3.1. Microscopic Investigation of the Changes in the Crack Width
Crack width images were obtained via microscopic observation at each specified time interval. Figure 5
shows the changes in the crack width of each mixture (Plain, Granule-EA, Granule-Cm#1, Granule-Cm#2, Granule-Cm#3, Granule-Cm#4, and Granule-Cm#5) with respect to the elapsed time. In Figure 5
, Plain shows that crack closing was not completely achieved in all the crack widths up to the 28 days elapsed time. Although the <0.1 mm crack width was not completely closed, as reported in the previous research [6
], it showed a similar crack closing. For the granulated CSA-based expansive agent with a PVA film coating (Granule-EA), the <0.1 mm, 0.1–0.2 mm, and >0.2 mm crack widths indicated complete crack closing within 11, 14, and 16 days, respectively. For the granulated cementitious material with a PVA film coating (Granule-Cm), the <0.1 mm crack widths indicated complete crack closing within 14, 11, 11, 14, and 14 days, respectively, in the following order: #1, 2, 3, 4, and 5. The 0.1–0.2 mm crack widths indicated complete crack closing within 15, 14, 14, 17, and 16 days, respectively. Finally, the >0.2 mm crack widths indicated complete crack closing within 21, 16, 21, and 21 days, respectively, in this order: #1, 2, 3, 4, and 5. It was noticed that the 0.2–0.3 mm crack widths of Granule-Cm#5 indicated complete crack closing within 18 days.
Therefore, it was verified via microscopic investigation that all the mixtures, except Plain, showed complete crack closing within 21 days. In addition, the time of the crack closing of Granule-EA (the CSA-based expansive agent) was slower than that of Granule-Cm (the cement + CSA-based expansive agent + sodium carbonate). This indicates that the formation rate of the healing products formed on the crack faces by the hydration of the healing material (the CSA-based expansive agent and the cementitious material) was different. Therefore, it was verified that the interior-healing material might have reacted with the water molecules as the water that migrated through the cracks dissolved the water-soluble PVA film and crack closing was achieved.
shows the process of crack closing via self-healing with regard to the typical specimens Plain and the granules with the PVA film coating (Granule-EA and Granule-Cm), which involved 0.1–0.2 mm crack widths within the crack width range. Plain in Figure 6
shows the occurrence of crystals on both faces of the crack after 14-day immersion, and crack closing was observed in some parts of the crack after 28-day immersion. As the specimens were dried before the microscopic investigation, these crystals were assumed to have been CaCO3
, which could be formed by the action of the leached Ca(OH)2
out of the cracks and the CO2
in the air [6
]. The specimen that incorporated Granule-EA was observed to have had crystals, except for some parts of the crack, after seven-day immersion. It also showed complete crack closing after 14-day immersion. Therefore, it can be supposed that crack closing was achieved because the CSA-based expansive agent had reacted due to the removal of the PVA film coating by the water that migrated through the crack faces.
For Granule-Cm, although the specimens that incorporated it (Granule-Cm#1, Granule-Cm#2, Granule-Cm#3, Granule-Cm#4, and Granule-Cm#5) were similar to those in the crack closing process of Granule-EA, the time of their complete crack closing differed. As shown in Figure 6
c–g, after seven-day immersion, the extent of the crack closing of the specimens that incorporated Granule-Cm was smaller than that of Granule-EA. Moreover, complete crack closing was observed after 14- to 17-day immersion. This can be supposed as indicative that the cementitious material, used as the healing material, had formed various hydration products on the crack faces. Therefore, the microscopic images verified that the healing material had reacted after the removal of the PVA film coating by the migrated water via the crack faces.
3.2. Evaluation of the Internal Crack Closing via the Dynamic Modulus of Elasticity
While the recovery of the crack on the surface of the specimen could be monitored with a microscope, the crack inside the specimen could not. Therefore, the dynamic modulus of elasticity was measured in conjunction with the microscopic investigation. Subsequently, the internal crack closing was evaluated based on the relative dynamic modulus of elasticity, which was determined based on the percentage of the dynamic modulus of elasticity before and after the crack introduction depending on the elapsed time.
