The impact of graphene oxide on cementitious composites

The impact of graphene oxide on cementitious composites

4

Alyaa Mohammed, Jay G. Sanjayan, Ali Nazari, Nihad T.K. Al-Saadi Swinburne University of Technology, Hawthorn, Melbourne, Victoria, Australia

4.1

Introduction

Service life and design of concrete structures should normally take into account both strength and durability to control the efficiency of the final product. All sorts of applied loads including compressive, shear, tensile, etc. can show the capacity of concrete structure under specific requirements. Different factors including but not limited to mix design, structural design, and curing affect the strength of the final concrete product (Mehta and Monteiro, 1993; Neville, 2011). A literature survey shows that in the past two decades, there have been extensive efforts to formulate the strength properties of concrete. Keeping on researching for suitable formulations with high attention shows the importance of finding reliable concrete in every practice. Attempts to increase the strength of concrete has led to the advent of ultra-high performance concrete with compressive strength values of 150 MPa or higher (Allena and Newtson, 2011; EL-Attar et al., 2015). Although improvements in attaining high compressive strength are remarkable, other strength features of unreinforced concrete including tensile and flexural capacities show less success. This is because of the heterogeneous nature of concrete where a weak point can initiate the crack and abrupt fracture of unreinforced concrete. Additives such as nanomaterials can assist raising tensile and flexural strength of concrete significantly. Durability, the other primary property of cementitious materials, is the ability of concrete to resist chemical attack, alteration of weather condition, and any other service life challenges. To have a durable concrete structure, planned functions must be constantly maintained for the predicted service life of the structure (Mehta and Monteiro, 1993). Although Ordinary Portland cement (OPC) is a significant contributor of greenhouse gas emission to the environment, concrete technology commonly uses it as the binder of most concrete structures. OPC concrete has always been considered as a sustainable material that can resist various environmental conditions. However, during harsh environmental situations, it is very hard to have sustainable concrete without modifying the mixture by relevant and advanced additives. However, OPC concrete itself cannot be considered as a durable material in many ecological circumstances. Enormous studies have been conducted on improving the strength and durability of OPC concrete; however, slightly effective progresses have been reported yet (Gjørv, 2011; Tang et al., 2015; Hensher, 2016). The aim of this chapter is to investigate the new approach of enhancing the durability of concrete structures by incorporating Nanotechnology in Eco-efficient Construction. https://doi.org/10.1016/B978-0-08-102641-0.00004-9 Copyright © 2019 Elsevier Ltd. All rights reserved.

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graphene oxide (GO) reinforcements. A literature survey shows that a wide variety of nanoscale particles and nanomaterials such as nanosilica, nanoalumina, carbon nanotubes, and GO have been used in concrete structures (Nasution et al., 2015). Nanoparticles are able to improve the concrete pore structure and hence, enhance the resistance of concrete structure to chemical attacks through reducing permeability of corrosive ions (Guo et al., 2017; Ghosal and Chakraborty, 2017). They have also the ability of making mechanical or chemical bonds with the cementitious paste and increase concrete strength by appropriate mechanisms. The difference between GO and other nanomaterials is in the form of the structure where GO has a sheet-like structure with a microscale size and nanoscale thickness and other materials are in terms of few molecules and spatial nanoscale size (Kim et al., 2010a, 2011). Therefore, effects of incorporating such an interesting material into concrete structures could represent a prolific research area. Various microscale particles such as fly ash and carbon fibers have been used in concrete structures to enhance their properties (Pelisser et al., 2010; Yusof et al., 2013). There is also significant attention to the use of nanoscale materials such as carbon nanotubes and nanosilica to attain a durable concrete (Du et al., 2014; Siddique and Mehta, 2014). Using graphene-based materials including GO in concrete has been seriously examined in the past decade. This chapter reviews the effects of incorporation of GO on properties of concrete specimens. First, some brief information about nanotechnology is given; it is obligatory because nanotechnology demonstrates the ways of production and characterization of nanomaterials. After that, properties of graphene and GO and the method of synthesis of GO will be addressed. A cutting-edge usage of GO in cementitious materials will then be discussed. Finally, a discussion about the influence of GO in developing an outstanding new construction material is presented. The term “nanotechnology” in the literature is a representative of multiple applications because various fields of study such as construction, materials science, biotechnology, medicine, and so on use it (Mohammed et al., 2018c). However, it is widely accepted that nanotechnology recognizes very small particles where the size of consideration is generally less than 100 nm (Ramsden, 2016). Any material with this dimension has different characteristics, features, and functions which differentiate it from its original bulk material. Therefore, the incorporation of these very tiny materials in an appropriate matrix can result in promising outcomes that are unattainable through adding mesoscale or even microscale additives. Gogotsi (Nanomaterials handbook, 2006) has presented a thorough definition of nanotechnology: If size and shape could be managed in nanodimension, then the resultant technology for designing, manufacturing, and characterizing of materials applicable in any equipment, system, or structure is called nanotechnology (Mohammed et al., 2018c). Countless additional inclusive representations of the nanotechnology concept could be found in the literature, for instance, it is the generation of materials and equipment and prevailing of the target substance in very small scales such as atoms and molecules, and in a more general case, supramolecular structure (Ramsden, 2016). In a more nonexclusive sense, it is the manner of extremely tiny elements of substances to produce innovative large-scale components (Singh, 2014).

