Performance of Capsules in Self-Healing Cementitious Material
Abstract
:1. Introduction
2. Performance of Self-Healing Cementitious System
2.1. Experimental Studies
2.1.1. Capsule
2.1.2. Healing System
- i.
- Capsules formed using UF are found to range between 10 to 1000 mm in diameter, 0.2 to 8 mm in thickness, and 8 to 39 in the ratio of radius to thickness except for the capsules that were used by Gilford et al. [71] whose ratio is 107 to 5000. Both the radius-to-thickness ratio and diameter of the capsules affect their ability to withstand forces, to develop a mechanical bonding, as well as to effectively deliver the healing agent. The spectrum provides little information and thus confidence on what geometrical properties the capsules need to possess for an effective self-healing system.
- ii.
- UF encapsulating epoxy resin [39,40,53,65,66], Dicyclopentadiene (DCPD), Sodium Silicate [71], and Calcium Nitrate Tetrahydrate [41], have been added to the mortar with varying mixture composition and properties. The reported 28-day mortar compressive strength ranges from 28 to 56 MPa, and the flexural strength from 8.4 to 10.6 MPa. The cementitious mix design is seldom documented in these studies and only some studies reported the mechanical properties of the hardened mixture. The ratio of water to cement and cement to sand, the cement content, capsule content, and other additives are found to significantly vary among the documented studies without any rationale to the design.
- iii.
- Capsules contents are found to range between 0.5 and 12% of the cement content. The broad range of the capsule content used in these studies combined with the absence of any rational to designing self-healing system can discourage the concrete construction industry from experimenting with the self-healing system.
- iv.
- UF, MUF, and PF are used for encapsulation, with UF being the most common, and epoxy resin, DCPD, Sodium Silicate, and Calcium Nitrate Tetrahydrate used as healing agents with epoxy being the most common. The diverse chemical composition and properties of the healing agents provide options, but with no justification or guidance on how to select the healing agents.
- v.
- Test methods not only vary in scale from recovery of mechanical properties and transport properties to recovery of matrix microstructure which includes pores size distribution and porosity, but also the varying ages at which matrix was pre-cracked and tested. These variabilities raise many questions: Is there a difference in material response between mechanically and chemically triggered cracks, i.e., between cracks induced by external loads versus those caused by dimensional changes? Does the cementitious material degree of hydration affect the healing efficiency of the system, specifically the capsule bond strength? The aim of these performance tests appears to test the mechanical and/or durability recovery of mature concrete and provide zero measure of the healing performance at an early age when the cementitious system is most vulnerable to cracking.
- vi.
- Healing performance indicators of the systems appear to be all over the place where the following measures have been reported: average recovery rate, recovery rate, healing rate, crack healing ratio, and healing ratio. For reference, the rate is a measure of two unlike units and should not be used to compare two measurements of the same units. Alternatively, the ratio can provide a measure of the healed system to the uncracked system. Moreover, the reported experimental measurements are concerning as without a measure of certainty in the form of standard deviation, there is zero confidence in their measured values.
2.2. Numerical Studies
3. Methodology
3.1. Finite Element Model
3.2. Damage Model
4. Results, Analyses, and Discussion
5. Conclusions
- (1)
- There is a need for developing standard test methods to measure the capsules geometry, being diameter and thickness, and mechanical properties, and the mechanical properties of the interface between the mortar and the capsule.
- (2)
- There is a need for developing standard test methods for measuring the survival rate of capsules during mixing and placing of concrete as a pre-requisite to determining efficiency of the self-healing cementitious system.
- (3)
- There is no clear definition of self-healing efficiency nor a define method for measuring self-healing efficiency of mortar and other cementitious systems.
- (4)
- Inconsistencies in the reported self-healing mortar performance are attributed to the inter-relationship between the geometry of the capsules, the properties of the capsules, the properties of the mortar, and the pre-crack width induced in the mortar.
- (5)
- The capsules’ radius to thickness is found to significantly affect the capsule mode of failure.
