Integration of FDM and self-healing technology: evaluation of crack sealing by durability and mechanical strength

The tensile zone of concrete is prone to cracking due to its limited ability to withstand tension. To address this issue, steel reinforcement is used in these specific regions. The occurrence of little cracks might potentially facilitate the ingress of liquids and gases into the reinforcing material, hence inducing corrosion. Self-healing concrete can repair and seal minuscule cracks, thus impeding the formation of corrosion. This study investigates the potential application of fused deposition modeling (FDM) for generating novel vascular networks and tubes using polylactic acid (PLA) as the material. Poly (lactic acid) (PLA) was fabricated using three-dimensional (3D) printing techniques, and its properties were compared to those of one-dimensional (1D) and two-dimensional (2D) networks. The external diameter measured 5.6 mm, while the internal diameter measured 4 mm. utilized a 10 ml volume to apply healing agents, specifically organic polyethylene glycol liquid and nano-powder (fly ash) derived from recycled materials, to all vascular structures (1D, 2D, and 3D). This application was carried out using a planetary ball mill. Following this, the prepared tubes were incorporated into a concrete beam to introduce self-healing capabilities. The water-to-cement ratio (W/C) utilized for all concrete mixtures was 0.6%, while the definite mixture proportions were 1:2.16:2.98. The quantification of the self-healing phenomenon was conducted by evaluating the restoration of load-carrying capacity following the application of a repaired specimen to a four-point bending test. Furthermore, these enhancements resulted in improved durability, increased compressive strength, and enhanced other physical characteristics. The pipes that are manufactured can be utilized to produce innovative concrete that possesses the ability to undergo self-healing processes by combining low-viscosity healing solutions (PEG) with powders (nano fly ash) that are appropriate for this application by injection into the vascular network , making it well-suited for various self-healing applications.


