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 be 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

Concrete is the resulting material from mixing cement, fine and coarse aggregates, and water [29]. Portland cement is the most common cement used in concrete mixtures and it is considered hydraulic since it “forms a water-resistant product” [30]. Upon mixing with water, the cement undergoes a chemical reaction, at which point it can bind to the aggregates before setting and hardening in a process called hydration [31]. The aggregates are generally both fine and coarse, with sand being the former variation and gravel being the latter [32].

Water to Cement Ratio

The water to cement ratio is an important factor in determining the quality of concrete. This is calculated by dividing the water in one cubic yard of the mix (in pounds) by the cement in the in the mix (in pounds) [34]. Low water to cement ratios generally produce low quality concrete as it affects all the desired properties generally sought in concrete. The following is a short list of some regulations stated in the 1997 Uniform Building Code in order to avoid a low water to cement ratio:

• • When exposed to freezing and thawing or to de-icing chemicals, the water to cement ratio should not exceed 0.50 [34]
  • Concrete with severe sulfate conditions should not exceed a water to cement ratio of 0.45 [34]

In general, a water to cement ratio exceeding 0.50 will exponentially increase the water permeability of the concrete which in turn decreases its durability. For example, a concrete mix with a water to cement ratio of 0.45 will have a compressive strength of up to 4,500 psi whereas a water to cement ratio of 0.50 will have a compressive strength of up to 4,000 psi [34]. As such, careful attention must be made when mixing concrete because a good balance must be made between durability and strength without comprising either property.

Admixtures

Variations of concrete with differing properties arise from adding chemical and/or mineral admixtures. There are 10 classifications of admixtures, of which the three most used are accelerators, retarders, and air-entraining agents.

• • Accelerators, as implied by their name, accelerate the rate of strength development and reduce the setting time of the concrete to speed up the process from start to finish. The most common accelerator mixed into concrete is calcium chloride, unless it is to be added to reinforced concrete which requires a non-chloride accelerator as to prevent corrosion of the steel [31, 32]
  • Retarders are used to delay the setting time of concrete, specifically during hotter periods where the temperature would naturally accelerate this process [31]
  • Air-entraining agents increase the concrete’s resistance to damage dealt by the combination of salt and snow during the winter [31]

Types of Cement

According to the ASTM Specification C-150, there are eight variations of Portland cement [30]:

• • Type I – used for general purposes in many applications including buildings, floors, pavements, bridges, etc
  • Type IA – Type I with the addition of air-entraining admixtures
  • Type II – used in applications where sulfate exists in water or soils as it is resistant to sulfate attacks and applied in hotter climates as it is slower at generating heat
  • Type IIA – Type II with the addition of air-entraining admixtures
  • Type III – similar to Type I using more finely ground aggregates, used when high strength is desired at earlier rates
  • Type IIIA – Type III with the addition of air-entraining admixtures
  • Type IV – used in applications where less heat is desired as it generates low amounts of heat
  • Type V – used in applications where high quantities of sulfates exist

Types of Concrete

Different variances in the composition of concretes have allowed for the creation of many types of concrete, each suitable for different purposes.

Plain Concrete

The most common occurring concrete worldwide used in pavement construction, and for buildings requiring low tensile strength. The general cement, aggregate, water ratio lies at 1:2:4. Without reinforcement, plain concrete is not suitable for structures that may be subjected to stresses from vibrations or wind loads. The k value for plain concrete can go up to 10-12 W/m·K [36].

Lightweight Concrete

As implied by its name, lightweight concrete uses aggregates that are lighter in weight which helps increase its flowability, making it a self-leveling concrete mix. With better flowability than other concrete mixes and its low thermal conductivity, lightweight is optimal for use as floor slabs, window panels or in roofing. The k value of lightweight concrete sits at about 0.3 W/m·K [36].

High Density Concrete

As the name implies, high density concrete has a much higher density than plain concrete, using more coarse aggregates in its mixture. Properties include protection from x-rays and radiation due to its density, allowing for good application in nuclear power plants and similar buildings [36].

Reinforced Concrete (RC)

Reinforced concrete is concrete embedded with steel rods, wires, mesh or cables, called rebar, to increase its strength. Alone, concrete resists compressive forces rather than tensile forces, but with the strengthening from rebar, reinforced concrete can resist many applied forces as a single structural element. Its versatility as a composite has made it a popular material selection in modern construction [36].

Precast Concrete

Precast concrete is pre-made at an off-site location—usually formed in moulds—before being shipped or transported to the site as a larger component [36]. As a single component, precast concrete elements can be joined with other elements as required. Typical applications of precast concrete include panels, beams, columns, walls, staircases, pipes, tunnels, etc.

Pre-stressed Concrete

Pre-stressed concrete uses engineered stresses along its structure to counteract the eventual stresses upon which it will be subjected. Similar to reinforced concrete, prestressed concrete has both high compressive and high tensile strength which comes from the induced stresses applied by the steel elements [36]. Typical applications of pre-stressed concrete include floor beams, piles, railway sleepers, bridges, roofs, etc.

Glass Reinforced Concrete (GRC)

Glass reinforced concrete uses alkali-resistant glass fibers in its mix to act as the load-bearing component which gives it its high-strength. Both the concrete and glass fibers retain their own properties to create a high-performance composite [36]. Glass reinforced concrete is most used as cladding panels for building exteriors.