- (6)
- The crack opening affects the capsule failure mode differently depending on the age and composition of the mortar, and properties of the capsule.
- (7)
- The age of the mortar is important when testing the self-healing system, especially when mortar is susceptible to cracking at early age. It is pivotal to check the status of the capsules due to early-age cracking before moving forward with measuring the efficiency of the self-healing system.
Author Contributions
Funding
Informed Consent Statement
Conflicts of Interest
References
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Shell Material | Healing Agent | Reference |
---|---|---|
Perspex | Epoxy Resin | [62] |
Ceramic | Polyurethane (PU) | [17] |
Methyl Methacrylate (MMA) | [29] | |
Glass | Epoxy Resin | [62] |
Methyl Methacrylate (MMA) | [25] | |
Cyanoacrylates (CA) | [26] | |
Borosilicate glass | Methyl Methacrylate (MMA) | [29] |
Polyurethane (PU) | [17,38,63] | |
Cyanoacrylates (CA) | [15,16] | |
Quartz glass | Polyurethane (PU) | [50] |
Urea-formaldehyde (UF) | Epoxy Resin | [39,40,53,64,65,66] |
Dicyclopentadiene (DCPD) | [67,68,69,70,71] | |
Melamine Urea-formaldehyde (MUF) | Epoxy Resin | [33] |
Dicyclopentadiene (DCPD) | [72] | |
Epoxy Resin | [73] | |
Phenol-formaldehyde (PF) | Dicyclopentadiene (DCPD) | [32,34] |
Epoxy Resin | [74] | |
Dicyclopentadiene (DCPD) | [75,76,77] | |
Polystyrene (PS) | Methyl Methacrylate (MMA) | [42] |
Polyurethane (PU) | [36] |
Shell Material [Test Method] | Average Size (D) (µm) | Shell Thickness (t) (µm) | Elastic Modulus (GPa) | Bursting/Rupture Stress (MPa) | Reference |
---|---|---|---|---|---|
Poly-Urethane (PU) | 50–100 | 1–2 | 0.0029 | 0.026 | [78] |
Urea-Formaldehyde (UF) | 65 ± 7 dry | 0.175 ± 0.033 | 3.7 ± 0.5 | 0.8 ± 0.3 | [77] |
187 ± 15 dry | 3.6 ± 0.4 | 0.24 ± 0.04 | |||
213 ± 12 immersed | 3.9 ± 0.7 | 0.14 ± 0.02 | |||
Poly-Melamine-Formaldehyde (PMF) | 10–150 | 0.2 | 4.66 | - | [73] |
Phenol-Formaldehyde (PF) | 50–200 | 29.96 | 2.2 ± 0.8 | 68.5 ± 41.6 | [32] |
200–400 | 96.8 ± 23.5 | ||||
400–600 | 198.5± 31.6 mN |
Healing Agent | Shell Material | Performance Criteria | Capsule Size (μm) | Capsule Content (%) | Pre-Loading Condition | Curing Conditions and Testing Age | Testing Methodology | Reference |
---|---|---|---|---|---|---|---|---|
