Effective Crack Control of Concrete by Self-Healing of Cementitious ...

materials Article

Effective Crack Control of Concrete by Self-Healing of Cementitious Composites Using Synthetic Fiber Heesup Choi 1 , Masumi Inoue 1 , Sukmin Kwon 2 , Hyeonggil Choi 3, * and Myungkwan Lim 4 1 2 3 4

*

Department of Civil and Environmental Engineering, Kitami Institute of Technology, Hokkaido 090-8507, Japan; [email protected] (H.C.); [email protected] (M.I.) Public Housing Division, Korea Land & Housing Institute, Daejeon 34047, Korea; [email protected] Faculty of Environmental Technology, Muroran Institute of Technology, Hokkaido 090-8585, Japan Department of Architectural Engineering, Songwon University, Gwangju 61756, Korea; [email protected] Correspondence: [email protected]; Tel.: +81-157-26-9474

Academic Editor: Prabir K. Sarker Received: 2 March 2016; Accepted: 24 March 2016; Published: 30 March 2016

Abstract: Although concrete is one of the most widely used construction materials, it is characterized by substantially low tensile strength in comparison to its compression strength, and the occurrence of cracks is unavoidable. In addition, cracks progress due to environmental conditions including damage by freezing, neutralization, and salt, etc. Moreover, detrimental damage can occur in concrete structures due to the permeation of deteriorating elements such as Cl´ and CO2 . Meanwhile, under an environment in which moisture is being supplied and if the width of the crack is small, a phenomenon of self-healing, in which a portion of the crack is filled in due to the rehydration of the cement particles and precipitation of CaCO3 , is been confirmed. In this study, cracks in cementitious composite materials are effectively dispersed using synthetic fibers, and for cracks with a width of more than 0.1 mm, a review of the optimal self-healing conditions is conducted along with the review of a diverse range of self-healing performance factors. As a result, it was confirmed that the effective restoration of watertightness through the production of the majority of self-healing products was achieved by CaCO3 and the use of synthetic fibers with polarity, along with the effect of inducing a multiple number of hairline cracks. In addition, it was confirmed that the self-healing conditions of saturated Ca(OH)2 solution, which supplied CO2 micro-bubbles, displayed the most effective self-healing performance in the surface and internal sections of the cracks. Keywords: micro-crack; synthetic fiber; PVA; cementitious composite materials; CO2 micro-bubble; self-healing; Ca(OH)2 ; CO3 2´ ; CaCO3

1. Introduction Concrete and cementitious construction materials are essential construction materials for buildings, civil engineering, and general construction in modern society, and it is deemed that the development of construction materials that can completely substitute for concrete is very problematic, even in the future. Meanwhile, since concrete is a material with tensile strength that is substantially low in comparison to its compression strength, the occurrence of cracks in concrete structures is unavoidable. In the case of Japan, cracks generated by the aforementioned reasons and having a width less than the allowable level are determined to impart no major effects on, or problems in, the durability of a structure [1]. However, although such micro-cracks in concrete are not in themselves a threat to the safety performance of structures, deteriorating elements such as CO2 and Cl´ are permeated into the body of the concrete by the micro-cracks, and in turn increase the water permeability, which is an index of durability evaluation [2]. In addition, repetitive permeation of such deteriorating elements expands Materials 2016, 9, 248; doi:10.3390/ma9040248

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the width of the cracks, thereby accelerating the deterioration of the concrete [3]. Due to this process, it is determined that there is an increased likelihood of a detrimental effect on the safety performance of concrete structures [4,5]. Accordingly, there is a need for the prevention of micro-cracks in concrete structures at a more fundamental stage. Meanwhile, under a moist environment, the phenomenon of filling in of portions of the cracks in concrete due to rehydration of the cement particles and precipitation of CaCO3 or calcite, particularly when the width of the crack is small, is being confirmed [6]. The self-healing products are closely related to hydrates such as C-S-H hydrate, ettringite, and calcium hydroxide, etc. [7,8], along with the calcium carbonate newly generated from the surfaces of the crack [3]. The self-healing mechanism of concrete generates CaCO3 , which is a carbon compound that does not dissolve well in water, through the reaction between Ca2+ in the concrete with the CO3 2´ dissolved in the water. This phenomenon leads to the reclamation or in-filling of the cracked portions [9]. The following are the crystalline reaction Equations (1)–(3) of calcite: H2 O ` CO2 ô H2 CO3 ô H+ ` HCO3 ´ ô 2H+ ` CO3 2´

(1)

Ca2+ ` CO3 2´ ô CaCO3 tpHwater ą 8u

(2)

Ca2+ ` HCO3 ´ ôCaCO3 ` H+ t7.5 ă pHwater ă 8u

(3)

Also, various self-healing approaches attempt to promote autogenous healing, such as the use of bacteria, crystalline admixtures, and superabsorbent polymers [10–14]. This self-healing phenomenon can delay the permeation of Cl´ by reducing the cracks in the concrete, and lead to a reduction in the permeability coefficient by partially restoring the permeability, which had been markedly increased due to the cracks [2,15]. In addition, it can almost fully restore the elastodynamic coefficient of concrete in which deterioration had occurred due to freeze–thawing, and also partially restore the deteriorated strength of the concrete [16]. Therefore, if a crack generated by the aforementioned causes can be restored through self-healing, it is possible to manage the progress of cracks effectively at the initial stage of their occurrence and, ultimately, to achieve the suppression of degradation in the safety performance of concrete structures. In addition, self-healing can make substantial contributions toward the ease of maintenance of concrete structures and ensure a reduction in environmental load, along with the prolongation of the lifespan of structures. Existing research reports that engineered cementitious composites (ECC) with multiple fine cracking have the greatest potential for the practical achievement of self-healing in concrete [17,18]. Especially, ordinary concrete restores cracks of 50 µm [19] and 30 µm [14] width through self-healing [9]. In addition, the self-healing performance can be made more efficient by effectively dispersing the cracks and thereby substantially reducing the width of the cracks generated. This can be achieved by mixing synthetic fibers such as polyvinyl alcohol (PVA), polyethylene (PE) and polypropylene (PP), etc. into the concrete [20,21]. In particular, it has been confirmed that by using PVA fibers with polarity-induced OH´ radicals, much better self-healing, along with effective dispersion of cracks, can be achieved even for cracks with a width of more than 0.1 mm where the permeation of CO2 gas and chloride ion (Cl´ ) is a concern [22]. This study assessed the composite self-healing performance of the cementitious materials to which a diverse range of synthetic fibers had been added, and the effect on micro-cracks with a width of more than 0.1 mm, which can result in serious degradation of the durability of the concrete structure. After manufacturing the specimen using synthetic fiber-reinforced cementitious composite materials, micro-cracks were induced in the specimen by applying tensile force. This was followed by the measurement of the changes in the physical characteristics of the specimen, structural changes in the surface and internal sections of the cracked sections, and the types and quantities of products arising from the self-healing of the introduced micro-cracks. In addition, it was concurrently attempted to identify the optimal self-healing conditions by inducing self-healing under an extensive range of conditions.

