Applications of Aluminum in Structural Design

Applications of Aluminum in Structural Design Eric Fraser Student ID: 260413230

McGill University Department of Civil Engineering and Applied Mechanics

Montréal, Québec October 9th 2015

1 Abstract l Aluminum is a relatively new construction material, and its unique properties make it extremely versatile and useful for a wide variety of applications. This paper evaluates the use of aluminum as a construction material. This evaluation will focus on the mechanical properties and various structural applications of aluminum. A history of aluminum structures and the objectives of this paper are given in the introduction. The strength to weight ratio and corrosion resistance of aluminum alloys, as well as interesting thermal and fatigue properties, are examined to show why aluminum is a unique construction material. The available strengths and types of aluminum alloys and the properties of aluminum foam are discussed. The benefits and costs of using aluminum are compared with those of steel and concrete. The structural applications of aluminum are then examined in detail to showcase both its potential and versatility as a construction material in two different areas; seismic engineering and bridge decks. Its uses in roof systems and concrete composites, as well as current and potential applications of aluminum foam are briefly disused. The environmental impact and embodied energy of aluminum production is covered. The retrofit of a steel pony truss bridge with an aluminum bridge deck is presented as a case study.

2 Acknowledgements l I would like to thank Professor Yixin Shao for his guidance and advice. His solid mechanics course showed me how interesting civil engineering is, and was a major factor in my decision to transfer into the faculty. I would also like to thank my mother for taking the time to proofread this paper and making several valuable suggestions to improve the quality of the writing.

Table of Contents l Abstract I Acknowledgements II Table of contents III List of figures V List of tables VI 1.0 Introduction 1 1.1 Historical Use of Aluminum in Structures 1 1.2 Objectives 1

2.0 Properties of Aluminum 2

3 2.1 Strength to Weight Ratio 2 2.2 Corrosion Resistance 2 2.3 Thermal Properties 3 2.4 Fatigue 3 2.5 Aluminum Alloys 4 2.6 Aluminum Foam 4

3.0 Comparison between Aluminum and Other Construction Materials 5 3.1 Steel 5 3.2 Concrete 6

4.0 Structural Applications 7 4.1 Seismic Engineering 7 4.2 Bridges 7 4.3 Long span roof systems 9 4.4 Concrete Aluminum Composites 9 4.5 Aluminum Foam 9

5.0 Environmental Impact and Sustainability 10 5.1 Environmental Impact 10 5.2 Embodied Energy 10

6.0 Case Study: Retrofit of a Steel Truss Bridge with an Aluminum Deck 11 6.1 Increased Load Bearing Capacity 12 6.2 Ease of Construction 12 6.3 Durability 12

7.0 Conclusions 13 References 14 Statement of Authorship 17

List of Figures l Numbe Pag Caption r e 1 Thickness of Aluminum Oxide Layer 2

2 Galvanic Corrosion of Aluminum 2

3 Fatigue Stress-Cycle Diagram of Aluminum and Steel 3

4 Specific Tensile Strengths of Different Alloys 3

5 Tensile Strength of Different Alloys 4

6 Cross Section of Aluminum Foam 4 Compressive Strength of Cylindrical Aluminum Foam Sample, 30mm 7 4 diameter and height

4 8 Stiffness of Steel and Aluminum Cross Sections 5

9 Aluminum Shear Link 6 Base Shear and Roof Displacement of Shear Linked Braced Frame and 10 6 Ordinary Concentric Braced Frame 11 Svensson Aluminum Deck Design 7

12 Composite Concrete Aluminum Beam 9

13 Aluminum Foam Acoustic Insulation 9

14 Sandwich Panel with and without Aluminum Foam Core 10

15 KY 974 Bridge Elevation View 11

16 KY 974 Bridge Removal of Concrete Deck 11

17 Dimensions of Aluminum Deck Design 11

18 Fabrication of Aluminum Deck Panel 11

19 Rubber Insulation Pads to Prevent Galvanic Corrosion 12

20 Clamps Connecting Aluminum Bridge Deck to Steel Supports 12 List of Tables l

Number Caption Page Canadian Bridge Inventory 1 6 Comparison of the Cost of Timber versus 2 Aluminum Deck 7 Replacement Cost

