1. Introduction
Concrete structures are susceptible to cracking due to various factors such as shrinkage and thermal stresses, mechanical loading, chemical attack, and environmental exposure. The presence of cracks could significantly compromise the durability of concrete structures since they create pathways for the ingress of detrimental agents such as chloride and sulfate ions. Therefore, periodic inspection, preventive maintenance, and repair and rehabilitation are often required. According to the latest Canadian infrastructure report card published in 2019 [
1], a concerning portfolio of public infrastructure is in critical condition, requiring immediate rehabilitation. Moreover, most civil infrastructure assets continue to deteriorate prematurely and are expected to fall into similar conditions if appropriate repairs are not made in a timely manner. Similar cases have been reported in other countries. For instance, it is estimated that £40 billion is spent annually in the UK for infrastructure maintenance, of which a substantial portion is used to repair deteriorating concrete structures [
2]. On the other hand, according to the ASCE, the US needs to spend
$4.59 trillion by 2025 to fix its deteriorating civil infrastructure [
3].
In recent years, major efforts have been devoted to developing concrete with self-healing abilities [
4,
5,
6,
7,
8,
9,
10,
11,
12,
13]. Several studies have investigated the effects of various additives on crack self-healing in concrete under a single environmental condition (typically when concrete is submerged in water). For example, Li et al. [
13] studied the effect of ground granulated blast furnace slag on the crack self-healing of cement mortar incorporating crystalline additives under full water submersion. The crack self-healing ability in specimens containing a crystalline admixture was significantly increased compared with that of the control specimens without additives. Similarly, Rong et al. [
7] explored the effect of the concentration of Bacillus pasteurii on the crack self-healing of cement-based materials and found that the healing efficiency for wide cracks was much lower than that of thin cracks. Qureshi et al. [
6] reported that partially replacing Portland cement with expansive minerals, including bentonite clay, magnesium oxide, and quicklime, improved the self-healing properties of cement paste prisms submerged in water.
Limited studies have so far investigated the efficacy of self-healing in concrete under variable environments. For instance, Roig-Flores et al. [
14] studied the self-healing of concrete samples incorporating crystalline additives (CA) exposed to various environments. Their results showed that depending on the exposure condition, the self-healing of concrete incorporating CA could exhibit different healing behavior. Negligible self-healing was found in all samples exposed to constant temperature and relative humidity, demonstrating that the presence of liquid water is essential for self-healing. Sisomphon et al. [
15] investigated the effect of environment exposure on the self-healing of cementitious composites with strain hardening ability and made with different cementitious materials. They reported that air curing adversely affected the self-healing process. Likewise, Wang et al. [
16] reported no crack healing in bacteria-based concrete maintained at 60% RH and 95% RH. Due to drying shrinkage, the final crack in specimens stored at 60% RH was even wider than their initial size. According to Wang et al. [
16], the deficiency of water supply to cracked specimens was the main reason for the lack of self-healing.
The influence of superabsorbent polymers (SAPs) as an admixture in cementitious materials to mitigate shrinkage cracking via providing internal curing has been a subject of research for many years [
17,
18,
19,
20,
21]. For instance, Jensen and Hansen [
18] reported that autogenous shrinkage after the setting of hardening high-performance concrete and subsequent cracking due to restraint could be prevented when SAPs were used as water-entraining admixtures. Similar findings were presented by Soliman and Nehdi [
20] and Wang et al. [
19]. Superabsorbent polymers are cross-linked and copolymerized polyacrylates that can absorb more than 100 times their own mass of liquid supplied by the surrounding environment and then retain it intact without dissolving [
19].
