Impermeability (Watertightness) Test for Elastomeric Modified Bitumen Waterproofing Membranes

The impermeability test is one of the most important quality verification procedures for elastomeric (SBS) modified bitumen waterproofing membranes, confirming that the reinforced sheet can withstand sustained hydrostatic pressure without water penetration. Performed in accordance with GB 18242 and the test method defined in the GB/T 328 series, this test is essential for product certification, factory quality control, and construction site acceptance. This article explains the test principle, specimen requirements, apparatus, operating procedure, acceptance criteria, and typical applications of the watertightness test for SBS modified bitumen membranes.

Technical Overview

Elastomeric modified bitumen waterproofing membranes, commonly referred to as SBS (styrene-butadiene-styrene) modified asphalt membranes, are polymer-modified waterproofing sheets widely used in roofing, basement, tunnel, and bridge deck construction. The impermeability test, also known as the watertightness test, evaluates whether a membrane can resist sustained hydrostatic pressure without allowing water to pass through the reinforced bitumen sheet.

In China, the product specification is GB 18242-2008, and the corresponding test method is GB/T 328.10-2007, part of the GB/T 328 series covering test methods for building sheets for waterproofing. Comparable international approaches include EN 1928 for flexible sheets for waterproofing in Europe and ASTM D5385 for hydrostatic pressure resistance of waterproofing membranes in the United States. Accredited third-party laboratories routinely perform this test as part of type testing, factory production control, and engineering acceptance.

The test principle is straightforward: a square specimen is clamped over a perforated plate inside a hydraulic impermeability tester, a defined water pressure is applied to one face of the specimen, and the pressure is held constant for a specified duration. Any seepage, damp patch, or water droplet appearing on the unexposed face indicates penetration and constitutes a failure.

Testing Samples

Product and Surface Types

GB 18242-2008 classifies elastomeric modified bitumen membranes by thickness and by the finish of the upper surface. The surface finish directly determines the specified test pressure, because smooth polyethylene film surfaces are exposed to a higher pressure than granular surfaces.

• Polyethylene (PE) film surfaced membranes: tested at 0.30 MPa for 120 minutes
• Fine sand surfaced membranes: tested at 0.20 MPa for 120 minutes
• Mineral (slate) granule surfaced membranes: tested at 0.20 MPa for 120 minutes

Specimen Preparation and Conditioning

Specimens are cut as squares of approximately 150 mm × 150 mm, large enough to fully cover the sealing gasket of the clamping head. At least three specimens shall be taken, distributed across the full width of the roll and positioned away from damaged or irregular areas. Care must be taken during cutting to avoid pinching, stretching, or otherwise damaging the specimen edges, since edge defects can cause false leaks.

Prior to testing, specimens are conditioned in a standard laboratory atmosphere, typically at (23 ± 2) °C for at least 24 hours, so that the bitumen sheet reaches a stable and reproducible temperature. For membranes with coarse mineral granules on the upper face, the test is normally performed with the reverse, smooth face exposed to the water pressure, because the granular surface cannot form a reliable seal against the gasket.

Testing Items

The impermeability test is a mandatory physical property item in the full evaluation program for elastomeric modified bitumen membranes. The specific test items are as follows:

• Impermeability under a hydrostatic pressure of 0.30 MPa maintained for 120 minutes, applicable to PE film surfaced products
• Impermeability under a hydrostatic pressure of 0.20 MPa maintained for 120 minutes, applicable to fine sand and mineral granule surfaced products
• Visual assessment of the unexposed face for water penetration, seepage marks, or droplet formation during the pressure hold period

Other items in the complete type testing program, such as tensile strength, ultimate elongation, low-temperature flexibility, and heat resistance, are complementary to the impermeability test but fall outside the scope of this article.

Testing Methods

Test Apparatus

The test is performed using a hydraulic impermeability tester, which consists of a water reservoir, a pressure-generating pump (manual or motor-driven), a calibrated pressure gauge with sufficient resolution, and several clamping test heads, typically three. Each test head is fitted with a rubber sealing gasket and a perforated backing plate that supports the specimen while allowing free contact between the water and the exposed face. The apparatus must maintain the set pressure with minimal fluctuation over the full 120-minute hold period, and the pressure gauge must be within its calibration validity period.

