Flexural Strength Test of Ordinary Concrete: Standards, Procedures, and Applications

The flexural strength test of ordinary concrete, also known as the modulus of rupture test, is a core mechanical evaluation used to determine the bending resistance of hardened concrete beams under transverse loading. This article provides a comprehensive technical overview covering specimen requirements, testing procedures in accordance with GB/T 50081, ASTM C78, and EN 12390-5, calculation methods, result evaluation, and typical engineering applications. It serves as a practical reference for laboratory engineers, quality control professionals, and construction project stakeholders.

Technical Overview

The flexural strength test of ordinary concrete measures the ability of a hardened concrete beam to resist failure when loaded in bending, and the resulting value is commonly referred to as the modulus of rupture. Because concrete is significantly weaker in tension than in compression, flexural performance is a critical design parameter for structures such as concrete pavements, airport runways, and slabs on grade, where bending stresses govern the structural behavior.

In this test, a simply supported prismatic beam specimen is loaded at mid-span or at two points symmetrically placed within the middle third of the span. The maximum tensile stress in the extreme fiber at failure is calculated from the breaking load and the specimen geometry. The test is widely performed by accredited third-party laboratories, ready-mix concrete producers, and on-site quality control teams as part of mix design qualification and acceptance testing.

The most commonly referenced standards for this test include:

• GB/T 50081 — Standard for Test Methods of Concrete Physical and Mechanical Properties (China)
• ASTM C78 — Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)
• ASTM C293 — Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Center-Point Loading)
• EN 12390-5 — Testing Hardened Concrete: Flexural Strength of Test Specimens
• ISO 1920-4 — Testing of Concrete: Strength of Hardened Concrete

Testing Samples

Standard Prismatic Beam Specimens

The standard specimen for the flexural strength test is a prism with a cross-section of 150 mm × 150 mm and a length of 550 mm or 600 mm, suitable for concrete containing aggregate with a maximum nominal size not exceeding 40 mm. This specimen geometry is specified in GB/T 50081 and produces the reference test result for structural acceptance.

Small-Size Specimens

Where the maximum aggregate size is 31.5 mm or smaller, a reduced prism of 100 mm × 100 mm × 400 mm may be used, particularly for laboratory mix design studies and screening tests. Results obtained on non-standard specimens should be interpreted with care, since size effects can influence the measured modulus of rupture.

Curing and Conditioning

Specimens are cast in steel or rigid molds, demolded after 24 hours, and cured under standard conditions at a temperature of 20 ± 2 °C with relative humidity not less than 95% until the test age. The standard test age is 28 days, although tests at 3, 7, or 90 days may be performed to monitor strength development. A test set normally consists of three companion specimens. Before testing, specimens should be removed from the curing room, wiped to a saturated surface-dry condition, and tested promptly to minimize moisture loss.

Testing Items

The primary testing item is the flexural strength (modulus of rupture) of hardened ordinary concrete. Related evaluations commonly performed alongside or derived from this test include:

• 28-day flexural strength for mix design qualification and acceptance
• Early-age flexural strength at 3 or 7 days for formwork removal and traffic-opening decisions
• Verification of pavement concrete against specified design flexural strength grades
• Comparative evaluation of concrete mixtures, admixtures, and curing regimes
• Indirect assessment of tensile performance for structural analysis inputs

Testing Methods

Apparatus

The test is performed using a compression or universal testing machine equipped with a flexural loading device. The device consists of two support rollers and two upper loading rollers, arranged so that the specimen is loaded as a simply supported beam with two loading points located in the middle third of the span. The typical support span is 450 mm for 150 mm prisms and 300 mm for 100 mm prisms. Roller alignment must be maintained within the tolerances specified by the applicable standard to avoid twisting or uneven loading.

