Before you can design with any material, you need to fully understand its engineering properties. This chapter provides the technical specifications for Reinforced Polyethylene (RPE) required to perform the design calculations we’ll cover in the chapters ahead.
Let’s move beyond general descriptions and look at the specific, quantifiable metrics that define RPE’s performance. We’ll break down key properties—from tensile strength and puncture resistance to long-term durability—and describe the industry-standard ASTM tests used to verify them. You’ll need this data to specify the material with confidence.
Let’s begin by digging into the numbers.
Composition: Reinforced Polyethylene’s Tri-Laminate Structure
The key to RPE’s performance lies in its composite, or tri-laminate, structure. It’s not a single, homogenous sheet of plastic like many common alternatives. Instead, it’s an engineered sandwich of three distinct layers fused together under heat and pressure.
The Reinforcement Scrim
RPE’s core is a woven grid of high-strength, high-density polyethylene (HDPE) tapes. It performs the same function as rebar in concrete—it’s the backbone that gives the material exceptional tear resistance, puncture strength, and dimensional stability. The scrim is what prevents a small cut or puncture from expanding into a big headache.
The Impermeable Coatings
Laminated onto both sides of the scrim are durable, flexible coatings of low-density polyethylene (LDPE). These outer layers make the liner impermeable and provide the primary defense against UV degradation and chemical exposure.
The real-world engineering advantage of this composite design is a dramatic increase in performance-per-pound. It allows an RPE liner to provide comparable, or even superior, puncture and tear resistance to an HDPE liner that is nearly twice as thick. This creates a massive logistical benefit: a 30-mil RPE liner, for example, delivers the durability of a 60-mil HDPE but weighs only about a third as much. Hauling and deploying a lighter, more manageable material saves a tremendous amount of time, fuel, and heavy equipment hours on site.
Key Performance Metrics: Strength and Durability
You’ll often see different ASTM tests cited for the same property. For tensile strength, some specs use D6693 (a general geomembrane test), while others use textile tests like ASTM D5034 (Grab Method) and ASTM D5035 (Strip Method), which are very appropriate for reinforced materials. The key isn’t that one is “better,” but that you should compare results from the same test method when evaluating different products.
Tensile Strength
Because RPE is a reinforced composite with a woven scrim at its core, its tensile strength—its ability to resist being pulled apart—is measured using standard textile tests.
The Grab Test (ASTM D5034)
The Grab Test is designed to measure the material’s strength in a “real-world” pulling scenario. In this test, a wide piece of the liner is clamped by jaws that are narrower than the sample itself, effectively “grabbing” it in the middle. The machine then pulls the sample until it breaks.
This test simulates the types of localized pulling forces the liner might experience during installation, for example, if a section of the material snags or is pulled from a specific point. It measures the strength of the yarns being pulled and the contribution of the surrounding material.
The Strip Test (ASTM D5035)
The Strip Test is a more straightforward measurement of the fabric’s baseline strength. A narrow “strip” of the material is cut, and the entire width of that strip is clamped in the jaws and pulled to its breaking point.
This test gives you a pure, apples-to-apples strength value per inch of material width. It’s a fundamental data point for engineers to calculate the liner’s ability to handle the immense, evenly distributed gravitational loads on the steep slopes of the reservoir.
Why it Matters for PSH
High tensile strength is what allows a massive, multi-ton panel to be dragged across the subgrade and pulled up a steep 3:1 slope without ripping apart under its own immense weight. The Grab Test is particularly relevant here, as it demonstrates the liner's ability to handle the inevitable localized pulling forces and stress points that occur during deployment.
But tensile strength is also critical for the next 50 years. The immense weight of the water and protective cover creates a constant gravitational load on the liner. The Strip Test provides engineers with the pure, baseline strength data they need to verify that the material can support this load for decades without stretching, deforming, or failing.
Puncture Resistance
Puncture resistance is the liner’s ability to resist being pierced by a sharp, pointed object—a critical property for any material that will have millions of tons of water pressing it down onto a soil subgrade.
The Standard (ASTM D4833)
This is a straightforward test, where a probe is pushed through the material to determine the force required to puncture it.
Why it Matters for PSH
Resistance to puncture is your primary defense against leaks.
