Concrete bending test

Concrete is one of the most widely used construction materials worldwide. It has high compressive strength but relatively low tensile and flexural strength. Therefore, it is important to understand how concrete behaves under flexural loading. Flexural strength testing is a method for determining the flexural strength of concrete and assessing its durability and structural performance.

This test is typically performed on standard-size prismatic concrete beams and provides an indirect measurement of concrete tensile strength. The results are particularly important for the design of concrete pavements, slabs, and beams .

Purpose of the concrete bending test

The main objectives of this test are:

Determine the flexural strength (modulus of rupture) of concrete.
Checking the quality and homogeneity of the concrete in different areas of the plant.
Evaluation of the influence of water-cement ratio, type of additives and fillers on the strength of concrete.
Comparison of the flexural properties of different mixture compositions
Quality control of concrete in workshops and plants for the production of precast concrete elements

Experimental theory and justification

denoted by $the symbols RR$ or $f_r$ The formula for calculating flexural strength varies depending on the type of load.

1. Transfer two points

At this stage, the load is applied symmetrically to two points on the beam. The calculation formula is as follows:

$\frac{P \times L}{b \times d^2}$

Where:

$R$ : Flexural strength (MPa)
: Maximum load of a beam before failure (Newton $)$
$L$ : Distance between supports (mm)
$b$ : Sample width (mm)
$d$ : sample height (mm)

2. Move the third point

Some standards place the load at the mid-span. In this case, the formula is as follows:

$\frac{3P \times L}{2b \times d$

Necessary equipment

The following equipment is required to carry out a concrete bending test:

Bending testing machine with precise load adjustment
Standard prism template (usually 15 x 15 x 50 cm or 10 x 10 x 50 cm)
Strain gauges (if necessary for recording stress-strain curves)
Concrete mixers and vibrating tables
Spirit levels and calipers are used to check the exact dimensions of the sample.
Process tanks or wet rooms are used to store samples prior to testing .

How to perform the test

To perform a concrete flexure test, the following steps must be followed:

1. Create samples

First, create a concrete mix table with specific proportions (cement, water, sand, gravel and additives).
Clean and lubricate the prism mold to prevent the concrete from sticking.
Fresh concrete is poured into the mold in three layers, with each layer being vibrated or compacted.
Make the top smooth and level.

2. Sample processing

After 24 hours, the samples were removed from the mold and placed in saturated lime water at a temperature of 20–25 °C.
The treatment duration is usually 7, 14 or 28 days, depending on the purpose of the trial.

3. Prepare the equipment.

For packages measuring 15x15x50, the distance between the supports is usually 40 cm.
The specimen should be positioned so that the loading axis is perpendicular to the concreting direction.

4. Conduct an experiment.

The load was applied uniformly at a constant rate of increase (approximately 0.06 MPa/s according to ASTM C78).
In case of a fault, the maximum load (P) is displayed on the device screen.
Check and document the location and shape of the sample defect.

5. Calculate and report the results.

Calculate the bending force $R using the appropriate formula.$
Finally, the average value of the results of three samples was used as the flexural strength of the concrete.

Standard for flexural tests on concrete

The most important national and international standards in this area are:

ASTM C78/C78M: Standard Test Method for Flexural Strength of Concrete Using Third Span Load
ASTM C293: Alternative Method for Concentrating Loads at the Center of a Beam
IS 516 (India)
BS 1881: Part 118 (United Kingdom)
Iranian National Standard 4906

How to analyze and interpret the results

The failure mode in a concrete flexure test is usually one of the following:

Failure in the central strain zone → Ideal and normal failure
Destruction in the area of the column → indicates a deficiency in the load or quality of the concrete
Diagonal or shear failure → may occur due to poor adhesion between the aggregate and the cement paste

Typically, the flexural strength of concrete is 10 to 20% of its compressive strength. For example, if the compressive strength of concrete is 30 MPa , its flexural strength is 3 to 6 MPa.

Factors influencing the flexural strength of concrete

Flexural strength depends on several factors, including:

Water-cement ratio (W/C) : The lower the water-cement ratio, the higher the strength.
Mass and total volume
Type and dosage of cement
Processing conditions and ambient temperature
chemical or mineral additives
Age of the concrete at the time of testing

Safety and operating tips

Avoid sudden shocks and vibrations during charging.
To ensure the load is distributed symmetrically, the equipment must be correctly aligned.
Anomalous results (e.g., errors outside the curvature range) should be excluded.
The samples must not have any cracks or visible defects.

Dissolved Dissolved Flotation (DAF) Suez Aquadav

Finally

Flexural tensile testing of concrete is one of the most important quality control methods in civil engineering. It provides valuable information about the behavior of concrete under indirect loading. The results of this test help engineers select appropriate concrete mixes and ensure the reliability and durability of the structure.

In short, the flexural strength of concrete is an important indicator for assessing its structural properties . Careful execution of this test according to applicable standards ensures accurate and reliable results.