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2. Materials and Method  2. Materials and Method 2.1. Materials  2.1. Materials The mixture proportions of the mortar are summarized in Table 1. Portland cement (C, density:  3,  3.16 g/cm , mean diameter: 10 μm), quartz sand as the fine aggregate (S, surface‐dry density: 2.61 g/cm The 3mixture proportions of the mortar are summarized in Table 1. Portland cement (C, density: 3 mean  diameter:  180  μm),  and  high‐performance  water  reducing  agent (S,as surface-dry an  admixture  (SP,  3.16 g/cm , mean diameter: 10a µm), quartz sand as the fine aggregate density: density: 1.05 g/cm , main constituent: polycarboxylate‐based superplasticizer) were used. As for the  2.61 g/cm3 , mean 3diameter: 180 µm), and a high-performance water reducing agent as an admixture synthetic  fibers,  PVA 3 ,(fiber  diameter:  40  μm,  fiber  length:  12  mm,  density: were 1.3  used. g/cm3As )  and  (SP, density: 1.05 g/cm main constituent: polycarboxylate-based superplasticizer) for 3 3 polyethylene  (PE)  (fiber  diameter:  12  μm,  fiber  length:  12  12 mm,  g/cm ) ) and  the synthetic fibers, PVA (fiber diameter: 40 µm, fiber length: mm,density:  density:0.97  1.3 g/cm and 3) were used. The  polypropylene (PP) (fiber diameter: 65 μm, fiber length: 12 mm, density: 0.91 g/cm polyethylene (PE) (fiber diameter: 12 µm, fiber length: 12 mm, density: 0.97 g/cm3 ) and polypropylene 3 ) were properties  of  the  employed  fibers  are 12presented  in  Table  2.  The  chemical  components  (PP) (fiber diameter: 65 µm, fiber length: mm, density: 0.91 g/cm used. The propertiesof  of the  the employed  synthetic  fibers  are in characterized  polar  components groups  as  shown  Figure  1.  PVA  has  the  employed fibers are presented Table 2. The by  chemical of the in  employed synthetic fibers highest polarity strength owing to the OH radical (indicated by the circle), whereas PE and PP have  are characterized by polar groups as shown in Figure 1. PVA has the highest polarity strength owing no polarity strength. to the OH radical (indicated by the circle), whereas PE and PP have no polarity strength. Table 1. Mixture proportions of the mortar.  Table 1. Mixture proportions of the mortar.

Type  S/C (wt.%)  Type S/C (wt.%) PVA  PVA 0.4  PE  PE 0.4 PP  PP

W/C (wt.%)

W/C (wt.%)

0.3 

0.3

SP/C (wt.%) Fiber (vol.%)  Fiber (vol.%) 0.4  0.4 1.2  0.45  0.45 1.2 0.3  0.3

SP/C (wt.%)

Note: wt.: Weight; vol.: Volume; S: Quartz sand; C: Portland cement; W: Water; SP: High‐performance  Note: wt.: Weight; vol.: Volume; S: Quartz sand; C: Portland cement; W: Water; SP: High-performance water water reducing agent; PVA: Polyvinyl alcohol; PE: Polyethylene; PP: Polypropylene.  reducing agent; PVA: Polyvinyl alcohol; PE: Polyethylene; PP: Polypropylene. Table 2. Properties of employed fibers. Table 2. Properties of employed fibers.  

Tensile Strength  Length  Density  Tensile Strength (N/mm2 ) Length (mm) 3 2 (N/mm )  (mm)  (g/cm )  16001600  12 1.30  12  0.97  2580  12  2580 12 500 500  12 0.91  12 

Type Type  Type of Fiber Density (g/cm3 ) Type of Fiber  Polyvinyl PVA PVA  1.30 Polyvinyl alcohol  alcohol PE  Polyethylene  PE Polyethylene 0.97 PP PP  Polypropylene 0.91 Polypropylene 

( CH2

CH )n

( CH2 CH2 )n

CH )n

( CH2

CH3

OH PVA

Diameter  Diameter (µm) (μm)  40 40 12 12 65 65

PE

PP

Figure 1. Characteristic part of the chemical components of each fiber. Figure 1. Characteristic part of the chemical components of each fiber.

2.2. Specimen Overview  2.2. Specimen Overview The  dimensions  of of  the the specimens specimens were were 85 85 ˆ ×  80 80 ˆ ×  30  (L  ˆ ×  B  The dimensions 30 mm  mm (L B ×  ˆ H).  H). Two  Two specimens  specimens were  were fabricated for each series. After mixing the mortar with the fibers, water curing was performed in a  fabricated for each series. After mixing the mortar with the fibers, water curing was performed in tank at 20 °C for 28 days. A universal testing machine (UTM) was used to apply a tensile load at a  a tank at 20 ˝ C for 28 days. A universal testing machine (UTM) was used to apply a tensile load speed  of  0.2  and and the the crack  width  was  adjusted  so so that  the  at a speed of mm/min,  0.2 mm/min, crack width was adjusted that thedisplacement  displacementof ofthe  thePI  PI (π)  (π) displacement  transducer  would  be  about  0.3  mm.  Figure  2a,b  show  the  mimetic  diagram  of  the  displacement transducer would be about 0.3 mm. Figure 2a,b show the mimetic diagram of the specimens, used in crack introduction, and the tensile load test.  specimens, used in crack introduction, and the tensile load test.

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Tension H

B

Crack L

Tension

  (a) 

(b)

Figure  2.  Specimen  overview.  (a) Geometry  of  the  specimen,  (b) Direction  of  the  load  during  the  Figure 2. Specimen overview. (a) Geometry of the specimen; (b) Direction of the load during the tensile tensile test. PI: π.  test. PI: π.

2.3. Experimental Method  2.3. Experimental Method The  order  and  evaluation  items  of  the  experiment  for  the  assessment  of  the  composite  The order and evaluation items of the experiment for the assessment of the composite self-healing self‐healing performance of the concrete, in accordance with the addition of synthetic fibers, are as  performance of the concrete, in accordance with the addition of synthetic fibers, are as follows. First, in follows.  First,  in  Step  A  (prior  to  self‐healing),  an  analysis  of  the  permeability  coefficient  Step A (prior to self-healing), an analysis of the permeability coefficient immediately following the immediately following the introduction of the cracks by the tensile loading test, observation of the  introduction of the cracks by the tensile loading test, observation of the internal sections of cracks using internal  sections  of  cracks  using  microfocus  X‐ray  computed  tomography  (CT)  scans,  and  an  microfocus X-ray computed tomography (CT) scans, and an analysis of the types and quantities of the analysis  of  the  types  and  quantities  of  the  hydrates  prior  to  self‐healing  using  the  thermo  hydrates prior to self-healing using the thermo gravimetric-differential thermal analysis (TG-DTA) gravimetric‐differential  thermal analysis  (TG‐DTA)  measurement, were  executed.  In  Step  B  (After  measurement, were executed. In Step B (After self-healing), a comparison was made using the method self‐healing), a comparison was made using the method applied in Step A in order to evaluate the  applied in Step A in order to evaluate the changes in the permeability of each of the specimens changes in the permeability of each of the specimens due to self‐healing and changes in the structure  due to self-healing and changes in the structure within the cracks, a quantitative evaluation of the within  the  cracks,  a  quantitative  evaluation  of  the  self‐healing  precipitated  substances  was  self-healing precipitated substances was undertaken. Especially, the coefficient of water permeability undertaken.  Especially,  the  coefficient  of  water  permeability  was  calculated  by  the  water  flow  was calculated by the water flow speed through the plate specimen in Step A. Then, all specimens speed through the plate specimen in Step A. Then, all specimens were kept for 7 days in a water  were kept for 7 days in a water tank at 20 ˝ C. After the curing for self-healing, the coefficient of tank at 20 °C. After the curing for self‐healing, the coefficient of water permeability was evaluated  water permeability was evaluated by the water flow speed through the plate specimen in Step B. by  the  water  flow  speed  through  the  plate  specimen  in  Step  B.  The  apparatus  used  in  the  water  The apparatus used in the water permeability tests in this study is shown in Figure 3 [23]. Furthermore, permeability  tests  in  this  study  is  shown  in  Figure  3  [23].  Furthermore,  in  Step  B,  the  self‐healing  in Step B, the self-healing characteristics and the types of precipitated substances at the surface section characteristics  and  the  types  of  precipitated  substances  at  the  surface  section  of  the  cracks  were  of the cracks were assessed by optical microscopic observation and Raman spectroscopy analysis. assessed  by  optical  microscopic  observation  and  Raman  spectroscopy  analysis.  The  experimental  The experimental factors and conditions are summarized in Table 3. As the conditions for self-healing factors  and  conditions  are  summarized  in  Table  3.  As  the  conditions  for  self‐healing  in  this  in this experiment, two conditions were used in existing research; namely, water (tap water) that experiment, two conditions were used in existing research; namely, water (tap  water) that supplied  supplied CO2 micro-bubbles (W + MB) and saturated Ca(OH)2 solution (Ca + MB) were employed. CO2 micro‐bubbles (W + MB) and saturated Ca(OH) 2 solution (Ca + MB) were employed. The water  The water temperature was adjusted to 20˝ C for both, and the pH to 6.0 and 8.5, respectively, for temperature was adjusted to 20°C for both, and the pH to 6.0 and 8.5, respectively, for the evaluation  the evaluation of the self-healing performance of each of the specimens in accordance with each of of  the  self‐healing  performance  of  each  of  the  specimens  in  accordance  with  each  of  the  the aforementioned conditions of self-healing, with the setting of the period of self-healing to 7 days. aforementioned conditions of self‐healing, with the setting of the period of self‐healing to 7 days. It is  It is thought that the Ca2+ in a saturated Ca(OH)2 solution promotes self-healing and that there is thought  that  the  Ca2+  in  a  saturated  Ca(OH)2  solution  promotes  self‐healing  and  that  there  is  a  a possibility of promoting the generation of self-healing precipitated substances through an increase in possibility of promoting the generation of self‐healing precipitated substances through an increase in  the quantity of CO3 2´ supplied by CO2 micro-bubbles at the time of self-healing [24,25]. Therefore, the quantity of CO32−  supplied by CO2 micro‐bubbles at the time of self‐healing [24,25]. Therefore, this  this was applied in this study to maximize the self-healing performance. was applied in this study to maximize the self‐healing performance.   

