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