Modulus is the force (stress) in pounds per square inch (psi) required to produce a certain elongation (strain). 100% is the most widely used figure for testing and comparison purposes of commercially available waterstops. Generally speaking, the harder a waterstop (ASTM D2240), the higher its modulus. Because it is basically a measure of tensile strength at a particular elongation (rather than at rupture), modulus is also known as tensile modulus or tensile stress.
Ultimate Elongation — ASTM D412
Elongation is the percentage increase in original length (strain) of a rubber or plastic waterstop specimen as a result of tensile force (stress) being applied to the specimen. Elongation is inversely proportional to hardness, tensile strength, and modulus. Therefore, the greater a waterstop’s hardness, tensile strength, and modulus, the less it will elongate under stress. It takes more force to stretch a hard waterstop having high tensile strength and high modulus than to stretch a soft material with low tensile strength and low modulus. Ultimate elongation is the elongation at the moment the specimen breaks. Per ASTM D412, ultimate elongation is expressed as a percentile; therefore, when comparing waterstops the higher the number (%) the better.
Tear Resistance (aka Tear Strength) — ASTM D412
Tear resistance (also known as tear strength) is resistance to the growth of a cut or nick in a waterstop specimen when tension is applied. Values are usually expressed in pound force per inch (lbf/in), so again, the waterstop with the higher value is superior, as it is more resistant to tearing.
Compression Set — ASTM D395
Compression set is the permanent deformation remaining when a force applied to a waterstop specimen for a period of time is removed. Tested under ASTM D395, compression set is expressed as a percentile that relates to the percentage of deformation compared to the waterstop’s original thickness. Therefore, a low value is better as it denotes the waterstop did not “take a set” and returned close to its original shape and size after the force is removed. A high value means the waterstop became deformed (squished) under pressure and did not return to its original shape and size. Compression set represents the percent of deflection that did not return.
There are a number of physical properties you should consider when choosing the right material for your waterstop application. These include hardness, tensile strength, modulus, elongation, tear resistance, and compression set.
Resistance to Chemicals — ASTM D471
Resistance to Ozone — ASTM D1171