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Table 3. Experimental factors and conditions. Table 3. Experimental factors and conditions. 

Experimental Factors Experimental Factors Fiber  Fiber Water + Micro‐bubble    (W + MB)  Water + Micro-bubble (W + MB) Self-healing Self‐healing  Ca(OH) + Micro-bubble (Ca + MB)   (Ca + MB)  Ca(OH)2 + Micro‐bubble  2 Temperature  Temperature Crack (Target of crack width: 0.3 mm) Crack (Target of crack width: 0.3 mm)  Period of self-healing Period of self‐healing 

Conditions  Conditions PVA, PE, PP  PVA, PE, PP pH 6.0pH 6.0  pH 8.5pH 8.5  20 ˝ C 20 °C  TensileTensile load  load 7 Days7 Days 

Note: W + MB: Water + Micro-bubble; Ca + MB: Ca(OH)2 + Micro-bubble. Note: W + MB: Water + Micro‐bubble; Ca + MB: Ca(OH) 2 + Micro‐bubble. 

  Figure 3. Apparatus used in the water permeability tests [23].  Figure 3. Apparatus used in the water permeability tests [23].

3. Results and Discussion  3. Results and Discussion 3.1. Permeability Coefficient  3.1. Permeability Coefficient Figures 4–6 illustrate the results of the water permeability test. Here, Figures 4 and 5 display the  Figures 4–6 illustrate the results of the water permeability test. Here, Figures 4 and 5 display the permeability coefficients of each of the fiber series prior to, and following self‐healing in accordance with  permeability coefficients of each of the6 fiber series to, and coefficient  following ratio  self-healing the  conditions  of  self‐healing.  Figure  displays  the prior permeability  for  each in of accordance the  fiber  series  basis  of  the  permeability  coefficient  values  of  Step  A.  In  addition,  lower  with the computed  conditionson  ofthe  self-healing. Figure 6 displays the permeability coefficient ratio forthe  each of the permeability coefficient of each of the graphs signifies improvement in the resistance to permeability.  fiber series computed on the basis of the permeability coefficient values of Step A. In addition, the lower The  results  of  the  (W  +  MB)  in  Step  B,  when  compared  with  that  Step  A,  permeability coefficient ofexperiment,  each of the graphs signifies improvement in the resistance to in  permeability. displayed  the  trend  of  an  increase  in  the  resistance  to  permeability  by  about  40‐fold  for  PVA,  The results of the experiment, (W + MB) in Step B, when compared with that in Step A, displayed 3.5‐fold  for  PE,  and  1.5‐fold  for  PP  (Figure  4).  Meanwhile,  (Ca  +  MB)  in  Step  B,  when  compared  the trend of an increase in the resistance to permeability by about 40-fold for PVA, 3.5-fold for PE, with  that  in  A,  displayed  the  trend  of  an +increase  in  the  to  permeability  and 1.5-fold forStep  PP (Figure 4). Meanwhile, (Ca MB) in Step B,resistance  when compared with thatby  inabout  Step A, 460‐fold for PVA, 60‐fold for PE, and 6‐fold for PP (Figure 5). In addition, in the comparison of the  displayed the trend of an increase in the resistance to permeability by about 460-fold for PVA, 60-fold permeability coefficient following self‐healing as illustrated in Figure 6, (Ca + MB), in comparison to  for PE, and 6-fold for PP (Figure 5). In addition, in the comparison of the permeability coefficient (W + MB), displayed the trend of improvement in the resistance to permeability by approximately  following self-healing as illustrated in Figure 6, (Ca + MB), in comparison to (W + MB), displayed 15‐fold  for  PVA,  17‐fold  for  PE,  and  4‐fold  for  PP.  From  the  aforementioned  results,  it  can  be  the trend of improvement in the resistance to permeability by approximately 15-fold for PVA, 17-fold discerned that the resistance to permeability is improved in the order of PVA > PE > PP, regardless  for PE, and 4-fold for PP. From the aforementioned results, it can be discerned that the resistance to of  the  conditions  of  self‐healing.  In  particular,  PVA  with  OH−radical  displayed  a  more  effective  permeability is improved in the order of PVA > PE > PP, regardless of the conditions of self-healing. self‐healing  performance,  and  it  was  confirmed  that  the  conditions  of  (Ca  +  MB)  were  more  ´ radical displayed a more effective self-healing performance, and it was In advantageous  particular, PVA withthe  OH than  conditions  of  (W  +  MB)  for  the  promotion  of  self‐healing  performance.  confirmed that the conditions of (Ca + MB) were more advantageous than the conditions of (W + MB) Therefore, for micro‐cracks with a width of more than 0.1 mm, for which substantial permeation of  fordeteriorating elements from the external into the internal sections of the concrete was anticipated, it  the promotion of self-healing performance. Therefore, for micro-cracks with a width of more than 0.1was deemed that the generation and precipitation of the self‐healing substances were promoted due  mm, for which substantial permeation of deteriorating elements from the external into the internal − radical, along with the enhancement of the conditions of  sections of the concrete was anticipated, it was deemed that the generation and precipitation of the to the mixing of the PVA fiber with the OH 2+) that contained CO self-healing substances were promoted due to the mixing of the PVA fiber with the OH´ radical, along self‐healing by the saturated Ca(OH) 2 solution (Ca 2 micro‐bubbles (CO 32−) [8,24].  with the enhancement of the conditions of self-healing by the saturated Ca(OH)2 solution (Ca2+ ) that contained CO2 micro-bubbles (CO3 2´ ) [8,24].

