1. Introduction
Concrete, as the most consumed building material, has been in constant demand along with the development of the global construction industry since the invention of cementitious materials. Natural sand and gravel originate from environmental mineral deposition weathering and are important components that make up the concrete skeletal system [
1]. The overuse of concrete materials has led to a scarcity of the remaining quantity, which is obviously contrary to the current progress of green and sustainable development of buildings. In addition, cementitious materials are subject to diversified degrees of cracking influenced by the hydration level, preparation process and maintenance environment. These microcracks reduce the strength and durability of concrete structures [
2]. Concrete internal rebar faces the threat of de-passivation, and the rusted state weakens the integrity of the reinforcement concrete, causing structural failure due to the appearance of penetration cracks. In order to reduce the harm caused by the cracking, traditional remedial methods are used that mostly consist of external mortar-filling of concrete surface micro-cracks. Yet, the internal cracks and damages are not healed by this method. This method can only maintain the external cracks for a short period of time until they are disrupted again due to the presence of internal inherent cracks [
3]. Therefore, the traditional repair effect is limited and high-cost, so it cannot effectively solve the problem. As an alternative, higher-quality NRM and concrete design ratios have been persistently developed to reduce the impact of cracking on overall strength reduction [
4]. However, this usually means more cement consumption and extraction of NRM, which is not conducive to green and sustainable development in the construction industry.
Based on the premise of maintaining structural safety, the resource recovery of C&DW has become a hot topic of concern in the industry, mainly in the exploration of reducing natural material consumption [
5,
6]. Granular and powder products are obtained through the process of cutting, crushing and screening primary C&DW. As the main product derived from this process, RAs’ replacement of NS&G to prepare RAC has become a way to resolve the scarcity of natural resources [
7]. According to their physical properties, RCA (particle size greater than 4.75 mm, Recycled Coarse Aggregate for Concrete GB/T 25177-2010) is used to replace natural coarse aggregates, and RFA (particle size less than 4.75 mm, Recycled Fine Aggregate for Concrete and Mortar GB/T 25176-2010) replaces natural sand. The development and application of RCA are in relatively early stages, and the performance of RCA concrete has been explored by scholars. Studies have shown that the RCA particle size, the amount of old mortar and the replacement ratio all affect the concrete mixes’ properties [
8]. The mechanical properties of RCA concrete are mainly influenced by the aggregate state [
3], type [
9], particle size [
10], replacement ratio [
11], and fatigue life of the concrete [
12]. Lower aggregate properties and higher replacement ratios lead to lower compressive, tensile and flexural strengths, and modulus of elasticity of the concrete [
2]. The properties of RAC, such as carbonation [
13] and freeze–thaw [
14], deteriorate due to the incorporation of RCA compared to natural concrete. The more RCA is mixed and the lower the strength of the old mortar, the greater the dry shrinkage of the concrete within a certain dry shrinkage age for the same mix ratio [
15,
16]. With the surge in demand for natural sand, research on the performance of RFAs for concrete preparation has also emerged. However, it was found that the higher water absorption and poorer performance of RFAs made them less applicable than RCAs [
17]. Their negative effects on concrete strength [
18,
19], shrinkage [
20] and durability performance [
21,
22] are more significant at high substitution rates due to the increase in microcracks and porosity. Therefore, the higher concrete cracking and porosity degree due to the macro performance loss caused by the replacement of RCA and RFA is the main reason to limit the high added value of recycled products [
23].
For concrete cracking, which is a time-dependent process, internal and external fine cracks cannot be detected in time by visual inspections, which results in a serious lag in manual repair [
24,
25,
26]. Self-healing technology [
27,
28], as an intelligent phase change regulation mode, is suitable for the repair of materials due to its characteristics of “autonomous excitation, passive repair”. The suitability of polymeric materials [
29,
30], shape-memory alloys [
31,
32] and microorganisms [
33,
34,
35] for concrete crack repair has been investigated by many scholars. The polymer material is pre-injected with microcapsules and hollow fiber tubes embedded in concrete to achieve the repair effect. The dry shrinkage and cracking of the concrete cause the thin wall of the package to break, and the polymer fills and hardens along the microcracks, providing good bonding and strength at the cracked interface [
36]. Yet, the high cost and casting process requirements lead to poor engineering adaptability [
37]. Shape-memory alloys are materials composed of two or more metallic elements that have a shape-memory effect [
38]. The material deformation characteristics are the result of the reversible transformation of the low-temperature phase (martensite) to the high-temperature phase (austenite) when heated by the thermoelastic martensitic phase transformation, which enables crack and defect repair by alloy deformation inside the concrete [
39]. Moreover, additional repair energy consumption and alloy location differences result in a large dispersion of healing effectiveness. Bioconcrete involves the repair of micro-cracks by the production of mineral materials through microbial activity in the concrete [
40]. This process reduces concrete cracks to improve the strength and durability of the structure and addresses the structural maintenance needs of reinforced concrete. Microbial mineralization highlights the green properties of concrete more than other self-healing methods, as it is a natural process and environmentally friendly [
41]. The self-healing process is directly related to calcium carbonate (CaCO
3) production, which depends on many factors, including the concrete pH, microbial carriers and the amount of Ca
2+ (free-state) in the concrete [
42,
43]. In addition, the type and concentration of bacteria, the type of carrier and the method of incorporation also affect the effectiveness of healing [
44].
