**4.4 Characterization of self-healing concrete**

The major cause of mechanical failure in concrete is cracks. A crack not only lowers strength, but exposes the reinforcing steel components to corrosion. Selfhealing targets crack closure or prevention of crack propagation to retain strength and reduce water permeability in order to maintain durability. Self-healing efficiency of concrete in hardened form has been determined by conducting tests at macro-, micro-, and nanoscale levels. Majority of the researchers evaluated the self-healing efficiency at macrostructural level, some at microscale level and very few authors at nanostructure level [42].

#### *Advanced Functional Materials*

Just like other hard materials, quantification of healing efficiency on one hand relies on fracture mechanics tests, which involve creation of controlled cracks on concrete. The mechanical characterization procedure follows the sequence of controlled crack initiation in the matrix using the standard compression and bending tests, healing processing and retesting of the healed concrete using the same pre-cracking procedure [91]. The initiation of cracks without causing failure at certain level of stress and detection of crack development are very important in testing of concrete. To control crack initiation, some authors have applied a notch in the middle point of test sample [92] while some used three-point bending or four-point bending technique found to be more effective in crack initiation without causing failure [93–95].

Besides conducting compressive and flexural tests [45, 96, 97], the performance of concrete has also been assessed by other mechanical tests such as split tensile and toughness tests [98, 99] and stiffness tests [100]. Detection of crack initiation and its degree have been carried out using nondestructive complementary tests such as acoustic emission analysis [101], linear variable differential transformer [102] and ultrasonic pulse velocity [103, 104].

On the other hand, efficiency is also evaluated by conducting permeability tests on the pre-cracked concrete. Permeability test aims at determining how effective self-healing concrete can shield steel bars from corrosion [105]. Permeability tests are performed in simulated environments containing fluids such as chlorine or water under certain temperature [94, 105].

Microscale tests are employed to identify and characterize the deposited materials within cracks in the concrete after self-healing and are used to complement and reinforce reliability of macroscale tests [42]. These deposited materials are the calcium carbonate precipitation by different bacterial strains, hydration product as well as polymerized products. Several of these tests are conducted using the following sensitive equipment: scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), and X-ray diffractometer (XRD). SEM is used to identify the morphology of the deposited materials within the cracks [106]. Selfhealing performance is also assessed using Raman spectroscopy [16]. Furthermore, nanostructure test has been used to evaluate self-healing efficiency of concrete [107]. Tests conducted at nanoscale help in the determination of bonding strength at the interface between the deposited materials in the cracks and the concrete.

## **4.5 Characterization of self-healing coatings**

The basic function of a traditional coating is to shield material surfaces, especially metal surfaces from fast corrosion in the environment. Smart coatings provide a spontaneous protection to metal surfaces upon chemical or mechanical damage [108]. This is achieved by release of inhibiting species in the coating architectures, which inhibits electrochemical interaction between the metal substrates and the environment [109]. The characterization techniques highlighted here are those traditionally used to study corrosion, but adapted for studying self-healing coatings on metals.

The testing techniques used to ascertain the self-healing properties of coatings have been generally grouped into two: electrochemical [55, 109, 110] and non-electrochemical techniques or physicochemical characterization as shown in **Figure 3** [55, 109]. Electrochemical techniques enable the quantification of self-healing efficiency by providing important information about kinetics of protection; formation of protective films and isolation of redox species [109]. The electrochemical techniques are further divided into conventional (global) electrochemical methods and localized electrochemical techniques [55, 109].

**227**

efficiency

Eq. (8):

**Figure 3.**

η

current density without coating.

