*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

**Table 2.** *Autonomic*

 *self-healing:*

*Encapsulation*

 *materials and techniques used ('–' means 'not reported', 'x' means 'not applicable', '*√*' means 'yes' and '/' means 'no'). (upgraded from [53]).*

**Shell material**

> Capsule for self-healing

**202**

 Spherical

 Expanded clay Expanded clay Expanded clay

Diatomaceous

 earth

> Gelatin

Gelatin Gelatin Gelatin

Wax Paraffin

Cement + paraffin

UF UFF

PU Silica gel

Silica Silica

Gelatin + acacia gum

> Cylindrical

Glass Glass Glass

epoxy

Na2FPO3 Bacteria CaC6H10O6

Bacteria Acrylic resin

Epoxy

Tung oil

Ca(OH)2 Retarder agent

Water

SAP Epoxy Epoxy Na2SiO3 MMA/ TEB

Epoxy Na2SiO3

–

5000

–

x

∕

√

x

5–20

300–700

–

800

1000

100

100

∕

Mineral oil+ Na2SiO3

CA CA

800, 1500,

—

 —

75, 75, 100

∕

3000

3000–4000

 5000–7000

—

250

∕ –

—

—

—

—

——

4.15

—

 —

x

√

x

√

40–800

—

x

√

20–70

—

x

—

120

4

x

√

x x

x

x

—

—

—

—

—

—

——

900

—

 —

x

—

x

—

120

—

x

√

50

—

x

√

50

—

x

√

50

—

x

√

125–297

—

x

—

\_

x

x

√

 1000–4000

 1000–4000

4000

x x x

x

√

*Advanced Functional Materials*

x

√

x

√

**Core material**

**Øi (μm)**

 **Øo (μm)**

 **Wall thickness**

**Length**

**Mixed**

**(mm)**

**in**

**(μm)**

**Figure 10.**

*(a) Vascular network in concrete slab panel and (b) vascular network combination with PET in field trial (Reproduced from [56]).*

#### **4.2 Autonomic microcapsule self-healing system**

Microcapsules are developed to avoid challenging issues in tubes-based capsulation systems incorporation in bulk concrete production. In this healing technique, microcapsules preserving reactive healing agents are ruptured by the forces imposed on capsules'shell due to the cracks propagation in the matrix. The released healing agent then reacts with the cementitious matrix crack surface to form healing compounds that bridge the gap and eventually heal the cracks.

Further advancement with sodium silicate encapsulated in gelatin and gum arabic shell materials (**Figure 11**) was found in recent studies [57, 60]. These microcapsules survive mixing with cement and rupture successfully upon crack formation and release sodium silicate solution. Although increasing microcapsules volume fractions in a 24% reduced the mechanical properties, the crack sealing was just under 100%. Besides, the crack depth and sorptivity coefficient were decreased by 70 and 54%, respectively. These microcapsules were also successfully implemented

*(a) Microscopic image of microcapsules (scale bars correspond to 500 μm) and (b) ruptured microcapsules*

The colloidal silica solution capsules up to 16 vol% in PC grout increased the sealing efficiency from 20% for the only PC to 85% in 28 days [62]. However, monodisperse photo-polymerised acrylate shell with hydrophilic mineral core microfluidic droplets are further advancement in the self-healing microcapsule

Although the direct addition of potential minerals to the concrete mix improves autogenous self-healing performance, protecting those minerals in initial mixing may further enhance the healing process. With this in mind, pellets of potential healing mineral agents have been used for improved concrete self-healing.

Sisomphon et al. [47] used expanded lightweight clay aggregates (LWAs) impregnated with a solution of sodium mono fluorophosphate (Na2FPO3, Na-MFP) and coated by cement paste layers. The entire mechanism is schematically presented in **Figure 12a**. Pellets with expansive minerals such as a reactive MgO were spraycoated (10–50 μm) with polyvinyl alcohol (PVA) to produce PVA-coated MgO pellets for self-healing concrete applications (**Figure 12b**). A PVA-coated granulated CSA (calcium sulpho aluminate)-based expansive mineral was used for improving the self-healing performance of cementitious materials [48]. Replacement of CSA pellets was up to 10% by wt. of cement and mortar was prepared with 1:3 cement-to-sand ratio and w/c = 0.5. Cracks in the range of 0.1–0.2 mm were healed completely within 14 days whereas larger crack >0.2 healed within 16 days. Granules of expansive self-healing agent coated with an extra layer of cement compounds were investigated by [64]. The self-healing concept is schematically

in the filed trail of a road improvement scheme by M4L project [61].

*appearing as 'wet' spots on the digital image of the split face (Both reproduced from [60]).*

*Self-Healing Concrete and Cementitious Materials DOI: http://dx.doi.org/10.5772/intechopen.92349*

**4.3 Coated minerals (pellets and granules) for self-healing**

field [63].

