Bacteria for Concrete Surface Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 343

of different immobilization technology (clay capsules, silica gel, or polyurethane
encapsulation). The use of bacteria in concrete mix also needs further research
efforts. Results from practical applications in which there is exposure to envi-
ronmental conditions are still needed in order to confirm the importance of this
new approach (Pacheco-Torgal and Labrincha 2013 ).
Microbially induced calcium carbonate precipitation is a naturally occurring
biological process that has various applications in remediation and restoration of
range of building materials. In the present study, the role of bacteria Bacillus sp. on
the durability properties and remediation of cracks in cementitious structures were
studied. Biocement induced by a Bacillus sp. lead to more than 50 % reduction in
the porosity of mortar specimens, while chloride permeability of concrete changed
from moderate to very low as indicated by rapid chloride permeability test. The
bacteria successfully healed the simulated cracks of depths including 27.2 mm in
cement mortars with increase in the compressive strength as high as 40 % of that
of control. The results clearly showed that microbially induced calcium carbonate
precipitation can be applied for various building materials for remediation of
cracks and enhancement of durability (Achal et al. 2013 ).
15.4 Bacterial Self-healing Concrete
Besides external application of bacteria in the case of remediation of cracks,
bacteria have also been applied in the concrete mixture. Until now, research has
mainly focused on the consequences of this addition on the material properties of
concrete, i.e., strength and durability. Both properties depend on the microstruc-
ture of the concrete. However, the effects of the presence of the bacteria and/or
biodeposition on the microstructure still need to be elucidated, especially the
interaction between the biomass and the cement matrix. Ramachandran et al.
( 2001 ) investigated the use of microbiologically induced mineral deposition for the
improvement of the compressive strength of Portland cement mortar cubes. This
study identified the effect of the buffer solution and type and amount of S. pasteurii
and P. aeruginosa used. Furthermore, in order to study the effect of the biomass,
the influence of both living and dead cells was investigated. Before addition to the
mortar mixture, bacteria were centrifuged and washed twice. The final pellets were
then suspended in either saline or phosphate buffer, which was subsequently added
to the mixture. After demolding, the mortar specimens were stored in a solution
containing urea and calcium chloride for 7 days. Subsequently, the specimens
were cured in air until the measurement of the compressive strength. At lower
concentrations, the presence of S. pasteurii was shown to increase the compressive
strength of mortar cubes. While the 28-day compressive strength of the control
cubes amounted to about 55 ± 1 MPa, specimens treated with 103 cells cm -3 had
a compressive strength of about 65 ± 1 MPa. The contribution of P. aeruginosa to
the strength was found to be insignificant. From the X-ray diffraction (XRD)
analysis, no significant increased amounts of calcite could be found in mortar