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials*

The global electrochemical methods provide kinetic and deterministic informa-

*<sup>p</sup>* of a protective polymer coating on an iron electrode surface using

<sup>0</sup> 100 *corr corr*

*x*

*<sup>i</sup>* (8)

*corr i* the is corrosion

*corr i i*

Information about evolution of damage and healing of damage is provided by the local electrochemical methods, including micro-capillary cell, scanning vibrating electrode technique (SVET), scanning ion-selective electrode technique (SIET), scanning electrochemical microscope (SECM) or scanning probe technique (SKP)

Each of these methods has advantages and limitations and should be used in combinations for detailed study of self-healing processes in coatings. Besides this,

tion on self-healing processes, but they do not supply information about local reactions taking place at the site of damage. They include potentiodynamic polarization (PP), open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and they are the popular methods used for the study of corrosion and selfhealing research [109]. For instance, PP and EIS give quantitative results about self-healing process enabling the determination of corrosion rate and protection efficiency [55]. Aramaki [111] employed the PP method to estimate the protection

*Techniques for evaluating self-healing coatings adapted from [55].*

η <sup>−</sup> <sup>=</sup> 0

*p*

where *corr i* is the corrosion current density with coating and <sup>0</sup>

and localized electrochemical impedance spectroscopy (LEIS).

*DOI: http://dx.doi.org/10.5772/intechopen.93031*

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials DOI: http://dx.doi.org/10.5772/intechopen.93031*

#### **Figure 3.**

*Advanced Functional Materials*

causing failure [93–95].

ultrasonic pulse velocity [103, 104].

water under certain temperature [94, 105].

**4.5 Characterization of self-healing coatings**

Just like other hard materials, quantification of healing efficiency on one hand

Besides conducting compressive and flexural tests [45, 96, 97], the performance of concrete has also been assessed by other mechanical tests such as split tensile and toughness tests [98, 99] and stiffness tests [100]. Detection of crack initiation and its degree have been carried out using nondestructive complementary tests such as acoustic emission analysis [101], linear variable differential transformer [102] and

On the other hand, efficiency is also evaluated by conducting permeability tests on the pre-cracked concrete. Permeability test aims at determining how effective self-healing concrete can shield steel bars from corrosion [105]. Permeability tests are performed in simulated environments containing fluids such as chlorine or

Microscale tests are employed to identify and characterize the deposited materials within cracks in the concrete after self-healing and are used to complement and reinforce reliability of macroscale tests [42]. These deposited materials are the calcium carbonate precipitation by different bacterial strains, hydration product as well as polymerized products. Several of these tests are conducted using the following sensitive equipment: scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), and X-ray diffractometer (XRD). SEM is used to identify the morphology of the deposited materials within the cracks [106]. Selfhealing performance is also assessed using Raman spectroscopy [16]. Furthermore, nanostructure test has been used to evaluate self-healing efficiency of concrete [107]. Tests conducted at nanoscale help in the determination of bonding strength at the

interface between the deposited materials in the cracks and the concrete.

The basic function of a traditional coating is to shield material surfaces, especially metal surfaces from fast corrosion in the environment. Smart coatings provide a spontaneous protection to metal surfaces upon chemical or mechanical damage [108]. This is achieved by release of inhibiting species in the coating architectures, which inhibits electrochemical interaction between the metal substrates and the environment [109]. The characterization techniques highlighted here are those traditionally used to study corrosion, but adapted for studying self-healing

The testing techniques used to ascertain the self-healing properties of coatings have been generally grouped into two: electrochemical [55, 109, 110] and non-electrochemical techniques or physicochemical characterization as shown in **Figure 3** [55, 109]. Electrochemical techniques enable the quantification of self-healing efficiency by providing important information about kinetics of protection; formation of protective films and isolation of redox species [109]. The electrochemical techniques are further divided into conventional (global) electro-

chemical methods and localized electrochemical techniques [55, 109].

relies on fracture mechanics tests, which involve creation of controlled cracks on concrete. The mechanical characterization procedure follows the sequence of controlled crack initiation in the matrix using the standard compression and bending tests, healing processing and retesting of the healed concrete using the same pre-cracking procedure [91]. The initiation of cracks without causing failure at certain level of stress and detection of crack development are very important in testing of concrete. To control crack initiation, some authors have applied a notch in the middle point of test sample [92] while some used three-point bending or four-point bending technique found to be more effective in crack initiation without

**226**

coatings on metals.

