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Performance optimization of one-component polyurethane healing agent for self-healing concrete
Construction and Building Materials 179 (2018) 151–159
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Performance optimization of one-component polyurethane healing agent for self-healing concrete Zun-Xiang Hu a, Xiang-Ming Hu a,c,⇑,1, Wei-Min Cheng a,c,⇑,1, Yan-Yun Zhao b, Ming-Yue Wu a a
Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China c College of Resources and Environmental Engineering, Binzhou University, Binzhou, Shandong 256603, China b
h i g h l i g h t s One-component polyurethane healing system is proposed. The strength recovery rate observed after 48 h of manual crack healing is 75%. The highest degree of healing efficiency of 67 ± 6% was observed. After cracking of the glass capsules, the PU healing agent was released uniformly.
a r t i c l e
i n f o
Article history: Received 30 July 2017 Received in revised form 28 April 2018 Accepted 24 May 2018
Keywords: Self-healing Capsule One-component polyurethane Healing agent Compatibility Healing efficiency
a b s t r a c t In this work, glass capsules containing one-component polyurethane healing agents are embedded in concrete to realize its self-healing properties. First, one-component polyurethane is diluted with acetone, and the viscosity, area of dispersion, surface tension, and bond strength of the resulting mixture are investigated. By encapsulating the diluted polyurethane inside quartz glass tubes, two different package types of capsules are prepared (capsules I and II), and their compatibility with concrete and survival rate are analyzed. In particular, the microscopic interface between the polyurethane and concrete phases is studied via scanning electron microscopy, while energy-dispersive X-ray spectroscopy is used to determine their elemental compositions. The mechanical properties of the produced self-healing concrete are examined by performing three-point bending tests, and the self-healing efficiency of the polyurethane healing agent is determined. When the acetone/polyurethane mass ratio is equal to 1:5, the viscosity and surface tension of the resulting mixture are low, while its healing effect is the strongest one. The strength recovery rate observed after 48 h of manual crack healing is 75%, and after two different types of glass capsules were placed into concrete, its flexural strength increased by 6–30%. However, the embedded glass capsules break after the crack formation in the self-healing concrete block. Finally, the strongest healing effect is observed for the self-healing concrete containing two type II capsules, which corresponds to the healing efficiency of 67%. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The active treatment of concrete cracks is a time-consuming, labor-intensive, and expensive process that is difficult to perform for certain structures (such as underground ones and bridges). ⇑ Corresponding authors at: Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China. E-mail addresses: [email protected] (X.-M. Hu), [email protected] (W.-M. Cheng). 1 These authors contributed equally to the work and should be regarded as co-first authors. https://doi.org/10.1016/j.conbuildmat.2018.05.199 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
Because the development of concrete with bionic self-healing functionality [1] is expected to solve this problem effectively, bionic self-healing concrete [2–5] has become a popular research topic in recent years. Based on existing self-healing polymer mechanisms, self-healing in cementitious materials can be classified broadly into the following four groups: intrinsic healing [6,7], microbial healing [8], capsule-based healing [3,9], and vascular healing [10]. Intrinsic self-healing materials (such as nonhydrated cement particles) exhibit self-healing properties due to the composition of their cementitious matrix. However, most of these agents are suitable only for repairing microcracks, as indicated by the study of Heide et al. [11]. During capsule-based and
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vascular healing, cracks are healed through interactions with a chemical healing agent. Thus, the selection of an appropriate healing agent is critical for improving the material self-healing characteristics. Currently, the healing agents used for bionic self-healing concrete are primarily of two types: two-component and onecomponent. In the case of two-component healing agents, the binder can react only when it is in contact with the second component. Such agents primarily consist of epoxy resin/tetraethylene pentamine [12], methyl methacrylate/benzoyl peroxide [13], and polyether glycol/isocyanate [14] compounds. Epoxy resin is a commonly used two-component healing agent. When blended at an appropriate fraction, it exhibits relatively high bond strength and toughness; however, it also requires long curing times [15]. According to Mihashi et al., the curing of epoxy resins significantly depends on the correct mix stoichiometry [16]. Methyl methacrylate is a low-viscosity healing agent that exhibits a relatively high bonding strength (50–75 MPa) [17] after curing, and at the polymerization reaction of this agent has been shown to be relatively insensitive to the mix ratio of both components [1]. However, its curing rate is low; as a result, it can be easily absorbed by the base materials [18]. In addition, as observed by Dry and McMillan, the agent may leak out of the crack, which will also result in incomplete crack filling [19]. Two-component polyurethane (PU) exhibits high expansibility, which not only provides a driving force for its flow, but also ensures its complete filling of the entire fracture space [20]. However, the mixing uniformity of the two components of PU significantly impacts its bonding strength [1,21–23]. Thus, ensuring the uniform mixing of the constituents of twocomponent healing systems represents a significant challenge that is difficult to achieve in construction applications. In contrast, one-component healing agents do not require uniformly mixed components. As a result, these agents can undergo curing during their contact with moisture or air. For example, acyanoacrylate is a common one-component healing agent. Lark et al. [24] sealed a-cyanoacrylate in glass capillary tubes, which were subsequently embedded into concrete. They found that, as a healing agent, a-cyanoacrylate cures quickly, and its curing process is not affected by the pH of the concrete matrix. However, fast curing also results in poor permeability (dispersivity) and low healing efficiency of the material; therefore, Sun et al. [25] concluded that the best results were obtained for crack widths not exceeding 0.3 mm. In addition, sealing in air or external air exposure in capsules may result in the premature hardening of the healing agent and related loss of its self-healing functionality [26,27]. In one recent study, Maria et al. investigated the suitability of other acrylate-end-capped precursors, including siloxane-, epoxy- and polyester-based materials, for the self-healing of active cracks and assessed the strain capacity of the healing agents both qualitatively and quantitatively after widening of the healed cracks [28]. However, because in practical applications, the healing agent needs to be encapsulated to enable concrete self-healing, the viscosity of the healing formulations should be further optimized (e.g., by increasing the amount of solvent or adding other inert components). Thus, the existing one-component healing agents have various disadvantages, and the development of healing agents with improved characteristics remains a challenging task. The one-component healing agent PU (MP 355; different from MEYCO MP 355 1K used by the Belgian research group [20]) can be cured in air at room temperature. Further, its curing rate is moderately high, which makes it suitable for filling cracks. Because this agent exhibits stable chemical properties and a long shelf life, it can be easily transported and stored; hence, it is generally considered an ideal healing agent for the fabrication of self-healing concrete [29,30]. However, the viscosity of one-component PU is high; as a result, it does not easily flow into concrete cracks. Therefore, in
