Concrete is one of the most widely used constructional materials which is disposed to cracking for various reasons. Cracking is one of the unavoidable defects in concrete. When micro-cracks develop and join together, a continuous network of cracks is formed inside the concrete. Cracks increase the permeability and reduce the impermeability against moisture and aggressive substances such as sulfate ions, chloride ions, and acids. These factors affect the structure durability and reinforcement corrosion and destroy the concrete matrix. The concrete self-healing approaches appear to be an appropriate idea to remove this damage. Among the different self-healing ways which are basically chemical, the Calcium Carbonate precipitation, resulting from the micro-organisms metabolic activities, is a new environmentally friendly strategy. Their ecological variety is high and can be found in different natural settings. In this way, to treat damaged structures, a microbial process is applied wherein the combination of bacteria, urea, and a calcium source forms calcium carbonate crystal that results in crack reduction, impermeability, and improved concrete mechanical properties. Biologically, the calcium carbonate precipitation helps heal the small cracks, fill pores, and bind other materials such as sand and gravel in concrete. These precipitations are the byproducts of the usual metabolic processes such as photosynthesis, urea hydrolysis, and sulfate reduction. To obtain a useful insight in this important researching area and to protect the environment, this article investigates different approaches of using bacteria in concrete, the bacteria potential to heal the cracks. Improving the properties of concrete was examined, and the laboratory results were interpreted. Investigation of the concrete micro-structure indicated the formation of the calcite crystals in the samples and confirmed the promising performance of bacteria in healing micro-cracks, improving the mechanical properties, and the concrete durability in the destructive environments. By reducing structural pores, bacterial participation at a concentration of 105-107 cells/ml led to an increase in compressive strength by and a decrease in the penetration of chloride ions and water absorption to and , respectively. Moreover, the maximum crack healing width at a concentration of 107-109 cells/ml about was mm.

Dense clay layers are one of the most common impermeable layers in most geotechnical structures such as earth dams and urban and radioactive waste landfills. Due to the specific geotechnical properties, these layers are damaged by cracking over their lifetime. These cracks increase the permeability of the layer and reduce its efficiency. Considering the necessity of knowledge and understanding of factors affecting the stability of geotechnical structures made with these layers, in this research, it has been tried to use a self-resilient property of clay soils to improve soil yield in different conditions. In the present study, it has been shown by numerous experiments that increasing the plasticity index's clay soils can increase the crack restoration rate. The results show that the addition of nanomorillonite to soil improves the self-healing process of the cracks; the time elapses the effect of this increase in the process of repairing cracks created in the specimens. The discharge rate of the samples is measured relative to the time elapsed during the self-repair and are examined according to the graphs. The experiments showed that the percentage of nano-clay montmorillonite is unaffected and reduces the amount of flow in the specimens, which is the self-restorative symptom of cracking by the addition of nano-clay. The total amount of discharge outflow in nano-clay samples from the non-additive samples was much lower, and nano-clay can be used as repairing cracks in clay soils at a depth of 5 cm.

To improve mechanical properties and increasing useful life of metal parts, different methods of welding are used for repairing surface crack of metal parts. In this research, performance of flame welding by spraying pure iron powder and diffusion welding of pure iron powder through magnetic induction evaluated for repairing surface cracks of structural steel. First, eight specimens prepared including two control specimen and other six specimens grooved specimens in depth of 1mm and in length of 12.5mm and groove width in the sizes of 0.5, 0.75 and 1mm. then, two methods of repairing had done. Results showed that after repairing surface groove by using flame welding method and diffusion welding method, tensile strength of the repaired specimens reached to the tensile strength of control specimen with the margin of 2.5% and 7.5%, respectively. Therefore, flame welding method was safer than diffusion welding method for repairing surface groove of structural steel by using pure iron powder.

