Experimental characterization of the self-healing capacity of cement based materials: an overview <s> Experimental Quantification of the Healing Capacity of Cement-Based Materials <s> Abstract This paper investigated non-ureolytic bacterially-induced precipitation of calcium carbonate as a self-healing strategy for concrete cracking. By incorporating bacteria and calcium source nutrients as a two-component healing agent in concrete matrix, the process of bacterially mediated calcium carbonate deposition was trigged upon crack formation and self-sealing of cracks can be expected. The effectiveness of healing was evaluated by mechanical tests in macroscale (flexural and ultrasonic pulse velocity) and nanoscale (nanoindentation), respectively. Both the repair method and the type of calcium source have a significant impact on the healing efficiency. The healing ratio and recovery ratios of flexural strength and modulus of the two-component self-healing with calcium glutamate were higher than that of control series by a factor of 2. A transition zone, whose average nanomechanical values were approximately 20% higher than that of outer precipitates, acted as a strong bond between the matrix and deposited layer from calcium glutamate. <s> BIB001 </s> Experimental characterization of the self-healing capacity of cement based materials: an overview <s> Experimental Quantification of the Healing Capacity of Cement-Based Materials <s> Abstract The aim of this study is analyzing the self-healing effect of a crystalline admixture in four types of environmental exposure comparing with a reference concrete. Healing was studied by means of permeability tests on cracked specimens and physical closing of the crack was observed by optic microscope and quantified through crack geometrical parameters. The studied crack openings were under 300 μm and the time set for healing was 42 days. The results show a different healing behavior depending on the exposure and the presence of the crystalline admixture, demonstrating that the presence of water is necessary for the healing reactions. <s> BIB002 </s> Experimental characterization of the self-healing capacity of cement based materials: an overview <s> Experimental Quantification of the Healing Capacity of Cement-Based Materials <s> Abstract This paper analyzes the self-healing properties of early-age concretes, engineered using a crystalline admixture (4% by the weight of cement), by measuring the permeability of cracked specimens and their crack width. Two concrete classes (C30/37 and C45/55) and three healing exposure conditions have been investigated: water immersion at 15 °C, at 30 °C and wet/dry cycles. Specimens were pre-cracked at 2 days, to values of crack width in the range of 0.10–0.40 mm. The results show almost perfect healing capability for specimens healed under water at 30 °C, better than for specimens healed under water at 15 °C, while insufficient for the wet/dry exposure. <s> BIB003 </s> Experimental characterization of the self-healing capacity of cement based materials: an overview <s> Experimental Quantification of the Healing Capacity of Cement-Based Materials <s> Abstract Autogenous healing of cracks may offer a solution for brittle cementitious materials. In this study, the healing building blocks are available through the well-designed ultra-ductile microfibre-reinforced mixture with a low water-to-binder ratio and water is available through the inclusion of superabsorbent polymers. As visual inspection demonstrates that the crack is completely closed at the surface, one may ask whether this healing also is present in the interior of the crack. X-ray computed microtomography was therefore used to study the extent of autogenous healing in cracked cylindrical specimens. It was found that the extent of autogenous healing in a cementitious material depends on the crack depth. Only near the crack mouth (0 till 800–1000 μm) the crack is closed by calcium carbonate formation in case of wet/dry cycles. In combination with superabsorbent polymers, the extent of healing was more substantial. For mixtures containing superabsorbent polymers there was even partial healing in the interior of the crack when stored at a relative humidity of 60% or more than 90%. Energy-dispersive spectroscopy combined with microscopic analysis showed that the healing products were mainly calcium carbonate. The smart cementitious material with superabsorbent polymers is thus an excellent material to use in future building applications as the healing capacity is improved. <s> BIB004 </s> Experimental characterization of the self-healing capacity of cement based materials: an overview <s> Experimental Quantification of the Healing Capacity of Cement-Based Materials <s> In this paper a methodology is proposed to investigate the influence of healing capacity of lime-based mortars in masonry specimens, by means of shear bond test. A first series of “brick-mortar” triplets – after 28 days curing in lab environment – were tested in shear, with and without transverse normal stress, to determine the shear bond strength. Then, on a second series of samples, a damage is induced by loading them to a prescribed fraction (70%) of the shear bond strength, determined as above; samples were subsequently immersed in water for 3 months and re-tested at the end of this curing period. Reference undamaged samples undergoing the same curing history as the damaged ones were tested as well. The effectiveness of the self-healing capacity has been evaluated through the recovery of shear bond strength. The reliability of this approach has been investigated also by comparing the experimental results with a simplified FE model. <s> BIB005
The quantification of the healing capacity of cement-based materials, be it based either on (stimulated) autogenous mechanisms or on an engineered process, involves a comprehensive set of activities. The range from creating a controlled "damage state" (a crack) in the material to the implementation of "healing conditions" suitably representing the intended functional and structural service scenario, to the evaluation of the sealing of the crack and of the recovery of the properties of interest, supported by the comprehension of the underlying mechanisms through the characterization of the healing products. As a matter of fact, the role of the experimental mechanics is crucial for the successful accomplishment of the aforementioned tasks. The choice of the most appropriate experimental methodology to perform these tasks has to consider the property whose healing-related recovery has to be measured together with the fracture toughness of the investigated material which governs the stability of its post-cracking response and the whole cracking and healing evaluation procedure. Such a choice is the outcome of a complex decision making process which has to obey to a "5W" concept structured as follows: "WHO": the material and its fracture toughness: under the denomination of cement-based construction materials a broad range of cementitious composites is nowadays numbered, which, starting from the quasi-brittle "conventional" concrete, has progressively evolved towards tougher and tougher ones, from ordinary Fiber Reinforced Concrete (FRC) to High Performance Fiber Reinforced Concrete and Cementitious Composites (HPFRC/HPFCRCs). These witness the progressive refinement of the "crack opening control concept", evolving up to a material