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3.1 Introduction

Composite is a material made by combining two or more constituent materials – often ones that have very different properties, when combined, produce a material with completely different characteristics from the individual components. However, within the composite, constituent materials do not dissolve or blend into each other. The first man-made composite is straw and mud combination to form bricks or walls for building construction [00Lop]. Thousand years ago in ancient Greece, metals were used to reinforce the tension face of concrete beams. Since then the invention of new class of composite materials for structure and construction application is gradually gaining importance to the civil engineers, both for the rehabilitation of existing structures and for the construction of new facilities. So, the glorious entry of polymers in structure and construction application was obvious with its discovery, and first polymer composites were developed during the 1940s for military and aerospace applications. After that polymer composites are used in numerous civil applications, and the most visible application is the pave of roadways in the form of either steel and aggregate reinforced Portland cement or asphalt concrete. The construction sector is the large consumer of polymer composites, and about 30 % of all polymers produced each year are used in the civil engineering and building industries [90Hol].

Polymer matrix-based nanocomposites have generated a significant amount of attention in civil infrastructure application. This area developed with the recognition that nanoscale fillers could yield significant mechanical property advantages as a modification of polymeric systems [08Nju]. The significance of the nanometric size is not only the small scale, but the materials obtain huge specific surface areas at this level, and this huge surface area offers the unique properties of the nanofillers such as mechanical properties and low concentrations along with the advanced characterization, and simulation techniques can bring abrupt changes in the polymer nanocomposites [14Dor]. Among the polymer nanocomposites, fiber-reinforced polymer (FRP) composites are most common and effective for this particular purpose. FRP composites are the combination of polymeric resins, which act as matrices or binders, with strong and stiff fiber assemblies which act as the reinforcing phase [05Tuk], and the combination of the matrix phase with a reinforcing phase produces a new material system with unique properties. Its manufacturing process is also “greener” than concrete and steel, as manufacturing of concrete and steel requires much more energy and water than composite manufacturing. The use of FRP composites can improve stiffness/strength, durability, whole-life cost benefit, environmental impact, and innovation. FRPs provide nonmagnetic and noncorrosive products with higher strength to weight ratios, and it could be applicable for highly seismic zone [97Ham]. So, the use of FRPs has been increasingly considered for structural load-bearing applications by the construction industry and has established themselves as a viable selection for rehabilitation and retrofit of existing civil structures. Depending on the application environment, load performance, and durability requirements, potential applications of FRP composites have been found mainly in bridge decks and superstructures, highways, corrosion-resistant reinforcing bars, structures with longer spans resulting in reduced self weight, seismic column, utility poles, and towers, strengthening and wrapping of in-service structures. Therefore, selection of the polymer matrix and reinforcing components depends on the environment, performance, and end user requirements intended use of the product. Polymer nanocomposites are also used to improve the properties of asphalt, an important construction material for pavement.

3.2 Fiber-Reinforced Polymer

3.2.1 Constituents of Fiber-Reinforced Polymer

3.2.1.1 Polymer Matrix

Polymer matrix plays an important role in FRP. Polymer matrix binds the fibers together, transferring the force between the individual fibers. It disperses the fibers within the composite and protects them from mechanical and environmental damage, delamination, water absorption, chemical attack, high-temperature creep, etc. that eventually cause failure of the composite. And finally, the matrix part can only give shapes to the FRP component. Matrices are made of resins selected for civil infrastructure on the basis of the application, environment, and type of processing. Polymeric resins exhibit an extensive variation of mechanical and physical properties over a wide cost range. A polymer matrix can be made of by both thermoplastic polymer and thermoset polymer [05Tuk]. The basic requirements for a matrix material are it must have lower modulus and greater elongation at break than the fibers it is holding, so that the fibers can carry maximum load. Type of matrix material and its compatibility with the fibers also significantly affect the properties of the composite.

