Cement Mortar Flexural Strength Testing Machine Testing: Principles, Standards, and Procedures

Cement mortar flexural strength testing machine testing determines the bending resistance of standardized 40 mm x 40 mm x 160 mm mortar prisms and is a core quality control procedure for cement manufacturers, construction contractors, and accredited third-party laboratories. Governed by GB/T 17671, ISO 679, EN 196-1, and ASTM C348, the test applies a controlled three-point bending load to evaluate early-age and 28-day mechanical performance. This guide explains the equipment, sample types, testing methods, calculation procedures, and typical applications of cement mortar flexural strength determination.

Technical Overview

The cement mortar flexural strength test measures the capacity of hardened cement mortar to resist failure under a transverse bending load. The test is performed on standardized prismatic specimens measuring 40 mm x 40 mm x 160 mm using a dedicated flexural testing machine that applies a three-point bending load through a central loading roller and two support rollers spaced 100 mm apart. The load is applied at a constant rate of 50 N/s ± 10 N/s until fracture occurs, and the maximum load at failure is used to calculate the flexural strength of the mortar.

The flexural test is normally conducted as the first stage of a complete strength determination: each prism is first broken in flexure, and the two resulting half-prisms are subsequently tested in compression. Together, these results define the mechanical performance profile of a cement and serve as the primary basis for assigning and verifying cement strength grades. The governing standards include GB/T 17671, ISO 679, EN 196-1, and ASTM C348.

A typical flexural testing machine consists of the following key components:

• A rigid load frame, either single-column or dual-column, designed for capacities of 5 kN to 10 kN
• An electromechanical or servo-controlled drive system that maintains a constant loading rate of 50 N/s ± 10 N/s
• A high-precision load cell with force measurement accuracy conforming to ISO 7500-1 Class 1 or better
• A flexural attachment with two support rollers and one central loading roller, each 10 mm ± 0.1 mm in diameter, with a support span of 100 mm ± 0.2 mm
• A control unit and data acquisition system that records load, displacement, and maximum failure load
• Safety guards and overload protection devices to protect operators during specimen fracture

Testing Samples

Common Hydraulic Cements

Standard testing covers the full range of commercially produced hydraulic cements, including Portland cement, ordinary Portland cement, and blended cements incorporating ground granulated blast furnace slag, pozzolana, fly ash, limestone, or composite mineral additions. These cements represent the vast majority of routine production testing performed at cement plants and construction laboratories.

Special and Functional Cements

Specialty binders such as rapid-hardening cement, sulfate-resisting cement, low-heat cement, road cement, white cement, and masonry cement are also evaluated using the same flexural test method. For rapid-hardening or fast-setting products, additional early-age test ages may be specified by the relevant product standards.

Sampling and Sample Conditioning

Cement samples shall be taken in accordance with the applicable sampling standard, such as GB/T 12573, and thoroughly mixed before testing to ensure representativeness. Samples must be protected from moisture and contamination during transport and storage, and testing should commence as soon as practicable after sampling. Standard sand meeting ISO 679 grading requirements and potable water are used to prepare the mortar.

Testing Items

The primary testing items evaluated with the flexural strength testing machine include:

• 3-day flexural strength of cement mortar prisms
• 7-day flexural strength of cement mortar prisms
• 28-day flexural strength of cement mortar prisms
• Optional early-age flexural strength at 24 h, 48 h, or 72 h for special or rapid-hardening cements
• Flexural-to-compressive strength ratio analysis, calculated by combining flexural results with the compressive strength values obtained from the broken half-prisms

Test ages are counted from the moment mixing water is added to the cement, and each age carries a defined tolerance window to ensure comparability of results between laboratories.

Testing Methods

Mortar Preparation and Molding

One standard mortar batch consists of 450 g ± 2 g of cement, 1350 g ± 5 g of ISO standard sand graded between 0.08 mm and 2.0 mm, and 225 g ± 1 g of water, giving a water-to-cement ratio of 0.50 and a cement-to-sand ratio of 1:3 by mass. The materials are mixed in a planetary-type mixer following a standardized program of low-speed and high-speed mixing stages with a defined rest period for scraping the bowl. The fresh mortar is then compacted in triple-gang 40 mm x 40 mm x 160 mm molds on a jolting apparatus, applying 60 jolts in 60 seconds to each of the two layers. The surface is struck off level, the specimens are marked for identification, and the filled molds are placed in a moist cabinet for initial curing.

