This article is about flexural test of concrete procedure, flexural strength of concrete, flexural strength of concrete lab report, flexural strength formula, flexural strength of concrete beam, flexural strength of concrete vs compressive, flexural strength of concrete at 7 days.
What is Flexural Test?
The flexural test on concrete is a common procedure used to evaluate its strength and deformation characteristics. ASTM standards provide guidelines for conducting this test. However, there may be differences in specifications or other aspects of the test between ASTM, Indian, and British standards.
ASTM standards outline the specific procedures and requirements for performing a flexural test on concrete. These standards cover aspects such as specimen preparation, loading configuration, span-to-depth ratio, loading rate, and calculation of flexural strength. They provide a standardized approach to ensure consistency and reliability in the test results.
In contrast, Indian standards may have their own specifications and variations for the flexural test on concrete. These variations could include differences in the size and shape of the specimens, the span-to-depth ratio, the type of loading (either two-point or third-point loading), and the calculation methods for determining flexural strength. These variations reflect the specific requirements and practices followed in the Indian construction industry.
Similarly, British standards may also have their own set of specifications and differences for the flexural test on concrete. These differences might include variations in specimen dimensions, loading arrangements, loading rates, and calculation methods. British standards are designed to align with the practices and requirements of the construction industry in the United Kingdom.
It is important to consult the relevant standards and specifications when conducting a flexural test on concrete, taking into account the specific requirements outlined by the respective standards. By adhering to the appropriate standards, engineers and researchers can ensure accurate and comparable test results, as well as compliance with regional construction practices and regulations.
Following points related to flexural tests on concrete will be discussed here:
- What is flexural test on concrete?
- What is the application of flexural test on concrete?
- What are the factors that cause variability in flexural test results?
- Size of concrete specimen for flexural test
- Apparatus required for flexural test on concrete
- Sample preparation
- Test procedure
- Computation of modulus of rupture
What is Flexural Test on Concrete?
The flexural test is used to indirectly assess the tensile strength of concrete by evaluating its ability to resist bending failure. It involves subjecting an unreinforced concrete beam or slab to bending forces. The test results are expressed as a modulus of rupture (MR), which is measured in MPa or psi.
There are two methods to conduct the flexural test on concrete: the three-point load test (ASTM C78) and the center-point load test (ASTM C293). The setup for each test is depicted in Figure-2 and Figure-3, respectively. This article specifically describes the test procedure outlined in ASTM C78.
The modulus of rupture value obtained through the center point load test arrangement is approximately 15 percent smaller than that obtained through the three-point load test configuration. Additionally, when larger size concrete specimens are used, a lower modulus of rupture is observed.
The modulus of rupture generally ranges from 10 to 15 percent of the compressive strength of the concrete. This value is influenced by factors such as mixture proportions, size of the specimen, and the volume of coarse aggregate used in its construction. It should be noted that the equation provided can be used to calculate the modulus of rupture, but its significance for design purposes must be determined through laboratory testing.
Where:fr: Modulus of rupture fc‘: concrete compressive strength
Applications of Flexural Test on Concrete?
The flexural test has several applications. One of its key uses is in specifying compliance with standards. By conducting this test, engineers can determine if a material meets the necessary requirements outlined in industry standards.
Additionally, the flexural test is an essential requirement for concrete mix design. It provides valuable data that aids in the development of concrete mixes with desired strength and durability properties. Moreover, the test is commonly employed in testing concrete for slab and pavement construction. It allows engineers to assess the material’s ability to withstand bending forces, ensuring it is suitable for use in these applications.
What Factors Cause Variability in Flexural Test Results?
Concrete specimen preparation involves several key factors, including the size of the specimen, its moisture condition, curing process, and whether the specimen is molded or sawed to the desired size. The specimen size is an important consideration as it determines the amount of concrete needed and affects the testing procedure.
The moisture condition of the concrete specimen refers to its level of hydration, which can impact its strength and other properties. Proper curing of the concrete specimen is essential to ensure optimal development of strength and durability. Lastly, depending on the requirements, the specimen can be either molded or sawed to the specified size for testing purposes.
Size of Concrete Specimen for Flexural Test
ASTM specifies that the concrete specimen should have a width and depth of 150mm. Additionally, the length should be at least three times the depth of the specimen.
Indian standard determines the size of the concrete specimen as 150mm width, 150mm depth, and a span of 700mm. However, if the maximum aggregate size used is not greater than 19mm, a smaller size of 100mm width, 100mm depth, and a span of 500mm can be utilized.
