180 / 2022-04-30 18:13:08

Development of a pH-responsive hydrogel with high moisture absorption for bacteria-based self-healing concrete

Hydrogel; pH responsive; Moisture uptake capacity; Bacterial CaCO3; Self-Healing

In the last decade, bacteria based self-healing concrete has gained more and more attention from researchers and civil engineers because of its great potential to reduce the high maintenance and repair cost of concrete structures. This type of self-healing strategy is mainly based on the microbial induced carbonate precipitation process. Carbonate precipitating bacteria Carbonate precipitating bacteria are added into mortar specimens to in-situ heal concrete cracks when cracking occurs. Due to the high alkalinity and small pore sizes of concrete, bacteria cannot be added directly. Therefore, an immobilization process is preferable. And a suitable bacterial carrier is of crucial importance. In our previous study, hydrogel has shown its superiority over other carriers due to its excellent biocompatibility and water absorption and retention property, which facilitate the revival of bacterial activity and carbonate precipitation since water is an essential element for microorganisms. However, also due to its water absorption or swelling property, hydrogel absorbs water and swells during the mixing process, while release the water and shrinks during the hydration process, resulting in the formation of macropores inside cementitious matrix, and hence a significant decrease of strength (Fig.1).



In order to solve this problem, a pH responsive hydrogel based on chitosan-carraeenan was synthesized. The hydrogel had high swelling property under moderate alkali pH conditions, and limited swelling when pH was higher than 12, and in the filtered cement solution (Fig.2). This indicates that there would be quite limited swelling during the mixing process, and hence no significant volume changes. The formation of macropores due to the addition of hydrogel was therefore greatly decreased. Because of the pH responsive swelling, the negative effect of hydrogel on the mechanical properties was greatly diminished. As shown in Fig.3, there was no significant decrease in compressive strength of the mortar specimens with the hydrogel addition of 0.5~1% by cement mass. Furthermore, with the planted carrageenan network, the moisture absorptivity was remarkably improved as well (Fig.4). Bacillus sphaericus spores were encapsulated into the hydrogel matrix during the crosslinking process. The revival test showed that the spores survived the encapsulation process and showed carbonatogenesity (Fig.5, SEM).



Cracked mortar specimens with hydrogel encapsulated bacterial spores added have also shown an enhanced crack-sealing efficiency in the aspects of crack sealing ratio (Fig.6) and maximum crack width sealed. Specimens exposed to wet-dry cycles showed a decreased sealing efficiency compared to the ones subjected to full submersion. This is more obvious in the specimens without the addition of hydrogels. The specimens added with hydrogels and hydrogel encapsulated bacteria all showed improved crack sealing efficiency, due to the water absorption and retention property of the hydrogels.