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6 of 14 6 of 14  6 of 14  6 of 14  Coefficient of water permeability Coefficient ofof water permeability Coefficient water permeability (m/sec) (m/sec) (m/sec)

PVA PVA PVA

11 1 0.1 0.1 0.1 0.01 0.01 0.01 0.001 0.001 0.001 0.0001 0.0001 0.0001 0.00001 0.00001 0.00001 0.000001 0.000001 0.000001 0.0000001 0.0000001 0.0000001

Step. Step.A A Step. A

PE PE PE

PP PP PP

Step. Step. B B Step. B

   

Coefficient of water permeability Coefficient ofof water permeability Coefficient water permeability (m/sec) (m/sec) (m/sec)

Figure 4. Permeability coefficientof Water + Micro bubble (W + MB).  Figure 4. Permeability coefficientof Water + Micro bubble (W + MB).  Figure 4. Permeability coefficientof Water + Micro bubble (W + MB).  Figure 4. Permeability coefficientof Water + Micro bubble (W + MB). PVA PVA PVA

11 1

0.1 0.1 0.1 0.01 0.01 0.01 0.001 0.001 0.001 0.0001 0.0001 0.0001 0.00001 0.00001 0.00001 0.000001 0.000001 0.000001 0.0000001 0.0000001 0.0000001

PE PE PE

Step. Step.A A Step. A

Step. Step. B B Step. B

PVA PVA PVA

PE PE PE

PP PP PP

Ratioofof ofcoefficient coefficientofof ofwater water Ratio Ratio coefficient water permeability permeability permeability

     Figure 5. Permeability coefficientof Ca(OH) 2  + Micro‐bubble (Ca + MB).  Figure 5. Permeability coefficientof Ca(OH)  + Micro‐bubble (Ca + MB).  Figure 5. Permeability coefficientof Ca(OH)222 + Micro‐bubble (Ca + MB).  + Micro-bubble (Ca + MB). Figure 5. Permeability coefficientof Ca(OH) 11 1

PP PP PP

0.1 0.1 0.1 0.01 0.01 0.01

0.001 0.001 0.001

0.0001 0.0001 0.0001

W+MB W+MB W+MB

Ca+MB Ca+MB Ca+MB

11 == Standard Standard    1 = Standard   Figure 6. Comparison of permeability coefficient ratio.  Figure 6. Comparison of permeability coefficient ratio.  Figure 6. Comparison of permeability coefficient ratio. Figure 6. Comparison of permeability coefficient ratio. 

3.2. Microscopic Review of the Crack Section Due to Self‐Healing  3.2.3.2. Microscopic Review of the Crack Section Due to Self‐Healing  Microscopic Review of the Crack Section Due to Self-Healing 3.2. Microscopic Review of the Crack Section Due to Self‐Healing  3.2.1. Surface Section of the Cracks  3.2.1. Surface Section of the Cracks  3.2.1. Surface Section of the Cracks  3.2.1. Surface Section of the Cracks Microscopic  Microscopic  observation  observation  of  of  the  the  surface  surface  section  section  of  of  the  the  cracks  cracks  was  was  executed  executed  using  using  an  an  optical  optical  Microscopic  observation  the  surface  section  of  Microscopic observation ofof the surface section of the  the cracks  cracks was  wasexecuted  executedusing  usingan anoptical  optical microscope and Raman spectroscopy in order to check the precipitated substances generated by the  microscope and Raman spectroscopy in order to check the precipitated substances generated by the  microscope and Raman spectroscopy in order to check the precipitated substances generated by the  microscope and Raman spectroscopy in order to check the precipitated substances generated by the self‐healing at the surface of the cracks. Since a white‐colored precipitated substance was observed at  self‐healing at the surface of the cracks. Since a white‐colored precipitated substance was observed at  self‐healing at the surface of the cracks. Since a white‐colored precipitated substance was observed at  self-healing at the surface of the cracks. Since a white-colored precipitated substance was observed at the surface section of the cracks, which exhibited self‐healing for both the (W + MB) and (Ca + MB),  the surface section of the cracks, which exhibited self‐healing for both the (W + MB) and (Ca + MB),  the surface section of the cracks, which exhibited self‐healing for both the (W + MB) and (Ca + MB),  themicroscopic  surface section of the cracks, which exhibited for both (W for  + MB) (Ca + MB), of  surface  section  of  was  made  the  presence  of  microscopic  observation  observation  of  the  the  surface  section self-healing of  the  the  cracks  cracks  was  the made  for  the and presence  of  microscopic  observation  of  the  surface  section  of  the  cracks  was  made  for  the  presence  of  microscopic observation of the surface section of the cracks was made for the presence of self-healing self‐healing (Ca + MB), which was determined to be advantageous in the promotion of self‐healing  self‐healing (Ca + MB), which was determined to be advantageous in the promotion of self‐healing  self‐healing (Ca + MB), which was determined to be advantageous in the promotion of self‐healing  (Caon the basis of the outcomes described in Section 3.1.  + MB), which was determined to be advantageous in the promotion of self-healing on the basis of on the basis of the outcomes described in Section 3.1.  on the basis of the outcomes described in Section 3.1.  The results of the optical microscopic observation are illustrated in Figure 7. In the case of PVA,  the outcomes described in Section 3.1. The results of the optical microscopic observation are illustrated in Figure 7. In the case of PVA,  The results of the optical microscopic observation are illustrated in Figure 7. In the case of PVA,  the surface of cracks was completely blocked by a white‐colored precipitated substance, obscuring  The results of the optical microscopic observation are illustrated in Figure 7. In the case of PVA, the surface of cracks was completely blocked by a white‐colored precipitated substance, obscuring  the surface of cracks was completely blocked by a white‐colored precipitated substance, obscuring  observation  of  the  configuration  of  fibers  themselves.  In  PE,  the  observation  of was the completely configuration  of  the  the by fibers  themselves. precipitated In  the  the  case  case  of  of  PE,  a  a  white‐colored  white‐colored  thethe  surface of cracks blocked a white-colored substance, obscuring the the  observation  of  the  configuration  of  the  fibers  themselves.  In  the  case  of  PE,  a  white‐colored  precipitated  substance  was  observed  in  portions  of  the  surface  of  the  cracks,  with  a  substantial  precipitated  substance  was  observed  in  portions  of  the  surface  of  the  cracks,  with  a  substantial  observation of the configuration of the fibers themselves. In the case of PE, a white-colored precipitated precipitated  substance  was  observed  in  portions  of  the  surface  of  the  cracks,  with  a  substantial  quantity attached to the area around the fibers. Meanwhile, in the case of PP, in comparison to PVA  quantity attached to the area around the fibers. Meanwhile, in the case of PP, in comparison to PVA  substance was observed in portions of the surface of the cracks, with a substantial quantity attached quantity attached to the area around the fibers. Meanwhile, in the case of PP, in comparison to PVA  and PE, there was almost no attachment of a white‐colored precipitated substance to the area around  to and PE, there was almost no attachment of a white‐colored precipitated substance to the area around  the area around the fibers. Meanwhile, in the case of PP, in comparison to PVA and PE, there was and PE, there was almost no attachment of a white‐colored precipitated substance to the area around  the  and  themselves  could  be  under  observation.  From  these  the  fibers,  fibers,  and  the  the  fibers  fibers  themselves  could  be  clearly  clearly  distinguished  distinguished  under  observation.  From  these  almost no attachment of a themselves  white-colored precipitated substance to under  the area around the fibers, and the  fibers,  and  the  fibers  could  be  clearly  distinguished  observation.  From  these  the fibers themselves could be clearly distinguished under observation. From these results it can be

