Epoxy Resin Strengths Explained

Compressive, Tensile, Flexural, and Bond Strengths of Epoxy Resin

What do the Strength Numbers for Epoxy Mean?

There are 4 primary strengths that are very important for most civil and mechanical engineering applications of epoxy: compressive strength, tensile strength, flexural strength, and bond strength. The test methods used to get the specific strength of the epoxy is done by the ASTM testing method listed on the epoxies technical data sheet. The master test standard for epoxy is ASTM C881. Norm Lambert the author of this page is a voting member of ASTM Technical Subcommittee C09.25 that sets the ASTM C881 standard.

Compressive Strength of Epoxy

This strength is the ability of the material to handle weight. Compressive strength tests are ran by pushing both sides of a cylinder of the epoxy (or concrete etc.) on the top and bottom of the cylinder. Typical numbers most non-flexible epoxies start at slightly under 10,000 psi (pounds per square inch) and work their way up from that. To put this into perspective typically quality concrete has a compressive strength of about 3,000 psi. Very high end concrete can have compressive strength of about 5,000 psi but concrete at that high a strength is not typical. Most concrete is 3,000 psi plus or minus.

Some people think that the higher the tensile strength the "better" the material. That is not necessarily true. As some epoxies get to very high compressive strength the may become brittle. If a product is brittle it can still break easily. That is because flexural strength has been compromised for the sake of getting a higher compressive strength number. More on this under tensile Strength below.

Epoxy.com Products are formulated to balance all the various strengths needed for a specific application. For example if you are installing an Epoxy.com Product over concrete, there is no need for the epoxy to have a compressive strength of more than a couple times more than the concrete. That gives you a high safety margin so that if there is a high compressive load the concrete will fail before the epoxy. Once the concrete has failed the epoxy is just along for the ride.

Interestingly enough in the real world materials don't really fail in compressive, they typically break on the side opposite the side of the load first, which means it actually fails in tension.

Tensile Strength of Epoxy

The Tensile of epoxy is basically the load in just the opposite direction of compressive strength. As you remember compressive strength tests are ran by pushing both sides of a cylinder of the epoxy or other material. Tensile strength is tested by pulling on the top and the bottom of the cylinders.

Materials do not break in compression, they break in tension. Prove this to yourself. Take a tooth pick or some material that you can bend in your hands until it breaks. It does not break on the side you are pushing on (the compressive side) first. It brakes on the side you are pushing towards first. The side that is in tension is the side you are pushing to. That makes the tensile strength the most important one to consider when comparing materials.

In the case of concrete the compressive strength normally tests out at about 10% of the compressive strength. So concrete with 3,000 lb. compressive strength typically only has about 300 lbs. of tensile strength. By comparison Epoxy.com epoxy has a much higher tensile strength verses compressive strength. By comparison Epoxy.com Product #10 Epoxy Mortar has a compressive strength of 10,000 PSI (about 3 times that of standard concrete) but Product #10 Epoxy Mortar has a tensile strength of 3,500 psi (at least 10 time stronger than concrete). So since materials break in tension first Epoxy.com Product #10 Epoxy mortar and most Epoxy.com Products are 10 times higher than standard concrete.

Flexural Strength

Flexural Strength is measured by supporting a bar of the epoxy or other material on both ends and then applying a load in the middle. Compressive strength and tensile strengths are measured differently, but in epoxy they really are talking about the same thing. These two tests measure the ability of the epoxy to resist being pulled apart. This test is much like what you are doing when you bend the toothpick in your hands, like we discussed earlier

Bond Strength

Bond strength is the measurement of how well an epoxy bonds to the substrate. Epoxy.com resins are designed to have a bond strength on concrete that is greater than the tensile strength of the concrete. If the bond strength of the epoxy or epoxy like material is greater than the tensile strength of concrete than if there is a failure it will be a failure of the limits of the strength of the concrete not the epoxy. Epoxy bond strength is typically measured under ASTM D-882. Bond strength to concrete is measured under an American Concrete Institute test method "Adhesion to Concrete ACI Committee 403".

Cite: References Used on this Page

ACI Guide for Use of Epoxy Compounds with Concrete: "Adhesion to Concrete ACI Committee 403"
ASTM C109 / C109M - Standard Test Method for Compressive Strength Concrete
ASTM C881 / C881M - Standard Specification for Epoxy Resin Base Bonding Systems for Concrete
C882 Test Method for Bond Strength of Epoxy-Resin Systems By Slant Shear
D638 Test Method for Tensile Properties
D695 Test Method for Compressive Properties