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Besides these thorough definitions, it is worthy to mention that nanotechnology incorporates the practice of using nanoparticles as appropriate additives/reinforcements in suitable host substances.

4.2

Graphene materials

Graphene is a 2D material and its atoms are structured in a hexagonal lattice. It can be considered that all types of graphite-based materials are composed of graphene where the size and arrangement of graphene sheets alter. Many forms of graphene are used nowadays by changing the size or shape of graphene sheet in an appropriate manner. Some examples include carbon nanotubes which are rolled-up graphene sheets with metallic and semiconducting properties, zero-dimensional fullerenes which are obtained by wrapping up graphene layers, and 3D graphite structures which are a form of stacked graphene sheets to form a semimetallic structure (Geim and Novoselov, 2007; Stoller et al., 2008). There are a wide variety of 2D materials other than graphene such as borophene, germanene, silicene, etc., however, the primary focus of research community is on graphene and its components. This is as a result of the unique properties of graphene including its superior strength, and suitable electrical and heating properties which make it appropriate for various fundamental applications (Lee et al., 2008). Graphene is used in different forms including but not limited to GO, reduced GO, and nanoparticle of graphene (Allen et al., 2010). Majority of the aforementioned graphene types exhibit superior properties, which enable them to improve mechanical, chemical, and electrical properties of a structure (Balandin, 2011; Kim et al., 2010b). Among them, GO, which was first employed to create graphene particles, could be taken as one of the most common derivatives of graphene. Nonetheless, GO is now considered as a prominent material because of its unique behavior in a wide variety of applications (Mohammed et al., 2018c; Stankovich et al., 2007; Dreyer et al., 2010). It is not only due to appropriate mechanical, electrical, and physical properties of GO, but as a result of the functionalizing ability of this interesting material.

4.3

Graphene oxide

GO is one of the proficient derivatives among a wide range of graphene-based substances. To produce GO, graphite is oxidized into graphite oxide and then, GO is derived by exfoliation of the resultant oxide (Geim and Novoselov, 2007; Gilje et al., 2007). In fact, synthesis of GO is in compliance with graphite oxide because the latter is a stacked structure of GO single layers. Although there are extensive research on GO in the literature, attempts to synthesize graphite oxide date back to 1850s. An Oxford Chemist, Brodie Dating, oxidized graphite by a combination of KClO3 and HNO3 to produce graphite oxide (Brodie, 1859). Staudenmaier then used an H2SO4 oxidizing component to modify Dating’s method (Staudenmaier, 1898). Although both methods were effective, they were left because of their timeconsuming nature and hazardousness of the chemical agents. Hummers and Offerman

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were the next researchers who, in 1957, introduced a faster procedure (Geim and Novoselov, 2007). They changed the oxidizing components and used sulfuric acid (H2SO4) and sodium carbonate (NaNO3) as their chemicals (Hummers and Offeman, 1958). Hummers’ method is now one of the prominent methods which is extensively used to fabricate GO; the method is constantly improving and the most recent ones are supplemented by preoxidation phases to the original Hummers’ method (Mohammed et al., 2018c; Chen et al., 2013; Shahriary and Athawale, 2014; Marcano et al., 2010).

