Concrete – Physical properties and its relevance in engineering design
The sustained use of concrete within construction is well established and has been a mainstay in the construction industry. The adoption of this material as the basis of many structures is distinctly due to its varying physical properties that have been worked upon and subsequently, diversified over time. Furthermore, as a result of this, there are a number of physical properties of concrete that I would like to review and additionally, assess their relevance towards engineering design.
Strength (Compressive and Tensile)
It is a well-documented fact that concrete fails detrimentally under tensile loading, however; under compressive loads, concrete performs to considerable effect. Adequately, the above statements are true due to the fact that concrete is a composite material that is unable to resist direct tension; Its “direct tensile strength is only about 7 to 15 percent of its compressive strength” (The Constructor, n.d.). In essence, the compressive strength of any formed concrete depends mainly on its constituents i.e. the materials used to produce it. Primarily, the aggregate and water/cement ratio used are of most concern to engineers when determining the compressive strength of concrete.
It is widely acknowledged that there are a number of ways in which the aggregate used to produce concrete can have an effect on its strength. The shape, texture, size, grade, moisture content, specific gravity, reactivity and bulk unit weight will all impact, in their own way, the resulting strength of the concrete. To demonstrate this, it is known that aggregates with differing surface textures may be used to produce concrete, each providing varying properties; “a smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength” (Engr.psu.edu, n.d.). Hence, aggregates with rougher surfaces will increase the strength of concrete, while aggregates with smoother surfaces will reduce the strength of the concrete. Effectively, this is due to the fact that, as stated in the above quotation, a rougher surface generates a stronger bond between the cement paste and the aggregate.
Alternatively, the water/cement ratio used in the concrete mixture can also have an effect on the strength of the concrete produced as these two materials are responsible for binding everything together. In general, it is thought that; the more water that is added for a fixed amount of cement i.e. the water/cement ratio, the more the strength of the resulting concrete is reduced. Essentially, this is mostly owed to the fact that “adding more water creates a diluted paste that is weaker and more susceptible to cracking and shrinkage” (Concrete Countertop Institute, n.d.) and this shrinkage leads to micro-cracks, which are deemed zones of weakness in concrete, and thus, reduces the strength. Hence, it is said that “using a low w/c ratio is the usual way to achieve a high strength and high quality concrete” (Concrete Countertop Institute, n.d.).
With respect to engineering design, it is of unquestionable importance for engineers to comprehend this physical property and the way in which it is affected as it is
Workability
This physical property is a term that describes “how easily freshly mixed concrete can be mixed, placed, consolidated and finished with minimal loss of homogeneity” (GlobalGilson.com, n.d.). Furthermore, it is a property that is of great interest to engineers as it directly influences many other factors such as strength and quality however, is directly affected by a number of factors such as water content, aggregate/cement ratio as well as others.
In general terms, it is said that the workability of concrete increases with an increase in water content. Essentially, this is due to the fact that “high water content results in a higher fluidity and greater workability” (Aboutcivil.org, 2017) and ultimately, this enhances the inter-particle lubrication within the concrete matrix. However, while stating this, it is also substantially important to note that if too much water is added to the mixture this may result in concrete bleeding (surface water) as well as a reduction in the strength of the concrete. Thus, an optimum water/cement ratio should be maintained to balance both the workability and strength of concrete; “A water to cementitious material ratio (w/cm) of 0.45 to 0.6 is the sweet spot for production of workable concrete” (GlobalGilson.com, n.d.).
Additionally, the shape, size, grade and surface texture of the aggregate used in the cement mixture will also have an effect on this property. Generally, it is known that porous aggregates require more water in comparison to impervious aggregates in order to achieve the same degree of workability. Essentially, this is due to the fact that porous aggregates possess voids that will be occupied by the water added whereas impervious materials possess a reduced number of voids and hence, less water is required to achieve the same degree of workability.
Lastly, the ambient temperature at which the concrete is produced also has an effect on this property as high temperatures generally tend to reduce workability. Effectively, this is owed to the fact that water evaporates at an accelerated rate at high temperatures and thus decreasing the rate of hydration in the mixture. Furthermore, this hardens the concrete sooner which in turn reduces its workability; “when temperature increases then evaporation rate also increases due to that hydration rate decreases and hence, concrete will gain strength earlier” (Afsar, 2012).
