What Is Flexural Strength Of Concrete?
What Is Flexural Strength Of Concrete?
Flexural strength is a measure of the ability of concrete to withstand bending without breaking.
It is determined by testing 6 x 6 inch concrete beams using either third-point or center-point loading methods, as defined by ASTM C 78 or ASTM C 293, respectively.
The resulting value is expressed in psi (MPa) and is known as the Modulus of Rupture (MR). The flexural strength of concrete is typically 10-20% of its compressive strength, but this can vary depending on the type, size, and volume of coarse aggregate used.
Laboratory tests can be used to determine the specific MR for a given material and mix design.
Flexural strength is used in the design of pavements and can also be used for quality control and acceptance of concrete in the field, although it is less commonly used than compressive strength for structural concrete.
How To Use Flexural Strength
To ensure accurate testing of the flexural strength of concrete beams, it is important to follow proper procedures when making and preparing the specimens in the field.
The concrete mixture used for pavement should have a slump of between 1/2 and 2 1/2 inches and should be consolidated by vibration and tapping the sides to remove air pockets.
If a higher slump is needed, the molds should be tapped and the sides should be spaded to consolidate the mixture after rodding.
It is important to keep the beam surfaces moist at all times, and the specimens should be immersed in saturated limewater for a minimum of 20 hours before testing.
Variability in flexural strength results can be significant, with standard deviations ranging from 40 to 80 psi for projects with good control and flexural strengths up to 800 psi.
Higher standard deviation values may indicate problems with the testing process. Factors such as testing problems or differences in moisture content within a beam due to premature drying can result in low strength.
In cases where a correlation between flexural and compressive strength has been established in the laboratory, core strength testing using ASTM C 42 can be used to verify the desired compressive strength using ACI 318 criteria.
It is not advisable to saw beams from a slab for flexural testing, as this can significantly reduce the measured flexural strength.
In some cases, splitting tensile strength of cores may be used for testing, but experience with this method is limited.
How Do You Calculate The Flexural Strength Of Concrete?
To calculate the flexural strength of concrete, you can use a simple formula. This test measures the tensile strength of concrete by testing a beam of specific dimensions, typically 15x15x7 millimeters or 100x100x500 millimeters.
The flexural strength can be determined using either central point loading or two-point loading.
The test is performed on a machine with two steel rollers of 38 millimeters in diameter placed a specific distance apart, depending on the size of the specimen.
The concrete is prepared to a specific grade and placed in layers in a mold, which is removed after a day and the specimen is cured in a tank at a specific temperature.
After drying for 14 and 28 days, the specimen is placed on the rollers and a load is applied at a specific rate.
The load at which the specimen cracks is recorded, and the distance between the fracture line and the nearest support is measured.
The flexural strength is then calculated using one of two equations, depending on the size of the specimen and the distance between the fracture line and the nearest support.
In this process, the load is applied at a rate of 400 kilograms per minute to 150 millimeter specimens and at a rate of 180 kilograms per minute to 100 millimeter specimens.
The flexural strength of the concrete is then calculated based on the point at which the cylinder cracks under the applied load.
If the distance between the line of fracture and the nearest support (I) is greater than 200 millimeters for 150 millimeter specimens or greater than 130 millimeters for 100 millimeter specimens, the flexural strength is calculated using the first equation provided.
If (I) is less than 200 millimeters and greater than 170 millimeters for 150 millimeter specimens or less than 133 millimeters and greater than 110 millimeters for 100 millimeter specimens, the flexural strength is calculated using the second equation provided.
In both equations, (b) represents the width of the beam in millimeters, (d) represents the failure point depth in millimeters, (L) represents the supported length in millimeters, (Pa) represents the maximum load applied to the beam in kilograms, and (a) represents the distance between the line of fracture and the nearest support in millimeters.
