Healed Specimens Research Articles

Method for evaluating healing state of self-healing ceramics using acoustic emission

Previous studies have evaluated the volume expansion rate of oxidation products and healing temperature and time. However, the bonding strength between the product and the matrix is yet to be appropriately evaluated. To develop a self-healing agent selection method for ceramics, this study proposed a method for examining the healing state and fracture behavior of self-healing ceramics through acoustic emissions (AE) generated during three-point bending tests. This study fabricated completely and incompletely healed SiC30 vol%/Al2O3 composite specimens by adjusting the healing time, and the crack length was assessed employing linear fracture mechanics, and effect of healing state on cumulative AE energy and AE frequency. The crack lengths were compared with those of the kinetic model of crack healing. The crack lengths of the 5-h healed specimens were estimated by linear fracture mechanics to be 12.5 μm for incomplete and 10.4 μm for complete healing, while that estimated by the kinetic model were 13.5 μm, indicating the usefulness of the kinetic model in the initial stages of crack healing. Furthermore, while the AE frequency at the initial stage of fracture was dominated by peaks below 100 kHz in damaged or incompletely healed specimen, there was a relatively high frequency peak of over 200 kHz in smooth and completely healed specimen, The average cumulative AE energies of smooth, damaged, 5 h-healed (incompletely), 5 h-healed (completely), and 24 h-healed specimens were 395, 0.89, 26.8, 189, and 318 V・μs, respectively. Thus, the AE energy that accumulated until rupture tended to increase as healing progressed, these findings suggest that the AE method can be used to determine healing conditions. These results will help establish a method for selecting self-healing agents according to the environment in which they are used.

Macroscopic experimental study and microscopic phenomenon analysis of damage self-healing in salt rock

Salt rock is characterized by a low porosity, low permeability and self-healing ability, and it is one of the most common materials used for underground energy storage and construction in underground nuclear waste repositories. Studying the healing of macroscopic cracks in salt rock under different conditions is important for ensuring the stability of the rock surrounding a salt cavern. To evaluate the healing ability and effect of cracks in salt rock at the macroscale, two tests were designed to directly assess the healing ability of salt rock by analyzing the tensile strength and permeability recovery of healed crack surface. Furthermore, the microstructure of the healed macroscopic fractures in salt rock was observed using scanning electron microscopy (SEM), and the microscopic mechanisms of salt rock damage healing were analyzed. (1) The results show that the presence of water is an important condition for the healing of cracks in salt rocks. Fully fractured salt rock cracks can heal at a low pressure of 160 kPa, and the uniaxial tensile strength of the healed specimens can reach a maximum of 121.1 kPa. It was found via computed tomography (CT) that a healed crack was still a weak surface in the whole specimen. (2) Increases in temperature, environmental humidity, and normal stress all promote the self-healing of salt rock fractures. Based on the experimental data, a model describing the evolution of salt rock damage healing variables with respect to temperature, humidity, and stress was established. It was also observed that the change in salt rock damage healing variables over time during the healing process can be represented by a first-order exponential decay function. (3) Using SEM, detailed micro healing experiments were conducted on NaCl single crystal and impurity-containing salt rock specimens. The healing characteristics of intracrystalline cracks and intercrystalline cracks were studied. It was observed that salt rock damage healing manifests at the microscopic level as the filling of microcracks and crack segmentation caused by grain growth. Additionally, the presence of a certain amount of impurities was found to promote the growth of internal structures that heal cracks within salt rock. These findings are of great significance for evaluating the stability and tightness of the surrounding rock of salt cavern reservoirs.

