Expt 2. Flexural Strength Test of Concrete Beam

VIRTUAL LABS

11 Apr 202305:19

Summary

TLDRThis video demonstrates the flexural strength test of concrete according to IS 516 standards. The test assesses the concrete's ability to resist failure under bending, using a plain concrete beam subjected to two-point loading. It outlines the steps from preparing the concrete mix, curing the specimen, to testing the beam by applying a gradual load. The video explains the two types of beam failures, Type A and Type B, and how the modulus of rupture is calculated based on these failure types. The final flexural strength is expressed as the modulus of rupture.

Takeaways

🧪 The flexural strength test evaluates the bending strength of concrete indirectly.
🔨 It assesses the ability of an unreinforced concrete beam or slab to resist failure in bending.
📏 The test can be conducted using either a third-point load test or a two-point load test.
🧱 The concrete specimen size for testing is standardized: 150mm width, 150mm depth, and a span of 700mm, or 100mm width, 100mm depth, and a span of 500mm.
🔧 For sample preparation, mix cement, sand, aggregate, fly ash, and water by hand or with a mixing machine.
🛠️ The fresh concrete is poured into molds, compacted in three layers, and cured at 27°C for 24 hours.
🕒 After curing, the specimen is moist-cured at the same temperature until the testing time.
📊 During the test, the specimen is placed correctly, and the load is applied continuously at a constant rate until the beam fractures.
⚠️ Two types of failure are noted: Type A failure (fracture in the middle third of the span) and Type B failure (fracture outside the middle third but within 5% of the span).
🧮 The modulus of rupture is calculated using different formulas depending on the failure type, as per IS 516 standards.

Q & A

What is the purpose of the flexural strength test on concrete?
-The flexural strength test evaluates the tensile strength of concrete indirectly by testing the ability of an unreinforced concrete beam or slab to withstand failure in bending.
Which two methods can be used to conduct a flexural strength test on concrete?
-The flexural test on concrete can be conducted using either the third-point load test or the two-point load test.
What are the dimensions of the concrete specimen used in this test according to Indian Standard IS 516?
-According to IS 516, the dimensions of the concrete specimen are either 150 mm width, 150 mm depth, and 700 mm span, or 100 mm width, 100 mm depth, and 500 mm span. In this test, a 100 mm x 100 mm x 500 mm plain concrete beam is used.
What materials are used to prepare the concrete mix for the test?
-The materials used for the concrete mix include cement, sand, aggregate, fly ash, and water.
What steps should be followed after preparing the concrete mix before testing?
-Once the concrete mix is prepared, it should be poured into lubricated molds in three layers, each layer compacted on a vibrating table. After compaction, the molds should be left undisturbed for 24 hours at 27°C, followed by moist curing of the specimen at the same temperature until testing.
How is the specimen positioned in the machine for testing?
-The test specimen should be placed in the machine correctly, centered with the longitudinal axis of the specimen at a right angle to the longitudinal axis of the upper and lower rollers.
What is the rate of loading applied during the flexural test?
-The load should be applied without shock and increased continuously at a constant rate. For a smaller beam, the rate is 30 Newtons per minute, and for a larger beam, it is 67 Newtons per minute.
What are the types of failure observed in flexural strength testing?
-The two types of failure are: Type A, where the fracture initiates in the tension surface within the middle third of the span between the loading points, and Type B, where the fracture initiates in the tension surface outside the middle third of the span but within 5% of the span length.
How is the modulus of rupture calculated in case of Type A and Type B failure?
-In Type A failure, the modulus of rupture (fcr) is calculated using the formula: fcr = PL / BD², where P is the failure load, L is the span length, B is the width, and D is the depth. In Type B failure, the formula accounts for the distance from the fracture line to the nearest support.
What should be done if the fracture occurs outside the acceptable range?
-If the fracture initiates outside the middle third of the span length by more than 5% of the span length (unsatisfactory failure), the test results should be discarded.

Outlines

00:00

🛠️ Introduction to Flexural Strength Test on Concrete

The flexural strength test assesses the tensile strength of concrete indirectly by evaluating its ability to resist bending failure. This test is usually performed on unreinforced concrete beams or slabs using either the third-point or two-point load method. The script introduces the standard dimensions for the concrete specimen based on Indian Standard IS 516, where the experiment uses a 100x100x500 mm concrete beam.

05:02

🔬 Preparing the Concrete Specimen for Testing

Details on the preparation of the concrete specimen are provided, including selecting materials like cement, sand, aggregate, fly ash, and water, followed by mixing them either by hand or with a machine. The fresh concrete is poured into molds, compacted in three layers, and allowed to set for 24 hours at 27°C. After curing, the specimen is kept moist until testing time.

📏 Setting Up the Concrete Specimen for Testing

Before testing, the concrete specimen is cleaned and aligned properly in the machine. The setup must be precise, with the longitudinal axis of the specimen perpendicular to the loading rollers. The loading should be gradual, with different rates of loading for beams of varying sizes. After reaching the maximum load, the failure type and details are recorded.

🧪 Recording and Analyzing Beam Failure

The script explains the classification of failure types: Type A, where the fracture occurs in the middle third of the beam, and Type B, where the fracture is slightly outside this range. Specific guidelines for the allowable distance of the fracture for different beam sizes are provided, along with conditions for discarding unsatisfactory results.