**205**

**Figure 11.**

The compatibility of microcapsules with bulk concrete depends on a wide variety of factors. Major influencing factors are the size and volume fraction of microcapsules used, the capsules' mechanical properties and interlock properties between the capsules and the surrounding materials [57]. The shape of the embedded capsule is another major factor that should be considered for compatibility issues. Spherically shaped capsules provide a more controlled and enhanced release of the healing agent upon breakage. It also reduces the stress concentrations around the void left from the empty capsule. However, a tubular capsule can cover a larger internal area of influence on the concrete for the same volume of a healing agent (higher surface area to volume ratio).

Yang et al. have investigated methyl methacrylate (MMA) as a monomer and triethylborane (TEB) as the healing agent and the catalyst [25]. In the investigation, about 50.2 and 66.8% reduction in permeability has been achieved within 3 and 30 days, respectively. Microscopic imaging confirms that some ruptured microcapsules existed and filled the cracks of the sample after 80% ultimate compressive strength at 28 days.

About 2% crystalline sodium silicate in polyurethane-encapsulated microcapsules with a diameter ranging from 40 to 800 μm increased 24% mechanical load recovery compared to 12% in the control samples [58]. However, the compressive strength of the composite reduced by 12% compared to that of the control mix. In the concrete containing microcapsules, sodium silicate reacts with calcium hydroxide of cement and produces a calcium-silica-hydrate (C-S-H) gel that heals the cracks partially. The C-S-H further reacts with dissolved CO2 in water and sodium oxide, which produced calcium carbonate. This is similar to the main hydration phase of cement, which causes strengthening.

Sodium silicate encapsulated in double-walled polyurethane/urea-formaldehyde (PU/UF) was reported in [59]. The addition of 2.5 and 5% microcapsules resulted in about 24 and 35% healing efficiency based on the crack depth measurements.

**Figure 11.**

**4.2 Autonomic microcapsule self-healing system**

area to volume ratio).

**Figure 10.**

*(Reproduced from [56]).*

*Advanced Functional Materials*

strength at 28 days.

**204**

phase of cement, which causes strengthening.

Microcapsules are developed to avoid challenging issues in tubes-based capsulation systems incorporation in bulk concrete production. In this healing technique, microcapsules preserving reactive healing agents are ruptured by the forces imposed on capsules'shell due to the cracks propagation in the matrix. The released healing agent then reacts with the cementitious matrix crack surface to form healing compounds that bridge the gap and eventually heal the cracks.

*(a) Vascular network in concrete slab panel and (b) vascular network combination with PET in field trial*

The compatibility of microcapsules with bulk concrete depends on a wide variety of factors. Major influencing factors are the size and volume fraction of microcapsules used, the capsules' mechanical properties and interlock properties between the capsules and the surrounding materials [57]. The shape of the embedded capsule is another major factor that should be considered for compatibility issues. Spherically shaped capsules provide a more controlled and enhanced release of the healing agent upon breakage. It also reduces the stress concentrations around the void left from the empty capsule. However, a tubular capsule can cover a larger internal area of influence on the concrete for the same volume of a healing agent (higher surface

Yang et al. have investigated methyl methacrylate (MMA) as a monomer and triethylborane (TEB) as the healing agent and the catalyst [25]. In the investigation, about 50.2 and 66.8% reduction in permeability has been achieved within 3 and 30 days, respectively. Microscopic imaging confirms that some ruptured microcapsules existed and filled the cracks of the sample after 80% ultimate compressive

About 2% crystalline sodium silicate in polyurethane-encapsulated microcapsules with a diameter ranging from 40 to 800 μm increased 24% mechanical load recovery compared to 12% in the control samples [58]. However, the compressive strength of the composite reduced by 12% compared to that of the control mix. In the concrete containing microcapsules, sodium silicate reacts with calcium hydroxide of cement and produces a calcium-silica-hydrate (C-S-H) gel that heals the cracks partially. The C-S-H further reacts with dissolved CO2 in water and sodium oxide, which produced calcium carbonate. This is similar to the main hydration

Sodium silicate encapsulated in double-walled polyurethane/urea-formaldehyde (PU/UF) was reported in [59]. The addition of 2.5 and 5% microcapsules resulted in about 24 and 35% healing efficiency based on the crack depth measurements.

*(a) Microscopic image of microcapsules (scale bars correspond to 500 μm) and (b) ruptured microcapsules appearing as 'wet' spots on the digital image of the split face (Both reproduced from [60]).*

Further advancement with sodium silicate encapsulated in gelatin and gum arabic shell materials (**Figure 11**) was found in recent studies [57, 60]. These microcapsules survive mixing with cement and rupture successfully upon crack formation and release sodium silicate solution. Although increasing microcapsules volume fractions in a 24% reduced the mechanical properties, the crack sealing was just under 100%. Besides, the crack depth and sorptivity coefficient were decreased by 70 and 54%, respectively. These microcapsules were also successfully implemented in the filed trail of a road improvement scheme by M4L project [61].

The colloidal silica solution capsules up to 16 vol% in PC grout increased the sealing efficiency from 20% for the only PC to 85% in 28 days [62]. However, monodisperse photo-polymerised acrylate shell with hydrophilic mineral core microfluidic droplets are further advancement in the self-healing microcapsule field [63].