Variation of microbial self-healing performance of cementitious composites with their biogranule content

Existing studies on microbial self-healing concrete often employed an arbitrary approach to the initial amount of healing agents, leading to significant performance variations. The amount of bacteria becomes a crucial parameter affecting the overall cost of the material. Therefore, assessing the effect of initial bacteria content on microbial self-healing performances is essential. This study presents the variation of self-healing performances of biomortars with initial bacteria content ranging from 0.05 % to 2.50 % (0.07 %–3.20 % biogranule) w/w cement. All biomortars outperformed the crack healing performance of abiotic control specimens. The net contribution of microbial healing (MH‾net) was determined by analysing autogenous healing and microbial healing periods separately. The variations in initial bacteria content did not significantly affect MH‾net which could reach up to 125 μm in 2-weeks of water immersion. The minimum effective bacteria dosage in the form of biogranules for developing microbial self-healing concrete was determined as 0.05 % bacteria w/w cement with an overall crack healing limit of 300 μm. Microbial healing of cracks up to 300 μm crack width led to 50–80 % higher water tightness regain compared to the autogenously healed specimens. The purity of CaCO3 minerals sealing the cracks decreased with the increasing initial bacteria content of the biomortars. Overall, the initial bacteria content did not significantly affect the microbial self-healing performance in the tested dosage range. At the minimum effective bacteria dosage, consistent self-healing of cracks up to 300 μm became possible at the expense of approximately 20 % increase in the overall cost of the material.

Effects of healing start time and duration on conventional and high-performance concretes incorporating SAP, crystalline admixture, and sepiolite: A comparative study

This research investigates the self-healing capability of conventional and high-performance concrete containing either a superabsorbent polymer (SAP), a crystalline admixture (CA), or water-encapsulated in sepiolite by quantifying the recovery of water tightness through the water permeability test and chloride permeability via cracks and matrix penetration on healed specimens. Specimens were pre-cracked to a crack width range of 50–450 µm at 28 days. Specimens were healed under water in four time combinations: healing starting at the concrete age of 28 days or 56 days and a healing duration of 28 or 56 days. Additionally, a healing condition of presaturation during 1 day of water immersion and storage in a humidity chamber for 27 days was studied. The results show that delayed healing reduced the self-healing efficiency, and extending the duration to 56 days enhanced healing, especially for narrow cracks (≈100 µm), and reduced chloride permeability. Specimens with SAP showed superior early-stage healing, but low delayed healing. Sepiolite enhanced healing for delayed cracks with variable effectiveness. CA improved healing for both early and delayed healing, exhibiting low chloride penetration in healed specimens.

Self‐healing thermoplastic elastomeric blends of zinc‐ionomer and styrene–butadiene–styrene block copolymer and their characterization

AbstractWe report the development of novel self‐healing binary thermoplastic elastomeric blends of Zn2+‐neutralized poly[ethylene‐co‐(methacrylic acid)] (Zn‐ionomer) and styrene–butadiene–styrene block copolymer (SBS). Zn‐ionomer/SBS blends were prepared by the melt‐blending technique using a twin‐screw extruder with three mass ratios of 75/25, 50/50 and 25/75. Structure, phase morphology and thermal as well as tensile properties of the samples were characterized. Fourier transform infrared spectroscopy results indicated the presence of two types of polymeric materials and their interactions. Scanning electron microscopy images confirmed the miscibility of blend components which remarkably influenced the properties of the blends. The optimum peak degradation temperature of Zn‐ionomer/SBS (75/25 wt%) blend was observed to be 484 °C. The ratio of the tensile strength of the healed specimens to that of the original specimens was implemented to quantitatively evaluate the self‐healing efficiency of the materials. The self‐healing efficiencies were calculated to be 55.42%, 58.36% and 60.75% for Zn‐ionomer samples healed for 4, 8 and 16 h, respectively. All of the blend samples exhibited self‐healing behavior and the self‐healing efficiency was more than that of virgin Zn‐ionomer which may be credited to the high mobility of SBS segments in the blends. The healing efficiency was increased with an increase of SBS content in the blends and also heat energy facilitated the healing process. The self‐healing efficiencies of Zn‐ionomer/SBS (25/75 wt%) blend sample were calculated to be 75.09%, 80.27% and 80.87%, respectively, for 4, 8 and 16 h of healing time at 100 °C. A comparison of optical micrographs of the cut area of samples (before healing and after the healing process) was also made and self‐healing of the blend materials was observed. © 2023 Society of Industrial Chemistry.