this study, acetone (AC) was used to dilute one-component PU, and the viscosity, area of dispersion, surface tension, and bonding strength of the resulting mixture were investigated. In addition, the efficiency of PU as a healing agent was evaluated by encapsulating it in glass capsules, which were subsequently embedded into test concrete samples. 2. Experimental 2.1. Materials The one-component healing agent PU (MP 355) was purchased from Shanghai Xiyou Chemical Co., Ltd. It contained a prepolymer synthesized via the addition polymerization of isocyanate groups and a polyether (its physical and mechanical properties are listed in Table 1). AC (analytical grade) was acquired from a fine chemical plant located in the Laiyang Economic and Technological Development Zone. Quartz glass tubes were obtained from Jiangxi Huayuan Glass Products Co., Ltd. (their physical and chemical properties are listed in Table 2). A circular organic glass sheet with a thickness of 4 mm and diameter of 10 mm was purchased from Zibo Shudong Organic Glass Co., Ltd. The utilized cement was 325# slag cement, which was purchased from Shandong Shanshui Cement Group Ltd. Its specific surface area was 3885 cm2/g, flexural strength after 28 d was 6.8 MPa, and compressive strength was 36.3 MPa. a-cyanoacrylate glue (502 Super Glue) was purchased from Zhejiang Dongyuan City Adhesive Industry Co., Ltd. 2.2. Preparation of capsules and concrete specimens First, 0, 1, 2, 3, 4, and 5 g of AC were added to 20 g of PU (the resulting samples are further referred to as healing agents A, B, C, D, E, and F, respectively; see Table 3). Each sample was thoroughly mixed until a homogeneous solution of PU in AC was obtained. 2.2.1. Fabrication of type I and II capsules In a recent study, Branko et al. assessed the fitness of five types of polymeric capsules (inner diameter: 6–8 mm) for the delivery of self-healing agents using a numerical model to screen the best performing capsules and verifying their fitness with experimental methods [31]. Based on those results, we conducted a preliminary experiment, which indicated that based on the properties of the PU used in this study, the optimal inner diameter of the capsule was 8 mm. Open-ended glass tubes with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 30 mm (Fig. 1-b) were used for capsule fabrication. To fabricate type I capsules, an organic (acrylic) glass sheet (Fig. 1-a) with a diameter of 10 mm and thickness of 4 mm was used to seal one end of the glass tube; one of the diluted PU samples was then injected into the tube, and its other end was also sealed in a similar manner (Fig. 1-c). The cementing material used for sealing the tubes was the a-cyanoacrylate glue. To fabricate type II capsules, glass tubes (Fig. 2-b) with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 30 mm, which had been previously sealed at one end and left open at the other end, were used. First, each glass tube was filled with one of the diluted PU samples, after which its end was sealed by another glass tube with a circular seal and length of 10 mm (Fig. 2-a) using the a-cyanoacrylate glue as a sealing agent (Fig. 2-c). 2.2.2. Fabrication of concrete specimens Specimen a (area dispersion test): The fabricated sample consisted of two blocks of concrete with dimensions of
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Z.-X. Hu et al. / Construction and Building Materials 179 (2018) 151–159 Table 1 Physical and mechanical properties of one-component PU used in this study. Viscosity (mPas)
Tensile strength (MPa)
Elongation at break (%)
Tear strength (N/mm)
Solid content (%)
Drying time (h)
812
1.90
550
12
80
24
Table 2 Characteristics of the quartz glass tubes used in this study. Mohs hardness
Poisson’s ratio
Density (g/cm3)
Compressive strength (MPa)
Tensile strength (MPa)
5.5
0.17
2.2
1100
180
Table 3 AC/PU dilution ratios utilized in this study. Name
A
B
C
D
E
F
AC/PU ratio
0:20
1:20
1:10
3:20
1:5
1:4
Fig. 1. Fabrication of the type I capsules.
Fig. 2. Fabrication of the type II capsules.
the area of dispersion under 300-lm cracks, which were produced after treatment with different healing agents, six layers of 50-lm aluminum foil were used to control the crack width between the A and B surfaces. As shown in Fig. 3-a, once the capsule was inserted into the hole, the leakage process could be triggered by tapping the top of the concrete specimen. The capsule was then broken by the applied shearing force, and the healing agent inside flows out into the specimen. Fig. 3-b shows the two rubber bands utilized to fix the concrete block and prevent the separation of its two parts. Although the concrete sections could move relative to one another and break the capsule, these rubber bands maintained sufficient stability and strength. Specimen b (manual crack healing, control): Cement mortar was prepared at a water/cement/river sand mass ratio of 1.2:1:4 and subsequently poured into a mold (160 mm 40 mm 40 mm) to fabricate a control concrete specimen without PU-containing capsules. The latter was cured in a constant temperature/humidity curing box at 20 °C and a relative humidity of 90%. Specimen c (bionic self-healing concrete specimen): One or two glass capsules containing the diluted PU healing agents were placed horizontally at the center of a half-filled mold (160 mm 40 mm 40 mm, Fig. 4). The placement direction of the capsules was consistent with that of the mold. Finally, the mold was filled with the remaining mortar and then placed on a vibrating table, where it was subjected to small vibrations and compaction. Demolding was performed after 12 h of curing. In order to achieve stress concentration, grooves with widths of 2 mm and depths of 1.5 mm were cut on the specimen’s bottom to ensure the formation of cracks in the middle part. Specimen d (cylindrical concrete specimen): Cement mortar was poured into a cylindrical iron mold with a diameter of 400 mm and height of 60 mm. A glass tube with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 100 cm was inserted vertically into the mortar layer with a depth of 20 mm. Polystyrene foam was used to fix the glass tube in place (Fig. 5). Specimen demolding was performed after 3 d, and the specimen itself was cured in a constant temperature/humidity curing box at 20 °C and a relative humidity of 90% for 14 days. 2.3. Testing of prepared concrete samples
70.9 mm 70.9 mm 70.9 mm. As shown in Fig. 3-a, a drill bit with a diameter of 10 mm was used to drill two holes in the concrete block (the controlled drilling depth was 20 mm, the height of the first hole was 40 mm, and the height of the second hole was 30 mm) for inserting the glass capsules with diameters of 10 mm and lengths of 40 mm. In addition, to simulate and compare
2.3.1. Optimization of healing agent composition The viscosities of six different healing agents were determined using an LVDT-2 digital rotational viscometer with an L3 rotor (Shanghai Jingtian Electronic Instrument Co., Ltd). The experiments were conducted at a PU volume of 200 mL and rotation speed of
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Fig. 3. a. A model describing the area of dispersion b. A test specimen with a area of dispersion.
Fig. 4. Fabrication of the self-healing concrete samples.
150 r/min. The results of three different measurements were averaged for each tested specimen. The surface tensions of six different healing agents were determined using a microcontroller-based fully automatic surface tension meter (VZL2000, Changzhou Dedu Precision Instruments Co., Ltd.). Each data point was obtained by averaging the results of three measurements. To examine the area of dispersion, twenty minutes after the capsules were broken, as described in Section 2.2.2., the concrete blocks were separated, and the block with a larger area of dispersion was selected. The surfaces of the resulting cracks were photographed, and the corresponding area of dispersion was calculated using the GIMP (GNU Image Manipulation Program) image editing software. To determine the manual bond strengths of the healing agents, three-point bending tests were performed on the type b specimens after their curing for 14 d. The specimens were strained at the rate of 10 lm/s to induce cracks and determine the load at each point in time. The cracked specimens were docked, and an injector was used to inject 3 mL of healing agents A, C, E, or F into the cracks. Next, three-point bending tests were performed on the healed specimens after 24 h to obtain the corresponding loads at each point in time. Three different measurements were performed for each sample, and the obtained results were averaged. The flexural strengths of the specimens were calculated using the following expression:
Rf ¼
1:5F f L b
3
ð1Þ
where Ff denotes the load at at each point in time (N); L corresponds to the distance between the supporting cylinders (mm); and b is the side length of the square section of the tested concrete specimen (mm).
Fig. 5. Fabrication of the cylindrical concrete specimen.