4.3.1

GO synthesis

Because the method introduced by Hummers and Offerman is one of the most acceptable methods in research, a brief description of it is given in this section: 23 g of sulfuric acid (H2SO4) with 98% purity is kept below 0 C for 30e45 min using an ice bath. A mixture of 1 g of graphite (powder form) and 0.5 g of NaNO2 is then progressively added to H2SO4. Mixing of these three chemical agents takes place for 45 min under continuous stirring. During this 45 min, the mixture container is placed in the ice bath to acquire the desirable mixing. At the next stage, 3 g of another chemical agent namely potassium permanganate (KMnO4) is steadily supplemented to the mixture. Occurrence of a strong oxidation in the mixture due to the addition of KMnO4 caused a color change to greenish black (from black). Temperature increases due to this reaction but it must be kept under 20 C, by adding some more ice to the ice bath for instance. Stirring for 15 min is still necessary to have a homogeneous mixture. Then, the temperature of the mixture must be reached to the ambient temperature so it is detached from the ice bath and is put in an oil bath. The stirring again continues and a color change to brown occurs. To finalize the reaction, 140 mL of ultrapure water together with 10 mm of hydrogen peroxide (H2O2) is introduced into the mixture. The constant stirring of the mixture will be maintained for another 15 min until its color changes to dark brown. The mixture is then left until the next day. During this time, GO is settled and is separated from the mixture by centrifugation. To remove impurities from the produced GO, the resultant material is washed by 5% HCl solution several times (Hummers and Offeman, 1958). Bothe Hummers and modified Hummers methods are presented in Fig. 4.1 (Marcano et al., 2010).

4.3.2

Structure of GO

Carbon atoms are arranged in a hexagonal packed structure to form GO. Further, GO has lots of functional groups. Epoxide and hydroxyl functional groups are mostly within GO’s basal plane while GO edges mostly incorporate carboxyl and carbonyl functional groups (Yang et al., 2009; Li et al., 2013; Zubir et al., 2014). Functional groups have the ability to react/interact with a wide variety of inorganic or organic molecules. Therefore, GO functional groups can react with many materials and form strong chemical bonds such as ionic or covalent bonds (Ong et al., 2015). A monolayer of GO which is deposited on a silica substrate is shown in Fig. 4.2 (G
omez-Navarro et al., 2007). It is very hard to observe GO sheets in mixture through optical or electron microscopy because it has a pale

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NOx Hummers

3 KMnO4 H2SO4, 0.5 NaNO3

Improved

6 KMnO4 9:1 H2SO4/H3PO4

Sifted/filtered Oxidized materials HGO IGO HGO+

Hummers modified

6 KMnO4 H2SO4, 0.5 NaNO3 NOx

Hydrophobic carbon material recovered

Figure 4.1 Production of GO by Hummers and modified Hummers methods (Marcano et al., 2010).

Figure 4.2 A monolayer of GO deposition on a silica substrate (G
omez-Navarro et al., 2007).

nature; it is solely observable when the deposition of GO occurs on an appropriate substrate. There are some proposals on the structure of GO and some of them are depicted in Fig. 4.3. Although literature reports a wide range of GO application in various fields of study, the molecular structure of GO has not been finalized yet. Many reasons have caused this deficiency of knowledge and among them are dissimilar synthesis methods, degree of oxidation, and nonstoichiometric and amorphous nature of GO (Szab
o et al., 2006; Talyzin et al., 2014). Moreover, the current techniques to identify 2D materials have not been well-developed and the resolution of equipment might not be sufficient to characterize GO sheets (Casabianca et al., 2010; Becerril et al., 2008).

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Hofmann

Scholzboehm

Nakajima-matsuo

Ruess

Lerf-klinowski

Décány

Figure 4.3 GO molecular structure proposed in the literature (Szab
o et al., 2006).

4.3.3

Hygroscopic nature of GO

As mentioned before, GO contains various functional groups in both basal plane and edges and therefore, GO has a hygroscopic nature and absorbs water. Water molecules are primarily trapped in GO’s interlayer vacancies. GO monolayers tend to agglomerate and stack; the interaction between water molecules and GO oxygenated groups (epoxide, hydroxyl, carboxyl, and carbonyl) creates hydrogen bonds. This reaction results in significant changes in the physical, mechanical, and structural properties of GO (G
omez-Navarro et al., 2007; Si and Samulski, 2008a). The concentration of available GO solutions ranges between 1 and 4 mg/mL (Si and Samulski, 2008a,b). It is also interesting that besides water, GO is capable of dispersing in many solvents including dimethylformamide, tetrahydrofuran, N-methyl-2-pyrrolidone, etc. (Chuah et al., 2018; Paredes et al., 2008).

