You often hear of building with concrete and steel, but what about concrete or steel? Whoa, you can make a choice? Sure. But how do they stack up against each other as building materials for the 21st century?
Let’s take a look.
Safety of the completed facility
Concrete, when it is in thick columns or cores, is sturdy protection against debris and high winds. The core, containing the elevator, stairs, and power systems, is typically encased in 2-foot thick concrete that is capable of protecting the area from fire and some explosions.
Concrete is fairly fire-proof as well without the need for additional fire-resisting materials. It can endure some very high temperatures and meet stringent fire codes. However, it can be somewhat difficult to attach fireproofing to concrete, a detriment to other materials in proximity to the hot surface.
With careful design, concrete is somewhat ductile but, overall, is a rather brittle and a concrete facility has a massive footprint.
Steel is easily fireproofed with a variety of spray on or attached materials that can be matched to the need. Structural steel is very heat resistant on its own and the added fireproofing prevents widespread damage to other materials.
Steel frame buildings have redundancy of structure that prevents the type of collapse seen in earthquakes and bombings. The facility can be designed with progressive collapse to enhance safety in the event of disaster.
Steel, known for its ductility, is also proof against high wind loads and is the material of choice in seismic zones. Steel also has a lighter footprint that doesn’t need a thick foundation.
Cost of construction
The cost of ready-mix has remained stable; however, supply and demand has had an impact on price and availability in the past decade, especially during times of extensive construction after natural disasters. Since most structural concrete contains steel reinforcement, any change in steel prices will affect it as well.
Cast-in-place, reinforced concrete can be more quickly started on the jobsite but over time will take a larger crew longer to complete than steel, meaning higher labor bills.
While structural steel experienced a 50% increase over mill prices from 2003-2005 it still represented less than 20% of all steel used in building construction. While overall project costs have risen for all materials, the increased cost of a steel framing system is still less than 2% of the increase in costs. This is because a majority of construction today uses recycled steel, which is less expensive than virgin steel.
Prefabrication off-site reduces labor costs since the crew won’t be needed as long. The building arrives ready to erect and there is little to no on-site metal work or waste.
Some sources say up to 50% of concrete is crushed and recycled; 40% is down-cycled to be used for hardcore in substructure works or road construction. The remaining 10% is waste that goes to the landfill. The BRE group has developed a rating system called Ecopoints to compare the environmental impact of certain materials. The higher the number, the higher the impact.
Reinforced concrete has 12.57 Ecopoints per ton.
Concrete structures have the advantage of thermal inertia which can save on heating and cooling costs if the building is ventilated correctly. Concrete floors take up heat during the day but must be discharged in the cool of the night. This can only occur if air can move freely during the nighttime hours in order to continue cooling the next day.
Concrete ceilings, while easily painted, tend to produce loud echoes not heard in buildings with acoustical tile ceilings. Acoustical ceilings can be added in concrete buildings but this will eliminate any floor height advantage.
Steel is 100% recyclable and 85% of steel is recycled at the end of its life. Due to its magnetic properties, steel is easy to separate from other debris and making the recycling process more efficient. In addition, the energy used to produce recycled steel is about one-third of what is required to produce virgin steel from iron ore.
This translates into less CO2, less energy, and fewer resources being used.
Even iron ore mining companies have gotten in on the green movement and made efforts to decrease the environmental impact of producing new steel.
Other ways steel is environmentally friendly:
- It’s flexible in adapting a structure for a different use
- There is little to no construction waste
- There is less need for maintenance or repair
- Steel buildings help reduce the “heat island” effect
According to the BRE index, steel only has 11 Ecopoints per ton, lower and better than concrete.
In the early 2000s, there was a shortage of cement, the binding ingredient in concrete, due to heavy construction needs in Florida after hurricanes ravaged the coastal cities. Shipping rates also went up and transport ships were of limited availability. The result was another hike in prices and an unreliable supply of concrete for building.
The impact of this shortage was greatest on smaller companies, builders, and contractors, all of whom had lower cash reserves. While cement companies began to expand and were expected to increase domestic capacity by 2008 the supply continues to suffer in times of natural disaster.
The steel industry shows no shortage, even though there has been a large increase in construction activity in Asia, especially in China. The U.S. produced 86 million tons in 2014; 1.6 billion tons were produced worldwide.
According to Alfred G. Gerosa, President of Concrete Alliance Inc. in New York, concrete buildings can be constructed up to twice as fast as a steel structure. In a process called the 2-Day Cycle, cast-in-place reinforced concrete buildings can rise as fast as one floor every other day.
While this can place up to 20,000 square feet of floor space every two days, it is extremely labor intensive, which may drive costs up. Gerosa also believes concrete is easier to use in construction where there is limited space surrounding the jobsite for cranes, staging, and equipment.
Accelerated scheduling is possible with steel. Prefabrication allows for shorter on-site construction and safely allows other trades to work in structures that were just erected, unlike concrete. Integrated 3-D modeling (design, detailing, and fabrication) creates the building more quickly, increasing overall productivity.
In fact, prefabrication can compress the steel erection portion of a project by 40-50% according to John P. Cross, Vice President of Marketing for American Institute of Steel Construction, Chicago.
Concrete can be molded into any shape and pre-casting of walls for tilt-up construction has become common. Construction with cast-in-place reinforced concrete for high rises can yield more rentable space due to lower floor to floor heights.
Concrete can even offer uninterrupted floor plates if it is carefully designed and properly engineered.
Steel has the highest strength to weight ratio of any construction material available today. Lower floor to floor heights can easily be created using girder slab, staggered truss, and castellated beam construction. Extremely long open spans are possible using steel that would not be feasible in concrete.
Concrete and steel are both common and useful construction materials. There are reasons for choosing one over the other, but as you can see, steel looks like a clear winner in our book. It is safer, is more flexible in design, easier to schedule, and has a higher availability than concrete. Plus, since steel is 100% recyclable with no loss of strength, the environmental impact is much lower.