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results it can be proposed that the white colored substance is precipitated by self‐healing and that the  results it can be proposed that the white colored substance is precipitated by self‐healing and that the  proposed that the white colored substance is precipitated by self-healing and that the promotion of promotion of self‐healing would be possible in the order of PVA > PE > PP in all of the test specimens.  promotion of self‐healing would be possible in the order of PVA > PE > PP in all of the test specimens.  self-healing would be possible in the order of PVA > PE > PP in all of the test specimens. Figure 8a,b show the experimental overview of the Raman spectroscopy analysis, and Figure 9  Figure 8a,b show the experimental overview of the Raman spectroscopy analysis, and Figure 9  Figure 8a,b show the experimental overview of the Raman spectroscopy analysis, and Figure 9 displays the experimental results of Raman spectroscopy. A comparison was made of the locations  displays the experimental results of Raman spectroscopy. A comparison was made of the locations  displays the experimental results of Raman spectroscopy. A comparison was made of the locations of of  the  occurrence  of  the  peak  of  the  wave  generated  by  the  laser  at  the  crack  section  of  PVA  of  the  occurrence  of peak the  peak  the generated wave  generated  by  the  laser  at  the  crack  of  PVA  the occurrence of the of theof  wave by the laser at the crack section of section  PVA specimen specimen to which the white‐colored precipitated substance was attached, and that at the sections  specimen to which the white‐colored precipitated substance was attached, and that at the sections  to which the white-colored precipitated substance was attached, and that at the sections without without cracks. With the peak of the wave indicating CaCO3 powder as the subject of comparison, it  without cracks. With the peak of the wave indicating CaCO cracks. With the peak of the wave indicating CaCO3 powder3 powder as the subject of comparison, it  as the subject of comparison, it can be can be seen that there was almost no peak in the wave that coincided with that of the CaCO 3in the  can be seen that there was almost no peak in the wave that coincided with that of the CaCO 3in the  seen that there was almost no peak in the wave that coincided with that of the CaCO3 in the sections sections  without  cracks.  However,  the  peak  of  the  wave  in  the  crack  section  accurately  coincides  sections cracks. without  cracks.  the However,  wave section in  the  accurately crack  section  accurately  without However, peak ofthe  the peak  waveof  in the  the crack coincides with coincides  the peak with the peak of the wave of CaCO3 powder. Accordingly, it is concluded that the majority of the  with the peak of the wave of CaCO 3 powder. Accordingly, it is concluded that the majority of the  of the wave of CaCO3 powder. Accordingly, it is concluded that the majority of the white-colored white‐colored precipitated substance was CaCO3generated during self‐healing.  white‐colored precipitated substance was CaCO 3generated during self‐healing.  precipitated substance was CaCO3 generated during self-healing.

  Figure 7. Surface section of the cracks of each of the specimens(Ca + MB). Figure 7. Surface section of the cracks of each of the specimens(Ca + MB).  Figure 7. Surface section of the cracks of each of the specimens(Ca + MB). 

1.0cm 1.0cm

Crack Crack

Non crack Non crack

1.0cm 1.0cm (a) 

(b) 

(a)  (b)  Figure 8. Experimental overview of the Raman spectroscopy analysis. (a) Geometry of the specimen,  (b) Raman spectroscopy.   Figure 8. Experimental overview of the Raman spectroscopy analysis. (a) Geometry of the specimen,  Figure 8. Experimental overview of the Raman spectroscopy analysis. (a) Geometry of the specimen; (b) Raman spectroscopy. (b) Raman spectroscopy.  

 

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CaCO3

Intensity (counts)

25

Non crack

20 15 10 5 0 0

500

Intensity (counts)

25

1000

1500

Raman shift/cm-1

20

2000

Crack

15 10 5 0 0

500

Intensity (counts)

1000

1500

2000

Raman shift/cm-1

25

CaCO3 powder

20 15 10 5 0 0

500

1000

1500

Raman shift/cm-1

2000

 

Figure 9. PVA (Ca + MB). Figure 9. PVA (Ca + MB). 

3.2.2. Internal Aspect of the Cracks 3.2.2. Internal Aspect of the Cracks  In this experiment, the internal sections of the cracks in the specimens were observed using In  this  experiment,  the  internal  sections  of  the  cracks  in  the  specimens  were  observed  using  microfocus X-ray CT scans in order to assess the status of the progress of self-healing within the microfocus  X‐ray  CT  scans  in  order  to  assess  the  status  of  the  progress  of  self‐healing  within  the  cracks. As the conditions of the experiment, an X-ray of 200 kV and 100 µA was used. As illustrated cracks. As the conditions of the experiment, an X‐ray of 200 kV and 100 μA was used. As illustrated in  in Figure 10, a screen image interpretation domain of the X-ray CT scan was set. Here, the 3D screen Figure 10, a screen image interpretation domain of the X‐ray CT scan was set. Here, the 3D screen  image of each of the specimens obtained through the X-ray CT scan was composed of a voxel, and the image of each of the specimens obtained through the X‐ray CT scan was composed of a voxel, and  width of the cracks and the volume of the crack sections of each of the specimens were computed using the  width  of  the  cracks  and  the  volume  of  the  crack  sections  of  each  of  the  specimens  were  this voxel (Figure 11) [26]. In addition, Figure 12 illustrates the conceptual diagram of the histogram of computed using this voxel (Figure 11) [26]. In addition, Figure 12 illustrates the conceptual diagram  the luminance (CT-value) and frequency of the 3D screen image. Here, the boundaries of the luminance of  the  histogram  of  the  luminance  (CT‐value)  and  frequency  of  the  3D  screen  image.  Here,  the  (CT-value) of the void and substance (cement matrix) were clearly distinguished through the regular boundaries  of  the  luminance  (CT‐value)  of  the  void  and  substance  (cement  matrix)  were  clearly  distribution of each of the peaks. The crack section prior to the self-healing had the same density distinguished through the regular distribution of each of the peaks. The crack section prior to the  as the void, and the volume of the void was computed. Moreover, regarding the crack section, the self‐healing had the same density as the void, and the volume of the void was computed. Moreover,  changes in the volume of the void section prior to, and following, the self-healing were compared regarding the crack section, the changes in the volume of the void section prior to, and following, the  and assessed using the difference in the densities of the void section and the section filled in by the self‐healing were compared and assessed using the difference in the densities of the void section and  precipitated substance. Figures 13–15 illustrate the changes in the volume of the void sections prior to, the section filled in by the precipitated substance. Figures 13–15 illustrate the changes in the volume of  and following, self-healing in accordance with the width and the conditions of self-healing of each the void sections prior to, and following, self‐healing in accordance with the width and the conditions  of the specimens. In addition, the graph in Figure 16 compares the ratio of changes in the volume of of self‐healing  of  each  of  the  specimens.  In  addition,  the  graph  in  Figure  16  compares  the  ratio  of  the void section following self-healing with the volume of the void section of Step A as the reference. changes in the volume of the void section following self‐healing with the volume of the void section  In the PVA series in Figure 13, the volume of the void section was reduced by approximately 62% for of  Step  A  as  the  reference.  In  the  PVA  series  in  Figure  13,  the  volume  of  the  void  section  was  the (W + MB) and by 67% for the (Ca + MB) in Step B in comparison to those in Step A. In the case reduced  by  approximately  62%  for  the  (W  +  MB)  and  by  67%  for  the  (Ca  +  MB)  in  Step  B  in  of the PE series in Figure 14, the volume of the void section was reduced by approximately 44% for comparison  to  those  in  Step  A.  In  the  case  of  the  PE  series  in  Figure  14,  the  volume  of  the  void  the (W + MB) and 67% for the (Ca + MB) in Step B in comparison to those in Step A. In the case of section was reduced by approximately 44% for the (W + MB) and 67% for the (Ca + MB) in Step B in  the PP series seen in Figure 15, there was the trend of reduction in the volume of the void section by comparison to those in Step A. In the case of the PP series seen in Figure 15, there was the trend of  approximately 44% for the (W + MB) and 46% for the (Ca + MB) in Step B in comparison to those in reduction in the volume of the void section by approximately 44% for the (W + MB) and 46% for the  (Ca  +  MB)  in  Step  B  in  comparison  to  those  in  Step  A.  Meanwhile,  in  terms  of  the  ratio  of  the  changes in the volume of the void section of all the specimens as illustrated in Figure 16, although 