- Subgrade Imperfections: Even with perfect site prep, a single sharp rock or root left just under the surface can become a significant threat. Under immense hydrostatic pressure, that rock will try to push its way directly through the liner. High puncture resistance is the liner’s ultimate defense.
- Construction Damage: A dropped tool or a sharp rock in the tread of a worker’s boot can cause a puncture. A tougher material is more forgiving of the inevitable accidents that happen on a busy construction site.
- Ice Action: In cold climates, the sharp, focused pressure from shifting ice sheets can exert enough force to puncture a weaker liner.
Tear Resistance
Tear resistance is the liner’s ability to keep a tear from developing from an existing nick or cut. For a shirt sleeve, it’s the difference between dealing with a tiny hole from a stray wire and ripping your sleeve in half. RPE has a considerable advantage here since its reinforcing scrim functions as a ripstop—think parachute material or even Kevlar.
The Standard (ASTM D5884)
For reinforced materials, a common test is the Tongue Tear method. This is measured using ASTM D5884, where a sample with a pre-cut ‘tongue’ is pulled apart to determine the force required to propagate the tear.
Why it Matters for PSH
On a busy construction site, small cuts from a dropped tool or a sharp rock are a real possibility, especially when the multi-ton panel is pulled into position. Later on, if any parts of the protective cover shift, ice action or debris movement could snag or gouge the liner. A liner with high tear resistance prevents minor, easily repairable nicks from zippering into a catastrophic failure.
Dimensional Stability
Dimensional stability is a material’s ability to maintain its shape and size when exposed to temperature changes. It’s a measure of how much the liner will shrink or expand.
The Standard (ASTM D1204)
In this test, a sample is carefully measured before being subjected to high temperatures. The before and after measurements are compared, and the difference is described in percentage of change.
Why it Matters for PSH
Dimensional stability is a critical property for both the short-term challenges of installation and the long-term stresses of operation. During construction, a newly-installed liner with poor stability can shrink significantly after a hot day, putting immense stress on new seams and anchor points.
This importance extends throughout the facility’s 50-year life. On a hot day, as the water is drawn down and the slope is exposed, a liner with poor stability will expand as it heats up, causing large wrinkles to form. When the reservoir refills, the water pressure flattens these wrinkles into sharp creases. This daily cycle of wrinkling and creasing creates material fatigue, leading to premature failure. A liner with excellent dimensional stability, however, will remain smooth and tight against the slope through thousands of temperature and water level cycles.
Environmental Stress Cracking Resistance (ESCR)
Stress cracking arises when a plastic material loses flexibility and becomes brittle under specific environmental conditions. When the material is exposed to long-term or repeated stress, it’s much more likely to form cracks that can slowly spread. ESCR is a critical indicator of a liner’s ability to avoid premature aging and perform through its predicted service life.
The Standard
The key test is ASTM D5397, which determines how long it takes for a stressed sample to fail in a controlled environment.
Why it Matters for PSH
You are designing an asset to last for 50 years or more. Over those decades, the liner must withstand the powerful and cyclic forces created by the immense weight of the water and protective cover (link to chapter 7) on those same steep slopes. A material with poor ESCR can become brittle and fail under these changing loads, resulting in catastrophic leaks decades after installation. Polyethylene-based materials like RPE are known for their exceptional ESCR, which makes them an excellent solution for long-service-life applications.
UV Resistance
UV resistance is the material’s ability to withstand degradation from long-term exposure to ultraviolet (UV) radiation from sunlight. For black geomembranes, this protection is primarily provided by the quantity and quality of carbon black mixed into the polyethylene.
The Standard
Carbon black content is measured via ASTM D1603, and UV degradation can be simulated with tests like ASTM D5596.
Why it Matters for PSH
One might assume that UV resistance isn’t important for a liner that will be covered for the length of its service life, but it is a critical factor during the construction phase. A PSH reservoir lining is a massive undertaking, and the RPE panels may be exposed to direct sunlight for weeks or even months between deployment and final covering. A liner with poor UV resistance can begin to lose its physical properties and durability before it’s even put into service. High UV resistance ensures the material you install is just as strong as the day it left the factory.
