What ASTM D3039 Tensile Testing Reveals About Material Behavior Under Stress

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On Spec #3 — published by Finish + Form

Author's note: This article interprets published laboratory testing comparing epoxy terrazzo and Durabella Biopolymer Terrazzo under controlled conditions. Its purpose is to provide context for architectural specification, not to prescribe a particular solution. The discussion that follows is based on the following sources:

• Primary Source: Intertek USA, Inc., Pittsfield, Massachusetts. Project Number P20220434.

• Testing Standard: ASTM D3039/D3039M-17, Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials (modified material).

• Comparative Systems: Epoxy terrazzo and Durabella Biopolymer Terrazzo, tested side by side under identical laboratory conditions.

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Terrazzo floors are not static once installed. Substrates move under thermal expansion, structural settlement, seismic activity, and ordinary building deflection. Because the finish is bonded to that substrate, it has to absorb some share of that movement as well before it cracks. Whether a flooring system can stretch slightly under that stress or resists that stretching until it suddenly fails is one factor influencing crack behavior, joint design, and long term performance in buildings that are never perfectly still.

Understanding that behavior requires looking beyond a single mechanical property. A material that resists bending is not necessarily the same material that resists pulling. Two terrazzo systems can behave very differently when stretched, even if their flexural numbers have already revealed part of the story.

This distinction can often be overlooked because specifiers can assume that a stiffer floor finish is automatically the safer choice. Stiffness reads as strength. But a finish bonded to a substrate is rarely loaded in pure bending. It also experiences tension, the pulling force that occurs when a substrate expands, contracts, or flexes and drags the bonded finish along with it. A finish that cannot accommodate any stretch under that pulling force has only one way to release the stress: to crack.

This article builds directly on the flexural analysis presented in On Spec no. 2. There, we examined how each material responds to bending. In this article, we examine how each material responds to being pulled apart. Together, the two sets of results offer a more complete picture of the same underlying behavior: 1) how much can the material deform before it resists further, and 2) how much energy does it store while doing so?

The Standard

ASTM D3039/D3039M, Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials, measures how a material responds when pulled along its length until failure. The test produces several values, but two matter most for specification purposes: tensile strength and tensile modulus.

Tensile strength is the maximum stress the material can withstand before failure, reported in pounds per square inch. Tensile modulus describes a different characteristic: stiffness under tension or how much stress is required to produce a given amount of stretch. A high modulus means the material resists stretching and behaves rigidly under tension. A low modulus means the material stretches more readily under the same load before it reaches its breaking point.

The distinction matters because strength alone does not describe how a material reaches failure. A material can be strong and still behave rigidly, reaching its limit with very little stretch beforehand. A material can also be comparatively less strong while still deforming more gradually, giving a substrate room to move before stress concentrates at a single point.

The Evidence

Testing was performed by Intertek USA, Inc., Pittsfield, MA (Project Number

P20220434), in accordance with ASTM D3039/D3039M-17, modified material. Five

specimens of each material were tested at 23°C ± 2°C and 50% ± 10% relative

humidity, the standard ambient test condition. Specimens were tested as received,

unconditioned, on an Instron 5985 at a cross-head speed of 2 mm/min.

Property	Epoxy Terrazzo	Biopolymer Terrazzo
Tensile strength, average (psi)	860	496
Tensile modulus, average (psi)	3,290,000	145,000
Tensile strain at max load, average (%)	0.0303	1.07
Standard deviation, strength (psi)	263	53
Standard deviation, modulus (psi)	354,000	25,000
Source: Intertek Tensile Report, ASTM D3039/D3039M-17, Project P20220434, dated May 13, 2022.

What the Results Mean

Epoxy terrazzo tested with a tensile modulus of 3,290,000 psi against biopolymer

terrazzo’s 145,000 psi, showing that epoxy is approximately 23 times stiffer under tension than the biopolymer system. This shows that epoxy resists stretching to a much greater degree than biopolymer does when both are pulled under the same load.

The specification implication is that a substrate experiencing thermal movement or

minor structural flexing transmits that movement differently into each finish. Epoxy’s

high modulus and low strain at max load (0.0303 percent) means it reaches its tensile limit after very little elongation, a behavior consistent with a more brittle response to movement. Biopolymer’s strain at max load (1.07 percent) is roughly 35 times higher, meaning the material accommodates substantially more elongation before reaching peak load.

This pattern is consistent with On Spec no. 2’s flexural findings, where epoxy measured approximately 18 times stiffer than biopolymer in bending (175,490 psi versus 9,803 psi modulus). The tensile data shows a similar relationship, approximately 23 times, in a different loading direction. Two independent test methods (one measuring bending and the other pulling) point toward the same underlying material behavior: epoxy is the more rigid system and biopolymer is the more flexible one.

What this test does not prove is how either material performs once installed, bonded to a real substrate, under field conditions of temperature cycling, structural movement, or years of service. Tensile coupon testing isolates the material itself. It does not simulate the bond line, the substrate, or the installation method.

Specification Considerations

Design teams working on projects with anticipated substrate movement— whether from thermal cycling, seismic design category, or structural deflection— may wish to consider how a finish’s tensile behavior, not only its tensile strength, factors into long-term crack performance. A lower-modulus, higher-elongation system is not inherently the correct choice for every application. But the relationship between modulus, strain capacity, and expected substrate movement is a variable worth evaluating during specification rather than assuming away.

Teams should also note the strength-versus-stiffness distinction directly. Epoxy’s higher tensile strength (860 psi versus 496 psi) does not by itself indicate better field

performance under movement-driven stress. A system that resists pulling to a much

greater extent, then fails abruptly with little warning strain, presents a different risk

profile than a system that yields gradually under the same forces. Both behaviors have legitimate applications depending on project conditions, substrate design, and

movement expectations.

Honest Limits

This test isolates tensile behavior of the cured material in coupon form. It does not

simulate bond strength to a substrate, the influence of reinforcement or membrane

systems such as Duraforce, long-term fatigue under repeated cyclic loading, or actual crack behavior in an installed floor. It also doesn't test compressive behavior, which governs different failure modes entirely and is addressed separately in a future article in this series.

Tensile coupon data should be combined with project-specific structural analysis,

substrate assessment, and engineering review before any specification decision is

made. This article presents one input among several that a design team should weigh, not a standalone basis for material selection.

Across two independent test methods, bending and pulling, epoxy terrazzo consistently tests as the stiffer, more rigid system, while biopolymer terrazzo consistently tests as the more flexible one, accommodating greater deformation before reaching peak load.

Full laboratory reports are available on request for design teams who want to review the underlying data directly.

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