You often hear of building with concrete and steel, but what about concrete or steel? Whoa, there’s a choice? Sure is. But how do they stack up against one another as building materials for the 21st century?
If you’re planning a new structure, you’ve likely wondered which is best suited to serve your needs. Both are known for their durability and cost-effectiveness and it may seem like a difficult choice to make.
Before we begin to look at the pros and cons (or plusses and minuses) of both, let’s describe exactly what we mean by concrete and steel structures.
Plain cement concrete is a hardened mass created by a mixture of cement, sand, gravel and water in definite proportions. Plain cement concrete has good compressive strength but little tensile strength, limiting its use in construction. Its best uses are in roads, concrete blocks for walls, etc.
To improve the tensile strength of concrete some sort of support is needed to take up the tensile stresses developed in a structure. The most common type of reinforcement is steel bars that are quite strong in tension. This reinforced concrete has many uses including buildings, water tanks, etc.
Prestressed concrete is a form of concrete used in construction that while under construction is substantially “prestressed” (compressed) in the areas that will be subjected to tensile forces while in service to strengthen it against these forces. It’s used in structures where tension develops or is subjected to vibrations, impact and shock like girders, bridges, electric poles, gravity dams, etc.
The material steel is an alloy of iron and carbon percentages and other elements, e.g., silicone, phosphorous and Sulphur in varying percentages. Depending on the chemical composition, the different types of steel are classified as mild steel, medium carbon steel, high carbon steel, low alloy steel and high alloy steel, with the mild, medium carbon and low alloy types generally used for steel structures.
Okay, let’s take a closer look at each type of material, their features and benefits and their plusses and minuses.
Safety of the completed building
Every building owner wants a strong, durable structure that stands up to nature’s challenges. So, which material promises greater building safety, concrete or steel?
Fire resistance. Both concrete and steel are resistant to fire. Both are not likely to burn and won’t spread flames. Especially with steel structures, innovative fire-retardant coatings and insulation increase its fire resistance to higher temperatures.
Of course, increased fire resistance provides more time to evacuate a structure, and there is less chance the building will collapse.
Many people point to the disaster at the World Trade Center of 9/11 as proof that steel does not have the fire resistance or strength that is professed. However, structural engineers point out that the design of the twin towers did not account for explosive detonation. The experience has led to embracing blast-resistant principles in the building code of high-rise structures.
Even so, the design of the new World Trade Center, or Freedom Tower, includes safety measures such as 24-inch thick concrete wall enclosures for its building core where the elevators, stairs and power system are placed for protection from fire and terrorist attack.
Earthquake resistance. Generally, it’s believed that steel’s biggest advantage over concrete may be in earthquake country. Under the pressure of a strong quake, concrete can crack and crumble. It’s argued that the extreme weight of concrete structures makes injuries or death of occupants more likely in a collapse.
According to the Portland Cement Association, however, a structure’s likelihood of surviving an earthquake is determined more on how well the structure is engineered than on what type of material is used to build it. The association points out that during a severe earthquake that struck Kobe, Japan, concrete buildings and steel buildings in the downtown area shared comparable rates: just 4.9 percent of concrete buildings and 5.3 percent of steel buildings collapsed.
These numbers would seem to indicate that the material is not as important as previously thought and demonstrates that either material can be just as strong when engineered properly.
The price of concrete itself is fairly constant. Plus, concrete components are reasonably priced and readily available.
The biggest secret to steel’s affordability is its strength. Steel enjoys the greatest strength-to-weight ratio of any building material. As a result, steel produces a stronger building with considerably less material than concrete. Less material equates to lower material costs.
Moreover, advances in steel production enable a ton of steel to be produced in less than two man-hours, creating an exceptionally cost-efficient construction material.
Using recycled materials also lowers material costs. Domestic structural steel comprises over 90% recycled steel. The industry’s well-organized steel scrap collection system recaptures 98% of all unused or discarded steel products for recycling. As a result, steel prices remain rather constant over time.
One glitch: tariffs put in place last year will begin affecting steel prices, generating 11% to 13% price increases. Nevertheless, steel remains competitively priced.
First, you need to consider which material best suits your design. Does concrete or steel offer the greatest flexibility?
You can create and shape most anything out of concrete. It simply molds to the form fashioned for it, making advanced design possibilities a reality. Concrete also has the benefit of offering extra space possibilities. Cast-in-place reinforced concrete can give more rentable space owing to lower floor-to-floor heights.
As a prime example, architects in Chicago switched from steel to concrete, so they could add two extra stories to the new building at the former Chicago Sun-Times site.
