Abstract
Micro-cracks, which develop during the service life of reinforced concrete structures, reduce the durability of concrete through the penetration of fluids. Microbially-induced calcium carbonate precipitation (MICP) occurs naturally in the presence of ureolytic bacteria which precipitate calcium carbonate (CaCO3) through urea hydrolysis. This deposition leads to the filling of micro-cracks and sealing of pores, reducing ingress of fluids into the concrete. The research aims were to assess the potential of Lysinibacillus sphaericus for healing cracks in concrete and to study the effects of this treatment on the absorption properties of treated concrete. Lysinibacillus sphaericus was cultivated in vitro and induction of MICP through urea hydrolysis was tested on cement paste with two different calcium sources. The calcium precipitates where characterised by light microscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy and Fourier Transform Infrared Spectroscopy. The final phase of the study involved testing of the crack healing capacity and the effect on absorption of the MICP process on mortar samples. These parameters were measured by means of visual examinations, light & digital microscopy, Ultrasonic Pulse Velocity (UPV) and absorption tests. The study confirmed that MICP is induced successfully on concrete using Lysinibacillus sphaericus. Samples exposed to repeated treatment cycles of Lysinibacillus sphaericus in the presence of a calcium source, exhibited a more extensive and even coating of CaCO3 crystals on the surface confirming that repeated cycles of treatment are more effective in increasing the amount of CaCO3 deposition and therefore increasing crack healing capacity. Digital microscopy and UPV analysis proved that this precipitate was successful in partially healing cracks in samples. Sorptivity tests confirmed this and showed that it was also successful as a surface treatment to reduce absorption.
Highlights
The development of new advanced cement-based materials leads to more durable concrete and reinforced concrete structures, with better performance throughout their intended life-time (Borg et al, 2018a)
Fourier Transform Infrared Spectroscopy (FTIR) was performed on samples of the Ca(CH3COO)2 and CaCl2 used during treatment to ensure that the crystals on the surface of samples treated with bacteria and a calcium source were not precipitates of these compounds
First and foremost, through this research, it was confirmed that Microbially-induced calcium carbonate precipitation (MICP) could be induced successfully on concrete using L. sphaericus and CaCl2 or L. sphaericus and (Ca(CH3COO)2) in the presence of urea
Summary
The development of new advanced cement-based materials leads to more durable concrete and reinforced concrete structures, with better performance throughout their intended life-time (Borg et al, 2018a). The durability of concrete is greatly reduced by the presence of micro-cracks within the material (Newman and Seng Choo, 2003). Micro-cracks may develop due to various processes. Surface Treatment and Crack Healing in Mortar and can occur at any time during the service life of the structure. They have a negative effect on the concrete as they allow ingress and transport of fluids into the concrete matrix, causing corrosion of reinforcement and degradation of the cementitious matrix. Crack widths must be controlled and healing processes may be promoted to reduce the ingress of fluids. Autogenous healing is only possible in small cracks and takes a long time to occur (De Belie et al, 2018)
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