By Sue McCraven
Reinforced concrete is the yin-yang composite of building materials. An ancient Chinese science and philosophy, yin and yang are complementary, interacting forces that form a whole greater than their separate parts. Likewise, concrete and steel are totally different materials, but when combined, they create reinforced concrete, the best composite construction material in the world.
Separate strengths & weaknesses
Taken separately, concrete and steel have their respective strengths and weaknesses. We all know concrete is king of compressive strength. A block of 4,000 psi concrete can resist tremendous compressive forces. But if we stretch concrete out into more linear shapes, like beams, it cannot resist the tensile – or bending – forces that are created. This is because concrete’s tensile strength is nil, at least for design purposes.
Fresh concrete is malleable as it flows into forms. It can be shaped into a myriad of designs, depending on the formwork geometry. New developments in formwork technology are leading to unprecedented architectural designs(1). Integral color, new techniques for shaping concrete’s surface, and the use of exposed granite and marble aggregates present designers with an endless array of exterior finishes and gorgeous facade possibilities.
Structural steel is extremely strong. But in addition to bearing tremendous tensile loads, steel has amazing ductility, and is therefore termed a “tough” material. Along with toughness, steel demonstrates an elastic range under stress – it can deflect under tensile forces without failure. But all by its lonesome, steel doesn’t stand up to the forces of fire and water. Naked steel is vulnerable to corrosion. Also, let’s face it: Plain structural steel has practically no aesthetic appeal – unless you like rust.
Stress & material properties
All engineers are taught that, in order to build with reinforced concrete, they must first understand the individual material properties of concrete and steel. For design purposes, normal-weight concrete has a density of approximately 145 lb/cu ft, and its compressive strength is about 4,000 psi. For various compressive strengths, graphs of concrete stress (in kips per sq in., or ksi)(2) versus strain (change in length divided by the original length) demonstrate a linear relationship up to the point where the strain measures around 0.002. This means that when forces create a strain of 0.002 or greater, the direct relationship (the straight line portion of the graph in Figure 1) between stress and strain is lost.
Concrete’s modulus of elasticity varies with compressive strength, loading, and the characteristics of the cement and aggregate. Concrete’s modulus of elasticity is taken from the slope of the stress-strain diagram in Figure 1, whose values are based on actual compressive tests of 28-day-old cylinders.
Reinforcing steel is taken as Grade 60. This means the rebar in reinforced concrete has a yield strength of 60 ksi, which represents the steel stress that corresponds to a strain of 0.35%. The nominal weight of a No. 3 bar is 0.376 lb/ft and 0.668 lb/ft for No. 4 bar. The modulus of elasticity of steel is taken as 29 x 106 psi for design purposes.
Once we have a feel for the material characteristics of concrete and steel, we try to determine the anticipated loading that reinforced concrete must resist in service.
Dead loads. Dead loads are easily calculated using density, mass and volume. Dead loads are static, unchanging or permanent forces, such as:
- Self weight of steel-reinforced concrete
- Building or superstructure loads
- Soil pressure
Live loads. More difficult to calculate, live loads are often unpredictable and vary in their intensity and magnitude. Examples include:
- Wind forces
- Snow and ice loads
- Seismic forces
- Construction and traffic loads
Reinforced concrete’s superior advantages as a construction material are:
- Strength and ductility
- Fire resistance
- Water resistance
- Low maintenance
- Long service life
- Design flexibility
- Most economical foundation structures
- Thermal efficiency
United strength in service
When concrete contributes its renowned compressive strength with steel’s unmatched toughness and ductility, it becomes a dynamic duo, a structural workhorse – the ultimate yin-yang composite. And its proven dependability and durability over time have earned reinforced concrete its position as the most preferred building material in the world.
Its combined material strengths, however, wouldn’t matter at all without one shared characteristic: similar coefficients of thermal expansion. As the mercury rises and falls, in the tropics or in the Arctic, steel and concrete not only bond to each other, they move in perfect unison with each other! In his book, “Design of Reinforced Concrete,”(3) J.C. McCormac captures this structural synergy better than anyone:
“Concrete and steel work together beautifully in reinforced concrete structures. The advantages of each material seem to compensate for the disadvantages of the other. For instance, the great shortcoming of concrete is its lack of tensile strength; but tensile strength is one of the great advantages of steel.
The two materials bond together very well so there is no slippage between the two, and thus they will act together as a unit in resisting forces.”
There you have it: the yin and yang beauty of reinforced concrete.
Sue McCraven, NPCA technical consultant and Precast Inc. technical editor, is a civil and environmental engineer.
(1) See “Breaking the Mold: Explorations Shaping Architectural Precast,” by Matt Roper, M.Arch., in the Spring 2013 issue of Precast Solutions.
(2) A kip = 1,000 lbs
(3) McCormac, J.C., “Design of Reinforced Concrete,” 2nd edition, Harper Collins Publishers Inc., 1986. The 9th edition of McCormac’s book, co-authored by Russell H. Brown, is currently available.