5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair This chapter addresses materials,particularly composite materials,that are impor- tant for lightweight structures,the civil infrastructure,joining,and repair.Since the fabrication of a composite material involves the joining of components,an understanding of joining is necessary for the development of composite materials. 5.1 Materials for Lightweight Structures A low mass is desirable for numerous structures,including aircraft,automobile, bicycles,ships,missiles,satellites,tennis rackets,fishing rods,golf clubs,wheel chairs,helmets,armor,electronics,and concrete precasts.In the cases of aircraft and automobiles,reducing the mass saves on fuel.Steel is a widely used structural material,but it is heavy,with a density of 7.9g/cm3. Composite materials with continuous fiber reinforcement and with lightweight matrices are the most attractive of all lightweight structural materials.Composites with discontinuous fillers tend to be inferior in strength and modulus than those with continuous fibers,but they are amenable to fabrication through the use of a wider variety of techniques. 5.1.1 Composites with Polymer,Carbon,Ceramic and Metal Matrices Lightweight matrices are those that exhibit a low density.Examples include poly- mers(with a typical density of less than 1.5 g/cm),carbons(with a typical density of 1.8 g/cm3,which is below the value of 2.26g/cm3 for ideal graphite due to incom- plete crystallinity),ceramics(e.g.,silicon carbide,with a density of 3.3g/cm3 if it is made by hot pressing rather than just sintering),and lightweight metals(e.g., aluminum,with density of 2.7 g/cm',and titanium,with a density of 4.5g/cm). Among metal matrices,aluminum is the most common,due to its low density,its high processability(associated with its low melting temperature of 660C),and its high ductility (associated with its face-centered cubic-fcc-crystal structure). Magnesium is even lower in density(1.7 g/cm3)than aluminum and also has a low melting temperature(650C),but it suffers from its relatively low ductility,which is a consequence of its hexagonal close-packed(hcp)crystal structure.In general, materials with noncubic unit cells have fewer slip systems and hence lower duc- tility than materials with cubic unit cells.Ductility is an important factor for the 131
5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair This chapter addresses materials, particularly composite materials, that are important for lightweight structures, the civil infrastructure, joining, and repair. Since the fabrication of a composite material involves the joining of components, an understanding of joining is necessary for the development of composite materials. 5.1 Materials for Lightweight Structures A low mass is desirable for numerous structures, including aircraft, automobile, bicycles, ships, missiles, satellites, tennis rackets, fishing rods, golf clubs, wheel chairs, helmets, armor, electronics, and concrete precasts. In the cases of aircraft and automobiles, reducing the mass saves on fuel. Steel is a widely used structural material, but it is heavy, with a density of 7.9g/cm3. Composite materials with continuous fiber reinforcement and with lightweight matrices are the most attractive of all lightweight structural materials. Composites with discontinuous fillers tend to be inferior in strength and modulus than those with continuous fibers, but they are amenable to fabrication through the use of a wider variety of techniques. 5.1.1 Composites with Polymer, Carbon, Ceramic and Metal Matrices Lightweight matrices are those that exhibit a low density. Examples include polymers (with a typical density of less than 1.5g/cm3), carbons (with a typical density of 1.8g/cm3, which is below the value of 2.26g/cm3 for ideal graphite due to incomplete crystallinity), ceramics (e.g., silicon carbide, with a density of 3.3g/cm3 if it is made by hot pressing rather than just sintering), and lightweight metals (e.g., aluminum, with density of 2.7g/cm3, and titanium, with a density of 4.5g/cm3). Among metal matrices, aluminum is the most common, due to its low density, its high processability (associated with its low melting temperature of 660°C), and its high ductility (associated with its face-centered cubic – fcc – crystal structure). Magnesium is even lower in density (1.7g/cm3) than aluminum and also has a low melting temperature (650°C), but it suffers from its relatively low ductility, which is a consequence of its hexagonal close-packed (hcp) crystal structure. In general, materials with noncubic unit cells have fewer slip systems and hence lower ductility than materials with cubic unit cells. Ductility is an important factor for the 131