Thermoplastic resins can be softened repeatedly on application of thermal energy. Thermoplastic polymers consist of linear molecules which are not interlinked as shown in Fig. 3.1. The intramolecular force in the chain is extremely strong, but the intermolecular forces of attraction between the adjacent chains are weak. Commonly used thermoplastics are nylon, polyetheretherketine (PEEK), and polyphenylene sulfide (PPS). The long, disconnected polymer chains melt to a viscous liquid at the high processing temperature and after forming are cooled to an amorphous, semicrystalline, or crystalline solid. The degree of crystallinity has a strong effect on the final matrix properties. Thermoplastic resins provide excellent damage tolerance property due to its high-impact strength and fracture resistance [93Mal]. Thermoplastic resins also provide higher strains to failure, which gives better resistance to micro-cracking in the matrix of a composite and a minimum stiffness to prevent buckling of the fibers. Moreover, thermoplastic resins can be recyclable, and fabrication time is less and easy to handle, but its creep resistance and thermal stability are poor. Mainly thermoset resin-based FRPs are used in civil infrastructures. In thermosetting, polymer chains are cross-linked to each other so that the polymer cannot flow or soften by heating as shown in Fig. 3.1. Thermoset resins give better mechanical properties, chemical and thermal stability, and high rigidity and dimensional stability. Thermosets achieve good wet-out between fibers and resins, which results in better creep resistance than thermoplastics. Thermosets are usually brittle in nature. However, stress relaxation behavior is poor in thermoset resin which lowers its strain to failure. They are usually made from liquid or semisolid precursors and manufacture by polycondensation, polymerization, or curing to get tightly bound three-dimensional, hard, and solid polymer. Unlike thermoplastic polymers, once thermosetting polymers are cured, they cannot be reheated and reformed. Common resin materials include polyester, epoxy, polyurethane, vinyl ester, phenol, etc. [03Fib].

Fig. 3.1
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Thermoplastic and thermoset polymers

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Currently polyesters are the most widely used matrix of FRP in construction application. Polyesters are produced by the condensation polymerization of dicarboxylic acids and difunctional alcohols (glycols) [12Sal]. Depending upon the structures of their basic building blocks, unsaturated polyesters are divided into orthopolyester, isopolyester, and vinyl ester. Besides of their good mechanical, chemical, electrical properties, easy processing and relatively low cost make them popular throughout the world [08Nju]. In composite structure isopolyester increases impact resistance, provides greater flexibility and thermal stability, and increases resistance to corrosion than orthopolyester. And vinyl ester has even better impact and corrosion-resistant, superior fatigue, and thermal properties.

Epoxies are mainly used for high-performance composites with superior mechanical properties, resistance to corrosive liquids and environments, superior electrical properties, good performance at elevated temperatures, good adhesion to a substrate, and low shrinkage during the cure. A wide variety of epoxy resin formulations are available providing a broad spectrum of properties.

Phenolics are a class of resins commonly based on phenol and formaldehyde. Phenol is used when there are requirements for high fire resistance, low smoke generation, and flame retardation when subjected to fire. They also have many desirable performance qualities including high-temperature resistance, creep resistance, excellent thermal insulation, sound damping properties, and corrosion resistance.

Polyurethane is prepared by an exothermic reaction of polyisocyanates with polyols. Polyurethane has good impact resistance and rapid curing of the resin, and the resin bonds nicely to a variety of different surface but costs about twice as much as polyester-based composites (Table 3.1).

Table 3.1 Properties of unfilled polymer matrix [02Maz]
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3.2.1.2 Fibers

The primary function of fibers is to carry load along the length of the fiber to provide strength and stiffness in one direction, thermal stability, and other structural properties to the FRP. Reinforcement effects can be oriented to provide tailored properties in the direction of the loads imparted on the end product. A fiber is a material made into a long filament that can be both amorphous and crystalline and usually has a diameter up to 15 μm [89Fel]. The aspect ratio, i.e., the length-to-diameter ratio, can be ranging from thousand to infinity in continuous fibers. There are three types of fibers dominating civil engineering industry: carbon, glass, and aramid fibers [11Pot].

