Adding rebar reinforcement to concrete provides building elements with tensile strength as well as compressive strength. Photo Credit: Joe Nasvik
People who love concrete tend to believe that it’s magic because it performs well under a wide variety of conditions: fires, earthquakes, high winds, hurricanes, and tornadoes. It can do all this and more, but it requires a special ingredient: reinforcement. Reinforcement added to concrete, designed by structural engineers, provides concrete structures that are safe and resilient.
Concrete has great strength under compression but little strength in tension. Reinforcement, especially steel, offers great tensile strength. Bringing concrete and steel together creates a material with both compressive and tensile strength. Placement of rebar in a slab can even increase concrete’s performance under load.
When you build a concrete house, local building codes require engineers to design the reinforcement in concrete walls, floors, beams, and columns to provide the tensile strength needed for homes to resist the natural forces common to an area. For example, much of California experiences earthquakes, so local codes require engineers to design structures that can resist seismic forces.
The mix design requirement for concrete homes remains fairly constant and compressive strengths of approximately 4000 pounds per square inch (psi) are usual. It’s the reinforcement that changes to meet different needs.
To understand how reinforcing requirements change when engineers design structures to resist natural forces assume that a concrete wall is being engineered to meet the objectives for each of the following conditions.
Fire resistance
A primary function for concrete in a fire situation is to protect steel reinforcement. When the temperature exceeds 800 degrees fahrenheit steel rebar becomes soft and pliable and moves in whatever direction forces move it. Concrete, on the other hand, is a good insulator and it takes approximately one hour for a temperature of 800 degrees to penetrate one inch of concrete thickness. So, for maximum fire resistance, steel reinforcement placed in the middle of a concrete wall thickness gives it the most protection for the longest time period. Bikash Sigdel, an engineering team leader for Tamarack Grove Engineering, Meridian, ID, says current Los Angeles (LA) building code fire ratings require 1-1/2 inches of cover. This means the minimum concrete wall thickness allowed by code would have to be about 4-1/2 inches. It also means it’s important to extinguish fires within that time period.
Using a removable form house building system workers prepare to cast the reinforced second floor of a home. The vertical rebar will tie the second floor walls to the floor. Photo Credit: Joe Nasvik
High wind speeds
Sigdel says that when his company designs concrete walls to resist static winds in LA, they usually design for wind speeds 100 mph and higher. He adds that designing for wind resistance is fairly easy because wind tends to come from a single direction and is therefore fairly easy to plan for. Typically only one “mat” of rebar is needed to provide the tensile strength needed.
Earthquakes
Seismic forces are dynamic, in that they involve forces coming from all directions. Sigdel calls them “rock and roll forces” and they tend to dominate all other natural events with regards to reinforcement requirements. Engineers have to plan for changing compression and tension forces all along a concrete wall. But Sigdel adds that engineers can design concrete walls to withstand any earthquake level.
If you design a wall to withstand earthquakes, you are also protecting for wind and fire. Two rebar mats are typically required by code for seismic protection, and this means that concrete wall thickness increases to provide for the required cover protection of steel reinforcement for fire resistance. This means that wall thicknesses can be 8 inches thick or more.
Tornadoes
Tornado winds are much like seismic forces; they are dynamic forces that push on concrete walls from all directions. Wind speeds can be much higher than straight-line winds, so Sigdel says they often design for wind speeds of 200 mph or more. Engineers can design structures to withstand any tornado force but it comes at a price—and it can get quite expensive.
Structural Fibers
Engineers are now able to specify structural-rated steel fibers to replace some of the rebar reinforcement for concrete walls, saving both money and time. The fibers shown here are twisted, providing a significantly higher bond to the concrete. Photo Credit: Joe Nasvik
Another way to structurally reinforce concrete is with steel fibers that are rated for their structural ability. Dan Bromley, president of ABI Corporation, Kansas City, MO, says his company installs footings and foundations. In 2013, he heard about Helix steel fibers, one of only a few companies making steel fibers rated for structural use. They cost more than rebar (he typically adds 9 pounds per yard of concrete) but the installed cost is less when labor to install rebar is factored in. “The length of time to complete a job is less, and that’s a benefit too,” he adds.
Luke Pinkerton, president of Helix, based in Ann Arbor, MI, says the University of Michigan developed and researched twisted steel fibers in the 1990’s. “Twisted fibers lock into concrete better allowing them to be classified as structural,” he says. Structural steel fibers can replace rebar in some applications but he adds this doesn’t apply to seismic reinforcement. Bromley says they still use rebar around windows and other locations where sheer forces can crack concrete.
Finding less expensive ways
Brent Anderson is the president of BDA Engineering Consulting Group, Minneapolis, MN, and has engineered concrete for home construction for much of his career. During that time, he has seen requirements for residential concrete wall thicknesses go down to as little as 4 inches, making them more affordable. As a structural engineer, he says there are ways to keep the costs of concrete homes as reasonable as possible. For example, several years ago, designing wind-resistant walls required a thickness of 6 inches, but today, this can be accomplished with 4-inch-thick walls. He also replaces portions of rebar reinforcement with fibers to reduce costs, and when concrete is used to construct ceilings (also referred to as decks) costs can be reduced by shortening the length of spans. He cautions that home designs that include all kinds of angles add to construction costs. Another way to lower costs is to build interior non-structural walls with either steel or wood studs—also reducing the installation cost of plumbing and electrical.
Reinforcing concrete to increase tensile strength is important for concrete homes. It provides the needed protection against earthquakes, high winds, and tornadoes. It’s the primary material that structural engineers work with and it’s invisible in the finished product. It’s the component that makes your home a safe place for you and your family.
Anderson believes that 100 years from now all homes in the US will be built with concrete.
RELATED:
Part 1: The Case for Fire-Resistant Concrete Homes
Part 2: Concrete and Fire
Part 3: Reinforcement for Concrete Homes
Part 4: Resiliency
About Joe Nasvik
Joe Nasvik has been a prominent figure in the concrete industry for over four decades. Before transitioning to editorial roles, he owned and operated a concrete construction company for nearly 20 years. In his editorial career, Nasvik served as a senior editor for Concrete Construction magazine, contributing extensively to the field through articles and insights.