In single fiber pullout tests, three types of steel fibers—smooth (S-), hooked (H-), and twisted (T-)—were investigated because they have different pullout mechanism. And, two types of sand with different sizes, normal crushed sand (type 1) and very fine silica sand (type 3), were employed in direct tensile tests whereas one type of sand, fine silica sand (type 2), was used in single fiber pullout tests in order to focus on the effect of different pullout mechanisms on the pullout resistance of pre-debonded fibers after healing. The cumulative passing ratios of three different types of sand were illustrated in Figure 2. The composition and compressive strength of the mortar matrix are provided in Table 1 while the properties, including geometry, strength, stiffness and density, of steel fibers investigated are given in Table 2. In the direct tensile test specimens, the amount of those steel fibers added in mortar matrices was 2% by volume. T-fibers had a triangular section with three ribs per 30 mm fiber length. A mortar mixer was used to prepare the cement mixture for both the single-fiber pullout and tensile tests. The detailed procedure for preparing both pullout and tensile specimens can be referred in Kang et al. (2012) [15]. Cement, fly-ash, silica fume and sand were dry mixed for 3 to 4 min and then superplasticizer was added with 5 min further mixing. In preparing single-fiber pullout test specimens, the embedment length and inclination angle could be successfully controlled using a device for holding fibers. For preparation of tensile specimens, fibers were distributed by hand when the mortar showed suitable workability and viscosity for uniform fiber distribution. Then, the mortar mixture with fibers was carefully placed in molds using a wide scoop. All specimens were covered with plastic sheets and stored at room temperature for 24 h prior to demolding. After demolding, the specimens were placed in a water tank at room temperature for an additional 2 weeks in a laboratory.

The detailed procedure for single-fiber pullout and direct tensile tests can be found in Kang et al. (2012) [15] and the test set-ups for them were provided in Figure 3. Briefly, a fiber was embedded at the center of a pullout test specimen with a section of 25 × 25 mm²; and, the embedment length of the fiber was maintained as 15 mm, which was a half of fiber length. To produce interfacial debonding, the fiber embedded was first pulled out until 1 mm slip at 28 days. The 1 mm slip was applied to ensure interfacial debonding along the entire embedded length of fiber by fully activating the mechanical interaction between the fiber and the matrix. Since H-fibers, among the fibers investigated, generally showed the highest pullout resistance when the slip reached 0.5–0.8 mm, 1-mm slip was applied. Then, the pre-damaged pullout specimens were stored in air or water for 3, 7, and 14 days healing. To obtain water healing conditions, pH 7 was maintained to consider continuous water leakage. After the healing periods, the pullout specimens were dried for 1 day at room temperature with 50%–60% relative humidity and then re-pulled out until complete fiber pullout.

In direct tensile tests, to investigate the mechanical recovery of HPSFRCs after healing, all tensile specimens were pre-tensioned at the age of 90 days until the tensile strain reached 0.3% to generate multiple micro-cracks in the specmens. Then, the pre-cracked tensile specimens were stored in both water and air for 3, 7, and 14 days, respectively. As in the pullout test specimens, for water healing, pH 7 was also maintained in the direct tensile test program. After healing in water, the specimens were removed from a water tank and dried for 1 day in air. Then, the healed specimens were re-tensioned using the tensile test set-up shown in Figure 3b. To observe the damaged interface between the fiber and the matrix after fiber pullout and the morphology and shape of the healed materials according to the crack width, the pre-damaged specimens were cut using oil as a lubricant and polished. A digital 3D microscope (Keyence VHX-1000, Tokyo, Japan) and electron probe microanalyzer (Shimadzu EPMA-8705, Tokyo, Japan) were used to investigate the morphology of the cross section containing steel fibers and for the chemical analysis of self-healing products. 3D X-ray microcomputer tomography (micro CT) (Bruker, Delft, The Netherlands) was also used to investigate the cracks and damaged area at the interface after pullout.

