Self-healing concrete-based composites

Self-healing concrete-based composites

15

Wei Zhang, Danna Wang and Baoguo Han School of Civil Engineering, Dalian University of Technology, Dalian, P.R. China

15.1

Introduction

Generally, materials will degrade over time due to fatigue, environmental actions, or damage incurred during operation. Self-healing materials have the self-repairing ability to themselves and can restore some of their properties after damage. They are expected to be applied in a broad range of fields, for instance, military equipment, electronic products, and construction engineering. Concrete (mainly including cement concrete and asphalt concrete) has become one of the mainstream construction materials at present, because of its favorable compressive strength, excellent water resistance, and good economy. However, cement concrete usually works with cracks, which is associated with its low tensile strength, self-shrinkage, and dry shrinkage. As the crack develops further, the internal transport of moisture, oxygen, and corrosive medium accelerates inside the concrete. As a result, concrete structures may be damaged and the internal steel bars are likely to be corroded, thereby affecting the durability and safety of infrastructures [1,2]. In addition, asphalt concrete has temperature instability (i.e., easy to brittle in winter and to soften in summer) and uneven compaction, resulting in rutting, cracking, and serious wear on asphalt concrete pavement. Moreover, as a polymer material, asphalt has poor aging resistance which is detrimental to the durability of infrastructures. The traditional repair technologies for concrete mainly include grouting, surface strengthening, and surface coating. Whereas these artificial methods cannot repair the unreachable places, and the treatment is often not timely. What’s more, it is very difficult to deal with microcracks. The presence of self-healing concrete-based composites provides an effective approach to solve problems of concrete cracking and damage. The self-healing concrete-based composites are smart materials that can heal microcracks in concrete and even improve material properties by recombining repair adhesive and concrete materials [3]. In 1925 Abram first found the self-healing phenomenon of cracks in concrete and its strength after self-repairing was doubled. Since then, researchers have conducted a series of researches to fabricate selfhealing concrete and prove its feasibility and effectiveness. For instance, adding short fibers to concrete-based composites is beneficial to improve its self-healing ability, according to the study of Qian et al. [4,5]. Yang et al. demonstrated the feasibility of introducing microcapsules with oil core and silica shell to heal microcracks in cementitious composites [6]. Moreover, the self-healing ability to heal Self-Healing Composite Materials. DOI: https://doi.org/10.1016/B978-0-12-817354-1.00015-6 © 2020 Elsevier Inc. All rights reserved.

260

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Table 15.1 Self-healing concrete-based composites with different additives [8 12]. Types of selfhealing concrete-based composites

Curing condition

Crack width

Self-healing products

Results

References

Cement paste with fly ash Engineered cementitious composites

Sealed cure

Microcrack Average 60 µm

In water, 20 C (70 days) In water (28 days)

5 15 µm

Heal the microcracks Recover the initial resonant frequency and tensile strain Regain initial stiffness

[9]

Five wet-dry cycles (10 days)

Hydration of fly ash CaCO3

Repair tensile strength

[12]

Highperformance concrete Fiber-reinforced cementitious composites

, 100 µm

CaCO3

[10]

[11]

cracks of bacteria-based concrete can be effectively improved in damp conditions [3,7]. In addition, improving the self-healing capability of concrete-based composites by incorporating mineral admixtures, nanofillers, and curing agents as well as embedding shape memory alloy (SMA) has been proven to be effective. In addition to these methods, electrodeposition technology, vascular technology, microwave technology, and light repair technology can also be used to fabricate the concretebased composites with the self-healing capability. Self-healing concrete-based composites have great potential for timely repair of infrastructures, which is of great significance to ensure the safety and durability of buildings, pavements, dams, underground structures, etc. According to the type of concrete, self-healing concrete-based composites are also divided into selfhealing cement concrete composites and self-healing asphalt concrete composites. Their self-healing capacities are summarized in Tables 15.1 and 15.2 [8 18]. In this chapter, the mechanism, repair effectiveness, and merit and demerit of cement/ asphalt concrete-based composites with self-healing capacity were comprehensively reviewed.

15.2

Self-healing cement concrete composites

15.2.1 Self-healing cement concrete incorporating with fibers Previous studies have shown that the incorporation of fibers contributes to promote the self-healing ability of cement concrete. The most commonly used fibers in selfhealing cement concrete include steel, polyethylene (PE), and polyvinyl alcohol (PVA). The self-healing mechanisms of fiber-modified concrete are summarized as follows. On the one hand, fibers can bridge cracks and attach crystallization products by serving as nucleation sites. This phenomenon was observed in PE

Table 15.2 Self-healing concrete-based composites based on different methods [13 18]. Types of selfhealing methods

Curing condition

Other condition

Failure mode

Results

References

Capsule method

In air (24 h)

220 µm crack

In air (24 h)

Electrodeposition method Bacterial method

In solution (28 140 days) In tap water (8 days) In air

Constant current

Regain initial peak strength; crack opening displacement $ 0.3 mm 0.4 0.6 mm crack

Recover $ 50% of original strength and stiffness Under second load new cracks formation Close cracks

[13]

Vascular method

MEYCO (healing agent) Healing agent

Spore bacteria

Break to pieces

[16]

SMA

Static loading test

In air

Conductive fillers and current

Produce more crackplugging minerals Reverse deflection and close cracks after unloading Recover cracks

Shape memory alloy embedding method Induction energy/ microwave method

[14] [15]

[17]

[18]