132 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair matrix of a composite material because the reinforcement tends to be strong and brittle,and a brittle matrix causes the composite itself to become very brittle.In contrast,a ductile matrix blunts crack tips upon the emergence of the cracks from the reinforcement,thus making the composite less brittle.Titanium has a relatively high density(4.5g/cm3)and is relatively brittle (due to its hcp structure),but it is still attractive due to its high temperature capabilities(melting temperature 1,668C). Due to the wide spectrum oftemperature capabilities exhibited by the lightweight matrices mentioned above,the choice of matrix material often depends on the tem- perature requirement.Composites with carbon and ceramic matrices are the most attractive for high-temperature applications.Composites with polymer matrices are used for applications that do not involve high temperatures.Composites with metal matrices are used for applications that involve moderately high tempera- tures.However,all of these composites are valuable for some room-temperature applications,as they provide certain special properties.For example,metal-matrix composites are attractive due to their electrical and thermal conductivities,while carbon-matrix composites are attractive due to their corrosion resistance. The carbon matrix has an even lower density than the silicon carbide matrix or the silicon nitride matrix,though it is inferior to them in terms of elastic modulus and tensile strength.The modulus of graphite (isostatically molded)is 12 GPa, compared to 207-483 GPa for silicon carbide.The high modulus of silicon carbide compared to graphite is due to the partially ionic character of the covalent bonding in silicon carbide,in contrast to the absence of ionic character in the covalent bonding in graphite.The tensile strength of graphite(isostatically molded)is 31-69 MPa,compared to 230-825 MPa for silicon carbide(hot pressed). There are a wide range of materials within each class of matrix materials,and the various matrix materials can have different temperature capabilities.For exam- ple,semicrystalline thermoplastic polymers can withstand higher temperatures than amorphous thermoplastic polymers (Fig.4.22),and furthermore,heavily crosslinked polymers can withstand higher temperatures than lightly crosslinked polymers(Fig.4.21),although the temperature capability of any polymer is limited and tends to be inferior to that of a metal. 5.1.2 Cement-Matrix Composites The density of concrete is typically 2.4 g/cm3,which is lower than that of aluminum (2.7 g/cm).However,this density is still higher than those ofpolymers.Lightweight concrete refers to concrete that is lower in density than conventional concrete (achieved through the use of lightweight aggregate). The elastic modulus of concrete is low(25-37 GPa),compared to 380 GPa for aluminum,and 207-483GPa for silicon carbide.The tensile strength of concrete is also low(37-41 MPa),compared to 90 MPa for annealed aluminum alloy 1,100, and 230-825 MPa for hot-pressed silicon carbide.In spite of its low modulus and strength,concrete is attractive as a structural material due to its processability in the field(outside a factory)-it simply requires mixing and pouring,without any need for heating or the application of pressure
132 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair matrix of a composite material because the reinforcement tends to be strong and brittle, and a brittle matrix causes the composite itself to become very brittle. In contrast, a ductile matrix blunts crack tips upon the emergence of the cracks from the reinforcement, thus making the composite less brittle. Titanium has a relatively high density (4.5g/cm3) and is relatively brittle (due to its hcp structure), but it is still attractive due to its high temperature capabilities (melting temperature = 1,668°C). Duetothewidespectrumoftemperaturecapabilitiesexhibitedbythelightweight matrices mentioned above, the choice of matrix material often depends on the temperature requirement. Composites with carbon and ceramic matrices are the most attractive for high-temperature applications. Composites with polymer matrices are used for applications that do not involve high temperatures. Composites with metal matrices are used for applications that involve moderately high temperatures. However, all of these composites are valuable for some room-temperature applications, as they provide certain special properties. For example, metal-matrix composites are attractive due to their electrical and thermal conductivities, while carbon-matrix composites are attractive due to their corrosion resistance. The carbon matrix has an even lower density than the silicon carbide matrix or the silicon nitride matrix, though it is inferior to them in terms of elastic modulus and tensile strength. The modulus of graphite (isostatically molded) is 12GPa, compared to 207–483GPa for silicon carbide. The high modulus of silicon carbide compared to graphite is due to the partially ionic character of the covalent bonding in silicon carbide, in contrast to the absence of ionic character in the covalent bonding in graphite. The tensile strength of graphite (isostatically molded) is 31–69MPa, compared to 230–825MPa for silicon carbide (hot pressed). There are a wide range of materials within each class of matrix materials, and the various matrix materials can have different temperature capabilities. For example, semicrystalline thermoplastic polymers can withstand higher temperatures than amorphous thermoplastic polymers (Fig. 4.22), and furthermore, heavily crosslinked polymers can withstand higher temperatures than lightly crosslinked polymers (Fig. 4.21), although the temperature capability of any polymer is limited and tends to be inferior to that of a metal. 5.1.2 Cement-Matrix Composites The density of concrete is typically 2.4g/cm3, which is lower than that of aluminum (2.7g/cm3).However,thisdensityisstillhigherthanthoseofpolymers.Lightweight concrete refers to concrete that is lower in density than conventional concrete (achieved through the use of lightweight aggregate). The elastic modulus of concrete is low (25–37GPa), compared to 380GPa for aluminum, and 207–483GPa for silicon carbide. The tensile strength of concrete is also low (37–41MPa), compared to 90MPa for annealed aluminum alloy 1,100, and 230–825MPa for hot-pressed silicon carbide. In spite of its low modulus and strength, concrete is attractive as a structural material due to its processability in the field (outside a factory) – it simply requires mixing and pouring, without any need for heating or the application of pressure
