## 9 Effect of Parameters on Performance of Cementitious Self-Healing Agents
Under cryogenic temperatures, the autonomous healing capacity of the fiber-reinforced concrete examined [109]. The samples were first cured in water for 28 days at 20 \({}^{\circ}\)C and then cured again under air exposure for 3 days at room temperature. A week was needed to seal the cracks of widths in the range of 10 and 50 \(\upmu\)m for the pre-cracked samples cured for 28 days. The sealing of the cracks of widths up to 50 \(\upmu\)m was achieved when the samples were subjected to 24 h of cryogenic temperatures. The FS properties of the specimen were affected by the fibers' lengths, wherein shorter fibers showed weaker FS. Three days of air cured specimen under cryogenic temperatures showed an improved FS wherein the pre-cracked samples were made with steel fibers with an aspect ratio of 97.5. The pre-cracked samples containing steel fiber (with an aspect ratio of 100) revealed weakest energy absorption capability. Some studies were performed to determine the effects of cellulose fibers mixing into concrete on the water permeability and autonomous healing efficacy [14]. The autonomous healing rate of the specimen was estimated via the water penetrability measurement and other tests. Concrete specimen prepared with 0.5% of cellulose fibers showed a decrease in the CS value. Contrastingly, the FS of the standard concrete was found to be 7.84% lower than that of the specimens made with fibers. The effects of microfibers and SAP inclusion in concrete on the autonomous healing mechanisms were examined [47], wherein the cracks' sealing ability was improved via the incorporation of SAP. Upon exposure to a wet environment, SAP was swelled. When subjected to the wet and dry cycles, a crack of size 138 \(\upmu\)m was healed when 1% of SAP was included in the specimen. In addition, the SAP inclusion into the concrete was responsible for the cracks' healing and water permeability reduction. The feasibility of enhancing the autonomous healing capacity of cracks through the incorporation of diverse bacteria and fibers in the concrete were studied [110]. A decrease in the concentration and growth of the bacteria was observed, while the cracks with a width up to 500 \(\upmu\)m were successfully sealed. Following one week of curing, the pre-cracked control samples without bacteria or fiber demonstrated about 30% healing, which was further improved by 2% after 28 days of curing. Following 28 days of curing, the control sample demonstrated higher crack repair ratio than those designed with PP1 (polyppropylene) fiber. After 28 days of curing,a crack healing ratio of 31% was achieved in the sample designed with PP2 fiber. In the first week of curing, an improvement in the crack healing ratio of 36% was observed. The incorporation of only PP1 and PP2 fibers into the mortar produced weaker effects than the mortar prepared with polyvinyl alcohol (PVA) fiber. The crack healing ratio of mortars containing PVA fibers after curing for 7 and 28 days was enhanced from 44% to 50%, respectively. Enhanced healing rates were observed when microscopic organisms such as bacteria were mixed in the mortar. Samples containing only fiber displayed lower healing rates than those made with fiber and bacteria. Concrete with these fiber and bacteria mixes showed reduced water absorption and improved FS. Many investigations have been conducted to determine the effects of fibers, polymers, and diverse crystalline materials incorporation into concrete on the autonomously healing efficiency of cracks. The mechanical properties of autonomously healing concrete were measured using the split tensile, water tightness, flexural, permeability test, and CS tests. Elemental compositions and microstructures were analyzed using EDS and SEM measurements. Diminished water permeability was achieved via the surface layer cracks in the concrete when sealed after the reactions. Despite enough air exposure, such samples could seal the cracks by absorbing the moisture from the environment because of the hydrophilic properties of the admixtures.