4.4 4.4.1

Effects of GO incorporation into cementitious composites Effects on mechanical properties

The usage of GO-incorporated concrete dates back to less than a decade ago. Lv et al. (2013a,b, 2014) seem to be the first researchers who initiated using GO in cementitious

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materials. They studied mechanical properties as well as cement hydration of GOincorporated cement matrix. The results show that the addition of GO in very small quantities (0.01e0.03 wt%) can increase strength values significantly; this was found in the form of 143% increase in flexural strength and 129% increase in compressive strength of control mix (Al-Saadi et al., 2017d). One of the aspects that Lv et al. (2013a,b, 2014) emphasize is the formation of flower-like crystals because of the nucleation of cement hydrate and CeSeH gel on GO nanosheets. Gong et al. (2014), Babak et al. (2014), Chuah et al. (2014), and Sedaghat et al. (2014) were next researchers who studied effects of GO incorporation on the compressive strength of concrete and achieved more than 40% increase in strength values in their studies. All these three groups mention that the main reason to increase the strength is the formation of cement hydrates and strengthening gels. Gong et al. (2014), Babak et al. (2014), Chuah et al. (2014) mention that the increase in strength is also related to the refinement of pore structure. Sedaghat et al. (2014) also emphasize that the adhesion between GO nanosheets and cement paste causes this increase in strength. The reason presented by Sedaghat et al. (2014) is claimed by Duan et al. (2018) as well. Horszczaruk et al. (2015) studied the effect of adding high dosages of CO (3%) to cement paste. Surprisingly, they could not achieve any remarkable strength raise. The only success was the increase in Young’s modulus which was at least double the value of their control sample. Some other studies were conducted in 2015 (Li et al., 2015; Pan et al., 2015; Sharma and Kothiyal, 2015; Wang et al., 2015) and all demonstrated that GO is more effective on increasing flexural strength rather than compressive strength. There was a boom in studying GO/cement composites from 2016 onward and lots of research (Duan et al., 2018; Sharma et al., 2016; Abrishami and Zahabi, 2016; Meng and Khayat, 2016; Lu et al., 2016; Wang et al., 2016a,b; Cao et al., 2016; Zhao et al., 2016, 2017; Qin et al., 2017; Lu and Ouyang, 2017; Yang et al., 2017; Li et al., 2017a,b,c; Mokhtar et al., 2017; Kang et al., 2017; Gholampour et al., 2017; Han et al., 2017; Tragazikis et al., 2018; Kim et al., 2018; Long et al., 2018) were carried on to study strength capabilities of these interesting materials. Almost all of these studies show a remarkable increase in all strength values. Most of them mention that adding GO particles as the nucleation sites for cement hydrates is the main reason for increasing strength values.

4.4.2

The influence of GO on durability

The structure of concrete could be considered as a heterogeneous composite with a huge percentage of pores. Concrete is normally faced with aggressive environmental conditions and hence, the durability of this material primarily depends on its transport properties. Sorptivity, permeability, and diffusion are nominated as crucial mechanisms that can describe the movement of aggressive elements in pore structures of cement paste (Basheer et al., 2001; Yang et al., 2004). These transport properties are affected by the microstructure of cementitious matrix expressed by both volume and connectivity of the pore network (Marchand et al., 2001; Shekarchi et al., 2010). Thus, it can be concluded that the most effective way to improve concrete

