Tensile Strength & Elongation Of Waterproofing Membranes

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:

1. 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,
2. 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 for pre-formed membranes during handling and installation 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. 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 consider 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-

1. Over the expected service life of the waterproofing membrane, stress cycles in the support structure are repeated countless times. Therefore, plastic elongation with no recovery does not measure the membrane’s performance durability;
2. Elongation is achieved at the expense of the membrane thinning out. For materials with very high elongation values, this thinning effect would have exceeded its useful functional thickness limit well before breakage.