Cement Mortar Strength Compaction Test (Jolting Table Method): Complete Technical Guide per ISO 679

The cement mortar strength compaction test, commonly performed with a jolting table, is the internationally standardized method for preparing 40 mm × 40 mm × 160 mm mortar prisms and determining the flexural and compressive strength of cement. Specified in ISO 679, EN 196-1, and GB/T 17671, the procedure ensures uniform specimen density through 60 controlled jolts applied within 60 seconds. This technical guide explains the apparatus, sample requirements, testing procedure, quality criteria, and practical applications of the method for cement producers, laboratories, and construction professionals.

Technical Overview

The cement mortar strength compaction test, widely known as the jolting table method, is the globally recognized procedure for producing prismatic mortar specimens and quantifying the mechanical strength of hydraulic cement. The method is prescribed by ISO 679 (Cement — Test methods — Determination of strength), EN 196-1 (Methods of testing cement — Part 1: Determination of strength), and GB/T 17671 (Test method of cement mortar strength, ISO method). It forms the technical basis for strength grade classification — such as 32.5, 42.5, and 52.5 — under EN 197-1 and GB 175.

At the core of the procedure is the jolting apparatus, also called a compaction table: a rigid tabletop that carries the specimen mold and falls from a defined height onto a resilient stop, transferring a precisely controlled compaction energy into the fresh mortar. Each of the two mortar layers placed in a 40 mm × 40 mm × 160 mm triple-gang mold receives 60 jolts at a drop height of 15 mm within 60 seconds. This controlled densification expels entrapped air, produces uniform density along the prism, and is the single most important factor governing the reproducibility of strength results between laboratories.

• Specimen geometry: 40 mm × 40 mm × 160 mm prisms cast in a three-compartment steel mold
• Standard mix proportions: 450 g of cement, 1,350 g of graded ISO standard sand, and 225 g of water (water/cement ratio of 0.50 and sand/cement ratio of 3.0)
• Compaction: two layers of mortar, each densified by 60 jolts of 15 mm fall height in 60 seconds
• Curing: moist air at 20 °C ± 1 °C and relative humidity of at least 90 % for the first 20 to 24 hours, followed by immersion in water at 20 °C ± 1 °C until the test age
• Strength determination: center-point flexural loading of full prisms, followed by compression of the resulting half prisms over a 40 mm × 40 mm bearing area

Testing Samples

Portland and Blended Cements

The method applies to ordinary Portland cement, rapid-hardening Portland cement, and blended cements incorporating ground granulated blast-furnace slag, pozzolana, fly ash, or limestone. Each submitted sample should originate from a single production batch and provide sufficient material for all required mixing batches, typically at least 1,800 g of cement for a complete two-age test program plus retention material for verification.

Special Binders and Cementitious Systems

Supplementary cementitious materials, composite binders, masonry cements, and experimental formulations can also be evaluated with this method. Where a specific system demands a different water content, sand type, or compaction effort, any deviation from the reference procedure must be clearly documented in the test report so that results remain traceable and comparable.

Auxiliary Constituents

The test requires ISO or CEN standard sand — a rounded, siliceous natural sand with a tightly controlled graded particle size distribution — and potable or distilled water at 20 °C ± 1 °C. These constituents must be identical across all batches, because sand grading, particle shape, and water temperature directly influence mortar workability and compaction behavior.

Testing Items

The compaction stage prepares the specimens; the subsequent mechanical loading quantifies the following items, each of which is reported in megapascals (MPa) to one decimal place:

• Flexural strength at 3 days — an indicator of early hydration and binding capability
• Compressive strength at 3 days — the early-age criterion for rapid-hardening classifications
• Flexural strength at 28 days — the standard reference age for conformity assessment
• Compressive strength at 28 days — the principal value used to assign cement strength grades
• Optional ages — 24 hours, 7 days, or 90 days for special cements, research programs, or client-specified schedules
• Derived parameters — flexural-to-compressive strength ratio, rate of strength development, and within-batch statistical scatter

Testing Methods

Mixing Procedure

The mortar batch is prepared in a planetary mixer following the fixed sequence of ISO 679. Water is charged into the bowl first, cement is added, and mixing proceeds at low speed for 30 seconds. Standard sand is introduced steadily over the following 30 seconds, after which the mixer runs at high speed for 30 seconds, pauses for 90 seconds (with mortar adhering to the bowl walls scraped down during the first 15 seconds of the pause), and finishes with a further 60 seconds at high speed. The total mixing time is 4 minutes.