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durability can be through careful design and selection of materials of the concrete mix which result in high quality concrete less prone to deterioration by chemical attacks (Shi et al., 2012). In practical terms, fluids do not transport in concrete in a single phenomenon nor in a single type of ion. Thus, fluids movement through concrete occurs not only within the porous system but also by sorption and diffusion (Lane et al., 2010; Zhutovsky and Kovler, 2012). The transport mechanisms that are associated with the movement of different chemical species in concrete can be described as follows: There is a direct influence on concrete durability involving the mobility of fluids within the concrete. Thus, it is better to have a sound understanding of this phenomenon (Hilsdorf and Kropp, 2004; Mehta, 1991). There are three major fluids including water (either in the pure case or with aggressive ions), oxygen and carbon dioxide, which have the ability to enter the concrete and affect its durability. Durability can be connected to the ease with which both liquids and gasses can enter concrete, in another term “permeability” (Abobaker, 2015). It is measurable by calculating the rate of penetration of soluble ions as well as oxygen and water into the interior structure of concrete under a pressure gradient to reach a certain level. In other words, permeability is the transport of a fluid under hydrostatic pressure, it can be described by Darcy’s law (Mehta, 1991). The permeability of cementitious materials depends mostly on their porosity, tortuosity, and continuity of pores aligned with size, shape and pore distribution. Durability of cementitious materials incorporated with GO nanosheets has been studied by some researchers and their results indicate that some improvement happens. Water sorptivity of cementitious mixtures has been shown to reduce by adding GO nanosheets especially when a dosage of 0.03% is used (Mohammed et al., 2015). It has been also shown that chloride ingress in the pastes decreases with adding any amount of GO. Our previous studies (Mohammed et al., 2016, 2017d) also considered freezeethaw properties of cementitious mixtures containing different concentrations of GO. Those studies showed the remarkable effects of GO on increasing freezeethaw cycles of concrete especially when 0.06% of GO was used. The mechanism described in those studies was the creation of air voids due to GO reactions; surprisingly, the additional air voids showed no negative impact on the mechanical performance of the studied mixtures. This is where that in normal concrete without GO, the addition of air entraining agents and subsequent increase in air voids significantly reduce the strength performance. Tong et al. (2016) have also studied the effect of adding GO in different mortars and have concluded that weight loss during freezeethaw cycles and chemical attack occasions rescues by 80% and 30% respectively. Du et al. (2016) and Du and Dai Pang (2015) studied the effect of GO in concrete samples on chloride ion penetration. They achieved 80% reduction in chloride penetration as a result of pore refinement and increased tortuosity of cement matrix. However, higher dosages of GO beyond 1.5% caused agglomeration of these particles and hence reduction in the effectiveness of GO. Resistance to carbonation is another durability index of concrete structures. Carbon dioxide diffuses into the concrete structure and reacts with the cement mixture. Due to the occurrence of various reactions, the nature of cement matrix changes and reduction in strength values happens. This results in rapid degradation of the concrete mixture.

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Lower carbonation depth shows higher resistance to aggressive environmental conditions. There are some studies from authors of this chapter in this area and their results show the huge improvement in the reduction of carbonation depth (Mohammed et al., 2017a, 2018a). The very limited carbonation has been related to the refinement of pore structure and interlocking of GO nanosheets to carbonate and calcium ions. GO has the ability of increasing resistance of concrete to high temperature exposures (Mohammed et al., 2017b). The refinement of pore structure which was in the form of a raise in gel pores and reduction in capillary pores was considered as the main reason for this performance.

4.5

Some structural applications of GO/cement composites in repairing of reinforced concrete

Carbon fiber reinforced polymer (CFRP) is one of the composite materials which is used in both repairing and strengthening of reinforced concrete structures. The usage of epoxy-based adhesives (and organic ones in general) incorporates disadvantages such as flammability of the resin or the issue of poisonous fumes (T€aljsten and Blanksv€ard, 2007). Moreover, there are other issues such as susceptibility to exposure to very low temperatures such as 70 C (Gamage et al., 2006), sunlight, and UV radiations (Ombres, 2011). When the surface has some humidity or the environmental temperature is lower than 10 C, the application of these composites is problematic (D’Ambrisi and Focacci, 2011). Therefore, it seems necessary to use an alternative adhesive such as polymer cement-based to overcome these issues. There are some evidences that show the success of using these alternative materials in the method of strengthening by CFRP (Al-Saadi et al., 2016). Nonetheless, polymer cement-based adhesives might have performance affected by the situation in which the polymer is used such as its hydrothermal situation. Another type of adhesive is a nonpolymer cementitious one, which has remarkable properties such as suitable bonding and appropriate resistance to ecological situations. Moreover, disadvantages such as flammability or release of poisonous fumes are not associated with these adhesives (Hashemi and Al-Mahaidi, 2012). This section presents some applications in which GO has been used to produce nonpolymer cementitious adhesives. Innovative high-strength self-compacting nonpolymer cement-based adhesive (IHSSC-CA) is a mixture incorporating GO nanosheets and was used in a previous study (Mohammed et al., 2016a). It is a very high strength mixture showing outstanding tensile and compressive strength values of 18 and 116 MPa after 28 days respectively (Mohammed et al., 2018c). Other properties of IHSSC-CA are given in Table 4.1. IHSSC-CA has been used in our previous studies and the results obtained for all applications including stiffness, ductility, bond strength, etc. are promising. Strengthening of fiber-reinforced concrete with the near-surface mounted CFRP (NSM-CFRP) technique at different loading conditions has been studied. We showed that it is more convenient to apply IHSSC-CA for NSM-CFRP than other common adhesives such as epoxy-based and cement-based polymer ones. This is because of the unique properties