3.2.1.2.1 Carbon Fiber

Carbon fibers are a type of high-performance fiber available for civil engineering application. Carbon fibers are hollow cylinders with diameters typically in the range of 50–500 nm and lengths of a few tens of microns giving high aspect ratios (length/diameter >100) with parallel and homogeneous alignment of nanoscopic graphene layers along the axis. Polyacrylonitrile (PAN) and mesophase pitch (MP) are the two most important carbon fiber precursors. Carbon fibers are manufactured by controlled pyrolysis and crystallization followed by carbonization and graphitization of these organic precursors at temperature range of 1500–2000 °C in inert atmosphere, and the schematic diagram is presented in Fig. 3.2. In this process, carbon crystallites are produced and oriented along the fiber length [88Bun, 09Xia]. Carbon and graphite are both prepared in similar way. Only extended time and temperature processing is required to form this ordered graphite fibers, and these are based on graphene (hexagonal) layer networks present in carbon. When the graphene layers or planes are stacked in three-dimensional orders, graphite formed. The structure of carbon fibers is similar to that of graphite, consisting of carbon atom layers (graphene sheets) arranged in a regular hexagonal pattern, as shown in Fig. 3.3 [67Joh].

Fig. 3.2
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Schematic of PAN and pitch-based carbon fiber manufacturing procedure

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Fig. 3.3
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Chemical structure of carbon fiber

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Carbon fibers have excellent elastic modulus, fatigue strength, low densities, and excellent creep resistance [11Jai]. Carbon fibers are very stiff and strong, 3 to 10 times stiffer than glass fibers. Carbon fiber-reinforced polymers have more service life than aramid and glass fibers. Carbon fiber composites are more brittle (less strain at break) than glass or aramid. Carbon fibers can cause galvanic corrosion when used next to metals. Their disadvantages include inherent anisotropy (reduced radial strength), poor thermal and chemical stabilities in the presence of oxidizing agents, electrical conductivities, galvanic corrosion when used with metals, and costs. Carbon fiber is gray or black in color and is available as dry fabric and prepreg form.

3.2.1.2.2 Glass Fibers

Glass fibers are manufactured by continuously drawing molten glass through small holes in electrically heated platinum bushings at high speeds, approaching 200 miles per hour as shown in Fig. 3.4. And immediately the filaments are cooled from a liquid state at about 1200 °C to room temperature in approximately 10−5 s. After that they are bundled together and bonded to one another by a lubricant or “size” to reduce the abrasive effect of the filaments rubbing against one another, damage to fibers during mechanical handling. Sizing also improves wettability of the fiber surface with the matrix and creates stronger bond between them [93Mal]. There are four types of glass fibers, A, E, C, and AR glass fibers, available in five forms such as chopped fibers, chopped strands, chopped strand mats, woven fabrics, and surface tissue to reinforce the matrix material. “E” glass fiber which is basically an alumino-borosilicate glass is most widely in the construction industry especially with polyester and epoxy resins. “AR” glass fiber has been developed for reinforcing cements, mortars, and concrete due to the presence of increased amount of zircon oxide, and that gives highly resistant to alkali attack. The disadvantages of glass fibers are a relatively low modulus, the low humidity resistance, and the stress rupture on long-term use.

Fig. 3.4
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Schematic diagram of manufacturing of glass fiber

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3.2.1.2.3 Aramid Fiber

Aromatic polyamide fibers, commonly known as aramid fibers, are prepared by the polycondensation reaction between appropriate diamines (m or p-phenylene diamine) and acid chlorides (isophthaloyl or terephthaloyl chloride) followed by the wet spinning from a solution of concentrated sulfuric acid. The procedure is depicted in Fig. 3.5. It has highest strength to weight ratio with the added advantage of toughness or damage/impact resistance. Aramid fibers are insulators of both electricity and heat. They are resistant to organic solvents, fuels, and lubricants. Aramid composites are high temperature and flame resistant but not as good in compressive strength as glass or carbon composites. Strong amide linkage (−CO-NH-), aromatic rings, hydrogen bonds, and chain rigidity in aramid fibers are responsible for all these properties [02Wu]. But aramid fibers are sensitive to moisture, acids, salts, and ultraviolet radiation. There are two forms of fibers, rovings and fabrics, mainly used in civil infrastructure to obtain maximum reinforcement. In roving a bundle of fibers having low twist reinforce the polymer composite in a single dimension, where in order to strengthen the composite in more than one direction of reinforcement, fabric forms are used.

