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As is known, the autonomous self-healing of concrete could be achieved by adding foreign aid materials. Roig-Flores et al. [1] found that the self-healing ability of concrete was enhanced with the addition of crystallization. Feiteira et al. [2] used the encapsulation of repair agents to achieve effective healing of cracks. Furthermore, bacteria-based microcapsules [3,4] and super absorbent polymers [5] were also used to promote the autonomous self-healing of concrete. It is concluded that the self-healing of cement-based materials is affected by foreign substance. However, the addition of foreign aid materials brings the extra cost of concrete and the easy failure of these materials in harsh environments restricts their promotion and application. Therefore, the improvement of autogenous self-healing is an important research direction for cement-based materials.
Autogenous self-sealing is an inherent characteristic of cement-based materials, which is regarded as a kind of self-healing performance and also an important performance of the environmental adaptability. Under suitable conditions, the micro-cracks of cement-based materials can be sealed autogenously by the continued hydration of unhydrated cement particles and the carbonation of hydration products [6].
Schematic diagram of a single crack preparation: (a) a single crack prepared in prisms by three-point bending procedure, (b) a single crack prepared in cylinders by split test procedure, (c) a single crack prepared in cubes by split test procedure.
where Mc is the mass of passed water per minute of the sample after making the crack, and Mh is the mass of passed water per minute of the same sample after the curing period for the self-sealing process.
The arrival times of the head wave for the uncracked samples were about 14 μs. At the same time, their amplitudes are between 3~5 mV. For the cracked samples, the arrival times of the head wave extended to 34 μs and the amplitudes dropped to ~1 mV. With the self-sealing of samples, the arrival time of the head wave was shortened and the amplitude increased compared with the cracked samples. In terms of the arrival time of the head wave, the following sequence was observed: static water (13 μs) < wet-dry cycles (15 μs) < humid air (22 μs) < flowing water (26 μs) < mild air (31 μs) < dry air (34 μs). At the same time, in terms of the amplitude of the head wave, the following sequence was also observed: static water (~4.7 mV) > wet-dry cycles (~3.4 mV) > humid air (~2.6 mV) > flowing water (~2.5 mV) > mild air (~1.9 mV) > dry air (~0.9 mV). The shorter arrival time and the greater amplitude demonstrate a better self-sealing ability of mortars. Therefore, the static liquid water is also conducive to the self-sealing of cracks inside the mortar.
where Tu, Tc, and Th are the arrival times of head waves for the uncracked, cracked, and sealed samples, respectively. Au, Ac, and Ah are the amplitudes of head waves for the uncracked, cracked, and sealed samples, respectively.
The maximum amplitudes of ultrasonic waveform are close to ~30 mV for the uncracked samples. When the samples cracked, the maximum amplitudes decreased to ~5 mV. With the self-sealing of the cracks, the maximum amplitude increased and the energy loss of ultrasound decreased. In terms of the maximum amplitude of the sealed samples cured in different conditions, the following sequence was observed: static water (25.9 mV) > wet-dry cycles (23.4 mV) > humid air (17.6 mV) > flowing water (11.5 mV) > mild air (7.6 mV) > dry air (5.3 mV). For the samples cured in liquid water, a good self-sealing behavior was obtained due to the continuous hydration and the formation of CaCO3. The self-sealing of mortar cured in the flowing water was not as good as that cured in the static water, which is due to the loss of Ca2+. The self-sealing of mortar is difficult to achieve in the samples cured in air with 30% RH and 60% RH, but the samples cured in the air with 95% RH had a better self-sealing behavior. A possible reason is that the air with 95% RH and above can condense into liquid water on the crack surface.
The dominant frequencies of the uncracked samples were in the range of 40~55 kHz and the amplitudes of dominant frequency were about 1.0~1.2 mV. For the cracked samples, the amplitudes of dominant frequency were below 0.4 mV. The decrease of amplitude indicates that the cracks caused a serious attenuation of ultrasonic wave. As the cracks sealed, the attenuation of ultrasonic wave was weakened, which shows the increase of the amplitude of dominant frequency. In terms of the amplitudes of dominant frequency for the samples self-sealed in different conditions, the following sequence was observed: static water (0.81 mV) > wet-dry cycles (0.75 mV) > humid air (0.58 mV) > flowing water (0.51 mV) > mild air (0.43 mV) > dry air (0.41 mV). The great amplitude of dominant frequency shows a good self-sealing behavior of the mortar cured in water. The results are similar to that of the head wave and ultrasonic waveform.
The crack surfaces were covered by the dense CaCO3 when the cracked samples experienced the flowing water, as shown in Figure 17c. The main self-sealing product was calcite. However, the self-sealing behavior was not good. On the one hand, the continue hydration products were dissolved by flowing water. On the other hand, the loss of Ca2+ produced by the dissolution of Ca(OH)2 is not conducive to the precipitation of CaCO3. Furthermore, the dense calcite blocked the leaching of Ca2+ [35]. 2b1af7f3a8