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Self-Healing Composite Materials

fiber reinforced self-healing concrete by Homma et al. [12]. On the other hand, the addition of fibers limits the width of the crack in concrete [19]. Vast researches have been carried out on self-healing of concrete with fibers, particularly for engineered cementitious composites (ECCs). Besides, fibers affect not only the crack morphology but also the properties of self-healing concrete. Homma et al. found that the recovery rate of tensile strength of hybrid fiber (i.e., steel cord and PE fibers) modified self-healing concrete is over 100%, that is, the tensile strength of concrete after self-healing can be restored to the initial strength. Calcium carbonate (CaCO3) crystals are confirmed as the self-healing products [12]. The self-healing of ECC in the environment of wetting drying cycles was studied by Yang et al. The initial resonant frequency value of ECC damaged by crack can be recovered up to 76% 100% by self-healing and an apparent stiffness rebound was obtained. Although the specimens were predamaged by intentionally loading 3% tensile strain in advance, its recovery capacity of self-healing tensile strain still approaches 100% [10]. The components of selfhealed cracks are CaCO3 and calcium hydroxide (Ca(OH)2). The influences of service environment, curing ages, and crack width on selfhealing property of ECC have also been investigated. For example, Zhu et al. conducted a comparative research on self-healing of ECC under water and deicing salt freeze thaw cycles. The results showed a preferable self-healing ability of ECC under water freeze thaw cycles rather than deicing salt freeze/thaw cycles [20]. Kan et al. found that through four or five wetting drying cycles, the self-healing of ECC is prominent [21,22]. Yang et al. explored the self-healing behavior of damaged ECC at curing ages of 3 days with different self-healing conditions (i.e., water/air cycles, water/high-temperature air cycles, 90% RH/air cycles, and water submersion). When the preloading strain is restricted to 0.3% and under aqueous conditions, the self-healing of the specimens with short-term curing ages is robust [23]. In addition, according to the study of Kan et al., the self-healing degree of ECC at curing age of 90 days is slightly superior to that at 3 days owing to the smaller width of crack. Besides, ECC with multiple microcracks is beneficial for self-healing, especially ECC with crack widths below 50 µm possess supernal selfhealing ability. Calcium silicate hydrates (C S H) and CaCO3 formed through further hydration and carbonation are identified as the products produced during the self-healing process. Furthermore, PVA fibers can act as nucleation sites in ECC for healing products, which may promote the self-healing [21,22].

15.2.2 Self-healing cement concrete incorporating with mineral admixtures The mineral admixtures are summarized in Table 15.3. The self-healing process of concrete incorporating with mineral admixtures can be explained as follows. Firstly, concrete with mineral admixtures cracks after being damaged. Secondly, when water fills crack, expansive agents and geomaterials begin to swell, carbonate gradually precipitates, and eventually the cracks will be shrunk or even filled [19].

Table 15.3 Summary of self-healing concrete-based composites with mineral admixtures. Type of concrete-based composites

Mineral admixtures

Results

Products

References

Precracked fiberreinforced strain hardening cementitious composites Self-compacting concrete

Blast furnace slag

Deflection recovery of 65% 105%

CaCO3

[4]

Fly ash

Cement paste

Fly ash

Engineered cementitious composites

Class-F fly ash Class-C fly ash Slag Blast furnace slag

After preloaded, initial strength reduces from 27% to 7% After preloaded, initial strength reduces from 19% to 13% Self-healing property improves as the fly ash content increases Healing cracks up to 30 µm Healing cracks up to 50 µm Healing cracks up to 100 µm 10 µm wide artificial gap is filled about 60% 1. Chemical expansive agents provide the greatest crack selfhealing ratio 2. Mineral combination further enhances the self-healing capacity 3. Flowing water, high pH, and high temperatures accelerate selfhealing process Maximum healing crack width/ length: 20 µm/5 mm

Cement paste Cementitious materials

Silica-based Chemical expansive agents Swelling minerals Crystalline components

Concrete

Carbonated steel slag

[24]

[9] [26]

[27] [28]

CaCO3, C S H, Ca(OH)2, calcium-aluminate-ferrite hydrate amorphous silica

[29]

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Self-Healing Composite Materials

Sahmaran et al. concluded that the pozzolanic reaction of unhydrated fly ash is the main reason for the self-healing of fly ash reinforced self-compacting concrete [24]. Similarly, Tittelboom et al. reported that the main mechanism by which blast furnace slag or fly ash enhances the self-healing ability of concrete cracks is continued hydration rather than CaCO3 precipitation. Besides, it is more effective to partially replace cement with blast furnace than with fly ash [25]. A summary of previous studies on the self-healing property of concrete-based composites with mineral admixtures is presented in Table 15.3.

15.2.3 Self-healing cement concrete incorporating with microcapsule Adding microcapsules containing healing agent and catalyst into concrete is another means for achieving self-healing. The healing process follows two steps. To begin with, when the microcapsule breaks under the crack propagation, the internal healing agents will flow to the crack. Then the healing agents chemically react with matrix material, which makes the crack surface to bond together and changes the crack tip shape. Consequently, the development of cracks can be restrained and the recovery of properties including stiffness, strength and fracture toughness of concrete is achieved. Numerous researches have been carried out on the preparation and the optimum dosage of microcapsules as well as its self-healing efficiency to concrete. Excellent microcapsules may have the following characteristics: satisfactory shell thickness and size, healing agent, viability of mixing, acceptable compatibility and interfacial adhesion with concrete matrix, along with good crack sensitivity. In 2001, ureaformaldehyde microcapsules filled with epoxy resin with a diameter of 20 70 µm and gelatin microcapsules filled with acrylic resin with a diameter of 125 297 µm were added into concrete by Mihashi et al., and the prepared concretes were verified to have the ability of self-healing under compression and splitting [30]. A low content of the silica gel shell microcapsules containing oil enhances the resistance to cracks and toughness of carbon microfiber modified cement mortar according to fatigue tests [6]. Van Tittelboom et al. found that the stiffness and strength of cement mortar incorporating with tubular capsules filled with healing agents recover up to 50% of the original values after self-healing. The presence of healing agents in the cracks was observed by using high-resolution X-ray computed tomography (XCT) [13]. The feasibility of ceramic tubes with polyurethane/the mixture of water and accelerator as healing materials was also investigated [31]. It was found that the stiffness and strength of the self-healing beam are restored to over 80% of original values, and the same stiffness and strength of the self-healing beam are recovered after autonomous crack healing by employing polyurethane or highstrength epoxy resin as the healing agents. Gilford III et al. studied that incorporating 5.0% sodium silicate microcapsules produced at pH 3.1 increases the elastic modulus of concrete by 11%, while 0.25% dicyclopentadiene microcapsules