Fig. 3.5
figure 5

Chemical structure of aramid fiber

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3.2.2 Reinforcement Mechanism

The matrix part of the composite has low strength and low modulus, and the fiber is the high-strength and higher modulus component. Under stress the matrix flows and transfers the load to the fiber which finally results in a high-strength and high modulus composite. In a composite fiber is the primary phase that bears the load due to its high mechanical properties and aspect ratio. But this primary phase must be well dispersed and bonded properly with the matrix, the secondary phase. Hence, the fiber–matrix interface plays the most important role in reinforcement mechanism [01Hol]. The interface provides adequate chemical and physical bonding and stability between the primary and secondary phases by coupling between these two phases. It is a dominant factor in the resistance of the composite to corrosive environments. The interface area is small as per Fig. 3.6, but it has significant role in controlling the overall stress–strain performance of the composite.

Fig. 3.6
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Schematic representation of polymer–fiber interaction

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This interface will allow stresses to be dispersed through the matrix and then to be transferred to the reinforcement. It also has a vital role in the failure of the polymeric composites and fracture toughness also. Mainly matrix controls the interfacial phenomena, serves as a medium of load transfer to the fabric, and separates the individual fibers, thereby preventing brittle crack. The mechanical performance and structural integrity of the FRP composites depend on the effectiveness of the bond between polymer and fiber. So the selectivity of the resin phase and its consequent fiber is of significant importance. This interfacial interaction can be improved by wetting the reinforcement with the matrix in the molten or low viscosity state [05Hig].

3.2.3 Composite Fabrication

The fabrication and shaping of composites into finished products often combine the formation of the material itself during the fabrication process. While choosing the technology of manufacturing fiber-reinforced composite for civil infrastructure, number of factors should be considered including final properties of the composite, mechanical compatibility between the reinforcement and the matrix, accuracy of dimensions, quality of the surface, coefficient of thermal expansion, etc. Fabrication of composite materials is accomplished by a wide variety of techniques but few methods related to civil engineering, including hand layup, spray-up, pultrusion, filament winding, and resin transfer molding [14Com].

3.2.3.1 Hand Layup

The oldest, the simplest, and the most commonly used method for the manufacture of both small and large reinforced products is the hand layup technique. In this manual processes, liquid resin is applied to the mold, and fiber reinforcement is placed manually on top as shown in Fig. 3.7. The fibers on this exposed surface must be protected with a resin-rich coating known as a gel coat, and a metal laminating roller is used to impregnate the fiber with resin and remove any trapped air. Several layers of resin and fiber are then applied until the composite attains desired thickness. But in this technique, inconsistency in quality of produced parts, low fiber volume fraction, and environmental and health hazards are concerned.

Fig. 3.7
figure 7

Schematic representation of hand layup process [14Pla]

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3.2.3.2 Spray-Up

Figure 3.8 shows the preparatory stages of spray-up process which are similar to those for hand layup, but the actual technique is much faster and less expensive. In this process, a spray gun is used to apply resin and chopped reinforcements to the mold. During the spray-up preparation, glass fiber roving is fed continuously through a chopping unit, and the resulting chopped strands are projected onto the mold together with a resin jet. The glass fiber resin matrix is then merged with rollers. But in this process, controlling of the fiber volume fraction and thickness is difficult, and therefore dimensional accuracy hampers.

Fig. 3.8
figure 8

Schematic representation of spray-up process [14Pla]

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3.2.3.3 Pultrusion

It is an automated manufacturing process of composites to continuous and constant cross-sectional shape. In this technique, fiber bundles and slit fabrics are pulled from the die through a wet bath of resin and formed into the rough part shape. At this stage appropriate dies can be used to get various types of profiles such as rods, tubes, etc. After that excess of matrix is picked up, and the saturated fiber is extruded from a heated closed die curing while being continuously pulled through die. To cure the formed composite, it is held tightly between rubber blocks and pulled through the die at a predetermined speed and then cut to the desired length by a cutoff wheel. When the fiber is pulled into the processing equipment, the matrix is added by injection. The injection method is an entirely enclosed process in which minimum evaporation of solvents is found. Pultrusion by injection is helpful to control and check the reinforcement. The properties of the finished product depend on the pulling speed and degree of impregnation of the fibers.