Costing of Aluminum 3 Deck 8

4 Compressive Strength 9 and Absorbed Energy of Empty and Aluminum Foam Filled

5 Sandwich Panels

6 1.0 Introduction l 1.1 History Aluminum was first produced industrially in 1886 after the electrolytic process was patented, and its first structural use was in 1891 as a solid cast aluminum staircase in the Monadnock hotel.(Merwood-Salisbury, 2009) Due to the higher initial cost and lack of design codes and experience, aluminum was not widely adopted throughout the twentieth century. The first use of aluminum in a bridge in 1933 when the deck of Pittsburgh’s Smithfield Street Bridge was replaced with an aluminum bridge deck. (Thompson et al, 1996). The first highway bridge made completely of aluminum was constructed in 1950 in Arvida, Quebec by the Aluminum Company of Canada to showcase the potential of aluminum and remains in service today. From 1958 to 1965 there were long lead times to obtain steel due to economic factors, and as a result five new bridges were constructed in the U.S. with aluminum as major structural components. (Das & Kaufman, 2007) Since then aluminum has increasingly been used in structures, and research into the properties and mechanics of aluminum structures expanded in 1970. Comprehensive design codes for aluminum began to develop in 1978 in Europe, continuing up to the preparation of Eurocode 9 “Design of Aluminum Structures” in 1999. As of 1997, approximately 11 bridges had been built using aluminum components in America. Aluminum bridge design has seen wider adoption in Europe, with at least 30 bridges having been rehabilitated with aluminum bridge decks in Sweden alone. As of 2013, the Quebec Ministry of Transportation is replacing the Saint-Ambrose bridge deck with an aluminum deck as a pilot project to evaluate the further use of aluminum in bridges.(Mazzolani, 2008; Arrien et al, 2001; Viami International Inc, 2013)

1.2 Objectives The primary objective of this paper is to demonstrate that aluminum is an incredibly practical construction material, and that in some cases is a better choice than other commonly used materials such as steel or concrete. Aluminum is a relatively new construction material and although it has been used in structural design it has the potential to be used more often. One of the major drawbacks to aluminum designs is simply the lack of experience. The case study at the end of the paper showcases how an aluminum bridge deck is more effective than a reinforced concrete deck. The secondary objective is to demonstrate the versatility of aluminum by showing different applications for aluminum in structures. 2.0 Properties of Aluminum l 2.1 Strength to Weight Ratio Aluminum is both strong and lightweight, which makes it extremely useful as a structural material. Aluminum is unique because it has a very low density, 2700kg/m^3, compared to most metals. Pure aluminum has low tensile strengths of around 90 MPa, but with alloying and heat treatment it can be extraordinarily strong with tensile strengths approaching 690 MPa. The reduced weight of aluminum makes it very easy to transport and install, and reduces the dead load applied on structures. (Davis, 1999) 2.2 Corrosion Resistance Aluminum reacts with air and water to rapidly form a small oxide layer ranging from 2 to 50 nanometers thick, which protects the metal from any further corrosion by drastically reducing the rate of oxidation. This oxide layer remains intact within a pH range of 4.5 to 8.5, and if the oxide layer is damaged due to abrasion it will quickly regenerate itself. Due to this oxide layer,

aluminum is incredibly resistant to atmospheric corrosion. However, it is extremely susceptible to galvanic corrosion and will act as a sacrificial anode when it comes into contact with almost any metal, with only magnesium and zinc being more anodic. Anodizing aluminum can be used to artificially increase the thickness of the aluminum oxide layer to increase protection and durability, which can be seen in figure 1. (Crissinger, 2013; Davis, 1999) 2.3 Thermal Properties Aluminum is very resistant to low ductility fracture as it does not have a sharp transition from ductile to brittle failure at lower temperatures. It is also a very good thermal conductor and has a high heat capacity of 900 J/kg/K. It has a relatively high coefficient of thermal expansion, 22.2*10^-6 m/(mK). Its high heat capacity and conductivity help increase its somewhat poor fire resistance due to its low melting temperature of 660°C, as it will efficiently distribute localized heating throughout the structure. (Davis, 1999; CST Industries)