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fabricated at the same pH value improves the elastic modulus by 30% through selfhealing [32]. The release of healing agent from the microcapsules is one key issue in achieving self-healing of microcapsule-filled cement concrete. The intelligent release process of chemical self-healing microcapsules in the stimulated pore solution of concrete was investigated by Dong et al. It was demonstrated that more corrosion inhibitors flow out of the polystyrene resin capsule as time increases and pH value decreases [33]. Therefore it is recommended to realize an intelligent release control of microcapsules in the alkaline cementitious environment. Perez et al. developed the self-healing concrete with epoxy encapsulating silica microcapsules and amine-functionalized silica nanoparticles [34,35]. The silica microcapsule shells can react with Ca(OH)2 in the cement matrix to make the microcapsules combine firmly with cement matrix. Aminefunctionalized silica nanoparticles can induce pozzolanic reaction as well, which leads to the formation of an amine-functionalized cementitious matrix. It was concluded that the microcapsules are still steady after mixing and pozzolanic reaction. They also confirmed the self-healing efficiency of 150 µm wide cracks in concrete. Kanellopoulos et al. studied the mechanical and self-healing properties of cement mortar containing different amounts of polymeric microcapsules filled with sodium silicate [36]. Results manifested that hydration and setting time are not affected by the addition of microcapsules, but the viscosity is increased. As more microcapsules are added, the self-healing property of cement mortar is significantly enhanced and nearly 100% of areal crack mouth is healed, although the mechanical properties are reduced. A new kind of polymeric microcapsule for self-healing cement concrete, which used phenol-formaldehyde resin as shell and the healing agent as dicyclopentadiene, was designed by Lv et al. [37]. The fracture and status behavior of microcapsules in cement matrix were investigated by XCT, and the three-dimensional (3D) images showed that microcapsules are well dispersed and sensitive to cracks. The stability of microcapsules during mixing and in concrete matrix is another important factor influencing the self-healing efficiency of microcapsule-reinforced cement concrete. Favorable chemical stability of the microcapsules in simulated pore solution or cementitious materials was observed by Lv et al. [37]. Polymer capsules designed by Hilloulin et al. are resistant to concrete mixing and can be ruptured in the presence of cracks [38]. All three polymers selected have a low glass transition temperature. Therefore the polymers can be changed to rubbery state by preheating to survive the mixing and then become brittle by cooling. The capsules become brittle again. Dong et al. filled urea-formaldehyde/epoxy microcapsule into cementitious materials to repair cracks [39]. The prominent surface texture, satisfactory size, and excellent thermal stability of the microcapsules can ensure that they are compatible with cement paste and not damaged during mixing. It was observed that healing cracks account for 20.71% 45.59%, and compressive strength and impermeability are healed separately by approximately 13% and 19.8%, respectively. Even though self-healing cement concrete incorporating with microcapsule has been widely studied, there are still many issues to be explored. Initially, the

266

Self-Healing Composite Materials

effective microcapsules with practical applicability should be fabricated. In addition, the possibility of microcapsules’ cracking needs to be improved by rational design. Finally, theoretical analysis including mechanism and numerical modeling [40] should be researched in depth.

15.2.4 Self-healing cement concrete embedding with shape memory alloy Owing to the shape memory effect, the prestrained SMA wire can restore its tensile length in martensitic phase as the phase changes from martensite to austenite. Embedding SMA in concrete can cause the healing of cracks and the recovery of deflection because of shape memory effect. It is worth noting that the self-healing process of cement concrete embedded with SMA requires the induction of thermal energy. Previous researches show that SMA wire can close cracks because of its super elastic behavior, thus timely repairing damage in concrete structures [17,41]. The research of Sun et al. indicates that long unbonded length leads to better crack repair for embedded-type SMA wire beams [42]. Li et al. explored that the selfhealing performance of SMA bundles embedded smart concrete beams, which were merged with an intelligent bridge on a highway. Experimental results show that SMA bundles can provide prominent and controllable resilience. The SMA bundle beam generates an approximately 0.44 mm deflection at the midspan, and each beam has the antioverload ability of around 2.98 kN [43]. Kim et al. found that the tensile cement mortars mixed with SMA fibers at a content of 1.5 vol.% are significantly shortened after heat treatment for 10 min, which is believed to be related to the shape memory effect [44]. Then the prestress of the tensile specimens is generated. In addition, SMA causes a remarkable increase in Young’s modulus of tensile specimens. Choi et al. compared the crack-closing ability of beams by adding four different kinds of SMA fibers [45]. They are straight-shaped fiber, dog-bone-shaped fiber, straight-shaped fiber with paper wrapping in the middle, and dog-bone-shaped fiber with paper wrapping in the middle, and they are placed at the bottom of the center of the beam specimens. The crack widths before and after healing are shown in Fig. 15.1, and the degrees of crack recovery and deflection recovery are diagramed in Fig. 15.2. The degrees of crack recovery values for beams with straight fibers, dog-bone fibers, straight-paper fibers, and dog-bone-paper fibers are 0.500, 0.605, 0.678, and 0.910, respectively. In addition, the deflection-recovery factor of straight-paper fiber is 1.54 and for dog-bone-paper fiber, 1.77, both exceeding 1.0. Whereas, the deflection-recovery factors of straight-shaped fibers and dog-boneshaped fibers are lower than 1.0, which are 0.72 and 0.88, respectively. To sum up, the incorporation of SMA can bring self-healing capacity to concrete, that is, to shrink cracks and recover deflection in a timely and effective manner, but it cannot eliminate cracks. Furthermore, high price of SMA limits its application.