Epoxy resin | UF | Mechanical properties and durability | 73–309 | 3, 6, 9 | 30%, 50%, 70% of maximum compressive/flexural strength | Pre-cracked after curing for 28 d (RH > 90%, 20 °C), then left to heal for 3 d (same curing conditions) |
| [39] |
Epoxy resin | UF | Mechanical properties and durability | 45–185 | 3, 6, 9 | 60% of maximum compressive strength | Pre-cracked after curing for 28 d (RH > 90%, 20 °C), then left to heal for 7 d (cured in a box at temp. < 50 °C) |
| [53] |
Epoxy resin | UF | Mechanical properties and durability | 132, 180, 230 | 2, 4, 6, 8 | 60% of maximum compressive strength | - |
| [40] |
Epoxy resin | UF | Mechanical properties and durability | 132, 180, 230 | 2, 4, 6, 8 | 30–70% of maximum compressive strength | Cured for 60 d in the curing chamber (95 ± 5% RH, 20 ± 2 °C), then pre-cracked and left to heal at a temperature of 30–60 °C for 3 d, 5 d, 7 d, 14 d and 28 d |
| [65,66] |
DCPD and Sodium Silicate | UF | Mechanical properties | 75–1000 | 0.5, 1.0, 2.5, 5.0 (Sodium Silicate), 0.25 (DCPD) | 70% of maximum compressive strength. | Steam cured for 7 d at 20–25 °C, then reloaded three cycles before left to heal in curing room for 48 h |
| [71] |
Calcium Nitrate Tetrahydrate | UF | Visual and crack width | 22–59 | 0.5, 0.75 | Flexural damage up to sudden change in the displacement | Cured for 28 d (95% RH), oven-dried for 3 d (60 °C), then pre-cracked, water immersed for 7, 21, 42 d, and oven-dried again for 3 d (60 °C) |
| [41] |
Sodium Silicate | Double-walled PU/UF | Visual and crack depth | - | 2.5, 5 | Flexural damage up to load of 500 kg | Cured water for 7 d, pre-cracked then left to heal for 2 weeks |
| [37] |
Epoxy resin | MUF | Mechanical properties | 10–1800 | 1, 2, 4 | 30, 60, 80% of maximum load resistance | Cured for 28 d (≥95% RH, 20 ± 2 °C), left to heal for 2 h after pre-cracked, then tested up to failure |
| [33] |
DCPD | PF | Mechanical properties | 50–600 | 4–12 | Loaded up to failure | Cured for 28 d in wet chamber (25 °C, 95% RH) |
| [32] |
MMA | PS | Mechanical properties and durability | 4.15 | 1.5 | 80% of maximum compressive strength | Cured in wet chamber for 28 d (≥95% RH, 20 ± 2 °C. Samples of 1 d and 28 d are pre-cracked, rest for 24 h, then cured for another 24 h in vacuum-dried room (for permeability)/subjected to cyclically loading between 25–95% of maximum compressive strength (for fatigue) |
| [42] |
CSA | PS | Visual and crack volume | 200–500 | 5 | Up to compressive strength of 11 MPa | Cured for 28 d in curing chamber, then pre-cracked and immersed in water for 21, 42, 63, 84 and 105 d |
