**3.1 Existing condition influence in autogenous self-healing**

Autogenous self-healing of concrete is significantly influenced by its age, internal stress and curing conditions. Early age concrete naturally heals rapidly due to autogenous healing. Concrete prisms with cracks up to 50 μm were autogenously healed under 0.1, 1 and 2 Mpa compressive stresses [5] (**Figure 5a**). The crack face comes into contact by the impelled compressive stress. Hence, the concrete specimens cured under any amount of compressive stress healed much better than specimens cured under no compression stress (**Figure 5b**). Only a specific amount of compression is required to keep the crack faces in contact. Samples that are submerged in water during curing recovered their strength. In contrast, specimens stored in 95% RH for 3 months did not heal at all. This is due to insufficient hydration in the high humid condition, which is not enough to trigger the healing process.

### **3.2 Fibre action in autogenous self-healing**

Fibres can restrict the propagation of crack width, and smaller crack width is favourable for enhanced autogenous healing in concrete. Fibre is a common feature in Fibre-Reinforced Composite Concrete (FRCC) and ECC. Randomly distributed fibres can bridge over cracks, which can decrease the crack width and block the migration of aggressive agents (e.g. chloride ions and CO2) [6, 37]. These properties improve the autogenous self-healing capacity of concrete and composites. A series of wetting and drying cycles on ECC was carried out by [6] to mimic self-healing performance in outdoor environments. Through self-healing, crack-damaged ECC recovered 76–100% of its initial resonant frequency value and attained a distinct rebound in stiffness. The tensile strain capacity after self-healing recovered close to 100% that of virgin specimens without any preloading. This was found even for the specimens deliberately pre-damaged with microcracks by loading up to 3% tensile strain. It takes about four to five wet-dry cycles to attain the full benefit of self-healing. The use of high cement content, low water-to-cement ratio also increases the autogenous self-healing capacity of ECC. However, FRCC, ECC and

#### **Figure 5.**

*(a) Application of compression and (b) stress-displacement curve of specimens after healing with and without applied compressive stress. (Both figures reproduced from [5]).*

HFRCC are costly and maintaining homogeneity of fibres in the matrix for consistent self-healing is challenging.

**Minerals Composition Damage type Curing**

Up to 10% (concrete) 3 PB,

cement (concrete)

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

> PC with 10% CSA and 1.5% CA

8% individual combination up to

CSA 4.44 and 15.24% of

14%

FA, SF, CA OPC, OPC + 30%FA, OPC + 10%SF, OPC + 1%CA

FA 15–20% with PC (paste)

FA 5–15% wt. of sand (concrete)

FA, slag 30–40% of cement (mortar)

L, slag, FA 30, 50% FA; 50, 75, 85% slag (paste/ mortar)

Slag 66% of cement (paste)

Bentonite Nanoclay in mortar

as internal water reservoir

2% PVA by vol. Length = 8 mm, dia = 40 μm

(3%) on fly ash-PC cement pastes

Na-MFP and PC coated (mortar)

10% by wt. of cement (mortar, 1:3)

CSA PVA coated, up to

mechanical

Compression, sp. tensile

Splitting tensile test

Shrinkage microcracks

BFS OPC + 50% BFS Mechanical Water Product formation is

3 PB, mechanical

Sliced, mechanical

1–2% of cement 4 PB Water, open

Tension force Still/

CSA, <sup>a</sup> H, <sup>b</sup> A,

CSA, CA, a H, <sup>b</sup> A, <sup>c</sup> L,

Mont.

Silica, <sup>d</sup>

c

Bentonite, slag, <sup>c</sup> L

Quicklime, FA

Expanded clay LWAs

CA: cement + sand + microsilica

**197**

CEA, bentonite, CA

c L, Mont. **condition**

continuous flow water

Water, wetdry, air, freeze–thaw

Freeze–thaw Water Improve <sup>e</sup>

Shrinkage Water Improvement in

Ca(OH)2 solution

dry cycle, air

Mechanical Water Increased SiO2

Mechanical Water Absorption decrease

3 PB Water <100 μm in 11d,

air

4 PB Water, wet-

Mechanical Water Enhanced hydration for

Sp. tensile test Water 100–400 μm in 56 d

**Performance (healed crack width in time**

Reduced flow in 100 μm cracks, continuous flow is efficient

220 μm in 2 weeks

larger cracks heal efficiently with SF

91, 182 and 364 d

three times faster for

compressive strength

60% of 10 μm in 240 h C-S-H, ettringite, hydrogenate etc.

Nanoclay improves the reloading deflection

sodium, phosphorous and fluoride, CH

100–200 μm in 14d, >200 μm in 16d

60% cracks sealed under open air condition

Water 200 μm in 42d [12]

self-healing

capacity

solubility extra Ca(OH)2

90% in 28d

CEM I

DME over

CEA (individually

**Source**

[8]

[38]

[9]

[11]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[14]

[47]

[48]

[49]

**etc.)**

Water 160–220 μm in 33d Calcite, CASH

Calcite

efficient) silica, bent., CA (combination is efficient)

d

Water 50 μm in 12d

Water Meso-macro pores at