Aging in asphalt pavements results in reduced serviceability and flexibility of pavements. Aging is not commonly considered as distress, but it substantially effects the rate of evolution of various distresses. One of the common distress observed in aged asphalt pavements is cracking. If cracks/micro-cracks are healed during their initial formation, the service life of the pavement will be increased. Otherwise, there will be the risk of crack propagation that results in more cracking and loss of pavement strength. It is well known that asphalt mixes have capability of self-healing their cracks/micro-cracks when they are exposed to high temperatures. Cracks/micro-cracks in asphalt mixes can be healed through an induced healing process. Induced healing of asphalt mixes by applying external electromagnetic radiation is an innovative technique to repair cracks/micro-cracks. Applying external energy through electromagnetic radiation increases the temperature of the asphalt binder in mixes, allowing it to move and fill the cracks/micro-cracks. Flowing and crack filling of asphalt binder play a significant role in induced healing characteristics of mixes. As temperature of the asphalt binder is increased, its viscosity will be decreased drastically. When asphalt binder gets to Newtonian fluid temperature or higher, the melted binder moves inside the cracks and micro-cracks and subsequently, the cracks will be healed. The aim of this research was to evaluate the effects of different aging levels on induced heating-healing of asphalt mixes. In order to impose different aging levels, asphalt mixes were aged in oven for 3, 5, 7 and 9 days at 85 ºC. Activated carbon was added to mixes so that to enhance electromagnetic sensitivity of mixes. In addition, effects of activated carbon on mechanical properties and microwave heating rate of mixes were determined. Results indicated that activated carbon, as a powder-based additive, improves electrical conductivity, induced heating-healing rate of asphalt mixes. In addition, it was shown that aging phenomenon in asphalt mixes decreases their heating rate, which was more pronounced in higher aging level. Lower heating rate of asphalt mixes resulting in lower efficiency of induced healing of mixes. For evaluating healing capability of mixes that were subjected to different aging levels, Semicircular Bending tests (SCB) was conducted at intermediate and low temperatures. It presented that induced healing efficiency of mixes decreased as the aging level and the notch length in SCB testing were increased. The adverse effects of aging on induced healing process can be attributed to increased viscosity of the asphalt binder in mixes, which limits moving capability of melted asphalt binders to move through damages and properly heal the cracks. Moreover, it resulted that, lower heating rate of aged mixes can be considered as another reason of reduction in induced healing efficiency. The results indicated that increased notch lengths not only affects load at fracture and fracture energy of mixes, but also it plays a significant role in induced healing efficiency of mixes. For further evaluation of the healing ability of asphalt mixes, combination effects of aging, notch lengths and testing temperature parameters were also investigated. Notch length and testing temperature was found to have significant effects on induced healing efficiency of mixes. In addition, the results indicated that induced healing efficiency of low temperature cracked asphalt mixes were more than that of asphalt mixes that were cracked at intermediate temperatures. The results suggest that the necessity of considering aging level in analyzing induced heating-healing process of asphalt mixes.

This study was conducted to investigate the effect of Si addition (1, 2, 3, and 5 wt. %) on the microstructure and castability of hypoeutectic Al-4.5Cu-Si alloys. According to the results, Si addition increased the average size and volume fraction of eutectic Si platelets in the microstructure. Adding Si also improved the fluidity and decreased the porosity content of the alloy, and enhanced its resistance against hot tearing. According to the constrained rod test results, the hot tearing sensitivity index (HTS) of the alloys containing 1, 3, and 5 wt. % Si, was decreased by 48, 78, and 88%, respectively. In agreement with hot tearing testing results, the extensive existence of dendrites and shrinkage micropores on the hot torn surface of the alloy without Si implies on its low potential in feeding the solidification shrinkages and healing hot cracks. Adding Si decreased the amount of shrinkage micropores on the hot torn surface and due to improving the fluidity as well as increasing the amount of Al-Cu-Si ternary eutectic at the last stage of solidification, improved the feeding characteristics of alloy. Therefore, the amount of free dendrite arms was significantly decreased on the fracture surface. However, adding 5 wt. % Si increased the amount of eutectic Si on the hot torn surface giving rises to a more brittle fracture mode.