3.2.3.4 Filament Winding

Filament winding is a technique in which fiber bundles are pulled through a wet bath of resin and wound over a rotating mandrel at the desired angle to produce high-performance hollow symmetrical products such as pipes, tanks, pressure vessels, and load-bearing tubes. High-speed precise laydown of continuous fiber in predetermined patterns is the basis of the filament winding method. Therefore, starting materials for this process are continuous glass, carbon, or aramid fibers, and liquid thermoset epoxy, polyester, and vinyl ester resins are used in this process. The composite unit is then removed from the mandrel and cured by being placed in an oven enclosure at 60 °C for 8 h. It is a comparatively low-cost process.

3.2.3.5 Resin Transfer Molding

Resin transfer molding (RTM) is a low-cost process in which large, integrated, high-performance products are fabricated. In this process dry reinforced material has been cut and shaped into a preformed piece which is generally called “perform.” These preforms are then more easily laid into the prepared mold cavity. A second mold tool is then clamped over the first, and a pressurized mixture of thermoset resin, catalyst, color, filler, etc. is injected into the cavity using dispensing equipment to form structural parts. Once all the reinforced material is wet out, the resin inlets are closed, and the laminate is allowed to cure. The mold can also be heated to accelerate the cure of the resin and so reduce cycle times.

3.2.4 Properties of Carbon Fiber-Reinforced Composites

Carbon fibers are a type of high-performance fiber available for civil engineering application. Carbon fiber consists of very thin strands of the element carbon. Carbon fibers have high tensile strength and are very strong for their size. In fact, carbon fiber might be the strongest material. Carbon fibers have high elastic modulus and fatigue strength. They are highly chemically resistant and have high-temperature tolerance with low thermal expansion and corrosion resistance. These improve the service life of carbon fiber-reinforced composites.

Advantages of carbon fiber used for construction application

3.2.4.1 High Specific Strength

Carbon fiber has high strength to weight ratio, i.e., high specific strength. Carbon fiber is strong and light and due to these, it has a favorable strength/weight ratio. So, the composite usually provides these properties at substantially less weight than metals: their specific strength and modulus per unit weight is almost five times that of steel or aluminum.

3.2.4.2 Rigidity

Deflection of carbon fiber under stress is very less which can be measured by its Young’s modulus.

3.2.4.3 Corrosion Resistant

Carbon fiber has high corrosion resistance. Carbon fiber does not easily deteriorate under UV radiation so it can protect the composite from sunlight.

3.2.4.4 Chemical Stability

Carbon fiber has good chemical resistance. Carbon fibers are made of carbon where carbon content is more than 95 %. Long carbon polymer chain is present in carbon fiber and does not have any free bonds or sites to react. Hence, most of the chemicals in low concentration have no effect on the fibers. This makes it possible to use carbon fiber in concrete structures, which works in aggressive environment conditions.

3.2.4.5 Fatigue Resistance

Many civil construction experiences fatigue loading, in which the internal stresses vary with time. These fatigue stresses can lead to failure, even when the maximum stress is much less than the failure strength of the material. Carbon fiber has good fatigue resistance, and failure is unlikely to be a problem when cyclic stresses coincide with the fiber orientation. The orientation of the fibers and the different fiber layer orientation have a significant influence on fatigue.

3.2.4.6 Tensile Strength

Carbon fiber has good tensile strength, but its brittleness does not always fail at the same stress level due to both surface and internal flaws and fail at small strains. Composites can be made anisotropic and have different tensile properties in different directions, and this can be used to design a more efficient structure.

3.2.4.7 Fire Resistance/Nonflammable

Depending upon the manufacturing process and the precursor material, carbon fiber can be quite soft and can be made into or more often integrated into protective clothing for firefighting. Nickel-coated fiber is an example. Because carbon fiber is also chemically very inert, it can be used where there is fire combined with corrosive agents. Carbon fiber blanket is used as welding protection.

