The layering degree test is a fundamental quality-control procedure used to evaluate the homogeneity, water-retention capacity, and anti-segregation stability of freshly gauged dry-mix mortar. Determined in accordance with the Chinese standard JGJ/T 70 (Standard for Test Method of Performance of Building Mortar) together with the product requirements of GB/T 25181, this simple 30-minute test quantifies how much a mortar's consistency changes while the material is at rest. This article explains the underlying principle, apparatus, sample types, step-by-step procedure, acceptance criteria, and practical applications of layering degree determination for dry-mix mortars.
Technical Overview
Layering degree (also referred to as stratification degree or segregation degree) is a key index of the stability of fresh mortar. It is defined as the difference between the consistency of a freshly mixed mortar and the consistency of the same mortar after it has been allowed to stand undisturbed for 30 minutes. Because heavier aggregate particles tend to settle while water migrates upward (bleeding) while mortar is at rest, a large drop in consistency indicates poor internal stability.
For dry-mix (pre-blended, bagged or silo-delivered) mortars, the test is performed on mortar that has been gauged with water in the laboratory according to the manufacturer's recommended mixing ratio. A low layering degree demonstrates that the mortar retains a uniform composition and workable consistency during transport, pumping, and waiting time on site, which is essential for reliable placement, compaction, and bond strength development.
- Core parameter: layering degree K = K1 − K2, expressed in millimeters (mm), where K1 is the initial consistency and K2 is the consistency after 30 minutes of standing.
- Primary method standard: JGJ/T 70, Standard for Test Method of Performance of Building Mortar (layering degree determination).
- Product and acceptance requirements: GB/T 25181 for pre-mixed (prepared) mortars and related project specifications.
- Internationally comparable fresh-mortar stability indicators include the water-retention tests described in EN 1015-8 and ASTM C1506.
- Mortars with layering degrees in the range of approximately 10–20 mm are generally regarded as well balanced between stability and normal setting behavior.
Testing Samples
Layering degree determination is applicable to cement-based dry-mix mortars that are mixed with water before application. Typical sample categories submitted for testing include the following.
Dry-Mix Masonry Mortar
Bagged or silo mortar designations used for laying bricks, blocks, and stones. These mortars must remain stable from mixing until placement, and typical project specifications limit their layering degree to no more than 25 mm.
Dry-Mix Plastering and Rendering Mortar
Mortars applied in thin or thick layers to walls and ceilings, where bleeding or aggregate settlement directly impairs adhesion and surface finish. A stricter limit of no more than 20 mm is commonly specified for these products.
Dry-Mix Floor and Screed Mortar
Mortars for horizontal surfaces, where segregation during standing can cause dusting, delamination, or reduced abrasion resistance. Acceptance values are normally defined in the product standard or the project specification.
Specialty and Modified Dry-Mix Mortars
Repair mortars, thermal-insulation plasters, and other formulated products, for which the layering degree may be assessed during formulation development or type-testing even when the governing acceptance criterion is water retention.
Testing Items
A complete layering degree determination comprises the following measured and derived quantities:
- Initial consistency K1 (cone penetration depth in mm), measured immediately after mixing.
- Consistency K2 (cone penetration depth in mm), measured on the lower portion of the mortar after 30 minutes of standing and remixing.
- Layering degree K = K1 − K2, reported in millimeters as the arithmetic mean of duplicate determinations.
- Supporting observations where required: mixing water ratio, mixing time, laboratory temperature, and visual records of bleeding or segregation.
Testing Methods
Apparatus
The test requires a layering degree cylinder consisting of two detachable sections with an internal diameter of 150 mm, an upper section 200 mm high and a lower section 100 mm high (total height 300 mm) joined by a quick-release clamp with a sealing gasket; a mortar consistency tester (cone penetrometer with a 300 g cone) complying with JGJ/T 70; a laboratory mortar mixer; a tamping rod; a mixing pan and trowel; and a timing device.
Sample Preparation
The dry-mix mortar is gauged with the quantity of mixing water recommended by the manufacturer, using the water-to-mortar ratio and mixing procedure specified for the product. Mixing is carried out at a standard laboratory temperature of approximately 20 ± 5 °C, and the test should start promptly after mixing is completed.
Test Procedure
- Step 1: Determine the initial consistency K1 of the freshly mixed mortar using the consistency tester, allowing the cone to penetrate for the standardized 10 seconds and recording the penetration depth in millimeters.
