Cement Mortar Flexural Strength Test: Principles, Standards, and Procedures

The cement mortar flexural strength test measures the bending resistance of standardized 40 mm × 40 mm × 160 mm mortar prisms and is one of the most widely performed procedures for cement quality assessment worldwide. Governed by international standards such as ISO 679, EN 196-1, ASTM C348, and GB/T 17671, the test provides essential data for cement classification, conformity assessment, and concrete mix design. This article offers a comprehensive technical guide covering the test principle, specimen preparation, apparatus, calculation methods, and practical applications for laboratory professionals.

Technical Overview

Flexural strength, also known as modulus of rupture or bending strength, represents the capacity of hardened cement mortar to resist deformation and fracture under transverse bending loads. Because mortar and concrete are brittle, quasi-brittle materials that are strong in compression but relatively weak in tension, flexural strength is a critical indicator of structural performance, crack resistance, and durability.

In the standardized cement mortar flexural strength test, a prism specimen is placed on two cylindrical support rollers spaced 100 mm apart, and a steadily increasing load is applied through a central loading roller until the specimen fractures. The maximum load at failure is used to calculate the flexural strength. The test is specified in several equivalent or closely related standards, including ISO 679 (Cement — Test methods — Determination of strength), EN 196-1 (Methods of testing cement — Determination of strength), GB/T 17671 (Test method of cement mortar strength, ISO method), and ASTM C348 (Standard Test Method for Flexural Strength of Hydraulic-Cement Mortars).

An important feature of the ISO/EN/GB test scheme is that the flexural strength test is performed first, and the two half-prisms produced by each fracture are subsequently used for the compressive strength test. A single set of three prisms therefore yields both of the key strength parameters required for cement classification, making the procedure highly efficient for routine quality control and conformity assessment.

Testing Samples

Standard Mortar Prisms

The standard specimen is a mortar prism measuring 40 mm × 40 mm × 160 mm, cast in a triple-gang mold so that three companion prisms are produced simultaneously. Three prisms are required for each test age, with the most common test ages being 3 days and 28 days of curing. The mortar is prepared with a fixed composition using ISO standard sand and a water-to-cement ratio of 0.50, ensuring that strength differences reflect the properties of the cement itself rather than variations in mix design.

Cement Types Suitable for Testing

• Ordinary Portland cement and rapid-hardening Portland cement (for example, strength grades 42.5, 42.5R, 52.5, and 52.5R)
• Blended cements incorporating fly ash, granulated blast-furnace slag, pozzolana, or limestone
• Composite cements and special-purpose cements such as sulfate-resistant, low-heat, and white cements
• Hydraulic cement and cementitious binders covered by ASTM, EN, ISO, or GB product standards

Sample Submission Requirements

For a complete test covering multiple ages, laboratories typically request a cement sample of approximately 10 kg or more, portioned and sealed in airtight moisture-proof packaging. The sample should be stored in a cool, dry environment and thoroughly blended before test portions are taken. A retained sample should be kept for potential repeat or arbitration testing by accredited third-party laboratories in the event of disputes.

Testing Items

The primary testing item is the determination of flexural strength at specified ages, typically including:

• 3-day flexural strength (early strength indicator)
• 28-day flexural strength (standard age strength indicator)
• Additional ages such as 2 days, 7 days, or later ages where specified by the client or product standard
• Conformity assessment of flexural strength against strength class requirements in product standards such as GB 175 and EN 197-1
• Combined flexural and compressive strength evaluation using the same specimen set

For example, under the Chinese standard GB 175, a 42.5 grade cement must achieve a minimum 3-day flexural strength of 3.5 MPa and a 28-day flexural strength of 6.5 MPa, while a 52.5 grade cement must reach at least 4.0 MPa and 7.0 MPa respectively. Test results are compared against these class thresholds to determine conformity.

Testing Methods

Apparatus and Materials

• Planetary mortar mixer conforming to the reference standard
• Compaction apparatus, typically a jolt (dropping) table or vibrating table
• Triple-gang prism molds with nominal dimensions of 40 mm × 40 mm × 160 mm
• Flexural test fixture with two support rollers and one central loading roller, span of 100 mm
• Compression testing machine with force verification traceable to ISO 7500-1 or equivalent calibration standards
• ISO or CEN graded standard sand and potable or distilled water
• Temperature-controlled curing cabinet and water bath maintained at 20 ± 1 °C

Mortar Proportioning

One batch of mortar for three prisms consists of 450 ± 2 g of cement, 1350 ± 5 g of standard sand, and 225 ± 1 g of water, giving a water-cement ratio of 0.50 and a sand-cement ratio of 3:1 by mass. This fixed composition is essential for comparability of results between laboratories.