Self-healing concrete-based composites

267

1.0 Before heating w0

Cracks (mm)

0.8

After heating w1

0.6 0.58

0.56

0.38

0.4 0.29

0.2

0.28 0.15 0.09

0.05

0.0

Straight Dog-bone Straight-paper Dog-bone-paper Figure 15.1 Cracks of beams before and after heating. Degree of crack recovery Degree of deflection recovery

1.8 1.6

0.8 1.4 0.7

1.2 1.0

0.6

0.8

Degree of deflection recovery

Degree of crack recovery

0.9

0.5 0.6 Straight Dog-bone Straight-paper Dog-bone-paper

Figure 15.2 Degree of crack recovery and deflection recovery.

15.2.5 Self-healing cement concrete incorporating with nanofillers Due to the promotion of cement hydration and the improvement of microstructure by nanofillers, cement concrete incorporating with nanofillers also possesses selfhealing property. Wang et al. studied the self-healing properties of reactive powder concrete (RPC) modified with different types of nanofillers, including nano-SiO2, nano-TiO2, and nano-ZrO2 [46]. Water- and air-curing methods were also investigated as influencing factors. Experimental results show that RPC with nanofillers cured in water or in air obtains higher self-healing coefficient of compressive strength (except for nano-ZrO2) and flexural strength than the control sample, manifesting that the addition of nanofillers improves the self-healing capacity of RPC. Moreover, the specimens with water curing are more effective in enhancing the self-healing property of RPC than that with air curing. RPC incorporating with 3%

268

Self-Healing Composite Materials

of nano-SiO2 reaches the maximum self-healing coefficient of compressive/flexural strength of 1.31/1.19, which is enhanced by 39.4%/33.7% corresponding to the control specimens without nanofillers, respectively. Under compressive load, RPC incorporating with nanofillers has lower accumulative energy compared with RPC without nanofillers under the same curing methods, and the degree of energy attenuation is far above that during preloading. Under flexural strength, the accumulative energy of RPC modified with nanofillers cured in water or in air (except for nano-ZrO2-filled RPC cured in water) is also lower than that of control sample. These demonstrate that the damage of RPC is prominently reduced due to the addition of nanofillers. According to the ring count-time curves of RPC with and without nanofillers under 28 days preloading and 90 days secondary loading, Kaiser effect is weakened or even eliminated by adding nanofillers, manifesting that the internal cracks are healed to some extent. Three mechanisms are considered to be responsible for the self-healing property of RPC incorporating with nanofillers. Firstly, the nanofillers can absorb water in matrix and act as nucleation sites to promote hydration of cement particles, and the hydration products will heal cracks within RPC [47 49]. Secondly, a new 3D network structure is established inside the RPC matrix due to the incorporation of nanofillers; thus more fine cracks are formed and the propagation and expansion of cracks are limited [50 52]. Lastly, nano-SiO2 and nano-TiO2 (especially for nanoSiO2) with pozzolanic reactivity can react with Ca(OH)2 to produce additional calcium silicate hydration, thereby improving the compactness of RPC [53 55].

15.2.6 Self-healing cement concrete incorporating with curing agents Self-healing concrete can also be developed by the method of incorporating curing agents, and the commonly used curing agents are presaturated porous lightweight aggregate (LWA) (such as ceramsite and pumice) and chemical admixtures including super absorbing polymer (SAP) and shrinkage reducing admixture (SRA) [56]. Curing agents work as internal water reservoirs, and LWA can store water by weight of 5% 25%, while the water absorbed by SAP and SRA is 1000 times heavier than their own weight [57]. Contrary to conventional water curing or membrane curing, the internal curing process provided by curing agents takes place in the contact zone between cement paste and self-curing materials, and it happens from the inside to outside. When early hydration causes a gradient in humidity, the water contained in curing agents will be transported to unhydrated cement to support continuous hydration. Therefore, the chemical shrinkage and self-desiccation due to low water binder ratio can be significantly reduced, that is, concrete performs self-healing capacity. According to Powers’ model, for high-performance concrete (HPC) with water binder ratio of 0.30 under sealed curing condition, when LWA supplements the system with additional curing water from 0% to 3.20% and 7.36% of total water volume, the hydration degree can be enhanced from 0.73 to 0.77 and 0.83, respectively. Moreover, the chemical shrinkage is reduced and totally eliminated, eventually [58]. Castro et al. measured the moisture transport between LWA and cement in