3.2.4.8 Low Coefficient of Thermal Expansion

This is a measure of how much a material expands and contracts when the temperature goes up or down, and this is important. Thermal expansion is an important property for extraordinary cases, as it determines saving of shape and rigidity under high temperatures. Also this property is very important for long-span structures. Carbon fiber can have a broad range of coefficient of thermal expansion −1 to 8+, depending on the starting source and manufacturing process. Low coefficient of thermal expansion makes carbon fiber suitable for applications where small movements can be critical.

3.2.5 Disadvantages of Carbon Fiber Used for Construction Application

  • Cost of carbon fiber is relatively high.

  • Carbon fiber is brittle in nature and carbon fiber-reinforced composite has low strain to failure.

  • Compressive strength of the composite is lower, and larger diameter fiber does not give improved compression properties. It also has poor impact strength.

  • Carbon fiber is electrically conductive. This feature can be useful and be a nuisance. Accumulated carbon fiber dust can cause sparks and thereby short circuits in electrical appliances and equipment. Careful preparation and installation of composite can reduce this problem.

  • Thermal conductivity of carbon fiber is also a disadvantage for civil applications. The composite oxidizes in air at temperatures above 450 °C.

3.2.6 Factors Affecting Mechanical Properties of the Composites

The properties of FRP-reinforced composites depend on the mechanical properties of matrix and fibers used in the composite, properties of the fiber–matrix bond, fiber orientation within the matrix material, and volume ratio of fiber to matrix and cross-sectional area of fiber.

3.2.6.1 Properties of Matrix and Fibers

Properties of matrix and fibers are already discussed in previous sections. Fibers with high strength and high stiffness are inserted and merged into the low modulus continuous polymeric matrix in fibrous polymeric nanocomposites; each of the individual phases must accomplish particular functional requirements based on their mechanical properties so that the system containing them may succeed satisfactorily as a composite.

3.2.6.2 Properties of the Fiber–Matrix Bond

The behavior of a composite material is depended on the combined behavior of the reinforcing fiber, the polymer matrix, and the fiber/matrix interface. To attain superior mechanical properties, the interfacial adhesion via the fiber/matrix interface should be strong. Fibers are embedded into the matrix molecules by chemical reaction or adsorption, which determine the extent of interfacial adhesion. Good fiber–matrix interaction results strong interface and overall a good composite.

3.2.6.3 Fiber Orientation Within the Matrix Material

In the case of FRP composites, the reinforcing fibers constitute the backbone of the material, and they determine its strength and stiffness in the direction of fibers. Fibrous reinforcement represents physical rather than a chemical means of changing the composite to suit various engineering applications. Discontinuous fibers or whiskers having either random or biased orientation are mainly used to improve properties or lower the cost of isotropic materials. Continuous fiber composites can be either single layer or multilayered. Orthotropic composites with long fibers in one direction are called continuous fiber reinforcement, and composite in which short or staple fibers are embedded in the matrix has discontinuous fiber reinforcement. Higher mechanical properties can be obtained in a particular composite by varying the fiber orientation than by fiber aspect ratio.

The composite is stronger along the direction of orientation of the fibers and weakest in a direction perpendicular to the fiber. Figure 3.9 depicted different fiber orientation, and Fig. 3.10 shows the effect of fiber orientation on tensile strength. In continuous fiber composites, fibers are oriented in one direction to produce enhanced mechanical strength. In short fiber composites, the length of short fiber does not allow to entangle with each other. Hence, moderate strength composite is obtained, and well-dispersed short fibers provide uniform reinforcement. To achieve effective strengthening and stiffening, the fibers must be larger than a critical length, which is defined as the minimum length at which the center of the fiber reaches the ultimate (tensile) strength when the matrix achieves the maximum shear strength.

Fig. 3.9
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Different fiber orientation

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Fig. 3.10
figure 10

Effect of fiber orientation on tensile strength

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A laminate is fabricated by stacking a number of laminae in the thickness direction. Multilayered composite, i.e., laminate, is fabricated by stacking polymer and fibers alternatively in the thickness direction, and it gives good compression strength. But for flexural strength, the laminates with fiber orientation of 45° have more flexural strength than the laminates with 0° or 90° orientation for the same type of the fiber reinforcement. Matrix and fiber behavior follows iso-strain approximation until the onset of failure; it is possible to predict the tensile and compressive strength in the fiber direction [89Sel]. The damage tolerance of polymeric materials can be enhanced by improving the interlaminar properties of the polymer composites [02Wan].