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Table 4.1 Properties of the innovative cementitious adhesive (IHSSC-CA) (Mohammed et al., 2016a)

Flow

Initial setting time (min)

Final setting time (min)

28-day tensile strength (MPa)

28-day compressive strength (MPa)

Bond strength (MPa)

Permeability 3 10L16 m2

7.5%

120

420

13.8

101

1.2

0.023

of IHSSC-CA including better flowability and workability (Al-Saadi et al., 2018). Further, IHSSC-CA showed better pull-out and bond strength results in NSM-CFRP applications than epoxy-based and cement-based polymer adhesives (Al-Saadi et al., 2018). Fig. 4.4 shows the application of IHSSC-CA with CFRP strip and Table 4.2 shows the pull-out test results. Another feature of using IHSSC-CA is its ability to reduce stress concentration. Physical analysis (Fig. 4.5) of CFRP connected to reinforced concrete through the application of IHSSC-CA showed a lower stress concentration than samples with epoxy-based and cement-based polymer adhesives (Mohammed et al., 2017c). After the pull-out test, a remarkable change in the pore network and structure of IHSSCCA was observed. The behavior is in contrast with epoxy-based and cement-based polymer adhesives which show slight change in the pore structure under loading. This implies the suitable behavior of the applied IHSSC-CA adhesive under the applied load (Fig. 4.6) (Mohammed et al., 2017c). Further, 3D laser profilometry analysis results shown in Fig. 4.7 reveal that a very rough surface is acquired for IHSSCCA adhesive after pull-out tests rather than epoxy-based and cement-based polymer adhesives (Mohammed et al., 2018b).

Figure 4.4 Application of IHSSC-CA with CFRP strips (Al-Saadi et al., 2018).

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Table 4.2 Pull-out test results (Al-Saadi et al., 2018) Ultimate pull-out force (kN)

Ultimate bond strength (MPa)

Ultimate axial stress (MPa)

CFRP strip utilization

Innovative cementitious adhesive (IHSSC-CA)

34.5

4.80

1233

0.34

Rupture of CFRP strip

Polymer cement adhesive

22.3

3.09

794

0.21

Pull-out of CFRP strip

Epoxy adhesive

41.1

5.70

1467

0.39

Debonding of CFRP strip

Specimens ID

(a)

Failure mode

(b)

Almost even thickness of a thin layer of IHSSC-CA

Uneven thickness of PCA layer with a number of voids disturbed randomly

(c)

Gravel particles from concrete substrate

Unsmooth surface of epoxy adhesive layer with uneven thickness

Figure 4.5 Images of the bond area after pull-out testing (Mohammed et al., 2017c). 16

14

IHSSC-CA-after the testing

12

IHSSC-CA-before the testing

14 Va/cm3(STP) g-1

Va/cm3(STP) g-1

16

10 8 6 4

PCA-after the testing PCA-before the testing

12 10 8 6 4 2

2

0

0 0

0.2

0.4

0.6 p/p0

0.8

1

1.2

0

0.2

0.4

0.6 p/p0

0.8

1

1.2

Figure 4.6 Nitrogen adsorption isotherms for IHSSC-CA and PCA (Mohammed et al., 2017c).

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(a)

0.20

(b)

(c)

Figure 4.7 3D and 2D images of the topographic surface of CFRP strip: (a) with IHSSC-CA, (b) with PCA, and (c) with epoxy (Mohammed et al., 2018b).

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High temperature testing of IHSSC-CA showed that this adhesive is able to maintain around 60% of its strength even at elevated temperatures (Mohammed et al., 2016b). Fig. 4.8 shows the compressive and tensile strengths at different temperatures for normal-strength concrete (NSC) and IHSSC-CA. The adhesive used in this study had a thickness of 5 mm. Further experiments showed that this thickness is insufficient to protect samples from the heat. An increased thickness of 20e25 mm was then applied as the protective heat-resistance cover. Test results showed better bond strength when the thickness increased (Fig. 4.9). It was revealed that higher temperatures require higher thickness of the IHSSC-CA adhesive where for 600 and 800 C heat exposure, the application of at least 20 and 25 mm (respectively) of adhesive is essential (Mohammed et al., 2016b).

Compressive strength (Mpa)

120 IHSSC-CA

100

NSC 80 60 40 20 0 21 °C

400 °C 600 °C Temperature

800 °C

Tensile strength (Mpa)

16 14

IHSSC-CA

12

NSC

10 8 6 4 2 0 21 °C

400 °C 600 °C Temperature

800 °C

Figure 4.8 Compressive and tensile strengths of tested specimens (Mohammed et al., 2016b).