2.3.2. Compatibility of concrete and glass capsules To determine the effect of the embedded glass capsules on the concrete strength, three-point bending tests were performed on the type c specimens (which contained different glass capsules and were previously cured for 14 d) using a WDW-20E universal testing machine. The peak load at fracture was recorded for each specimen in order to calculate its flexural strength. The observed effect of the capsule addition on the concrete strength was analyzed, and the obtained results were compared with those for the control specimen. To measure the bond strength between the concrete block and the capsule, the type d specimens that had been previously cured
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for 14 d were subjected to tensile testing using an open-loop electronic universal testing machine. The strain rate was 2 mm/min, and the values of the bond strength were calculated using the following expression:
C¼
F
pdL
155
removed from the concrete samples and subjected to gold metallization. The PU/concrete interfacial microstructure was investigated via scanning electron microscopy (SEM; JSM6380LV) in order to analyze the elemental compositions of two different regions (PU-covered and non-covered concrete).
ð2Þ
where C is the bond strength (N/mm2); F denotes the force applied to the glass tube during pulling (N); d represents the external diameter of the glass tube (mm); and L denotes the length of the glass tube embedded into a concrete specimen (mm). At present, there are two methods of placing healing capsules in large concrete specimens: gluing the healing capsules onto a network of wires with MMA glue [32], or distributing the healing capsule randomly in the concrete [33]. In this study, the larger capsule size and thicker capsule wall was more suitable for random distribution. To verify the suitability of the glass capsule in the concrete mixing process, their survival rates were determined by preparing 10 filled capsules of types I and II and 2 L cement mortar with a water/cement/river sand mass ratio of 1.2:1:4, and then mixing the cement mortar and 10 type I (or type II) capsules inside a mixing pot (JJ-5 cement mortar mixer; Wuxi Xiyi Building Materials Co., Ltd.) at a rotation rate of 65 r/min for 3 min to observe the capsule crushing process. 2.3.3. Efficiency of healing agent To examine the self-healing efficiency of PU, the type I and II capsules were filled with healing agent E (after the optimization of the healing agent composition, agent E was found to have the best healing effect and mobility). Subsequently, type c specimens containing one or two capsules were prepared. Specimen cracks were generated in a controlled manner by performing a threepoint bending test at a loading speed of 15 N/s. The open displacement (crack width) of the fracture area was measured using a standard KTRC-10.0 linear displacement sensor, as shown in Fig. 6. The peak load F1 was recorded for each crack. After the crack width reached 400 lm, the tested specimen was unloaded, which resulted in the crack contraction and reduction in the crack width down to approximately 300 lm. The cracked specimen was fixed and stored at a constant temperature of 20 °C and relative humidity of 90%. Each specimen was reloaded again after 1 d, 7 d, and 28 d, and the average of three measurements was obtained for each sample. After determining the peak load Fn, the healing efficiency g was calculated using the expression g = Fn/F1. 2.3.4. Characterization of healing interface Reloading tests were conducted after 1 d for the SHC I-2 and SHC II-2 self-healing samples, and the obtained areas of dispersion after fracture were compared. A small PU-containing block was
Fig. 6. A linear displacement sensor utilized during controlled crack testing.
3. Results and discussion 3.1. Effect of AC amount on PU physical properties Diffusion problems that could potentially occur with the healing agents in the cracks were investigated. The high viscosity (approximately 812 mPas) of pure PU inhibits its diffusion through the cracks; therefore, the viscosity of the healing agents was varied by diluting PU with different amounts of AC (see Fig. 7). The obtained results show that as the amount of AC increased, the viscosity of PU gradually decreased because of the increased spacing between individual PU molecules. A relatively low viscosity of the healing agent (approximately 268 mPas) was observed at an AC/PU ratio of 1:5. The observed changes in the surface tensions of the six healing agents at room temperature are presented in Fig. 7. Similar to viscosity, the surface tension of PU decreased with an increased amount of AC, indicating that the addition of AC affected the Van der Waals interaction forces between PU molecules and thus changed the macroscopic surface tension [28]. At an AC/PU ratio of 1:5 (healing agent E), the surface tension was approximately 38 mN/m. Because the latter was below the magnitude measured for the solid concrete, the added healing agent could easily spread across the concrete surface. The smaller the surface tension of the PU-based agent is, the more uniform is its flow across the solid surface and inside concrete cracks. The capillary force represents the driving force that extracts the fluid from the capsule (it is counter-balanced by the viscous and gravity forces). In this study, the distance between different crack surfaces was fixed to approximately 300 lm in all cases. However, the spreading capability of the fluid between these surfaces can be modified by adding various amounts of AC. The photographs of each crack surface shown in Fig. 8 illustrate the extent of hardening of each healing agent. The PU-covered regions were colored by the GIMP software, which also automatically identified the PU pixels, enabling determination of the PU coverage area with relatively high accuracy. Fig. 8 shows that the area of dispersion inside the cracks increased with an increased amount of AC, and the observed trend was negatively correlated with viscosity and surface tension. At a very small amount of added AC (Fig. 8-a,b), the area of dispersion was also small (approximately 300–400 mm2), and a
Fig. 7. Viscosities and surface tensions of different healing agents.
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Fig. 8. The area of dispersion obtained after using different healing agents.
relatively large quantity of the PU healing agent remained inside the capsule. At an AC/PU mass ratio of 3:20 or more, the area of dispersion gradually increased up to approximately 2000 mm2. Hence, a one-component PU healing agent was able to flow out of the ruptured capsule because of the presence of the added solvent.
reduction in the strength recovery. At this time, the healing agent has a lower viscosity and the healing area is still increasing.
3.3. Compatibility between concrete and glass capsules
Fig. 9 shows the manual healing efficiency and healing area curves of the artificially healed specimens containing the PU healing agents, which were recorded after 24 h. The ratio of the flexural strength of the healing specimen to the initial specimen is strength recovery. They show that the addition of healing agent A (AC/PU = 0:20) resulted in a strength recovery of approximately 69%, while the flexural strength of the test specimen was equal to 1.35 MPa. The highest strength recovery (approximately 79.0%) was observed for healing agent C (AC/PU = 1:10). This was because the addition of AC increased the molecular spacing of PU, resulting in better mobility. In addition, the healing area between the PU and concrete surfaces increased as well, which resulted in a significant increase in the bonding strength. As the amount of added AC increased, the magnitude of the strength recovery started to decrease, reaching the values of 75% and 72% for agents E and F, respectively. This was because the volume of the healing agent injected was the same for all cases. When more AC was added, it resulted in a decrease in the proportion of PU in the agents, resulting in a
Fig. 10 shows the changes in the flexural strength observed after embedding different glass capsules in concrete. Compared to the control group, the flexural strengths of specimens with the embedded capsules were significantly enhanced. Because glass is a typical brittle material, it does not exhibit any yield extension or plastic deformation, and its compressive strength (500–2000 MPa) and tensile strength (50–100 MPa) are much higher than those of concrete. When the external load is small, the concrete block does not crack because of adhesion, bond stress, friction, and other factors; as a result, the glass capsule is capable of bearing the load applied to the concrete to a certain extent (during this process, some tensile stress is generated inside the capsule) [34]. After further increasing the load, the concrete block cracks, and the tensile stress generated inside the glass increases dramatically. As the load continues to increase, the glass capsule experiences stress concentration, and after reaching the critical stress, the continuously expanding crack ultimately causes capsule breakage. The influences of different types and quantity of capsules on the mechanical properties of concrete are different, and therefore the capsule volume, length, diameter, and other factors must be analyzed. In this study, the effect of a single capsule on the flexural strength of concrete was relatively small, and the observed increases in the concrete strength were 6–10%. However, the
Fig. 9. Manual healing efficiency and healing area curves recorded for the concrete samples.