5.2 Materials for Civil Infrastructure 133 Concrete is not attractive for lightweight structures because its mechanical properties cannot compete with continuous fiber polymer-matrix composites, even if the cement-matrix composite contains continuous fiber reinforcement. This problem with cement-matrix composites arises because it is difficult for the cement matrix-which is relatively high in viscosity compared to polymer resins-to penetrate the fine space between continuous fibers during composite fabrication(even in the absence of aggregate),in contrast to the relative ease with which a polymer matrix can penetrate this fine space.Inadequate penetration of the fine space means poor bonding between the fibers and the matrix,in addition to high porosity.As a consequence of this inadequate penetration,the fibers are not able to act very effectively as a reinforcement.In other words,the modulus and strength of the resulting composite are lower than the theoretical values obtained by assuming perfect bonding between the fibers and the matrix. 5.2 Materials for Civil Infrastructure Civil infrastructure refers to structures that support the operation of a society. They include highways,bridges,buildings,water pipes,sewage pipes,oil pipes, and electric power distribution lines.Materials used for highways and bridges in- clude concrete,steel,continuous fiber polymer-matrix composites,asphalt(pitch- matrix composites containing aggregates),aggregates,and soil.Materials used for pipes include concrete,iron,and polymers(such as polyvinyl chloride).Most of these materials are composite materials,including particulate,fibrous and layered composites,as described below. Concrete is a cement-matrix composite that contains both fine aggregate(sand) and coarse aggregate(gravel).These aggregates make concrete a particulate com- posite.The use of both fine and coarse aggregates in the same composite allows the total aggregate volume fraction to be higher than what would be obtained if only the fine aggregate or only the coarse aggregate was used.The fine aggregate fills the space between the units of the coarse aggregate,as illustrated in Fig.1.12. The resulting high total aggregate volume fraction leads to a high compressive strength and modulus,in addition to a low drying shrinkage.Concrete provides an example of a particulate composite with multiple particle sizes. Mortar is a cement-matrix composite that contains only the fine aggregate.As a result,mortar has a lower aggregate volume fraction than concrete and is thus not as strong as concrete.However,the absence of the coarse aggregate allows mortar to be used as a relatively thin layer,such that the layer of mortar between two bricks can be used to join the bricks by cementitious bonding. Concrete is much stronger under compression than under tension due to the brittleness of the cement matrix.The aggregates are not sufficient to provide con- crete with the required tensile or flexural properties.Therefore,steel reinforcement is necessary.Concrete with steel reinforcing bars(rebars)is widely used for high- way pavements and bridge decks(Fig.5.1).The rebars make the concrete a fibrous composite.Since the rebars are long (e.g.,as long as the height of a concrete
5.2 Materials for Civil Infrastructure 133 Concrete is not attractive for lightweight structures because its mechanical properties cannot compete with continuous fiber polymer-matrix composites, even if the cement-matrix composite contains continuous fiber reinforcement. This problem with cement-matrix composites arises because it is difficult for the cement matrix – which is relatively high in viscosity compared to polymer resins – to penetrate the fine space between continuous fibers during composite fabrication (even in the absence of aggregate), in contrast to the relative ease with which a polymer matrix can penetrate this fine space. Inadequate penetration of the fine space means poor bonding between the fibers and the matrix, in addition to high porosity. As a consequence of this inadequate penetration, the fibers are not able to act very effectively as a reinforcement. In other words, the modulus and strength of the resulting composite are lower than the theoretical values obtained by assuming perfect bonding between the fibers and the matrix. 