Introduction
Concrete is extensively utilized in the field of construction due to its exceptional compressive strength and comparatively affordable price [1].The inherent brittleness of concrete results in the inevitability of cracking, which subsequently provides pathways for aggressive chemicals to deteriorate the reinforcement and compromise the structural integrity [2].The occurrence of most of these cracks can be attributed to factors such as limited tensile strength, creep, excessive load, permeability, thermal stress, seismic activity, and other similar causes [3][4][5].Implemented through the utilization of diverse autogenous or autonomous methods.Nevertheless, the crack width constrains the effectiveness of autogenous healing.The crack should have a maximum width of approximately 200 μm or smaller [6].In autonomous healing, a notable illustration pertains to the controlled release of a stored healing agent through various types of capsules, such as polymer, glass, ceramic, and ZnO capsules [7][8][9][10][11].Capsule-based self-healing offers limited healing agents, thereby mitigating the need for repetitive damage repair and the restoration of larger cracks [10].The vascular systems provide a significant advantage by enabling multiple cycles of cracking and healing, as a continuous supply of the healing agent can be provided through the flow networks [12].Studies explore the impact of the vascular system on crack healing in cementitious materials and the use of recycled materials in concrete.
The investigation focused on evaluating the flexural properties of self-healing concrete railway sleepers (SHCRS) after their reinforcement with glass tubes filled with polyurethane (PU).Revealing an 11% increase in applied stress by Mohammad Siahkouhi et al [13].T. Selvarajoo et al developed a delivery system for cyanoacrylate by integrating PET tube networks into cementitious matrices, undergoing experimental tests on unreinforced notched prisms [14].Zhi Wan and colleagues utilized dissolvable polyvinyl alcohol filament to create vascular networks coated with wax and incorporated into cementitious mortar.Epoxy resin was used as a healing agent, and experiments were conducted to investigate its efficacy [15].The Murray circulatory blood volume transfer equation was used to design and build a biomimetic 3D PLA vascular network.In addition to fracturing and impregnating the sample with sodium silicate for four weeks,examined its mechanical characteristics using load recovery analysis by Zijing Li et al [10].Eleni Tsangouri et al used 3D printing technology to create therapeutic polymer tubes that ruptured upon fractures, allowing for the application of polyurethane healing foam, restoring strength and resilience [16].This study developed a composite material using cement and superabsorbent polymers to address cracks with depths not exceeding 400 μm.The granulation process used polyethylene glycol as a binding agent.Improved sealing ratio, water tightness, compressive strength, and flexural properties [17].A study examined the impact of self-curing chemicals on concrete strength, involving the addition of polyethylene glycol (PEG), sodium polyacrylate, perlite, propylene glycol (PPG), and vermiculite to M30 concrete mix.Results showed PEG and PPG as the most effective selfcuring agents [18].Jianhang Feng et al successfully created hydrogels by encapsulating sodium alginate within polyethylene glycol granules coated with epoxy resin and calcium sulfoaluminate cement.This resulted in crack closure and reduced water permeability within 24 h [19].Azam Torkan et al developed a model to predict the minimum critical crack radius using PEG/SiO2 nanoparticles in fibrous media.The media showed improved healing efficiency due to a reduced crack radius, as evaluated through ultrasonic tests, FESEM, and EDS analysis [20].A study by Wu et al incorporated OH-rich PVA, sisal fibers, and healing agents into cement mortar to improve its mechanical properties.The combustion of MgO and -OH in fibers facilitated crack self-repair, resulting in calcite-type CaCO3, hydromagnesite, and dypingite [21].developed by Jian Gao et al a new fastresponsive capsule for self-healing concrete, uses poly (ethylene glycol) (PEG) to granulate healing agents coated with epoxy resin and fine sands.This capsule offers superior alkali resistance and waterproofing qualities, allowing crack healing under 200 μm in three days [22].The study examined the impact of fly ash and steel/ polypropylene fibers on concrete self-healing.Results showed that 15% fly ash reduced permeability by 94%, and the re-curing process restored secondary tensile strength, as reported by Seyedehtina.S. and Madandoust R. [23].The study investigates the impact of incorporating mechanically activated fly ash (MAFBC fly ash) into cement-based composites on self-healing.Results show that MAFBC fly ash substitution alters self-healing materials' properties.Furthermore, hydrated calcium aluminates were found in fractures, resulting in the closure of fissures up to 110 mm wide [24].Polyethylene glycol (PEG) enhances concrete self-curing, shrinkage, and durability by increasing pore solution viscosity, limiting chloride corrosion, and reducing particle size and durability [25].
This study aims to develop a three-dimensional vascular network that enhances the coverage and distribution of healing agents.appropriate can be filled with various healing chemicals and repaired samples again .The vascular network was constructed using polylactic acid (PLA) and 3D printing, and its efficacy was evaluated in comparison to the corresponding one-dimensional (1D) and two-dimensional (2D) networks for their potential application in concrete self-healing.The study demonstrated that the structures of polylactic acid (PLA) exhibit a satisfactory interfacial connection in terms of mechanical triggering while also displaying a brittle fractural response.The specimens were embedded within concrete prisms and exposed to four-point bending tests to experimentally evaluate their capacity to form a strong bond with the cement matrix.The initial fractures observed in all systems had widths ranging from 600 to 800 μm.The experimental findings revealed that the transportation of the pumped polyethylene glycol liquid, containing powdered nano fly ash as a healing agent, was effectively achieved within the vascular network of all three systems for a duration ranging from seven to twenty-eight days.The mechanism behind the self-healing effect was clarified in relation to the restoration of load-bearing capacity observed during the mechanical evaluation subsequent to the reapplication of force on the specimen through four-point bending.The investigation has the potential to encompass a broader range of therapeutic agents and will contribute to the improvement of cement formulations to increase their resistance to chlorine penetration, which is an essential aspect in the development of concrete technology and construction practices.

Experimental work 2.1. 3D printing vascular tubes and networks
The vascular tubes and networks were created via 3D printing with various patterns (1D, 2D, and 3D).The Creality CR-10S 3D printer with a 0.80 mm nozzle printed the vasculature.The printer has a maximum printing size of 300 mm × 300 mm X 400 mm, a maximum print resolution of 60 μm, and a maximum print speed of 80 mm/s.Corn and sugarcane are used to make biodegradable thermoplastic aliphatic polyester polylactic acid (PLA).PLA has a low Tg (60°C) and elongation at break (10%) [26].tubes with a capacity of 30 ml are 100 mm long, 4 mm internal diameter, and 5.6 mm external diameter.Table 1 lists the filament printing settings used to produce FDM samples.
Tube and vasculature capability to protect the healing agent and chemical stability of PLA from the cementitious environment and prevent agent loss from pipes before rupture is crucial to self-healing tubes.FDM pipes have strong interlayer adhesion.There are no gaps between the pipes and concrete.
To test mechanical recovery after breaking and embedding into a concrete cube during casting, tubes constructed with each plastic filament were filled with The nanopowder was combined with a polyethylene glycol (PEG) solution using a magnetic stirrer as a healing agent, then injected into the tubes by syringe.The specimens were kept in water for 28 days before being sliced in two to test the healing agents.The healing ingredient was still liquid; thus, polyethylene glycol might promote self-healing.