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Step A. Meanwhile, in terms of the ratio of the changes in the volume of the void section of all the specimens as illustrated in Figure 16, although there was almost no(W  difference between PE and the  PP there  was almost  almost  no difference  difference  between  PE and  and  PP in  in  the  case of  of  MB), PVA  PVA displayed  displayed  there  was  no  between  PE  PP  the  case  (W  + + MB),  the  in the caseof  of (W + MB), PVA displayed thevolume  tendency of a reduction in the ratio of the volume of the tendency  reduction  in the  the  ratio of  of the  the  of the  the  void section  section  by approximately  approximately  1.5‐fold  in  tendency  of  a a reduction  in  ratio  volume  of  void  by  1.5‐fold  in  void section to  bythat  approximately 1.5-fold in comparison to that of PE+ MB),  and PP. In addition, in the casethe  of comparison  of  PE  and  PP.  In  addition,  in  the  case  of  (Ca  PVA  and  PE  displayed  comparison  to  that  of  PE  and  PP.  In  addition,  in  the  case  of  (Ca  + MB),  PVA  and  PE  displayed  the  (Ca + MB), PVA and PE displayed the tendency of reduction in the ratio of volume of the void section tendency of reduction in the ratio of volume of the void section by about 1.6‐fold in comparison to PP.  tendency of reduction in the ratio of volume of the void section by about 1.6‐fold in comparison to PP.  by about 1.6-fold in comparison to PP. Based on the above results, the performance of self‐healing by each of the fibers was found to  Based on the above results, the performance of self‐healing by each of the fibers was found to  Based on the above PE > PP. From the existing researches [13,27], internal cracks were blocked  results, the performance of self-healing by each of the fibers was found to be in the order of PVA ≥ be in the order of PVA ≥ PE > PP. From the existing researches [13,27], internal cracks were blocked  be in the order of PVA ě PE > PP. From the existing researches [13,27], internal cracks were blocked   by self‐healing with a significantly longer time. However, in this study, it was determined that the  by self‐healing with a significantly longer time. However, in this study, it was determined that the    by self-healing with a significantly longer time. However, in this it was determined that the self‐healing  performance  of internal  internal  cracks  can be  be maximized  maximized  by study, concurrently  and appropriately  appropriately  self‐healing  performance  of  cracks  can  by  concurrently  and  2+ and CO self-healing performance of internal cracks can be maximized by and appropriately using using the (Ca + MB) conditions of supplying the Ca 32−concurrently necessary in self‐healing along with  2+ and CO using the (Ca + MB) conditions of supplying the Ca 32−necessary in self‐healing along with  2 ´ 2+ − the (Ca + MB) conditions of supplying the Ca and CO3 necessary in self-healing along with the the use of PVA fiber with an OH  radical [28].  − radical [28].  the use of PVA fiber with an OH use of PVA fiber with an OH´ radical [28].

2cm 2cm

8.5cm 8.5cm

Analysisarea area Crack Analysis Crack

2cm 2cm Scanningarea(φ=4.8×H=3cm) area(φ=4.8×H=3cm) Scanning

8.5cm 8.5cm

  

Figure 10. Area of X-ray computed tomography (CT) scanning. Figure 10. Area of X‐ray computed tomography (CT) scanning.  Figure 10. Area of X‐ray computed tomography (CT) scanning. 

  

Frequency Frequency

Figure 11. Method of calculating the crack width [26]  Figure 11. Method of calculating the crack width [26]. Figure 11. Method of calculating the crack width [26] 

Cementmatrix matrix Cement Void Void

Threshold Threshold

00

CT-value   CT-value  

Figure 12. Conceptual diagram of the frequency of CT-value. Figure 12. Conceptual diagram of the frequency of CT‐value.  Figure 12. Conceptual diagram of the frequency of CT‐value. 

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1.5

Step. A

1.21.5

1.5 1.5 0.91.2 1.2 1.2 0.9 0.60.9 0.9 0.6 0.30.6 0.6 0.3 0.3 00.3 0 0 0

Step. B Step. B Step. B

1.01 1.01 1.01 1.01

0.39

0.33

0.39 0.39 0.39

0.15

0.33 0.33

0.33 Ca+MB (0.30mm)

0.15 W+MB (0.34mm) 0.15 0.15

 

W+MB (0.34mm) Ca+MB (0.30mm) W+MB (0.34mm) Ca+MB (0.30mm) Figure 13. PVA fiber.  W+MB (0.34mm) Ca+MB (0.30mm)

    

Figure 13. PVA fiber.  Figure 13. PVA fiber. Figure 13. PVA fiber. 

Figure 13. PVA fiber.  Step. A Step. B

1.5 1.5 1.5 1.21.5 1.2 1.2 0.91.2 0.9 0.9 0.9 0.6 0.6 0.6 0.6 0.30.3 0.3 0.3 00 0 0

Step. A Step. A Step. A

Volume ofofvoid+crack (%) Volume ofvoid+crack void+crack (%) Volume (%)

Volume of void+crack (%)

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Step. B

Step. A Step. A Step. A

Volume ofofvoid+crack (%) Volume ofvoid+crack void+crack (%) Volume (%)

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Step. B Step. B Step. B

0.66

0.66 0.66 0.66

0.4 0.4 0.4 0.4

0.37 0.37

0.37 0.37

W+MB W+MB(0.35mm) (0.35mm)

0.13 0.13 0.13 0.13

Ca+MB(0.36mm) (0.36mm) Ca+MB Ca+MB (0.36mm) Ca+MB (0.36mm)

W+MB (0.35mm) W+MB (0.35mm)

      

Figure 14. PE fiber.    Figure 14. PE fiber. Figure 14. PE fiber. Figure 14. PE fiber.   Figure 14. PE fiber. 1.51.5

1.5 1.5 1.2 1.21.2 1.2 0.9 0.90.9 0.9 0.6 0.60.6 0.6 0.3 0.3 0.30.3 0 0 00

Volume of void+crack (%)

Volume ofofvoid+crack (%) Volume ofvoid+crack void+crack (%) Volume (%)

Step.AA Step. Step. B Step. Step. A Step. B B Step. A

Step. B

1.32 1.32 1.32 1.32

1.1 1.1 1.1 1.1

0.7 0.7 0.7 0.7

0.61

0.61 0.61 0.61

W+MB (0.25mm) W+MB (0.25mm)

Ca+MB (0.32mm) Ca+MB (0.32mm)

      

Ratio of volume of void+crack

Ratio ofofvolume ofofvoid+crack Ratio of volume ofvoid+crack void+crack Ratio volume

W+MB(0.25mm) (0.25mm) Ca+MB W+MB Ca+MB(0.32mm) (0.32mm) Figure 15. PP fiber.  Figure 15. PP fiber.  Figure 15. PP fiber. Figure 15. PP fiber. Figure 15. PP fiber.    0.1 0.1 0.1

0.1

W+MB W+MB W+MB

W+MB

1 1 1

1

PVA PVA PVA

Ca+MB Ca+MB Ca+MB

1 = Standard line 1 = Standard line 1 = Standard line

1 = Standard line

Ca+MB

PE PE PE

PP PP PP

Figure 16. Ratio of volume of (void + crack).   PVA PE PP Figure 16. Ratio of volume of (void + crack).   Figure 16. Ratio of volume of (void + crack). 