5.2 Materials for Civil Infrastructure Civil infrastructure refers to structures that support the operation of a society. They include highways, bridges, buildings, water pipes, sewage pipes, oil pipes, and electric power distribution lines. Materials used for highways and bridges include concrete, steel, continuous fiber polymer-matrix composites, asphalt (pitchmatrix composites containing aggregates), aggregates, and soil. Materials used for pipes include concrete, iron, and polymers (such as polyvinyl chloride). Most of these materials are composite materials, including particulate, fibrous and layered composites, as described below. Concrete is a cement-matrix composite that contains both fine aggregate (sand) and coarse aggregate (gravel). These aggregates make concrete a particulate composite. The use of both fine and coarse aggregates in the same composite allows the total aggregate volume fraction to be higher than what would be obtained if only the fine aggregate or only the coarse aggregate was used. The fine aggregate fills the space between the units of the coarse aggregate, as illustrated in Fig. 1.12. The resulting high total aggregate volume fraction leads to a high compressive strength and modulus, in addition to a low drying shrinkage. Concrete provides an example of a particulate composite with multiple particle sizes. Mortar is a cement-matrix composite that contains only the fine aggregate. As a result, mortar has a lower aggregate volume fraction than concrete and is thus not as strong as concrete. However, the absence of the coarse aggregate allows mortar to be used as a relatively thin layer, such that the layer of mortar between two bricks can be used to join the bricks by cementitious bonding. Concrete is much stronger under compression than under tension due to the brittleness of the cement matrix. The aggregates are not sufficient to provide concrete with the required tensile or flexural properties. Therefore, steel reinforcement is necessary. Concrete with steel reinforcing bars (rebars) is widely used for highway pavements and bridge decks (Fig. 5.1). The rebars make the concrete a fibrous composite. Since the rebars are long (e.g., as long as the height of a concrete
134 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair Concrete Steel Figure5.1.Concretereinforced with anembedded steelrebar.Abeamunder flexure is undertension onone side and under compression on the other side.The rebar is positioned in the part of the beam that is under tension;it is not positioned in the middle plane column),they are considered a form of continuous reinforcement.Thus,steel- reinforced concrete is both a fibrous composite and a particulate composite.In this composite,the particulate composite is concrete,which may be considered the matrix,while the steel rebars are the reinforcement. Cementitious bonding refers to bonding resulting from the adhesiveness of cement.The bonds between aggregate and cement and between a steel rebar and concrete are cementitious.However,this cementitious bonding is weak compared to the bonding resulting from a polymeric adhesive such as epoxy. A steel rebar exhibits surface deformation such that the surface has undulations like ridges.These ridges allow mechanical interlocking between the rebar and the concrete.This mechanical interlocking makes it difficult to pull the rebar out from the concrete.Since the cementitious bonding between the rebar and concrete is not very strong,the mechanical interlocking is a valuable way of enhancing the bond.This provides an example of a reinforcement with a rough surface. Only one rebar is shown in Fig.5.1,but in practice a steel rebar mat is commonly used.A mat is a grid consisting of rebars in two directions that are perpendicular to each other,such that those in one direction are above those in the other direction and are tied (fastened using wires)to those in the other direction.A bridge deck typically has a steel rebar mat in its upper part and another steel rebar mat in its lower part.Vertical concrete beams,called bulb tee beams,with layers ofembedded steel strands placed at selected critical positions at the bottom part of the beam (Fig.5.2)are commonly used for bridges.These are examples of fibrous composites in which the reinforcement is not uniformly distributed but is instead judiciously positioned.For concrete columns,vertical steel rebars and spiral steel wire are commonly used in combination(Fig.5.3),thus providing an example of a fibrous composite that involves multiple geometries of fibrous reinforcement. Prestressed steel strand Figure 5.2.The bottom part of a vertical concrete beam,known as a bulb tee beam,with numerous embedded steel strands(indicated by solid circles)in the direction perpendicular to the page
134 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Steel Concrete Figure5.1. Concretereinforcedwithanembeddedsteelrebar.Abeamunderflexureisundertensionononesideandunder compression on the other side. The rebar is positioned in the part of the beam that is under tension; it is not positioned in the middle plane column), they are considered a form of continuous reinforcement. Thus, steelreinforced concrete is both a fibrous composite and a particulate composite. In this composite, the particulate composite is concrete, which may be considered the matrix, while the steel rebars are the reinforcement. Cementitious bonding refers to bonding resulting from the adhesiveness of cement. The bonds between aggregate and cement and between a steel rebar and concrete are cementitious. However, this cementitious bonding is weak compared to the bonding resulting from a polymeric adhesive such as epoxy. A steel rebar exhibits surface deformation such that the