Fig. 10. Flexural strengths of the concrete specimens of various types with different numbers of embedded capsules.
3.2. Effect of AC amount on PU bonding strength (manual healing efficiency)
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Table 4 Bond strength between the cylindrical concrete specimen and the glass capsules. Pipe diameter (mm)
Embedded length (mm)
Test force (N)
Cohesive strength (MPa)
10
20
392
0.63
impact caused by embedding double capsules is very significant (the corresponding strength increase is equal to about 25–30%). A comparison of the effects of capsules I and II on the concrete mechanical properties revealed that the embedment of capsules II increased the flexural strength of concrete to a much higher extent. Since the geometries and materials of both capsules are the same, the observed phenomenon can be caused by the difference in their encapsulation methods. Hence, the encapsulation method utilized for capsule II leads to a higher flexural strength of concrete, and the resulting structure is more stable as compared to that obtained after embedding capsule I. In addition, the cement type (325# slag cement) used in this study exhibited a small degree of contraction during the setting and hardening processes. However, the contraction of concrete after 28 d did not affect capsule storage. Table 4 lists the bond strengths between the glass capsules and the cylindrical concrete specimens (the average value of the bond strength was 0.63 MPa). This result is consistent with the study of Van and Benoit [35,36]. The linear expansion coefficient a of glass is approximately 1.0 105, and the linear expansion coefficient of concrete is almost the same, ranging between 1.0 105 and 1.5 105. When the temperature changes, a system composed of glass and concrete does not undergo a relative temperature deformation that destroys the bond between them. In addition, the high chemical stability of glass indicates that it can be resistant to water, acids, alkali elements, and other chemical reagents as wells gas exposure, and thus can be stored in concrete for a long time without affecting its mechanical performance. When three-point bending tests were performed on the self-healing concrete specimens, a distinct noise related to the cracking of the glass capsules was detected. Therefore, the glass capsules did not move inside the concrete blocks, thus satisfying the requirement related to the cracking of capsules during the generation of cracks. In addition, experiments were performed to determine the survival rate of different types of capsules. After 3 min of stirring, all 10 capsules of type I were intact, and their survival rate was 100%. Because one capsule of type II was broken at the sealing site, the survival rate of this capsule type was approximately 90%. These results indicated that the sealing site of the type II capsule represented this type’s weak link, and the encapsulation method utilized to manufacture type I capsules was more robust than that used to fabricate type II capsules. This result is consistent with that presented by Gruyaert et al., and it satisfies the conditions for the type I capsule’s suitability in the concrete mixing process [37].
Fig. 11. Self-healing efficiency of the concrete specimens containing different capsules.
recovery after healing for 1 d; its flexural strength was approximately 1.15 MPa, and the healing efficiency was approximately 63 ± 5%. Because the volume of the type II capsules was larger than that of the type I capsules, the type II capsules contained a larger amount of the PU healing agent, which increased their healing strength. These results were consistent with the sizes of the healing areas of the two tubes depicted in Fig. 12, which shows that the healing area of the SHC II-2 specimen was larger than that of the SHC I-2 specimen. Hence, the magnitude of the healing strength was closely related to the size of the healing area. This result is also consistent with those described in Section 3.3, which showed that the concrete block containing capsules of type II exhibited higher flexural strength and durability. In addition, the results of reloading tests performed on the specimens healed for 7 d and 28 d showed that their healing efficiencies were slightly improved compared to that of the specimen treated for 1 d. This result was attributed to the secondary hydration, which was considered to improve the healing strength of concrete cracks under the curing conditions. As a result, the healing efficiency of SHC II-2 reached a magnitude of 67 ± 6% after 28 d. Fig. 12 displays the diffusion areas of the SHC I-2 and SHC II-2 specimens. The figure shows that the healing agent E could flow out of the capsule after the rupture of the glass tube and become evenly dispersed on the inner surface of the fracture, with the diffusion
3.4. Efficiency of self-healing concrete Fig. 11 describes the healing efficiency of self-healing concrete after embedding the capsule containing healing agent E. After the reloading of the self-healing specimens, crack formation was observed at the healed locations, suggesting that the latter remained the points of weakness in the concrete specimens. As indicated by the diagram, when a single capsule is added to concrete, the healing efficiency of the SHC II-1 specimen after 1 d became higher (40 ± 3%) than that of SHC I-1; the flexural strength of the former was equal to approximately 0.73 MPa. In comparison, the self-healing efficiency of concrete increased after embedding two capsules. The SHC II-2 sample exhibited a higher strength
Fig. 12. Diffusion areas of the SHC I-2 and SHC II-2 specimens.
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Fig. 13. SEM images of the concrete/PU interface.
dispersal mainly due to gravity and capillary forces. The healing area of the type II capsule (897 mm2) was larger than that of the type I capsule (742 mm2) and was equal to more than half of the total crack surface area (1600 mm2). In other words, the higher fracture filling degree leads to a higher healing strength, which is the same as the above conclusion. 3.5. SEM studies of concrete/PU interfaces Fig. 13 shows the SEM images of the concrete/PU interfaces, indicating that relatively large amounts of the PU curing products were tightly cemented together with fine sand as well as with the C3S and C2S on the concrete surface [38]. From these images, it can be concluded that after the glass capsules cracked, the healing agent was released uniformly into the formed cracks, resulting in their re-cementation, which not only greatly improved the strength of the concrete, but would also prevent the intrusion of external substances that would cause corrosion [39,40]. 4. Conclusions The conclusions of this study can be summarized as follows: (1) The presence of AC can significantly reduce the viscosity and surface tension of one-component PU agent and increase its area of dispersion within 300 lm of the produced crack in concrete blocks. The healing agent corresponding to an AC/ PU ratio of 1:5 exhibited the best performance in terms of these parameters. The strength recovery rate observed after manual crack healing for 24 h is 75%. (2) The bond strength between the quartz glass capsules and concrete was approximately 0.63 N/mm2, which satisfied the requirements for the cracking of the capsules caused by the crack generation. (3) When two type II capsules (AC/PU = 1:5) were embedded horizontally into the concrete specimens, the highest degree of self-healing of the cracked specimens corresponding to the healing efficiency of 67 ± 6% was observed. (4) According to the results of SEM analysis, after the cracking of the glass capsules, the PU healing agent was released uniformly into the cracks, which were closely cemented (self-healed) with concrete. Moreover, this work only investigates the short-term behavior of the healing systems since crack creation and loading were performed at early age. Because concrete structures are generally designed for a service life of 50 years or more, the healing performance at the long-term should be evaluated in future work.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled ‘‘Performance Optimization of One-component Polyurethane Healing Agent for Selfhealing Concrete”.
Acknowledgements This study was supported by the National Natural Science Foundation of China (51674038); Shandong Province Natural Science Foundation (ZR2018JL019); the China Postdoctoral Science Foundation (2014M560567 and 2015T80730); the Shandong Province Science and Technology Development Plan (2017GSF220003); the State Key Program for Coal Joint Funds of the National Natural Science Foundation of China (U1261205); Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2017RCJJ010, 2017RCJJ037).