    

 

Figure 16. Ratio of volume of (void + crack). Figure 16. Ratio of volume of (void + crack).  3.3. Chemical Evaluation of the Precipitated Substances of Self‐Healing  3.3. Chemical Evaluation of the Precipitated Substances of Self‐Healing  3.3. Chemical Evaluation of the Precipitated Substances of Self‐Healing  For the types of, and quantitative evaluation of, the quantities of the substance precipitated by  For the types of, and quantitative evaluation of, the quantities of the substance precipitated by  3.3. Chemical Evaluation of the Precipitated Substances of Self‐Healing  3.3. Chemical Evaluation of the and  Precipitated Self-Healing For the types of, and quantitative evaluation of, the quantities of the substance precipitated by  self‐healing  at  the  surface  internal Substances sections  of ofthe  cracks,  a  comparison  and  evaluation  was  self‐healing  at  the  surface  and  internal  sections  of  the  cracks,  a  a  comparison  comparison  and  and  evaluation  evaluation  was  was  self‐healing  at  the  surface  and  internal  sections  of  the  cracks,  made  by  means  of  TG‐DTA  for  the  quantitative  changes  in  Ca(OH) 2  and  CaCO 3  prior  to  and  For the types of, and quantitative evaluation of, the quantities of the substance precipitated by  For the types of,of and quantitative evaluation of,changes  the quantities of the substance precipitated made  by  means  TG‐DTA  for  the  quantitative  in  Ca(OH) 2  and  CaCO 3  prior  to  and  by made  by  means  of  TG‐DTA  for  the  quantitative  changes  in  Ca(OH)2  and  CaCO3  prior  to  and 

self‐healing at at the the surface surface  and  internal  sections  of  the  cracks,  a  comparison  and  evaluation  was  self-healing and internal sections of the cracks, a comparison and evaluation was made made  by  means  of  TG‐DTA  for  the  quantitative  changes  in  Ca(OH) 2   and  CaCO 3   prior  to  by means of TG-DTA for the quantitative changes in Ca(OH)2 and CaCO3 prior to and followingand  the

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self-healing for each of the fiber series. Also, TG-DTA samples in the experiment were collected from each specimen, as shown in Figure 17. Figure 18 illustrates the outcome of the TG-DTA experiment on the sections without Materials 2016, 9, 248  11 of 14  cracks (Non-crack) and the sections with cracks (Crack) prior to, and following, the self-healing for each of the Materials 2016, 9, 248  11 of 14  following the self‐healing for each of the fiber series. Also, TG‐DTA samples in the experiment were  fibers and the conditions of self-healing. The quantity of the Ca(OH)2 displayed a tendency to decrease, collected from each specimen, as shown in Figure 17.  while that of CaCO3 increased for the Crack sections in comparison to those prior to self-healing following the self‐healing for each of the fiber series. Also, TG‐DTA samples in the experiment were  Figure  18  illustrates  the  outcome  of  the  TG‐DTA  experiment  on  the  sections  without  cracks  (Before) and to those of the Non-crack sections   in all the fiber series. Here, quantities of CaCO3 , collected from each specimen, as shown in Figure 17. (Non‐crack) and the sections with cracks (Crack) prior to, and following, the self‐healing for each of  presumed to be substance precipitated by self-healing, increased in the without  order ofcracks  PVA > PE > PP. Figure  18 the illustrates  the  outcome  of  the  TG‐DTA  experiment  on  the  sections  the fibers and the conditions of self‐healing. The quantity of the Ca(OH) 2 displayed a tendency to  (Non‐crack) and the sections with cracks (Crack) prior to, and following, the self‐healing for each of  In particular, in the case of of  theCaCO PVA3 series, thefor  quantity of Ca(OH) incomparison  the crack section reduced by decrease,  while  that  increased  the  Crack  sections 2in  to  those was prior  to  the fibers and the conditions of self‐healing. The quantity of the Ca(OH) 2 displayed a tendency to  self‐healing (Before) and to those of the Non‐crack sections in all the fiber series. Here, quantities of  about 7%, while the quantity of CaCO3 was increased by about 6% in comparison to those prior to decrease,  that  of  CaCO3  increased  for  the  Crack  sections  in  comparison  to  those  prior  to  CaCOwhile  3, presumed to be the substance precipitated by self‐healing, increased in the order of PVA >  self-healing. The tendency of a reduction in the quantity of Ca(OH)2 and an increase in the quantity self‐healing (Before) and to those of the Non‐crack sections in all the fiber series. Here, quantities of  PE > PP. In particular, in the case of the PVA series, the quantity of Ca(OH)2 in the crack section was  of CaCO was also displayed in comparison to the Non-crack sections following self-healing for CaCO 33, presumed to be the substance precipitated by self‐healing, increased in the order of PVA >  reduced  by  about  7%,  while  the  quantity  of  CaCO3  was  increased  by  about  6%  in  comparison  to  (Figure 18c) (W + MB). In addition, for (Figure 18f) (Ca + MB), there2 in the crack section was  was substantial increase in the PE > PP. In particular, in the case of the PVA series, the quantity of Ca(OH) those prior to self‐healing. The tendency of a reduction in the quantity of Ca(OH) 2 and an increase  reduced  by  about  7%,  quantity  of  3  was  increased  by  about  comparison  to  quantity CaCO PVA3the  was used in CaCO comparison with the use of6%  PPin  and PE, and compared to 3 when in of the  quantity  of while  CaCO   was  also  displayed  in  comparison  to  the  Non‐crack  sections  following  those prior to self‐healing. The tendency of a reduction in the quantity of Ca(OH) 2 and an increase  self‐healing for (Figure 18c) (W + MB). In addition, for (Figure 18f) (Ca + MB), there was substantial  (W + MB). From such results, it was determined that self-healing was further promoted. Therefore, in  the  quantity  of  CaCO3  was  also  displayed  in  comparison  to  the  Non‐crack  sections  following  increase in the quantity of CaCO as the results of the aforementioned3 when PVA was used in comparison with the use of PP and PE,  comparison and evaluation of the types and the quantities of self‐healing for (Figure 18c) (W + MB). In addition, for (Figure 18f) (Ca + MB), there was substantial  and  compared  to  (W  +  MB).  From  such  results, itit was was  determined  self‐healing  was  further  the increase in the quantity of CaCO substances precipitated due3 when PVA was used in comparison with the use of PP and PE,  to self-healing, possible tothat  generate a greater quantity of promoted. Therefore, as the results of the aforementioned comparison and evaluation of the types  precipitated substances of self-healing under the condition of (Ca + MB) in comparison to that under and and  compared  to  (W  + of  MB).  From  such precipitated  results,  it  was  determined  that  it  self‐healing  was  the quantities  the substances  due  to  self‐healing,  was  possible  to further  generate a  the condition (W + MB) for each ofsubstances  the fiber series (PVA, PE, andthe  PP). Moreover, it was determined promoted. Therefore, as the results of the aforementioned comparison and evaluation of the types  greater of quantity  of  precipitated  of  self‐healing  under  condition  of  (Ca  +  MB)  in  the quantities  the substances  precipitated  to  CaCO self‐healing,  it  was  possible  to  generate a  thatand  thecomparison to that under the condition of (W + MB) for each of the fiber series (PVA, PE, and PP).  majority of of  this precipitated substancedue  was . In addition, it was possible to achieve 3 quantity it of  precipitated  substances  of  self‐healing  under  the  of  (Ca  +  MB)  was  determined  that  the  majority  of  this  precipitated  was  CaCO 3.  In  not greater  onlyMoreover,  self-healing of the cementitious composite materials, butcondition  alsosubstance  improvement ofin self-healing comparison to that under the condition of (W + MB) for each of the fiber series (PVA, PE, and PP).  ´ addition,  it  was  possible  to  achieve  not  only  self‐healing  of  the  cementitious  composite  materials,  performance by synthetic fibers by using PVA with the OH radical. Therefore, it was determined that Moreover,  it improvement  was  determined  that  the  majority  of  this  precipitated  substance  CaCO In OH−  but  also  of  self‐healing  by  synthetic  fibers  by  using was  PVA  with 3. the  more effective self-healing performance isperformance  possible for micro-cracks with a width ofmaterials,  more than 0.1 mm addition,  it  was  possible  to  achieve  not  only  self‐healing  of  the  cementitious  composite  radical.  Therefore,  it  was  determined  that  more  effective  self‐healing  performance  is  possible  for  for which substantial permeation of deteriorating elements from the external sections internal but  also  improvement  self‐healing  performance  by  synthetic  by  using  PVA  with  the into OH−the   micro‐cracks  with  of  a  width  of  more  than  0.1  mm  for  which fibers  substantial  permeation  of  deteriorating  radical.  Therefore,  it  was  determined  that  more  effective  self‐healing  performance  is  possible  for  sections of the concrete is anticipated. elements from the external sections into the internal sections of the concrete is anticipated.  micro‐cracks  with  a  width  of  more  than  0.1  mm  for  which  substantial  permeation  of  deteriorating  elements from the external sections into the internal sections of the concrete is anticipated.  Crack Cutting

