Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 336

Promising results of techniques based on microbial mineralization have lead to
several investigations on the use of bacteria in concrete. Alkali-resistant spore-
forming bacteria represent promising candidates for application in concrete and
probably other cement-based materials. The controlled use of bacteria offers new
approaches

for conservators to help preserve, protect, and restore building

materials.
Permeability of concrete is believed to be the most important characteristic of
concrete that affects its durability. The principal result of the intrusion of chloride
(i.e., salt water) into concrete is the corrosion of the reinforcing steel. Once this
occurs, the structure will no longer maintain its structural integrity; the lifespan is
reduced, and the general safety of the public is severely degraded. It is increasingly
apparent that for many concrete members, the ability of the concrete to resist
chloride penetration is an essential factor in determining its successful perfor-
mance over an extended period. The decrease in gas permeability due to the
biodeposition treatments resulted in an increased resistance toward carbonation
(Siddique and Kaur Chahal 2011 ).
Sorptivity coefficient K for different grades of mortar applied with different
types of surface treatments helped to determine the increase in resistance toward
the water penetration which is best depicted (Fig. 15.2 a). So, absorption of bac-
teria and precipitation of carbonate crystals resulted in weight increase of the
mortar specimens. So, absorption of biodeposition resulted in weight increase of
the mortar specimens. The most pronounced reduction in water absorption com-
pared to untreated samples was reached for the most porous mixture w/c (0.6) and
where urea, nutrient broth, and external calcium source were provided, the most of
water absorbed by the mortar samples after 2 h was the decreased by factor 5
(Fig. 15.2 b). Except for the water repellents, similar tendencies were observed
between the gas permeability and carbonation rate results. The rate of carbonation
and the performance of the surface treatment were correlated with the water-
cement ratio. Carbonation was shown to be related to the nature and connectivity
of the pores, with larger pores giving rise to higher carbonation depths. Significant
differences in carbonation depth between treated and untreated specimens were
already noticeable after 2 weeks of accelerated carbonation. The protective effect
of the biodeposition treatment toward carbonation could be improved by additional
treatments with bacteria and a calcium source or an increased concentration of
calcium ions. Film forming coatings and sealants to be effective against carbon-
ation, the thickness of the treatment should be at least 200 lm (Basheer et al.
2001 ). The mean thickness of deposition layer was about 30-50 lm; nevertheless,
an improved resistance toward carbonation was already noticed in Fig. 15.2 c.
Resistance toward chloride penetration of biodeposition treated samples was
measured with the use of an accelerated migration test, chloride migration coeffi-
cient. The increased resistance toward the migration of chlorides of cubes treated
with biodeposition was similar to that of the acrylic coating and the water repellent
silanes and silicones and larger than in the case of the silanes/siloxanes mixture,
which were all reported to be effective in decreasing the rate of reinforcement
corrosion (Fig. 15.2 d). Based on the studied properties, the conventional methods