composition designed through micromechanical concepts and based on the balance between fiber pullout and crack-tip toughness. This composition is able to provide, after the formation of the first crack, a through-crack stress redistribution of such extent that the formation of new multiple cracks is favored instead of the further opening of the previously formed one(s). The aforesaid mix composition, characterized by high binder content and low water-binder ratios, on the one hand makes it possible to achieve a strain hardening behavior characterized by stable multiple cracking before the onset of unstable crack localization. This results into the possibility of performing, in a stable manner and with a quite accurate control of the crack opening, tests such 4-point bending on un-notched prisms and even direct tension tests, which would be otherwise quite unstable in the case of plain concrete. On the other hand, the same composition is highly conducive to self-healing since, even after quite a long time, may leave significant amounts of binder un-hydrated and hence potentially available to undergo, together with carbonation, a delayed hydration reaction. This also makes it worth to tackle the quantification of self-healing through the recovery of mechanical properties. In fact, the postcracking strength at different levels of crack opening, are the parameters which have to be employed in the design of FRC/HPFRCC structures  . It is hence of the utmost importance not only to experimentally identify those parameters but also to assess, through likewise sound experimental methods, the possibility and extent of maintaining their "instantaneous" values along the structure service life. This calls into play the second "W": "WHAT": the requested performance recovery which has to be obtained thanks to combined effects of crack sealing and material healing, which could be either a durability or a mechanical one. In the first case the healing is evaluated by measuring a durability related parameter (e.g., sorptivity, water permeability, chloride ion penetration resistance) on the same specimen immediately after precracking and after the scheduled "curing time" under the selected exposure conditions. The durability property to be measured dictates the test methodology to be employed to measure the healing together with the methodology to be employed to create the crack to be healed. For example, chloride penetration tests generally employ cylinder specimens which could be pre-cracked by splitting tensile tests, though, in the case of plain concrete, such a test is quite unstable and producing a crack with a controlled width may be difficult. The same holds for water penetration tests. In case, fibers can help to stabilize the post-cracking response BIB002 BIB003 . In the case of sorptivity, the crack through which the specimen water gain is triggered could be induced by means of 3-point bending on either notched or un-notched prisms, by performing the test in a crack-opening controlled mode through closed loop feedback systems. On the other hand, the evaluation of the healing through the recovery of mechanical properties is based on the comparative evaluation of the mechanical response of the same specimens to the same action, applied for pre-cracking the specimen first and after the scheduled healing time. Hence, it becomes of the utmost importance to test companion specimens which initially remain undamaged and undergo the same curing history as the cracked/healed ones and are then tested at the same age of the healing specimens. This only can allow to discriminate between healing-induced recovery and aging-induced evolution of the mechanical property of interest. In view of this, the boundary conditions in which the healing recovery has to be evaluated ("WHERE" and "WHEN") are of paramount importance since they are meant to replicate at the lab-scale the structural service scenario under which the healing has to take place with reference to the intended application. The scenario encompasses the exposure conditions, with main reference to the (dis)continuous presence of water and/or moisture, temperature, pressure and possible presence of aggressive substances. Moreover, it has to consider the type and duration of actions which have caused cracking, and at which age, and which the structure may fully or partially continue to sustain along its service life. To the current state of knowledge, the foremost, though quite challenging, experimental need is the ability to replicate, as long as the healing goes, the presence of sustained and/or repeated through-crack stress states so to correctly reproduce their interaction with the healing phenomena and the long-term behavior of the material. The aforementioned through-crack stress states, which are implied by the intrinsic cohesive nature of concrete cracks as well as by the presence of conventional reinforcing bars or dispersed fiber reinforcement, may even not be tensile. Applications exist, such as in tunnel rings made of precast elements, where a crack formed at earlier ages in transient situations may later experience, in the persistent service design scenario, throughcrack compressive stresses, whose effect on the healing may be favorable. The interaction between the characteristics of the crack pattern, in terms of width and spacing, the exposure conditions and the nature and frequency of the sustained and/or repeated actions may also imply, in the case of R/C and FRC thanks to, e.g., improved bond, a stress redistribution capacity such that new freshly healable cracks may form in the non-crack-saturated free space between the existing cracks, instead of the progressively reopening of the same crack(s), which will obviously feature a lower and lower healing potential. In this respect it would also be advisable to study the effects of healing not only on the fundamental mechanical and durability related properties of the material but also on "mesoscale" structural mechanisms, such as the bond to reinforcement, which stands as a transfer to the structure scale of the healing-related performance recovery of the materials. An attempt in this direction has been made with reference to masonry structures by De Nardi et al. BIB005 , who studied the effects of healing of lime mortars on the brick-mortar interface bond in structure subassemblies. The complexity of the scenarios described above requires a likewise comprehensive characterization of the healing mechanisms  , and hence the last but not least "W" "WHY", which can rely upon methods ranging from classical ones, such as Electronic Scanning Electron Microscope (SEM-ESEM), as well Thermo-Gravimetry Analysis (TGA), X-ray Diffraction and Fourier Transform Infrared (FTIR) Spectroscopy to less common ones, which may go even down to the visualization of the healing products through X-ray microtomography BIB004 and the characterization of the signature properties of the healing products through nano-indentation BIB001  . As a matter of fact, the experimental characterization of the healing products has to be based not on a single test but rather on a thorough methodology combining visual crack sealing evaluation, measurement of durability related and/(or) mechanical recovery and establishment of mutual correlation between the garnered measurements, all completed by the micro-and, in case, nano-scale characterization of the healed cracks and healing products.