Bacillus pasteurii, a kind of self-healing bacteria, have a high alkaline activity and oxygen sensitivity, which are compatible with the properties of concrete materials.
Unlike the previous study [
45], which considered the effect of microbial carriers of only RCA on the performance of concrete, this paper investigates the properties of RCA and RFA as carriers acting together in concrete (
Figure 1). More importantly, the improvement in the performance of SHC due to differences in carriers was evaluated. By using an orthogonal experimental design, the effects of the carrier particle size, bacterial solution concentration and carrier replacement ratio on the repair efficiency were evaluated. The optimal solution for repair efficiency was also proposed. The time-dependent repair model at the crack was developed, and the effect of the crack repair trend at the interface with the carrier was summarized. The usefulness of the model was verified by designing the permeability test. The higher water absorption of RFAs results in a typically lower replacement ratio than RCAs when concrete performance was previously considered. Being a self-healing carrier provides a new avenue for high value-added applications, and RAs serve as a means of enhancing smart materials, playing a “turn waste into treasure” role.
4. Time-Dependent Model for SHC after Cracking
4.1. Optimization of Time-Dependent Characterization Methods for Repair Efficiency
The time-dependent measurement of repair performance was based on the standard rectangular crack assumption, and the repair trend was characterized by the change in the area of the marked points, as shown in
Figure 9. In fact, all cracks showed an irregular, jagged development, which cannot be accurately characterized by the area method. In addition, a limitation of the area method is that only the surface repair product volume can be considered, lacking the influence of internal parts. Additionally, the interior is complicated further by the crack depth, carrier distribution location and water–oxygen conditions. Therefore, the method cannot truly measure the variation in repair efficiency.
The crack water flux is the amount of water that passes through the penetration path per unit of time and can reflect the actual damage within the concrete. Water flux differences were used to describe changes in crack surfaces and internal repair properties. The correlation between the microbial activation, water–oxygen environment and time effects was constructed to model the temporal variations in the cracking of microbial self-healing materials.
4.2. Time-Dependent Model for Repair after Cracking
Combined with the foundation of the previous study, the time-dependent model of microbial SHC repair based on RA carriers considered four main factors: bacterial solution concentration, carrier substitution rate, repair time and temperature. Assuming that the ionic reaction time conditions are neglected and the temperature is set to the peak microbial activity temperature (water temperature) of 25 °C, the variation in crack repair width is influenced by two factors, including the concentration of the bacterial solution
Cb and carrier utilization
rRA.
rRA—the carrier replacement ratio, which includes both rRCA (the RCA carrier) and rRFA (the RFA carrier)
The concentration of the bacterial solution and the carrier replacement ratio are influenced by the results of the orthogonal design tests, the economics of the microorganisms and the mechanical properties of the concrete. The values selected for both are recommended to be taken from the following ranges.
The crack repair width W
r boundary and initial conditions were set as follows.
The resulting time-dependent deposition adjustment factor β was expressed as follows.
β—the deposition adjustment factor.
t—the repair time.
Kt—the temperature coefficient.
Wr—the repair’s remaining width.
W0—the initial crack width.
Cb—the bacterial concentration.
rRA—the RA carrier replacement ratio.
The conditions for the deposition of CaCO
3 molecules per unit area–time are as follows when neglecting the ionic reaction time.
F—the CaCO3 deposition per unit area–time.
α—the deposition rate, α = D/ε.
D—the diffusion coefficient of Ca2+ in the crack solution.
ε—the thickness of the diffusion layer.
C0—the concentration of Ca2+ in the crack solution, mol/L.
Cε—the concentration of Ca2+ during precipitation, mol/L.
