We defined the following glossary terms in earlier posts a few years ago. These follow-up posts expand on how each of these factors is tested, and ratings determined for its intended application.
In this first post of the series, we’ll address testing for
- Compressive and Tensile Strength
- Flexural Modulus
- Tensile Modulus
Compressive and Tensile Strength
Both compressive and tensile strengths are linear measurements of how a given material behaves in the pulling (tensile) and crushing (compression) along the axial center of that material. The ASTM standards utilized in the determination of these values are ASTM D638 (Tensile) and ASTM D695 (Compression).
Both tests take the material to failure to determine the respective values. In the U.S., these values are generally measured in pounds per square inch (psi.).
In most applications of material behavior, compression and tensile values are used in conjunction with each other to calculate the flexural strength of a material as it receives loads, because flexural strength more accurately models what is actually occurring in the material’s behavior under a given load scenario.
An example would be an I-Beam pinned at both ends, with a load being generated at the center part of the span. As the load is applied, the top of the beam is in compression, while the bottom of the beam is under a tensile force. As the load is applied, the force needed to bring that I-Beam to failure is called Flexural Strength (watch video). Though this is a simple example, it illustrates how tensile and compressive linear forces are utilized in a three-dimensional world.
Another example would be the use of steel rebar in a concrete structure. Concrete is strong in compression but is relatively weak in tension (tensile strength). This is why steel reinforcing bar (rebar) is added to concrete, to reinforce its ability to distribute or bear tensile loads.
The flexural (or bending) modulus (or value) of a given material is defined as the area under the linear portion of the stress/strain curve when the material bears a load. This is determined as flexural deformation as described in ASTM D790.
Flexural Modulus is a key material behavior characteristic considered by design engineers when calculating required material thicknesses in buried structures. Flexural Modulus is a more accurate measurement than any other test of what is actually happening in a buried structure.
As discussed in the previous section, flexural strength is a measurement of the interaction of the compressive and tensile forces being applied to the structure. All materials react differently in the ASTM D790 testing protocol, with the greatest variance being in plastic materials.
More rigid plastics generally have a higher flexural modulus, and therefore can be employed in the structural rehabilitation of a structure. More elastic polymers exhibit elongation under load, and are therefore unable to create a linear portion of the stress/strain curve. This characteristic prevents the material from offering load-bearing capacity, so they can’t be used as structural rehabilitation materials.
Tensile Modulus is derived in the same manner as Flexural Modulus, in that it is the area beneath the linear portion of the stress/strain curve generated when the material is tested in tension (tensile) per ASTM D638. This factor is applied when evaluating a material’s property where internal or external circumference or hoop stresses may apply.
Hoop stress is the stress around the outside of a pipe wall, acting perpendicular to the long axis of the pipe, and produced by the pressure of the flow in the pipe. This would occur in pressure pipe from the internal or external pressure that may be applied in an out-of-round condition created by external buckling forces. In other words, if excessive hydraulic pressure from a high water table, or crushing pressure from traffic loads above begin to “squash” the pipe, this out-of-round condition is generally termed “ovality,” because it turns a previously round pipe shape into an oval one.
In our next post, we’ll address testing and/or ratings for
- Manning’s N-Factor
- Build Capability
- Creep Factor
- Moisture Sensitivity