Test Procedure

The general procedure is as follows:

• Measure and record the width and height of the specimen at the section of expected fracture, accurate to 1 mm.
• Place the specimen in the loading device with the casting surface (top during casting) oriented as required by the standard, ensuring the span and loading points are correctly positioned.
• Apply the load continuously and uniformly without shock. In accordance with GB/T 50081, the loading rate is typically 0.02 to 0.05 MPa/s for strength grades below C30, 0.05 to 0.08 MPa/s for grades C30 to below C60, and 0.08 to 0.10 MPa/s for grades C60 and above.
• Record the maximum load carried by the specimen during the test and observe the location of fracture.
• Continue until all three specimens in the set have been tested.

Calculation of Flexural Strength

For third-point loading, the flexural strength is calculated using the formula:

ff = F × L / (b × h²)

where ff is the flexural strength (MPa), F is the failure load (N), L is the span length between supports (mm), b is the specimen width (mm), and h is the specimen height (mm). The result is reported to the nearest 0.1 MPa. The reported flexural strength of a concrete set is derived from the three specimen results in accordance with the outlier provisions of the applicable standard; where an individual value deviates from the median beyond the permitted tolerance, the median is adopted, and where two values deviate, the test set is rejected and repeated.

Validity of Fracture Location

For third-point loading, the test is valid when fracture occurs within the middle third of the span. If the fracture occurs outside the middle third by more than the tolerance allowed by the standard (for example, more than 5% of the span length in ASTM C78), the result is discarded because the stress distribution at the fracture section no longer corresponds to the constant maximum-moment zone. Center-point loading, as specified in ASTM C293, yields a single maximum-moment section and is sometimes used when fracture location control is difficult, although results from the two methods are not directly interchangeable.

Applications

The flexural strength test of ordinary concrete is applied across a wide range of civil engineering and infrastructure scenarios:

• Cement concrete pavements for highways and urban roads, where flexural strength is the governing design parameter
• Airport runways, taxiways, and apron slabs subjected to concentrated aircraft gear loads
• Bridge decks and floor slabs on grade where bending resistance controls performance
• Factory and warehouse industrial floors designed for heavy wheeled traffic
• Ready-mix concrete production control and delivery acceptance testing
• Research and development of concrete mixtures, supplementary cementitious materials, and admixtures
• Quality disputes and failure investigations requiring verified mechanical performance data

FAQ

What is the difference between flexural strength and compressive strength of concrete?

Compressive strength measures the resistance of concrete to crushing loads and typically ranges from 20 to 60 MPa or higher for ordinary concrete, while flexural strength measures resistance to bending and usually falls between about 3.5 and 6.5 MPa. Flexural strength reflects the tensile capacity of the concrete matrix and is more sensitive to curing quality, moisture condition, and aggregate bonding at the extreme fibers.

Why is flexural strength the key indicator for concrete pavements?

Concrete pavements fail primarily by bending under wheel loads rather than by crushing. Therefore, pavement design codes specify a required design flexural strength, commonly 4.0 to 5.5 MPa depending on traffic class, and acceptance is based on beam flexural tests rather than cube or cylinder compressive tests alone.

Which specimen size is considered standard?

The 150 mm × 150 mm × 550 mm (or 600 mm) prism is the standard specimen when the maximum aggregate size does not exceed 40 mm. A 100 mm × 100 mm × 400 mm prism may be used for aggregate sizes up to 31.5 mm, but the size difference should be considered when comparing results.

Can flexural strength be converted to compressive strength?

No direct conversion exists, because the two properties depend on different failure mechanisms. Empirical relationships, such as flexural strength being roughly proportional to the square root of compressive strength, are used only for estimation and must never replace actual test data in acceptance or design decisions.

What happens if the beam breaks outside the middle third of the span?

When fracture occurs outside the middle third beyond the tolerance allowed by the standard, the stress at fracture cannot be computed from the simple bending formula, and the result is discarded. The specimen is replaced or the test repeated until valid fractures are obtained.

At what age is the flexural strength test normally performed?

The standard test age is 28 days after casting under standard moist curing. Earlier tests at 3 or 7 days are frequently conducted to monitor early strength development for traffic opening or formwork removal, while later-age tests may be used to evaluate long-term strength growth.