Considering the effects of the replacement ratio, the concentration of the bacterial solution, the temperature and the time, the above equation is modified as follows.
The CaCO
3 deposition caused by
Bacillus pasteurii was mainly in the calcite state, and its linear rate expression equation is as follows.
v—the linear deposition rate of CaCO3
Vm—the molar volume of CaCO3 and the molar volume of calcite is 36.93 cm3/mol.
Then, the remaining width of the crack repair,
Wt, was as follows.
Due to the small crack width, the repair surface boundary could be defined as a parallel relationship (i.e., no angular difference between the planes on both sides of the repair area). Based on the parallel plate theory, the following relationship was derived.
q—the water flux (L/h).
I—the hydraulic gradient, m/m. I = ΔH/L.
ΔH—the difference in water level elevation between two points on the isobath, m.
L—the horizontal distance between two points, m.
Combining Equations (7), (9)–(12), the model for CaCO
3 deposition at cracks in SHC was as follows.
Simplify Equation (12) is as follows.
Therefore, based on the measured initial crack water flux q0, the water flux value qt of the crack in the specimen during the repair period T can be calculated.
Microbial SHC crack repair effectiveness factor α
r can characterize the degree of concrete repair at different times, as in Equation (15).
5. Results and Discussion
5.1. Effect of Carrier Interface on Repair Efficiency through SEM
The SEM sample preparation process consists of the following main steps.
1. Locate the repair crack area and mark a 10 × 10 × 20 mm3 rectangular area;
2. Obtain the marked area cube using a concrete cutter;
3. Fix the sample with waterproof tape and soak it in anhydrous ethanol to prevent continuous hydration/repair of the sample;
4. Send the sample to the carrier table. The crack surface should face the electron microscope and gold spraying should be performed;
5. Scan and collect morphological data.
The carrier interface is the initial area where repair behavior is stimulated to cause healing action. It includes the raw aggregate (gray basalt, massive structure, the minerals include plagioclase, olivine and pyroxene), the OM and the ITZ between the raw aggregate and the OM. The interfaces that affect the time-dependent repair efficacy include the mortar and ITZ (old and new), as displayed in
Figure 10. Among them, the OM and the old ITZ are used as microbial attachment areas, which affect the time-dependent repair efficiency through microbial activity. The new ITZ serves as an excitation condition transfer pathway that affects time-dependent repair efficacy through moisture and oxygen concentrations. At the early stage of crack extension to the carrier, moisture and oxygen are sufficient in the crack, and the repair efficacy is mainly determined by microbial activity. The process of microbial awakening and proliferation leading to repair expenditure at this stage forms the early time-dependent trend of repair efficacy. At the late stage of healing at the carrier towards the crack, the microbial activity reaches its peak and the repair efficacy is mainly determined by the amount of H
2O and O
2 transmitted by the progressively smaller crack. The time-dependent late development trend of the repair efficacy of microorganisms at this stage of the process of repair results from the lack of H
2O and O
2 causing the transition to dormancy again.
The RCA and RFA at the fractures were wrapped by the repair products, which were found via SEM, as shown in
Figure 11. There were almost no repair products or hydration products (C-S-H) at the raw aggregate interface, which was due to the density and high strength of the raw aggregate, and it was extremely difficult for microorganisms to adhere to the exposed surface of the raw aggregate. The OM was abundantly distributed on the carrier surface, and the repair products were attached to the old mortar and extended outward. The larger the volume of the OM, the larger the particle size and total repair products, which indicates that the old mortar directly influences the distribution pattern and the total amount of CaCO
3 produced by the attached microorganisms.
The sample group with RCA as the carrier had irregular aggregated CaCO3 distributions on the attached mortar and the ITZ. Among them, only a small amount of repair products appeared on the aggregate due to the dense and smooth texture, and the repair products were sporadically distributed with small particle sizes. The CaCO3 generated on the attached mortar, as observed through the RCA carrier profile, had the characteristics of large particle sizes and tight aggregation, which indicated that the attached mortar gave a better repair environment for microorganisms. The aggregated CaCO3 obviously improved the resistance to penetration at the concrete cracks, which coincides with the results of impermeability tests.
The RFA group specimens show that more adhered mortar from RFA did not make more repair products appear, while the area occupied by cementitious materials (such as C-S-H, etc.) was larger and fewer repair products adhered to the cementitious materials. This may be due to the fact that concrete cracking only had a small chance of occurring in the RFA. The RCA had more microbial volume than the RFA, which had a larger particle size and more adherent mortar monomer volume. Further hydration of the cementitious material with the RFA also inhibited the microbial repair effect, i.e., it hindered microbial contact with O2 and H2O. The lower remediation efficiency and the smaller distribution density of individual microorganisms resulted in smaller particle sizes and distribution areas for the products. It is worth noting that the areas where CaCO3 was generated are mostly places where less gelling material was present.
