Biaxial Geogrid Tensile Testing: Standards, Specimens, Methods, and Performance Parameters

Biaxial geogrid tensile testing measures the load-carrying capacity and deformation behavior of polymer geogrids in both the machine direction and the cross-machine direction, providing the key strength parameters used for base reinforcement and subgrade stabilization design. This article explains specimen preparation, testing items, and procedures in accordance with ASTM D6637, ISO 10319, EN, and GB standards, together with guidance on result interpretation for quality control and engineering acceptance.

Technical Overview

A biaxial geogrid is a polymeric reinforcement product, typically manufactured from polypropylene or high-density polyethylene by punching a polymer sheet and stretching it in two perpendicular directions. The resulting square or rectangular apertures and stiff, oriented ribs give the material high tensile stiffness in both the machine direction (MD) and the cross-machine direction (CMD). In service, the geogrid interlocks with aggregate particles, laterally confines granular layers, distributes applied loads over a wider area, and reduces the required base course thickness.

Because the reinforcement function in base and subgrade applications is mobilized at very small strains, tensile testing is the single most important laboratory procedure for characterizing biaxial geogrids. The test produces a complete force-versus-strain curve for each direction, from which engineers extract the tensile strength at 2% strain, tensile strength at 5% strain, peak tensile strength, and elongation at maximum load. The governing methods are ASTM D6637 (determination of tensile properties of geogrids by the single-rib or wide-width multi-rib method) and ISO 10319, also published as EN ISO 10319, for the wide-width tensile test of geosynthetics. In the Chinese system, GB/T 15788 specifies the equivalent wide-width procedure and GB/T 17689 defines performance requirements for plastic geogrid products. Manufacturers, contractors, and accredited third-party laboratories use these tests for factory production control, type testing, lot acceptance, and conformity assessment.

Testing Samples

Single-Rib Specimens

A single-rib specimen consists of one complete, full-depth rib cut with transverse junction stubs at each end, prepared with a gauge length of typically 100 mm. This configuration corresponds to Method A of ASTM D6637 and is widely used for routine factory quality control because specimen preparation is quick and material consumption is low. The mean single-rib strength is converted to strength per unit width by multiplying by the number of ribs per meter.

Wide-Width Multi-Rib Specimens

Wide-width specimens are cut at least 200 mm wide across the full aperture pattern, with edges trimmed so that only complete ribs and junctions are loaded, and mounted with a gauge length of 100 mm. This configuration corresponds to Method B of ASTM D6637 and to ISO 10319, EN ISO 10319, and GB/T 15788. Wide-width testing engages several ribs simultaneously and better represents the distributed load transfer that occurs in the field, so it is generally preferred for type testing, certification, and dispute resolution.

Complementary Junction Specimens

Junction (node) specimens are prepared to evaluate the bond between a rib and the transverse junction, following the single-rib-to-junction concept of ASTM D7737. Junction integrity is essential because load transfer between crossing ribs, and therefore between the geogrid and the confined aggregate, depends on the connection strength at each node.

Sampling and Conditioning

Samples are taken from production rolls across the full width, avoiding the outermost wrap and any visibly damaged areas. Specimens for the two directions are cut at least one roll width apart where practical. Before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity, per the reference atmospheres of ISO 291 or ASTM D618, normally for at least 24 hours.

Testing Items

A complete biaxial geogrid tensile test program reports the following properties for each direction:

• Peak tensile strength in MD and CMD, expressed in kN/m
• Tensile strength at 2% strain, the primary design index for base reinforcement
• Tensile strength at 5% strain, used for deformation-controlled design checks
• Elongation at peak load, in percent
• Secant tensile stiffness between specified strain limits, in kN/m
• Junction strength, expressed in N and as a percentage of the rib tensile strength
• The full load-strain curve for each valid specimen, with individual and mean values
• Optional long-term characterization, including sustained-load creep testing per ASTM D5262 or ISO 13431

Testing Methods

Apparatus and Gripping

Testing is performed on a constant-rate-of-extension (CRE) universal testing machine of verified calibration, equipped with grips suitable for stiff polymer grids. Wedge grips with serrated or grooved inserts, pneumatic clamping grips, or roller grips are all acceptable provided they transmit load without rib slippage, rib crushing, or premature jaw-face rupture. Careful specimen alignment is essential so that all ribs are loaded uniformly and simultaneously.

