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
136 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair Longitudinal Pavement 日ase Subgrade Pavement Subgrade Figure 5.5.A pavement with base(gravel layer)undemeath and subgrade(soil layer)undemeath the base.The base layer allows water to flow through it in order to avoid the local collection of water.The longitudinal direction refers to the direction of traffic precipitate-known as ductile iron-is used instead.Ductile iron is akin to but different from cast iron,which has flake graphite precipitate.Due to the spheroidal graphite precipitate,ductile iron is stronger and more ductile than cast iron.This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection.Corrosion is particularly serious when the water is acidic.It causes tuberculation,which refers to the formation of small mounds of corrosion products on the inside surface of a pipe.Due to the centrifugal casting used to manufacture iron pipes,there is variation in the wall thickness along the length of a pipe.As a result,the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement,but may have asbestos re- inforcement.Asbestos fibers are an effective reinforcement,but their carcinogenic character results in health concerns.Due to the water in the pipe and in the soil surrounding the pipe,corrosion is an issue.As a result,the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication,since the creation of a composite involves the joining of various components,such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite
136 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Pavement Base Subgrade Pavement Subgrade Base Longitudinal Figure 5.5. A pavement with base (gravel layer) underneath and subgrade (soil layer) underneath the base. The base layer allows water to flow through it in order to avoid the local collection of water. The longitudinal direction refers to the direction of traffic precipitate – known as ductile iron – is used instead. Ductile iron is akin to but different from cast iron, which has flake graphite precipitate. Due to the spheroidal graphite precipitate, ductile iron is stronger and more ductile than cast iron. This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection. Corrosion is particularly serious when the water is acidic. It causes tuberculation, which refers to the formation of small mounds of corrosion products on the inside surface of a pipe. Due to the centrifugal casting used to manufacture iron pipes, there is variation in the wall thickness along the length of a pipe. As a result, the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement, but may have asbestos reinforcement. Asbestos fibers are an effective reinforcement, but their carcinogenic character results in health concerns. Due to the water in the pipe and in the soil surrounding the pipe, corrosion is an issue. As a result, the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication, since the creation of a composite involves the joining of various components, such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite
5.3 Materials for Joining 137 Due to the limited size of a structural component (for example,the size of a panel is limited by the equipment used to fabricate it),most structures involve the joining of components.The structural integrity of the resulting joints is critical to the mechanical integrity of the overall structure. Joining may be achieved by sintering,welding,brazing,soldering,adhesion,or fastening.This section covers various types of joints. 5.3.1 Sintering or Autohesion Diffusive adhesion refers to bonding that results from the diffusion of certain species from one party to another.Sintering is one of the forms of diffusive adhe- sion.It involves solid-state diffusion.Sintering commonly refers to powder met- allurgy-the bonding of solid particles due to the solid-state diffusion of atoms between adjacent particles,as illustrated in Fig.1.7 and shown in Fig.5.6 for the sintering of silver particles.In the field of thermoplastic polymers,sintering is known as autohesion,which refers to diffusive adhesion at temperatures above the glass transition temperature Tg and below the melting temperature Tm. In liquid-state sintering,at least one but not all of the elements exist in the liquid state.This means that liquid-state sintering (a rather confusing term)involves solid-state diffusion that is supplemented by the presence of a liquid,the flow of which aids the sintering.For example,the fabrication of a silicon carbide whisker copper-matrix composite may be achieved by sintering a mixture of silicon car- bide whiskers and copper particles at a temperature slightly below the melting temperature of copper,so that only solid-state diffusion occurs(i.e.,the admix- ture method).The diffusion involves copper and does not involve silicon carbide, which has a much higher melting temperature than copper.The addition of a mi- nor proportion of zinc particles to the mixture causes the presence of zinc liquid during the sintering,since the melting temperature of zinc is much less than that of 1Hm30000X Figure 5.6.Scanning electron microscope photograph of the surface morphology of a silver particle organic-based thick-film paste on an alumina substrate after heating(known as firing)in air at 300C for 30 min.The heating causes the buring out of the organic vehide,in addition to the sintering of the silver particles.The arrows show the necks between adjacent silver partices.The silver particles are irregularly shaped,with sizes ranging from 1.5 to 2.5 um.(From [1])