Molding and Jolting Compaction

Immediately after mixing, the mortar is placed into the mold mounted on the jolting table. The first layer fills each compartment to approximately half the mold depth, is spread evenly across the width, and is compacted by 60 jolts in 60 seconds. The second layer is then placed with a slight excess, spread, and compacted with a further 60 jolts. Excess mortar is struck off flush with a straightedge using a sawing motion, the surface is lightly smoothed, and the mold is covered and marked for identification. From mold filling to the start of jolting, the operation must be completed promptly so that compaction occurs while the mortar remains fully workable.

Curing Regime

The molds are stored in a moist cabinet or box at 20 °C ± 1 °C and a minimum relative humidity of 90 % for 20 to 24 hours. After demolding, the prisms are immersed in water at 20 °C ± 1 °C, resting horizontally on suitable supports with adequate spacing so that all faces remain in contact with water; the water is renewed at regular intervals, typically every 15 days. Testing is performed within strict time tolerances — for example, ±15 minutes at the 24-hour age and ±8 hours at the 28-day age — because cement hydration is time and temperature dependent.

Strength Determination

For flexural testing, each saturated surface-dried prism is loaded at mid-span on supports 100 mm apart, with the loading rate controlled at 50 N/s ± 10 N/s until fracture. The flexural strength is calculated from the peak force and the specimen cross-section. Each half prism produced by the flexural test is then tested in compression between 40 mm × 40 mm auxiliary platens at a loading rate of 2,400 N/s ± 200 N/s. One set of three prisms therefore yields three flexural results and six compressive results per age, which are evaluated as a mean or median according to the statistical rules of the governing standard, with outlying values rejected where permitted. A full program at multiple ages requires an independent mixing batch for each age.

Apparatus Verification

The jolting table must maintain a drop height of 15.0 mm ± 0.3 mm and a jolt frequency of 60 jolts in 60 seconds. Verification is performed through periodic calibration of dimensional and kinetic parameters and through periodic comparative testing with a reference cement of known strength; accreditation bodies require such verification records as part of routine laboratory quality management.

Applications

The cement mortar strength compaction test is the cornerstone of cement quality evaluation worldwide, supporting decisions from the production line to the construction site.

• Factory production control — daily or per-lot monitoring of strength development in cement plants to keep production within specification limits
• Certification and conformity assessment — assignment of strength grades (32.5, 42.5, 52.5, and rapid variants) for product certification and market release under EN 197-1 and GB 175
• Construction acceptance testing — incoming inspection of cement delivered to ready-mix plants, precast factories, and project sites before use in structural concrete
• Research and development — evaluation of supplementary cementitious materials, low-carbon and blended binders, and chemical admixture compatibility
• Laboratory benchmarking — verification of jolting apparatus performance and participation in interlaboratory proficiency comparisons by accredited third-party laboratories
• Education and standardization — training of technical personnel and generation of reference data for specification development

FAQ

Why is jolting used instead of internal vibration or hand tamping?

The standardized jolting motion delivers identical compaction energy to every specimen regardless of the operator, eliminating human variability and ensuring that measured strength differences reflect the cement itself rather than preparation technique. Reproducibility studies underpinning ISO 679 established the jolt count, drop height, and two-layer filling regime as the combination giving the lowest between-laboratory scatter.

What tolerances must the jolting table satisfy?

The table must produce a drop height of 15.0 mm ± 0.3 mm at a frequency of 60 jolts in 60 seconds, with the moving assembly maintained within the mass limits specified in the standard. Deviations in drop height directly change specimen density and can bias compressive strength by several percent, which is why regular calibration and reference-cement checks are mandatory.

How many specimens are tested and how are results reported?

One mixing batch produces three prisms. Each prism supplies one flexural result and, after fracture, two compressive results from its half sections. Flexural strength is typically reported from the three values and compressive strength from the six values, applying the mean or median rule with outlier rejection defined by the applicable standard.

Why is testing performed at 3 days and 28 days?

Three-day results verify early strength development and distinguish rapid-hardening cements, while 28-day results represent the essentially stabilized strength used for structural design and grade classification. Additional ages may be tested when a specification, research plan, or contractual requirement demands them.

Can the method be used for mortars containing chemical admixtures?

Yes. The specimen preparation, compaction, curing, and loading stages remain unchanged, but the mix proportion of the admixture must be documented precisely, and any effect on workability should be noted because extremely stiff or highly fluid mortars may not compact identically to reference mortars.

What common factors cause low or scattered results?

Frequent causes include jolting tables out of calibration, non-standard or degraded sand, incorrect water temperature, delayed molding after mixing, inadequate first-layer spreading, curing water drifting outside 20 °C ± 1 °C, and loading rates outside the specified ranges. Systematic control of these parameters keeps within-batch coefficients of variation at the low levels expected of a competent laboratory.