Self-healing concrete-based composites

269

different relative humidity environments, and found that a great amount of water can be released by an efficient aggregate at 93% humidity, which means that this part of water can contribute to further cement hydration and consequently self-healing property [59]. Bentz and Snyder reported that fine LWA provides more effective curing effect, which means better healing effect with respect to equal mass coarse LWA, since fine LWA enables a more uniform distribution of water in cement paste of concrete owing to its larger specific surface area [60]. For chemical curing admixtures, studies indicate that the amount of chemical admixture needed is greatly affected by the mortar strength grade [61,62]. Schlitter et al. replaced a partial normal aggregate in concrete with LWA of different volumes, and researched the width and distribution of plastic shrinkage crack in concrete [63]. Results indicated that the increase of LWA replacement volume leads to the decline of cracks formation probability, and the crack caused by plastic shrinkage is eliminated when LWA volume is up to 18.0%. Furthermore, with the increasing LWA volume, the probability of cracks narrower than 0.0 and 0.2 mm is significantly decreased. Durability and mechanical properties can also be used to evaluate the selfhealing property of concrete-containing curing agents. Tyagi et al. reported that as the content of polyethylene glycol-400 (PEG-400) increases, both slump and compaction ratio improve, and M40 concrete has a lower increase rate of slump and compaction factors compared with M25 concrete. The optimum dosages of PEG400 for M25 and M40 grades are 1% and 0.5%, respectively [64]. Hajazin et al. observed that the internal curing caused by expanded clay sand results in a 30% decrease in chloride ion permeability of concrete, owing to the decreased percolation in interfacial transition zone between cement paste and aggregate [65]. The research of Dhir et al. also manifested that the incorporation of chemical curing agents provides concrete enhanced durability in comparison with concrete with air curing, although the durability is not better corresponding to that of film-cured concrete [66]. Some studies show that the compressive strength of concrete with curing agents can reach an increase of 10% 20%, while others indicate a decline of 8% 31% [67 71]. This is because an appropriate amount of additional curing water plays a positive role in improving the degree of cement hydration, while redundant curing water may have negative effect, such as causing some spherical capillary pores [72]. Thus it is of great importance to add appropriate curing agents to achieve enhanced mechanical properties. It can be seen from Fig. 15.3 that a small quantity of PEG results in the enhancement of compressive strength, split-tensile strength, and flexural strength of concrete. The PEG dosages of 1%, 1%, and 0.5% give M20, M25, and M40 grade concrete the highest strength, respectively [61,62]. The self-healing concrete incorporating with curing agents has been applied in infrastructures, especially bridge decks and pavements, and it can also be employed with recycled aggregate. In January 2005, a large railway transit yard in Texas, United States, was casted by approximately 190,000 m3 of HPC filled with presoaked LWA. Owing to the improvement of the cement hydration process, the flexural strength at 7 days achieved 90% 100% of that required at 28 days, and only very small shrinkage cracks occur [73]. In 2010, nine bridges were constructed using the concrete with a special mixture design similar to the one of conventional

270

(B)

55

M20 M25 M40

50 45 40 35 30 25 20

0.0

0.5 1.0 1.5 Content of PEG (%)

2.0

Split tensile strength (MPa)

Compressive strength (MPa)

(A)

Self-Healing Composite Materials

3.0

M20 M25 M40

2.5

2.0

1.5 0.0

Flexural strength (MPa)

(C) 5.5

0.5 1.0 1.5 Content of PEG (%)

2.0

M20 M40

5.0 4.5 4.0 3.5 3.0

0.0

0.5 1.0 1.5 Content of PEG (%)

2.0

Figure 15.3 Mechanical properties of self-healing cement concrete with different PEG dosages: (A) compressive strength; (B) split-tensile strength; and (C) flexural strength.

deck design, in addition to the extra 120 kg/m3 of fine LWA. It was reported that a remarkable improvement of 2% 10% in strength is obtained for Count Street Bridge and the strength of Bartell Road Bridge is enhanced by 15% at 28 days. In addition, according to the analysis of life-cycle cost of HPC bridge with internal curing in New York, Cusson et al. pointed out that nevertheless there exist slightly higher initial costs, the life-cycle costs can be cut down by 63% maximally. Porous crushed brick with large surface area can play a role of internal curing in concrete [74]. Rani added crushed spent fire brick to concrete to replace part of sand, and similar mechanical properties and workability as normal concrete are obtained. This can not only effectively consume vast waste bricks, but can also alleviate the large demand for fine LWAs in infrastructures [75].

15.2.7 Electrodeposition technology enabled self-healing cement concrete Electrodeposition technology can endow reinforced concrete with self-healing property by filling the cracks in concrete and coating the concrete surface by electrodeposits of chemical compounds.

Self-healing concrete-based composites

271

Otsuki et al. presented the electrodeposition method to repair cracked reinforced concrete, and experimental results indicate that electrodeposits formed on the concrete surface can close crack and coat the concrete surface [15]. Ryu reported that the self-healing ability provided by electrodeposition technology can improve the durability of reinforced concrete [76]. It was observed that almost drying shrinkage cracks with a width of 0.05 0.10 mm are completely closed after test period of 14 days, and around 70% of surface area is coated. In addition, the presence of electrodeposits prevents water from entering the concrete and hinders carbonation of the concrete. Ryu et al. investigated the crack repair based on electrodeposition technology by analyzing the crack closure rate and the chloride concentration on the steel surface [77]. Specimens were immersed in ZnSO4 solution with a concentration of 0.1 mol/L for 18 months and were applied with a constant current for 8 weeks. The results indicate that cracks with a width of 0.2 mm show a higher rate of crack closure than cracks with a width of 0.6 mm. Moreover, a rapidly increasing rate of crack closure is observed during the first 2 weeks and nearly 90% of the cracks are closed at the end of the 8-week test period. Chloride concentration on the steel surface continues to decrease with time, meaning that the corrosion resistance of steel is enhanced. Ryu et al. conducted a research about the ability of electrodeposition technology to repair cracks, and considered the effects of water cement ratio and solution temperature [78]. A larger water cement ratio results in a higher rate of crack closure. This is because that high water cement ratio results in more pore formation in concrete, thus reducing resistance and causing larger electric current. In addition, higher solution temperature is more conductive to increasing the crack closure rate. Otsuki and Ryu found that ZnSO4 solution shows the best electrodeposition effect on the self-healing of cement concrete, compared with external solution such as MgCl2, AgNO3, CuCl2, Mg(NO3)2, CuSO4, Ca(OH)2, and NaHCO3 [79]. Jiang et al. employed porous concrete as a simulation of cracks within reinforced concrete [80]. Greater current density or electrolyte solution concentration leads to more products deposited on the specimen in a certain period of time, but makes them bigger and looser. However, according to the research of Chu et al., as the concentration of ZnSO4 or MgSO4 electrolyte solution increases, the weight gain rate, crack closure rate, surface coating rate, as well as the filling depth of cracks all reduce [81]. The electrolyte solution concentration only influences the sediment morphology without affecting the composition of the sediment. The crack self-healing speed improves rapidly in the first 5 days then decreases, and the cracks nearly completely healed within 20 days. To sum up, electrodeposition technology can not only help heal cracks and provide a protective layer to improve watertightness and carbonation resistance properties of concrete, but can also enhance the corrosion resistance of steel. Besides, this electrodeposition method belongs to an electrochemical technique, so it works only under the conditions of conductive concrete, electric current, and electrolyte solution.

