Decode tensile strength, permittivity, and ASTM specs like a pro—before they cost you time, money, or credibility.
Spec sheets aren’t just paperwork—they’re the difference between a job that holds and one that fails inspection. This guide breaks down the jargon and shows contractors how to spot red flags before they pour concrete. Learn how to read between the lines, validate performance claims, and avoid costly mistakes on site.
Why Spec Sheets Matter More Than You Think
“It looked good on paper”—that phrase has cost contractors thousands. A spec sheet might say “high strength” or “meets ASTM standards,” but unless you know what those numbers mean in the real world, you’re gambling with your reputation. Geosynthetics aren’t forgiving. If you install the wrong product, it doesn’t just fail quietly—it fails publicly, and often expensively.
Let’s say a contractor installs a geotextile for slope stabilization. The sheet says it has a tensile strength of 1,200 lbs, which sounds solid. But the fabric also has a low permittivity, meaning water doesn’t pass through easily. After a few heavy rains, water builds up behind the slope, pressure increases, and the whole thing slumps. The client calls, the engineer investigates, and suddenly the spec sheet becomes the center of the conversation. That’s when you realize: the numbers weren’t wrong—they were just misunderstood.
Spec sheets are more than technical data—they’re legal documents. Once you install a product, you’re on the hook for its performance. If it fails, the first thing anyone looks at is whether the product matched the design intent. And if you didn’t catch a mismatch between the spec and the site conditions, it’s your liability. That’s why reading spec sheets isn’t just about checking boxes—it’s about protecting your work, your margins, and your name.
Here’s the real kicker: most spec sheets are written for engineers, not contractors. They assume you know the difference between grab tensile strength and wide-width tensile strength, or how permittivity changes with soil type. But on site, you don’t have time to decode every line. You need to know what matters, what’s fluff, and what could burn you later. That’s what this guide is for—to help you read spec sheets like someone who’s been burned before and learned how to avoid it.
The Big Three: Tensile Strength, Permittivity, and ASTM Standards
Tensile strength is often the first number contractors look at—and for good reason. It tells you how much force the material can take before it stretches or breaks. But here’s the catch: not all tensile strength values are created equal. You’ll see terms like “grab tensile strength,” “wide-width tensile,” or “machine direction.” If you don’t know which one applies to your application, you’re guessing. Grab tensile strength (ASTM D4632) is the most relevant for field conditions because it simulates how the fabric behaves when pulled at a single point—like during installation or under localized stress.
Let’s say you’re reinforcing a slope and the spec sheet says the geotextile has a tensile strength of 1,200 lbs. That sounds solid, but it’s only in the machine direction. The cross-machine direction might be 400 lbs. If your slope sees lateral forces, that lower number is what matters. And if you didn’t catch that, you’re now relying on a product that’s underperforming in the direction that counts. That’s how failures happen—not because the product is bad, but because the spec wasn’t fully understood.
Permittivity is another number that gets overlooked, but it’s critical for drainage and filtration applications. It measures how fast water passes through the fabric, usually in sec⁻¹. High permittivity means water flows freely; low permittivity means it doesn’t. If you’re installing fabric behind a retaining wall or under a roadbed, and water needs to move through it, you want high permittivity. ASTM D4491 is the standard test method here. If the sheet doesn’t list permittivity—or lists it without the ASTM reference—you’re flying blind.
ASTM standards are the backbone of spec sheet credibility. They tell you how the numbers were measured. Without them, the values are just marketing fluff. For example, a spec might say “puncture resistance: 500 lbs,” but unless it’s tested using ASTM D6241, you can’t compare it to other products. Always match the property to the test method. If the sheet doesn’t list ASTM standards, ask for the full test report. If they won’t provide it, that’s a red flag. You’re not just buying material—you’re buying verified performance.
How to Spot Red Flags in Spec Sheets
Spec sheets are designed to sell, not to protect you. That’s why you need to read them with a contractor’s eye, not a marketer’s. One of the biggest red flags is vague language. If a sheet says “high strength” or “durable” without numbers, it’s not a spec—it’s a brochure. You want hard data, minimum values, and ASTM references. Anything less is a liability waiting to happen.
Another warning sign is broad ranges. If a sheet says “tensile strength: 400–800 lbs,” that’s not helpful. You build to worst-case, not best-case. What happens if the roll you get is on the low end? You’re still responsible for performance. Always look for minimum values, not averages or ranges. And if the sheet doesn’t specify which value it’s reporting, ask. You don’t want to find out the hard way that the “average” strength was based on ideal lab conditions, not field reality.
Marketing fluff is another trap. Words like “engineered for performance” or “premium quality” mean nothing without data. If the sheet doesn’t list test conditions—dry vs. saturated, temperature, strain rate—you can’t trust the numbers. For example, a geogrid might show high tensile modulus, but if it was tested at a slow strain rate, the numbers are inflated. You need real-world relevance, not lab perfection.
Finally, watch for missing information. If the sheet doesn’t list roll size, coverage area, or installation guidelines, that’s a problem. You need to know how much material you’re getting, how it’s packaged, and whether it requires special handling. If the supplier can’t provide that, they’re not serious about supporting contractors. And if they’re vague on specs, they’ll be vague when things go wrong.
Matching Specs to Site Conditions
The most common mistake contractors make is assuming that a strong product is a good product. But strength alone doesn’t make a geosynthetic suitable. You have to match the specs to the site conditions. That means understanding soil type, water flow, load conditions, and installation method. A fabric that works great under a parking lot might fail in a wetland. The numbers don’t lie—but they don’t tell the whole story either.
