National Standard (GB) Testing of Cement Mortar Strength: Compressive and Flexural Strength Determin

Cement mortar strength testing to the Chinese national standard GB/T 17671 is the fundamental method for determining the flexural and compressive strength of hydraulic cements using standardized 40 mm × 40 mm × 160 mm prisms. This article explains the testing principle, specimen preparation, apparatus, procedures, and acceptance criteria defined in the national standard, providing a practical technical reference for engineers, laboratory technicians, and quality control managers.

Technical Overview

Cement mortar strength testing in China is governed by the national standard GB/T 17671, titled "Test Method of Cement Mortar Strength (ISO Method)". The current edition, GB/T 17671-2021, is technically aligned with ISO 679 and follows the same fundamental principle as EN 196-1: a standard mortar consisting of cement, water, and graded standard sand is compacted into 40 mm × 40 mm × 160 mm prisms, cured under strictly controlled temperature and humidity conditions, and loaded to failure at prescribed ages to determine flexural strength first and compressive strength afterwards.

The results obtained by this method serve as the technical basis for the mandatory national standard GB 175 for common Portland cements, which defines minimum strength requirements for each strength grade, such as 32.5, 42.5, and 52.5, including early-strength "R" variants. Because the mortar composition, sand grading, compaction energy, curing regime, and loading rates are all fully standardized, the method produces results that are directly comparable between manufacturers, accredited third-party laboratories, and regulatory authorities.

• Standard followed: GB/T 17671-2021 (equivalent in principle to ISO 679 and EN 196-1)
• Specimen type: 40 mm × 40 mm × 160 mm mortar prisms
• Standard mix: 450 g cement, 1,350 g standard sand, 225 g water (water/cement ratio 0.50, sand/cement ratio 3.0)
• Standard test ages: typically 3 days and 28 days
• Reported results: flexural strength and compressive strength, expressed in MPa to 0.1 MPa

Testing Samples

Common Portland Cements

The method applies to the full family of common Portland cements covered by GB 175, including ordinary Portland cement (P·O), slag Portland cement (P·S), pozzolanic Portland cement (P·P), fly-ash Portland cement (P·F), and composite Portland cement (P·C). For each of these cement types, the standard mortar proportioning is used without modification.

Special and Characteristic Cements

Special cements, such as rapid-hardening cement, sulphoaluminate cement, white Portland cement, road cement, and moderate-heat cement, may also be evaluated with this method. For certain special cements whose standard mortar flow falls below the specified threshold of 180 mm, the standard permits the water/cement ratio to be adjusted in accordance with the flow-determination procedure given in the annex, and the adjusted ratio must be clearly reported.

Sampling and Sample Handling

Samples should be taken in accordance with the national sampling standard GB 12573 to ensure representativeness. A minimum sample quantity of approximately 10 kg is typically required for a complete strength determination. The sample is thoroughly blended, divided into sealed portions for testing and retention, and protected from moisture and carbonation. Cement that has hardened, become lumpy, or been exposed to humid air for an extended period must not be tested.

Testing Items

The core deliverables of national standard cement mortar strength testing are the flexural and compressive strengths at specified ages, which together determine whether a cement satisfies its declared strength grade.

• Flexural strength at 3 days and 28 days (MPa)
• Compressive strength at 3 days and 28 days (MPa)
• Optional or supplementary ages, such as 24 hours or 7 days, for special cements or research purposes
• Strength grade conformity evaluation against GB 175 requirements

As a typical example, for a 42.5 grade cement the minimum requirements are a compressive strength of 17.0 MPa at 3 days and 42.5 MPa at 28 days, together with a flexural strength of 3.5 MPa at 3 days and 6.5 MPa at 28 days. Early-strength "R" grades impose higher 3-day minimums, while all grades must satisfy the corresponding 28-day limits. Test ages must be respected within the tolerances specified by the standard, for example ±45 minutes for the 3-day age and ±8 hours for the 28-day age.

Testing Methods

Apparatus and Materials

The test requires calibrated equipment conforming to the specifications of GB/T 17671, including a planetary mortar mixer, a jolting (compaction) table delivering 60 jolts in 60 seconds with a 15 mm drop height, three-gang steel prism molds of 40 mm × 40 mm × 160 mm internal dimensions, a moisture cabinet, a water curing bath, a flexural testing device with 100 mm support span, a 40 mm × 40 mm compression jig, and a compression testing machine of verified accuracy with controllable loading rate. The sand must be graded siliceous standard sand in the range of 0.08 mm to 2.00 mm, supplied in 1,350 g portions, and the water must be potable or distilled water.