Asphalt pavement cracking is an unavoidable phenomenon and the main factor of failure in pavement, which if followed, will lead to high costs and yet, with preventative measures, these costs can be reduced very effectively. Crack sealing is one of the preventive maintenance methods and the simplest and most economical way to repair recently cracked surfaces of asphalt pavement. This study was conducted to investigate the effect of discontinuity dimensions and their repair on the mechanical behavior of asphalt. Therefore, asphalt samples with treated and untreated discontinuities were made. Then dynamic creep, resilient modulus, fatigue, and durability testing were performed by the Lottman method on the samples. The results show that for a layer with finer granulation, sealing of narrow cracks (up to 20 mm wide) improves the corrosion resistance of asphalt concrete. Also, sealing cracks can increase the resistance of asphalt concrete to fatigue, regardless of the dimensions of the cracks. For a coarser-grained layer, sealing cracks deeper than width also reduces the resistance of asphalt concrete to sealing. Sealing cracks with overall width and depth of 20 mm reduces fatigue performance. Regarding resilience modulus for a layer with finer granulation and a layer with coarser granulation, crack sealing increases the modulus of resilience of asphalt concrete by 7.7% and 8.9%, respectively. The findings also indicate a more prominent role of discontinuity width than its depth on rutting behavior and durability. The results of this study are key in deciding the appropriate time to seal the cracks after their formation and improving some pavement properties in parallel with the treatment of cracks. Even though, generally, the desired effects of treatment on rutting, fatigue, and durability of pavements are partial and mostly negligible. However, sealing can be considered as a way to increase the resilient modulus of cracked pavements.

Concrete is one of the most used materials in the world. Cracking in concrete is a common problem that occurs due to its relatively low tensile strength. The cracks created in concrete are the main reason for the reduced durability and useful life of concrete structures. Proper and timely repair of primary cracking is necessary in order to prevent its expansion in concrete and reduce the high cost of maintenance. For this purpose, self-healing methods have been developed. Adding microorganisms to concrete mixing and bio-concrete production is an intelligent and environmentally friendly strategy for the repair of concrete cracks. Bacterial species resistant to hard and rough concrete environment such as bacillus, with the production of calcium carbonate precipitation, can be repaired in a continuous hydration process by filling freshly formed fine cracks. The aim of this study was to identify the effective bacterial species and its effect on the strength properties of concrete as a restorative agent.The results show that the changes in the strength properties of bio-concrete by adding Bacillus subtelius at a concentration of 〖10〗^8 (Cells⁄ml), increase the compressive strength by 24.24% and also improve the durability of concrete by bacterial self-healing process.

IntroductionConcrete elements can be easily cracked due to several reasons, such as tensile stresses. When these cracks are connected to each other, permeability will increase and bearing capacity will decrease. Therefore, cracks may decrease the durability of concrete structures. Repair of cracked concrete structures is often expensive and sometimes impossible. Therefore, researchers by inspiration from autogenous recovery of small scratches in the body of live creatures, have presented self-healing concrete based on concrete characteristics.  Utilization of such concretes may increase the durability of concrete structures and minimize repair and rehabilitation costs. Self-healing phenomenon is not new; Academy of science in France has observed the healing of micro cracks in 1836 (Hearn, N, Morley, 1997). The self-healing refers to the reduction of crack width itself. Systematic investigation of self-healing was started by Glanville in 1926. Autogenous self-healing is a case of healing that happens without using external agents (Glanville, 1931). Cementitious materials have such ability, because rehydration of cement particles may be continued and reaction products such as C-S-H and Portlandite will fill the cracks. Of course, when the unhydrated cement particles and water are available in the cement paste, this type of self-healing will occur. Parameters such as crack width, pH of curing water, temperature, the chemical composition of cement and etc. may influence on the self-healing process.

With urbanization expansion, application of concrete and construction materials is widely increasing throughout the world. Therefore, the use of a mechanism that can effectively extend the life of concrete structures is essential. Durable reinforced concrete structures are generally affected by the crack. Cracks in concrete are caused due to various reasons such as an environmental attack, overloading or accidental damage. Surface cracks in concrete facilitate the penetration of chemicals and corrosive chlorine, so as a result of these factors steel rebars corroded and caused the destruction of concrete structures. Calcium carbonate precipitates have proved their ability as a microbial sealant to fill the cracks and the gaps in Granites and sand. In this method, urea is hydrolyzed by the urease secreting bacteria and calcium carbonate is formed in the presence of calcium ion, which improves the stability and properties of concrete in the long term. Therefore, the use of microbial precipitation in concrete restoration can be considered as a natural and environmentally friendly strategy. This paper reviews current progress and potential of this technology.