2.4 Fatigue Aluminum is unique as it does not have a true endurance limit, and will eventually fail if it is subjected to a high number of cycles. As a result, the fatigue strength of aluminum is chosen based on a high cycle level, usually 5x10^8 cycles. Design practices to account for this include damage tolerant design, safe lifecycle design, and fail-safe design. Safe lifecycle design assumes the structure is initially crack free and has a finite lifetime before failure occurs when a crack of a certain size is detected. Fail-safe design assumes that cracks will be detected and repaired, and includes multiple load paths so one crack will not cause the structure to fail. Damage tolerant design is an improvement of safe lifecycle design and assumes that cracks are initially present so that their growth rate can be calculated to schedule inspections to repair or retire the structure

before the crack becomes critical. (Cambell, 2008) 2.5 Aluminum Alloys The main alloying elements used in aluminum alloys for structural purposes are magnesium and silicon. (Gitter, 2008) Commonly used alloys for bridges and their properties are

listed below. Aluminum alloys have a wide range of strengths. As shown in figure 5, some alloys are actually stronger than the weakest steels, and most alloys have specific strengths that are much larger than commonly used structural steels 2.6 Aluminum Foam Aluminum foams are manufactured aluminum matrices with cellular pores and can be divided into two types of foam, open and closed cell. It is produced by bubbling gas into molten aluminum and allowing it to cool. Aluminum foams have serval distinct properties from pure aluminum. The foams are extremely light due to the high porosity .The density of the foam can be controlled by changing the porosity of the foam. The density and structure of the foam both have an impact on the mechanical behavior of the foam. The largest factor in increasing the compression resistance is the density of the foam. Due to its structure and low density, they are extremely effective at energy dissipation, thermal and sound insulation, and lightweight structural applications. (Duarte & Oliveira, 2012) 3.0 Comparison Between Aluminum and Other Construction Materials l 3.1 Steel Aluminum alloys have a higher specific strength than steel, and weight savings up to 50% can be achieved by replacing steel with aluminum without reducing stiffness, despite aluminum’s lower Young’s modulus. A general design rule is that by increasing all the dimensions of a steel cross section by 1.4 except the width, the cross section has the same

stiffness but half the weight. Because of this it is not always necessary to select the highest strength alloys in design. (Gitter, 2008) Another large advantage aluminum has over steel is its corrosion resistance. Steel structures will corrode unless preventative measures are taken, and maintenance for steel structures is expensive while aluminum requires no maintenance and is much more durable. Aluminum can be made into much more complex cross sections easily by extruding it through a die due to its lower modulus of elasticity, whereas most steel sections are hot rolled. Aluminum is more expensive than steel, but can be more economic when considering the maintenance required for steel structures. The other major drawback is that aluminum is relatively new resulting in a lack of design experience and specifications compared to steel structures. (Gitter, 2008; Mazzolani, 2008)

3.2 Concrete Concrete is much cheaper than aluminum and more fire resistant, but it is also significantly heavier. Concrete suffers from durability problems like freeze thaw cycling, and deicing salt penetration whereas these issues do not affect aluminum. Unlike concrete, aluminum can be prefabricated, transported to site and assembled rapidly to minimize construction time. (Mazzolani, 2008; Das, 2007)

4.0 Structural Applications l 4.1 Seismic Applications Aluminum can be used as a shear link in buildings to improve the seismic resistance of the structure. The shear link yields plastically in shear to reduce the amount of lateral force distributed through the structure and absorb a large amount of energy that would otherwise be absorbed by the main structural elements. Aluminum is useful since it has a low yield strength, so the thickness of the web of the shear link can be increased to prevent failure due to buckling . Using soft aluminum alloys also allow the shear link to endure more plastic deformation cycles before tearing. Finally, since aluminum experiences large amounts of strain hardening , after yielding the shear links ultimate shear strength increases significantly. This ensures the shear links on lower stories yield and absorb energy which reduce the transmission of forces, undergo strain hardening to increase their shear strength, and then allow all the other shear links on higher floors to absorb energy and deformations. This behavior prevents deformation from being

