Choosing construction materials can be difficult, especially when trying to balance quality and price along with any other factors that may be important to the buyer. Here is a breakdown of some of properties to consider, including weight, strength, corrosion resistance and cost, of aluminum, iron, steel, wood and concrete.
Metals & Alloys: Strength to Cost
Metals & Alloys: Elasticity to Cost
Wood & Wood Products: Strength to Cost
Wood & Wood Products: Elasticity to Cost
Ceramics: Strength to Cost
Ceramics: Elasticity to Cost
*Young’s Modulus from the charts above can also be referred to as Elastic Modulus, which determines the elasticity of a certain material. Charts courtesy of the Department of Engineering from Cambridge University.
The best-known properties of aluminum are its light weight yet high tensile strength, which gives it an ideal weight to strength ratio in construction applications. It is approximately one third the density of steel and much lighter than steel. Aluminum alloys commonly have tensile strength between 70 and 700 MPa, and the range for alloys used in extrusion is normally between 150 – 300 MPa. What is tensile strength? It is the maximum amount of tensile (tension) stress a material can endure before failure, such as breaking or permanent deformation. Tensile strength is commonly measured using MPa, which stands for megapascals. As a point of reference, the tensile strength for structural steel is 400MPa.
Aluminum is also corrosion resistant because when it reacts with oxygen in the air, a thin layer of oxide is formed. Anodizing aluminum will increase the thickness of the natural oxide layer which will further protect the material from corrosion, especially corrosion due to outdoor elements. This layer is self-repairing when damaged and provides better adhesion for paint primers and glues than other bare metals. Aluminum is extremely durable in neutral and slightly acidic environments, but corrosion occurs quickly in high acidic environments. High acidic environments can consist of sulfuric pools and geysers along with areas polluted by acid mine drainage.
Another bonus of aluminum is that it is the third most common element on Earth’s crust, and aluminum compounds occur naturally in our food. Thus, it has zero toxicity and is completely recyclable.
For everything considered, including its light weight, high tensile strength, corrosion resistance and recyclability, aluminum is very cost-effective.
Iron & Steel
Iron is generally cheaper than aluminum because aluminum is more expensive to extract from its ore.
Iron is slightly stronger than aluminum in terms of tensile strength, but it is much more dense and heavy, making it more difficult to install in certain applications. Cast iron’s tensile strength is commonly between 60 and 800MPa while mild steel is around 300MPa. So what’s the difference between iron and steel? Iron is an element while steel is an alloy that is comprised of iron and carbon.
Different grades of steel exist, each with varying amounts of carbon in them. Carbon is incorporated into the iron during a smelting process which involves controlled heating and cooling of molten iron. A higher level of carbon in steel means that it will be harder, but it will also be more brittle. Whereas lower amounts of carbon allow steel to be softer but more malleable. In general, alloys are much stronger than pure metals, so steel is stronger than iron and consequently more expensive.
Iron is commonly used in construction applications and is usually covered with a strong protective coating or buried within other building materials. Why? Because iron alone is not weather-resistant. The surface of the material readily combines with the oxygen in the air in the presence of moisture, thus, creating rust. In completely dry air, however, iron does not rust. That said, consumers typically opt for a galvanized product. Galvanization is the process of applying a protective zinc coating to iron or steel to prevent rusting; the thicker the zinc coating, the longer it will resist corrosion. This process will increase the initial cost of the product, but will increase the life of the product.
The main concern with galvanization is that the zinc coating eventually develops a natural carbonate on its surface by exposure to the atmosphere and rainwater. The carbonate can become brittle and crusty and eventually split, exposing fresh zinc for corrosion. Since the zinc coating is thin, it can corrode up to the base metal exposing the base to the atmosphere and corrosion as well.
Wood is typically weaker than steel, iron and aluminum, yet it is not much cheaper on average. Pine and Oak are the strongest woods that would be used for structural applications with tensile strengths between 70 and 90MPa.
It is known that no matter what type of wood is used, it is always stronger when cut with the grain. So why is wood stronger when cut with the grain? Wood is a natural substance that is much stronger when the grain is continuous. Think about packing together a bunch of straws; each individual straw is weak, but they become quite strong when altogether. Wood works the same way with its strands of cellulose fibers – when fibers are continuous (packed together like straws) and cut with the grain they are stronger.
Wood is cheaper than the other materials discussed above, but the lower cost comes with a maintenance price. Like iron and steel, wood must be treated to prevent corrosion. People typically use a sealer or varnish to prevent rotting and warping, but the sealer must be reapplied every year to maintain its appearance. However, even with treatment, wood does not have anywhere near the lifetime of aluminum; wood will expand in the heat and shrink in the winter even with finishes. Along with rotting and warping, wood can splinter, which can be hazardous if being used by the public.
Concrete is weak in terms of tensile strength with a range of 2-5MPa. Recall tension and compression are not the same; tension forces materials apart whereas compression forces materials together. Concrete weight is typically measured in terms of compressive strength because most concrete applications don’t experience tension. The compressive strength, which is usually about 10x the tensile strength, of concrete is 20 to 40MPa, which is still much less than other materials. In addition, concrete has a very low thermal coefficient of expansion which means that it is highly vulnerable to cracking. Cracking happens more quickly in environments where the temperature is constantly rising and falling.
Another cause of corrosion can be the expansion of the reinforcement steel. If the steel is located too close to the surface of the concrete, it is exposed to air and spalling can occur. Spalling is a process where flat fragments of the concrete chip off from the mass by the structural steel.
Concrete is a relatively cheap option, but there are many factors that can lead to quick corrosion.
All of these materials are good construction materials, but each one functions best in different environments and applications. The optimal material for an outdoor location with fluctuating temperatures would be aluminum followed by steel. Wood is a cheaper option that can also be used outdoors but would require yearly maintenance due to the likelihood of corrosion and warping. If price is the most important factor and other properties don’t matter, concrete would suffice.
Pick the characteristics that are important to you as a buyer or user, and choose the associated material that fits your need.