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Once I verified that I could log in to my Cucumber dashboard site, I wanted to add the external antenna to my device. I followed the steps shown in the aforementioned video to install my external antenna. Here is the antenna (two for under $10) I selected, which I found on Amazon:
For crack repair, a variety of techniques are available and the repair is mostly conducted manually. The typical repair involves chemical materials (e.g., epoxy systems, polymers, Na2CO3, Ca(NO3)2, Ca(HCOO)2 et al. [1,2,3,4,5,6]), which is normally expensive, time-consuming, non-compatible with concrete, lacking durability, and hazardous to the environment and health.
As an alternative strategy to address this crack repairing issue, recent studies on self-healing concrete have drawn lots of attention in the past decade. Under moist environments, concrete has the potential to heal cracks autogenously by the continuous reaction of unhydrated cementitious materials and the precipitation of CaCO3, which has been observed in a bridge in Amsterdam in the 1830s [7]. However, such a naturally occurring, self-healing phenomenon is rarely observed since it requires the crack width to be controlled below 100 μm while the crack width in concrete often reaches millimeter sizes even if restrained by a steel rebar [8,9].
To address this issue, Engineered Cementitious Composites (ECC), a unique type of fiber-reinforced concrete with tight crack width control, has been used to study the self-healing behavior. It forms multiple micro-cracks typically below 100 μm instead of localized single cracks in the concrete [10,11]. It was found that the micro-crack could heal itself effectively as the transport properties of pre-cracked ECCs recovered to that of virgin ones [12,13]. Meanwhile, the combination of C-S-H and CaCO3 were observed within the crack space, and they are believed to come from the further hydration of unhydrated cement particles and supplementary cementitious ingredients, and the precipitation of CaCO3 through the wet-dry curing cycle [9,13].
Additionally, incorporating bacteria into concrete could also achieve self-healing due to the microbial activity. The bacterial cells inside the concrete is likely to be activated when the air and water get into concrete through cracks. Afterwards the bacteria start producing mineral compounds due to the microbiologically induced calcite precipitation (MICP) process [14,15,16], and subsequently seal the crack naturally. Many researches have been reported to demonstrate that this bio-concrete technology can heal the crack at around 1 mm wide, efficiently.
As aforementioned, research on self-healing concrete has made great strides; nevertheless, existing infrastructures built with conventional concrete cannot heal cracks autogenously. Hence manual repair is still the most practical way to seal the cracks in most cases. However, it is not a trivial task to look for an environmentally friendly and economical repair material that can repair concrete efficiently before it deteriorates from the initial micrometer-sized cracks to millimeter ones.
In this paper, Bacillus halodurans, a kind of carbonic anhydrase positive bacteria, is adopted as a repair liquid for crack-sealing in concrete. The carbonic anhydrases from the metabolism of bacteria cells could capture CO2 efficiently and convert it to carbonic acid by the catalysis. This characteristic has made carbon capture a much more viable option for energy firms that use fossil fuels [20]. It is also expected to facilitate the generation of calcite by combining the Ca2+ (widely exist on the crack surface) and atmospheric CO2, which could avoid the typical usage of urea, thus addressing a number of aforementioned issues with its usage. Furthermore, the transposon mutagenesis method is proposed to create a genetically modified bacteria strain aiming to improve the activity of bacteria, subsequently shorten the period of crack-repairing. In a previous study, the directed addition of Bacillus halodurans has already shown good compatibility with the cement system, resulting in increased compressive/tensile strength [21].
The transposon mutagenesis method has been widely used in other applied microbiology fields (e.g., environmental engineering, electricity generation, bio-production). For example, Ding et al. used the transposon mutagenesis method to create a Shewanella oneidensis mutant (CP2-1-S1) leading to a much higher efficiency of heavy metal removal [22]. Yang et al. used the same Shewanella oneidensis mutant to improve the electricity generation in the microbial fuel cell (MFC) [23]. Shi et al. also reported that transposon mutagenesis was employed in Escherichia coli to achieve high-yield pyruvate production [24]. Lin et al. applied the Tn5 transposon mutagenesis to improve butanol production by Escherichia coli [25]. Hemarajata et al. applied the transposon mutagenesis method to the bacteria Lactobacillus reuteri and found that the gene eriC greatly affects histamine production [26]. Overall, the transposon mutagenesis method has been widely proved as a method to enhance the bacterial process greatly. However, this method has never been used in the healing/repairing of cracks in the concrete to the best of our knowledge.