| [81] |
Constant Values | |||
---|---|---|---|
w/c | 0.3 | ||
Cementing (kg/m3) | 550 | ||
Sand/cementing | 3 | ||
Variables Values | |||
Mortar age (day) | 2 | 2 | 28 |
SCM (% of cement) | 0 | 22%GGBFS + 8%SF | 0 |
f’c (MPa) | 24.1 | 17.0 | 50.8 |
Em (GPa) | 30 | 30 | 39 |
ftm (MPa) | 1.6 | 1.3 | 4.0 |
Gm (J/m2) | 30 | 20 | 60 |
Variable | Values Used | Range in the Literature | References | |
---|---|---|---|---|
Shell geometry | Rs (μm) | 50, 60, 100 | 5–1000 | [32,33,34,67,71,84,86,87] |
ts (μm) | 1, 2, 3, 8 | 1–200 | [32,34,39,53,79,82,84,87,95] | |
Shell properties | Es (GPa) | 4 | 2.25–12 | [32,33,34,39,53,73,77,79,82,84,86,87,96,97,98,99,100,101,102,103] |
frs (MPa) | 30, 50 | 23–90 | [36,84,99,100,101,102,103] | |
Gs (J/m2) | 100 | 40–500 | [84,86,97,98,101,102,104,105,106] | |
νs | 0.3 | 0.3–0.36 | [84,86,87,96,97,98,99,102] | |
Interface properties | σbi (MPa) | 0.9–3.4 | 0.1–15 | [36,79,82,84,86,87,107,108,109,110,111,112] |
Gi (J/m2) | 20–80 | 0.1–100 | [82,84,86,87] |
Variables | 1 Level | 2 Levels | 3 Levels | Star Point |
---|---|---|---|---|
2d Mortar without SCM | ||||
Rs (mm) | 0.06 | 0.1 | 0.05 | |
ts (mm) | 0.002 | 0.003 | 0.008 | 0.001 |
Es (GPa) | 4 | |||
frs (MPa) | 30 | 50 | ||
Gs (J/m2) | 100 | |||
σbi (MPa) | 1.1 | 1.3 | ||
Gi (J/m2) | 20 | 50 | ||
2d Mortar with SCM | ||||
Rs (mm) | 0.06 | 0.1 | 0.05 | |
ts (mm) | 0.002 | 0.003 | 0.008 | 0.001 |
Es (GPa) | 4 | |||
frs (MPa) | 30 | 50 | ||
Gs (J/m2) | 100 | |||
σbi (MPa) | 0.9 | 1.0 | ||
Gi (J/m2) | 20 | 50 | ||
28d Mortar without SCM | ||||
Rs (mm) | 0.06 | 0.1 | 0.05 | |
ts (mm) | 0.002 | 0.003 | 0.008 | 0.001 |
Es (GPa) | 4 | |||
frs (MPa) | 30 | 50 | ||
Gs (J/m2) | 100 | |||
σbi (MPa) | 2.9 | 3.4 | ||
Gi (J/m2) | 40 | 80 |
Run | Rs (mm) | ts (mm) | frs (MPa) | σbi (MPa) | Gi (N/mm) | Failure Mode | CMOD (μm) |
---|---|---|---|---|---|---|---|
1 | 0.06 | 0.003 | 30 | 1.1 | 0.02 | D | 5.187 |
2 | 0.10 | 0.003 | 30 | 1.3 | 0.02 | D | 5.148 |
3 | 0.06 | 0.008 | 30 | 1.1 | 0.02 | D | 5.296 |
4 | 0.10 | 0.008 | 30 | 1.3 | 0.02 | D | 5.145 |
5 | 0.06 | 0.003 | 50 | 1.1 | 0.02 | D | 5.142 |
6 | 0.10 | 0.003 | 50 | 1.3 | 0.02 | D | 5.108 |
7 | 0.06 | 0.008 | 50 | 1.1 | 0.02 | D | 5.258 |
8 | 0.10 | 0.008 | 50 | 1.3 | 0.02 | D | 5.180 |
9 | 0.06 | 0.003 | 30 | 1.1 | 0.05 | D | 5.156 |
10 | 0.10 | 0.003 | 30 | 1.3 | 0.05 | D | 5.121 |