concentrated on one floor. (Rai, 1998; Matteis et al, 2008) Figure 9 shows the base shear and roof displacement experienced by an ordinary concentric braced frame (OCBF) with a chevron type brace and a shear linked braced frame (SLBF) shown in figure 8. The base shear experienced by the SLBF is much lower than the OCBF due to the shear links. In addition to lower base shear, SLBFs have a more uniform story drift than OCBFs due to the strain hardening of the shear links, and absorb much more energy. Another advantage of aluminum shear links is that they are easy to replace after an earthquake, and the strength can be adjusted easily by increasing the length. One disadvantage of using aluminum shear links is the increase in roof displacement due to the decreased stiffness, and the shear links have to be protected from galvanic corrosion. (Rai, 1998; Matteis et al, 2008) 4.2 Bridge Decks Aluminum bridge decks can be used to cost effectively rehabilitate structurally deficient bridges, and offer important advantages compared to traditional steel and reinforced concrete bridge decks. Replacing a heavier reinforced concrete deck with an aluminum deck reduces the dead load on foundations, and increases the load carrying capacity of the bridge. Aluminum bridge decks are extremely resistant to corrosion and other durability problems suffered by concrete such as freeze thaw cycling. Since it requires little maintenance and is extremely durable it has a long service life and lower life cycle cost than traditional bridge decks despite the higher initial cost. They are also easy and quick to prefabricate, transport, and assemble which can greatly reduce construction time. Complex structural shapes can be produced because of the extrudability of aluminum, and it does not suffer brittle fracture from low temperatures. Most importantly, bridge decks made from aluminum have sufficient stiffness, strength, and load carrying capacity to allow their use in design.(Siwowski, 2009) One of the main projected markets for aluminum bridge decks is in Quebec, where a large number of bridges are structurally deficient and suffer from durability and maintenance issues. (Walbridge & Chevrotiere, 2012; Viami International Inc, 2013) Sweden has widely adopted aluminum bridge building techniques, having built 35 bridges using a Svensson deck system. It is composed of aluminum extrusions that have female and male connectors which snap together and are then bolted to the substructure. Since there is no welding involved and it is simple to produce, the deck is more economic and does not suffer from the durability issues inherent in other designs. Rehabilitation of some bridges with this deck

system have taken less than 24 hours to complete. The deck weighs around 50 to 70 kg/m^3, which is around 1/10th the weight of a reinforced concrete slab bridge deck. With asphalt pavement the deck weighs around 150 kg/m^3, which is still lighter than anything used by the Quebec Ministry of transportation. Reducing the deal load on foundations that are inadequate for current loads prevents the need for reinforcement. It allows bridges that were designed for lower loads to carry the weight of heavier trucks and vehicles, and also allows for lightweight bridge decks such as timber or steel grid decks to be replaced with aluminum. (Arrien et al, 2001) Two of the largest durability issues for concrete and steel bridges are freeze-thaw cycling and the use of deicing salts on roads. Aluminum decks are waterproof, protecting the substructure below them from exposure to water and deicing salts, do not experience freeze-thaw cycling, and will not experience oxidation from salts. The risk of fatigue failure is greatly reduced as there are no welds. The deck does not require protective coatings or maintenance to prevent corrosion due to the oxide layer. As a result, the service life of this type of aluminum bridge deck is projected to be around 80 years according to the designers. The long service life and the lack of maintenance make the bridge deck economically competitive despite the higher initial cost of aluminum when looking at the total lifecycle cost compared to concrete, steel, or timber bridge decks. (Arrien et al, 2001)

A comparison between the costs of replacing the bridge deck of the Nicolet River Bridge in Quebec with a timber or aluminum deck showed that the Svesson deck design is slightly cheaper. When taking into account that a timber deck has a very short service life and does not protect the substructure due to high permeability, the only advantage it has is that it is a proven technique while the Svensson deck is a relatively new technique. One of the potential obstacles to wider adoption of aluminum bridge decks in Quebec is the current shortage of suppliers in Canada capable of producing large aluminum extrusions. (Arrien et al, 2001; Viami International Inc, 2013) 4.3 Long Span Roof Systems Aluminum is particularly useful in roof systems such as geodesic domes or other roof systems where live loads are small, with the dead loads comprising the majority of the loading. The large area exposed to the environment will not need any protective coating or maintenance, and the light weight of aluminum will help reduce the dead load on the structure. (Mazzolani, 2008) 4.4 Concrete Aluminum Composites As early as the previous century, research was conducted on the possibility of using hybrid concrete-aluminum beams. Several of the bridges built in America during the early 1960s used the fairchild design, with triangular aluminum box girders and a reinforced concrete deck connected together to form a composite system. (Szumigala, 2015) Since the Young’s modulus of aluminum is much closer to concrete than steel, cooperation between the two materials may be increased. The steel sheeting is used to connect the concrete with the aluminum beam, and is zinc plated to increase corrosion resistance. Aluminum has also been used together with glass fibers

to reinforce concrete beams. (Hong, 2014)

4.5 Aluminum foam Aluminum foam is a novel material with very promising prospects as a structural material, and its use has already been adopted in other fields. Research is being conducted on the use of aluminum foam to reinforce sandwich panels. Compressive strength tests were conducted on a stainless steel panel with aluminum foam cores epoxied into the corrugated core, and on empty panels. It was shown that the compressive strength and energy absorption of the hybrid aluminum foam sandwich increased by 211% and 300% respectively compared to the empty panel. Potential uses in structures include crash absorption and extremely lightweight structural elements, in addition to their current use as thermal and acoustic insulation. Aluminum foam, which is a good acoustic and thermal insulator, is placed under a causeway in figure 12 to reduce noise reflection as it is lightweight and resistant to mechanical damage.(Yan et al, 2013; Zu et al, 2012; Kammer, 1999) Table 4 Compressive Strength and Absorbed Energy of Empty and Aluminum Foam Filled Sandwich Panels (Yan, 2013)

