Structural Materials

What are Structural Materials?

The basics of all structures boil down to the materials used in its construction—these are called structural materials. Structural materials come in various shapes, sizes and forms, each with their own properties, suitable for different purposes. The most common materials are stone, steel, wood, concrete and glass, among many others.

In general, structural materials are classified into two distinct categories: natural and synthetic. Natural materials are extracted directly from natural resources, such as wood which is derived from trees. Synthetic materials, on the other hand, are industrially manufactured—concrete and steel are examples of synthetic materials. Often times, structural materials will require some sort of treatment before they are ready for use [2].

Stone

Stone—a natural material derived from naturally occurring types of rock at earth’s crust—has been among the first few materials used in construction [3]. In the past, stone was sought out for its ability to act as the basic support for structures since it is “strong, durable, and very resistant to weather conditions” [4]. Well-known examples of stone structures include Stonehenge, the Pyramids, the Great Wall of China, and various ancient temples across Italy and Greece, like the Temple of Hercules and the Acropolis, from each respective country. As a natural material, it is readily available for use and does not require any special processing aside from being cut, shaped and polished [4]. The term that defines the extraction of stone is quarrying which is different from mining as it removes rock from the surface-level as opposed to deeper, underground sites [5].

Recently, stone’s popularity as a main structural material has dwindled with the release of newer technologies that can be shaped as needed, like steel. Nowadays, it is commonly used in construction applications such as masonry for bridges or dams, or for decorative purposes as a kitchen countertop or for landscaping.

Types and Uses

As previously mentioned, stone is derived from naturally occurring rock types which are either igneous, metamorphic, or sedimentary. Igneous rocks are formed from cooled magma or lava, and they mostly include hard rocks like granite and basalt, and some soft rocks. Metamorphic stone is derived from pre-existing stone that has been subjected to physical or chemical changes under intense heat or pressure. Examples of popular metamorphic stone include the easily carved marble and slate which is commonly used as plates for roofing. Sedimentary is compacted stone commonly found near a body of water where deposits form strata under heat and pressure [7]. The best example of sedimentary rock that is commonly used in buildings is limestone. More information regarding the different types of rocks and their properties can be found in the following section.

Granite

Granite is a type of igneous rock found deep beneath earth’s crust. It varies in both colour and density depending on its mineral composition which mainly contains quartz and feldspar among other minerals—giving granite its recognizably mottled appearance [5, 8]. Generally, the amount of quartz embedded in the granite will determine its colour where lighter colours of granite will have more quartz as opposed to darker granites which contain less quartz. The hardness rating of all granite lies at around 6 or 7 on the Mohs scale—a scale to determine the hardness of a mineral by comparing its resistance to scratching with ten existing minerals rated from 1 through 10 (1 being talc and 10 being diamond) [9]. Granite is also incredibly strong “with compressive strength of 19,000 pounds per square inch (psi)”, making it the optimal material “for countertops, steps, driveway curbing and fireplaces” [5, 7]. As for construction applications, granite is commonly used for “bridge components, retaining walls, stone columns, road metal, ballast for railways, foundation, stonework and coarse aggregates in concrete.” [7]

Basalt

Basalt is another type of rock derived from igneous rocks—more specifically volcanic rocks. It is dark gray or black in colour and “rich in iron and magnesium” with a 45 to 52 percent silica content, making it less viscous than other volcanic rocks with higher silica content [11, 12]. Given its compressive strength of about 29,000 to 50,760 psi, basalt serves great use as “road metals, aggregates for concrete… rubble masonry works for bridge piers, river walls and dams”, and in pavement [7].

Marble

Formed from heated and pressurized sedimentary rocks, marble is a metamorphic rock, characterized by the crystallization of minerals during its formation as evidenced by its veiny appearance. Marble is a soft stone, ranking at a 3 or 4 on the Mohs scale with a compressive strength of 7,500 psi [5]. In ancient times, marble was made popular by the Greeks and Romans as they used it in a variety of their structures for its beauty and its ease of carving [14]. Nowadays, applications of marble are mainly for ornamental purposes which include “stone facing slabs, flooring, facing works… columns,… steps etc.” [7]. In case of its use as a kitchen countertop, marble must be sealed as it is vulnerable to acids and will be damaged upon contact.