272

Self-Healing Composite Materials

15.2.8 Vascular technology enabled self-healing cement concrete Vascular technology refers to a one-channel or multiple-channel vascular system consisting of hollow tubes with the healing agent stored therein. The long vascular channel can connect the inside and the outside of the structure, so that the selfhealing agent can be continuously supplemented. When the passing crack causes the tube to rupture, the self-healing agent is released to repair the crack under the action of capillary force, gravity, etc. [14]. The vascular technology was first proposed by Dry for healing cracks in concrete [82]. Joseph et al. prepared concrete beams embedded with hollow glass tubes containing healing agent and studied their self-healing behavior [14]. During the test, the healing agent can be replenished through the open end of the tube to support crack repair. Both primary and secondary healing are observed during the first and second loading cycles, respectively. The porous concrete core is also employed as a vascular channel, which acts as a sensor for detecting cracks and can be used to inject healing agents to fill voids and cracks in concrete body [83]. This method heals a macrocrack and leads to strength recovery. Huang et al. compared selfhealing in concrete materials realized by using capsules and a vascular system as the carrier of Ca(OH)2 solution [84]. Since the vascular system can continuously provide the healing agent, it helps achieve higher self-healing efficiency. After continuously supplying saturated Ca(OH)2 solution for 250 h, about 80% of the original ultrasonic pulse velocity through the specimens is recovered. Vascular technology can provide repeatable and continuous self-healing for concrete. However, they generally have a diameter of no less than 3 mm, which may damage the mechanical properties of concrete.

15.2.9 Bacterial technology enabled self-healing cement concrete The principle of self-healing caused by ureolytic bacteria can be explained as follows. Because the bacterial cell wall is negatively charged, calcium ions in the solution are attracted to it. In addition, ureolytic bacteria decompose urea into ammonium and inorganic carbon. When the concentration of CaCO3 exceeds its solubility, it will precipitate on the cell wall of the bacteria [85]. CaCO3 can be generated by bacteria through a variety of metabolic pathways, mainly including urea hydrolysis and organic acid oxidation. The advantages and shortcoming of these two pathways are given in Table 15.4 [86]. CaCO3 precipitation induced by bacteria can be used to seal cracks [87]. Proper bacteria and calcium sources are prerequisites for self-healing. Jonkers et al. incorporated alkaliphilic spore-forming bacteria of the genus Bacillus in concrete as selfhealing agent. They found that high numbers of bacteria (109 cm23) and some suitable organic growth substrates do not negatively affect compressive and flexural tensile strength [88]. Besides, bacterial spores can be activated by crack ingress water and meanwhile the bacteria convert incorporated calcium lactate to calcium

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Table 15.4 The metabolic approaches of CaCO3 precipitation [86]. Metabolic approaches

Advantages

Shortcomings

Hydrolysis of urea

Large amounts of carbonate are rapidly produced Catalyzed by urease Less environmental impact CO2 is produced and it can also react with portlandite

Excessive ammonium is produced

Oxidation of organic acids

Need more time to produce carbonate

carbonate based minerals, thereby plugging cracks, reducing permeability, and further preventing corrosion of steel reinforcement [16]. Krishnapriya et al. investigated the feasibility of four types of calcite precipitating bacteria isolates as healing materials, including Bacillus licheniformis BSKNAU, Bacillus megaterium BSKAU, B. megaterium MTCC 1684, and Bacillus flexus BSKNAU [89]. The results manifest that strength of concrete with the first three bacteria is enhanced and cracks in concrete are completely repaired by calcite. Nonureolytic bacteria and calcium source nutrients as a two-component healing agent were also studied [90]. It was found that healing efficiency is significantly affected by the repair method and the type of calcium source. External treatment with nutritional calcium source media together with bacteria allows concrete to obtain the maximum healing ratio and recovery ratio of flexural strength and modulus. Second is calcium glutamate, which is significantly superior to control samples and is also more effective than calcium lactate. High internal pH, relative dryness, and lack of nutrients required for bacteria make concrete a pretty rugged environment for common bacteria. Besides, the study of Jonkers et al. shows that bacterial spores without any protective measures can survive in the cement paste mixture for 4 months. Whereas if the pore width in the matrix is less than the typical size of Bacillus spores (1 µm), the life span of spores may be limited [16]. For these reasons, some scholars have carried out researches about carrier materials for protecting bacteria in concrete. Bang et al. immobilized the Bacillus pasteurii cells with polyurethane foam [91]. They found that the immobilized bacteria exhibit the same rates of calcite precipitation and ammonia production as those of the free bacteria. Calcite in polyurethane improves the compressive strength of concrete samples. Wiktor et al. quantified the crack-healing potential of a novel two-component biochemical self-healing agent consisting of bacterial spores and calcium lactate, which are embedded in porous expanded clay particles [3]. Experimental results indicate that the maximum width of healing cracks in bacterial concrete reaches 0.46 mm, but it is only 0.18 mm in control samples after 100 days of water curing. Wang et al. found that diatomaceous earth has a superduper protective effect for bacteria and its optimal mass content is 60% of the volume of bacterial suspension [92]. The concrete incorporating with diatomaceous earth immobilized bacterial cells completely repairs the cracks with a width