Curing Conditions

Filled molds are stored in a moist cabinet or curing box at 20 ± 1 °C with a relative humidity of at least 90 percent for 20 to 24 hours. After demolding, the prisms are immersed in water tanks at 20 ± 1 °C until the designated test age. Test age tolerances are 24 h ± 15 min, 48 h ± 30 min, 72 h ± 45 min, 7 days ± 2 h, and 28 days ± 8 h. Immediately before testing, each specimen is removed from the water and wiped clean of surface moisture.

Flexural Test Procedure

The prism is placed on the two support rollers with a molded longitudinal side face resting on the supports, and the central loading roller is aligned at mid-span. The load is applied at a constant rate of 50 N/s ± 10 N/s until the specimen fractures, and the maximum load at failure is recorded. Each mortar batch yields three prisms, providing three individual flexural strength determinations. The two halves of every broken prism are preserved for subsequent compressive strength testing in accordance with the same strength determination standard.

Calculation and Result Evaluation

Flexural strength is calculated using the following formula:

Rf = 1.5 x Ff x L / b³, where Rf is the flexural strength in megapascals, Ff is the maximum load at fracture in newtons, L is the distance between the support rollers (100 mm), and b is the side of the square cross-section (40 mm). For the standard configuration, this simplifies to Rf = 0.00234 x Ff, with the result reported to the nearest 0.1 MPa.

The reported flexural strength is the arithmetic mean of the three prism results. If any individual value deviates from the mean by more than 10 percent, that value is discarded and the mean of the remaining two results is reported, in accordance with the rejection rules of GB/T 17671 and ISO 679.

Applications

Cement mortar flexural strength testing supports a wide range of industrial and research activities:

• Daily quality control at cement production plants and verification of declared strength grades
• Acceptance testing of incoming cement at construction sites, ready-mix concrete plants, and precast element factories
• Compliance verification and certification testing performed by accredited third-party laboratories
• Research and development of new clinker compositions, supplementary cementitious materials, and chemical admixtures
• Concrete mix design optimization and durability studies where cracking resistance and tensile performance are critical
• Interlaboratory comparison programs and proficiency testing for laboratory competence evaluation
• Academic instruction and standardized training in construction materials laboratories

FAQ

What loading rate is used for cement mortar flexural strength testing?

GB/T 17671, ISO 679, and EN 196-1 specify a constant loading rate of 50 N/s ± 10 N/s applied through the central roller until fracture. Maintaining this rate is essential because loading speed directly influences the measured strength value.

What are the standard specimen dimensions?

The standard specimen is a prism measuring 40 mm x 40 mm x 160 mm, molded from a 1:3 cement-to-standard-sand mortar with a water-to-cement ratio of 0.50 and cured under controlled conditions until the test age.

Which standards govern this test?

The principal standards are GB/T 17671 in China, ISO 679 internationally, EN 196-1 in Europe, and ASTM C348 in North America. All four standards use the same 40 mm x 40 mm x 160 mm prism geometry and three-point bending configuration.

How are abnormal test results handled?

The flexural strength result is the mean of three prisms. If one individual value differs from the mean by more than 10 percent, that value is rejected and the mean of the remaining two values is reported as the final result.

How often should the flexural testing machine be calibrated?

The force measuring system should be verified in accordance with ISO 7500-1 at least annually, and additionally after relocation, major repair, or any indication of measurement drift. Support span, roller diameter, and loading rate should also be checked as part of routine equipment verification.

What is the difference between flexural and compressive strength testing?

Flexural testing applies a bending load to a full prism to evaluate tensile resistance at the bottom fiber, while compressive testing crushes the two half-prisms produced by the flexural test using 40 mm x 40 mm loading platens. The flexural test is always performed first, and both results together characterize the strength grade of the cement.