The effect of microbiological agents on the efficiency of bio-based repair systems for concrete

Abstract

The induction of calcium precipitation via bacterial action has been studied increasingly in past years for self-(healing/sealing) concrete applications. Several of these studies have presented promising conclusions that microbiologically induced calcite precipitation might be a useful approach for remediation and rehabilitation of shallow cracks on existing structures. Such studies have noted the necessity to encapsulate the ingredients (bacteria, nutrients and organic precursors) separately for self-healing concrete using microbiologically induced calcite precipitation. However, during mixing there is a chance that capsules or other carriers of self-healing agents may release their cargoes and affect the properties of the concrete.Based on the above-mentioned information, the objective of this research was to evaluate whether or not shallow concrete cracks can be remediated using a bacteria-based system of repair. This research also aims to develop a new bacterial agent for use in the remediation of concrete cracks and to understand the effect of bacterial agents on the properties of cement-based mortar. The scope of this research is diverse; it requires an understanding of the factors that affect durability, water permeability, and cement properties such as initial and final setting time as well as the quality and quantity of the precipitated materials from the bacteria-based healing/sealing system.This research is broadly divided into four stages. In spite of a number of studies on the mechanism and efficiency of bacterial self-(healing/sealing) concrete, stage one investigates the effect of bacterial self-(healing/sealing) agents on the properties of fresh and hardened concrete. This information is critical for further research and implementation of this novel material. As with any additives, this may have a negative effect on the concrete’s final properties. This will be viewed with skepticism and limited uptake.This stage included the effects of the self-(healing/sealing) agents individually and as a combined medium on the mechanical properties of fresh concrete, the hydration kinetics, and early and final setting times, as well as strength and microstructure development over time. In addition, the effects of self-(healing/sealing) agents on hardened concrete were investigated to determine whether or not capsule rupture in response to a crack would have a detrimental effect on concrete properties in the area surrounding the crack. The results showed that self-(healing/sealing) agents such as sodium citrate greatly influenced hydration kinetics when the concentration exceeded 0.05% of the cement mass. Although the self-(healing/sealing) agents at 0.5% by binder mass retarded lightly of setting time, they had littlenegative effect on either 3- or 28-day strength. Calcium acetate, the dominant self-(healing/sealing) agent, acts as an accelerator while other components of the medium can have detrimental effects on the properties of fresh and hardened concrete. However, provided the quantity of self-healing/sealing agents released is below a certain threshold, it is unlikely that any detrimental effects will limit the application of bacterial self-healing/sealing concrete.Stage two included applying the main components of the self-sealing agents (calcium lactate and yeast extract) with the ingredients of the mortar mix and as a combined medium with mortar mix (two-stage bio-concrete/mortar) to investigate the ability of B. cohnii, B. halodurans and B. pseudofirmus to induce calcite precipitation through the cracks. The results showed that the sealing materials using each one of the three bacteria with the main components of self-sealing agents were very weak and were not distributed along the crack. Moreover, the primary components of self-sealing agents in 5% bio-cement mortar of the combined medium by binder mass distributed in entire samples were unable to seal cracks. Results showed that the samples dissolved in water, meaning that with more self-healing agent (SHA-1) ingredients added to the bio-mortar, the weaker and more ineffective it was with cement.Remediating cracks of hardening concrete with three different types of bacteria (B. cohnii, B. halodurans, and B. pseudofirmus) was performed during Stage three. Factors affecting the quantity and quality of healed materials included start and end healing time, the growth of each bacterium, and the viability of each bacterium in alkaline environment, all of which were also studied experimentally. Results showed that the three bacteria can produce calcium carbonate in a self-healing/sealing process, although B. pseudofirmus is the most suitable, efficient, and economical for remediating concrete cracks.The delivery of bacteria spores inside the concrete environment has always been the most challenging task. The main objective of stage four is to study the possibility of using successful previous delivery system used to remediate concrete cracks by using mineral agent to be used as delivery systems of bacteria spores. This study investigated three different encapsulate techniques within cement mortar namely calcium alginate beads (CAB), vascular tubes, and perlite.Results showed that CAB is very weak and very light due to low density, which cause decrease in their size, floating on the surface of mortar and poor distribution in mortar matrix.In spite of some passive mode effect variables such as tube length and diameter, the viscosity of bacteria solutions and their agents (SHA) and the ability of the glass tube to resist internal stresses were investigated. The results showed there was not enough data to demonstrate its ability to heal cracks.The mechanical and physical properties of uncoated and coated perlite, the ability of perlite to carry bacteria and its SHA, and the ability of bacteria and its SHA to form calcite out of perlite were investigated. The results demonstrated the ability of perlite to inoculate bacteria and its SHA, and the ability of this system to heal cracks. It is clear from the above perlite is the most suitable delivery system.

Date of Award	27 Jun 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Kevin Paine (Supervisor) & Andrew Heath (Supervisor)