In terms of its application to engineering design, engineers are concerned with how compact the concrete mixture is or its consistency. Essentially, a test known as the ‘Compaction factor’ test is conducted in order to find the compacting factor and ultimately, determine the workability of the concrete mixture being tested. While, a test known as the ‘Slump’ test is conducted by engineers in order to find the mixtures consistency. Critically, these applications allow for alterations if need be.
Durability
Concrete durability is often defined as the ability of concrete to “resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties” (Cement.org, n.d.). Conversely, it is also often termed as the ability of concrete to withstand the conditions for which it is designed for a substantial amount of time without significant deterioration. As with all other physical properties, the durability of concrete varies depending on a number of factors that are both internal and external i.e. the materials used to produce the concrete as well as the aggressiveness of the environment in which it is being used.
When assessing this physical property, engineers are often concerned with permeability as it is often considered the most important factor for the durability of concrete. Essentially, this factor is directly linked to the water/cement ratio as a high water/cement ratio will produce a higher coefficient of permeability which in turn will reduce the durability of the concrete mixture produced. Due to the direct link between factors, engineers are able to produce permeability vs w/c ratio graphs that allow them to view the effect of one on another.
Often, engineers tend to specify the materials used to produce a specific concrete. Essentially, this is due to the fact that different concretes require different degrees of durability depending on the environment it is exposed to as well as the properties desired. Thus, this physical property is relevant to engineering design as it allows engineers to avoid unnecessary time and costs due to the fact that it can be pre-determined as no single concrete mixture will be durable in every situation that it could be exposed to e.g. “when concrete is exposed to sulphate contaminated soil, the mix design should consider using sulphate resistant cement. However this mix design is not appropriate when a concrete structure is exposed to the cycles of freezing and thawing in a wet environment” (Kryton, 2014).
Concrete – Case study: White Collar factory
Old Street Roundabout, London, UK
White collar factory, located in in the heart of London’s Tech City, is a modern-day engineering project that provides office, retail and residential spaces as well as a 150m running track. It is a recent and rather notable development in the city of London that makes use of novel as well as traditional methods of engineering. However, it is the amount of research and time invested on this project that makes it rather extraordinary; “The project started in 2008, but was only completed and handed over in 2017” (Designingbuildings.co.uk, 2017). The overall concept of the 16-storey building centres on tried and tested environmental principles that were requested by the client; Derwent London.
The significant amount of time taken to complete the project was partly due to the fact that a “£1m, 320 sq. m prototype ‘slice’ of a floorplate” (Designingbuildings.co.uk, 2017) which was “led by AHMM director Simon Allford and Simon Silver and Paul Williams, directors of developer Derwent London, working in collaboration with engineering firms AKT II and Arup” (ArchDaily, 2017). Essentially, the prototype was constructed in order to view if the concrete core cooling and other design principles would perform as expected.
Appropriately, the architecture of the 16-storey building constructed makes use of concrete in a number of varied ways. Firstly, and undeniably its most important use is to provide the structure of the building. However, due to its minimalistic look, much of the concrete is visible and therefore, the concrete mix was of paramount importance; “The construction team aimed for (and achieved) an Outstanding BREEAM rating and 50% of the cement was replaced with ground granulated blast-furnace slag (GGBS) to reduce the carbon content. But, says Taylor, the GGBS resulted in a “whitish” finish: “We wanted a warmer, more traditional concrete look, so 200kg/m3 of FA (fly ash) was added to darken the mix.”” (Concretecentre.com, n.d.).
Furthermore, another way in which concrete has been utilised in the construction of this building is through concrete core cooling, which functions by circulating chilled water through pipes that are embedded within the concrete. The then cooled concrete is able to cool the office spaces in order to simulate an environment that could otherwise be cooled through the use of conventional air conditioning systems. Furthermore, “it is a passive cooling system which is quieter to run with less air movement within the space” (Constructionmanagermagazine.com, 2017).