Test Procedure

After conditioning, the specimen width and the rib pitch are measured and recorded. The specimen is mounted in the grips at the specified gauge length, and a small preload is applied to remove slack and straighten the ribs. The machine is then operated at a constant rate of extension, typically corresponding to a strain rate of about 20% per minute, which equals 20 mm/min for a 100 mm gauge length under ISO 10319. Loading continues until the specimen ruptures or the force falls substantially below the peak. The maximum force, the force at 2% strain, the force at 5% strain, and the strain at maximum force are recorded for each specimen. A minimum of five valid specimens is tested in each direction; specimens that fail inside the jaw faces are rejected and replaced.

Calculation and Reporting

For wide-width specimens, the tensile strength at a given strain is the corresponding force divided by the specimen width, reported in kN/m. For single-rib specimens, the mean rib force is multiplied by the number of ribs per unit width. The test report states the standard used, the specimen configuration, gauge length, conditioning atmosphere, extension rate, individual and mean results for each direction, the coefficient of variation, and the load-strain curves. Balanced biaxial products should exhibit comparable MD and CMD values, and any systematic imbalance is reported explicitly because it affects orientation-sensitive installation decisions.

Applications

The parameters obtained from biaxial geogrid tensile testing directly govern specification, design, and acceptance in a wide range of geotechnical and pavement engineering contexts:

• Base reinforcement of flexible and rigid pavements, where strength at 2% strain controls load distribution and layer-thickness reduction
• Subgrade stabilization over soft or variable soils for highways, airfields, and access roads
• Unpaved haul roads, temporary working platforms, and construction staging areas
• Parking lots, log yards, intermodal container yards, and interlocking concrete block pavements
• Railroad ballast confinement and trackbed stabilization
• Foundation mattresses, embankment basal reinforcement, and slope stabilization systems

Commercial biaxial geogrids are commonly classified by their strength at 2% strain, for example 15 x 15, 20 x 20, 30 x 30, or 40 x 40 kN/m in the two directions. Peak tensile strength is typically one and a half to two and a half times the 2% strain value, and peak elongation for stiff punched-and-drawn polypropylene grids generally remains below about 15%. These correlations help reviewers quickly judge whether a set of test results is plausible for the declared product grade.

FAQ

Why is the strength at 2% strain more important than the peak strength?

In base reinforcement and subgrade stabilization, the granular layer and the subgrade deform very little before serviceability is affected. The geogrid therefore works in the low-strain region of its load-strain curve, so the force available at 2% strain, rather than the ultimate strength, determines the reinforcement benefit. Peak strength is still reported, but design methods are calibrated against the low-strain stiffness and strength indices.

Are the machine direction and cross-machine direction results always equal?

For a balanced biaxial product they should be approximately equal, and most specifications allow only a limited directional imbalance. Differences arise from the drawing ratios and thermal history during manufacture, so consistent, well-controlled production is reflected in tightly matched MD and CMD results with low scatter.

Which standard should a project specification cite?

This depends on the region and the contract framework. North American projects commonly specify ASTM D6637, while European and many international projects specify EN ISO 10319, and Chinese projects specify GB/T 15788 with product requirements from GB/T 17689. Because gauge lengths, extension rates, and specimen widths differ slightly between methods, results are not strictly interchangeable, and a single method should be applied consistently for both type testing and lot acceptance.

How much sample is needed and what is the typical testing sequence?

Laboratories normally require a full-width sample of roughly one to two meters of roll, taken away from the roll ends. After at least 24 hours of conditioning at standard atmosphere, a typical program tests at least five wide-width specimens in each direction plus junction specimens, and reports are normally issued within a few working days of specimen receipt.

Can short-term tensile strength be used directly as the long-term design strength?

No. Long-term allowable strength must account for creep under sustained load, installation damage from aggregate placement and compaction, and chemical and environmental degradation, assessed for example through creep testing per ASTM D5262 or ISO 13431 and simulated damage testing per ISO 10722, followed by application of appropriate reduction or partial factors in the design method.