Materials 2.2.1. Cement
The present study employed Type I regular Portland cement for the investigation.The cement's physical characteristics and chemical composition, as presented in table 1, align with the requirements outlined in the Iraqi Specification (No. 5/1984) [27] and ASTM (C150-04) [28].

Aggregate
The coarse aggregates exhibit a rounded shape and possess an ultimate size of 20 mm.Conversely, the fine aggregates consist of natural sand sourced from the contemporary Mustafa plant in Karbala, with an ultimate size of 4.75 mm.The materials in question adhere to the ASTM (C33-03) [29] and the Iraqi Standard Specification (No. 45/1984) [30].

Fly ash
The used class (F) fly ash was within the requests of the american standard ASTM (C 618-15), as evinced in table 1 [31].

Polyethylene glycol
PEG-400 is a condensation polymer formed through the reaction of ethylene oxide and water.It can be represented by the general formula H(OCH2CH2)nOH, where the variable n denotes the average count of recurring ethylene groups, typically within the range of 4 to approximately 180.Polyethylene glycol (PEG) exhibits a range of desirable characteristics, including being water-soluble, neutral, non-irritating, lubricating, non-volatile, non-toxic, and odorless.Concrete that has been treated with a PEG-400 self-curing agent initiates a chemical reaction with the water content within the concrete, leading to the creation of a hydrogen bond.According to reference [32], during the hydration process, the self-curing agent exhibits an affinity for water molecules, leading to an augmentation in the water retention capability of the concrete and a reduction in water evaporation.

Proportions of mixture
The weight ratio of (1:2.16:2.98)that was highly utilized was employed for the purpose of mixing, with an intended strength of (30 MPa) to be achieved within a period of four weeks.The composite consists of 340 kilograms of cement, 734.4 kilograms of sand, and 1013.2 kilograms of gravel per cubic meter.The water-tocement ratio is 0.6.The study involved the incorporation of nano fly ash powder into the healing fluid within hollow tubes, at a proportion of 10%, to establish an autonomous self-healing system.The resulting mixture of concrete is presented in table 2, and mixtures of concrete beam compressive strength specimens listed in table 3.
The current study involved the preparation of various specimen shapes, including cubes, prisms, and cylinders, for the purpose of evaluating the mechanical properties of hardened concrete and measuring its physical characteristics and durability.Following a period of 7 and 28 days, as well as the introduction of cracked samples, the specimens of cured concrete were subjected to a further 28-day period in an air-conditioned environment to facilitate healing.Subsequently, the compressive strength of the specimens was evaluated through testing.The tests were specified by B.S. (1881-116) [33] for cubic specimens measuring 150 × 150 x 150 mm 3 .500 × 100 × 100 mm 3 beams were fabricated for the purpose of assessing the flexural strength of concrete samples at 7 and 28 day intervals, in accordance with ASTM (C78) [34], utilizing a four-point flexural machine.A distinctive vascular system was integrated utilizing various designs, including one-dimensional, twodimensional, and three-dimensional configurations.The specimens were reinforced with 4 mm smooth metal bars, which were positioned at a distance of 10 mm from the bottom of the specimen to prevent complete splitting during the cracking process.The specimens were subjected to an applied force until the beams, which were situated on two supports positioned 400 mm apart at both ends, experienced failure.Three cylindrical specimens, 100 mm in diameter and 200 mm in height, were prepared for the Rapid Chloride Penetration Test.These specimens were of the same age by the established ASTM (C 1202) [35].

Inspections and tests 2.4.1. Nano-powder inspections
The determination of particle size for the examined fly ash powder was conducted at the Centre of Nanotechnology and Advanced Research utilizing the Brookhaven Nano Brook 90 Plus device (Model: Brookhaven Nano Brook 90 Plus, US).The results revealed a particle size of 100 nm.