Recently, several studies have investigated the effect of SAPs on autogenous self-healing of cement-based materials [
22,
23,
24,
25,
26]. For example, Snoeck et al. [
22] examined the water penetration in cementitious materials incorporating SAPs using neutron radiography. Their results showed that specimens containing SAPs exhibited lower total moisture uptake than the reference specimens without SAPs. Snoeck and De Belie [
23] studied the repeated autogenous self-healing in strain-hardening cementitious materials containing SAPs. They found that SAPs promote autogenous self-healing and mechanical recovery of the strain-hardening cementitious materials. Lee et al. [
26] reported that SAPs can re-swell and seal cracks in concrete.
Several studies have also explored the influence of SAPs on autogenous self-healing under different environments such as cycles of wetting and drying, continuous exposure to a constant relative humidity, and/or continuous immersion in water. For example, Lefever et al. [
27] evaluated the self-healing of cracks in mortars incorporating SAPs and nano silica under cycles of wetting and drying using microscopy measurements and water permeability testing. Their results showed that self-healing efficiency was markedly increased in mortar specimens containing SAPs. Park and Choi [
28] investigated the ability of cementitious materials containing crystalline admixtures and SAPs to attain crack closing due to self-healing when immersed in tap water at a temperature of 20 ± 3 °C by means of water flow test and crack closing test using an optical microscope. They found that using crystalline admixtures with SAPs accelerated the crack sealing.
Sidig et al. [
29] investigated the effectiveness of SAPs and superplasticizer in the self-healing of cementitious materials immersed in water at room temperature using X-ray tomography images. They found that self-healing efficiency increased in specimens with 2.2% of SAPs. Snoeck et al. [
24] used X-ray computed microtomography to investigate autogenous healing of cementitious materials promoted by SAPs. In their study, three curing conditions under a constant temperature were used, including (a) storing the specimens at a relative humidity (RH) of 60%, (b) at RH > 90%, and (c) exposing the specimens to cycles of wetting and drying (1 hr in water and 23 hr in the lab at 60% RH). Their results showed that specimens exposed to cycles of drying and wetting achieved the highest crack self-healing. Similarly, Snoeck et al. [
30] used Nuclear Magnetic Resonance NRA to quantify autogenous self-healing in cement-based materials incorporating SAPs and exposed to similar environments by Snoeck et al. [
24]. Chindasiriphan et al. [
31] studied the effect of fly ash and SAPs on concrete self-healing ability exposed to either continuous water immersion or wet-dry conditions. They found that specimens exposed to continuous water immersion attained more self-healing than the specimens that were subjected to cycles of wetting and drying.
However, there is a lack of studies in the open literature that have investigated and quantified the change in the entire crack volume due to autogenous self-healing in cement-based materials containing SAPs when exposed to a combined change in temperature and relative humidity. Therefore, in the present study, the entire crack volume changes due to self-healing in cementitious materials incorporating SAPs exposed to an environment with a combined change in temperature and relative humidity was investigated and quantified using X-ray µCT scans. The results of segmentation and quantification analysis were compared with the influence of other environments, including cyclic wetting and drying, and/or continuous water submersion. In addition, the effect of different SAP contents on the mechanical properties and pore structure of concrete was evaluated. The results should provide a robust basis for the quantification of the self-healing potential in cement-based materials incorporating SAPs under combined environmental conditions.
2. Experimental Program
2.1. Materials
Mortar specimens reinforced with 1% of polyvinyl alcohol (PVA) fibers were made with general use normal Portland cement (CSA A3001). The physical and chemical properties of the cement and sand are summarized in
Table 1 and
Table 2.
In the present study, cross-linked sodium polyacrylate SAP (LiquiBlock™ HS Fines) was used. The physical and chemical properties of the SAP are given in
Table 3. As shown in
Table 4, a varying dosage of SAP expressed as a percentage of the cement mass was incorporated in the mortar mixture. In addition, the amount of water and sand were kept constant at a water-to-cement ratio (w/cm) of 0.35 and sand-to-cement mass ratio (s/c) of 2 in all mixtures to investigate the effect of each SAP dosage on the mechanical properties, pore structure, self-healing capacity of the tested mortars.