Optimization of concrete mix designs toward the bond properties of steel reinforcement in self-healing concrete by Taguchi method

The bond of steel reinforcement in normal concrete has been extensively studied, while its bond properties in self-healing concrete has rarely been investigated because the main focus of developing self-healing concrete relies on the improved healing performance such water tightness and strength regain. In fact, the mix design of concrete will greatly affect the bonding. This study elaborates on the optimisation of mix designs for self-healing concrete toward the bond properties of steel reinforcement. The proposed healing agents were bacterial healing agent (BAC) and crystalline admixtures (CA). Mix design parameters include water-cement ratio (w/c) (0.40, 0.50 and 0.60), fine ratio (FR) (0.34, 0.44 and 0.54), and healing agent (HA) (BAC, CA and REF, without healing agent). The Taguchi method was used to formulate nine self-healing concrete mixes by considering three factors and three levels in the design. The compressive strengths of all different concrete mixtures were initially assessed and the pull-out tests on a single steel rebar embedded in concrete were performed in three different states of concrete (uncracked, cracked and healed). The sizes of the cracks were varied and three groups were identified: cracks with a width of 200-300, 300–400 and 400-500 μm. Water immersion was selected as the healing regime with two healing periods lasting 28 and 112 days. The Taguchi analysis and in-depth statistical evaluations were performed for optimisation purpose. Results showed that the compressive strength considerably increased as lowering the w/c, increasing the FR and incorporating the healing agents. The bond of steel reinforcement with the concrete matrix in the uncracked state improved after the BAC or CA was added into the concrete. The formation of cracks in the concrete significantly reduced the bond strength. After being subjected to healing, the bond strength slightly recovered. A longer healing time induced a better bond strength recovery. The phenomenon of crack closure was found at all healed specimens and the healing products were confirmed to be calcium carbonates.

Low‐velocity impact and self‐healing behavior of CFRP laminates with poly(ethylene‐co‐methacrylic acid) filament reinforcement

AbstractAn experimental study is performed to explore the low‐velocity impact (LVI) and self‐healing behavior of carbon‐fiber‐reinforced‐polymer (CFRP) laminates reinforced by thermoplastic poly(ethylene‐co‐methacrylic acid) (EMAA) filaments. Firstly, differential scanning calorimetry (DSC), thermos‐gravimetric analysis (TGA) and dynamic mechanical analysis (DMA) are used to determine the process parameters. Thereafter, the unreinforced laminates and EMAA reinforced laminates are prepared via phased hot‐processing curing process. The LVI tests with four energy levels (5, 15, 25 and 35 J) are then carried out on the two types of specimens. In these energy cases, the peak‐force values of the reinforced specimens are 2.2%, 1.9%, 2.3% and 7.5% higher than those of the unreinforced ones, while the energy‐absorption‐rate values of the reinforced ones are 6.02%, 3.53%, 4.24% and 2.32% lower. The comparisons demonstrate the impact‐resistance enhancement of EMAA. Moreover, multiple LVI tests are performed on the unhealed and healed specimens. In 15 and 25 J cases, around 6.92% higher peak load and 4.58% lower absorbed energy are observed within the healed specimens, which is attributed to the healing effect. The healing mechanism is investigated by utilizing attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), scanning electron microscope (SEM) and optical microscope (OM) technologies. It reveals that EMAA flows into the delaminations and cracks during the healing process, and thus accomplishing self‐healing behavior effectively.