However, the massive weight of concrete limits its ability to span great distances without support columns. Bulky concrete columns limit maneuverability and usable floor space within the structure.
Steel continues as a popular choice for office and multifamily developers. One plus: with the use of girder slab, castellated beam construction and staggered truss, lower floor-to-floor heights than previously expected have been achieved in structural steel buildings.
Steel’s greatest design asset, however, may be its ability to span long distances without interfering interior columns. A pre-engineered steel building can span up to 300 feet without any interior load-bearing columns. With minimal supports, a modular frame steel structure spans as much as 480 feet. Aircraft hangars, churches, manufacturing plants and warehouses often demand these soaring ceilings.
In addition, no limitations exist for the length of a pre-engineered steel building. The design simply adds more frames and bays to meet the owner’s needs.
Buildings with concrete can almost always be constructed faster, sometimes twice as fast, according to officials of the Concrete Alliance. On a two-day cycle, workers can pour up to 20,000 square feet of floor space.
While steel can’t top concrete’s two-day turnaround, it does provide benefits of its own. Some observers believe structural steel framing systems are the way of the future, and they will result in a faster construction schedule. CAD programs can pass their information straight through a database as a 3-D model and send it to both detailing and shop floor fabrication systems. These productivity increases help assure the future of steel as a viable construction material.
Both concrete and steel deliver benefits for the environment. Standard concrete preparations employ materials that are accessible most everywhere in the world. Plus, concrete produces negligible waste and can be made in the amounts required for specific projects. Green concrete stipulated by a concrete contractor can bring even greater protections for the environment.
In theory, 100 percent of steel can be recycled and used again. In practice, as noted above, approximately 90 percent of the steel on hand for a conventional metal building incorporates recycled materials.
One might not realize that more steel is recycled annually than all other materials combined, including the more well-documented aluminum, paper and glass. In fact, steel is the one material that can be recycled over and over and not lose its quality.
As for transporting materials to the construction site, concrete is often locally sourced and requires less fuel to move to a construction site. On the other hand, steel is frequently fabricated at a considerable distance from the site, increasing the use of fuel needed to transport it.
Steel may corrode when it makes contact with water and, if left without appropriate care, could eventually affect the structural safety of the builder. That’s why builders need to take precautions with such processes as water-resistant seals and paint upkeep.
With proper construction and care, reinforced concrete is water-resistant and will not corrode. However, it’s crucial to keep in mind that the concrete’s internal steel reinforcement should never be exposed to the elements. If exposed, steel can be compromised and easily corrode, affecting the strength of the structure.
So, are there any supply issues with concrete or steel building materials?
All the ingredients for producing concrete— cement, sand, gravel, and water— are most often readily available as well as inexpensive. The steel rebar for reinforced concrete is also in abundant supply.
Concrete’s one drawback, however, occurs when construction demand— such as after a major hurricane, earthquake, or other natural disaster— temporarily outstrips the supply capabilities for cement.
As for the availability of steel, U.S. steel companies turn out 7.2 million tons of structural steel each year. Presently, over 2,300 fabrication shops operate across the U.S., with a maximum capacity of 9 million tons per year. As one can see, American steel manufacturers can meet all building demands without difficulty.
What about combining concrete and steel?
In some instances, using concrete and steel in combination can provide added strength and greater flexibility in managing a building project. One popular configuration is to have the concrete contractor pour concrete for lower floors and use steel joints and beams to construct upper levels.
PDDM Solutions clan provide excellent guidance on combining steel and concrete in the most practical way.
Concrete or steel: some conclusions
As builders are aware, choosing the appropriate construction material is a huge step in achieving a successful project. And one of the more thought-out questions builders ask is if they should use steel or concrete. As we’ve seen, the plusses and minuses of each material can affect the cost, schedule and quality of the final product.
Usually acknowledged as the safer option, concrete is somewhat better able to guard against fire and impacts. However, concrete has several minuses related to cost and time. It can cause lengthy construction times, which can lead to delays in completion.
Steel, on the other hand, is recognized as a time-saving construction material. By deciding on steel, you can anticipate lower overhead costs and reduced construction times. However, the fabrication and delivery of steel can take up to 10 months, which is one of its principal minuses.
Truth is, there will probably never be a conclusive answer as to whether a builder should make use of steel or concrete. Truth is, the specifications of the project can offer more understanding into which material is more suitable in terms of the affordability, timetable and the quality control it can deliver.
In the end, a builder needs to consider the building’s function and requirements. Those will more likely determine whether to choose concrete or steel.
If you’d like to discuss how PDDM Solutions can help you on your next project, simply call us at (724) 788-4048 or visit us at www.pddmsolutions.com