Study: Analyzing Cracks in Concrete Tunnel Linings
Researchers from China have recently proposed a new method for analyzing cracks in plain concrete linings using the double-K fracture criterion. The study, which was recently published in Applied Sciences, was conducted by the Shanghai Jiao Tong University, Shanghai Marine Diesel Engine Research Institute and the China Academy of Transportation Sciences.
According to the study, cracks, leakage, spalling and displacements are the main indications of anomalies that affect the stability of the tunnel. The lining crack is reportedly the most common and adverse anomaly, providing a key indicator of safety.
Typically, safety evaluations utilize field measurement results with empirical criteria, such as the Japan Road Association’s maintenance manual of road tunnels and maintenance technical specifications proposed by the professional standards compilation group of China. Evaluations can also use the complex variable method, fracture mechanics or damage mechanics to analyze crack resistance of the lining.
Researchers argue, however, that the prior evaluations are semi-quantitative or quantitative because they neglect the mechanical aspect of crack generation or evolution. The latter method is more quantitative, but cannot take full advantage of field measurement results, like crack width or depth.
Based on the research, the study has proposed a method that combines the fracture mechanics of concrete with the field-measured data without complicated mechanical calculations, utilizing a modified double-K fracture criterion.
Researchers looked to concrete, the main ingredient of the tunnel lining, and its fracture characteristics. Because of the materials mixed together to form the concrete, including a fine aggregate, a coarse aggregate and cement paste, the poor bonding can create a large number of microcracks, leading to reduced stiffness and strength.
A typical fracture process for concrete consists of three stages:
For the double-K fracture criterion, the fracture process zone is reportedly equivalent to an elastic crack and the stress intensity factor is calculate by linear elastic fracture mechanics with an equivalent crack. To find these numbers, researchers measured the crack width and the load, calculated the crack depth and then calculated the stress intensity factor.
According to the study, this method introduces two parameters to evaluate the state of the cracks: initiation toughness and unstable toughness. By comparing these two factors against the stress intensity factors, the team is able to evaluate what stage of the fracture process the fracture is in.
The research team then developed a code based on the XFEM-FCM algorithm, a computational tool to detect and identify cracks in structures, to apply this method to tunnel linings and simulate the fracture process of concrete linings.
Recent Concrete Fracture Research
Last year, scientists from the Polytechnic Institute of the Far Eastern Federal University (FEFU) announced their development of a concrete material that can reportedly seal cracks and restore strength independently.
The study of self-healing concrete is especially relevant for construction in seismically hazardous areas where small cracks appear in structures, as well as in areas with high humidity or rainfall. The research was conducted alongside colleagues form Russia, India and Saudi Arabia, and has since been published in the Sustainability journal.
To create the self-healing concrete, scientists replaced ordinary water traditionally used when preparing the mixture with an aqueous concentrate containing the bacteria Bacillus cohnii, which can survive within the pores of the dried cement stone, filling any resulting damages with calcium carbonate (CaCO3).
Once the mixture had cured, scientists tested the material for compression until it cracked and then observed how the bacteria was able to fix the gaps and restore the strength of the material. According to the report, once the bacteria was able to access oxygen and moisture through the cracks, the bacteria became “awakened” and was able to successfully heal cracks with a 0.2-0.6 mm width within 28 days by releasing the CaCO3, which crystallized under the influence of water.
After the concrete slabs returned to its original compressive strength, the bacteria “fell asleep” again.
The university notes that spores of the bacteria Bacillus cohnii can live within concrete for up to 200 years, and in theory, could extend the life of structures for that same period—almost four times the service life of conventional concrete.
To conduct the experiment, scientists grew the bacteria Bacillus cohnii in the laboratory using an agar substrate and culture medium, as to force the bacteria to survive within the conditions of the cement stone pores. Crack repair was evaluated under a microscope, while the chemical composition of the repairing agent was isolated and studied using an electron microscopy in addition to X-ray images.
In reaching this discovery, scientists plan to develop a reinforced concrete next, which would further enhance the material’s properties using different types of bacteria with the potential of speeding up the recovery process.