MSc‐thesis
Self ‐healing of Engineered of Engineered Cementitious Composites (ECC) in Concrete Repair System
TABLE OF CONTENTS ACKNOWLEDGEMENTS
i
ABSTRACT
ii
1
INTRODUCTION
1
1.1
Problem definition
1
1.2
Objective of the research
2
1.3
Outline of the thesis
2
2
LITERATURE STUDY
3
2.1
Review of the Engineered Cementitious Composites (ECC) material
3
2.1.1
Characteristics of ECC
3
2.1.2
Additives in ECC
5
2.2
Self ‐healing in concrete materials
6
4.3
5
4.2.2
Materials and mix proportion
17
4.2.3
Specimen preparation
18
4.2.4
Three‐point bending test
19
4.2.5
Nano‐computer tomography (nano‐CT)
20
4.2.6
Environmental scanning electron microscopy (ESEM)
22
4.2.7
Light microscope
23
Short summary
23
RESULTS AND DISCUSSION
24
5.1
Recovered mechanical properties in ECC
24
5.1.1
Load‐displacement relation
24
5.1.2
General results of stress‐deflection curves
25
5.1.3
Deflection hardening behavior influenced by the capsules
26
5.1.4
Deflection capacity and recovery
27
5.1.5
Flexural strength and recovery
28
5.1.6
Flexural stiffness and recovery
30
5.2
Nano‐CT observation
32
ACKNOWLEDGEMENTS
This research has been carried out at the Microlab of the Faculty of Civil Engineering and Geosciences, Delft University of Technology, and continued for 8 months from December 2009 until July 2010. I wish to take this opportunity to express my gratitude to all those who helped me in this research and successful completion of this thesis, especially the following people: First of all, I would like to express my sincere thanks to Dr. G. Ye for introducing me to this project, for his daily supervisions, discussions and constant encouragements throughout the research. I am also very grateful for his effort to make this project go on smoothly. Second, I really appreciate Prof. K. van Breugel and Dr. P. C. J. Hoogenboom for providing me the general supervisions, for their valuable comments on the research. Third, I would like to thank PhD student H. Huang for his stimulation and collaboration on sealing material investigation and manufacture. During the study, PhD student J. Zhou also provided me some interesting ideas and guided me to perform some experiments. In addition, I also thank Mr. A. Thijssen from Microlab for his assistance on the nano‐CT scan investigation and the coordinator of Structural Engineering Ir. L. J. M. Houben for his
ABSTRACT
Since the concept of high performance concrete was raised in the late 1980’s, it is well known that concrete properties have been greatly improved. However, a larger number of existing concrete structures are suffering deterioration resulting from external or internal causes even during the early stage of service life. Durable repair of concrete has thus drawn more attention. To this end, the main objective of this research is to develop and test a novel method for promoting the self ‐healing behavior in concrete repair system. A new type high performance fiber reinforced cementitious composites called Engineered Cementitious Composites (ECC) has been developed in recent years, characterized by high ductility and improved durability due to the multiple micro‐cracking behavior. In this study, it was proposed that the original ECC with local waste materials was embedded with capsules to investigate the self ‐healing potential of this modified ECC material. To realize this self ‐healing concept, Super Absorbent Polymers (SAP) can be used as the water reservoir enclosed in the capsules, and then provide available water for self ‐healing process when the capsules are ruptured by cracking. Based on this idea, the preliminary experiments, concerning sealing materials and encapsulation procedure, were first carried out. Three ECC mixtures focusing on the influence of capsule content and capsule size were involved. In order to induce artificial cracks, three‐point bending tests have been used to preloaded ECC specimens to different