Slate

Slate is a metamorphic rock composed of “quartz, aluminum, chlorite magnetite, and other minerals” giving it a darker colour in shades of gray, red, purple, and green [5]. Its formation from layers of clays and volcanic ash allows for it to be easily broken into thin and durable planes. Slate is a hard rock, falling at 6 on the Mohs scale with a compressive strength varying from 14,500 to 29,000 psi [5, 7]. It is also “naturally resistant to chemicals and stains”, making for excellent applications “as roofing tiles, slabs, pavements etc.” [5, 7].

Limestone

Derived from the accumulated decay of ancient organisms along seabeds, limestone is a soft sedimentary rock with a 50% composition of calcium carbonate, and the rest being “other forms of calcium, magnesium, silica (which gives limestone much of its hardness), and other minerals” [5]. As a softer type of rock, limestone lies at a 3 or 4 on the Mohs scale with a compressive strength of 8,000 psi. Like marble, it’s vulnerable to acid which will cause the edges to round off with enough exposure to acid rain. Limestone is available in gray as well as various beige-tone colours, and similarly to granite, its colour depends on its mineral composition. Common applications of limestone include “interior surfaces, exterior cladding, flooring, landscaping and pavers” [5].

Steel

Prior to steel, iron was the go-to metallic element used in a variety of applications from weapon and tool forgery to structural support in buildings all over the world. In the mid 1850s, Henry Bessemer invented the blast furnace—the first method for mass producing steel—and around the year 1870, steel quickly replaced iron as a cheaper material with its versatility and superior properties [18].

General properties of steel include [19]:

• Density of 7.80 to 8.00 lb/in³
• Brinell hardness of 121
• Knoop hardness of 140
• Vickers hardness of 126
• Tensile strength ranging from 50800 to 60900 psi
• Elongation at break of 15%
• Modulus of elasticity of 200ksi
• Machinability of 65%
• Specific heat capacity of 0.460 J/g·ºC
• Thermal conductivity of 44.0-52.0 W/m·K

Composition of Steel

Steel is mainly composed of iron and carbon with “less than 2% carbon” and trace amounts of “silicon, phosphorous, sulfur and oxygen” [18, 20].

Production of Steel

There are two different processes by which steel can be produced. The first is a blast furnace-basic oxygen furnace (BF-BOF) and the second is an electric arc furnace (EAF) [18, 20].

Blast Furnace-Basic Oxygen Furnace (BF-BOF)

Invented by Henry Bessemer, the blast furnace in the 1850s used a blast of air to oxidize molten iron and separate impurities. Nowadays, the “blast furnace is a large steel shell shaped like a cylinder and lined with heat-resistant brick” that uses limestone instead of air to remove the impurities [20]. The raw materials consumed in the production of steel via the BF-BOF are “predominantly iron ore, coal and recycled steel” [18]. Approximately 70.7% of all steel is produced using this method [18].

Electric Arc Furnace (EAF)

EAF’s use electricity for heating the materials which is mainly sourced from recycled steel, making steel a sustainable material. Steels produced by the EAF account for about 28.9% of all steel production as they tend to be used for producing quality steel [18, 20].

Four Main Types of Steel

Currently, there are over 3,500 different grades of steel, each with their own unique properties [18]. These different grades of steel can be classified among four main groupings: carbon steels, alloy steels, stainless steels, and tool steels. The classification of each steel is dependent on the percentage composition of carbon and the addition of alloying elements such as nickel, copper, chromium, or cobalt.

Carbon Steels

As the most common type of steel, this group makes up 90% all steel production [23]. These steels can be characterized by their matte appearance and are known “to be vulnerable to corrosion” [24]. Carbon steels are mainly composed of iron and carbon, and depending on the percent composition of carbon, they can be sub-categorized in one of three sub-groups: “Low Carbon Steels/Mild Steels (up to 0.3% carbon), Medium Carbon Steels (0.3–0.6% carbon), and High Carbon Steels (more than 0.6% carbon)” [23].

Alloy Steels

Alloy steels are the result of mixing iron and carbon with several alloying elements like “nickel, copper, chromium, and/or aluminum” [23]. Unlike carbon steels, alloy steels are more resistant to corrosion, making for good application as “car parts, pipes, ship hulls, and mechanical projects” [24]. The strength of different steel grades within this category differs depending on the concentration of each alloying element.

