Testing standard
Standard Test Method for Determining Tensile Properties of Geogrids by the Single or Multi-Rib Tensile Method
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D6637 pulls geogrid specimens to failure by one of three procedures: a single rib, multiple ribs, or multiple layers of multiple ribs. Method A reports a force per rib; Methods B and C report kilonewtons per metre. The standard states its intent as quality control and conformance testing. The current edition is D6637/D6637M-15(2023).
Strips of geogrid are pulled in tension until they fail. Three procedures are defined and they differ only in how much of the grid is in the specimen: Method A tests a single rib and reports a force in newtons; Method B tests multiple ribs and reports a force per unit width in kilonewtons per metre; Method C tests multiple layers of multiple ribs and also reports kilonewtons per metre.
That choice is not a convenience. A geogrid is a structure, not a sheet, and how many ribs are gripped changes what the specimen is being asked to do.
Geogrids reinforce soil. In a retaining wall, a steepened slope or a road base, the grid carries tension that the soil cannot, and the design value is a tensile strength per metre of wall width. That is the number this method produces by Methods B and C.
Single-rib testing by Method A is a different animal. It gives a force per rib, which is useful for quality control and for comparing production against a specification, but it is not directly a design value: converting it to a per-metre strength by multiplying by rib count assumes every rib takes an equal share, and in a real grid under a real grip they do not.
The standard states its own intent plainly — quality control and conformance testing of geogrids. Long-term design strengths involve creep, installation damage and durability reduction factors that no short-term tensile test provides.
A geogrid is a structure, not a sheet. How many ribs are in the specimen decides what the test is measuring.
Maximum force divided by the specimen width
The form used in reinforced-soil design, produced by Methods B and C.
Maximum force carried by one rib
A quality-control figure. Useful for comparing production against a specification, and not a design value on its own.
Long-term design strength needs reduction factors
None of these come from a tensile test, and a short-term strength used directly as a design strength is a serious error.
Where the grid is wrapped rather than clamped, a capstan introduces load progressively and avoids crushing the junctions. Wrapping distributes the grip force over a length of rib instead of concentrating it at one bite.
Specifications
Strain measured optically from marks on the ribs, not from crosshead travel. On a specimen this compliant, machine compliance and grip slip are a large share of the movement, and a non-contact gauge is the only instrument that survives the elongation.
Specifications
For multi-rib specimens where a clamped grip is used, wide hydraulic faces take the full specimen width at the forces a heavy uniaxial grid demands. Clamping force has to be enough to hold without crushing the junction ribs.
SpecificationsGeogrid strengths are substantial. A multi-rib specimen of a heavy uniaxial grid can demand well over 100 kN, and a frame of 100 to 300 kN with wide grips and long travel covers most commercial products. Force accuracy to ASTM E4 is the requirement.
Travel is the constraint people underestimate. Geogrids elongate significantly before failure, so the crosshead has to have the stroke to take a specimen to break without restarting.
Strain measurement, where strength at a specified strain is reported, has to come from the specimen rather than from crosshead travel: grip slip and machine compliance are large fractions of the movement on a compliant specimen. A video or long-travel extensometer following marks on the ribs is the practical answer.
Specimens that pull out of the grips are the dominant failure, and a partially slipped specimen produces a low strength with a plausible curve. Junctions crushed by over-tightened clamps break there rather than in the rib. Strain taken from crosshead travel overstates elongation and understates modulus. And a single-rib figure multiplied up and presented as a per-metre design strength is a reporting error that reaches drawings.
Four products, four methods. Substituting one for another is the commonest specification error in this area.
| Geogrid | Geotextile | Geomembrane | |
|---|---|---|---|
| Tensile method | ASTM D6637 | ASTM D4595, ISO 10319 | ASTM D6693 and D4885 |
| Specimen | Ribs, with junctions | A wide strip of fabric | A dumbbell, or a wide strip |
| Reported as | N per rib, or kN/m | kN/m | Strength and elongation, or kN/m |
| Puncture method | Not usually applicable | ASTM D6241, ISO 12236 | ASTM D6241, ISO 12236 |
| Grip problem | Holding a specimen that is mostly hole | Slippage on a wide fabric | Slippage on a ductile sheet |
A grid is not a fabric and a fabric is not a membrane. Each method is written around how its product introduces load, and applying the wrong one produces a number that looks right and describes the grip.
ASTM D6637, published as D6637/D6637M, is the test method for the tensile properties of geogrids by the single or multi-rib method. Strips of grid are pulled to failure by one of three procedures: Method A on a single rib, Method B on multiple ribs, and Method C on multiple layers of multiple ribs. The current edition is D6637/D6637M-15, reapproved in 2023.
Whichever the specification names, and the choice is not cosmetic. Method A gives a force per rib in newtons and is a quality-control measure. Methods B and C give a force per unit width in kilonewtons per metre, which is the form reinforced-soil design uses. A laboratory that runs Method A because it is easier to grip has not produced the number the designer asked for.
Not reliably. Multiplying the single-rib force by the number of ribs per metre assumes every rib carries an equal share, and in a real grid clamped in a real grip they do not — the outer ribs and the ribs nearest the grip take more. The conversion produces a number that is usually too high, and it reaches drawings often enough to be worth stating plainly.
No. The standard says its intent is quality control and conformance testing. A long-term design strength for a reinforced-soil structure is derived by applying reduction factors for creep, installation damage and durability to a short-term strength, and none of those factors comes from this test. Using an ultimate tensile strength directly as a design value is a serious error, not a conservative simplification.
Because the specimen is mostly hole. Load has to be introduced into the ribs without the grip crushing the junctions, and a flat clamped face on an open structure either crushes what it holds or lets the specimen pull through. Capstan or roller arrangements that wrap the grid, or clamps that engage the transverse bars rather than squeezing the ribs, are the usual answers.
More than people expect. A multi-rib specimen of a heavy uniaxial geogrid can demand well over 100 kN, so a frame of 100 to 300 kN with wide grips is realistic for commercial products. Force accuracy to ASTM E4 applies. The other constraint is stroke: geogrids elongate significantly before failure and a short-travel frame cannot take the specimen to break.
Because they are not exact conversions of one another. The values stated in each system are to be regarded separately as standard, so a laboratory picks one and stays in it. Mixing them — measuring in one and reporting in the other by conversion — introduces small discrepancies against the tabulated requirements, and on a conformance test those discrepancies are exactly what is being judged.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | High — a multi-rib specimen of a heavy uniaxial grid can demand well over 100 kN | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Strain measurement | An extensometer of the class the method specifies | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Capstan or roller grips that wrap the grid, or clamps engaging the transverse bars rather than crushing the junctions | Our split capstan grips or a fixture built for this method, built to the specimen |
| Environment | Standard laboratory conditioning as the method requires | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