- Step 2: Fill the layering degree cylinder with mortar in two approximately equal layers, tamping each layer 25 times with the tamping rod, then strike off the surface flush with the top of the cylinder and clean the outside of the cylinder.
- Step 3: Allow the filled cylinder to stand undisturbed for 30 minutes in the laboratory.
- Step 4: After the standing period, remove the upper 200 mm section of the cylinder and discard the mortar it contains. Transfer the mortar from the lower 100 mm section into a mixing pan and remix it for 2 minutes.
- Step 5: Determine the consistency K2 of the remixed lower portion of mortar using the same cone penetrometer procedure as in Step 1.
- Step 6: Calculate the layering degree as K = K1 − K2, in millimeters.
Repeatability and Reporting
The determination is performed in duplicate. The arithmetic mean of the two results is reported as the layering degree of the mortar; if the difference between the two individual results exceeds 20 mm, the test shall be repeated. The test report should identify the mortar type and designation, the mixing water ratio, the laboratory temperature, the individual and mean consistency values K1 and K2, and the calculated layering degree.
Acceptance Criteria and Interpretation
- Layering degree ≤ 20 mm: generally acceptable for plastering and rendering mortars; indicates good cohesion and water retention.
- Layering degree ≤ 25 mm: generally acceptable for masonry mortars under GB/T 25181 and typical project specifications.
- Layering degree well above 25 mm: indicates poor stability, pronounced bleeding, and aggregate settlement; the mortar is prone to segregation during transport and placement and should be reformulated.
- Layering degree approaching zero, particularly when combined with markedly delayed setting: may indicate an excessive dosage of water-retaining or retarding admixtures, which can compromise early strength development and increase shrinkage risk.
- Whenever acceptance values differ between the product standard, the project specification, and the design documents, the governing project requirement takes precedence, and the current editions of the referenced standards should always be consulted.
Applications
Layering degree determination is widely used across the mortar industry and construction supply chain:
- Factory quality control in dry-mix mortar plants, as a routine fresh-mortar stability check for every production batch and formulation.
- Incoming inspection and acceptance testing of bagged or silo mortars at construction sites before large-scale placement.
- Formulation optimization, particularly for adjusting cellulose ether and thickener dosages, sand grading, and water demand during product development.
- Type testing, certification, and third-party verification of compliance by accredited third-party laboratories against GB/T 25181, JGJ/T 70, and project specifications.
- Root-cause analysis of site problems such as pumping blockages, bleeding, poor trowelability, weak bond, and surface defects.
- Technical comparison of dry-mix products with alternative fresh-mortar stability indicators such as water retention measured per EN 1015-8 or ASTM C1506.
FAQ
What does a high layering degree indicate?
A high layering degree means the mortar lost a significant amount of flowability while standing, which is caused by water bleeding and settlement of aggregate particles. Such a mortar segregates easily, is difficult to pump and apply, and develops weaker bond to the substrate.
What is the acceptable range for layering degree?
Under common Chinese requirements, masonry mortars are limited to no more than 25 mm and plastering mortars to no more than 20 mm, with a practical optimum band of roughly 10–20 mm. Always verify the limit applicable to the specific mortar type in the current edition of the governing standard and the project specification.
How long does the test take?
The core test takes about 45 minutes: initial consistency measurement, 30 minutes of standing, remixing of the lower portion, and the second consistency measurement. Including sample gauging and duplicate testing, a complete determination is typically completed within 1.5 to 2 hours.
How is layering degree different from water retention?
Layering degree measures the overall stability of the mortar by comparing consistency before and after standing, whereas water retention (for example per EN 1015-8 or ASTM C1506) measures the mortar's ability to retain mixing water against suction. The two properties are closely related and both are governed largely by water-retaining admixtures, but they are distinct tests with different apparatus and units.
What causes excessive layering degree in dry-mix mortar?
Common causes include insufficient cellulose ether or thickener content, unfavorable sand grading or excessive fines deficiency, over-gauging with water, prolonged or poorly controlled standing time before use, and incompatibility between admixtures. Corrective action usually involves adjusting the water-retention system or the water-to-mortar ratio and verifying the result with a repeated test.
Can the test be performed at any ambient temperature?
No. The determination should be carried out in the laboratory at approximately 20 ± 5 °C, because both bleeding and consistency are temperature-sensitive. Extreme hot or cold conditions distort the result and reduce repeatability.