Jing Yu

Microbial-induced carbonate precipitation (MICP) has outstanding characteristics in solidifying soil, such as good fluidity, ecological environmental protection, adjustable reaction, etc., making it have a good application prospect. As a typical silty clay, the composition of loess is fine, and the microstructure is quite different from that of sand. Previous research has found that the unconfined compressive strength of loess cured by MICP can be increased by nearly four times. In this paper, by comparing the changes of structural characteristics of undisturbed loess before and after MICP solidification, the mechanism of strength improvement of loess after MICP solidification is revealed from the microscopic level. Firstly, the microstructure of loess before and after solidification is tested by scanning electron microscope, and it is found that the skeleton particles of undisturbed loess are granular, and the soil particles coexist in direct contact and indirect contact, and the pores in soil are mainly overhead pores compared with the microstructure of solidified loess, it is found that the surface contact between aggregates increases obviously, and calcium carbonate generated by MICP is adsorbed around the point contact between aggregates, which makes the contact between soil particles change from point contact to surface contact. Then, Pores (Particles) and Cracks Analysis System (PCAS) is used to quantitatively analyze the pores of loess before and after solidification. The results show that the total pore area, the maximum total pore area and porosity of soil samples decrease, and the total number of pores decreases by 13.2% compared with that before MICP solidification, indicating that a part of calcium carbonate produced by MICP reaction accumulates in tiny pores, thus reducing the number of pores. One part is cemented between soil particles, which increases the contact area of particles. Therefore, some pores of loess solidified by MICP are filled and densified, the contact area between soil particles is increased, and the strength of loess under load is obviously improved.

According to the reinforcement principle of hybrid fiber, steel fiber and PVA fiber were used to prepare hybrid fiber reinforced concrete (HyFRC) with compression strength of 120 MPa (SPHFC120). The tensile properties of SPHFC120 were studied by uniaxial tensile test, and the uniaxial tensile constitutive equation was established according to the stress-strain curve. An efficient and applicable three-dimensional mesoscale model to simulate HyFRC was developed. It consists of two components involving concrete matrix and fibers, and the bond between concrete matrix and fibers was considered indirectly by defining the fiber constitutive model. The model was validated with uniaxial tensile test. Then parametric studies were conducted to investigate the effect of fibers content on the tensile performance of HyFRC. The results show that PVA fibers obviously improve the tensile strength, peak tensile strain and tensile toughness of steel fiber reinforced concrete.

A new model that can simulate the behavior of construction joint subjected to seismic forces was proposed. Nonlinear time-history analysis was carried out for reinforced concrete regular frame structures designed in different seismic intensity regions as well as with different height-to-width ratios. Two kinds of numerical models are adopted to simulate the seismic behavior of each frame, one with construction joint using the new proposed model and the other without construction joint using the conventional model. Results show that the influence of construction joint on the seismic behavior of reinforced concrete frame is strongly related to structural nonlinearity. It may increase the top displacement and the inter-story drift, change the inter-story drift distributions, and exacerbated the local reaction of key members. The influence of construction joint cannot be ignored for structures with low emergency capacity against major earthquake. Seismic design suggestions are proposed from the aspect of calculation analysis method.