The examination of physical features
The dry density, water absorption, and porosity were determined in accordance with the ASTM standard C642-97 [36].The broken pieces are collected after the compressive strength is measured while they are still saturated in the healing solution after the healing period.The examination procedures are carried out according to the standard given.The following equations were used to ascertain the outcomes of these experiments: Let w 1 represent the weight (in grams) of the dry specimen, w 2 represent the weight (in grams) of the wet specimen, w 3 represent the weight (in grams) of the water-submerged specimen, and ρw represents the water's density, which is 1 g cm −3

Compressive strength test
The compressive strength test was performed using the TONI PACT 3000, a compressive strength testing apparatus, as reported by B.S. (1881-116) [33].The loading velocity was approximately 0.25 MPa.The equation used to calculate the compressive strength is as follows: In the given equation, σ represents the compressive strength measured in megapascals (MPa), denotes the maximum compressive load expressed in Newton's (N), and signifies the area of the specimen measured in square millimeters (mm 2 ).The positioning of the tubes and the distribution of the cured material inside the cube are shown in figure 1, which serves the goal of investigating the compressive strength.The bending strength was assessed in accordance with ASTM (C78) [34] standards.Prior to conducting the fourpoint bending test, a notch with a depth of 3 mm was created.The download rate was measured at a rate of 0.2 MPa/s.Following the completion of the test, the specimens were subjected to an air curing process and allowed to remain undisturbed for an additional duration of four weeks.The bending strength of the specimens can be determined for both the first repetition (1st R) and the second repetition (2nd R) by utilizing equation (5).
In this context, the flexural strength (Fct ) is denoted in megapascals (MPa) or Newton's per square millimetre (N/mm2).The ultimate load (F) is measured in Newton's (N).The span (I ) refers to the distance in millimetres (mm) between the two supporting rollers.Furthermore, (d1) and (d2) reflect the specimen's lateral dimensions, which are likewise measured in millimetres (mm).Furthermore, the prism samples that were identical in both combinations were subjected to another examination after a period of four weeks of healing (referred to as the 2 nd R).During this examination, relevant equations were employed to assess the recovery of flexural strength, as outlined in reference [8].
In this context, we define fct1 as the maximum stress experienced by the original specimen during the first rupture (1st R), fct2 as the maximum stress reached by the specimen after healing has occurred (2nd R), and % h as the measure of healing efficiency.The fracture seal was subjected to multiple examinations using a crack detection microscope.The crack was documented through photography in order to quantify its dimensions, specifically its depth and width, with the purpose of determining the extent to which it recovered in terms of percentage strength over a given period of time.The location where the readings were obtained and the presence of cracked prisms were identified.figure 2 illustrates the spatial arrangement of the vascular network within the prism.

Fourier transform infrared (FTIR) spectra test
It is a method employed to obtain comprehensive insights into the molecular structure and chemical bonds of polymer materials.This test was conducted in accordance with ASTM E1252 standards, utilizing a Fourier infrared (FI) spectrometer manufactured by Brukeroptier, specifically the TENSOR-27 model.The infrared spectrum was utilized within the range of 400-4000 cm -I.

Energy-dispersive x-ray spectroscopy (EDS) and field emission scanning electron microscopy (FESEM)
We used (Zeiss -Sigma -VP FE -SEM, and EDS, Iran) to analyze the surface morphology of both healed and unhealed specimens using (FESEM) at magnifications ranging from l00 um to 10 m and to detect elemental composition (EDS).

Rapid chloride penetration test
Three cylindrical specimens, each having a diameter of 100 mm and a height of 200 mm, were manufactured in accordance with the established ASTM (C 1202) [35] standard.Subsequently, these cylindrical specimens were transformed into disc-shaped specimens with a diameter of 100 mm and a thickness of 50 mm.This procedure was conducted in order to evaluate the level of resistance to the penetration of chloride ions.After the application of a voltage of 60 V for a duration of 6 h, the anode and cathode compartments were then filled with a solution of 0.3 M NaOH and a solution of 3% NaCl, respectively.Figure 3 illustrates a schematic representation  of the test system.Further measurements of current were taken at regular intervals of 30 min to monitor the current passing value.The total amount of charge that went through was calculated using equation (7).