Long-term viable SF immobilized bacterial cells as sustainable solution for crack healing in concrete

Utilization of microbially induced calcium carbonate precipitation (MICCP) via biomineralization process has been considered a novel method in self healing of concrete structures. To develop MICCP as ready product for field scale construction, mineral based inoculum of higher shelf life is required. The present study focuses on this aspect of field based application of MICCP. In this study, mineral admixture (silica fume (SF), cement kiln dust (CKD) and rice husk ash (RHA)) based bacterial inoculum were developed to immobilize bacteria for self healing capabilities in concrete. The prepared inoculum was stored at varying temperature (4 °C and 25 °C) and the survival of bacterial cells in carrier based materials were tested on weekly basis until 180 days of storage at both temperature. After 180 days of storage, the most efficient bacterial inoculum based upon viability studies was incorporated in concrete and tested for crack healing performance. For this, crack of approximately 0.5 mm width was generated during the casting and healed using a bacteria-based healing agent (nutrient broth, urea and CaCl2). The healed cracked surface was examined through optical imaging to monitor the crack width reduction. Along with this, Electromechanical Impedance (EMI) technique was used to monitor the crack healing potential until the full healing of cracked surface was achieved. Statistical EMI indicators such as root mean square deviation (RMSD), mean absolute percentage deviation (MAPD) and correlation coefficient deviation (CCD) were employed to quantify crack healing process. RMSD was found to be promising parameter to monitor the crack healing process and reflected an increasing trend from 1 % to 14 % in healed bacterial specimen. At the end of test, the healed specimens were subjected to bending failure to assess the strength regain. Significant regain was noticed in healed bacterial specimen (approximately 33 %) in comparison to the control. The healing mineral precipitated inside the cracks was examined through field emission scanning electron microscopy(FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) to evaluate its physicochemical attributes. The results give clear proof that the SF-based carrier material can be effectively used in healing the cracks and the crack healing process can be efficiently monitored by the EMI technique. Results further indicated that RMSD values were very effective in quantifying the progressive crack healing achieved due to MICCP precipitation.

Experimental and numerical research on healing performance of reinforced microcapsule-based self-healing polymers using nanoparticles

The effect of enhancing multicore microcapsules with various nanoparticles on the healing efficiency of microcapsule-based self-healing polymers was investigated in this study. The incorporated polymers with enhanced microcapsules by MWCNT, nanoclay, and nanosilica were fabricated. Three volume fractions (VFs) of microcapsules, including 5, 7.5, and 10% were added to the epoxy matrix. The maximum tensile stresses of the virgin, damaged by controlled low velocity impact test, and healed specimens were obtained to calculate the healing efficiencies of fabricated self-healing polymers. Also, a multi-scale FE modeling was promoted using RVEs generation and ABAQUS-Explicit solver attached with VUSDFLD subroutines to predict the healing efficiencies. The results indicated that by increasing the VF of microcapsules, the healing efficiencies of specimens were increased. Increasing the VF of the microcapsules could increase the probability of hitting induced cracks due to the LVI test with microcapsules. Thus, further spreading of healing agents into the propagated cracks would increase the healing efficiency of specimens containing higher VF of microcapsules. Also, increasing the strength of microcapsules reduced the healing efficiency of specimens. For instance, in the specimens containing 10% VF of enhanced microcapsules with MWCNT, the healing efficiency was 45.90%, while it was 50.41% for enhanced microcapsules with nanosilica. Increasing the mechanical properties of microcapsules could decrease the probability of rupturing damaged microcapsules, which reduced the healing efficiency of these specimens. Finally, the predicted healing efficiency of simulated RVEs had good accuracy, indicating the reliability of introduced multi-scale FEM.

Evaluation of crack healing potential of cement mortar incorporated with blue-green microalgae

