Cement Mortar Strength Loading Test: Flexural and Compressive Strength Determination

The cement mortar strength loading test determines the flexural and compressive strength of standardized 40 mm × 40 mm × 160 mm mortar prisms under strictly controlled loading rates, in accordance with ISO 679, EN 196-1, and GB/T 17671. As the primary reference method for cement strength classification, factory production control, and conformity assessment, this test provides the mechanical performance data on which cement strength grades such as 42.5 and 52.5 are based.

Technical Overview

The cement mortar strength loading test is the internationally recognized procedure for quantifying the mechanical strength of hydraulic cements by applying a controlled, steadily increasing load to standardized mortar specimens until failure. A standard mortar is prepared from cement, graded standard sand, and water at a fixed proportion, molded into prismatic specimens, and cured under precisely defined conditions. Each prism is first loaded to failure in flexure, and the two resulting half-prisms are then loaded to failure in compression.

The defining feature of this test is the loading regime itself: the applied force must increase at a constant, tightly toleranced rate so that failure occurs within a predictable load range, ensuring that results are reproducible between laboratories and comparable against specification limits. The governing methods include ISO 679 and EN 196-1 at the international level, GB/T 17671 as the corresponding Chinese national standard, and ASTM C109/C109M, ASTM C348, and ASTM C349 as the North American cube-based and prism-based alternatives. JIS R 5201 defines the equivalent Japanese procedure.

Testing Samples

Standard Mortar Prism

The reference specimen is a prism measuring 40 mm × 40 mm × 160 mm. One standard batch, which fills a three-gang prism mold, consists of the following materials weighed to tight tolerances:

• Cement: 450 g ± 2 g
• Standard sand (graded, in accordance with ISO 679 or nationally certified equivalent): 1350 g ± 5 g
• Water: 225 g ± 1 g, corresponding to a water/cement ratio of 0.50 and a sand/cement mass ratio of 3:1

Specimen Quantity and Test Ages

A complete determination at any given age requires three prisms, yielding three flexural results and six compressive results. Cement specifications typically call for testing at an early age (commonly 3 days) and at the standard 28-day age, with rapid-hardening cements additionally controlled at intermediate ages. The permitted time tolerances are:

• 24 h ± 15 min and 48 h ± 30 min
• 72 h (3 days) ± 45 min
• 7 days ± 2 h
• 28 days ± 8 h
• Ages beyond 28 days ± 12 h

Curing Conditions

Molds are kept in a moist cabinet or room at 20 °C ± 1 °C with relative humidity of at least 90% for the first 20 h to 24 h. After demolding, prisms are submerged in clean water at 20 °C ± 1 °C until shortly before testing, and specimens are tested in the wet condition, with surface water wiped off immediately before loading.

Testing Items

The core measured and derived quantities of the loading test are:

• Flexural strength of the full prism, in MPa, at each specified age
• Compressive strength of each half-prism, in MPa, at each specified age
• Maximum failure load recorded during each loading cycle, in N or kN
• Verification of applied loading rate within the prescribed tolerance band
• Conformity evaluation of test results against cement strength grade requirements (for example, 32.5, 42.5, 52.5, and their rapid-hardening "R" variants)

Testing Methods

Specimen Preparation

Mortar is mixed in a planetary-type mixer following a fixed sequence: water is placed in the bowl, cement is added and mixed at low speed, then standard sand is introduced steadily during mixing at high speed, followed by additional high-speed mixing with a brief pause for scraping the bowl. The fresh mortar is compacted in the prism mold on a jolting table, delivering 60 jolts in 60 seconds, after which the surface is leveled with a straightedge. Accurate batching, controlled room temperature (20 °C ± 2 °C, relative humidity not less than 50%), and consistent compaction are prerequisites for valid strength results.

Flexural Strength Loading Procedure

The prism is placed on two coaxial support rollers 100 mm apart, with the 40 mm × 40 mm face in contact, and a loading roller applies force at mid-span. The load is applied steadily and without shock at a rate of 50 N/s ± 10 N/s until fracture. The flexural strength is calculated as:

Rf = 1.5 × Ff × L / b³, where Ff is the failure load, L is the distance between supports (100 mm), and b is the side of the square section (40 mm).