surface has undulations like ridges. These ridges allow mechanical interlocking between the rebar and the concrete. This mechanical interlocking makes it difficult to pull the rebar out from the concrete. Since the cementitious bonding between the rebar and concrete is not very strong, the mechanical interlocking is a valuable way of enhancing the bond. This provides an example of a reinforcement with a rough surface. Only one rebar is shown in Fig. 5.1, but in practice a steel rebar mat is commonly used. A mat is a grid consisting of rebars in two directions that are perpendicular to each other, such that those in one direction are above those in the other direction and are tied (fastened using wires) to those in the other direction. A bridge deck typically has a steel rebar mat in its upper part and another steel rebar mat in its lower part. Vertical concrete beams, called bulb tee beams, with layers of embedded steel strands placed at selected critical positions at the bottom part of the beam (Fig. 5.2) are commonly used for bridges. These are examples of fibrous composites in which the reinforcement is not uniformly distributed but is instead judiciously positioned. For concrete columns, vertical steel rebars and spiral steel wire are commonly used in combination (Fig. 5.3), thus providing an example of a fibrous composite that involves multiple geometries of fibrous reinforcement. Prestressed steel strand Figure 5.2. The bottom part of a vertical concrete beam, known as a bulb tee beam, with numerous embedded steel strands (indicated bysolid circles) in the direction perpendicular to the page
5.2 Materials for Civil Infrastructure 135 Figure 5.3.A concrete column reinforced with vertical straight steel rebars and a steel spiral Figure 5.4.A steel truss.Each line represents a steel beam Steel beams that are fastened together in the absence of concrete are used for trusses,such as that used in a truss bridge(Fig.5.4).The fastened joints in a truss allow deformation,thereby providing the structure with vibration damping. However,the joints tend to suffer from crevice corrosion. Immediately beneath a concrete pavement is a layer of aggregate with little or no cement.This layer is called the base(Fig.5.5),and it provides mechanical stability in addition to drainage.This drainage is enabled by the water permeability of the base and helps to avoid the collection of excess water that can degrade the pavement.Beneath the base is the subgrade (Fig.5.5),which is soil-the most abundant material on Earth.The base and subgrade are critical to the performance of a pavement.The combination of pavement,base,and subgrade provides an example of a layered composite. Asphalt is a particulate pitch-matrix composite.Pitch is a thermoplastic poly- mer that melts upon heating.Thus,the pouring of asphalt requires heating.Like concrete,asphalt has aggregates.It can be used in place of concrete as a pave- ment material.Compared to concrete,asphalt is not durable and is mechanically soft.However,it is advantageous in terms of its vibration damping ability and the consequent improvement of driving comfort.Therefore,asphalt is also used as an overcoat on a concrete pavement.This provides an example of a layered composite involving concrete as one layer and a polymer-matrix composite as the other layer. Cast iron,which is known for its corrosion resistance,has historically been used for water and wastewater pipes.Currently,iron with a spheroidal graphite
5.2 Materials for Civil Infrastructure 135 Figure 5.3. A concrete column reinforced with vertical straight steel rebars and a steel spiral Figure 5.4. A steel truss. Each line represents a steel beam Steel beams that are fastened together in the absence of concrete are used for trusses, such as that used in a truss bridge (Fig. 5.4). The fastened joints in a truss allow deformation, thereby providing the structure with vibration damping. However, the joints tend to suffer from crevice corrosion. Immediately beneath a concrete pavement is a layer of aggregate with little or no cement. This layer is called the base (Fig. 5.5), and it provides mechanical stability in addition to drainage. This drainage is enabled by the water permeability of the base and helps to avoid the collection of excess water that can degrade the pavement. Beneath the base is the subgrade (Fig. 5.5), which is soil – the most abundant material on Earth. The base and subgrade are critical to the performance of a pavement. The combination of pavement, base, and subgrade provides an example of a layered composite. Asphalt is a particulate pitch-matrix composite. Pitch is a thermoplastic polymer that melts upon heating. Thus, the pouring of asphalt requires heating. Like concrete, asphalt has aggregates. It can be used in place of concrete as a pavement material. Compared to concrete, asphalt is not durable and is mechanically soft. However, it is advantageous in terms of its vibration damping ability and the consequent improvement of driving comfort. Therefore, asphalt is also used as an overcoat on a concrete pavement. This provides an example of a layered composite involving concrete as one layer and a polymer-matrix composite as the other layer. Cast iron, which is known for its corrosion resistance, has historically been used for water and wastewater pipes. Currently, iron with a spheroidal graphite