Lastly, the concrete was additionally adopted in the design of the building due to its high heat capacity which ultimately, will aid in making the building more environmentally friendly as it will help to conserve energy in the form of heat. Essentially, its purpose here is to reduce the impact of daily temperature fluctuations. Additionally, it is known that “When used well and combined with passive solar design, thermal mass can play an important role in major reductions to energy use in active heating and cooling systems” (En.wikipedia.org, n.d.) and basically, this is what the design of the White Collar factory is trying to achieve.
Structural Timber – Physical properties and its relevance in engineering design
The adoption of structural timber as a structural material in construction is a method that was established a long time ago. Essentially, just like concrete, developments in timber component production have altered the physical properties derived and associated. Adequately, as conducted above, I will assess the physical properties associated with timber and discuss their relevance towards engineering design.
Specific gravity
Effectively, the specific gravity, SG, also known as relative density, of a material is a dimensionless unit, essentially, defined as the “ratio of density of the material to the density of water at a specified temperature” (Engineeringtoolbox.com, n.d.). With regards to timber, we would state that it is a measure of the density of timber relative to the density of water i.e. if a wood possesses a density that is of 1.5 times the density of water, its specific gravity is said to be 1.50.
Its use is practical, as often, wood density at differing moisture contents can vary significantly. Basically, this is due to the fact that for a hygroscopic material i.e. a substance “that readily attracts water from its surroundings, through either absorption or adsorption” (ScienceDaily, n.d.), such as timber, its density very much depends on two factors: “the weight of the wood structure and moisture retained in the wood” (Fpl.fs.fed.us, 1994). Hence, due to the variation in densities, it must be given relative to a specific condition that always remains constant i.e. water that possesses a density of 1000 kg/m3, in order to have practical meaning.
Furthermore, with relevance to its use in engineering design, it provides engineers with a relative measure of the amount of wood substance that is actually contained within a sample of wood.
Directional properties
It is said that wood is an orthotropic material i.e. “a material whose properties are unique and independent in three mutually perpendicular directions” (Help.solidworks.com, 2012). Due to the orientation of the wood fibres, and the way in which a tree increases in diameter as it grows annually, its properties vary along three mutually orthogonal axes; “longitudinal, radial, and tangential” (Fpl.fs.fed.us, 1994) and thus why it is said to be orthotropic. Effectively, this is shown by the image presented below:
Image source: https://www.researchgate.net/publication/281591470_Stress_simulation_in_layered_wood-based_materials_under_mechanical_loading
Furthermore, as is depicted in the image presented above, the radial axis is normal to the growth rings, the longitudinal axis is parallel to the fibre direction and the tangential axis is perpendicular to the fibre direction. Additionally, it is also said that wood is an anisotropic material i.e. a material whose properties such as young’s modulus change with direction along the material.
Essentially, this is effective, as ultimately, it allows engineers to determine properties that are associated with each of the different directions. For example, in general, it is said that the strength properties of wood are higher in its longitudinal axis compared to the strength achieved in either its radial or tangential axis. Furthermore, this property has allowed engineers to determine that it is able to be used as a structural column to withstand compressive loads without the need for additional supports.
Moisture content
Essentially, this property is termed as “the weight of water in wood expressed as a percentage of the weight of wood fibrous material (which is considered to be the oven dry weight of the sample)” (Timber.ce.wsu.edu, 2000) and is of major concern to engineers when opting to use timber for structural purposes. Basically, the above mentioned is due to the fact that in order to avoid problems owed to dimensional change and distortion in use, its moisture content, often documented as ‘MC’, must be controlled.
As mentioned previously, we are aware of timbers’ hygroscopic nature meaning that it will effectively gain or lose moisture from the surrounding environment. Due to these changes in moisture content, it will expand or contract. Unfortunately, these alterations in shape, effectively known as ‘swelling and shrinking’, are one of the main causes of failure when timber is used as a structural material. However, this cause of failure is preventable as “It is only once all the free water has been lost that the wood will reach what is called the fiber saturation point, or simply FSP” (Wood-database.com, n.d.) and failure will occur. Hence, for this reason alone, it is relevant towards engineering design.
