The skimming (leveling) operation is a decisive specimen-preparation step in cement mortar strength testing to ISO 679 and GB/T 17671, in which the fresh mortar surface of 40 mm × 40 mm × 160 mm prisms is struck off flush with the mold to ensure uniform load transfer during flexural and compressive strength determination. This article explains the purpose, samples, procedural steps, quality checkpoints, and practical applications of the skimming operation as performed by accredited third-party laboratories. Correct leveling directly governs the accuracy, repeatability, and comparability of reported strength values.
Technical Overview
In hydraulic cement strength testing, the skimming operation (also called striking off, screeding, or leveling) refers to the removal of excess fresh mortar from the top of compacted prism molds so that the finished specimen surface is plane, continuous, and exactly flush with the mold edges. The procedure is specified in ISO 679, GB/T 17671, and EN 196-1 for prismatic mortar specimens of 40 mm × 40 mm × 160 mm, which are subsequently tested in flexure and compression at specified ages. Because mortar is compacted in two layers on a jolting table, surplus material rises above the mold rim; if this surplus is not removed correctly, the bearing faces of the hardened prism become uneven or oversized, producing eccentric loading, stress concentrations, and biased strength results.
- The skimming operation is performed immediately after compaction of the second mortar layer, while the mortar is still fresh and workable.
- A rigid metal straightedge (skimming rule or scraper) is drawn across the mold with a controlled sawing motion to cut away surplus mortar and fill shallow depressions.
- The finished surface must be coplanar with the mold top faces so that the hardened prism presents two parallel, flat bearing surfaces for strength testing.
- Proper leveling is a primary contributor to low between-specimen scatter and is a key element of operator qualification in cement testing laboratories.
Testing Samples
Standard Mortar Prisms
Test mortars are batched to the ISO 679 and GB/T 17671 standard composition: 450 g of cement, 1,350 g of graded standard sand, and 225 g of water, giving a water-cement ratio of 0.50 and a sand-cement ratio of 3.0 by mass. Each batch yields sufficient mortar to fill one three-gang prism mold producing three 40 mm × 40 mm × 160 mm specimens, with a small surplus retained for the skimming operation.
Molds and Compaction State
Specimens are molded in rigid, leak-proof steel or cast-iron three-gang molds with accurately machined top edges. Immediately before skimming, each mold has been filled in two layers, each layer compacted by 60 jolts of the vibrating (jolting) table, leaving a mound of excess mortar standing above the mold rims. The skimming operation is applied to this freshly compacted, pre-leveling state, not to partially set mortar.
Cured Reference Specimens
After skimming, specimens remain in their molds in a moist cabinet at 20 °C ± 1 °C and relative humidity of at least 90% for 20 h to 24 h, are then demolded, identified, and cured under water at 20 °C ± 1 °C until the test age. These cured prisms constitute the samples on which flexural and compressive strengths are determined, typically at 3 days and 28 days.
Testing Items
The skimming operation test evaluates both the immediate quality of the struck surface and its downstream effect on measured strength. The principal items examined are:
- Surface flushness and flatness of the fresh mortar relative to the mold top faces.
- Geometric conformity of the hardened prism, including squareness of edges and parallelism of bearing faces.
- Absence of surface defects such as honeycombing, edge voids, drag marks, loose ridges, and mortar smears on the mold rims.
- Flexural strength of the prisms, determined by center-point loading at 50 N/s ± 10 N/s.
- Compressive strength of the half prisms, determined through 40 mm × 40 mm auxiliary platens at 2,400 N/s ± 200 N/s.
- Within-batch dispersion, expressed as the range or coefficient of variation of the results, which is a sensitive indicator of leveling consistency.
Testing Methods
Mixing and Molding
Mortar is mixed in a planetary mixer following the timed sequence of ISO 679 and GB/T 17671, then transferred to the molds in two approximately equal layers, each compacted with 60 jolts of the jolting apparatus operating at approximately 60 jolts per minute. Compaction ensures dense, repeatable packing, and the slight excess of mortar in the second layer guarantees that, after skimming, the finished surface is fully consolidated rather than mortar-starved.