11 | 0.06 | 0.008 | 30 | 1.1 | 0.05 | D | 5.267 |
12 | 0.10 | 0.008 | 30 | 1.3 | 0.05 | D | 5.112 |
13 | 0.06 | 0.003 | 50 | 1.1 | 0.05 | D | 5.170 |
14 | 0.10 | 0.003 | 50 | 1.3 | 0.05 | D | 5.164 |
15 | 0.06 | 0.008 | 50 | 1.1 | 0.05 | D | 5.297 |
16 | 0.10 | 0.008 | 50 | 1.3 | 0.05 | D | 5.147 |
17 | 0.06 | 0.003 | 30 | 0.9 | 0.02 | D | 4.218 |
18 | 0.10 | 0.003 | 30 | 1.0 | 0.02 | D | 4.221 |
19 | 0.06 | 0.008 | 30 | 0.9 | 0.02 | D | 4.371 |
20 | 0.10 | 0.008 | 30 | 1.0 | 0.02 | D | 4.216 |
21 | 0.06 | 0.003 | 50 | 0.9 | 0.02 | D | 4.207 |
22 | 0.10 | 0.003 | 50 | 1.0 | 0.02 | D | 4.188 |
23 | 0.06 | 0.008 | 50 | 0.9 | 0.02 | D | 4.286 |
24 | 0.10 | 0.008 | 50 | 1.0 | 0.02 | D | 4.204 |
25 | 0.06 | 0.003 | 30 | 0.9 | 0.05 | D | 4.192 |
26 | 0.10 | 0.003 | 30 | 1.0 | 0.05 | D | 4.236 |
27 | 0.06 | 0.008 | 30 | 0.9 | 0.05 | D | 4.242 |
28 | 0.10 | 0.008 | 30 | 1.0 | 0.05 | D | 4.193 |
29 | 0.06 | 0.003 | 50 | 0.9 | 0.05 | D | 4.228 |
30 | 0.10 | 0.003 | 50 | 1.0 | 0.05 | D | 4.213 |
31 | 0.06 | 0.008 | 50 | 0.9 | 0.05 | D | 4.230 |
32 | 0.10 | 0.008 | 50 | 1.0 | 0.05 | D | 4.227 |
33 | 0.06 | 0.003 | 30 | 2.9 | 0.04 | D | 9.523 |
34 | 0.10 | 0.003 | 30 | 3.4 | 0.04 | R | 8.132 |
35 | 0.06 | 0.008 | 30 | 2.9 | 0.04 | D | 9.578 |
36 | 0.10 | 0.008 | 30 | 3.4 | 0.04 | D | 9.556 |
37 | 0.06 | 0.003 | 50 | 2.9 | 0.04 | D | 9.629 |
38 | 0.10 | 0.003 | 50 | 3.4 | 0.04 | R | 9.025 |
39 | 0.06 | 0.008 | 50 | 2.9 | 0.04 | D | 9.744 |
40 | 0.10 | 0.008 | 50 | 3.4 | 0.04 | D | 9.515 |
41 | 0.06 | 0.003 | 30 | 2.9 | 0.08 | R | 9.886 |
42 | 0.10 | 0.003 | 30 | 3.4 | 0.08 | R | 8.158 |
43 | 0.06 | 0.008 | 30 | 2.9 | 0.08 | D | 9.614 |
44 | 0.10 | 0.008 | 30 | 3.4 | 0.08 | D | 9.504 |
45 | 0.06 | 0.003 | 50 | 2.9 | 0.08 | D | 9.564 |
46 | 0.10 | 0.003 | 50 | 3.4 | 0.08 | R | 9.056 |
47 | 0.06 | 0.008 | 50 | 2.9 | 0.08 | D | 9.639 |
48 | 0.10 | 0.008 | 50 | 3.4 | 0.08 | D | 9.552 |
49 | 0.10 | 0.002 | 30 | 1.3 | 0.02 | R | 5.115 |
50 | 0.10 | 0.002 | 50 | 1.3 | 0.02 | D | 5.270 |
51 | 0.10 | 0.002 | 30 | 1.3 | 0.05 | R | 5.139 |
52 | 0.10 | 0.002 | 50 | 1.3 | 0.05 | D | 5.202 |
53 | 0.10 | 0.002 | 30 | 1.0 | 0.02 | R | 4.219 |
54 | 0.10 | 0.002 | 50 | 1.0 | 0.02 | D | 4.261 |
55 | 0.10 | 0.002 | 30 | 1.0 | 0.05 | R | 4.167 |
56 | 0.10 | 0.002 | 50 | 1.0 | 0.05 | D | 4.255 |
57 | 0.10 | 0.002 | 30 | 3.4 | 0.04 | R | 9.479 |
58 | 0.10 | 0.002 | 50 | 3.4 | 0.04 | R | 9.726 |
