A Case Study: Bacterial Surface Treatment of Normal and Lightweight Concrete - Biotechnologies and Biomimetics for Civil Engineering - page 354

16.1 Introduction

There has been considerable effort to apply special types of bacteria which produce
calcium carbonate particles in various fields of civil engineering (Bellucci et al.
1999 ; Ramachandran et al. 2001 ; Beklemishev and Kozliak 2003 ; de Graef et al.
2005 ; Phutane et al. 2007 ; Van Tittelboom et al. 2010 ). Among these efforts,
bacterial surface treatment for concrete has been of particular interest given its
strong potential to improve the durability of concrete structures (De Muynck et al.
2008a , b ). By inoculating bacteria onto the surface of concrete, a thin and stable
layer of calcium carbonate crystals can be formed via the metabolism of the
bacteria. The layer of calcium carbonate crystals may diminish the uptake of water
or protect the concrete from chemical deterioration, leading to an improvement of
the durability of the concrete structures (Stocks-Fischer et al. 1999 ; De Muynck
et al. 2008a , b ; Kim et al. 2013 ).
Compared to the application of bacteria inside the cement matrix, bacterial
surface treatment is also considered as a promising method for practical reasons.
Regarding this approach, it is relatively easy to supply nutrients, oxygen, and
water to the bacteria, as the bacteria are applied onto the surface of the concrete,
compared to when the bacteria are inoculated into the cement matrix (Ghosh et al.
2005 , 2009 ; Qian et al. 2009 ; Jonkers et al. 2010 ; Chahal et al. 2012 ). Moreover,
the pH of concrete surfaces is generally neutral due to carbonation by CO 2 gas in
the atmosphere, making the surface a suitable environment in which to survive,
whereas the pH of the inner part of concrete is too basic for the bacteria to survive
(Wiktor and Jonkers 2011 ).
Compared to conventional surface treatment methods using polymer-based
coating materials, including water repellents or pore-blockers, the bacterial surface
treatment method has a number of advantages. Most of all, the problem of the
delamination of polymer-based coating materials from the concrete surface is
mitigated (De Muynck et al. 2008a , b ). The delamination of coating materials
stems from the different thermal expansion coefficients between the polymer and
the concrete (De Muynck et al. 2008b ). However, calcium carbonate crystals
precipitated by bacteria have a thermal expansion coefficient similar to that of the
cement matrix (Kim et al. 2013 ). Moreover, the risk of environmental pollution
induced by polymer-based coating materials can be eliminated. For example,
Bisphenol A, one of the components resulting from the manufacturing of epoxy
resin, can act as an endocrine disruptor in the human body (Maffini et al. 2006 ;
Kimura et al. 1998 ). However, the bacteria used in concrete surface treatments
only have limited health effects, such as only isolated cases of mild eye and skin
irritation (US EPA 1998 ). In addition, the bacterial surface treatment may have
self-healing characteristics if the bacteria survive after they are supplied with
nutrients,

whereas polymer-based coating materials become ineffective after

delamination from the concrete surface (c.f. Wiktor and Jonkers 2011 ).
In terms of energy and time consumption for surface treatments, there are no
significant differences between bacterial treatments and the use of a conventional