight preload to ensure proper contact between the specimen and the loading points. This step helps eliminate initial gaps or irregularities in the specimen’s contact surface.

Load Application

Gradually apply the load at a controlled rate using the testing machine. The loading rate is usually specified in the testing standard and may vary depending on the test’s purpose—typical rates of loading range from 20 to 50 N/s (4.5 to 11 lbf/s).

Deflection Measurement

The deflection curve and maximum deflection and measured to understand the bending strain and shear stress experienced by the specimen. Load deflection curves are particularly important in flexural tests for composite materials and plastic materials.

Load Increment

Continue applying the load until the specimen reaches failure. The load increment should be steady and uniform, allowing sufficient time for the representative to exhibit its whole behavior under increasing loads.

Failure Identification

Observe and record the mode of failure. It could be a visible crack, complete fracture, or any other form of failure. Note the maximum load applied at the time of failure.

Calculation of Flexural Strength

  • Formula: Flexural Strength = 3PL/(2bd^2)
  • P = the force required
  • L = the span length
  • B = the specimen width
  • D = the depth

Using this formula, you can calculate the flexural strength. This calculation is central to standard test methods in testing machines.

Report Generation

Compile the test results, including the flexural strength value, specimen age, and other relevant information. Prepare a test report that can be used for analysis, quality control, and structural design purposes.

These steps ensure consistent and accurate flexural strength testing of concrete specimens, providing valuable information about the material’s performance under bending or flexural conditions.

Flexural strength testing of concrete provides valuable information about the material’s behavior and ability to withstand bending or flexural stresses.

Test Result Insights

Strength Assessment

Flexural strength testing primarily aims to determine the maximum stress or load a concrete specimen can withstand before failure occurs. This value, known as the flexural strength or modulus of rupture, measures the material’s resistance to bending. It indicates the capacity of the concrete to bear loads and withstand bending forces, which is crucial for structural applications.

Quality Control

Flexural strength testing primarily aims to determine the maximum stress or load a concrete specimen can withstand before failure occurs. This value known as the flexural strength or modulus of rupture, measures the material’s resistance to bending. It indicates the capacity of the concrete to bear loads and withstand bending forces, which is crucial for structural applications.

Material Characterization

Characterizing materials like unreinforced and reinforced plastics, metallic materials, and brittle materials using flexural tests in common. Understanding the outer fibers and the outer surface behavior under stress is crucial. 

Flexural Performance Evaluation

Evaluating the bending moment and shear stress in materials like polymer matrix composite materials and corrugated boards is essential. The core shear properties of materials like metallic bond plates are assessed using flexural tests.

Structural Design

The flexural test values obtained from testing are utilized in structural design calculations. Engineers use this data to determine concrete members’ required dimensions and reinforcement to ensure they can safely carry the anticipated loads and resist bending. Flexural strength is an essential parameter in structural analysis and plays a crucial role in ensuring the safety and reliability of concrete structures.

By conducting flexural strength testing, engineers and researchers gain valuable insights into concrete behavior, quality, and performance under bending conditions. This information is critical for making informed decisions in construction projects, assessing structural integrity, optimizing design parameters, and ensuring the durability and safety of concrete structures.

ASTM International (formerly known as the American Society for Testing and Materials) has several standards specifically developed for testing the flexural strength of concrete.

ASTM Flexural Testing Standards

ASTM standards such as ASTM C78/C78M, ASTM C293/C293M, and ASTM C1609/C1609M specify the procedures for three-point and four-point flexural tests. These standards are critical to flexural tests and bend testing of various materials.

ASTM C293/C293M – Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Center-Point Loading): 

This standard describes the procedure for determining the flexural strength of concrete beams using a simple beam with center-point loading. It provides guidelines for specimen preparation, test setup, loading, and calculation of flexural strength.

ASTM C1609/C1609M – Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with a Three-Point Load):

This standard specifies the procedure for determining the flexural performance of fiber-reinforced concrete using a beam with a three-point load. It is specifically designed for testing concrete specimens with added fibers to enhance their toughness and crack resistance.

These ASTM standards outline the requirements and procedures for conducting flexural strength tests on concrete specimens. In addition, they provide guidelines for specimen dimensions, testing apparatus, loading configurations, rates of loading, and calculation methods to ensure standardized and reliable testing practices across different laboratories and projects.

It’s important to note that other ASTM standards may be related to flexural testing, such as those specific to fiber-reinforced concrete or different loading configurations. The appropriate standard to be followed depends on the project’s particular requirements and the type of concrete being tested. Therefore, consulting the relevant ASTM standards or project specifications is essential for accurate and consistent testing.

Flexural tests are vital in determining the mechanical properties of materials like concrete, plastic, and composites. These tests, performed at a constant rate of loading, provide essential data for structural design and quality control. The insights gained from flexural tests aid in understanding how materials behave under bending and flexural stresses, ensuring their suitability for various applications.