Residual indentation testing quantifies how well plastic (PVC/vinyl) floor coverings recover after being subjected to sustained static loads, making it one of the most important indicators of long-term surface durability and appearance retention. This article explains the testing principle, sample requirements, procedures and evaluation criteria for residual indentation determination based on EN ISO 24343-1, ASTM F1914, ASTM F970 and related GB/T standards, and outlines how manufacturers, specifiers and accredited third-party laboratories apply the results in quality control and product certification.
Technical Overview
Residual indentation (also called residual dent or permanent indentation) is the depth of the depression that remains on the surface of a plastic floor covering after a compressive static load has been applied and removed. Unlike the immediate, largely elastic indentation that disappears within seconds, residual indentation reflects the non-recoverable deformation caused by viscous and plastic flow within the wear layer, the plasticized PVC compound, the foam interlayer or the backing of the flooring.
In service, plastic flooring is constantly exposed to concentrated static loads from furniture legs, desks, filing cabinets, hospital beds, retail shelving and heavy equipment. A product with poor recovery develops visible, permanent dents that degrade appearance and complicate cleaning and furniture repositioning. Residual indentation testing therefore forms a core mechanical requirement in virtually all product standards for resilient and plastic floor coverings.
- Residual indentation is reported as a depth in millimetres (or inches in North American practice) and compared against a maximum permissible value.
- Key international references include EN ISO 24343-1 (residual indentation), ISO 24343-2 (recovery after a rolling load), ASTM F1914 (short-term indentation and recovery) and ASTM F970 (recovery after static loading).
- Chinese national standards for PVC flooring, such as GB/T 4085 and GB/T 11982, together with the Japanese standard JIS A 5705, incorporate equivalent residual indentation requirements.
- Performance depends strongly on wear-layer formulation and thickness, foam-layer density and thickness, backing construction, degree of plasticization and test temperature.
Testing Samples
Homogeneous and Heterogeneous PVC Flooring
Sheet and tile products with a single-layer (homogeneous) or multi-layer (heterogeneous) PVC structure are the primary scope of EN ISO 24343-1. Specimens are cut from production samples so that they represent normal product quality, avoiding areas close to sheet edges where thickness may vary.
Luxury Vinyl Tiles, Planks and Rigid-Core Products
LVT/LVP and rigid-core vinyl flooring (SPC/WPC constructions) are tested using the same principle. For click products, specimens are taken from the flat central area of the plank so that the indentation result is not influenced by the joining profile. Tests are performed on the flooring material itself, without adhesive or subfloor, unless a system-level assessment is specifically required.
Specimen Preparation and Conditioning
- A minimum of three specimens is normally required, each of sufficient size (commonly 60 mm × 60 mm or larger) to allow at least one test point located well away from the cut edges.
- Specimens are conditioned to equilibrium in the standard atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity for at least 24 hours before testing.
- The thickness of each specimen at the intended test point is measured and recorded before loading.
- Specimens must be flat and free from wrinkles, curling or surface defects that could distort the measurement.
Testing Items
Depending on the governing standard and the purpose of the evaluation, one or more of the following results are determined:
- Residual indentation depth in millimetres after a defined static load and recovery period, which is the principal result.
- Immediate (total) indentation under load, recorded in some procedures to characterize the compressive behaviour of the surface.
- Indentation recovery, expressed as a percentage of the initial indentation, particularly in ASTM F1914.
- Static load limit pass/fail classification at a specified load, as used in ASTM F970.
- Conformity assessment against the maximum residual indentation stated in the relevant product standard or project specification.
Testing Methods
EN ISO 24343-1 — Residual Indentation
The reference method for resilient floor coverings in Europe and many international markets applies a force of 500 N through a hardened steel, flat-ended cylindrical indenter with a diameter of 11.3 mm (contact area 100 mm²), producing a contact pressure of approximately 5 MPa that simulates a concentrated furniture foot. The force is maintained for 10 minutes and then removed; after the recovery period specified in the standard (150 minutes in the current edition), the depth of the remaining depression is measured with a precision gauge and reported in millimetres.
ASTM F1914 and ASTM F970 — North American Practice
ASTM F1914 determines short-term indentation and indentation recovery: a compressive load is applied through a flat indenter for a defined short dwell time, the immediate indentation is recorded, and the recovered depth is measured after a set recovery interval. ASTM F970 evaluates recovery after long-duration static loading: loads commonly ranging from 125 lbf to 500 lbf are applied through a flat circular indenter for 24 hours, and the residual indentation is measured after a 24-hour recovery period. Specifications frequently require a residual indentation no greater than 0.005 in (approximately 0.13 mm) at a stated load.