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Performance optimization of one-component polyurethane healing agent for self-healing concrete Zun-Xiang Hu a, Xiang-Ming Hu a,c,⇑,1, Wei-Min Cheng a,c,⇑,1, Yan-Yun Zhao b, Ming-Yue Wu a a
Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China c College of Resources and Environmental Engineering, Binzhou University, Binzhou, Shandong 256603, China b
h i g h l i g h t s One-component polyurethane healing system is proposed. The strength recovery rate observed after 48 h of manual crack healing is 75%. The highest degree of healing efficiency of 67 ± 6% was observed. After cracking of the glass capsules, the PU healing agent was released uniformly.
a r t i c l e
i n f o
Article history: Received 30 July 2017 Received in revised form 28 April 2018 Accepted 24 May 2018
Keywords: Self-healing Capsule One-component polyurethane Healing agent Compatibility Healing efficiency
a b s t r a c t In this work, glass capsules containing one-component polyurethane healing agents are embedded in concrete to realize its self-healing properties. First, one-component polyurethane is diluted with acetone, and the viscosity, area of dispersion, surface tension, and bond strength of the resulting mixture are investigated. By encapsulating the diluted polyurethane inside quartz glass tubes, two different package types of capsules are prepared (capsules I and II), and their compatibility with concrete and survival rate are analyzed. In particular, the microscopic interface between the polyurethane and concrete phases is studied via scanning electron microscopy, while energy-dispersive X-ray spectroscopy is used to determine their elemental compositions. The mechanical properties of the produced self-healing concrete are examined by performing three-point bending tests, and the self-healing efficiency of the polyurethane healing agent is determined. When the acetone/polyurethane mass ratio is equal to 1:5, the viscosity and surface tension of the resulting mixture are low, while its healing effect is the strongest one. The strength recovery rate observed after 48 h of manual crack healing is 75%, and after two different types of glass capsules were placed into concrete, its flexural strength increased by 6–30%. However, the embedded glass capsules break after the crack formation in the self-healing concrete block. Finally, the strongest healing effect is observed for the self-healing concrete containing two type II capsules, which corresponds to the healing efficiency of 67%. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The active treatment of concrete cracks is a time-consuming, labor-intensive, and expensive process that is difficult to perform for certain structures (such as underground ones and bridges). ⇑ Corresponding authors at: Key Lab of Mine Disaster Prevention and Control, College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China. E-mail addresses: [email protected] (X.-M. Hu), [email protected] (W.-M. Cheng). 1 These authors contributed equally to the work and should be regarded as co-first authors. https://doi.org/10.1016/j.conbuildmat.2018.05.199 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
Because the development of concrete with bionic self-healing functionality [1] is expected to solve this problem effectively, bionic self-healing concrete [2–5] has become a popular research topic in recent years. Based on existing self-healing polymer mechanisms, self-healing in cementitious materials can be classified broadly into the following four groups: intrinsic healing [6,7], microbial healing [8], capsule-based healing [3,9], and vascular healing [10]. Intrinsic self-healing materials (such as nonhydrated cement particles) exhibit self-healing properties due to the composition of their cementitious matrix. However, most of these agents are suitable only for repairing microcracks, as indicated by the study of Heide et al. [11]. During capsule-based and
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vascular healing, cracks are healed through interactions with a chemical healing agent. Thus, the selection of an appropriate healing agent is critical for improving the material self-healing characteristics. Currently, the healing agents used for bionic self-healing concrete are primarily of two types: two-component and onecomponent. In the case of two-component healing agents, the binder can react only when it is in contact with the second component. Such agents primarily consist of epoxy resin/tetraethylene pentamine [12], methyl methacrylate/benzoyl peroxide [13], and polyether glycol/isocyanate [14] compounds. Epoxy resin is a commonly used two-component healing agent. When blended at an appropriate fraction, it exhibits relatively high bond strength and toughness; however, it also requires long curing times [15]. According to Mihashi et al., the curing of epoxy resins significantly depends on the correct mix stoichiometry [16]. Methyl methacrylate is a low-viscosity healing agent that exhibits a relatively high bonding strength (50–75 MPa) [17] after curing, and at the polymerization reaction of this agent has been shown to be relatively insensitive to the mix ratio of both components [1]. However, its curing rate is low; as a result, it can be easily absorbed by the base materials [18]. In addition, as observed by Dry and McMillan, the agent may leak out of the crack, which will also result in incomplete crack filling [19]. Two-component polyurethane (PU) exhibits high expansibility, which not only provides a driving force for its flow, but also ensures its complete filling of the entire fracture space [20]. However, the mixing uniformity of the two components of PU significantly impacts its bonding strength [1,21–23]. Thus, ensuring the uniform mixing of the constituents of twocomponent healing systems represents a significant challenge that is difficult to achieve in construction applications. In contrast, one-component healing agents do not require uniformly mixed components. As a result, these agents can undergo curing during their contact with moisture or air. For example, acyanoacrylate is a common one-component healing agent. Lark et al. [24] sealed a-cyanoacrylate in glass capillary tubes, which were subsequently embedded into concrete. They found that, as a healing agent, a-cyanoacrylate cures quickly, and its curing process is not affected by the pH of the concrete matrix. However, fast curing also results in poor permeability (dispersivity) and low healing efficiency of the material; therefore, Sun et al. [25] concluded that the best results were obtained for crack widths not exceeding 0.3 mm. In addition, sealing in air or external air exposure in capsules may result in the premature hardening of the healing agent and related loss of its self-healing functionality [26,27]. In one recent study, Maria et al. investigated the suitability of other acrylate-end-capped precursors, including siloxane-, epoxy- and polyester-based materials, for the self-healing of active cracks and assessed the strain capacity of the healing agents both qualitatively and quantitatively after widening of the healed cracks [28]. However, because in practical applications, the healing agent needs to be encapsulated to enable concrete self-healing, the viscosity of the healing formulations should be further optimized (e.g., by increasing the amount of solvent or adding other inert components). Thus, the existing one-component healing agents have various disadvantages, and the development of healing agents with improved characteristics remains a challenging task. The one-component healing agent PU (MP 355; different from MEYCO MP 355 1K used by the Belgian research group [20]) can be cured in air at room temperature. Further, its curing rate is moderately high, which makes it suitable for filling cracks. Because this agent exhibits stable chemical properties and a long shelf life, it can be easily transported and stored; hence, it is generally considered an ideal healing agent for the fabrication of self-healing concrete [29,30]. However, the viscosity of one-component PU is high; as a result, it does not easily flow into concrete cracks. Therefore, in