Compressive Strength Loading Procedure

The two halves of each fractured prism are immediately tested in compression between hardened bearing plates with a contact area of 40 mm × 40 mm (1600 mm²), positioned so that the load is applied over the portion of the side face that was in contact with the mold wall during preparation. The load is increased smoothly at a rate of 2400 N/s ± 200 N/s until failure. The compressive strength is calculated as:

Rc = Fc / 1600 mm², where Fc is the maximum load sustained by the half-prism.

Processing and Validity of Results

Flexural strength is reported as the mean of the three prism results; if any single result deviates by more than 10% from the mean of the three, it is discarded and the mean of the remaining two is reported. Compressive strength is reported as the mean of the six half-prism results; if one result deviates by more than 10% from the mean of the six, the outlier is rejected and the mean of the remaining five is reported. If a second result again exceeds the 10% band, the entire determination is invalid and must be repeated. All strength values are reported to the nearest 0.1 MPa.

Equipment and Machine Requirements

Testing machines must be calibrated and verified against recognized force-measurement standards, with load-indication accuracy typically within ±1.0%, and must be capable of maintaining the prescribed loading rate within tolerance over the entire loading range in which failure is expected. The flexural device geometry (roller diameters, span, and coaxiality), the flatness and hardness of the 40 mm × 40 mm compressive platens, and the alignment of the loading column all directly influence the measured load, so their periodic verification is an integral part of the method.

Applications

The cement mortar strength loading test underpins a wide range of engineering, regulatory, and research activities:

• Strength grade classification and certification of cements, where minimum compressive and flexural requirements at specified ages define grades such as 42.5 and 52.5
• Factory production control by cement manufacturers, enabling statistical monitoring of strength development in real time
• Conformity assessment and market surveillance testing performed by accredited third-party laboratories and regulatory authorities
• Research and development of blended cements, supplementary cementitious materials, and chemical admixtures, where loading-rate-controlled results allow direct comparison of formulations
• Reference input for concrete mix design, supplier qualification, and incoming material acceptance on construction projects
• Forensic and dispute resolution investigations, where deviations in loading rate or curing history are evaluated when explaining strength discrepancies

FAQ

What are the required loading rates for flexural and compressive testing?

Under ISO 679, EN 196-1, and GB/T 17671, the flexural load must be applied at 50 N/s ± 10 N/s, and the compressive load at 2400 N/s ± 200 N/s. Deviation from these rates changes the measured failure load, so rate control is a core validity requirement of the test.

Why are flexural and compressive strengths obtained from the same specimen?

The 40 mm × 40 mm × 160 mm prism geometry allows a single specimen to serve both purposes economically and consistently: the full prism is broken in flexure, and the two halves, which present identical 40 mm × 40 mm loading faces, are then crushed in compression. This yields three flexural and six compressive results per batch with minimal material.

How does the ISO/GB prism method differ from the ASTM cube method?

ASTM C109/C109M determines compressive strength on 50 mm cube specimens using a different graded sand, mortar proportion, and loading configuration, whereas the ISO/GB method uses 40 mm × 40 mm × 160 mm prisms with ISO standard sand at a 0.50 water/cement ratio. Because specimen geometry and loading area differ, results from the two systems are close but not interchangeable, and comparisons must be made with caution.

At what ages must the test be performed?

Common cement specifications require testing at 3 days and 28 days, with 7-day results also widely used. The permitted time windows tighten at early ages (for example, ±45 min at 72 h) and relax at later ages (±8 h at 28 days), reflecting the decreasing relative rate of strength gain with time.

What causes invalid or scattered results, and how are they handled?

Frequent causes include incorrect batching, poor specimen compaction or curing outside 20 °C ± 1 °C, misaligned platens, and drifting loading rates. Statistically, any flexural or compressive result deviating more than 10% from its group mean is discarded per the result-processing rules; repeated outliers indicate an invalid determination requiring full retesting.