Microbial induced calcite (CaCO3) precipitation (MICP) is a biochemical process that induces calcite precipitation. MICP is considered a solution for remediation of concrete cracks using bio-mineralization. This study aims to use microalgae as an agent in the healing of micro-cracks. Microalgae were used in cement mortar to induce the formation of calcium carbonate to seal the cracks. Two microalgae species, namely Synechococcus elongatus (Syn. elongatus) and Spirulina platensis (S. platensis), were tested for their characteristics and then incorporated into the cement mortar. The specimens were cured under two different conditions, namely ambient and water curing. Next, the mechanical properties and crack healing of the cement mortar were examined. The cement mortars that were cured for 28 days in the air and water were subjected to a compressive load of 70% of its maximum threshold load, to induce micro-cracks. Subsequently, the specimens with pre-induced cracks were cured under ambient and water to check for the ability to seal cracks through bio-mineralization. The effect of replacing cement with 4, 8 and 12% of both species of microalgae were investigated. The results demonstrate that the mortars cured in water have a higher strength compared to the mortars cured in air. The investigation results also reveal that the mortars with S. platensis showed better strength and crack healing compared to mortars with Syn. elongatus. The water cured specimens with 12% S. platensis developed a compressive strength of 72% of the control specimen (100%), compared with 12% Syn. elongatus that exhibited only 36%. The healing potential was evident as the micro-camera images showed the narrowing of the induced cracks on the surface of the mortar after 14 days of water curing. Furthermore, the residual compressive strength of the biotically healed specimens showed 35% of the strength regain with 12% S. platensis as a cement replacement. The formation of crystalline calcium carbonate precipitates in the specimens with S. platensis and Syn. elongatus exhibits an increase in the derivatives of calcium ions; the enhancement in the strength of mortar due to the calcium carbonate (crystal) formation, which seals the surface of the crack, was supported by the SEM-EDS and XRD analysis. It was also found that the integration of microalgae into the cement had the effect of self-healing and could potentially improve the future direction of crack healing.

Self-Healing Potential and Post-Cracking Tensile Behavior of Polypropylene Fiber-Reinforced Cementitious Composites

The use of synthetic fibers as reinforcement in fiber-reinforced cementitious composites (FRCC) demonstrates a combination of better ductile response vis-à-vis metallic ones, enhanced durability in a high pH environment, and resistance to corrosion as well as self-healing capabilities. This study explores the effect of macro- and micro-scale polypropylene (PP) fibers on post-crack energy, ductility, and the self-healing potential of FRCC. Laboratory results indicate a significant change in fracture response, i.e., loss in ductility as curing time increases. PP fiber samples cured for 2 days demonstrated ductile fracture behavior, controllable crack growth during tensile testing, post-cracking behavior, and a regain in strength owing to FRCC’s self-healing mechanism. Different mixes of FRCC suggest an economical mixing methodology, where the strong bond between the PP fibers and cementitious matrix plays a key role in improving the tensile strength of the mortar. Additionally, the micro PP fiber samples demonstrate resistance to micro-crack propagation, observed as an increase in peak load value and shape deformation during compression and tensile tests. Notably, low volume fraction of macro-scale PP fibers in FRCC revealed higher post-crack energy than the higher dosage of micro-scale PP fibers. Lastly, few samples with a crack of < 0.5 mm exhibited a self-healing mechanism, and upon testing, the healed specimens illustrated higher strain values.

Biological Mechanism of Extracorporeal Shock Wave Combined with Platelet-Rich Plasma on the Healing of Reconstructed Tendon Bone in a Rabbit Model of Rotator Cuff Tear

To investigate the biological mechanism of the effect of extracorporeal shock wave combined with plateletrich plasma on the healing of reconstructed tendon bone in a rabbit model of rotator cuff tear. 40 New Zealand white rabbits aged 4-5 mo were selected, 32 rabbits were randomly selected and each rabbit was executed at 4-8 w postoperatively and these rabbits were randomly divided into control group, shockwave group, platelet-rich plasma group and shockwave+platelet-rich plasma group. The tendon bone healing site was observed at 2, 4 and 8 w after surgery and the content of transforming growth factor beta 1 and basic fibroblast growth factor were measured by reverse transcription-polymerase chain reaction and western blot, while the biomechanical test was performed on the tendon bone healing specimens. The results showed that the content of neovascularization at the tendon-bone interface at 4 w and 8 w was significantly increased in shockwave, platelet-rich plasma and shockwave+platelet-rich plasma groups. The basic fibroblast growth factor and transforming growth factor beta 1 messenger RNA and protein expression tended to increase after surgery in all four groups and the levels in shockwave, plateletrich plasma and shockwave+platelet-rich plasma groups was significantly increased, while the highest expression in shockwave+platelet-rich plasma group. The maximum load force of the 3 experiment groups was all significantly increased. Besides, the maximum tensile distance was significantly longer in shockwave+platelet-rich plasma group. The combination of extracorporeal shockwave and plateletrich plasma showed optimal performance in the tensile performance of the supraspinatus tendon-bone combination, the maturity of the tendon-bone interface and the content of growth factors that promote the healing of the tendon bone, indicating that the combination of extracorporeal shock wave and plateletrich plasma can promote the healing process of the tendon bone after rotator cuff tears and improve its strength and ultimate load.