1 INTRODUCTION 1.1 Problem definition Concrete is a strong, versatile and economical material that has been widely utilized for constructions over the world. Since the concept of high performance concrete was raised in the late 1980’s, concrete properties have been greatly improved. However, a large number of existing concrete structures, such as bridge decks and pavements are suffering deterioration resulting from external and internal causes even during the early stage of service life. In the civil engineering sector of the Netherlands, premature failure of structures leads to a situation where about half the budget is spent annually on maintenance and repair. For concrete structures in particular, 90% of the repair works focus on repair of cracks caused by reinforcement corrosion [1]. Due to the brittleness of concrete, cracking is unavoidable in concrete structures. Cracking can introduce chlorides, sulphates, oxygen, alkali or moisture into the concrete and accelerate further deterioration of the whole system. To address this problem, the durability of concrete repairs has drawn more attention. Even though the quality of concrete repairs has increased a lot in recent years, realizing durable repairs is still difficult. As stated above, people have tried to make better and stronger materials, which are capable of repairing cracks and restoring their functionality. These materials can be defined as self ‐healing
1.2 Objective of the research Since the ECC material has self ‐healing potential, the main objective of this research is to develop and test a novel method for promoting self ‐healing behavior in ECC materials. The encapsulation approach is considered as novel method of this study. More specifically, it is investigated that when cracks rupture embedded capsules inside the ECC mixture, whether this action can release healing agent (water) for further hydration of cement, without replying on external supply of water. This study will be conducted as a preliminary study to get more insight into a cement‐based self ‐healing coating of old concrete in concrete repair system.
1.3 Outline of the thesis This thesis consists of six chapters. Chapter 1 introduces the motivation, objective and overview of this research. Chapter 2 gives a review of the literature study about the development of cementitious materials and self ‐healing behavior. Chapter 3 explains the methodology, including the starting point, challenge and approach of the experimental research. Chapter 4 illustrates the experimental set‐ups, procedures of the research program. Chapter 5 presents the results from the mechanical tests and microscopic observations followed by the discussions concerning influencing factors and healing efficiency of the self ‐healing behavior. Chapter 6 summarizes the
2 LITERATURE STUDY This chapter intends to review the previous works on the ECC material including its characteristics and additives in ECC. Moreover, the literature study of self ‐healing phenomenon in concrete materials including the mechanisms, the conditions and the approaches of self ‐healing is introduced.
2.1 Review of the ECC material 2.1.1 Characteristics of ECC In the last decades, concrete with increasingly high compressive strength have been applied to civil engineering since modern building constructions rapidly grow towards high‐rise and diversity. The addition of steel fiber and powders improves a number of concrete properties. However, most of these materials still remain brittle. In some cases, the brittleness increases as the compressive strength goes up, which poses potential dangers or fracture failures of the concrete. A specially designed cementitious material termed as Engineered Cementitious Composites (ECC) has been developed by Li and continuously evolved over the last twenty years. ECC is characterized by a high ductility in range of 3‐7%, a tight crack width of around 60 μm and relatively low fiber content of 2% or less by volume [3]. In terms of main material constituents, ECC has characteristics similar to regular Fiber Reinforced Concrete (FRC), including water,
The crack width is another important indicator, reflecting the durability of a concrete structure. ECC exhibits a well crack width self ‐controlled in terms of a flat steady state microcracks propagation, see Figure 2.2. After the tensile deformation up to around 1% strain, the early microcracks stop widening and remain more or less constant with crack width of around 60 μm . ECC material can be tailored to form numerous closely spaced microcracks. The crack width in ECC is much smaller than the typical crack width observed in the reinforced concrete. Moreover, the self ‐control of crack width can be seen as intrinsic properties of ECC material, rather than depending on steel reinforcement ratio and structural dimensions [7]. Figure 2.2 also shows the tensile strain capacity of 5% that is about 300‐500 times great than normal concrete [8].