Timber – Case study: Marks & Spencer
Cheshire Oaks, Ellesmere Port, Cheshire, UK
In January 2007 Marks and Spencer launched Plan A; “a commitment to combat climate change, reduce waste, use sustainable raw materials and help its customers adopt healthier lifestyles” (Trada.co.uk, n.d.). As part of this plan, the company went on to develop a series of stores that they claimed to be ‘sustainable learning’ stores. The store built at Cheshire Oaks is the second largest M&S store in the UK and is the “company’s flagship for sustainability, carbon efficiency, biodiversity and material innovation, with the use of timber, both for construction and for fuel, as a central concept of the design” (Trada.co.uk, n.d.). Effectively, the building has been designed to be “42% more energy efficient and 40% fewer carbon emissions than an equivalent store” (Corporate.marksandspencer.com, n.d.). Furthermore, it also boasts a BREEAM ‘Excellent’ rating which further exemplifies its commitment to sustainability.
Structural steel – Physical properties and its relevance in engineering design
Lastly, the use of structural steel in construction is a relatively modern technique in comparison to the use of concrete or structural timber that were constriction techniques adopted traditionally. Due to its varying physical properties, it is an advantageous and essential construction material that enables modern-day engineers to produce state-of-the-art structures to meet present-day consumer needs. Consequently, as a result of this, I would like to discuss, as done previously, some of the physical properties associated to structural steel and their relevance towards engineering design.
Strength
Basically, strength in metallurgy is defined as the ability of a material to resist deformation from its initial shape upon the application of a load. Essentially, the above definition declares that; the higher the force required to alter the shape of the steel, the stronger the steel is. Ultimately, this physical property is determined as a result from both its chemical composition and the way in which it is manufactured i.e. heat treatments and manufacturing processes conducted at production. Furthermore, due to the differing procedures available, steels with a variety of strengths can be attained e.g. high strength low-alloy steels such as “Microalloyed ferrite-pearlite steels” (Asminternational.org, 2001) and low carbon steels such as “Interstitial-free (IF) steels” (Worldautosteel.org, n.d.).
With relevance to engineering design, engineers are often majorly concerned with the yield strength of steel as “it is the basis used for most of the rules given in design codes” (Steelconstruction.info, n.d.). Essentially, yield strength is termed as the maximum amount of stress that can be applied to a material before it begins to change shape. Hence, understandably being the property of most concern to engineers. Furthermore, the desirability of this property is unquestioned for any engineer at the design stage of any scale project. Ultimately, this is due to the fact that it will allow the engineer to determine which steel grade is most suitable to cater for all expected load cases without failure.
Ductility
In simple terms, ductility is a measure of a “metal’s ability to withstand tensile stress —any force that pulls the two ends of a material away from each other” (The Balance, 2017). Effectively, as can be derived from the above quotation, ductility of steel or any metal in fact, is a measure of the degree to which a material is able to strain or elongate i.e. plastically deform, until eventual failure or fracture caused due to the application of loads. Often, it is expressed as a percentage and is termed as either the ‘percentage elongation’ after the application of a tensile load or ‘reduction of area’ if necking occurs.
Additionally, to the above mentioned, the ductility of steel varies depending on the alloying constituents used. In general, it is said that “as the carbon content increases the hard-ness of the steel also increases, while the elongation decreases” (Pdfs.semanticscholar.org, 2008). Furthermore, this physical property can be changed if the conditions which it is subject to are altered i.e. the temperature and pressure at which it operates. The former of which is governed by a phenomenon known as the DBTT (Ductile-Brittle Transition Temperature) that is of utmost relevance to engineers.
Basically, this phenomenon directly correlates an increase in temperature with an increased amount of energy absorption that ultimately, enables engineers to view the ideal operating temperatures for particular grades of steel. In general, at low temperatures, steels are brittle, while, at high temperatures, steels are ductile on the DBTT graph.
Lastly, another application of ductility with reference to its use in engineering design, in a broader view, is its use in earthquake engineering where “the term is used … to designate how well a building will endure large lateral displacements imposed by ground shaking” (Air-worldwide.com, 2016). Understandably, this will account for all components of a structure, however; will be relevant to steel if steel reinforcements are used in the construction of a building or other structure. Ultimately, comprehending the way in which this physical property functions has allowed engineers to improve the durability of structures as these reinforcements will permit the structure to endure longer elongation without collapse.