Let’s say you’re working on a drainage layer under a roadway. You choose a woven geotextile because it has high tensile strength. But woven fabrics typically have low permittivity. Water doesn’t pass through easily, and over time, fines clog the openings. The result? Water backs up, the subgrade softens, and the pavement cracks. The fabric didn’t fail—it was just the wrong match for the job. That’s why understanding permittivity and apparent opening size (AOS) is just as important as tensile strength.
Here’s a simple framework to help match specs to applications:
- For separation, focus on tensile strength and puncture resistance.
- For filtration and drainage, prioritize permittivity and AOS.
- For reinforcement, look at tensile modulus and creep resistance.
If you’re not sure which property matters most, ask the engineer or supplier. And if they can’t explain it in plain language, that’s a sign they don’t understand the field conditions. You’re not just installing fabric—you’re solving a problem. The specs should reflect that.
How to Validate Manufacturer Claims
Trust is earned, not assumed. When a manufacturer says their product meets ASTM standards, ask for proof. That means third-party lab results, not internal testing. ASTM compliance isn’t just a checkbox—it’s a process. If the product was tested in-house, you need to know the lab’s credentials, the test conditions, and whether the results are repeatable. Otherwise, you’re relying on marketing, not engineering.
One of the best tools for validation is NTPEP data. The National Transportation Product Evaluation Program tests geosynthetics under standardized conditions and publishes the results. If a product is listed in NTPEP, you can compare it apples-to-apples with others. It’s not perfect, but it’s a lot better than relying on spec sheets alone. Ask your supplier if their product is NTPEP-listed—and if not, why.
Let’s say you’re choosing between two geogrids. One is cheaper and claims “similar specs.” But when you dig into the details, you find that the cheaper one was tested at a slower strain rate, which inflates the tensile modulus. The numbers look good, but they don’t reflect field conditions. If you install it and it stretches more than expected, your reinforcement fails. That’s not a cost savings—it’s a liability.
Always ask for the full test report. If the supplier won’t share it, walk away. You’re not just buying material—you’re buying confidence. And if the numbers don’t hold up under scrutiny, they won’t hold up under load either.
Spec Sheet Hacks: What Experienced Contractors Always Check
Experienced contractors don’t just glance at spec sheets—they scan them like a checklist. First, they check the units. Is tensile strength listed in pounds or kilonewtons? Is AOS in millimeters or inches? Mixing units can lead to major errors. Always convert to the units you use on site. And if the sheet mixes units without explanation, that’s a sign of sloppy documentation.
Next, they look for minimum values. Not averages, not ranges—minimums. You build to the worst-case scenario, not the best. If the sheet lists “average tensile strength,” ask for the minimum. That’s the number that matters when things go wrong. And if the supplier can’t provide it, they’re not serious about performance.
UV resistance is another overlooked detail. If the fabric will be exposed to sunlight—even temporarily—you need to know how long it can last. Some products degrade in weeks; others hold up for months. Look for ASTM D4355 testing, which measures UV resistance. If it’s not listed, ask. And if the supplier says “it’s not needed,” they’re guessing.
Finally, confirm roll size and coverage. You need to know how much material you’re getting, how it’s packaged, and whether it fits your installation plan. Some rolls are 12.5 feet wide; others are 15. If you don’t check, you might come up short—or waste material. And always ask about installation guidance. Some products require specific overlap, anchoring, or orientation. If it’s not in the sheet, get it in writing.
Top Takeaways
- Always match spec values to ASTM standards—numbers without test methods are just marketing.
- Permittivity and AOS matter more than tensile strength in drainage applications—water flow is the real test.
- Use a pre-installation checklist—verify units, minimum values, UV resistance, and roll dimensions before ordering.
FAQs Contractors Ask About Geosynthetic Spec Sheets
How do I know if a spec sheet is trustworthy? Look for ASTM standards, minimum values, and third-party test reports. If it’s vague or missing key data, ask questions—or walk away.
What’s the difference between grab tensile strength and wide-width tensile strength? Grab tensile simulates field conditions (ASTM D4632); wide-width is more lab-specific (ASTM D4595). Use grab tensile for most site applications.
Why does ASTM matter so much? ASTM standards are the only way to compare products reliably. They define how each property is measured—so when two products claim the same tensile strength, you can verify if they were tested the same way. Without ASTM references, you’re comparing apples to oranges.
Can I use the same geotextile for separation and drainage? Not always. Separation fabrics focus on strength and durability, while drainage fabrics prioritize water flow. Some nonwovens can do both, but you need to check permittivity and AOS to be sure. Don’t assume—verify.
What’s the risk of ignoring UV resistance? If the fabric is exposed to sunlight—even for a few weeks—it can degrade. That weakens tensile strength and leads to premature failure. Always check for ASTM D4355 UV resistance if exposure is expected.
How do I know if a product is NTPEP listed? Ask your supplier or check the NTPEP website. NTPEP testing is voluntary but highly respected. If a product isn’t listed, ask why. It doesn’t mean it’s bad—but it does mean you need more proof.
Summary
Reading geosynthetic spec sheets isn’t just about checking numbers—it’s about protecting your work. Every line on that sheet represents a potential risk or a hidden advantage. If you know how to decode it, you can build smarter, faster, and with fewer callbacks. If you don’t, you’re leaving your reputation in someone else’s hands.
The best contractors treat spec sheets like blueprints. They don’t skim—they scan for red flags, validate claims, and match specs to site conditions. That’s how you avoid costly mistakes and build trust with engineers, clients, and crews. It’s not about being perfect—it’s about being informed.
So next time you’re handed a spec sheet, don’t just glance at it. Read it like your next job depends on it—because it probably does. And if something doesn’t add up, ask questions. The best builders aren’t just skilled—they’re curious, cautious, and always one step ahead.