Test Conditions

The test is carried out at ambient laboratory temperature, normally (23 ± 5) °C, using clean water that is free of suspended particles. Trapped air in the hydraulic circuit shall be expelled before the pressure is applied, because entrained air causes pressure instability and may produce misleading leakage indications.

Operating Procedure

• Fill the water reservoir, prime the pump, and purge all air from the hydraulic circuit and test heads
• Place the conditioned specimen on the perforated plate with the designated face directed toward the water, ensuring the sealing gasket sits squarely on the specimen surface
• Clamp the specimen securely and uniformly so that no leakage can occur at the sealing edge rather than through the sheet itself
• Raise the water pressure steadily to the specified value of 0.30 MPa or 0.20 MPa, depending on the surface finish, typically within a few minutes
• Hold the pressure constant for 120 minutes while continuously observing the unexposed face of each specimen for signs of penetration
• Record the pressure, hold duration, test temperature, and any observations of seepage, damp patches, or droplets for each specimen

Evaluation of Results

A specimen is judged impermeable when no water penetration is observed on the unexposed face during the entire 120-minute hold period. Indicator paper placed beneath the specimen may be used to assist the detection of fine seepage. All specimens must remain watertight for the sample to be declared compliant; a single leaking specimen constitutes a failure of the impermeability requirement. The test report shall identify the standard applied, the membrane type and surface finish, the number of specimens, the test pressure and duration, which face was exposed to water, and all observations.

Influencing Factors and Precautions

Test reliability depends on correct gasket seating, complete air removal, stable pressure control, and careful specimen handling. Specimens that are tested too cold may exhibit artificially high resistance due to bitumen stiffening, while damaged edges or uneven clamping can produce false failures. Regular verification of gauge calibration and gasket condition is therefore an essential element of laboratory quality management.

Applications

Impermeability testing of SBS modified bitumen membranes is applied across the entire product life cycle, from development to installation, and supports decisions in many engineering contexts.

• Product certification and type testing against GB 18242-2008, including conformity assessment in accredited third-party laboratories
• Factory production control and routine batch release testing by membrane manufacturers
• Construction site acceptance testing, including witness sampling of delivered rolls before installation on roofs, basements, tunnels, and bridge decks
• Verification of suitability for applications with standing water or hydrostatic exposure, such as green roofs, plazas, podium decks, and below-grade structures
• Research and development of modified bitumen formulations, reinforcement carriers, and surface finishes
• Dispute resolution and post-installation investigation of waterproofing failures

FAQ

What pressure is applied in the impermeability test?

Under GB 18242-2008, PE film surfaced membranes are tested at 0.30 MPa, while fine sand and mineral granule surfaced membranes are tested at 0.20 MPa. The higher pressure for film surfaced products reflects their smoother surface, which can withstand a more demanding hydrostatic condition.

How long must the pressure be maintained?

The specified pressure is held constant for 120 minutes. The specimen must remain free of any water penetration for the entire duration to pass the test.

Which face of the specimen should face the water?

The upper surface, as manufactured, is normally exposed to the water. For membranes with coarse mineral granules on the upper face, the test is generally performed with the smooth reverse face exposed, because the granular surface cannot seal properly against the gasket.

How many specimens are tested, and what happens if one leaks?

At least three specimens are tested. If any single specimen shows seepage, damp patches, or droplets on the unexposed face, the impermeability requirement is not met and the sample is judged nonconforming.

Which international standards are comparable?

EN 1928 defines watertightness determination for flexible waterproofing sheets in Europe, while ASTM D5385 covers hydrostatic pressure resistance of waterproofing membranes in the United States. Although the apparatus and pressure levels differ, the underlying principle of applying sustained water pressure to one face of the membrane is the same.

Where can this test be performed?

The test can be conducted in manufacturer laboratories for internal quality control and in accredited third-party laboratories for certification, type testing, and site acceptance purposes, provided the laboratory holds appropriate accreditation and uses calibrated impermeability testing equipment.