this study, acetone (AC) was used to dilute one-component PU, and the viscosity, area of dispersion, surface tension, and bonding strength of the resulting mixture were investigated. In addition, the efficiency of PU as a healing agent was evaluated by encapsulating it in glass capsules, which were subsequently embedded into test concrete samples. 2. Experimental 2.1. Materials The one-component healing agent PU (MP 355) was purchased from Shanghai Xiyou Chemical Co., Ltd. It contained a prepolymer synthesized via the addition polymerization of isocyanate groups and a polyether (its physical and mechanical properties are listed in Table 1). AC (analytical grade) was acquired from a fine chemical plant located in the Laiyang Economic and Technological Development Zone. Quartz glass tubes were obtained from Jiangxi Huayuan Glass Products Co., Ltd. (their physical and chemical properties are listed in Table 2). A circular organic glass sheet with a thickness of 4 mm and diameter of 10 mm was purchased from Zibo Shudong Organic Glass Co., Ltd. The utilized cement was 325# slag cement, which was purchased from Shandong Shanshui Cement Group Ltd. Its specific surface area was 3885 cm2/g, flexural strength after 28 d was 6.8 MPa, and compressive strength was 36.3 MPa. a-cyanoacrylate glue (502 Super Glue) was purchased from Zhejiang Dongyuan City Adhesive Industry Co., Ltd. 2.2. Preparation of capsules and concrete specimens First, 0, 1, 2, 3, 4, and 5 g of AC were added to 20 g of PU (the resulting samples are further referred to as healing agents A, B, C, D, E, and F, respectively; see Table 3). Each sample was thoroughly mixed until a homogeneous solution of PU in AC was obtained. 2.2.1. Fabrication of type I and II capsules In a recent study, Branko et al. assessed the fitness of five types of polymeric capsules (inner diameter: 6–8 mm) for the delivery of self-healing agents using a numerical model to screen the best performing capsules and verifying their fitness with experimental methods [31]. Based on those results, we conducted a preliminary experiment, which indicated that based on the properties of the PU used in this study, the optimal inner diameter of the capsule was 8 mm. Open-ended glass tubes with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 30 mm (Fig. 1-b) were used for capsule fabrication. To fabricate type I capsules, an organic (acrylic) glass sheet (Fig. 1-a) with a diameter of 10 mm and thickness of 4 mm was used to seal one end of the glass tube; one of the diluted PU samples was then injected into the tube, and its other end was also sealed in a similar manner (Fig. 1-c). The cementing material used for sealing the tubes was the a-cyanoacrylate glue. To fabricate type II capsules, glass tubes (Fig. 2-b) with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 30 mm, which had been previously sealed at one end and left open at the other end, were used. First, each glass tube was filled with one of the diluted PU samples, after which its end was sealed by another glass tube with a circular seal and length of 10 mm (Fig. 2-a) using the a-cyanoacrylate glue as a sealing agent (Fig. 2-c). 2.2.2. Fabrication of concrete specimens Specimen a (area dispersion test): The fabricated sample consisted of two blocks of concrete with dimensions of
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Z.-X. Hu et al. / Construction and Building Materials 179 (2018) 151–159 Table 1 Physical and mechanical properties of one-component PU used in this study. Viscosity (mPas)
Tensile strength (MPa)
Elongation at break (%)
Tear strength (N/mm)
Solid content (%)
Drying time (h)
812
1.90
550
12
80
24
Table 2 Characteristics of the quartz glass tubes used in this study. Mohs hardness
Poisson’s ratio
Density (g/cm3)
Compressive strength (MPa)
Tensile strength (MPa)
5.5
0.17
2.2
1100
180
Table 3 AC/PU dilution ratios utilized in this study. Name
A
B
C
D
E
F
AC/PU ratio
0:20
1:20
1:10
3:20
1:5
1:4
Fig. 1. Fabrication of the type I capsules.
Fig. 2. Fabrication of the type II capsules.
the area of dispersion under 300-lm cracks, which were produced after treatment with different healing agents, six layers of 50-lm aluminum foil were used to control the crack width between the A and B surfaces. As shown in Fig. 3-a, once the capsule was inserted into the hole, the leakage process could be triggered by tapping the top of the concrete specimen. The capsule was then broken by the applied shearing force, and the healing agent inside flows out into the specimen. Fig. 3-b shows the two rubber bands utilized to fix the concrete block and prevent the separation of its two parts. Although the concrete sections could move relative to one another and break the capsule, these rubber bands maintained sufficient stability and strength. Specimen b (manual crack healing, control): Cement mortar was prepared at a water/cement/river sand mass ratio of 1.2:1:4 and subsequently poured into a mold (160 mm 40 mm 40 mm) to fabricate a control concrete specimen without PU-containing capsules. The latter was cured in a constant temperature/humidity curing box at 20 °C and a relative humidity of 90%. Specimen c (bionic self-healing concrete specimen): One or two glass capsules containing the diluted PU healing agents were placed horizontally at the center of a half-filled mold (160 mm 40 mm 40 mm, Fig. 4). The placement direction of the capsules was consistent with that of the mold. Finally, the mold was filled with the remaining mortar and then placed on a vibrating table, where it was subjected to small vibrations and compaction. Demolding was performed after 12 h of curing. In order to achieve stress concentration, grooves with widths of 2 mm and depths of 1.5 mm were cut on the specimen’s bottom to ensure the formation of cracks in the middle part. Specimen d (cylindrical concrete specimen): Cement mortar was poured into a cylindrical iron mold with a diameter of 400 mm and height of 60 mm. A glass tube with an internal diameter of 8 mm, wall thickness of 1 mm, and length of 100 cm was inserted vertically into the mortar layer with a depth of 20 mm. Polystyrene foam was used to fix the glass tube in place (Fig. 5). Specimen demolding was performed after 3 d, and the specimen itself was cured in a constant temperature/humidity curing box at 20 °C and a relative humidity of 90% for 14 days. 2.3. Testing of prepared concrete samples
70.9 mm 70.9 mm 70.9 mm. As shown in Fig. 3-a, a drill bit with a diameter of 10 mm was used to drill two holes in the concrete block (the controlled drilling depth was 20 mm, the height of the first hole was 40 mm, and the height of the second hole was 30 mm) for inserting the glass capsules with diameters of 10 mm and lengths of 40 mm. In addition, to simulate and compare
2.3.1. Optimization of healing agent composition The viscosities of six different healing agents were determined using an LVDT-2 digital rotational viscometer with an L3 rotor (Shanghai Jingtian Electronic Instrument Co., Ltd). The experiments were conducted at a PU volume of 200 mL and rotation speed of
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Fig. 3. a. A model describing the area of dispersion b. A test specimen with a area of dispersion.
Fig. 4. Fabrication of the self-healing concrete samples.
150 r/min. The results of three different measurements were averaged for each tested specimen. The surface tensions of six different healing agents were determined using a microcontroller-based fully automatic surface tension meter (VZL2000, Changzhou Dedu Precision Instruments Co., Ltd.). Each data point was obtained by averaging the results of three measurements. To examine the area of dispersion, twenty minutes after the capsules were broken, as described in Section 2.2.2., the concrete blocks were separated, and the block with a larger area of dispersion was selected. The surfaces of the resulting cracks were photographed, and the corresponding area of dispersion was calculated using the GIMP (GNU Image Manipulation Program) image editing software. To determine the manual bond strengths of the healing agents, three-point bending tests were performed on the type b specimens after their curing for 14 d. The specimens were strained at the rate of 10 lm/s to induce cracks and determine the load at each point in time. The cracked specimens were docked, and an injector was used to inject 3 mL of healing agents A, C, E, or F into the cracks. Next, three-point bending tests were performed on the healed specimens after 24 h to obtain the corresponding loads at each point in time. Three different measurements were performed for each sample, and the obtained results were averaged. The flexural strengths of the specimens were calculated using the following expression:
Rf ¼
1:5F f L b
3
ð1Þ
where Ff denotes the load at at each point in time (N); L corresponds to the distance between the supporting cylinders (mm); and b is the side length of the square section of the tested concrete specimen (mm).
Fig. 5. Fabrication of the cylindrical concrete specimen.