*Techniques for evaluating self-healing coatings adapted from [55].*

The global electrochemical methods provide kinetic and deterministic information on self-healing processes, but they do not supply information about local reactions taking place at the site of damage. They include potentiodynamic polarization (PP), open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and they are the popular methods used for the study of corrosion and selfhealing research [109]. For instance, PP and EIS give quantitative results about self-healing process enabling the determination of corrosion rate and protection efficiency [55]. Aramaki [111] employed the PP method to estimate the protection efficiency η *<sup>p</sup>* of a protective polymer coating on an iron electrode surface using Eq. (8):

$$\eta\_p = \frac{\dot{t}\_{corr}^\circ - \dot{t}\_{corr}}{\dot{t}\_{corr}^\circ} \text{мом} \tag{8}$$

where *corr i* is the corrosion current density with coating and <sup>0</sup> *corr i* the is corrosion current density without coating.

Information about evolution of damage and healing of damage is provided by the local electrochemical methods, including micro-capillary cell, scanning vibrating electrode technique (SVET), scanning ion-selective electrode technique (SIET), scanning electrochemical microscope (SECM) or scanning probe technique (SKP) and localized electrochemical impedance spectroscopy (LEIS).

Each of these methods has advantages and limitations and should be used in combinations for detailed study of self-healing processes in coatings. Besides this, these electrochemical techniques should be complemented by conducting non-electrochemical or physicochemical analysis. Physicochemical analysis is majorly aimed at studying mechanism of self-healing coating protection—elemental analysis, interaction mechanisms between the metal substrate and the coating, morphology or phase transformation of the coating before and after healing. Surface analysis can be carried out with OM, SEM and CLSM, while energy dispersive X-ray (EDX), EPMA and XPS are useful for elemental analysis. The detailed description of the above test methods can be found in Refs. [55, 109, 112].

## **4.6 Challenges in the characterization of self-healing materials**

Developing an artificial material to self-heal like a biological system comes as a huge task to the scientist or engineer. But more challenging is proving the material's self-healing capability and suitability for a particular application via characterization. Different approaches are used to achieve self-healing in different material classes based on their inherent properties. A self-healing material is an advanced material, a new product with new properties different from its original properties. It is important that the characteristics of the new product should be determined qualitatively or quantitatively. It is also important to monitor the changes during processing that led to the new properties. As a result, specific characterization methods are required to quantify self-healing efficiencies in each of the material classes. Most times, the equipment might not be common and sometimes unavailable unless improvised.

Unlike other characterization methods, evaluation of self-healing materials in most cases entails inducing a minimal expected mode of damage or failure during utilization on the material and using different methods to determine the extent of recovery/restoration of properties compared to original or virgin materials properties and to understand the mechanism of recovery. The methods of inducing the damage are different for the various material classes and even in the same material group. Inducing the appropriate damage simulating the real-life scenario is challenging. Also tasking is the selection of an appropriate testing procedure from an extensive range of known materials testing procedures that is adaptable and suitable for a particular self-healing concept. The field of self-healing materials is relatively new but a richly rewarding venture. The understanding of self-healing mechanisms in a variety of self-repairing material classes is still evolving. So also are the characterization methods needed to elucidate the dynamics of self-healing process. The challenges of harmonization of these methods in various research groups are yet to be resolved.

More so, the mode of damage is different and unique to the damaged material and its intended applications. Even within the same material class, there are various self-healing approaches and evaluation strategies. This makes the adopted routine of assessing the performance of the modified material and comparing its properties to the unmodified, virgin material complex. This makes it equally arduous to establish a common testing procedure for similar or for different materials classes.