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between 0.15 and 0.17 mm because of CaCO3 precipitation, and its capillary water absorption is 50% or 70% lower than that of specimens without bacteria. Compared with silica gel, the use of polyurethane as a carrier for protecting the bacteria in concrete can result in more CaCO3 precipitation, greater strength recovery and lower water permeability coefficient [93]. Hydrogel can also be applied to carry bacteria and water. Mortar specimens containing hydrogel-encapsulated spores exhibit evident self-healing performance. Its water permeability is reduced by 68% and 0.5 mm wide cracks are completely repaired [94]. Because microcapsules are resistant to the high pH of concrete and are humidity sensitive, they can also be used to encapsulate bacterial spores to realize self-healing of concrete [7]. Mortar specimens with microencapsulated bacterial spores exhibit higher healing ratio, larger healing crack width, and much lower water permeability. However, selfhealing will not happen without liquid water. Er¸san et al. compared six concrete compatible materials (diatomaceous earth, metakaolin, expanded clay, granular activated carbon, zeolite, and air entrainment) for protecting either Bacillus sphaericus spores or Diaphorobacter nitroreducens and their respective nutrients and studied the effects of six protection materials on mortar setting and compressive strength [95]. Expanded clay and granular activated carbon are advantageous for microbial self-healing concrete applications and the latter is most promising. Activated compact denitrifying core and cyclic enriched ureolytic powder are considered as the optimum bacterial agents, since they have no significant influence on mechanical properties of the mortar specimens. Jonkers adopted a capsule made of biodegradable plastic to encapsulate Bacillus spores together with its nutrients calcium lactate and embed it into concrete [96]. Experimental results revealed that this biological concrete can heal up to 0.5 mm wide cracks in about 3 weeks. This bacterial technology has also been applied in an old building and has achieved good self-healing effects. In conclusion, the introduction of bacteria belongs to a kind of environmentfriendly concrete self-healing technology. It has broad application prospect in marine engineering environment and is expected to provide long-term self-healing due to the generation of new bacteria. However, water is an essential condition for self-healing behavior.

15.3

Self-healing asphalt concrete composites

15.3.1 Self-healing asphalt concrete incorporating with microcapsule Asphalt will age during use, causing a decrease in viscoelasticity. Even if the aging course is nonreversible, the original properties of asphalt concrete are able to be regained by supplementing the corresponding disappearing components or similarly effective components through microcapsules. The self-healing process of asphalt concrete incorporating microcapsule is similar to that of cement concrete containing microcapsule, which is mentioned in Section 15.2.3.

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Su et al. fabricated rejuvenator-filled microcapsules of which the shell was used as methanol-melamine-formaldehyde (MMF) prepolymer, and characterized their physicochemical properties [97]. The average size of microcapsules was reduced from 23.5 to 5.0 µm as the emulsion stirring rate was increased from 1000 to 6000 r
min21. MMF shell microcapsules have the characteristics of elastoplastic deformation and their micromechanical properties are greatly affected by size and shell thickness. Besides, the microcapsule decomposition temperatures surpass the asphalt melting temperature, which guarantees its recovery capacity. Sun et al. incorporated 11 microcapsules which were fabricated with different core shell thickness ratio at different stirring rates into asphalt. Then the recovery experiments of fatigue life were performed to assess their self-healing ability with suitable diameter and shell thickness of microcapsules [98]. The shell material of microcapsules is melamine-formaldehyde resin and rejuvenator is the core material. It was found that microcapsules prepared with 1:1 core shell thickness ratio under 800 rpm stirring rate has the greatest effect on improving the self-healing ability of asphalt concrete, resulting in a 20% increase in total loading cycles relative to asphalt concrete without microcapsules. Su et al. reported that the crack selfhealing rate of the bitumen/MMF shell microcapsule composites is fastest at 30 C compared with the healing temperatures of 10 C and 20 C. Furthermore, microcapsules can be punctured by microcracks and release repair agents in aged bitumen. The bitumen sample containing microcapsules with an average size greater than 20 µm can recover to its initial load values, as sufficient rejuvenators are released from microcapsules [99].

15.3.2 Self-healing asphalt concrete incorporating with nanofillers Nanofillers are commonly employed to prepare the self-healing capability of asphalt concrete, including nanoclay, nano-SiO2, carbon nanotubes, graphite and carbon black. The mechanism of nanofillers for the self-healing behavior of asphalt concrete may be explained as follows. As the crack develops, nanofillers tend to move to the crack tip due to their high surface energy, thus preventing the propagation of cracks and healing cracks [100]. Limited researches have been conducted on self-healing asphalt concrete incorporating with nanofillers. You et al. found that when adding 2% and 4% nanoclays, the shear composite modulus, the secant or direct tension moduli, and rotational viscosity of asphalt concrete are greatly improved, indicating that nanoclays help to avoid rutting and cracking of asphalt concrete. This is conducive to self-healing of asphalt concrete incorporating with nanoclays [101]. The addition of nano-SiO2 makes the viscosity of asphalt concrete slightly reduced, which is conducive to reducing the compaction temperature/energy consumption during construction. Meanwhile, the recovery ability and resistance to rutting/stripping of asphalt concrete can also be improved [102]. Nanotubes or nanoclays are always effective in improving the fatigue response of asphalt binders. Moreover, since the chemical

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structure of bituminous matrix plays a key role, the healing behavior is more sensitive to the interaction of asphalt and nanoparticles [103]. However, nanofillers have poor dispersion in asphalt and are easy to aggregate. This affects the microstructure of asphalt and causes brittle interface between nanofillers and asphalt, consequently leading to a decline in performance of asphalt concrete [104]. Therefore it is necessary to further study the preparation method and mechanism of self-healing asphalt concrete with nanofillers.