2.3.2. Compatibility of concrete and glass capsules To determine the effect of the embedded glass capsules on the concrete strength, three-point bending tests were performed on the type c specimens (which contained different glass capsules and were previously cured for 14 d) using a WDW-20E universal testing machine. The peak load at fracture was recorded for each specimen in order to calculate its flexural strength. The observed effect of the capsule addition on the concrete strength was analyzed, and the obtained results were compared with those for the control specimen. To measure the bond strength between the concrete block and the capsule, the type d specimens that had been previously cured
Z.-X. Hu et al. / Construction and Building Materials 179 (2018) 151–159
for 14 d were subjected to tensile testing using an open-loop electronic universal testing machine. The strain rate was 2 mm/min, and the values of the bond strength were calculated using the following expression:
C¼
F
pdL
155
removed from the concrete samples and subjected to gold metallization. The PU/concrete interfacial microstructure was investigated via scanning electron microscopy (SEM; JSM6380LV) in order to analyze the elemental compositions of two different regions (PU-covered and non-covered concrete).
ð2Þ
where C is the bond strength (N/mm2); F denotes the force applied to the glass tube during pulling (N); d represents the external diameter of the glass tube (mm); and L denotes the length of the glass tube embedded into a concrete specimen (mm). At present, there are two methods of placing healing capsules in large concrete specimens: gluing the healing capsules onto a network of wires with MMA glue [32], or distributing the healing capsule randomly in the concrete [33]. In this study, the larger capsule size and thicker capsule wall was more suitable for random distribution. To verify the suitability of the glass capsule in the concrete mixing process, their survival rates were determined by preparing 10 filled capsules of types I and II and 2 L cement mortar with a water/cement/river sand mass ratio of 1.2:1:4, and then mixing the cement mortar and 10 type I (or type II) capsules inside a mixing pot (JJ-5 cement mortar mixer; Wuxi Xiyi Building Materials Co., Ltd.) at a rotation rate of 65 r/min for 3 min to observe the capsule crushing process. 2.3.3. Efficiency of healing agent To examine the self-healing efficiency of PU, the type I and II capsules were filled with healing agent E (after the optimization of the healing agent composition, agent E was found to have the best healing effect and mobility). Subsequently, type c specimens containing one or two capsules were prepared. Specimen cracks were generated in a controlled manner by performing a threepoint bending test at a loading speed of 15 N/s. The open displacement (crack width) of the fracture area was measured using a standard KTRC-10.0 linear displacement sensor, as shown in Fig. 6. The peak load F1 was recorded for each crack. After the crack width reached 400 lm, the tested specimen was unloaded, which resulted in the crack contraction and reduction in the crack width down to approximately 300 lm. The cracked specimen was fixed and stored at a constant temperature of 20 °C and relative humidity of 90%. Each specimen was reloaded again after 1 d, 7 d, and 28 d, and the average of three measurements was obtained for each sample. After determining the peak load Fn, the healing efficiency g was calculated using the expression g = Fn/F1. 2.3.4. Characterization of healing interface Reloading tests were conducted after 1 d for the SHC I-2 and SHC II-2 self-healing samples, and the obtained areas of dispersion after fracture were compared. A small PU-containing block was
Fig. 6. A linear displacement sensor utilized during controlled crack testing.
3. Results and discussion 3.1. Effect of AC amount on PU physical properties Diffusion problems that could potentially occur with the healing agents in the cracks were investigated. The high viscosity (approximately 812 mPas) of pure PU inhibits its diffusion through the cracks; therefore, the viscosity of the healing agents was varied by diluting PU with different amounts of AC (see Fig. 7). The obtained results show that as the amount of AC increased, the viscosity of PU gradually decreased because of the increased spacing between individual PU molecules. A relatively low viscosity of the healing agent (approximately 268 mPas) was observed at an AC/PU ratio of 1:5. The observed changes in the surface tensions of the six healing agents at room temperature are presented in Fig. 7. Similar to viscosity, the surface tension of PU decreased with an increased amount of AC, indicating that the addition of AC affected the Van der Waals interaction forces between PU molecules and thus changed the macroscopic surface tension [28]. At an AC/PU ratio of 1:5 (healing agent E), the surface tension was approximately 38 mN/m. Because the latter was below the magnitude measured for the solid concrete, the added healing agent could easily spread across the concrete surface. The smaller the surface tension of the PU-based agent is, the more uniform is its flow across the solid surface and inside concrete cracks. The capillary force represents the driving force that extracts the fluid from the capsule (it is counter-balanced by the viscous and gravity forces). In this study, the distance between different crack surfaces was fixed to approximately 300 lm in all cases. However, the spreading capability of the fluid between these surfaces can be modified by adding various amounts of AC. The photographs of each crack surface shown in Fig. 8 illustrate the extent of hardening of each healing agent. The PU-covered regions were colored by the GIMP software, which also automatically identified the PU pixels, enabling determination of the PU coverage area with relatively high accuracy. Fig. 8 shows that the area of dispersion inside the cracks increased with an increased amount of AC, and the observed trend was negatively correlated with viscosity and surface tension. At a very small amount of added AC (Fig. 8-a,b), the area of dispersion was also small (approximately 300–400 mm2), and a
Fig. 7. Viscosities and surface tensions of different healing agents.
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Fig. 8. The area of dispersion obtained after using different healing agents.
relatively large quantity of the PU healing agent remained inside the capsule. At an AC/PU mass ratio of 3:20 or more, the area of dispersion gradually increased up to approximately 2000 mm2. Hence, a one-component PU healing agent was able to flow out of the ruptured capsule because of the presence of the added solvent.
reduction in the strength recovery. At this time, the healing agent has a lower viscosity and the healing area is still increasing.
3.3. Compatibility between concrete and glass capsules
Fig. 9 shows the manual healing efficiency and healing area curves of the artificially healed specimens containing the PU healing agents, which were recorded after 24 h. The ratio of the flexural strength of the healing specimen to the initial specimen is strength recovery. They show that the addition of healing agent A (AC/PU = 0:20) resulted in a strength recovery of approximately 69%, while the flexural strength of the test specimen was equal to 1.35 MPa. The highest strength recovery (approximately 79.0%) was observed for healing agent C (AC/PU = 1:10). This was because the addition of AC increased the molecular spacing of PU, resulting in better mobility. In addition, the healing area between the PU and concrete surfaces increased as well, which resulted in a significant increase in the bonding strength. As the amount of added AC increased, the magnitude of the strength recovery started to decrease, reaching the values of 75% and 72% for agents E and F, respectively. This was because the volume of the healing agent injected was the same for all cases. When more AC was added, it resulted in a decrease in the proportion of PU in the agents, resulting in a
Fig. 10 shows the changes in the flexural strength observed after embedding different glass capsules in concrete. Compared to the control group, the flexural strengths of specimens with the embedded capsules were significantly enhanced. Because glass is a typical brittle material, it does not exhibit any yield extension or plastic deformation, and its compressive strength (500–2000 MPa) and tensile strength (50–100 MPa) are much higher than those of concrete. When the external load is small, the concrete block does not crack because of adhesion, bond stress, friction, and other factors; as a result, the glass capsule is capable of bearing the load applied to the concrete to a certain extent (during this process, some tensile stress is generated inside the capsule) [34]. After further increasing the load, the concrete block cracks, and the tensile stress generated inside the glass increases dramatically. As the load continues to increase, the glass capsule experiences stress concentration, and after reaching the critical stress, the continuously expanding crack ultimately causes capsule breakage. The influences of different types and quantity of capsules on the mechanical properties of concrete are different, and therefore the capsule volume, length, diameter, and other factors must be analyzed. In this study, the effect of a single capsule on the flexural strength of concrete was relatively small, and the observed increases in the concrete strength were 6–10%. However, the
Fig. 9. Manual healing efficiency and healing area curves recorded for the concrete samples.