To warrant consistent distribution of SAPs in the cementitious matrix, the PVA fibers and SAPs were first dry mixed with the cement for about 1 min. The sand was then included, and dry mixing progressed for another 1 min. Subsequently, water was added, and a superplasticizer (MasterGlenium 7700—BASF) was used to adjust workability at a constant flow for all mixtures. The amount of water was kept constant in all mixtures to compare the effect of each SAP dosage on the pore structure, crack self-healing, tensile and compressive strength of the tested mortars. The flow was determined using Standard Test Method for Flow of Hydraulic Cement Mortar (ASTM C1437). The flow in all mortar mixtures was 110 ± 5%. All the ingredients were then mixed for 2 min using a Hobart N50 mixer (Hobart Corporation, OH, USA). Cylindrical specimens (50-mm in diameter and 100-mm in height) were cast to determine the compressive and splitting tensile strength of the tested mortars. Cylindrical specimens incorporating 1% SAPs with 50 mm in diameter and 25 mm in height were made for X-ray µCT scan examination. Specimens were cured for 28 d in an environmentally controlled room at a temperature of 21 ± 1 °C and relative humidity ≥ 95%. After 28 days, all specimens were removed from the curing room and cracked.
2.2. Crack Creation and Measuring
For the computed tomography scan samples, the crack was created using a screw jack (
Figure 1). The crack width was controlled via a calibration ruler as per the method described by Roig-Flores et al. [
14]. In addition, an optical microscope was used to measure the width of the surface part of cracks. Based on the crack width range, specimens were divided into three groups. For each environmental exposure, three groups of specimens with three different values of crack width were tested. The first group (small crack) consisted of three specimens having crack widths of 50 µm to 150 µm. The second group (medium crack) consisted of three specimens with crack widths of 150 µm to 300 µm. Finally, the third group (large crack) included three specimens with crack widths of 300 µm to 500 µm.
2.3. Compressive and Tensile Strength
Three samples from each mixture were tested for compressive strength after 28 days of curing according to ASTM C39. In addition, another three (3) samples from each mixture were tested for tensile strength after 28 days of curing according to ASTM C496.
2.4. Environmental Exposure
The present study explores the crack width changes due to the self-healing of cementitious materials incorporating SAPs under a combined change in temperature and relative humidity. A set of samples from each group (small, medium, and large crack width) were subjected to different temperature and relative humidity cycles using a controlled climate chamber, as shown in
Table 5. A similar set of samples were subjected to cyclic wetting and drying at 20 ± 2 °C, which consisted of storage in water for 4 d, followed by drying in the laboratory environment for 4 d. Finally, another set of samples (with similar crack width) was continuously submerged in water.
2.5. Pore Structure Analysis
The effect of SAPs on the mortar pore structure was examined using a Micromeritics AutoPore IV 9500 (Micromeritics Instrument Corporation, Norcross, GA, USA) mercury intrusion porosimeter. In addition, the effect of different environmental exposures on the pore structure of mortars was investigated. Before testing, pieces from previously cracked specimens were dried in a glass desiccator until a constant mass was reached.
2.6. Segmentation and Quantification Analysis of Crack
To measure the change in the entire volume of the crack due to self-healing, a Nikon XT H 225-ST industrial Computed Tomography (CT) scanning system was used. The XT H 225-ST provides detailed high-resolution inner and outer 3D images. The specimens were scanned before and after each environmental exposure. An advanced segmentation and quantification analysis software (Dragonfly 3.5) was used to calculate the change in the volume of crack due to self-healing.
2.7. Composition and Morphology of Healing Product
The composition and morphology of the healing products were investigated using a high-definition Hitachi SU3500 scanning electron microscope (SEM) imaging system coupled with energy dispersive X-ray (EDX) analysis. Before testing, all samples that exhibited self-healing were dried using a glass desiccator and then coated with gold.