15.3.3 Induction energy/microwave technology enabled selfhealing asphalt concrete Asphalt concrete employing induction energy/microwave technology to realize self-healing contains sufficient conductive fibers/fillers. The closed loops formed by conductive fibers/fillers around the cracks can induce the generation of electric current in magnetic field, which causes the asphalt to melt and eventually close cracks. Garcı´a et al. added graphite and steel wool as conductive fillers and fibers into asphalt mortar [105]. The results revealed that adding conductive fibers rather than fillers allows for a faster appearance of the percolation threshold. Asphalt mortar blended with the least amount of fibers can generate heat spontaneously, because fibers form electrically conductive closed loops which are susceptible to magnetic effects. Moreover, lower resistivity results in higher heating rate. The research of Liu et al. indicated that multiple healing of steel wool reinforced asphalt beams with entire fracture could be achieved, thanks to induction heating [106]. The research of Dai et al. revealed that electromagnetic induction heating can heal cracks in asphalt concrete more than six times. Besides, as the heating temperature (60 C, 80 C, and 100 C) increases, the healing performance is enhanced [107]. Schlangen et al. measured the self-healing property of tiny steel fiber modified asphalt concrete based on induction energy technology, and concluded that if the induction machine runs over the road made using this concrete every 4 years, the service life of the road will double or even longer [18]. The research of Garcı´a et al. manifested that the asphalt concrete conductivity improves as the content of steel fiber increases, and the speed of induction heating depends on the volume and diameter of the steel fiber [108]. The effects of different kinds of bitumen and different asphalt mixture porosities on the induction-healing rates were investigated by Garcı´a et al. [109]. They concluded that the ability of asphalt to flow at the threshold temperature determines the minimum temperature at which the asphalt mixture begins to heal. Porous asphalt concrete needs a higher temperature to reach the same healing level than dense asphalt concrete. The maximum healing recovery for porous asphalt mixtures and dense asphalt mixtures do not exceed 78% and 51%, respectively. The type of bitumen used affects the maximum degree of healing of the asphalt mixture after damage. Besides, structural breakage of the material is caused possibly due to continuous heating of asphalt mixture, and accompanied by a decrease in self-healing ability.

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In addition to electromagnetic induction, microwaves can also induce the selfhealing behavior of asphalt concrete. The study of Gallego et al. demonstrated that employing microwaves to promote self-healing of asphalt mixes with steel wool and graphite is more economic and effective than using electromagnetic induction [110]. The results show that optimal steel wool dosage is 10 times less than that required for electromagnetic induction heating and microwave devices consume less electricity. Induction energy/microwave technology is a promising approach for achieving self-healing, and it has been applied in practice. Nevertheless, repeated hightemperature heating for a long time will accelerate asphalt aging and volatilize toxic gases. Therefore the intelligent induction heating technology should be promoted to accurately determine the degree of asphalt damage, heating temperature and heating time.

15.3.4 Light repair technology enabled self-healing asphalt concrete The healing mechanism of light repair technology is that under the stimulation of ultraviolet light, the repairing agent molecules are converted from the ground state to the excited state to generate free radicals, and they undergo rearrangement reaction and paired bonds, thereby realizing the repair of cracks. Zhou et al. found that the incorporation of OXE-CHI-PUR healing agent synthesized by the chemical method can heal cracks in asphalt concrete under ultraviolet light. The synthesis conditions of this healing agent affect the healing efficiency of asphalt, and the best results are obtained when pH value is near neutral and the dosage of catalyst is around 2% [111]. Chen found that under the excitation of ultraviolet light, asphalt concrete incorporating with cross-linked polymers of trithiocarbonate units can achieve self-healing. The self-healing mechanism is that when excited by ultraviolet light, the sulfur radical and carbon-free radical generated are reconnected. Consequently, the cracks are repaired by the newly formed molecular chain network structure [112]. Nevertheless, it is still difficult to apply light repair technology to practical engineering, since the ultraviolet light irradiated onto the building in sunlight is very low [113]. In the future, it may be more practical to research the self-healing of asphalt concrete under visible light conditions.

15.4

Summary

Cement concrete is a brittle material with low tensile strength and is easy to crack due to shrinkage, creep, load and harsh service environment. Asphalt concrete pavement is likely to crack and wear due to the temperature instability and poor aging resistance of asphalt concrete. Crack accelerates moisture, oxygen and corrosive medium transports internally, reducing the durability and safety of infrastructures.

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In comparison with traditional repair method, self-healing technology can heal cracks that are invisible to the naked eye and it is expected to extend the service life of infrastructures. It has significant advantages for repairing marine infrastructures, underground concrete and concrete infrastructures storing hazardous materials. This chapter introduces various self-healing techniques used in cement concrete and asphalt concrete, and the conclusions are summarized as follows: 1. Self-healing concrete-based composites can repair the cracks in time based on different mechanisms and methods, thereby the strength is partially restored and the durability of concrete is improved. Common methods for evaluating self-healing capability of concrete-based composites include surface crack width, mechanical properties, permeability, microstructure and phase analyses. 2. Self-healing based on electrodeposition technology and induction energy/microwave technology has been applied in practice. However, most self-healing concrete-based composites are still at the stage of theoretical research and experimental exploration, so further efforts should be made to develop the self-healing concrete-based composites. Firstly, selfhealing detection and evaluation system should be improved. Secondly, further study should be focused on some key problems such as the compatibility of self-healing materials with concrete, durability of self-healing materials, self-healing speed, healing efficiency and repeatability. For capsules and bacteria, the protection method is also an important factor affecting the self-healing efficiency, which requires careful consideration.

Acknowledgment The authors thank the funding supported from the National Science Foundation of China (51578110) and the Fundamental Research Funds for the Central Universities in China (DUT18GJ203).

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