Mortar Proportioning and Mixing

One batch consists of 450 g of cement, 1,350 g of standard sand, and 225 g of water, giving a water/cement ratio of 0.50 and a sand/cement ratio of 3.0. Water is placed in the mixing bowl first, cement is added and mixed at low speed for 30 seconds, sand is steadily introduced during the following 30 seconds, the mixer runs at high speed for 30 seconds, then pauses for 90 seconds during which the bowl walls are scraped down, and finally runs at high speed for a further 60 seconds.

Specimen Molding and Curing

The mold is filled in two approximately equal layers, each compacted with 60 jolts of the jolting table. The surface is struck off level and covered. Specimens are kept in the moisture cabinet at 20 ± 1 °C with relative humidity of at least 90% for 20 to 24 hours before demolding, then submerged in a water bath at 20 ± 1 °C until the testing age, stored so that free water circulates around all surfaces.

Flexural Strength Determination

Each prism is broken as a simply supported beam under center-point loading with a 100 mm span. The load is applied at a rate of 50 N/s ± 10 N/s until failure. Flexural strength is calculated as Rf = (1.5 × Ff × L) / b³, where Ff is the failure load, L is the span between supports, and b is the 40 mm side of the prism; with the standard geometry this simplifies to Rf (MPa) = 0.00234 × Ff (N).

Compressive Strength Determination

The two half-prisms resulting from each flexural test are immediately tested in compression through 40 mm × 40 mm platens, giving a loaded area of 1,600 mm². The load is applied at a rate of 2,400 N/s ± 200 N/s until failure. Compressive strength is calculated as Rc = Fc / 1,600, with Fc in newtons, and is expressed in MPa.

Result Calculation and Validity Rules

Flexural strength is reported as the mean of the three prism results; if one value deviates from the mean by more than ±10%, it is discarded and the mean of the remaining two values is reported. Compressive strength is reported as the mean of the six half-prism results; if one value deviates from the mean of six by more than ±10%, it is eliminated and the mean of the remaining five is reported, whereas if two or more values exceed the limit, the test is considered invalid and must be repeated. All results are reported to the nearest 0.1 MPa.

Applications

National standard cement mortar strength testing is used throughout the cement and construction industries for quality control, product conformity, and engineering acceptance. Typical applications include:

• Factory production control and routine batch release testing by cement manufacturers
• Conformity assessment and certification of declared strength grades under the mandatory GB 175 requirements
• Acceptance inspection of cement delivered to construction sites, precast concrete plants, and ready-mixed concrete facilities
• Verification testing by accredited third-party laboratories in the context of market surveillance, dispute resolution, and import or export compliance
• Research and development activities, such as evaluating mineral admixtures, cement substitutes, and process changes, and comparing performance with international methods

Because 28-day mortar strength is the defining index of a cement grade in China, this test is one of the most frequently performed procedures in building materials laboratories and directly influences product labeling, pricing, and engineering approval.

FAQ

What is the relationship between GB/T 17671 and ISO 679?

GB/T 17671 adopts the ISO method for cement mortar strength determination and is essentially identical to ISO 679 and EN 196-1 in terms of prism dimensions, mortar proportioning, compaction, curing, and loading rates. Minor national differences exist mainly in annex provisions applicable to certain special cements, which are clearly identified within the standard.

Why must standard sand be used instead of local construction sand?

Standard sand has tightly controlled silica content, particle shape, and grading. Using it eliminates sand variability as an influence factor, so that strength differences between results reflect the cement itself, ensuring comparability across laboratories, regions, and time.

What curing conditions must be maintained?

Prisms are held at 20 ± 1 °C and at least 90% relative humidity for the first 20 to 24 hours, then cured underwater at 20 ± 1 °C until testing. Deviations in temperature or humidity during curing are among the most common causes of non-reproducible strength results.

Can GB/T 17671 results be directly compared with ASTM C109 results?

No. ASTM C109 determines compressive strength on 50 mm cube specimens with a different sand and procedure. Although both methods measure the same fundamental property, the numerical results differ systematically, and correlations must be established empirically when comparing data from the two systems.

How many specimens and results are produced per test?

One standard batch produces three prisms. Each prism yields one flexural result, and the six resulting half-prisms each yield one compressive result, giving three flexural and six compressive values that are evaluated under the outlier rules of the standard.

How long does a complete strength evaluation take?

Specimen preparation takes less than one hour, but the evaluation itself spans the curing ages. A standard 3-day and 28-day evaluation therefore requires roughly four weeks from molding to the final report, with the 28-day results serving as the basis for grade determination.