By 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 tornados. 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.

Bringing concrete and steel together creates a building material that has both great compressive and tensile strength.

Concrete has great strength under compression (forces that push concrete against itself), but it has little strength intension (forces that pull concrete apart). Reinforcement, especially steel reinforcement, has great tensile strength, while its compressive strength is much less. Sometimes the placement of rebar in a slab causes additional compression in the concrete when loads are applied, aiding in the overall performance.

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 4,000 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, Idaho, 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.

High Wind Speeds

Sigdel says that when his company designs concrete walls to resist static winds in LA, they usually design for wind speeds of 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. However, 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 eight inches thick or more.

Tornadoes

Tornado winds are much like seismic ones; 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 can become quite expensive.

Structural fibers

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 nine 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 Steel, based in Ann Arbor, Mich., says the University of Michigan developed and researched twisted steel fibers in the 1990s. “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 that this doesn’t apply to seismic reinforcement. Bromley says ABI still uses 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, Minn., 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 four inches, making concrete homes more affordable. As a structural engineer he says there are ways to keep the costs as reasonable as possible. For example, designing wind resistant walls required thicknesses of six inches several years ago, but today this can be accomplished with four-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 inside non-structure supporting 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 tornados. It’s the primary material structural engineers work with, and it’s invisible in the finished product. It’s the quality that makes your home a safe place for you and your family. Anderson believes that, 100 years from now, all homes in the U.S. will be built with concrete.

In the final article in this series, sustainable and resilient building systems will be discussed.