3.2.6.4 Volume Ratio of Fiber to Matrix

Short carbon fiber shows a great strengthening and toughening effect for a low volume percentage of fibers (3–5 vol.%). Although composites formed under pressure usually have a higher fiber volume fraction, and the strengthening and toughening effect of short carbon fibers reduces. The reason behind this may be due to damage in fibers, formation of high shear stresses of intersect between fibers and strong interface cohesion of fiber/matrix. The predominant strengthening and toughening effects at low volume % may be due to the formation of the reasonable network structure of short carbon fibers as well as the apparent fiber bridging and pulling out [09Lin].

3.2.6.5 Fiber Cross-Sectional Area

Composite with reduced fiber diameter provides considerably more fiber surface area than the composite with higher fiber diameter where in both cases keeping the fiber volume fraction same. The increased surface area can help to minimize the ineffective fiber length. The ineffective fiber length representing the fiber region where the axial stress is not fully developed and ineffective fiber length reduction will balance the imperfect bonding between the fiber–matrix interface. Therefore, uniform stress distribution will be found with less overall mechanical damage. Composites with reduced fibers also offered reduced shear stress developed in the fiber–matrix interface. Therefore, composite materials with reduced diameter fiber are expected to offer higher strength than a conventional composite prepared with the same volume fraction of fiber [10Hos].

3.2.7 Applications of Carbon Fiber-Reinforced Composite in Structure and Construction

In recent years, carbon fiber-reinforced composites have emerged as a potential solution to the problems associated with infrastructures. The usual applications of carbon fibers are strengthening constructions, retrofitting of old structures, reinforcing precast concrete. Carbon fibers are also utilized as a replacement for steel, pre-stressing materials and strengthening cast-iron beams. Its high strength, light weight and resistance to corrosion make it an ideal for structural engineering applications. The composites are used in form of tension elements, ropes, dismountable and permanent shuttering, and prestressing bars.

3.2.7.1 Strengthening of the Constructions

Since 1982, FRP composites have been successfully used to strengthen concrete structure, and these days applying carbon fibers in building engineering is one of the most common alternatives [95Mei]. Carbon fibers are applied in fabric or pultruded strip forms, and the product is a thin strip of unlimited length having unidirectional fibers. These prefabricated strips are glued to the surface, and the fabrics improve strength and rigidity. The glue is responsible only for the contact between the carbon strip and the surface of the member. The main advantages of this reinforcing for wooden structures in comparison with traditional methods are fast and easy to install, possible to make it invisible. Bidirectional canvas gluing onto the bottom surface of the beam is also very popular where tensile stresses cause cracks along the element axis. In another way, resin layer is applied on the surface, and the fibers are put into this resin layer in situ in the form of fabrics. The fabrics are covered again with a resin layer. This procedure can be repeated several times and provide strength to the construction [03Has]. Strengthening of the concrete deck in the transverse direction by this method is accepted in Europe, Japan, and North America due to its less electrolytic corrosion, high strength, low transportation cost, easy application even in small spaces, no need for scaffolding to support the strengthening materials during hardening of adhesive, no limitation in length, and reduction of height below the strengthened member.

3.2.7.2 Using of Carbon Fiber in Precast Concrete

This field is comparatively new and rapidly developing. Carbon fibers in precast concrete started to appear in quantity production from 2003. Now it is a very common material for precast elements in the USA. Carbon fiber used in precast concrete gives noncorrosiveness, low weight, and thermal efficiency and due to these carbon fiber grid is very useful in the panel faces to replace steel mesh reinforcement and as a mechanical link to the outer and inner sections of the concrete wall as shown in Fig. 3.11.

Fig. 3.11
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Carbon fiber used in precast concrete [14Acs]

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3.2.7.3 Carbon Fiber in Bridge Construction

A prospective field of applying carbon fiber is bridge construction. Easier, faster, and more economic installation makes this technique well conventional, and these types of bridges are applicable in any extreme environment (de-icing salts, chemicals). Minimal maintenance requirements, through-life costs, and disruption are the extra advantages of using carbon fiber in bridge [06Ban]. Carbon fiber-reinforced composites are thermally stable which may remove need for expansion joints. So, since 1992 there have been built several carbon fiber bridges. In Fig. 3.12 carbon fiber bridge in Spain is shown.

