Since concrete with bacteria incorporated in the matrix shows promising results in initial studies, more research has focused on using bacteria for the strength and durability enhancement of concrete. Bacterial concentrations varying from different positions on the crack surface were recorded and evaluated in conjunction with the amount of self-healing product formed. The change in bacteria concentration with the survival time in concrete was investigated and lasted for two years. The degree of mineralization of calcium carbonate from the microbial activity is also closely related to the level of survival and reduction of bacterial concentrations in concrete compared to the initial amount. These processes were tracked and analyzed through phase composition analysis and microstructure analysis. The role of nucleation sites of bacteria for accelerating mineral deposition was also investigated. The change in the content of hydrated cementitious minerals can be seen in groups of samples with different bacteria regarding cracking age (7-90 days). The increase in C-S-H content in the bacterial samples at early cracking age was significant compared with the control group. The effect on healed crack parameters through microscopic observation contributed to supporting and demonstrating the hypothesis of the combination of the formation of the calcium carbonate crystals around the bacterial cell as crystallization nuclei and the promotion of hydration for C-S-H formation.
This paper explains the experimental research on fracture properties of concrete using TiO2 nano powder (NT) (1, 2, 3, 4%) and fly ash (FA) (10, 20, 30, 40%). Three point bend tests were conducted on 153 notched beams in a closed loop servo control machine with crack mouth opening displacement (CMOD) control and opening rate of 0.0005 mm/s. A total 17 mixes were prepared with combination NT+ FA with the above percentages. Work of fracture method (WFM) and size effect method (SEM) was used to analyze fracture parameters. The results for WFM and SEM show that fracture energy, fracture toughness and brittleness number increase with increase in NT and FA percentages, while the characteristic length decreases. In each mix varying from NT1FA10 to NT4FA40, the fracture energy increased by 20.92% and fracture toughness increased by 34.61%. The mix with 4% NT showed significant improvement of 38.46% for fracture toughness and 20.93% for fracture energy. The mix indicating brittle behavior as the characteristic length (Lch) decreases with increase in percentage of NT and FA and the length of fracture process zone (CF) decreases with increase in percentage of NT and FA which indicates the brittle behavior of material.
Many advantages of concrete-filled steel tube columns have led to their use in a variety of distinctive and important structures in recent years, including high-rise buildings, piers, stairs and other structures. The exterior steel wall of these columns is uncoated, which is the main disadvantage. When these columns are subjected to external pressure on the external steel wall, such as collision, explosion, fire or other accidents, the confinement of concrete will be lost owing to the weakness generated in the steel wall, resulting in an abrupt loss in column strength. In this study, a new upgraded section using internal steel mesh is designed and introduced, with this steel mesh and the external steel wall acting as a double-skin steel tube to protect the concrete core from being destroyed if the external steel wall is destroyed and the column's strength drops suddenly. Under axial and cyclic loads, a comparison of the behavior of this innovative section (with internal steel mesh) with the most commonly used sections of concrete-filled steel columns was conducted. Following the verification of the finite element modeling, various other studies were carried out. The results of the analyses clearly show that the suggested CFT section has increased strength, improved resistance to progressive collapse, and energy absorption capacity under axial and cyclic loading, particularly abrupt loads such as fire or explosion, and thus its use in construction is recommended.