shows the relative dynamic modulus of elasticity in each mixture compared with that in Plain. Plain-1 represents the crack widths within the range less than 0.1 mm. In the same way, Plain-2 and Plain-3 show the crack widths with the 0.1–0.2 mm and greater than 0.2 mm ranges, respectively. As can be seen in Figure 7
, the mean relative dynamic modulus of elasticity in all the specimens showed a sharp decrease when the cracks appeared. The mean relative dynamic modulus of elasticity of Plain-1 reached about 80% of the state before the introduction of cracks, with a gradual increase depending on the elapsed time, and those of Plain-2 and Plain-3 reached about 60%. These indicate that Plain did not achieve complete internal crack closing. In Figure 6
a, Granule-EA-1, Granule-EA-2, and Granule-EA-3 show over 90% values for the mean relative dynamic modulus of elasticity, with a rapid increase from five days depending on the crack width range. Although a 100% dynamic modulus of elasticity was not reached, it can be supposed that complete internal crack closing was almost achieved. In Figure 6
b–f, the mean relative dynamic modulus of elasticity of Granule-Cm shows a pattern different from that of Granule-EA in the early stage. The mean relative dynamic modulus of elasticity of Granule-EA increased rapidly early then stabilized whereas that of Granule-Cm increased gradually and then stabilized. This is because the formation rates of the healing products that were formed by the healing material differed. In particular, the recovery of the relative dynamic modulus of elasticity of Granule-Cm#2 and Granule-Cm#3 were the most excellent. It should also be noted that Granule-Cm#2 and Granule-Cm#3 are superior to Granule-EA.
After the completion of the measurement of the dynamic modulus of elasticity, the presence of internal crack closing was observed with a microscope after the surface of the specimen was cut 10 mm deep using a cutter. Figure 8
shows a microscopic image of the specimen inside. As Granule-Cm showed a similar internal crack closing, Granule-Cm#2 and Granule-Cm#5 were observed with a microscope, respectively. In Figure 8
, Plain is shown to have hardly achieved internal crack closing. On the other hand, Granule-EA and -Cm are shown to have achieved internal crack closing because marks of the cracks could no longer be observed. This can be taken to mean that internal crack closing was achieved without difficulty because the crack width of the inside of the specimen was smaller than the width (0.1–0.2 mm) of the crack that appeared on the surface. Therefore, the self-healing efficiency of the concrete was verified on the internal crack of the specimen.
3.4. Analysis of the Healing Products Formed on the Crack Faces
SEM and XRD tests were conducted to analyze the healing products that were formed within the cracks. Figure 10
shows the XRD patterns of the healing products of Plain (with no healing material incorporated), the specimens that incorporated the granulated CSA-based expansive agent with the PVA film coating (Granule-EA), and the granulated cementitious material (the cement + CSA-based expansive agent + sodium carbonate) with the PVA film coating (Granule-Cm#2). In Figure 10
, Plain shows the significant peaks of CaCO3
in the 25°–30° and 15°–20° ranges, respectively. In particular, these indicate that the healing products, which were formed on the crack faces, were mainly CaCO3
because the peak of CaCO3
was remarkable. Granule-EA shows the peaks of ettringite, CaSO4
, and gypsum. Among them, ettringite and Ca(OH)2
may be formed by hydrating the CSA-based expansive agent. CaCO3
may be formed by the reaction with CO2
, as shown in Plain. The peak of gypsum may be supposed to be one of the main ingredients of the CSA-based expansive agent. In particular, the peak of ettringite was remarkable. Therefore, this indicates that the healing products that were formed on the crack faces were mainly ettringite, and that crack closing could be additionally achieved by CaCO3
Finally, Granule-Cm showed the peaks of C–S–H, C–A–H, ettringite, Ca(OH)2, etc., which were formed by cement hydration. The peak of CaCO3 was also shown in Plain and Granule-EA. This can be supposed to mean that CaCO3 was formed by the reaction between Ca(OH)2, CO2, and sodium carbonate. The peak of gypsum was included in the cement and the CSA-based expansive agent. Therefore, this indicates that the healing products that were formed on the crack faces were mainly hydrates formed by cement hydration, and that ettringite was formed by the CSA-based expansive agent. In addition, it can be supposed that crack closing was slightly achieved via CaCO3. In the case of cement, stable hydrate products were created, but they slowed down the generation rate, whereas in the case of the expansive agents, the generation rate of hydrate products such as ettringite was rapidly presented, but it had an unstable condition. Therefore, Granule-Cm#2 indicated a good result because the characteristics of both materials might have reacted well compared to the other mixtures.