For crack image analysis and X-CT scan technique, the uniaxial tensile test was employed on ECC dogbone specimens to produce cracks until failure. The test set-up could be found in reference [21]. After the testing, segments with cracks were cut from the dogbone specimen. The bio-liquid was sprayed on the surface of cracked ECC segments for a number of times (once a day) until the crack was filled by the sealing products. The test was ended after 20 times of spraying even if there was no crack sealing. The specimens were then cut into small pieces for the X-CT scan to observe the distribution of the sealing products within the crack space. Each piece containing one crack had dimensions of 10 mm 10 mm 15 mm.
Cracks in specimens were extracted from these pictures based on a self-compiled MATLAB program. The dimensions of pictures were 3891 pixels by 2917 pixels, whereby each pixel corresponds to 9 μm (resolution of the image). The gray value of the digital photograph was checked against a threshold for the identification of the crack region. A proper gray value must be selected as a threshold to differentiate the crack region from its background so that a binary image can be constructed. The range of gray value was from 0 to 255, and a gray threshold of 80 was selected in order to identify cracks effectively so that pixels with gray values less than 80 were identified as crack regions.
Figure 2a illustrates a raw image directly obtained by a camera device. It can be found that the color of the crack is dark. In this study, a raw, colorful image was transferred into a gray image first; thereafter, pre-processing work was conducted, which included contrast enhancement, removal of the noise points caused by the density mutant, and burr smoothing of the border. In the final step, the Otsu binary method was used to determine the optimal threshold [28], which can effectively distinguish the crack from the surrounding matrix. Consequently, a binary image of crack can be obtained, as shown in Figure 2b. By the use of morphological operation described above, cracks are effectively extracted from the surrounding matrix. Accordingly, geometry indicators, such as crack area, width, and length, are calculated based on the pixel number that the crack contains.
In previous studies, Jonkers et al. grew the Bacillus subtilis to perform the healing or repairing of the cracks of the cementitious materials [31,32,33]. This study initially attempted the same bacteria and found it cannot grow well in Singapore. Considering the local environmental conditions, Bacillus halodurans was selected for repairing of the cracks in cementitious materials since it is potentially more suitable for growth in the local environment in Singapore with relatively high temperature and humidity.
To be consistent with the sample preparation method used for image analysis, the sorptivity test was conducted on pre-cracked samples after three times of spraying for series M (as cracks are fully filled) and 20 times for series W.
In the comparison of the curves in Figure 10a,b, it can be observed that water absorbed in the pre-cracked specimens decreased after bio-liquid spraying, especially for series M. On the other hand, virgin specimens for both cases also presented less water absorption after bio-liquid spraying. This may be attributed to that the MICP products from bacterial metabolism fill in the pore on the specimen surface for the case M, which is shown in Figure 11. For the case W, no MICP products were found on the specimen surface, reduced water absorption may be explained that the wild type bacteria produce a kind of bio-film after 20 times of spraying, which seals the specimen surface and thereby reduces the absorption of water (will be discussed in details later).
Figure 12 displays the margin of sorptivity of pre-cracked specimens, which is defined as the difference in sorptivity of specimens before and after bio-liquid sprayings. It can be seen that, after spraying, the margin of sorptivity of the pre-cracked specimens for series M decreased very distinctly, while only a slight reduction achieved for series W. The decrease percentage for the series W and M, which reflect the repair efficiency, is 22.4% and 94.1%, respectively. This observation again demonstrated that the cracks in concrete could be sealed very effectively by spraying the genetic modified bacteria liquid. 2ff7e9595c
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