Particle size distribution (PSD)
The flow chart for the experimental strategy may be seen in figure 4. The fly ash nanoparticles derived from the planetary ball milling technique were assessed using a particle size analyze (PSA) to determine the particle size and size distribution during the milling process.The particle size of fly ash was effectively reduced to 95.3 nm through a milling process lasting for a duration of three hours, as determined by the PSA.The observation revealed that a narrow particle size distribution significantly enhances the efficacy of healing.Figure 5 illustrates the particle size distribution of the FA nano-powder that was prepared.

Characteristics of matter's physical properties
In the context of experimental methodologies for analyzing the physical features of concrete mixes after 7 and 28 days, it is seen that the dry density increases with an increase in the amount of the healing agent, as shown in figure 6.The formation of high-strength dense gel (C-S-H) is a result of the pozzolanic reaction, which occurs when particles of nano-fly ash (NFA) combine with calcium hydroxide (Ca(OH)2) crystals formed during hydration.The density of the transition zone increased as a result of this process, since it included filling the gaps that were created by the lack of material.The latest research has shown that, across all designs, an expansion in the region of distribution of the healing agent resulted in a decrease in porosity and water absorption, aligning with previous studies [37,38].The addition of 3D design with a healing agent resulted in the greatest rate of density growth at 1.2% and 3.8% after 7 and 28 days, respectively.However, the water absorption and porosity exhibited a reduction at rates of 7% and 14% after 7 days, and 15.8% and 28% after 28 days, respectively.

Compressive strength
The compressive strength test outcomes are presented in figure 7, encompassing all cube samples at the ages of 7 and 28 days.The results represent the mean values of the three respective samples.The increase in compressive strength resulting from the incorporation of nano-fly ash (NFA) particles in conjunction with polyethylene glycol can be attributed to the substantial surface area of NFA.This surface area can serve as a site for nucleation and promote the formation of hydration products, specifically the (C-S-H) gel.The utilization of PLA tubes,  which are composed of hydrophobic material, in conjunction with a healing agent results in the creation of a cohesive mixture.In the event of a crack, the healing agent is released, and the cube is subsequently left to undergo air curing for a duration of 28 days.Furthermore, the enhancement in compressive strength can be attributed to Concrete maintains high moisture levels well using polyethylene glycol (PEG).PEG is homogeneously distributed in concrete, enabling continuous gel hydration. .Fly ash and calcium hydroxide (Ca (OH)2) in concrete interact well with polyethylene glycol (PEG).It reduces pore size, closes capillary pores, and lowers permeability.PEG increases C-S-H gel density., which is consistent with findings from prior research [38][39][40][41].The incorporation of 3D design with a healing agent resulted in a maximum increase of 22% and 23% in compressive strength at 7 and 28 days, respectively.

Flexural strength
The efficacy of the mechanical triggering and healing of the vascular system was evaluated through the analysis of loading recovery subsequent to the healing process, as depicted in figure 8(a).The specimens containing vascular networks that were pre-fabricated in one, two, and three dimensions were subjected to four-point bending tests in order to induce crack formation.The results of these tests are presented in figure 10.The results of the initial load testing on plain concrete beams that were aged for 7 and 28 days indicated a minimal load response, with values of approximately 4.5 MPa and 5.5 MPa, respectively.In comparison to the control specimen, it was observed that the specimens implanted with vascular networks required a greater force for fracture, as depicted in figure 8(a).This could potentially be attributed to the physical activation or the tubes composed of PLA.Moreover, it was observed that the vascular systems (1D, 2D, and 3D) exhibited cracks under increased strain, indicating that the cement beams were strengthened by the plastic tubing, as reported in reference 44.The restoration of flexural strength was evaluated using equation (6).The prism specimens from both combinations were subjected to a re-evaluation process after a healing period of 28 days, as depicted in figure 8 (b).The results indicate that the recovered strength was 93% for the specimen treated with 3D technology, while the control specimen exhibited a recovery of only 43%.The analysis of the outcomes in relation to the control sample demonstrated that the utilization of MC as a therapeutic agent is efficacious, a finding that is in line with previous investigations.Small particles having a high  specific surface area and a mesoporous structure are porous superfine powders.It may absorb the aqueous phase, allowing water to be supplied for the PEG hydration process in the cementitious material.serve as sites to nucleate and promote the configuration of C-S-H products [42][43][44][45].In contrast to the 1D and 2D systems, the 3D vascular system exhibited a greater capacity for obstructing a larger area of the fracture.The inability of the capillary force to facilitate the upward movement of the healing agent and fill the crack tip resulted in the inadequacy of the (1D) and (2D) systems to deliver the healing agents to this particular location.The superior performance of the 2D and 3D networks under stress can be attributed to their numerous supporting structures and the absence of emphasis on a singular tube, in contrast to the 1D structure.

