Flexural strength of concrete tests how well a concrete slab or beam made without reinforcement will survive bending-related failures. ASTM C 293 or ASTM C 78 techniques are used to measure the Flexural strength. These measurements are expressed as MR (Modules of Rupture). Depending on how much coarse aggregate is used, these MR ranges are between 10% – 20% of compressive strength. Flexural strength is measured by using concrete beams.
Importance of flexural strength of concrete
- Some people utilise MR to approve and control pavements in the field. As a result, a laboratory mix design based on flexural strength tests may be necessary, or a cementitious material content may be selected based on prior experience to obtain the needed design MR.
- Flexural testing is seldom ever used for structural concrete. The use of compressive strength is typically convenient and trustworthy for organizations not employing flexural strength for field control to assess the quality of the delivered concrete.
Applications of flexural strength of concrete
The applications of the flexural strength of concrete test are as follows:
- Determining standards compliance
- It is a key necessity for the design of the concrete mix
- It is used to test concrete for the building of slabs and pavement
Flexural strength testing: Methods and procedures
- In the field, beam specimens must be made correctly. The concrete used for pavement is firm. Consolidate by shaking and taping the sidewalls to expel any air bubbles. After rodding, tap the moulds to release air bubbles and spade down the edges to consolidate for the higher slump. At no point can the beam surfaces be allowed to dry. Before testing, soak for at least 20 hours in saturated limewater.
- The standard deviation for concrete flexural strengths up to 800 psi is roughly 40 to 80 psi. Over 100 psi standard deviation numbers may be a sign of a testing issue. Testing issues or moisture discrepancies inside a beam brought on by early drying are most likely to result in low strength.
- When a connection between flexural and compressive strength has been demonstrated in a laboratory setting, compressive strength can be determined using ASTM C 42 core strengths, which can then be compared to the desired value using the ACI 318 criteria of 85% of the specified strength for the average of three cores. Sawing beams from a slab for flexural testing is not practical. Saw beams are not advisable since they will significantly diminish their measured flexural strength of concrete.
Concrete specimen size
- As per ASTM, the specimen – 150 mm wide, 150 mm deep, and its length – is not more than three times its depth.
- The concrete dimensions – 150 mm in width, 150 mm in depth, and 700 mm in span according to Indian standards.
- It indicates that sizes of 100 mm in width, 100 mm in depth, and 500 mm in span may be employed as long as the maximum aggregate size is 19 mm.
- Square specimen cross-sections with dimensions of 100 or 150 mm and spans between four and five times specimen depth are required by British standards. But it selected a specimen with dimensions of 750mm in length, 150mm in width, and 150mm in depth.
Equipment used to test flexural strength of concrete
- Moulds made of non-absorbing materials such as steel, iron, or cast with a size of (150mmX150mmX 750mm)
- Tamping rods: ASTM specifies large rod (16 mm in diameter and 600 mm in length) and small rod (10mm diameter and 300mm long)
- Testing device that can apply weights consistently without interrupting shocks
- Scoop
- Trowel
- An accurate balance of 1g
- Concrete mixer with power
- Table vibration when compacting concrete in moulds with vibration
Difficulties
- The handling, preparation, and curing of the specimen significantly impact flexural tests. When handled and moved from the workplace to the lab, beams might be damaged because of their weight. Lower strengths are produced by letting a beam dry. Beams need to be cured according to industry standards and tested while still wet. Due to the enormous difficulty in meeting all these parameters on a Jobsite, MR values are frequently unreliable and typically low. A sudden decrease in flexural strength might result from a brief drying process.
- For task management and quality assurance of concrete pavement, several state highway agencies previously used flexural strength but are now switching to compressive strength or maturity principles. The information suggests that a review of present testing practices is necessary. Additionally, they contend that while the flexural strength test is appropriate for research and laboratory evaluations of the components and proportions of concrete, it is too sensitive to testing variations to be used as a foundation for the acceptance or rejection of concrete in the field.
- Traditional cylinder compression tests for concrete control and acceptance are far more common in the concrete industry and among inspection and testing organizations. Flexure can be utilized in the design, but the matching compressive strength should be used to order and accept the concrete. Flexural and compressive testing should be performed on each trial batch to allow for the development of a correlation that may be used for field control.
The flexural strength of concrete is extremely important for construction purposes. Pavement designers employ a theory based on flexural strength, so laboratory mix design based on flexure may be necessary. To produce the required MR design, cement content may be chosen based on prior experience. Few try outthis test for structural concrete, but some additionally use MR for the acceptance of pavements and field control.
Flexural strength vs compressive strength of concrete
Flexural strength of concrete tests how well a concrete slab or beam made without reinforcement will survive bending-related failures while compressive strength can be defined as the capacity of concrete to withstand loads before failure. Flexural strength is typically from 10 to 15% of the compressive strength.
FAQs
What is concrete’s maximum flexural strength?
Depending on the particular concrete mix, flexural strength ranges from 10 to 15 per cent of compressive strength. The ASTM C78 and C293 tests are two common procedures used to gauge the flexural strength of concrete.
What is the flexural strength requirement for concrete?
The pavement must have a flexural strength of 4.1 MPa (600 psi) before 14 days have passed since installation. These specifications raise several concerns regarding the quality of concrete specified and the procedures used to assure compliance with the standards.
What influences the concrete’s flexural strength?
The flexural tensile strength of concrete affects the behaviour of concrete structures in terms of deflection and cracking. Numerous parameters, including the level of stress, size, age, and confinement to concrete flexure members, have been proven to affect the flexural tensile strength of concrete.
What is flexural failure?
This kind of failure happens when the loads on the beam are greater than its flexural strength. The beam’s shear strength must be more significant than its flexural strength to prevent shear failure from occurring before flexural failure.
Is tensile strength and flexural strength similar?
No, The tensile strength is weaker than the flexural strength of concrete.