These properties are often headlined as important features of waterproofing membranes:
a) tensile strength
b) elongation at break
How relevant are these properties for waterproofing performance?
In order to answer these questions, it is necessary to first look at these definitions:
Elastic Deformation and Elastic Limit
An elastic material deforms when a force is applied to it. So long as the deformation stays within the elastic limit, the material will revert to its original shape when the force is removed.
Many waterproofing materials which are described as elastic do not show perfect elastic behaviour i.e the recovery is not 100 % after the force is removed.
Plastic Deformation
A plastic material also deforms when a force is applied to it. However, when the force is removed the material will not recover its original shape.
Many plastic materials used for waterproofing exhibit a small recovery but not enough to justify calling them elastic.
Elastic materials which are stretched beyond the elastic limit but before the breaking point will deform plastically i.e. with little or no recovery.
Young’s Modulus of Elasticity, E
Young's modulus measures the stiffness of elastic materials or the force needed to deform an object along an axis when the force is applied along that axis; it is measured as the ratio of stress to strain (change in length over the original length).
Comparing two materials with differing E values, the material with lower modulus requires less force to elongate it by a defined amount. In other words, and this is relevant for assessing waterproofing membranes, the material with higher modulus is stressed more for the same elongation compared to a lower modulus material.
a) How important is the tensile strength?
For fully-adhered membranes, consider the stresses imposed on it when the supporting substrate moves due to, for example, settlement or temperature changes. Under these circumstances, the membrane’s tensile strength is irrelevant because its function is not to restrain the movement (it cannot, in any case), but to deform and accommodate the movement without suffering functional damage.
Comparing two materials installed on the same structure:
(i) The membrane with a lower tensile strength and lower E modulus may well fulfill this function and retain its waterproofing integrity so long as the movement is within its deformation limit,
(ii) The other material, on the other hand, with a higher tensile strength and higher modulus (stiffer) may fracture because the elevated stress needed to deform it exceeds its tensile strength. Or, if the built-up tensile stresses overcome its adhesion strength, it will delaminate, allowing the lateral migration of water between the membrane and the substrate in case of leaks thus compromising its waterproofing performance.
In general, tensile strength is important during handling and installation of pre-formed membranes and for loose-laid or mechanically-fixed membranes.
b) How important is the elongation value?
The ability of waterproofing membranes to stretch and elongate under stress is undoubtedly important in applications where the substrate is subjected to movement such as on roof slabs. However, the elongation value, taken in isolation, is often misleading.
Superficially, a material with 800 % ultimate elongation looks far superior to another with 300 % elongation. But conside
r these points:
Very high elongation values are usually achieved by materials with plastic deformation behaviour or elastic materials which have been stretched beyond their elastic limits. This raises two concerns-
(i) Over the expected s
ervice life of the waterproofing membrane, stress cycles in the support structure are repeated countless times. Therefore, plastic elongation with little or no recovery does not measure the membrane’s performance durability;
(ii) Elongation is achieved at the expense of the membrane thinning out. For materials subjected to very high elongation, this thinning effect would have stretched it way past its useful thickness well before breakage occurs.
In view of the above, waterproofing membranes are often reinforced with fabrics; these are built-in for preformed sheets and added during installation for liquid-applied membranes. They serve to add elasticity (recovery property) and/or restrict the elongation within useful limits.
Therefore, for a waterproofing membrane applied on structures subject to movement stresses, the elongation value on its own is not sufficient for assessing its fitness for purpose. The important performance property to look for is its dynamic crack-bridging or crack-cycling capability, tested to recognised standards eg EN 1062-7. This test measures the membrane’s ability to elongate and recover over repeated cycles, simulating conditions you would expect the membrane to undergo during its service life.
(Reference must be made to the manufacturer’s technical data sheet for instructions on use of particular products.)
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