Fig. 10. Flexural strengths of the concrete specimens of various types with different numbers of embedded capsules.
3.2. Effect of AC amount on PU bonding strength (manual healing efficiency)
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Table 4 Bond strength between the cylindrical concrete specimen and the glass capsules. Pipe diameter (mm)
Embedded length (mm)
Test force (N)
Cohesive strength (MPa)
10
20
392
0.63
impact caused by embedding double capsules is very significant (the corresponding strength increase is equal to about 25–30%). A comparison of the effects of capsules I and II on the concrete mechanical properties revealed that the embedment of capsules II increased the flexural strength of concrete to a much higher extent. Since the geometries and materials of both capsules are the same, the observed phenomenon can be caused by the difference in their encapsulation methods. Hence, the encapsulation method utilized for capsule II leads to a higher flexural strength of concrete, and the resulting structure is more stable as compared to that obtained after embedding capsule I. In addition, the cement type (325# slag cement) used in this study exhibited a small degree of contraction during the setting and hardening processes. However, the contraction of concrete after 28 d did not affect capsule storage. Table 4 lists the bond strengths between the glass capsules and the cylindrical concrete specimens (the average value of the bond strength was 0.63 MPa). This result is consistent with the study of Van and Benoit [35,36]. The linear expansion coefficient a of glass is approximately 1.0 105, and the linear expansion coefficient of concrete is almost the same, ranging between 1.0 105 and 1.5 105. When the temperature changes, a system composed of glass and concrete does not undergo a relative temperature deformation that destroys the bond between them. In addition, the high chemical stability of glass indicates that it can be resistant to water, acids, alkali elements, and other chemical reagents as wells gas exposure, and thus can be stored in concrete for a long time without affecting its mechanical performance. When three-point bending tests were performed on the self-healing concrete specimens, a distinct noise related to the cracking of the glass capsules was detected. Therefore, the glass capsules did not move inside the concrete blocks, thus satisfying the requirement related to the cracking of capsules during the generation of cracks. In addition, experiments were performed to determine the survival rate of different types of capsules. After 3 min of stirring, all 10 capsules of type I were intact, and their survival rate was 100%. Because one capsule of type II was broken at the sealing site, the survival rate of this capsule type was approximately 90%. These results indicated that the sealing site of the type II capsule represented this type’s weak link, and the encapsulation method utilized to manufacture type I capsules was more robust than that used to fabricate type II capsules. This result is consistent with that presented by Gruyaert et al., and it satisfies the conditions for the type I capsule’s suitability in the concrete mixing process [37].
Fig. 11. Self-healing efficiency of the concrete specimens containing different capsules.
recovery after healing for 1 d; its flexural strength was approximately 1.15 MPa, and the healing efficiency was approximately 63 ± 5%. Because the volume of the type II capsules was larger than that of the type I capsules, the type II capsules contained a larger amount of the PU healing agent, which increased their healing strength. These results were consistent with the sizes of the healing areas of the two tubes depicted in Fig. 12, which shows that the healing area of the SHC II-2 specimen was larger than that of the SHC I-2 specimen. Hence, the magnitude of the healing strength was closely related to the size of the healing area. This result is also consistent with those described in Section 3.3, which showed that the concrete block containing capsules of type II exhibited higher flexural strength and durability. In addition, the results of reloading tests performed on the specimens healed for 7 d and 28 d showed that their healing efficiencies were slightly improved compared to that of the specimen treated for 1 d. This result was attributed to the secondary hydration, which was considered to improve the healing strength of concrete cracks under the curing conditions. As a result, the healing efficiency of SHC II-2 reached a magnitude of 67 ± 6% after 28 d. Fig. 12 displays the diffusion areas of the SHC I-2 and SHC II-2 specimens. The figure shows that the healing agent E could flow out of the capsule after the rupture of the glass tube and become evenly dispersed on the inner surface of the fracture, with the diffusion
3.4. Efficiency of self-healing concrete Fig. 11 describes the healing efficiency of self-healing concrete after embedding the capsule containing healing agent E. After the reloading of the self-healing specimens, crack formation was observed at the healed locations, suggesting that the latter remained the points of weakness in the concrete specimens. As indicated by the diagram, when a single capsule is added to concrete, the healing efficiency of the SHC II-1 specimen after 1 d became higher (40 ± 3%) than that of SHC I-1; the flexural strength of the former was equal to approximately 0.73 MPa. In comparison, the self-healing efficiency of concrete increased after embedding two capsules. The SHC II-2 sample exhibited a higher strength
Fig. 12. Diffusion areas of the SHC I-2 and SHC II-2 specimens.
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Fig. 13. SEM images of the concrete/PU interface.
dispersal mainly due to gravity and capillary forces. The healing area of the type II capsule (897 mm2) was larger than that of the type I capsule (742 mm2) and was equal to more than half of the total crack surface area (1600 mm2). In other words, the higher fracture filling degree leads to a higher healing strength, which is the same as the above conclusion. 3.5. SEM studies of concrete/PU interfaces Fig. 13 shows the SEM images of the concrete/PU interfaces, indicating that relatively large amounts of the PU curing products were tightly cemented together with fine sand as well as with the C3S and C2S on the concrete surface [38]. From these images, it can be concluded that after the glass capsules cracked, the healing agent was released uniformly into the formed cracks, resulting in their re-cementation, which not only greatly improved the strength of the concrete, but would also prevent the intrusion of external substances that would cause corrosion [39,40]. 4. Conclusions The conclusions of this study can be summarized as follows: (1) The presence of AC can significantly reduce the viscosity and surface tension of one-component PU agent and increase its area of dispersion within 300 lm of the produced crack in concrete blocks. The healing agent corresponding to an AC/ PU ratio of 1:5 exhibited the best performance in terms of these parameters. The strength recovery rate observed after manual crack healing for 24 h is 75%. (2) The bond strength between the quartz glass capsules and concrete was approximately 0.63 N/mm2, which satisfied the requirements for the cracking of the capsules caused by the crack generation. (3) When two type II capsules (AC/PU = 1:5) were embedded horizontally into the concrete specimens, the highest degree of self-healing of the cracked specimens corresponding to the healing efficiency of 67 ± 6% was observed. (4) According to the results of SEM analysis, after the cracking of the glass capsules, the PU healing agent was released uniformly into the cracks, which were closely cemented (self-healed) with concrete. Moreover, this work only investigates the short-term behavior of the healing systems since crack creation and loading were performed at early age. Because concrete structures are generally designed for a service life of 50 years or more, the healing performance at the long-term should be evaluated in future work.
Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled ‘‘Performance Optimization of One-component Polyurethane Healing Agent for Selfhealing Concrete”.
Acknowledgements This study was supported by the National Natural Science Foundation of China (51674038); Shandong Province Natural Science Foundation (ZR2018JL019); the China Postdoctoral Science Foundation (2014M560567 and 2015T80730); the Shandong Province Science and Technology Development Plan (2017GSF220003); the State Key Program for Coal Joint Funds of the National Natural Science Foundation of China (U1261205); Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (2017RCJJ010, 2017RCJJ037).
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