Crack

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Mortar Non-Crack

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Mortar

Powder Mortar sample

Mortar

 

Mortar

Figure 17. Sampling of specimen for thermo gravimetric‐differential thermal analysis (TG‐DTA). 

  Figure 17. Sampling of specimen for thermo gravimetric-differential thermal analysis (TG-DTA).

20

Ca(OH)2.3 Ca(OH)2 Ca(OH) 22

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40

CaCO33 CaCO CaCO3 Ca(OH)22 Ca(OH)2 Ca(OH) 30 CaCO33 CaCO CaCO3

Rate of CH and CC (%)

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Figure 17. Sampling of specimen for thermo gravimetric‐differential thermal analysis (TG‐DTA).   40 40 CaCO3 CaCO3

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Ca(OH)2 Ca(OH)2 CaCO3 CaCO3 Ca(OH)2 Ca(OH)2.3 2

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Figure 18. Cont.

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22.6 15.4

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Before

Non-crack

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(f) 

Figure 18. Comparison of the self‐healing precipitated substances. (a)PP (W + MB), (b) PE (W + MB),  Figure 18. Comparison of the self-healing precipitated substances. (a)PP (W + MB); (b) PE (W + MB); (c) PVA (W + MB), (d) PP (Ca + MB), (e) PE (Ca + MB), (f) PVA (Ca + MB).  (c) PVA (W + MB); (d) PP (Ca + MB); (e) PE (Ca + MB); (f) PVA (Ca + MB).

4. Conclusions 

4. Conclusions

This study aimed to assess the changes in the physical properties and structure of the surface  and internal sections of cracks during composite self‐healing by cementitious composite materials  This study aimed to assess the changes in the physical properties and structure of the surface andand synthetic fibers. This included changes in the type and quantity of the precipitated substance,  internal sections of cracks during composite self-healing by cementitious composite materials and the optimal conditions of self‐healing. The study examined the effective dispersion of cracks in the  and synthetic fibers. This included changes in the type and quantity of the precipitated substance, cementitious  composite  materials  reinforced  with  synthetic  fiber,  and  demonstrated  self‐healing  of  andcracks  the optimal conditions of self-healing. The study examined the effective dispersion of  approximately  0.3  mm  width.  The  changes  in  the  structure,  observations  of  the  surface  and of cracks in the internal sections of the cracks using the permeability coefficient, optical microscope, and X‐ray CT scan,  cementitious composite materials reinforced with synthetic fiber, and demonstrated self-healing and  experimental  results  of  the  comparison  and  changes evaluation inof  the  types  and observations quantities  of  the  of cracks of approximately 0.3 mm width. The the structure, of the surface precipitated substance by using Raman spectroscopic analysis and TG‐DTA, are summarized as follows:  and internal sections of the cracks using the permeability coefficient, optical microscope, and X-ray (1)  It  was  confirmed  that  self‐healing  of  cementitious  composite  materials  alone  and  CT scan, and experimental results of the comparison and evaluation of the types and quantities of cementitious composite materials mixed with synthetic fiber with polarity progressed not only on  the the surface of the cracks but also in the internal sections of the cracks.  precipitated substance by using Raman spectroscopic analysis and TG-DTA, are summarized as follows: (2)  Composite  self‐healing  of  the  cementitious  composite  materials  and  synthetic  fiber  with  (1) It was confirmed that self-healing of cementitious composite materials alone and cementitious polarity generates a precipitated substance on the surface and internal sections of the micro‐crack,  and at this time, it was confirmed that the majority of the precipitated substance is CaCO composite materials mixed with synthetic fiber with polarity progressed not only3. on the surface of the (3) It is possible to restore micro‐cracks with width of more than 0.1 mm, for which substantial  cracks but also in the internal sections of the cracks. permeation of deteriorating elements from the external into the internal sections of the concrete is  (2) Composite self-healing of the cementitious composite materials and synthetic fiber with anticipated,  by  mixing  synthetic  fiber  with  the  concrete.  In  particular,  PVA  fiber  with  polarity  is  polarity generates a precipitated substance on the surface and internal sections of the micro-crack, and able to restore water tightness and precipitate large quantities of self‐healing substances to a greater  at this time, it the  wasPE  confirmed thatAccordingly,  the majorityit of the precipitated substance isachieve  CaCO3more  . extent  than  and  PP  fibers.  is  determined  that  PVA  is  able  to  effective self‐healing performance.  (3) It is possible to restore micro-cracks with width of more than 0.1 mm, for which substantial (4) Regarding the optimal condition of self‐healing, it was confirmed that applying conditions  permeation of deteriorating elements from the external into the internal sections of the concrete is of saturated Ca(OH)2 solution plus CO2 micro‐bubbles, which supply the Ca2+ and CO32− necessary for  anticipated, by mixing synthetic fiber with the concrete. In particular, PVA fiber with polarity is able to self‐healing,  is  effective  in  promoting  self‐healing  performance.  This  is  due  to  the  generation  of  the    restore water tightness and precipitate large quantities of self-healing substances to a greater extent self‐healing substance and an increase in the quantity of precipitation available for crack reclamation. 

than the PE and PP fibers. Accordingly, it is determined that PVA is able to achieve more effective self-healing performance. (4) Regarding the optimal condition of self-healing, it was confirmed that applying conditions of saturated Ca(OH)2 solution plus CO2 micro-bubbles, which supply the Ca2+ and CO3 2´ necessary for self-healing, is effective in promoting self-healing performance. This is due to the generation of the self-healing substance and an increase in the quantity of precipitation available for crack reclamation.

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Acknowledgments: This research was supported by the young researcher program through the LIXIL 2015 of Japan funded by LIXIL. Author Contributions: Heesup Choi and Hyeonggil Choi conceived and designed the experiments; Heesup Choi, Masumi Inou, Sukmin Kwon, Hyeonggil Choi, and Myungkwan Lim performed the experiments; Heesup Choi, Masumi Inou, Sukmin Kwon, Hyeonggil Choi, and Myungkwan Lim analyzed the data; All authors contributed in manuscript preparation and participated in revising the article critically for important intellectual content. Conflicts of Interest: The authors declare no conflict of interest.

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