Toughness
Essentially, this physical property, in materials science and metallurgy, is characterised as “the ability of a metal to deform plastically and to absorb energy in the process before fracture” (Nde-ed.org, n.d.) and alternatively, is often expressed in terms of the amount of energy a material can absorb before sudden fracture. Effectively, it is measured as a combination of both of the above mentioned physical properties; Strength and Ductility. Subsequently, it is a widely known fact that a steel with high strength and high ductility will possess more toughness than a steel with low strength and high ductility or conversely, a steel with high strength and low ductility.
Additionally, engineers are often most concerned with the fracture toughness, usually denoted KC, of a material. Essentially, it is an “indication of the amount of stress required to propagate a preexisting flaw” (Nde-ed.org, n.d.) i.e. it is essentially the amount of stress required to cause failure in a material that has endured cracking from previous loading (crack propagation). Generally, this property tends to vary with both thickness and grain direction. Ultimately, this physical factor is relevant as it provides engineers with an indication of the critical stress value required to cause failure in materials that possess cracks of a given length induced by a number of possible factors such as previous load cases. Conversely, this property also enables engineers to determine the critical crack length that would cause failure when a specified stress is applied to a component.
In terms of engineering design, this physical property is undoubtedly significant as it will allow any engineer to determine how much energy can be stored in every cubic metre of the material before eventual failure. Furthermore, it is a desirable material property as it allows a component to deform plastically, rather than crack and perhaps fracture.
Structural Steel – Case study: Cannon Place
Cannon street, London, UK
Cannon place, located in the central London financial district is a structure that adopts a rather unusual and unexplored method of architectural engineering. Construction works began in “January 2010” (Sharpfibre.com, n.d.) and initially comprised of the demolition of a 16-storey office building and parts of the former cannon street railway station that were already there with the aim to construct a “new 50,000 m2 eight-storey ‘air rights’ office block” (Laingorourke.com, n.d.) and reconfigure “both the mainline and underground stations” (Laingorourke.com, n.d.). Furthermore, the brief established by both Hines and Network Rail asked for a building “which would appeal to financial, legal and corporate tenants, be capable of multiple subdivision, would optimise the amount of lettable space on the site while at the same time maximise value, quality and, as a key element of these parameters, floor-to-ceiling heights” (Icevirtuallibrary.com, 2012).
Due to the location at which the building was to be constructed, there were a number of restrictions as well as limitations that had to be catered for by the engineers when conducting the design phase of this structural project. Firstly, due to the fact that the site sat in the foreground of protected views of St Paul’s Cathedral, there was an imposed limitation on the height of the building that could be built: “51.3 m above ordnance datum (AOD)” (Icevirtuallibrary.com, 2012). Additionally, the need to cater for a functioning railway system meant that a “minimum height of 5.1 m above the main-line running tracks” (Icevirtuallibrary.com, 2012) had to be maintained. Essentially, combining the above two factors meant that there was a resulting height of 32 m within which designers had to construct the eight floors of office space that would make the project commercially viable as per the brief. Furthermore, there were a number of other limitations, due to the historical nature of the site, that affected the design of the building.
Construction of this building was rather complex, making use of techniques that are used in major suspension bridges around the world. Due to the inability to lay comprehensive foundations, the engineers working on this project devised a structural solution “which balances a cantilevered 21m deep ‘strip’ of office space to the north with the equivalent accommodation in the south through the use of façade deep transfer structures” (Steelconstruction.info, n.d.). Furthermore, the use of steel was adopted as the main construction material due to the fact that upon completion of the design, they discovered that they required a lightweight solution for both the framing material as well as for the foundations; ““The cores are also steel because we needed a lighter solution as they are founded on old foundations and we had to limit the loads” (newsteelconstruction.com, 2011).
Unusually, both east and west elevations of the structure feature three large cross bracings, known as X-frames, that are formed by two 16 m long steel beams bolted together. The use of steel here was imperative due to the fact that it will be able to transfer loads onto the foundations under the central structural zone in order to keep the structure balanced without any physical failure due to its strength.
Effectively, as expected, the design solution produced was a direct response to the limitations and restrictions imposed by the site. Thus, making this structure, a remarkable feat of modern-day engineering.