5.3 Materials for Joining 137 Due to the limited size of a structural component (for example, the size of a panel is limited by the equipment used to fabricate it), most structures involve the joining of components. The structural integrity of the resulting joints is critical to the mechanical integrity of the overall structure. Joining may be achieved by sintering, welding, brazing, soldering, adhesion, or fastening. This section covers various types of joints. 5.3.1 Sintering or Autohesion Diffusive adhesion refers to bonding that results from the diffusion of certain species from one party to another. Sintering is one of the forms of diffusive adhesion. It involves solid-state diffusion. Sintering commonly refers to powder metallurgy – the bonding of solid particles due to the solid-state diffusion of atoms between adjacent particles, as illustrated in Fig. 1.7 and shown in Fig. 5.6 for the sintering of silver particles. In the field of thermoplastic polymers, sintering is known as autohesion, which refers to diffusive adhesion at temperatures above the glass transition temperature Tg and below the melting temperature Tm. In liquid-state sintering, at least one but not all of the elements exist in the liquid state. This means that liquid-state sintering (a rather confusing term) involves solid-state diffusion that is supplemented by the presence of a liquid, the flow of which aids the sintering. For example, the fabrication of a silicon carbide whisker copper-matrix composite may be achieved by sintering a mixture of silicon carbide whiskers and copper particles at a temperature slightly below the melting temperature of copper, so that only solid-state diffusion occurs (i.e., the admixture method). The diffusion involves copper and does not involve silicon carbide, which has a much higher melting temperature than copper. The addition of a minor proportion of zinc particles to the mixture causes the presence of zinc liquid during the sintering, since the melting temperature of zinc is much less than that of Figure 5.6. Scanning electron microscope photograph of the surface morphology of a silver particle organic-based thick-film paste on an alumina substrate after heating (known as firing) in air at 300°C for 30min. The heating causes the burning out of the organic vehicle, in addition to the sintering of the silver particles. The arrows show the necks between adjacent silver particles. The silver particles are irregularly shaped, with sizes ranging from 1.5 to 2.5μm. (From [1])
138 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair 25 ●Cu/SiCw Coated Cu/SiCw Admixture 20 ▲Brass/SiCw Coated △Brass/SiCw Admixture IOA) 15 10上 0 102030405060 Vol.SiCw Figure 5.7.Variation of the porosity with the silicon carbide whisker volume fraction in copper-matrix and brass-matrix composites made by the coated filler method and the admixture method.(From[2]) copper.Thus,the presence of zinc results in liquid-state sintering;in other words, the solid-state diffusion of copper that is supplemented by the dissolution of cop- per in the zinc liquid,which flows,thereby enhancing the mass transport during sintering.Figure 5.7 shows that,for the same filler volume fraction,the presence of zinc results in a composite of lower porosity,whether or not the conventional admixture method of powder metallurgy is used for the composite fabrication (Sect.1.3.2).If the coated filler method of powder metallurgy is used,the presence of zinc does not make much difference to the porosity of the composite since this method is highly effective at producing composites of low porosity.The presence of zinc in the copper matrix makes the matrix brass. 5.3.2 Welding A welded joint refers to a joint made by melting parts of the two members involved in the joint at their interface and their subsequent solidification upon cooling.The joining commonly arises from the presence of van der Waals forces-electrostatic interactions between the electric dipoles of one member and those of the other member.These dipoles may be permanent or temporary.Joining based on van der Waals forces is known as dispersive adhesion or adsorption.However,excess material (called the filler metal)that is ideally of the same composition as the members is placed at or around the joint to provide additional mechanical support. The filler metal undergoes melting during welding.Figure 5.8 shows an I-beam (i.e.,a beam in the shape of the letter I)obtained by welding a vertical member and two horizontal members together.The filler metal takes the form of a fillet at the
