[11, 207]. This has led to search for another kind of hydrogel, and a methacrylate
modified alginate-based hydrogel has been tried [207]. This hydrogel absorbs water
which is higher than its weight just at 98% relative humidity (RH) and nearly 50% of
its weight at 90% RH. This means it can feed water to fuel the cells activity even in
the absence of liquid water. The B. sphaericus spores encapsulated in the modifiedsodium alginate hydrogel precipitated a large amount of CaCO 3 , indicating the
efficiency of protection. However, the authors reported that this alginate-based
hydrogel also reduces the strength of the concrete and the preparation involves UV
treatment. Thus, although the studies indicated the potential of hydrogels, it seems
that an ideal hydrogel for practical application has yet to come.
The microbial-based self-healing systems described above are based on pure
microbial cultures (axenic cultures); however, recently non-axenic cultures have
also been considered [222]. The use of non-axenic cultures eliminates the need for
sterile production conditions and lowers the production cost. A further cost reduction
can be achieved by minimizing the immobilization cost. Lately, self-protected
non-axenic mixed cultures which is referred to as Cyclic EnRiched Ureolytic
Powder (CERUP) have been studied [222]. CERUP is an ureolytic community
protected by its high salt content and obtained from side streams of a vegetables
processing plant. The efficiency of this non-axenic culture is comparable to that of
the benchmark B. sphaericus. Direct addition of the self-protected CERUP at levels
of 0.5% and 1% of the cement weight was sufficient, and this healed the concrete
crack effectively within 28 days [222]. The use of this non-axenic self-immobilized
preparation substantially reduces the production cost. In fact, the cost analysis
revealed that it is 40 times cheaper than the operational expense cost of the axenic
B. sphaericus culture [222]. A self-immobilized bacterial preparation has also been
tried successfully in self-healing concrete [115].
Even though most of the studies made so far on development of microbial-based
self-healing concrete are based on this (ureolytic) system, it suffers from potential
risks which pose a serious threat on its future commercial-scale applicability. The
ureolytic activity produces ammonium ions (NH 4
+ ) which can result in nitrogen
oxides emission into the atmosphere. It is estimated that remediation of one m
2 of
concrete needs about 10 g of urea which releases about 4.7 g of nitrogen [18]. Nitrate
can also be formed in this process. Ammonium and nitrate can be toxic and
hazardous to human health at high concentrations [65]. Moreover, ammonium that
stays in the concrete is a risk for bacteria-mediated nitrification into nitric acid which
can lead to deterioration of building materials [223]. These risks resulted in a
dramatic shift from the once dominant ureolytic-based approach to the other two
systems described below.
3.3.2 Oxidation of Organic Compound-Based Self-Healing System
This system is based on the precipitation of calcite through consumption of
organic compounds by microbial agents added to the concrete [104, 171, 200,
224]. Aerobic oxidation of organic acids by microorganisms produces CO 2 which
forms CO 3
2À in an alkaline environment. In the presence of a calcium source, the
320
G. Mamo and B. Mattiasson
modified alginate-based hydrogel has been tried [207]. This hydrogel absorbs water
which is higher than its weight just at 98% relative humidity (RH) and nearly 50% of
its weight at 90% RH. This means it can feed water to fuel the cells activity even in
the absence of liquid water. The B. sphaericus spores encapsulated in the modifiedsodium alginate hydrogel precipitated a large amount of CaCO 3 , indicating the
efficiency of protection. However, the authors reported that this alginate-based
hydrogel also reduces the strength of the concrete and the preparation involves UV
treatment. Thus, although the studies indicated the potential of hydrogels, it seems
that an ideal hydrogel for practical application has yet to come.
The microbial-based self-healing systems described above are based on pure
microbial cultures (axenic cultures); however, recently non-axenic cultures have
also been considered [222]. The use of non-axenic cultures eliminates the need for
sterile production conditions and lowers the production cost. A further cost reduction
can be achieved by minimizing the immobilization cost. Lately, self-protected
non-axenic mixed cultures which is referred to as Cyclic EnRiched Ureolytic
Powder (CERUP) have been studied [222]. CERUP is an ureolytic community
protected by its high salt content and obtained from side streams of a vegetables
processing plant. The efficiency of this non-axenic culture is comparable to that of
the benchmark B. sphaericus. Direct addition of the self-protected CERUP at levels
of 0.5% and 1% of the cement weight was sufficient, and this healed the concrete
crack effectively within 28 days [222]. The use of this non-axenic self-immobilized
preparation substantially reduces the production cost. In fact, the cost analysis
revealed that it is 40 times cheaper than the operational expense cost of the axenic
B. sphaericus culture [222]. A self-immobilized bacterial preparation has also been
tried successfully in self-healing concrete [115].
Even though most of the studies made so far on development of microbial-based
self-healing concrete are based on this (ureolytic) system, it suffers from potential
risks which pose a serious threat on its future commercial-scale applicability. The
ureolytic activity produces ammonium ions (NH 4
+ ) which can result in nitrogen
oxides emission into the atmosphere. It is estimated that remediation of one m
2 of
concrete needs about 10 g of urea which releases about 4.7 g of nitrogen [18]. Nitrate
can also be formed in this process. Ammonium and nitrate can be toxic and
hazardous to human health at high concentrations [65]. Moreover, ammonium that
stays in the concrete is a risk for bacteria-mediated nitrification into nitric acid which
can lead to deterioration of building materials [223]. These risks resulted in a
dramatic shift from the once dominant ureolytic-based approach to the other two
systems described below.
3.3.2 Oxidation of Organic Compound-Based Self-Healing System
This system is based on the precipitation of calcite through consumption of
organic compounds by microbial agents added to the concrete [104, 171, 200,
224]. Aerobic oxidation of organic acids by microorganisms produces CO 2 which
forms CO 3
2À in an alkaline environment. In the presence of a calcium source, the
320
G. Mamo and B. Mattiasson