The NA interface of the specimens was observed to have only a small amount of cementitious material and no repair products on the larger aggregate area. This indicates that the smooth and dense nature of the natural aggregate (NA) is also not conducive to the adhesion of cementitious materials but effectively reduces the pores within the concrete and improves the overall impermeability. This also indicates that the loose and porous nature of the mortar to which the RA adheres contributes to the self-healing efficiency, a characteristic that is not present in concrete prepared with NA.
Contrast analysis of RA and NA interface characteristics found that the RA is rougher and old defects can be clearly observed at the ITZ between the aggregate and the repaired product, which is due to the poor process of the initial pouring of the RA (
Figure 12). No or little cementitious material or repair products appeared in the area of the unattached mortar, which is consistent with the concrete prepared with NA. The surface of the NA specimens was smooth, and the aggregate boundary was free of adherent mortar and repair products. In fact, the NA was wrapped by the cementitious material, and incomplete wrapping in the larger aggregate areas led to the appearance of larger voids or defects. It should be pointed out that the voids in RA are mostly due to the micropores of the cementitious material after hydration and not attachment mortar area voids. The RA attachment mortar area is affected by self-healing behavior without obvious defects. The voids in the NA group are mostly due to the dense and smooth texture, resulting in the cementitious material after hydration not being better wrapped in NA.
5.2. Effect of Carrier Interface on Repair Product via XRD
The calcite state of CaCO
3, as a repair product (SEM-observed morphology), is influenced by the carrier interface, and its particle size and generated amount have a direct impact on the effectiveness of the repair. The quantitative analysis of the physical phases at the repair cracks when the RCA and RFA were used as the carriers was performed via XRD. The calculation of the grain size requires obtaining the value of the full width at half maximum (FWHM) of the diffraction peaks. The sample is deconvoluted from it according to Equation (16) to obtain the broadened FW(S) due to grain refinement.
D—transposed convolution parameters, defined as a value between 1 and 2, where the diffraction peak pattern can be represented by either the Cauchy function or the Gaussian function. If the peak shape is closer to the Gaussian function, it is taken as D = 2. If it is closer to the Cauchy function, it is taken as D = 1. In this paper, the value of D is set to 2.
FWHM—the full width at half maximum of diffraction peaks.
FW(I)—the linear width value of the instrument.
FW(S)—the width value of the specimen.
After obtaining the specimen width value, the grain size is calculated according to the Scherrer formula (Equation (17))
Csize—the size of the crystal.
K—the constant, taken as K = 1.
λ—the wavelength of X-ray, nm.
θ—the angle of diffraction (two-theta), rad.
As shown in
Figure 13 and
Figure 14, the diffraction angle 2T (two-theta) and FWHM of the CaCO
3 in the RCA and RFA were 29.672, 0.109 and 29.317, 0.147, respectively. The products obtained from the physical phase analysis are mainly CaCO
3, SiO
2 and hydration products when the RCA is the carrier. Based on Equations (14) and (15), the average grain size of the repair products in RCA was calculated to be about 14.92 μm. The proportion of its physical phase is about 46.0% of the measured sample fraction, followed by SiO
2 contained in siliceous aggregates, with a proportion of about 28.0%. The products obtained from the RFA specimen physical phase analysis are mainly CaCO
3, SiO
2, hydration products, a small amount of H
2O and CO
2, etc. The average grain size of the RFA repair products is calculated to be about 13.83 μm. The proportion of its physical phase is about 28.7% of the measured sample fraction, followed by SiO
2 contained in siliceous aggregates, with a proportion of about 25.2%.
The analysis results showed that the RCA group generated a larger CaCO3 particle size than the RFA group, which indicated that the carrier interface of the RCA group enhanced the quality of the repair products more significantly. The larger particle size allowed the single-grain repair products to exhibit better resistance to deformation and better filling of the cracks. In addition, more CaCO3 was generated in the RCA group, about 1.6 times more than that in the RFA group, indicating that the amount of repair products was significantly enhanced. The greater level of production resulted in a faster repair speed and denser aggrege of repair products, which directly influenced the performance improvement in the repaired specimens. Small amounts of H2O and CO2 were found in the RFA specimens. On the one hand, this may be due to the higher water absorption rate, which makes the pore water in the adherent mortar migrate outward during the hydration process; on the other hand, it may be that the two were not completely absorbed during the repair process from the cracks into the concrete.