The mechanical recovery of HPSFRCs is definitely dependent upon the pullout resistance of pre-debonded fibers after healing. Firstly, to quantitatively estimate the effects of healing on the pullout resistance of pre-debonded steel fibers, the maximum pullout load (P_2,max) during the second pullout was compared with the maximum pullout resistance of fibers (P_1,max) during first pullout. To quantify the effects, the ratio, R_M, between P_2,max and P_1,max was calculated. Higher R_M is thought to generate higher post cracking tensile strength of pre-damaged HPSFRCs after healing because the post cracking tensile strength, σ_pc, strongly depends on the maximum pullout resistance of the fiber. And, to investigate whether the de-bonded interface between fiber and matrix can be healed, the ratio, R_H, between P_2,start and P_1,end was calculated. The ratio, R_H, was also believed to effect on the first cracking strength, σ_cc, of HPSFRCs after healing. Both P_2,start and P_1,end are identified in the pullout stress versus slip curves as shown in Figure 4: P_1,end is the end point during the first pullout while P_2,start is the limit of proportionality during the second pullout.

The R_M of deformed, T- and H-, steel fibers were much higher than that of smooth steel fibers as shown in Figure 5: those of deformed fibers were close to 1.0 and ranged between 0.77 and 1.20 whereas those of S-fibers varied between 0.43 and 0.71. The difference arose from their different pullout mechanisms. The R_M of deformed steel fibers was not clearly influenced by the condition and age of healing owing to the different pullout mechanisms whereas that of S-fibers was slightly enhanced as the age of water healing increased.

It was hard to find significant effects of healing on R_H, as shown in Figure 6 because of the difficulty in water penetration into the interface between the fiber and the matrix. The initial healing or sealing at the interface might prevent further water penetration into the interface. As shown in Figure 6, water healing produced slightly higher values of R_H than air healing, although the values differed slightly according to the type of fiber. The R_H of T-fibers subjected to water healing increased from 0.9 to 1.3 as the age of healing increased from 3 to 14 days. The R_H of S-fibers subjected to water healing also increased until 7 days; however, the S-fiber was corroded after 14 days of water healing. The higher R_H of water healing might arise from continual hydration rather than interfacial healing of the debonded zone. Further investigation is needed to quantify the effect of interfacial healing alone on the R_H.

The interfacial cracking behavior during the fiber pullout could be successfully captured by 3D X-ray micro CT, as shown in Figure 7 for the deformation of H- and T-fibers in the matrix after 1-mm fiber pullout. The end hook of the H-fiber was significantly straightened during 1 mm slip, as shown in Figure 7a; the damaged area of the matrix owing to the straightening of the end hook could be clearly observed; furthermore, it was observed that the H-fiber slipped by only 0.51 mm from the matrix due to stretching of the fiber although 1-mm slip was applied. In contrast, the T-fiber in the matrix showed interfacial cracking behavior with various tiny cracks with a zigzag path due to the twisted geometry, as shown in Figure 7b, although it did not show as severe damage at the interface as did the H-fiber. This figure suggested that the T-fiber should be untwisted to pull it out; furthermore, the T-fiber slipped by only 0.63 mm although a 1 mm slip was applied.

Figures 8a–d respectively show the tensile stress-strain curves for each series with H-fiber 2% in matrix A, T-fiber 2% in matrix A, H-fiber 2% in matrix B, and T-fiber 2% in matrix B. All the test series produced tensile strain hardening behavior accompanied with multiple micro-cracks as shown in Figure 8, although the tensile parameters differed according to the types of sand and fiber. Table 3 provides averaged values for the typical tensile parameters such as first cracking strength (σ_cc), post cracking strength (σ_pc), strain capacity (ε_pc), number of cracks within gauge length, average crack spacing, and average crack width. First cracking strength was identified at the limit of proportionality while the post cracking strength and strain capacity were determined at maximum tensile strength point in the stress versus strain curve. The number of multiple micro-cracks was counted from both front and back surfaces and then averaged. And, the average crack width was calculated using the average number of cracks within the gauge length and the elongation at the maximum tensile strength.

The average values of tensile parameters provided in Table 3 were averaged from at least three specimens. The series of matrix B containing type 3 silica sand produced more number of micro-cracks and smaller width of those cracks. T-fibers generally generated higher tensile strength, more number of multiple micro-cracks, and smaller width of crack than H-fibers. Thus, the use of T-fibers is expected to produce better conditions for self-healing than H-fibers. Moreover, the very fine silica sand (type 3) generated 17%–19% higher tensile strength and 51%–58% smaller width of crack.