138 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Figure 5.7. Variation of the porosity with the silicon carbide whisker volume fraction in copper-matrix and brass-matrix composites made by the coated filler method and the admixture method. (From [2]) copper. Thus, the presence of zinc results in liquid-state sintering; in other words, the solid-state diffusion of copper that is supplemented by the dissolution of copper in the zinc liquid, which flows, thereby enhancing the mass transport during sintering. Figure 5.7 shows that, for the same filler volume fraction, the presence of zinc results in a composite of lower porosity, whether or not the conventional admixture method of powder metallurgy is used for the composite fabrication (Sect. 1.3.2). If the coated filler method of powder metallurgy is used, the presence of zinc does not make much difference to the porosity of the composite since this method is highly effective at producing composites of low porosity. The presence of zinc in the copper matrix makes the matrix brass. 5.3.2 Welding A welded joint refers to a joint made by melting parts of the two members involved in the joint at their interface and their subsequent solidification upon cooling. The joining commonly arises from the presence of van der Waals forces – electrostatic interactions between the electric dipoles of one member and those of the other member. These dipoles may be permanent or temporary. Joining based on van der Waals forces is known as dispersive adhesion or adsorption. However, excess material (called the filler metal) that is ideally of the same composition as the members is placed at or around the joint to provide additional mechanical support. The filler metal undergoes melting during welding. Figure 5.8 shows an I-beam (i.e., a beam in the shape of the letter I) obtained by welding a vertical member and two horizontal members together. The filler metal takes the form of a fillet at the
5.3 Materials for Joining 139 Figure 5.8.A steel I-beam,where the components have been joined by welding junction between the vertical member and the horizontal member being joined. The molten filler metal penetrates the space between the two members,although this penetration is not shown in Fig.5.8.This type of weld is called a fillet weld, which is widely used to make lap joints,corner joints,and T-joints. Because the heat associated with welding affects the microstructure of the mem- bers near the joint,the members next to the joint region tend to have a different microstructure than the parts of the members away from the joint region.The zone where the heat has affected the microstructure is known as the heat-affected zone, which is immediately next to the fusion zone(the zone that underwent melting during welding).The change in microstructure is usually undesirable in terms of mechanical properties,as it can take the form of precipitate coalescence(thereby forming larger precipitates),grain growth,and recrystallization.In addition,ther- mal expansion of the members in the heat-affected zone during welding can cause thermal stress.Therefore,subsequent use of the welded structure under dynamic loading tends to cause fatigue cracks near the toe(tip)of a fillet weld,as illustrated in Fig.5.9. A special type of welding involves the joining of members in the form of a ther- moplastic polymer,which melts upon heating and subsequently solidifies upon cooling.The joining results from the presence of van der Waals forces,as in the Fatigue crack Weld Figure 5.9.A fatigue crack near the toe of the fillet of a weldjoining two members that are perpendicular to one another
5.3 Materials for Joining 139 Figure 5.8. A steel I-beam, where the components have been joined by welding junction between the vertical member and the horizontal member being joined. The molten filler metal penetrates the space between the two members, although this penetration is not shown in Fig. 5.8. This type of weld is called a fillet weld, which is widely used to make lap joints, corner joints, and T-joints. Because the heat associated with welding affects the microstructure of the members near the joint, the members next to the joint region tend to have a different microstructure than the parts of the members away from the joint region. The zone where the heat has affected the microstructure is known as the heat-affected zone, which is immediately next to the fusion zone (the zone that underwent melting during welding). The change in microstructure is usually undesirable in terms of