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5

Without protective cover

4.5

With protective cover fitting curve Without protective cover fitting curve

Bond strength (MPa)

4 3.5

T = 4.597–0.002T–4.88E–06T2 R2 = 0.96

3 2.5 2 1.5

T = 4.675–0.006T–2.893E–11T2 R2 = 0.97

1 0.5 0

0

100

200

300 400 500 600 Temperature (°C)

700

800

900

Figure 4.9 Average bond strength versus temperature relations (Mohammed et al., 2016b).

The results of fatigue tests on concrete samples joined to NSM-CFRP through the application of IHSSC-CA were better than samples with epoxy-based and cementbased polymer adhesives, especially at longer lives (Fig. 4.10). The analysis of pore structure of specimens subject to fatigue loading shows a better composite behavior for IHSSC-CA than other polymer-based adhesives (Al-Saadi et al., 2017a). Physical examination after fatigue and post-fatigue tests (Fig. 4.11) shows uniform stress distribution and therefore, suitable composite behavior is observed between NSM-CFRP and IHSSC-CA (Mohammed et al., 2018c). This is against epoxy-based and cement-based polymer adhesives where a nonuniform stress distribution was observed, as shown in Fig. 4.12 (Al-Saadi et al., 2017a).

Load range (La) (kN)

20

15 FR20IC-1 FR20C-1 FR10C-1 FS20C-1 FS10C-1

10

5

0 10

100

1000 10000 100000 Fatigue life (N) (Cycles)

1000000

10000000

Figure 4.10 Load range-fatigue life relationships for IHSSC-CA (FR20IC) and polymer cement-based (FR20 C) adhesives (Al-Saadi et al., 2017a).

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16 R20IC R20C

14 Va/cm3(STP) g-1

12 10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

1

1.2

P/PO

Figure 4.11 Nitrogen adsorption isotherms for IHSSC-CA (R20IC) and polymer cement-based (R20 C) adhesives (Al-Saadi et al., 2017a).

Figure 4.12 Images of CFRP strip and a bond area of tested specimens: (a) IHSSC-CA and (b) polymer cement-based adhesive (Al-Saadi et al., 2017a).

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Load (kN)

120 100 MC MSC MRC MSE MRE

80 60 40 20 0

0

20

40

60 Deflection (mm)

80

100

120

Figure 4.13 Load versus midspan deflection relations (Al-Saadi et al., 2017b).

In a previous study, deflection capacity (ductility) and flexural strength of reinforced concrete beams strengthened by NSM-CFRP using IHSSC-CA adhesive showed improvement compared to a control specimen (Al-Saadi et al., 2017b). Despite IHSSC-CA, strengthening by using epoxy-based adhesives showed decreased ductility and flexural strength as shown in Fig. 4.13 (Al-Saadi et al., 2017b). Fig. 4.14 shows the deformation behavior of reinforced concrete beams strengthened by both epoxy-based and IHSSC-CA adhesives. This figure shows the suitability of using IHSSC-CA adhesive in creating large deformations compared to epoxy-based adhesive which caused abrupt rupture (Al-Saadi et al., 2017b). Further, as Fig. 4.13 shows, the application of epoxy-based adhesive to strengthen reinforced concrete beams causes the appearance of no residual strength after the ultimate strength. The post-cracking behavior of specimens strengthened by IHSSC-CA is different and a large amount of residual strength (close to 87%) sustains after reaching its ultimate strength (Al-Saadi et al., 2017b). Results of our previous studies show the effectiveness of using IHSSC-CA as adhesive for repairing and strengthening of reinforced concrete beams. Values such as deflection and strain are higher in structures strengthened by epoxy-based adhesives. Further, the proposed IHSSC-CA adhesive has the capability for in-situ applications (Al-Saadi et al., 2017c,d). Analysis of the fatigue behavior of reinforced concrete beams strengthened by the IHSSC-CA adhesive shows the effectiveness of this binder as better stress transfer and bond strength are attainable compared to epoxy-based adhesives, as shown in Figs. 4.15 and 4.16. This behavior shows once again that the utilized IHSSC-CA adhesive has the ability of maintaining composite action even under fatigue loads (Al-Saadi et al., 2017c,d). Besides mechanical performance, IHSSC-CA is more convenient for on-site applications compared to epoxy-based adhesives because its workability, flowability, and self-compacting properties are better and create a uniform and smooth bonding layer (Al-Saadi et al., 2017c).