FTIR results
The observed spectral features in figure 9, include broad adsorption bands at 3504-3417 cm −1 , which were assigned to O-H, and a band at 874 cm −1 , which was attributed to C-H stretching vibration.The latter is believed to arise from the deformation vibration of the (-CH 2 , -CH3) moiety.In addition, the carbonate bands exhibit pronounced peaks in proximity to 1423 cm −1 and in proximity to 873 cm − 1 , which are identified as the antisymmetric stretching and the C=O angular deformation of calcite (CaCO 3 ), correspondingly.In contrast, the double bond of (C=O) exhibits a weak peak at a wavelength of (1798) cm-1 in the overlapping stretching bond.The hydration of calcium silicate (C-S-H) is characterized by a medium peak at 915 cm −1 and a strong peak at 1113 cm −1 , which correspond to the (Si-O) bond group.The spectral peaks detected within the 600-655 cm −1 range are indicative of the asymmetric stretching and bending of SO 4 .The (FTIR) spectroscopy of normal concrete reveals that the presence of silica (SiO 2 ) is indicated by the wavelength range of (420-690) cm −1 , specifically the (Si-O-Si) bond.The minor peak observed at 2513.82 can be attributed to the increased production of Ca(OH) within the sealed crack.The observed supplementary peaks detected at 2874 cm −1 are attributed to the presence of PEG400 within the composite, resulting from a chemical reaction with the cementitious material.In comparison to the concrete used in the control group.This spectral feature is indicative of the presence of calcium hydroxide (Ca(OH) 2 ).The reduction in the formation of calcium hydroxide and the concomitant increase in the formation of calcium silicate hydration (C-S-H) resulted in a slight decrease in the broad band.A novel peak was detected in the PEG composite sample upon the inclusion of fly ash nanoparticles.This occurrence can be attributed to the cross-linking phenomenon observed in these specimens as well as the discovery of a chemical bond.The peak observed at a wavelength of 2514 cm −1 in FTIR polyethylene glycol in concrete is attributed to the C-H bond group.The observed peak is attributed to the stretching vibration of the methylene (CH 2 ) group present in the PEG molecule.Infrared radiation absorption by specific functional groups or chemical bonds is indicated by strong peaks, which correspond to low transmittance values.Conversely, weak peaks correspond to high transmittance values and suggest lower absorption by the same functional groups or chemical bonds [46,47].

Microstructure development analysis
The analysis of fractured surfaces before and after self-healing was conducted using scanning electron microscopy, as illustrated in figures 10(a) and (b).A discernible stratum of self-repairing substances was observed on fractured surfaces, displaying a configuration that deviated from the internal composition of cement paste as illustrated in figures 10(c) and (d).The product layer demonstrated a radial expansion extending from the crack's surface towards its core.The analysis and assessment of the development and progression of microstructure.The experimental sample primarily generated discrete CH crystals and a voluminous CSH gel derived from unreacted cement particles as self-healing agents, occupying a minimal proportion of the crack voids.
The enhancement of water retention capacity in concrete and the reduction of water evaporation during the curing process can be achieved through the utilization of a self-curing agent.This agent has the ability to absorb water from the surrounding atmosphere, resulting in water conservation during the curing process when compared to conventional concrete.
An increase in strength.The occurrence of micro cracks resulting from aggregate restraint is mitigated due to the subsequent shrinkage of cement paste, which expands during the early stages and subsequently enhances the strength of concrete.The primary objective of incorporating polyethylene glycol (PEG) into concrete is to mitigate water evaporation and enhance the water retention capability of the concrete in comparison to traditional concrete.This alteration ultimately results in an enhancement of the concrete's compressive strength [48].The inclusion of this combination facilitates the uninterrupted process of cement hydration in concrete, incorporating nanopowder constituents derived from fly ash, while PEG is present, thereby achieving the desired material characteristics.By enhancing the gel density, the porosity and permeability of cement paste can be decreased, leading to an enhancement in compressive strength.
The EDX analysis yielded numerical data that enabled a thorough assessment of the chemical compositions produced.The atomic ratio of calcium to silicon (Ca/Si) was employed to determine the degree of hydration in the calcium-silicate-hydrate (C-S-H) gels.The higher consumption of Ca(OH)2 during cement hydration was observed as a consequence of a decrease in the Ca/Si ratio.This phenomenon led to the formation of C-S-H linkages, which played a significant role in the densification of the concrete matrix through the nanofiller effect.The occurrence of CH formation takes place in the initial phases of the curing process, thus initiating the pozzolanic reaction with NFA to produce supplementary C-S-H binder gel.This study employed scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) to evaluate the impact of PEG liquid containing nano fly ash on the dimensions of the microstructure [49][50][51].