59 | 0.10 | 0.002 | 30 | 3.4 | 0.08 | R | 9.714 |
60 | 0.10 | 0.002 | 50 | 3.4 | 0.08 | R | 9.649 |
61 | 0.06 | 0.002 | 30 | 1.1 | 0.02 | D | 5.170 |
62 | 0.06 | 0.002 | 50 | 1.1 | 0.02 | D | 5.168 |
63 | 0.06 | 0.002 | 30 | 1.1 | 0.05 | D | 5.179 |
64 | 0.06 | 0.002 | 50 | 1.1 | 0.05 | D | 5.161 |
65 | 0.06 | 0.002 | 30 | 0.9 | 0.02 | D | 4.195 |
66 | 0.06 | 0.002 | 50 | 0.9 | 0.02 | D | 4.188 |
67 | 0.06 | 0.002 | 30 | 0.9 | 0.05 | D | 4.267 |
68 | 0.06 | 0.002 | 50 | 0.9 | 0.05 | D | 4.232 |
69 | 0.06 | 0.002 | 30 | 2.9 | 0.04 | R | 8.031 |
70 | 0.06 | 0.002 | 50 | 2.9 | 0.04 | D | 9.602 |
71 | 0.06 | 0.002 | 30 | 2.9 | 0.08 | R | 8.014 |
72 | 0.06 | 0.002 | 50 | 2.9 | 0.08 | D | 9.627 |
73 | 0.10 | 0.001 | 30 | 1.3 | 0.02 | R | 4.546 |
74 | 0.10 | 0.001 | 50 | 1.3 | 0.02 | R | 5.112 |
75 | 0.10 | 0.001 | 30 | 1.3 | 0.05 | R | 4.549 |
76 | 0.10 | 0.001 | 50 | 1.3 | 0.05 | R | 5.208 |
77 | 0.10 | 0.001 | 30 | 1.0 | 0.02 | R | 3.846 |
78 | 0.10 | 0.001 | 50 | 1.0 | 0.02 | R | 4.791 |
79 | 0.10 | 0.001 | 30 | 1.0 | 0.05 | R | 3.838 |
80 | 0.10 | 0.001 | 50 | 1.0 | 0.05 | R | 4.757 |
81 | 0.10 | 0.001 | 30 | 3.4 | 0.04 | R | 7.370 |
82 | 0.10 | 0.001 | 50 | 3.4 | 0.04 | R | 8.111 |
83 | 0.10 | 0.001 | 30 | 3.4 | 0.08 | R | 7.376 |
84 | 0.10 | 0.001 | 50 | 3.4 | 0.08 | R | 8.099 |
85 | 0.05 | 0.008 | 30 | 1.1 | 0.02 | D | 5.189 |
86 | 0.05 | 0.008 | 50 | 1.1 | 0.02 | D | 5.238 |
87 | 0.05 | 0.008 | 30 | 1.1 | 0.05 | D | 5.154 |
88 | 0.05 | 0.008 | 50 | 1.1 | 0.05 | D | 5.179 |
89 | 0.05 | 0.008 | 30 | 0.9 | 0.02 | D | 4.242 |
90 | 0.05 | 0.008 | 50 | 0.9 | 0.02 | D | 4.236 |
91 | 0.05 | 0.008 | 30 | 0.9 | 0.05 | D | 4.219 |
92 | 0.05 | 0.008 | 50 | 0.9 | 0.05 | D | 4.216 |
93 | 0.05 | 0.008 | 30 | 2.9 | 0.04 | D | 9.678 |
94 | 0.05 | 0.008 | 50 | 2.9 | 0.04 | D | 9.597 |
95 | 0.05 | 0.008 | 30 | 2.9 | 0.08 | D | 9.728 |
96 | 0.05 | 0.008 | 50 | 2.9 | 0.08 | D | 9.785 |
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Reda, M.A.; Chidiac, S.E. Performance of Capsules in Self-Healing Cementitious Material. Materials 2022, 15, 7302. https://doi.org/10.3390/ma15207302
Reda MA, Chidiac SE. Performance of Capsules in Self-Healing Cementitious Material. Materials. 2022; 15(20):7302. https://doi.org/10.3390/ma15207302
Chicago/Turabian StyleReda, Mouna A., and Samir E. Chidiac. 2022. "Performance of Capsules in Self-Healing Cementitious Material" Materials 15, no. 20: 7302. https://doi.org/10.3390/ma15207302