Fig. 3.12
figure 12

Carbon fiber bridge in Spain [14Bus]

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3.2.7.4 Seismic Retrofit

One of the most severe problems faced by the earthquake engineers in the past decades was large number of masonry buildings in seismic regions that were damaged due to earthquake. A large lateral cyclic earthquake force can degrade strength of concrete and reinforcing bar that can result in premature failure of column. Seismic rehabilitation techniques for civil construction involving steel jacketing, concrete jacketing, and fiber-reinforced composite jackets for columns have been developed. Retrofit of columns to withstand earthquakes is a recent and extensive challenge for civil engineers. Seismic resistance of retrofitted columns by carbon fiber reinforcement improves significantly because of confining action of the carbon fiber jacket wraps [96Chr, 97Saa]. The technique has been examined to recover displacement ductility as well as strength [08San]. The repaired structures also exhibit lower rate of deterioration under large reversal cyclic loading than the virgin columns. Fiber-reinforced wraps consisting of carbon fibers, bonded with epoxy resins, have been successfully applied for seismic rehabilitation of bridge piers in the USA and Japan [92Muf].

3.2.7.5 Other Uses of Carbon Fiber

Carbon fibers can also be used for construction of fiber-reinforced plastic profiles. These high-strength aggregates with low weight and low deflection make this solution suitable for construction in intricate structures. High-loaded floors and roads can be constructed by carbon fiber because of low thermal conductivity in comparison with steel and good cohesion with concrete. In fiber–cement, carbon fibers replace asbestos, because carbon fiber can reduce inhalation problems. Considering all its very properties, it can be used to repair dams, tubes, high pressure pipes, and historic monuments.

3.3 Clay- and Carbon-Based Polymer Nanocomposites Used in Road Application

It is widely recognized that transport infrastructure plays a key role in economic growth and development of a country. In India, flexible pavements constitute over 95 % of total road network. Bitumen is widely used as a binder of mineral aggregates for flexible pavement construction. The rapid growth in traffic in terms of commercial vehicles, overloading of commercial vehicles, and significant variation in daily and seasonal temperatures are the causes of early development of rutting and cracking in the bituminous layers. Bituminous binders deform when subjected to loads. The tensile deformation is responsible for the fatigue damage and consequently results in the fatigue failure of the pavements. Similarly, compressive deformation causes permanent deformation or rutting [03Rea]. Bituminous pavements also get damaged by temperature and moisture. Moreover, it is also observed that the entry of water through the surface cracks during monsoon season leads to the rapid deterioration of the bituminous surfacing.

Nanoclay modifications of bitumen help to increase the stiffness and aging resistances. Nanoclay modification improves the rutting resistances of bitumen as well as cyclic fatigue resistance [11Zha]. The effective use of a montmorillonite (MMT) on properties of styrene–butadiene–styrene copolymer modified bitumen by melt blending increases the viscosity and gives higher complex modulus and lower phase angle, implying stiffer and more elastic asphalt. Therefore, the nanoclay-modified bitumen was determined to have good rutting resistance compared to the virgin bitumen [07Yu].

Various fiber modifiers have been used widely to enhance material strength, fatigue resistance, and ductility properties of bitumen [96Ree]. Fibers offer an excellent potential for bitumen modification by carrying tensile forces and also help in preventing the formation and propagation of cracks because of their excellent mechanical properties [96Ser]. The use of carbon fiber shows consistency of the binder, increases stability and voids, and decreases the flow value of the pavement. Further, carbon fiber has the potential to improve structural resistance to distress occurring in road pavement due to traffic loads. Further, addition of fiber improves fatigue life and permanent deformation of bituminous mixtures by improving mix stiffness [08Jah]. Similarly, the incorporation of carbon nanoparticles in bitumen matrix increases the failure temperature, complex modulus, and elastic modulus values and improves aging properties [11Xia].