Stainless Steels

Stainless steels are the most well-known types of steel with a 10-20% composition of chromium, giving them their shiny appearance. Stainless steels are highly resistant to corrosion and demonstrate properties of flexibility and malleability. The best application for stainless steels is in “surgical equipment, home applications, silverware, and… as exterior cladding for commercial/industrial buildings” [24].

Tool Steels

Tool steels contain elements like vanadium, “cobalt, molybdenum, and tungsten”, providing them with superior hardness as well as heat and scrape resistance [24]. Given these properties, tool steels make for excellent applications as metal tools and in equipment or machinery.

Steel Grades

Currently, there are over 3,500 different grades of steel, each with their own unique properties [18]. These different grades of steel can be classified among four main groupings: carbon steels, alloy steels, stainless steels, and tool steels. The classification of each steel is dependent on the percentage composition of carbon and the addition of alloying elements such as nickel, copper, chromium, or cobalt.

The ASTM Grading System

Each metal is assigned a letter prefix based on their category followed by a sequential number that indicates its specific properties [24].

The SAE Grading System

Each metal is assigned a four-digit number. The first two digits refer to the type of steel and concentration of alloying element while the last two digits indicate the concentration of carbon of the specific metal [24].

Resource

Steel Standards developed by ASTM

Wood

Wood seconds stone as another natural material that has been used in structures throughout much of history. Since wood is naturally sourced from trees, it is readily available to harvest, and also fairly inexpensive. Its versatility as a material, in terms of being machined and shaped, has allowed its application in a variety of projects from furniture to boats, to homes and shelters. Considering its weight, wood is quite strong and “it provides good insulation from the cold”, making it the optimal material selection for most home constructions in colder climates [25]. Wood is also a sustainable material as it naturally belongs to the cycle of earth, being both biodegradable and renewable. To top it off, the production of wood does not cost the earth, carrying a low carbon footprint as “no high-energy fossil fuels are required” [25].

Types of Wood

There are three types of woods, two of which are natural: hardwood and softwood, and the last, more recent type which is engineered wood. The general classification of woods between hardwoods and softwoods is differentiated between what the tree bears. Hardwoods are mostly those that bear leaves while softwoods likely bear cones, however, this is merely a generalization, meaning that there are many exceptions to these cases. Examples of popular hardwoods “include oak, maple, mahogany, cherry, walnut, and teak” while softwoods are generally derived from “pine, hickory, beach, ash, birch, and cedar” trees [25]. As for engineered woods, these are manufactured woods that are combinations of wood products and additives that may be adhesives or resin [27]. Common examples of engineered wood include plywood, MDF, and composite board.

Hardwood

In general, hardwoods are more durable and have higher densities than softwoods. Most tree species that fall under hardwoods, called angiosperms (trees that produce covered seeds that grow into fruits, nuts or seeds upon fertilization), have low sap content which increases its resistance to fire. Hardwoods are mostly used in projects where they are exposed such as “deck, flooring, beams, paneling, and high-quality furniture” [27].

Softwood

Softwoods tend to be the less expensive type of wood as they grow a lot faster than hardwoods. Their tall and upright growth permits the production of dimensional lumber which is the main material used in house frames. Most softwoods have a lower density than hardwoods allowing them to absorb adhesives but also stain a lot quicker. Softwoods are optimal for general construction purposes as they can be nailed without splitting as easily [27].

Engineered Wood

Engineered woods are composed of the waste wood from sawmills mixed with additives to form boards through chemical treatment or heat processes. Since engineered wood is not directly cut from hardwoods or softwoods, the boards produced can be shaped as needed to meet the size requirements [27]. Engineered woods have different properties depending on their composition, and so they can eb used in a variety of applications which include home constructions and industrial products.

Lumber vs Timber

These words are used to distinguish different pieces of wood based on their general sizes. Lumber generally refers to pieces of wood that are less than 5x5 inches while timber generally refers to pieces that are greater than 5x5 inches. Beyond 8x8 inches, these pieces of wood are generally referred to as beams [25].

Lumber Grades

Like steel, all lumber is given a specific grade in order to distinguish between the different pieces of lumber. At the mill, each dimension lumber is inspected and stamped by a lumber grader to mark the grading agency, mill designation, species group, moisture content and assigned grade. Lumber grading is based on appearances relating to its natural appearance as observed by the grader and in Canada, the grading follows the NLGA Standard Grading Rules for Canadian Lumber [28].

Resources

Dimensional Lumber Sizes: Nominal vs. Actual

Concrete

Glass