Air-Entrained Concrete

Air-entrained concrete is a variation of plain concrete with microscopic air bubbles interspersed within its composition from adding air-entrained admixtures. The air bubbles act as pockets for water to settle into where it may freeze without contributing additional internal pressure that would otherwise damage the concrete [36]. Air-entrained concrete is best used in colder climates.

Self-Compacting Concrete

Self-compacting concrete is a more recent development in the world of concrete. Like lightweight concrete, it has excellent flowing properties that allow it to spread, fill formwork, and surround reinforcements without the assistance of equipment [36]. Self-compacting concrete makes for good application in complex concrete frames and structures.

Smart Concrete

Smart concrete is embedded with trace amounts of short carbon fibers which alter its resistance to electricity upon being subjected to stress or strain. The change in electrical resistance can be monitored to determine the health of the concrete, and to identify any issues prior to failing [36]. This becomes useful as it can catch minor structural flaws that may appear after an earthquake. Currently, smart concrete is unavailable in the market as it is still undergoing extensive lab testing.

Polymer Concrete

Polymer concrete uses polymers as a binder as opposed to lime-type cement—the less common cement type. Among the different polymers, epoxy is the most popular as it demonstrates properties including, but not limited to “high impact strength, high vibration resistance, good bonding with concrete and metal surfaces” [44]. Recently, polymer concrete has broadened in variation with the addition of reinforced fibers which provide improvements to the pre-existing properties of concrete. Polymer concrete can be found in applications such as aircrafts, offshore platforms, biomedical devices, and various engineering structures.

Glass

Glass is a composition of natural and raw materials including sand, calcium carbonates, and waste recycled glass that are melted down to a molten liquid at around 1500ºC and then shaped and cooled to form an amorphous solid [46, 47]. Energy that goes into the manufacturing of glass is conserved with the addition of sodium carbonates which reduces the melting point of sand. Sodium carbonate, however, must be paired with limestone otherwise the resulting glass will dissolve in water [46].

As a structural material, structural glass is designed with the intent of bearing loads. The engineering of such glass will vary depending on the specific loads that they will be required to carry. For example, structural glass made for flooring will be designed to bear loads along its surface whereas structural glass used in framing that must stand vertically will be designed to bear loads in the perpendicular direction [48].

Properties of Glass and Glass Treatments

Glass is characterized by its mostly transparent and sometimes translucent appearance as well as its significant hardness. Though it may be hard, glass is also quite brittle which is why engineers must process the glass to optimize its strength according to protocols. There are seven processes by which glass can be strengthened: annealing, heat strengthening, tempering, heat soaking, chemical strengthening, laminated glass, and insulating glass units (IGUs).

• • Annealing is the process by which glass is slowly cooled to relieve internal stress, promoting durability [49]. It is both the least expensive and weakest treatment of the three [50].
  • Heat strengthening produces a stronger glass than the annealing process. It involves cooling the outer 20% from the surface of the glass with a blast of air which compresses and strengthens the outer surface [50].
  • Tempered glass follows a process similar to that of heat strengthening, however the cooling is done at a faster rate and with higher volumes of air [50]. This compresses the outer surface while putting the inner surface into tension which will cause the glass to break into “safety”, granular chunks as opposed to jagged shards formed from breaking annealed glass [49].
  • Heat soaking is the process by which toughened glass is heated to 290ºC and maintained at that temperature for a specific period before it begins a slow cooling [51].
  • Chemical strengthening is used exclusively for glass with high sodium content. The glass is immersed in hot potassium salt where sodium ions are exchanged, increasing molecular density and compressive stress at the surface of the glass [51]. Glass that undergoes chemical strengthening is generally more susceptible to surface-level flaws.
  • Laminated glass is the layering of glass and interlayers—commonly Ethylene Vinyl Acetate (EVA) film or Polyvinyl Butyral (PVB). The glass layers of laminated glass can be of any kind depending on the desired properties [51].
  • Insulating glass units (ISU) are sealed units that use air or gasses with higher densities than air to fill the spaces between two or more glass panes [51].

Factors that disrupt the strength of glass include any minor defects, thermal stress from rapid cooling, and the embedding of crystals during the annealing process [52].

Structural Glazing Systems

Structural glazing systems are types of glass curtain wall systems that are bonded or anchored to the structure without any gaskets, pressure plates, or caps [53]. These anchoring components tend to disrupt the clean-cut aesthetics of glass while increasing thermal bridging—a pathway for heat to flow across a thermal barrier caused by conductive materials—which is unwanted in any building as it wastes energy [54]. Instead, structural glazing systems use mullions, steel blades, cables, or stainless steel rods [53].

• • Four-Sided Structural Glazing – structural silicone is used to support glass panels from four sides [55].

• • Two-Sided Structural Glazing – two sides of the glass panel are supported by structural silicone while the other two are supported by “a mechanical frame or another non-structural method” [55].

• • Slope Glazing – glazing applied in a non-vertical manner which generally uses laminated glass [55].
  • Tooth-Shape Glazing – structural joint is attached to the interior panel which is built after the external panel is attached [55].