The Skimming Operation
Immediately after the second compaction, the operator places the rigid straightedge across the mold, contacting both machined top edges. The blade is held essentially perpendicular to the mold surface, or at a slight inclination in the direction of travel, and is advanced with a gentle transverse sawing motion that shears off surplus mortar while dragging material into any local depressions. A second, lighter pass along the length of the mold may be made to close the surface without overworking it. Excessive working, back-and-forth troweling, or pressing mortar down instead of cutting it off must be avoided, as these actions trap bleed water and disturb the compacted structure. Mortar adhering to the mold rims is wiped away so that the cover plate seals cleanly and no ridge is formed at the specimen edge.
Curing and Strength Determination
Skimmed specimens are covered, stored in the moist cabinet for 20 h to 24 h, demolded, marked, and water-cured at 20 °C ± 1 °C. At the designated ages, each prism is first broken in flexure; the resulting half prisms are then tested in compression with the skimmed bearing faces loaded through 40 mm × 40 mm platens over a bearing area of 1,600 mm². Because compressive failure is initiated at surface irregularities, the quality of the skimming operation is reflected directly in the measured values and their spread.
Equipment and Verification
- Planet-type mortar mixer, graduated apparatus, and timing controls.
- Jolting (vibrating) table delivering 60 jolts per minute with a drop height of 15 mm.
- Three-gang prism molds, 40 mm × 40 mm × 160 mm, with verified flat top faces.
- Rigid metal skimming straightedge or scraper with a straight, undamaged edge.
- Moist cabinet and water bath maintained at 20 °C ± 1 °C.
- Flexural and compressive testing machine with calibrated force indication and rate control.
Applications
Skimming operation competence and the associated strength test results are used across the cement and construction materials sector:
- Factory production control at cement plants, where daily strength trending depends on consistently prepared prisms.
- Conformity assessment of common cements against strength class requirements, such as those in GB 175 and corresponding EN product standards.
- Operator training, qualification, and internal proficiency programs in accredited third-party laboratories, where skimming quality is judged from result repeatability.
- Interlaboratory comparisons and proficiency testing schemes, in which leveling-induced bias is separated from material variability.
- Research and development of blended cements, supplementary cementitious materials, and chemical admixtures, where small strength differences must be resolved reliably.
- Acceptance testing of cement delivered to concrete plants and construction sites.
FAQ
What exactly is the skimming operation in cement mortar strength testing?
It is the controlled removal of excess fresh mortar from the top of a compacted prism mold with a rigid straightedge, using a sawing motion, so that the specimen surface finishes flush with the mold edges and the hardened prism has flat, parallel bearing faces.
When must skimming be carried out?
Immediately after compaction of the second mortar layer on the jolting table, while the mortar remains fresh. Delayed or reworked skimming disturbs setting mortar and produces weak surface layers, voids, and unreliable strength values.
How does leveling quality affect the test result?
High spots and ridges concentrate load and trigger premature cracking, understating compressive strength, while hollow or mortar-starved surfaces reduce the effective bearing area and increase scatter. Well-leveled surfaces distribute load uniformly, which is essential for accurate, repeatable results.
Which standards specify this operation?
ISO 679 and GB/T 17671 define the skimming operation, specimen preparation, curing, and strength determination for mortar prisms; EN 196-1 is the equivalent European method. ASTM C109/C109M covers a comparable cube-based mortar strength test with its own surface finishing requirements.
How is operator proficiency in skimming verified?
Laboratories compare flexural and compressive results, ranges, and coefficients of variation obtained by different operators on homogenized cement batches, and review the surface condition of demolded prisms. Consistently low scatter and defect-free surfaces demonstrate competent leveling technique.
What defects indicate improper skimming?
Typical signs include crowned or dished surfaces, ragged or chipped longitudinal edges, drag marks, embedded loose mortar, bleed-water channels, and mortar debris on the mold rims that prevents a clean seal during early curing.