5.0 Environmental Impact and Embodied Energy l 5.1 Environmental Impact Primary production of 1 ton of aluminum from bauxite produces 1.6 tons of CO2, and 0.6 tons of perfluorocarbons (PCFs). The amount of CO2 produced cannot be changed, but the aluminum industry has achieved an impressive 85% reduction in the level of PFCs produced since 1990. Around 80-95% of large item aluminum applications are recycled. (The Aluminum Association, 2012)

5.2 Embodied Energy Primary production of 1 ton of aluminum from bauxite requires 15,300 kWh of electricity. The smelting process is energy intensive, comprising 80% of the total energy demand of production. The cost of electricity is around 40% of the overall cost of primary aluminum production. Secondary production of aluminum by recycling is much less energy intensive, and accounted for 61% of production in 2010. (The Aluminum Association, 2012) 6.0 Case Study: Retrofit of a Steel Truss Bridge with an Aluminum Deck A one lane steel truss bridge with a reinforced concrete deck that has experienced severe deterioration is replaced with an aluminum bridge deck to increase the load carrying capacity of the bridge and better understand the performance of aluminum bridge decks. The KY 974 Bridge has an 81 foot deck length and is supported by six steel stringers. The reinforced concrete deck has experienced severe cracking and leaching and will be completely removed and replaced with the aluminum bridge deck. An aluminum bridge deck was chosen primarily due to the decrease in dead load, reduction in maintenance costs, short construction time, and cost effectiveness. Its secondary purpose was to provide an opportunity to gain experience with the design and analysis of the performance of the bridge deck under field conditions. All of the information in this section is taken from the report by the Kentucky Transportation Cabinet, 2012. The design of the aluminum deck is comprised of extruded aluminum profiles of alloy 6005 T6 welded together to form 2m long prefabricated sections that are clamped on the steel stringers, with the welds running perpendicular to the direction of traffic. Each panel has a drain in it that directs water away from the steel supports, and the panel is coated with epoxy to prevent surface wear during asphalt removal. A 3 dimensional finite element analysis was conducted to prove the bridge deck was of adequate stiffness and strength.

To prevent galvanic corrosion resulting from the aluminum touching the steel supports, 2mm rubber bearing pads were epoxied on top of the steel stringers prior to the installation of the deck, and the panels were clamped to the steel stringers with aluminum clamps that have a zinc coated steel nut and spring. 6.1 Increased Load Bearing Capacity The total dead load of the aluminum deck along with the epoxy coating and asphalt layer was 0.74 kN/m2, and this reduction in the dead load increased the load bearing capacity of the bridge by more than 80%. The old concrete bridge deck was designed to carry a 17 ton truck, and the new aluminum bridge deck was designed to carry a 32 ton truck. 6.2 Ease of Construction The panels were manufactured in the Netherlands and shipped to site, and the construction time was reduced due to the ease of installing the prefabricated members. As each panel was installed, it became a load bearing element, which helped with the installation of the next panel. 6.3 Durability The deck protects the steel supports from exposure to rain, and requires no maintenance itself. The bridge was monitored for four years, and the aluminum panels were described as being in perfect condition with no signs of oxidation or visible damage of any kind. Slight corrosion of the clamping mechanisms were reported, with only two clamps experiencing severe corrosion. 7.0 Conclusions It is clear that aluminum is a novel, versatile, and cost effective construction material. It has a wide range of applications from seismic protection, bridge design, roof systems, composite structures, and acoustic insulation. Until recently, it has not been widely adopted in structural design due to high initial costs and lack of design experience. However it is beginning to see wider adoption in design in European countries, and is being evaluated for use in bridges in Quebec because of its superior performance and cost savings. Aluminum offers important advantages such as reduced dead loads, increased durability, and reduced maintenance costs over traditional construction materials due to its unique properties. It is an incredibly useful and cost effective construction material that should be used more frequently in structural design.

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Statement of Authorship

I hereby certify that this technical paper,

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Name of student and has not previously been submitted as part of the requirements of any other course or project at McGill University or elsewhere.

All sources of information used have been fully referenced.

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