While concrete remains the most popular and widely used construction material, significant research efforts have been devoted to improving concrete properties to meet current construction needs. To this end, steel fiber reinforced concrete (SFRC) has been developed. Unlike conventional concrete, SFRC consists of fine, short and high-strength discontinuous steel fibers randomly distributed in its matrix. These fibers contribute to the superior toughness, strength and ductility properties of SFRCs, making them suitable for engineering structures.
Many parametric tests about the characteristics of steel fibers, such as slenderness and shape, have been extensively carried out to enhance the mechanical performances of SFRC under different loading conditions. Likewise, the amount of experimental and theoretical research on the properties of these structures under different loading conditions has increased in the past decades. However, most experimental studies, especially those on large-sized SFRC members, are still limited in terms of technique, high cost and test error.
Numerical simulation is acknowledged as an effective approach for understanding the mechanical properties and responses of SFRC members, but most existing numerical models are unsuitable for studying the size effect law of SFRC members in terms of fracture characteristics. With the growing demand for large-size concrete structural members, the size effect law is considerably significant to understand the fracture and tensile properties of SFRC. Moreover, it requires a high-precision modeling approach, which is currently lacking.
On this account, researchers at the Nanjing University of Aeronautics and Astronautics: Dr. Zhangyu Wu and Prof. Hongfa Yu, in collaboration with Prof. Jinhua Zhang from the Southeast University of China, Prof. Xinguo Liu at the PLA Rocket Force University of Engineering, and Prof. Qin Fang from Army Engineering University of PLA, numerically investigated the size effect on the fracture properties of notched SFRC structural members under quasi-static bending loading. A novel three-dimensional (3D) two-phase mesoscale model considering the effects of random orientation and distribution of steel fibers in the concrete matrix was developed by the research team to realize their numerical investigation. The work has been published in the international journal, Engineering Fracture Mechanics.
In their approach, a generation algorithm for the 3D mesoscale model as well as supporting methods was developed. The interfacial relationship between the concrete matrix and the steel fibers was described using a new coupling method. The proposed 3D mesoscale model was validated against the available test data and then employed to perform various numerical simulations of the flexural behaviors of the SFRC beams with different depths: 30, 60, 90, 120 and 150 mm. The numerical study was performed regarding the fracture process zone evolution, load versus cracking mouth opening displacement curve, fracture failure patterns, and nominal flexural strength.
The authors reported the negligibility of the size effect on the fracture patterns of the SFRC members and a decrease in the nominal flexural strength with an increase in the beam depth from 30 to 150mm. The effectiveness of the newly proposed coupling method in simulating the relationships and interfacial behaviors between the concrete matrix and steel fibers was demonstrated. The load versus cracking mouth opening displacement curve entailed three key segments: the hardening stage, linear elastic and post-softening segments.
The enhancing effects of the steel fibers was more pronounced in the hardening stage before being removed gradually from the matrix of the concrete during post-softening stage. The role of the discontinuous steel fibers in suppressing the cracking propagation was revealed and was attributed to the gradual propagation of the stress along the beam central line. As a result, distributing the steel fibers in the weak zone improved the fractural toughness properties of the structural members.
In summary, the analysis of the size effects provided an efficient approach for evaluating the fractural properties of notched SFRC beams using a 3D mesoscale modeling approach. The size effect on the nominal flexural strength of the beam could also be evaluated using Bazant’s size effect law and the simulation results agreed with the corresponding test data. In a statement to Advances in Engineering, the authors explained that the new proposed mesoscale modeling approach would contribute to advanced studies on the mechanical properties and performance of SFRC to expand their application scope.
Zhang, J., Liu, X., Wu, Z., Yu, H., & Fang, Q. (2022). Fracture properties of steel fiber reinforced concrete: Size effect study via mesoscale modelling approach. Engineering Fracture Mechanics, 260, 108193.