Most of the existing relevant literature claims that the primary mechanism of self-healing is the crystallization of calcium carbonate and calcium hydroxide [33
]. This view is supported by the fact that precipitated calcium carbonate can often be observed at the outside surfaces of the crack, as a white residue. As one of the cement hydration products dissolved in water, calcium hydroxide is liberated and dissipated along the cracking surfaces, after which the free calcium ions from the cement hydration react with the dissolved CO2
. Consequently, crystals are formed, growing at both surfaces of the cracks and finally filling the gaps [34
The CSA-based expansive agent consists of hauyne (3CaO·3Al2O3·CaSO4), free lime (CaO), and anhydrite (CaSO4). They mainly provide ettringite (3CaO·Al2O3, 3CaSO4·32H2O), a micrometer-sized acicular crystal, due to hydration. In particular, ettringite fills the micropores of concrete and induces its expansion. Also, the CSA-based expansive agent formed crystalline calcium hydroxide via the reaction between free lime (CaO) and water, after which the expansion occurred due to the growth of crystalline calcium hydroxide. Thus, the CSA-based expansive agent caused the expansion induced by the ettringite and calcium hydroxide that were formed due to the hydration reaction. Also, when the cement reacted with the water, various hydration products, such as C–S–H, C–A–H, CH, and ettringite, were formed, and insoluble calcium carbonate (CaCO3) was formed by the reaction between sodium carbonate (Na2CO3) and calcium hydroxide [Ca(OH)2], which was in turn formed by C3S and β-C2S hydration, and from the CSA-based expansive agent.
Ettringite is a needle-shaped prismatic crystal. It is seen in SEM micrographs in the literature of very young pastes as needles growing into the capillary pores between the cement particles [35
tends to form large crystals with a distinctive hexagonal-prism morphology, but it may vary in morphology as it is found as small equidimensional crystals; large, flat, platy crystals; large, thin, elongated crystals; and all variations in between. The morphology of CaCO3
is similar to that of Ca(OH)2
, and it is mainly shown as having a short hexagonal-prism morphology or a layer morphology. If the hydration further progresses, a membrane-shaped C–S–H will be formed around the particles. This membrane will be dense because it will become thick both inside and outside the particles over time. A C–S–H with a chestnut morphology is formed around the cement particles [36
shows the SEM micrographs of the healing products of Plain, the specimens that incorporated the granulated CSA-based expansive agent with the PVA film coating (Granule-EA), and the granulated cementitious material (the cement + CSA-based expansive agent + sodium carbonate) with the PVA film coating (Granule-Cm). As shown in Figure 11
, the crack face was clearly observed, and the original and healing zones can be distinguished from each other. CaCO3
, which was indicated as having a short hexagonal-prism morphology or a layer morphology, was seen at the healing zone in the SEM micrographs of Plain. Also, in the SEM micrographs of Granule-EA, ettringite, a needle-shaped prismatic crystal, and CaCO3
, a short hexagonal-prismatic crystal, were seen at the healing zone. Finally, in Granule-Cm, acicular ettringite was visible in the form of a spider web. Also, ettringite had become entangled with the C–S–H gel that was formed as a result of the hydration of the cement particles. Therefore, it was verified that crack closing was achieved by the healing products, as summarized in Table 3
The CSA-based expansive agent, cement, and sodium carbonate used as healing agents form various hydrates when they react with water. The CSA-based expansive agent causes the expansion induced by ettringite and calcium hydroxide, which are formed due to the hydration reaction. Calcium carbonate (CaCO3
) is formed by the reaction between sodium carbonate (Na2
) and calcium hydroxide (Ca(OH)2
). As is well known, cement compounds (C3
A, and C4
AF) form hydrates with low solubility with the elapsed time. Among the cement compounds, alite (C3
S) and belite (C2
S) form calcium hydroxide and calcium silicate hydrate via hydration reaction. Aluminate (C3
A) forms calcium aluminate hydrate (C4
) through hexagonal crystallization, and forms ettringite by reacting with gypsum. C4
AF forms ettringite through circular crystallization by reacting with gypsum, which is supplied to sulfate ion. If sulfate ion is not supplied, it is transformed into monosulfate (3CaO·Al2