Rebar is placed inside precast concrete forms in order to provide reinforcement, particularly when it comes to tensile strength. But does precast concrete need rebar?
Rebar is a steel reinforcing bar. The broad definition (from Merriam-Webster) is “a steel rod with ridges for use in reinforced concrete.” This type of reinforced concrete is everywhere.
There are basically two types of strength measurements when it comes to concrete: compressive strength and tensile strength. Learning a bit about each of these will help us better answer the question of whether precast concrete needs rebar.
We can test concrete’s compressive strength by squeezing it between hydraulic cylinders. Concrete excels when it comes to compressive strength. This is the most common measure used by engineers in determining appropriate design parameters.
Concrete’s compressive strength ranges from a few thousand pounds per square inch (psi) to strengths exceeding 10,000 psi.
Concrete fares less well, however, in tests of tensile strength. Tensile strength measures resistance to forces working to pull a piece of concrete apart. And for concrete, tensile strength is typically much less than its compressive strength.
As the National Precast Concrete Association, the leading advocate for the precast concrete industry asks, “Compressive strength is one test, but what happens to the strength of concrete if we pull on the ends of the sample, rather than squeeze them together? In other words, what if we put the concrete in tension, or stretch it, in what is referred to as a tensile test? Now we find the tensile strength is only one-tenth of its compressive strength. Concrete that has an impressive compressive strength of 4,000 psi has maybe 400 psi of tensile strength. Not so strong in tension!”
Enter rebar, which provides a tremendous boost in helping concrete improve its tensile strength. Rebar absorbs the stressors that would otherwise threaten to tear the concrete apart.
Concrete and Rebar
Now, back to the question at hand: Does precast concrete need rebar? Well, it depends on the project’s size and scope, plus other factors.
Construction sites, manufacturing plants, and concrete construction projects, in general, are filled with all manner of standard concrete structures — concrete foundations, retaining walls, concrete slabs — with reinforcing steel (often stainless steel rebar) holding it all together.
If the precast concrete piece is acting as a support system, then it must be designed to sustain vertical pressure and support. Many but not all construction and infrastructure plans call for reinforced concrete. This is because rebar reinforces concrete building blocks. This allows them to withstand the lateral stresses placed upon them. Stresses can arise due to ground motion, weather cycles (especially freeze and thaw), and other factors.
Vertical rebar is often recommended for builds such as precast concrete footers since the rebar can also be utilized to tie separate pieces together. For example, for subterranean applications, rebar encased in concrete with no air pockets through multiple inches of gravel and down to the bottom of the hole is a recipe for strength and durability.
Rebar is an inexpensive way to avoid costly problems that may crop up later when unreinforced concrete is used. So the answer to the question do precast concrete products need rebar may best be answered by simply saying Yes in most situations.
There may be situations in which pieces of rebar aren’t called for. However, rebar can be a project-saver for both the short- and long-term. Especially on unstable soil, when dealing with frost lines, when used with concrete footing, or on extreme slopes.