British standard specifies a square cross-section for the specimen, with dimensions of either 100mm or 150mm. The span is recommended to be four to five times the depth of the specimen. However, the preferred size according to the British standard is 150mm width, 150mm depth, and a span of 750mm for the specimen.
Apparatus for Flexural Test on Concrete
To conduct concrete testing, molds made of nonabsorbent materials such as steel, iron cast, or other similar materials with dimensions of 150mm x 150mm x 750mm are required. Tamping rods, specified by ASTM, consist of a large rod measuring 16mm in diameter and 600mm in length, as well as a small rod measuring 10mm in diameter and 300mm in length.
The testing machine should be capable of applying loads at a consistent rate without interruptions or shocks. Other necessary tools include a scoop, trowel, and a balance with a 1g accuracy. If vibration is used to compact the concrete in the molds, a table vibrator or a power-driven concrete mixer may be used.
Sample Preparation of Concrete
To prepare the concrete mixture, determine the proportions of cement, sand, aggregate, and water. Mix these materials either manually or using a suitable mixing machine in batches that are 10 percent larger than the size of the molding test specimen.
After blending the materials, measure the slump of each batch of concrete. Place molds on a horizontal surface and apply a proper lubricant material to the inside surface of the molds, ensuring that excessive lubrication is avoided.
Pour the fresh concrete into the molds in three layers. For each layer, compact the concrete using a 16mm rod and apply 25 strokes. Alternatively, fill the mold completely and compact the concrete using a vibration table. Remove any excess concrete from the top of the mold and smooth it without exerting pressure.
Cover the top of the specimens in the molds and store them in a temperature-controlled room for 24 hours. Afterward, remove the molds and continue to moist cure the specimens at a temperature of 23+/-2 degrees Celsius until the time of testing.
The testing should be conducted at two different ages: 14 days and 28 days. For each testing age, prepare three specimens. According to the Indian Code, the specimens should be stored in water at a temperature of 24-30 degrees Celsius for 48 hours before being tested.
Procedure of Flexural Test on Concrete
To prevent surface drying and a decline in flexural strength, it is important to conduct the test on the specimen immediately after removing it from the curing condition. When placing the specimen on the loading points, ensure that the hand-finished surface does not come into contact with them, which will ensure proper contact between the specimen and loading points. The loading system should be centered in relation to the applied force.
Bring the block applying force into contact with the specimen surface at the loading points. The loads applied should be between 2 to 6 percent of the computed ultimate load. Use 0.10 mm and 0.38 mm leaf-type feeler gauges to determine if there is any space between the specimen and the load-applying or support blocks over a length of 25 mm or more.
If there is a gap greater than 0.10 mm, use leather shims (6.4 mm thick and 25 to 50 mm long) that extend the full width of the specimen to eliminate the gap. Consider capping or grinding to remove gaps in excess of 0.38 mm.
Load the specimen continuously and without shock until it fails, using a constant rate of loading. The loading rate can be determined based on specific standards such as the Indian standard (loading rate of 400 kg/min for 150 mm specimen and 180 kg/min for 100 mm specimen) or the British standard (stress increase rate of 0.06 +/- 0.04 N/mm2.s). The loading rate as per ASTM standard can be computed using a provided equation.
In order to determine the loading rate, rate of increase of extreme fiber, average specimen width, average specimen depth, and span length, the cross section of the tested specimen needs to be measured at each end and at the center. These measurements will allow us to calculate the average depth and height of the specimen.
Computation of Modulus of Rupture
The expression provided is utilized to estimate the modulus of rupture.
The modulus of rupture (MR) is a measure of a material’s ability to resist breaking under an applied load. It is typically determined through testing, where the ultimate load (P) is applied to the specimen and the span length (L), average width (b), and average depth (d) of the fractured section are measured.
References
Several standards and publications provide methods for testing the flexural strength of concrete. In 1983, the British Standard BS 1881: Part 118 outlined a method for determining flexural strength, while in 2000, the ASTM Committee C09 published two standards: C 192/C 192M – 00 for making and curing concrete test specimens and C 293 – 00 for flexural strength testing using a simple beam with third-point loading. Another ASTM standard from 2002, C 78 – 02, specified the flexural strength test method using a simple beam with center-point loading.
The Bureau of Indian Standards released a method of test for concrete strength in 1959, reaffirmed in 1999, under the title IS: 516. Additionally, the National Ready Mix Concrete Association published a guide called “Concrete in Practice” in 2016, providing practical information on concrete. Finally, a book by I. Kett titled “Engineering Concrete: Mix Design and Test Methods” was published in 2000 by CRC Press LLC, focusing on concrete mix design and testing methodologies.