📊 Calculating the Modulus of Rupture

The flexural strength of the beam, expressed as the modulus of rupture, is calculated using formulas specific to Type A and Type B failures. For Type A, the modulus is derived using the beam's dimensions and the failure load. For Type B, additional factors such as the distance from the neutral support are considered. The test results are discarded if the fracture occurs outside the acceptable range.

📝 Conclusion: Recording Failure and Strength

Once the test is complete, the failure load is recorded in the observation sheet, and the type of failure is noted. The test sample shows a Type A failure, and the modulus of rupture is calculated accordingly, reflecting the specimen's flexural strength as per IS 516.

📐 Flexural Strength Calculation

The modulus of rupture is the key parameter in expressing the flexural strength of the concrete specimen, summarizing the final output of the test.

Mindmap

Keywords

💡Flexural Strength Test

The flexural strength test evaluates the tensile strength of concrete indirectly by measuring its ability to resist bending or flexing. This test helps determine how well an unreinforced concrete beam or slab can withstand failure due to bending forces. In the script, this test is the main procedure being conducted, following guidelines specified by the IS 516 standard.

💡IS 516

IS 516 refers to the Indian Standard code for testing the strength of concrete. It provides guidelines for performing various tests, including the flexural strength test, to assess concrete's quality and durability. The video script mentions that the flexural strength test on concrete is performed according to IS 516, indicating adherence to standardized procedures.

💡Two-Point Loading Test

The two-point loading test is one of the methods used to conduct the flexural strength test. It involves applying load at two points on the concrete beam, which helps to assess the concrete's ability to resist bending. The script specifies that the experiment is performed using this method on a standard plain concrete beam, making it a crucial aspect of the test process.

💡Plain Concrete Beam

A plain concrete beam is a concrete beam that does not have any reinforcement like steel bars. In the flexural strength test, this type of beam is used to evaluate the unreinforced concrete's ability to resist bending. The script describes that a plain concrete beam with dimensions 100 mm x 100 mm x 500 mm is used for the experiment.

💡Compaction

Compaction refers to the process of removing air voids from freshly mixed concrete, ensuring it is densely packed and achieves the desired strength. In the script, compaction is mentioned as an essential step, where the fresh concrete is poured into molds in three layers, each being compacted using a vibrating table to improve the overall strength of the specimen.

💡Curing

Curing is the process of maintaining adequate moisture, temperature, and time to allow the concrete to achieve its desired strength and durability. In the video, it is stated that after the concrete specimens are removed from the molds, they are cured at 27°C until testing, which ensures the concrete reaches optimal strength before the flexural strength test.

💡Modulus of Rupture (fcr)

The modulus of rupture (fcr) is a measure of the flexural strength of concrete, indicating its ability to resist failure in bending. It is calculated using the formula provided in the script based on the type of failure (Type A or B). The modulus of rupture serves as an important indicator of the tensile strength of the concrete specimen under flexural load.

💡Type A and Type B Failures

Type A and Type B failures describe the different ways a concrete beam can break during a flexural strength test. Type A failure occurs when the fracture starts within the middle third of the span, while Type B failure occurs outside this middle third but within a specified range. The script provides criteria for identifying these failure types, which help determine the correct calculation of the modulus of rupture.

💡Rate of Loading

The rate of loading refers to the speed at which the load is applied to the concrete specimen during the flexural strength test. According to the script, the rate of loading should be 30 Newton/min for smaller beams and 67 Newton/min for larger beams. This controlled loading rate ensures that the test results are consistent and accurate.

💡Fractured Specimen

A fractured specimen is the concrete sample that has undergone the flexural strength test and has broken due to the applied load. Observing the fracture pattern and recording the type of failure provides valuable information about the concrete's strength characteristics. The script emphasizes examining the fractured specimen to identify the type of failure and determine the modulus of rupture.

Highlights

Flexural strength test on concrete as per IS 516 evaluates the tensile strength of concrete indirectly.

Flexural test checks the ability of unreinforced concrete beams or slabs to withstand failure in bending.

The test can be conducted using either the third-point load test or two-point load test.

A standard plain concrete beam with dimensions of 100x100x500 mm is used for the experiment.

Material proportions include cement, sand, aggregate, fly ash, and water.

Concrete mix is prepared and poured into molds in three layers with each layer compacted.

After 24 hours, the molds are removed, and specimens are moist cured at 27°C until testing.

Before testing, the specimen is placed in the machine with correct alignment, and loading rollers are wiped clean.

The load is applied continuously at a constant rate until the specimen can no longer sustain the load.

For smaller beams, the loading rate is 30 N/min, and for larger beams, it is 67 N/min.

Type A failure occurs when the fracture initiates in the tension surface within the middle third of the span.

Type B failure occurs when the fracture initiates outside the middle third of the span but within 5% of the span length.

Type A failure modulus of rupture (fcr) is calculated using the formula fcr = Pl/BD².

Type B failure modulus of rupture (fcr) involves measuring the distance between the fracture and the support line.

Unsatisfactory failure is when the fracture initiates outside the permissible range, and test results should be discarded.