mechanical properties, as it can take the form of precipitate coalescence (thereby forming larger precipitates), grain growth, and recrystallization. In addition, thermal expansion of the members in the heat-affected zone during welding can cause thermal stress. Therefore, subsequent use of the welded structure under dynamic loading tends to cause fatigue cracks near the toe (tip) of a fillet weld, as illustrated in Fig. 5.9. A special type of welding involves the joining of members in the form of a thermoplastic polymer, which melts upon heating and subsequently solidifies upon cooling. The joining results from the presence of van der Waals forces, as in the Figure 5.9. A fatigue crack near the toe of the fillet of a weld joining two members that are perpendicular to one another
140 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair case of the welding of metals.Although a thermoplastic material softens at the glass transition temperature,it needs to be heated to temperatures close to or beyond the melting temperature in order for it to be sufficiently fluid for a strong bond to form.The required temperature depends on the pressure used to hold the adjoining surfaces together during the joining operation.The higher the pressure, the lower is the required minimum temperature. Polyphenylene sulfide(PPS)is a high-temperature thermoplastic polymer with a thermosetting/thermoplastic character.Its melting temperature is 280C and its glass transition temperature is 90C in the material used to obtain the results given below.Due to the CTE mismatch between crossply laminae of carbon fiber PPS-matrix composite,thermal stress arises during cooling after the bonding has been conducted at an elevated temperature.As a result,debonding can take place during cooling.This debonding can be monitored in real time by measuring the contact electrical resistance of the bonding interface.Upon debonding,the resis- tance increases abruptly.Figure 5.10 shows the effect of the bonding temperature on the bond between(PPS)surfaces,with the pressure fixed at 4.8 x 103Pa during the bonding and subsequent cooling.The heating rate up to the bonding temper- ature is 10C/min and the temperature is held at the bonding temperature for 5h. When the bonding temperature is 260,280,or 285C(i.e.,below the melting tem- perature,at the melting temperature,or just 5C above the melting temperature), debonding occurs during the cooling,so that the bonding process fails.However, when the bonding temperature is 290C(i.e.,10C above the melting temperature), 400 300 150 300 350 300 100 250 200 200 50 150 150 100 100 0 15 防 -100 10 15 20 Time (h) b Time(h) 300 300 300 250 250 200 200 20 200 100 150 150 100 100 50 50 分 250 -100 15 0 15 20 Time (h) d Time(h) Figure 5.10.Effect of PPS-PPS bonding temperature on the variation in contact electrical resistance at a constant pressure of4.8×103Pa.a260℃：b280℃c285℃d290℃.(From[3)
140 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair case of the welding of metals. Although a thermoplastic material softens at the glass transition temperature, it needs to be heated to temperatures close to or beyond the melting temperature in order for it to be sufficiently fluid for a strong bond to form. The required temperature depends on the pressure used to hold the adjoining surfaces together during the joining operation. The higher the pressure, the lower is the required minimum temperature. Polyphenylene sulfide (PPS) is a high-temperature thermoplastic polymer with a thermosetting/thermoplastic character. Its melting temperature is 280°C and its glass transition temperature is 90°C in the material used to obtain the results given below. Due to the CTE mismatch between crossply laminae of carbon fiber PPS-matrix composite, thermal stress arises during cooling after the bonding has been conducted at an elevated temperature. As a result, debonding can take place during cooling. This debonding can be monitored in real time by measuring the contact electrical resistance of the bonding interface. Upon debonding, the resistance increases abruptly. Figure 5.10 shows the effect of the bonding temperature on the bond between (PPS) surfaces, with the pressure fixed at 4.8 × 103 Pa during the bonding and subsequent cooling. The heating rate up to the bonding temperature is 10°C/min and the temperature is held at the bonding temperature for 5h. When the bonding temperature is 260, 280, or 285°C (i.e., below the melting temperature, at the melting temperature, or just 5°C above the melting temperature), debonding occurs during the cooling, so that the bonding process fails. However, when the bonding temperature is 290°C (i.e., 10°C above the melting temperature), Figure 5.10. Effect of PPS–PPS bonding temperature on the variation in contact electrical resistance at a constant pressure of 4.8 × 103 Pa. a 260°C; b 280°C; c 285°C; d 290°C. (From [3])