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(a) Crushing of concrete

Rupture of CFRP strips Rupture of CFRP strips

Rupture of CFRP strips

Transverse cracks

(b)

Beginning of concrete cover seperation at the cut-off point of the CFRP strips

Intersection between flexure and shear cracks

Main horizontal crack along the level of tension steel reinforcement

Figure 4.14 Failure modes of tested beams: (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017b).

Fig. 4.17(a) shows that reinforced concrete beams with IHSSC-CA adhesive upon the rupture of CFRP strip. This happens once the concrete cover in the compression zone crushes and reinforced steel bars reach to their yield strength. In other words, there is no degradation between the adhesive and CFRP strip under fatigue loads (Al-Saadi et al., 2017d). The results for CFRP strips joined to reinforced concrete

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18 16

Deflection (mm)

14 12 FRE FSE FC FRC FSC

10 8 6 4 2 0 0

0.5

1

1.5

2

2.5

3

Number of cycles (million)

Figure 4.15 Midspan deflection versus fatigue life relations for tested beams (Al-Saadi et al., 2017c). 0.8 0.7

Crack width (mm)

0.6 0.5 0.4 0.3 FC FRE FSE FRE FSC

0.2 0.1 0

0

0.5

1 1.5 2 Number of cycles (million)

2.5

3

Figure 4.16 Crack width versus fatigue life relations for tested beams (Al-Saadi et al., 2017c).

through epoxy-based adhesive in Fig. 4.17(b) show an abrupt failure after the crushing of concrete cover and the yielding of steel bars. In other words, because of bond degradation between CFRP and epoxy-based adhesive, this type of failure occurs (Al-Saadi et al., 2017d). Ductility of beams reinforced by IHSSC-CA also increased compared to epoxy-based adhesives (Al-Saadi et al., 2017d). Less deformation of

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Figure 4.17 Failure modes of tested post-fatigue beams: (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017d).

the surface of CFRP strips was proved by studying the results through 3D laser profilometry, as shown in Fig. 4.18. The less deformation means higher capacity of repaired and strengthened reinforced concrete to resist fatigue loading (Al-Saadi et al., 2017d).

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(a) µm

21.8 15.0 10.0 5.0 0.0 –5.0 –10.0 –15.0 –20.0 –25.0 –30.0

1.27 mm –35.0

0.95

–40.0 –46.0

(b) µm

38.5

30.0 25.0 20.0 15.0 10.0 5.0 0.0 –5.0 –10.0 –15.0 –20.0

1.27 mm 0.95

–25.0 –32.0

Figure 4.18 2D and 3D images of the topographic surface of CFRP strip (a) with IHSSC-CA and (b) with epoxy adhesive (Al-Saadi et al., 2017d).

4.6

Summary of the chapter

This chapter reviewed effects of incorporating GO on the performance of cementbased materials. Further, an innovative application of GO-incorporated mixture applicable in repairing and strengthening of reinforced concrete sections was presented. Review of the current literature showed that GO is suitable for the modification of cement paste microstructure. The reduction in the amount of capillary pores and the raise of gel pores are the direct benefits of using GO. As a result of this microstructural refinement, cement hydration improves and hence, mechanical properties of concrete are enhanced remarkably. GO addition was also effective in increasing durability of concrete by reducing water transport in concrete. It was also shown that concrete samples reinforced by GO has better freezeethaw behavior. This is attributed to the surfactant effect of GO. One interesting feature of adding GO was about sustaining mechanical strength of air-entrained concrete. Adding air to

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concrete to increase its freezeethaw ability reduced the mechanical strength of normal concrete, adding GO prevents this decrease in strength. Carbonation of concrete is also improved by adding GO. Large surfaces of GO trap carbon dioxide diffusing into concrete. GO-incorporated cement mixtures as adhesive for joining CFRP strips to reinforced concrete revealed unique results compared to common adhesives. Better adhesion, higher compressive and tensile strength, better ductility, lower deflection, and better fatigue performance of beams were the advantages of using this binder.

Acknowledgment A review paper on effects of graphene oxide on cementitious materials has been already published by the authors as “Mohammed, A., Al-Saadi, N.T.K., Sanjayan, J., 2018. Inclusion of graphene oxide in cementitious composites: state-of-the-art review. Australian Journal of Civil Engineering, 1e15.” This is to confirm that the current chapter again reviews this effect. However, the majority of information in this chapter are new.

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