GB/T and JIS Standards
Chinese PVC flooring standards such as GB/T 4085 (semi-rigid PVC flooring) and GB/T 11982 (PVC sheet flooring), together with the Japanese standard JIS A 5705, specify residual indentation limits and procedures that are technically aligned with the EN ISO approach, typically using the same 500 N load with an 11.3 mm indenter. Acceptance limits of 0.10 mm to 0.15 mm are common depending on the product type and use classification.
Apparatus
- A compression frame or dedicated residual indentation tester capable of applying and holding the specified force smoothly, without shock or overload.
- A hardened steel cylindrical indenter with a flat, polished end face of 11.3 mm diameter, together with a rigid support plate for the specimen.
- A displacement measuring device (dial gauge, micrometer gauge or LVDT) with a resolution of 0.01 mm or better, capable of measuring the depth of the indentation relative to the undeformed surrounding surface.
- A stopwatch or timer for the loading and recovery periods, and a conditioned laboratory maintained at 23 °C ± 2 °C and 50 % ± 5 % relative humidity.
Typical Test Procedure
- Condition the specimens in the standard atmosphere and record the initial thickness at the test point.
- Place the specimen flat on the rigid support plate, ensuring full contact and no rocking.
- Lower the indenter to the surface and apply the 500 N force smoothly and without impact.
- Maintain the force for 10 minutes, then remove it completely and start the recovery timer.
- After the specified recovery period, measure the depth of the residual indentation at the centre of the impression, using the undented surface as the reference plane.
- Repeat the test for each specimen, calculate the individual and mean values, and round the result to 0.01 mm.
Expression and Evaluation of Results
The mean residual indentation of the tested specimens is compared with the limit in the applicable product standard or purchase specification. Typical requirements are no more than 0.10 mm for demanding commercial classes, with values up to 0.20 mm permitted for some domestic or specialty product categories. If the mean, or any individual value as defined by the standard, exceeds the limit, the product is considered non-conforming with respect to residual indentation.
Applications
- Factory quality control and batch release for manufacturers of PVC sheet, homogeneous and heterogeneous tiles, LVT/LVP and rigid-core flooring.
- Third-party product certification and CE marking under EU harmonized product standards for resilient floor coverings, which reference EN ISO 24343-1 as the assessment method.
- Specification and procurement screening for hospitals, schools, offices, retail and industrial premises where heavy static loads from beds, cabinets, shelving and equipment are expected.
- Research and development of wear-layer compounds, foam interlayers and backing systems, and evaluation of the influence of plasticizer content and processing conditions on recovery behaviour.
- Benchmarking of competing products and investigation of field complaints involving visible, permanent dents in installed floors.
- Verification of flooring suitability under hospital beds, pianos, server racks, display fixtures and other high point-load items.
FAQ
The following questions address the most common technical and practical issues relating to residual indentation determination for plastic floor coverings.
What is the difference between indentation and residual indentation?
Indentation is the total deformation present while the load is still applied, whereas residual indentation is only the portion that remains after the load has been removed and the material has recovered for the specified time. A good flooring product may compress visibly under load yet show a very small residual indentation.
Why is a 500 N load on an 11.3 mm indenter used?
The 100 mm² contact area and 500 N force reproduce the stress produced by a typical narrow furniture foot of about 50 kg resting on roughly one square centimetre. This combination has proven over decades to correlate well with dents observed in service, which is why EN ISO 24343-1 and the related GB/T and JIS methods adopt it.
What residual indentation value is considered acceptable?
Most European product standards for commercial-class resilient flooring require no more than 0.10 mm, while domestic classes and certain specialty products may allow up to 0.15 mm or 0.20 mm. In North America, specifications based on ASTM F970 commonly demand a residual indentation not exceeding 0.005 in (approximately 0.13 mm) at a designated load. The applicable limit is always the value stated in the governing product standard or project specification.
How does residual indentation testing differ from rolling-load testing?
Residual indentation testing evaluates recovery after a static point load, whereas rolling-load recovery, covered by ISO 24343-2 and castor-chair test methods, subjects the flooring to repeated dynamic loading from castors to assess suitability under office chairs and mobile equipment. Both properties are usually required together in product standards.
Do the subfloor, adhesive and underlay affect the result?
Standard laboratory methods are performed on the flooring material alone on a rigid plate, so the results characterize the product itself. In practice, a compliant adhesive, an appropriate underlay and a flat, hard subfloor all influence how the installed system behaves under load, which is why system-level evaluation may be specified in addition to standard material testing.
What information does a test report contain?
A complete report identifies the product and sampling details, the test standard applied, the conditioning atmosphere, the number of specimens, the indenter geometry and load, the loading and recovery durations, the individual and mean residual indentation values, the applicable limit and a statement of conformity, together with the testing date and the identity of the accredited laboratory.