5.3. Verification of Time-Dependent Model for Repair after Cracking
The test values and calculated values are shown in
Table 6, and after calculating the standard deviation of the qt test values, the coefficient of variation of the calculated values for each group was obtained by bringing in the coefficient of variation formula.
Coefficient of variance (CV):
According to the analysis of the results, it is found that the time-dependent repair model has a higher accuracy when the calculated value is less than or equal to 0.5 mm, while the crack width of 1 mm is not applicable to the time-dependent repair model because the microorganisms basically do not appear or rarely appear in the repair products. In addition, the accuracy of the calculated value of the model decreases when the size of the carrier changes, so it is recommended to use this model. For the calculation, the carrier size of RCA should not exceed 15 mm.
The repair of each initial width at different sampling points with time was measured by a crack observer. The analysis revealed that the smaller specimens with initial cracks showed little repair behavior at 3 day. Some areas of the cracks healed from the sides to the middle, but a large number of pores still allowed water to flow into the concrete. The larger areas of the initial crack in the concrete show a more pronounced calcite state of CaCO
3 inside and extending to the outside of the crack at 7 day. This is probably due to the fact that the larger cracks did not allow the CaCO
3 generated on both sides to support its own weight while connecting to the middle and generating it inside the cracks. In addition, larger initial cracks allow moisture and oxygen fluxes to saturate the internal concentration faster than smaller ones, which favor the repair behavior of microorganisms, and again, larger parts also allow lower repair rates at the surface. This is detrimental to the corrosion resistance of concrete. At 14 day of repair, the filling of pores with repair material within the cracks could already be observed, but there were still a number of pores that were not filled. This improved the internal mechanical properties of the concrete to some extent, but the improvement in durability is still unfavorable due to the presence of voids. The observation of the smaller initial crack specimens at 28 day and 56 day revealed that the healed specimen generated repair material to cover and fill the crack surfaces, which effectively prevented the erosion of harmful substances into the concrete interior through the cracks. It should be noted that the initial crack width (>4 mm) in the 28 day to 90 day repair period remained unrepaired, and the internal repair was not significantly enhanced (
Figure 15). This may be due to the fact that the flux of H
2O and O
2 into the microbial repair area was weakened while the interior was better filled, which instead lessened the repair effectiveness and meant that the repair of the cracked fecal surface could not be completed.
6. Conclusions
The incorporation of RAs enhanced the resourcefulness, greenness and economic properties of microbial SHC. In addition, the experimental results demonstrate the positive repair effects of both RCA and RFA on cracking. The rate of RA as a repair carrier instead of NS&G can be increased with the design demand, which is beneficial to resolving the high replacement utilization problem of RA. Bacillus pasteurii, as a clean repair driver, meets the current demand for sustainable building development.
(1) In the comparison among RCA, RFA and NA as carriers, the loose and porous nature of the adhered mortar of RA makes the cracks extend more along the aggregate when cracking. The higher level of adhered mortar needed for RCA makes the local microbial density more obvious than that of RFA, and the smoothness of NA is not conducive to microbial adherence, which in turn leads to lower repair efficiency.
(2) The products of the specimens with RCA and RFA as carriers that were obtained in the physical phase analysis were mainly CaCO3, SiO2 and hydration products. The average particle size of the RCA group was calculated to be about 14.92 μm, according to the formula. The proportion of its physical phase accounted for about 46.0% of the measured sample fraction. In contrast, the average particle size of the repair products in the physical phase analysis of the RFA group was about 13.83 μm, which was 92.69% of the RCA group; the proportion of its physical phase was about 28.7% of the measured sample fraction, which was 62.39% of the total of the RCA group. This indicates that the RCA group is better than the RFA group in terms of particle size and the total amount of repair products.
(3) Both recycled coarse and fine aggregates as carriers enhanced the concrete crack repair rate, compared to the SC and NC groups. At 14 days of repair, the remaining crack, with an initial width of 0.15 mm, was 0.06 mm in the SCC group, thus 60% repaired, while the remaining crack width in the SFC group was 0.11 mm (only 26.67% repaired). The crack repair was difficult to achieve by microbial action alone; only 13.33% and 10% of the cracks with initial 0.15 mm and 0.2 mm widths were repaired in the SC group, respectively. The larger cracks continued to extend compared to the initial cracks, which aggravated the deterioration of the properties.