Influence of strain rate on mechanical characteristic and pore structure of self-healing cementitious composites with epoxy/urea-formaldehyde microcapsules

A physical triggered self-healing system was formed by mixing the epoxy/urea-formaldehyde microcapsules into cementitious materials. The effects of the microcapsule contents and strain rate on the mechanical properties, pore structure and self-healing efficiency were investigated in this study. Both the strain rate and microcapsule contents influenced the strength and pore structure. With increasing of the strain rate, the strength of specimens gradually increased. With increasing of microcapsule content, the strength of initial specimens declined to varying degree, the strength of healing specimens gradually increased. Microcapsules had an positively effects on the self-healing but negatively effects on the pore structure parameters of initial specimens. And the microcapsules also had effects of the deformations properties, and improved the brittleness characteristics of the specimens. These findings reflected the self-repairing effects of microcapsule on cementitious materials from a dynamic point of view.

The flexural properties of self healing fiber reinforced polymer composites

Fiber reinforced polymer composites have a complex damage prediction and repair mechanism. Such complexities made the researchers to implement new techniques like self healing. Amendments in the existing material can modify its properties. So, it is necessary to study the change in property due to inclusion of self healing in fiber reinforced composite materials. This work aims to investigate the self recovery response of composites prepared by reinforcing glass fibers and carbon fibers in epoxy matrix. The healing was achieved by inclusion of dual microcapsules made of epoxy and hardener encased in separate capsules. To investigate the recovery activity of the composite laminate, a 5mm diameter indentation was produced at the centre of specimen and the healing was monitored by measuring the diameter of indentation after 12 hrs and 24 hrs. To notice the influence of micro-inclusions on flexural strength, flexural test was conducted on neat, indented and healed specimens. It was observed that inclusion of microcapsules did not majorly alter flexural strength of glass fiber reinforced and carbon fiber reinforced polymer composites. Results also indicated complete healing of both carbon fiber reinforced polymer and glass fiber reinforced polymer composite specimens with healing efficiency of 103.4% and 101.8% respectively.

Microstructure optimizations for improving interlaminar shear strength and self-healing efficiency of spread carbon fiber/epoxy laminates containing microcapsules

The purpose of this study is to improve interlaminar shear strength and self-healing efficiency of spread carbon fiber (SCF)/epoxy (EP) laminates containing microcapsules. Microencapsulated healing agents were embedded within the laminates to impart a self-healing functionality. Self-healing was demonstrated on short beam shear specimens, and the healing efficiency was evaluated by strain energies of virgin and healed specimens. The effects of microcapsule concentration and diameter on apparent interlaminar shear strength and healing efficiency were discussed. Moreover, damaged areas after short beam shear tests were examined by an optical microscope to investigate the relation between the microstructure and the healing efficiency of the laminates. The results showed that the stiffness and the apparent interlaminar shear strength of the laminates increased as the microcapsule concentration and diameter decreased. However, the healing efficiency decreased with decreasing the microcapsule concentration and diameter.

Impermeability characteristics of cementitious materials with self-healing based on epoxy/urea-formaldehyde microcapsules using an immersion test

A cementitious self-healing system was built using epoxy/urea-formaldehyde microcapsules. The effects of microcapsule content and soaking in chloride on impermeability, pore structure, and self-healing efficiency were investigated. Both microcapsule content and soaking length influenced impermeability and pore structure. With increasing microcapsule content, the impermeability of sound and damaged cementitious composite material specimens increased and then decreased; the impermeability of healing specimens gradually increased. The healing and recovery rates of impermeability increased up to 90 days soaking and then stabilized. Microcapsules positively influenced self-healing but negatively influenced the microstructure of sound specimens. Models to predict microcapsules’ effect on pore structure and impermeability were established.