Rapid chloride penetration resistance
The test findings for chloride ion penetration resistance in plain concrete specimens, as shown in figure 11, indicate durability.Higher RCPT values indicate increased concrete diffusion, whereas chloride ions affect concrete pavement spalling below the pore solution's freezing point.It boosted rebar corrosion saturation.Variables affecting RCPT include compressive strength, mineral admixtures, aggregates, curing methods (normal, autoclaving, steam), specimen temperature, pore size, regional environmental conditions, test conditions [25,52].Adding a healing agent to concrete enhanced chloride ion resistance by 14%, 36%, and 46% for 1D, 2D, and 3D samples, respectively, compared to the reference sample.PEG complexes Ca2+ on calcium silicate hydrate (C-S-H) and spheroidizes it, loosening its packing in cementitious material and reducing chloride binding in the concrete matrix.Concrete specimens should have a total charge passing between 2000 and 4000 °C under appropriate conditions.

Conclusions
The proposed self-healing mechanism is deemed beneficial for incorporation in concrete infrastructures, particularly those situated in challenging environments, such as underwater structures, bridges, and dams, where maintenance and repair works are arduous.While the upfront costs may be higher due to immediate repairs, there is potential for reduced maintenance expenses and prolonged building lifespan.The potential of the Network to facilitate healing at multiple scales is enhanced due to its versatility in accommodating diverse healing agents and its reusability.The utilization of a vascular system results in improved loading efficiency of powders, thereby augmenting the properties and crack sealing ability of the concrete beam.The second loading cycle revealed the occurrence of mechanical repair, thereby providing evidence for the process of healing.The administration of the curative agent at multiple locations on the surface of the crack resulted in the augmentation of the repaired surface area by the 3D vascular system, surpassing the repair capabilities of the 1D and 2D systems.The incorporation of Nano powder and PEG as healing agents in concrete beams leads to an increase in density and compressive resistance, thereby enhancing pozzolan interaction and overall durability.The minimization of permeability in NFAs is achieved through the process of hole and crack filling.

Prospects for the Future
The use of monitoring mechanisms is essential for the advancement of self-healing concrete.These mechanisms may include the incorporation of optical fibers or piezoelectric transducers to establish a sensor system capable of detecting cracks and then initiating the release of the healing agent from the reservoir station.Subsequent research endeavors will prioritize the development of impactful vascular healing networks and their subsequent integration into architectural structures.

Figure 3 .
Figure 3.The measuring apparatus used for the assessment of rapid chloride penetration test (RCPT).

Figure 5 .
Figure 5. Particle size distribution for prepared FA Nano-powder.

Figure 6 .
Figure 6.Varying vascular tube designs effects on the concrete physical qualities.

Figure 8 .
Figure 8. Flexural strength for concrete with different designs of vascular tube (a) Flexural strength of all samples with a different design at 1st R and 2nd R (b) Flexural strength percentage recovery of all samples.

Figure 10 .
Figure10.The SEM -EDS Image of cementitious materials before and after self-healing (a) The Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) image of cementitious materials prior to undergoing self-healing after a period of 7 days (b) The scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) image of cementitious materials prior to selfhealing at the 28-day mark (c) The Scanning Electron Microscope-Energy Dispersive x-ray Spectroscopy (SEM-EDS) image depicts the cementitious materials following a self-healing process involving the utilization of polyethylene glycol (PEG) to hold nano fly ash powder.The image was captured after a period of 7 days (d) The SEM-EDS Image of Cementitious Materials after Self-Healing with (PEG holding nano fly ash powder) at 28 days.

Table 2 .
Chemical composition of the used cement and Fly Ash.

Table 3 .
Mix proportions of concrete beam compressive strength specimens.