Mixing Procedure

• Place the water in the mixing bowl, add the cement, and start the mixer at low speed for 30 seconds
• Add the sand steadily and evenly during the following 30 seconds while mixing continues
• Switch to high speed and mix for 30 seconds
• Stop the mixer for 90 seconds; during the first 15 seconds, scrape any mortar adhering to the bowl wall and lid into the mix
• Resume high-speed mixing for a final 60 seconds, then immediately proceed to molding

Molding and Compaction

The mortar is placed in the molds in two approximately equal layers, each layer being compacted with 60 jolts or vibrations to remove entrapped air and ensure uniform density. The surface is struck off level, the specimens are marked for identification, and the molds are covered to prevent moisture loss.

Curing Conditions

The filled molds are stored in a moist cabinet at 20 ± 1 °C with a relative humidity of at least 90% for 20 to 24 hours. After demolding, the prisms are immersed in a water bath at 20 ± 1 °C until the specified test age. At testing, specimens are removed from the water, wiped to a saturated surface-dry condition, and tested immediately while still in that moisture state.

Test Procedure

• Center the prism on the two support rollers of the flexural fixture, which are 100 mm apart, with the molded side faces oriented correctly
• Apply the load through the central loading roller at a constant rate of 50 ± 10 N/s until the specimen fractures
• Record the maximum load at failure for each of the three prisms
• Retain the six half-prisms for subsequent compressive strength testing

Calculation and Expression of Results

The flexural strength is calculated using center-point bending theory with the following formula, where Ff is the maximum load in newtons, L is the distance between support rollers (100 mm), and b is the side of the square cross-section (40 mm):

Rf = 1.5 × Ff × L / b³

For the standard geometry, this simplifies to Rf = 0.00234 × Ff, with the result expressed in MPa when the load is in newtons. The reported flexural strength is normally the mean of the three individual results, rounded to 0.1 MPa. If any single value deviates by more than 10% from the mean of the three, that value is discarded and the mean of the remaining two results is reported; if more than one value exceeds the 10% deviation limit, the entire test is considered invalid and must be repeated.

Applications

The cement mortar flexural strength test is applied across the entire lifecycle of cement production and construction, including:

• Factory quality control at cement plants for routine monitoring of grinding, burning, and blending processes
• Conformity assessment and product certification against strength class requirements in national and international standards
• Third-party inspection and arbitration testing performed by accredited third-party laboratories for construction disputes or batch rejection
• Research and development of new binders, supplementary cementitious materials, and chemical admixtures
• Reference data input for concrete mix proportioning and performance prediction
• Academic research on cement hydration, microstructure-property relationships, and long-term durability
• Acceptance testing for engineering projects where cement flexural performance is contractually specified

Because the test follows a strictly standardized mortar composition and curing regime, results obtained in different laboratories can be directly compared, providing a reliable technical basis for trade, certification, and regulatory compliance.

FAQ

What is the standard specimen size and how many specimens are required?

The standard specimen is a 40 mm × 40 mm × 160 mm mortar prism. Three prisms are tested at each age, and the reported flexural strength is the mean of the valid individual results.

What loading rate is used in the flexural strength test?

Under ISO 679, EN 196-1, and GB/T 17671, the load is applied at a constant rate of 50 ± 10 N/s until fracture. ASTM C348 specifies a comparable constant loading rate appropriate for its apparatus, so the governing standard should always be cited in the test report.

At what ages is flexural strength typically determined?

The most common ages are 3 days and 28 days after casting, which correspond to early strength and standard strength requirements. Other ages such as 2 days, 7 days, or later can be tested when specified by the client or the relevant product standard.

How is flexural strength calculated?

Flexural strength is calculated as Rf = 1.5 × Ff × L / b³. For the standard prism geometry with a 100 mm span and 40 mm cross-section, the formula simplifies to Rf = 0.00234 × Ff, giving the result in MPa.

What happens if one test result differs significantly from the others?

If an individual result deviates by more than 10% from the mean of the three results, that result is discarded and the mean of the remaining two is reported. If more than one result exceeds the deviation limit, the test is deemed invalid and must be repeated with a new batch of specimens.

Can the same specimens be used for compressive strength testing?

Yes. The two half-prisms generated by each flexural fracture are retained and tested in compression over the 40 mm × 40 mm fractured faces, so one set of three prisms provides both the flexural strength result and six compressive strength results.

What flexural strength values are required for cement conformity?

Requirements depend on the strength class in the applicable product standard. For example, GB 175 requires a 42.5 grade cement to achieve at least 3.5 MPa at 3 days and 6.5 MPa at 28 days in flexure, while a 52.5 grade must achieve at least 4.0 MPa and 7.0 MPa respectively.

Why is standard sand used instead of ordinary construction sand?

Graded standard sand with a controlled particle size distribution eliminates aggregate variability as an influence on strength. This ensures that measured differences in flexural strength are attributable solely to the quality and characteristics of the cement being tested.