Multiple damaging and self-healing properties of cement paste incorporating microcapsules

The multiple damaging and healing of cracks in cement paste by microcapsules was experimentally investigated under various flexural loading scenarios. The crack formation in the loading process was detected in situ using the acoustic emission (AE) technique, in which passive and active sensing modes were combined to provide complementary information on the overall damage and local cracking. The AE analysis was conducted based on the differentiated features of the signals in terms of the temporal and spectral descriptors to distinguish the cracking mechanisms. The results indicate the alleviated damage of the specimens due to healing of cracks based on the active sensing mode. The time-dependent hits and cumulative energy demonstrate new cracking sources in the healing specimens. The changes in the variation of hits (versus amplitude) and the AE energy (versus duration time) reveal that the new cracking sources were the failure of the pre-attached crack space by the adhesive according to the calibration. The distinguished characteristics of the maximum frequency of the waveforms provide further insights into the cracking mechanisms of matrix and interface. The plot of the average frequency (AF) versus the RA index illustrates that the cracking modes contribute to the characteristic spectrum.

Experimental investigation of the fatigue phenomenon in nano silica-modified warm mix asphalt containing recycled asphalt considering self-healing behavior

This study investigates the fatigue performance of nano silica-modified warm mix asphalt containing recycled asphalt pavement (RAP) with self-healing properties. To achieve this goal, Sasobit was added at 2 wt% and nitric oxide at 3 wt%, 5 wt% and 7 wt% to the bitumen and mixed with a high shear mixer. In order to simulate aging in warm asphaltic mixtures, 0, 70 and 100 wt% of the RAP were used to prepare four-point bending test slabs. Then, the beams obtained from the slabs were evaluated under constant strain at two strain levels of 400 and 800 µm. The results showed that addition of nano -silica can significantly improve the self-healing behavior of warm asphalt mixtures. Furthermore, the healed specimens showed significant flexural stiffness. On the other hand, the self-healing behavior of asphalt samples with and without RAP and nano-silica were substantially different under constant strains of 400 and 800 µm.

The effect of vascular self-healing pattern on mechanical behaviour and healing performance of epoxy/glass composite

ABSTRACTIn this research, the effect of vascular self-healing configuration on the healing behaviour of epoxy/glass (50/50) composite has been experimentally studied. Hollow glass fibres (HGFs) of 450 ± 10 μm diameter and 30–35% hollowness were produced by using an extruder. The hollow glass fibres were filled with healing components and subsequently employed as vascular healing system in the composite laminate. The orientations of embedded HGFs were selected at three levels of 0, 45 and 90°, and the distances of HGFs were chosen at three levels of 200, 500 and 800 μm in composites. A damage at tensile strain of 1.2% initiated, and following, the self-healing process took place. The virgin and healed specimens were characterised in terms of tensile behaviour, and subsequently, the healing performance was obtained. In addition, the formation of micro-cracks, fracture of filled HGFs and morphology of healed zones in self-healing composites were analysed by employing SEM.

Self-healing of impact damage in fiber-reinforced composites

Healing of impact damage in vascular fiber-reinforced composite beam specimens was explored using a flexure after impact testing protocol. Two-part epoxy and amine based healing agents were delivered to impact-damaged beam specimens using a novel air assisted reagent delivery scheme. The incorporation of microchannels into composite specimens did not alter the flexural stiffness or strength of the composites, however, the post impact flexural strength was reduced, on average, by 25.7%. Flexure testing of healed specimens demonstrated 47% recovery of strength and 83% recovery of moduli when compared to control samples in which no agents were delivered. Optical cross-sectional micrographs reveal that not all damage regions are infiltrated during healing as a result of incomplete damage connectivity. A damage filling efficiency during healing agent infiltration was calculated and was found to positively correlate with recovery of post-impact strength.