Figure 9a,b show the different healing speeds according to the crack width in pre-cracked HPSFRC with 2% H-fibers in matrix A after water healing. The crack with 200 μm width did not heal completely at the early stage (area A), as shown in Figure 9a; however, that with a width less than 100 μm healed considerably at the early stage through the filling of the crack with re-hydration products, as shown in Figure 9b. Furthermore, the crack was closed by the formation of self-healing products after re-curing for 7 days. The EPMA result in Figure 9a shows the entire self-healed area of the cracked specimen. It comprised different phases between the original and the self-healing zone. In particular, re-hydration products were mainly found to comprise calcium alumina silicate materials, as shown in the X-ray mapping results from the polished section (around 1–2 mm depth from the surface). This self-healing phenomenon seems to be related to crystallization by aluminosilicate with calcium ions based on unreacted fly ash. However, in the case of area B, the chemical compositions were found to be different as shown in the X-ray mapping results relative to previous results for the formation of calcite. X-ray spectra obtained from these phases revealed particular trends in the chemical composition as the formation of modified calcium alumina ferric salts and calcite phases proceeded. In this case, some Fe²⁺ and Fe³⁺ ions in the presence of steel fibers appear to have led to the formation of calcium ferric salts between cracks.

Rust generally forms in the presence of water and oxygen by the following reaction: Fe²⁺ + 2H₂O ⇌ Fe(OH)₂ + 2H⁺, Fe³⁺ + 3H₂O ⇌ Fe(OH)₃ + 3H⁺. Moreover, some hydration products in the cement matrix with pozzolanic materials have structures similar to that of garnet (3CaO·Al₂O₃·3SiO₂), in which Al³⁺ may be partially replaced by Fe³⁺ and Si⁴⁺ by 4H⁺ in the solution between cracks during self-healing. These complicated mechanisms are particularly affected by the presence of other ions such as Ca²⁺ and Al³⁺ in the matrix, and they combine with the hydroxides and oxides of iron to precipitate a variety of Ca-Fe-O-OH, Ca-Al-Fe-O-OH, or Ca-Al-Fe-Si-O-OH species. This phenomenon indicates that these products are sensitive to the corrosion, expansion, and bonding properties in the matrix. Therefore, the relationship between self-healing velocity and reasonable amounts of ferric ions should be considered in detail in order to understand the self-healing behavior of HPSFRCs. In other words, the addition of artificial ferric ions with chemical additives might lead to the formation of modified calcium ferric salts, one of the secondary phases, for self-healing. Thus, to improve the physical and chemical stability of these hydration products and precipitated products, the properties of calcium ferric salts in the initial term period should be further investigated; this will be addressed in a future study. As mentioned above, the effect of the crack width on the self-healing behavior of HPSFRCs using T-fibers is clearly shown in Figures 10 and 11. The self-healing capacity of HPSFRCs is confirmed to strongly depend on the crack width. Thus, a suitable sand, fiber, and mineral admixture should be carefully considered in the design of HPSFRCs to maximize their healing capacity.

Table 4 shows the influence of healing age on the mechanical healing capacity of HPFRCCs using T-fibers and very fine silica sand. Figure 12 shows the tensile and cracking behavior of HPFRCCs before and after healing. The standard tensile stress-strain curve shown in Figure 12 was averaged from at least three specimens. The average tensile parameters for the standard tensile stress-strain curve of HPFRCCs with 2% T-fiber and very fine micro silica sand are as follows: σ_cc, σ_pc, and ε_pc are 9.707 MPa, 15.361 MPa, and 0.519%, respectively.

The σ_cc of HPFRCCs after healing is much lower than that of the standard curve of HPFRCCs, as shown in Figures 12 and 13. However, the σ_pc at the healing period of 7 days is even higher than that of the standard curve of HPFRCC. The ε_pc of HPFRCCs after healing is clearly lower than the standard value, as shown in Figure 12 owing to the pre-tensioning until 0.3%.

The influence of healing period on σ_cc (first cracking strength before healing) is illustrated in Figure 13 using the ratio between σ_cc,H (first cracking strength after healing) and that of the same specimen before healing. As the healing period increases from 0 to 14 days, the ratio increased from 9.4% to 36%, as shown in Figure 13. Although HPSFRCs with T-fibers showed rapid crack healing behavior in the early stage, most healed specimens showed rusted areas on the specimen surface, as shown in Figure 12. Figure 14 shows the corrosion process of the cracked area with T-fibers in the presence of water. In this case, corrosion at the exposed area (over 200 μm) after the pullout test started after 3 days, as shown in Figure 14, and the exposed T-fibers were significantly corroded after re-curing for 14 days. This phenomenon indicates that the healing speed should be further increased to prevent any corrosion of steel fibers in HPSFRCs through micro-cracks.