WHY THE MEMBRANE’S CLASSIFICATION DOESN’T MATTER
(at least not the way you think it does)

In the 2016 & 2019 NCC, there is a requirement for the waterproofing membranes used both internally and externally to comply with appropriate Australian Standards.

AS 4654.2: Waterproofing membranes for external, above ground use is the standard for complying with Section F1.4: External above ground membranes.

AS 3740: Waterproofing of domestic wet areas is the standard for complying with Section F1.7: Waterproofing of wet areas in buildings.

The associated product testing standards (AS 4654.1 & AS 4858 respectively) have a variety of testing needing satisfactory results to ensure that a waterproofing membrane is fit for purpose.

One of the tests performed in these standards, that has been extrapolated incorrectly and used frequently by the industry to compare waterproofing membranes against one another is the elongation testing and the resulting classification.

For both standards, they classify the waterproofing membrane as a Class I, Class II, or Class III based on the membranes ability to stretch in a “dog bone” shape, per AS 1145 (image below of the specimen sample).

Upon stretching the sample, the membrane will achieve an elongation, and therefore classification, according to the following table requirements.

Class Elongation at Break
Class I < 60%
Class II 60 – 299%
Class III ≥ 300%

 

Most specifiers, builders, certifier and installers are under the (false) assumption that the higher the classification, the better the waterproofing membrane is. That is not an accurate assessment of a product’s ability to be a good waterproofing membrane. For example, wet chewing gum would achieve an elongation well in excess of 300% and yet makes a terrible waterproofing membrane.

The reason why is because of the test procedure. It is measuring the ability of the membrane to stretch as a free film, which the product is rarely installed as such. Looking at the diagram above, the specimen
is 25 mm wide in its measurement area. For a product that has 300% elongation (Class III), that means 25 mm x 300% = 75 mm, so the membrane that is a free film of 25 mm, can stretch to 75 mm.

However, when the membrane is fully bonded onto a concrete deck, that same equation is 0 mm x 300% = 0 mm. This is seen all too often when the membrane splits on a deck, and the engineer or consultant
point to the elongation classification and wonder why the membrane didn’t account for the poorly designed/constructed concrete slabs newly developed crack.

The classification of the membrane per this method has no bearing on how fit a membrane is for a particular application, it is merely to inform the interested party of the detailing requirements associated with the various classifications.

In both AS 3740 and AS 4654, there is the following table, dictating the required floor to wall bond breaker based on the membrane classification.

Class Elongation at Break Minimum Bond Breaker
Class I < 60% 100 mm or 75 mm with backing rod
Class II 60 – 299% 35 mm
Class III ≥ 300% 12 mm

 

That is the sole purpose of the elongation test value and resulting membrane class classification.

The entirety of AS 4858 and AS 4654.1 testing is needed to determine if a product is a) a code compliant material and b) the right product for your application.

SIDE NOTE: If you were interested in how a membrane could handle a newly developed crack after the membrane is installed, you are in luck.

A much better determination of the suitability of a waterproofing membrane to handle differential movement is in ASTM C 1305: Standard Test Method for Crack Bridging Ability of Liquid Applied Waterproofing Membrane or Appendix B in AS 4858: Assessment of resistance of waterproofing membranes to cyclic movement.

In either test method, the membrane is clamped with a minimal gap (hairline for ASTM C1305, 2 mm for AS 4858 Appendix B) and then is exposed to cyclic movement to ensure the membrane stays intact and keeps its continuity and therefore water resistance. This is a much better test indicator for determining if a membrane can bridge newly developed cracks after the membrane has been installed.

Peter Musso
TECHNICAL MANAGER
TREMCO AUSTRALIA