2.1.2 Additives in ECC 2.1.2.1 Blast furnace slag (BFS) and limestone powder (LP)
In order to develop a new version of ECC with locally available materials, a number of mixtures with blast furnace slag (BFS) and limestone powder (LP) instead of fly ash and silica sand respectively have been investigated at Delft University of Technology. Portland cement, BFS and LP are used to produce ECC as matrix materials, which can enhance the mechanical properties and durability of ECC [3]. There are only a small amount of LP reacting with cement clinker or hydration products, thus the limestone powder usually behaves as an inert filler material. The incorporation of limestone powder and Portland cement is conducive to early compressive strength, workability and durability of concrete. When BFS is mixed with Portland cement, it reacts with the calcium which is called the pozzolanic reaction. It was reported that the addition of BFS leads to a lower strength at early age, however it does not have any side effect on the final compressive strength. Besides LP, BFS is able to improve the durability of ECC and results in a well homogenous fiber distribution. The experiments with different BFS, LP contents and different water‐powder ratios were discussed [3]. The optimal results of the ECC mix proportion with Portland cement, BFS and LP were used as a reference in this study. 2.1.2.2 Super Absorbent Polymer (SAP)
Super Absorbent Polymer (SAP) is a low cross‐linked polyelectrolyte which starts to swell when it
2.2 Self ‐healing in concrete materials 2.2.1 Introduction Self ‐healing is generally defined as the ability to repair or heal damage of material itself [12]. In natural materials, skin tissue and bone structures are perfect examples of self ‐healing behavior. Although the mechanisms of healing in natural materials cannot be copied exactly, some forms of healing in concrete materials have been observed based on the similarity theory.
2.2.2 Mechanisms of self ‐healing Self ‐healing behavior in cementitious materials has been demonstrated by numerous experimental investigations and practical experiences [7,12‐14]. The autogenous healing phenomenon is that the material has the ability to seal itself without external monitoring or human intervention. Self ‐healing of cracks in concrete is a combination of the complicated chemical and physical processes. Up to now, several possible causes can be illustrated as follows (also schematised in Figure 2.5): i.
Formation of calcium carbonate or calcium hydroxide.
ii.
Blocking cracks by impurities in the water and loose concrete particles resulting from crack
Ca(OH)2
2+
↔ Ca
Ca2+ + CO32−
+ 2OH
‐
→ CaCO3
According to Neville [15], self ‐healing was mainly owing to continued hydration in his opinion at first. But later he stated that this is only applied to very young concrete [16] and believed that the formation of calcium carbonate is the most likely cause of self ‐healing. Besides, loose particles blocking the crack path was also mentioned in some studies as a reason for healing cracks. Since this was considered to cause the first fast decrease of water flow through the cracks [17].
2.2.3 Conditions for self ‐healing From the literature study, it is pointed out that five general criteria should be satisfied to ensure self ‐healing. These necessary conditions to experience healing of cracks are: i.
Presence of water
All the studies so far state that the presence of water is essential to facilitate healing of the cracks. Without water, it is impossible for the calcium hydroxide to be leached out of the bulk material into crack [13]. ii.
Presence of chemical species
Adequate concentrations of certain critical chemical species for instance carbonate ions or bicarbonate ions and free calcium ions dissolved in a flow of water, play a direct role to exhibit
2.2.4 Self ‐healing approaches With the developments of smart materials, several innovative approaches of self ‐healing have been promoted in recent years. The core of these approaches is capable of continuously offering materials or energy. For another, an ideal healing agent is supposed to continuously sense and respond to damage, and recover the material performance without adverse affecting the matrix material properties [18]. Several approaches based on this principle can be discussed below.
2.2.4.1 Encapsulation
The microcapsules can be defined as “particles, spherical or irregular, in the size range of about 50 nm to 2000 μm or larger, and composed of an excipient polymer matrix (shell or wall) and incipient active polymer (core substance) ” [19]. The microencapsulated approach of incorporation of healing agent was demonstrated by White [12], and Figure 2.6 illustrates this autonomic healing concept. When the crack ruptures embedded microcapsules, the healing agent is released into the crack plane through capillary action. Then the healing agent contacts the embedded catalyst, triggering polymerization that bonds the crack faces closed. However, a successful completion of the healing process is not easily realized since it combines a complex set of requirements on storage, rupture, release, transport and healing. Furthermore, some studies indicated that specific problems in terms of the size of microcapsules and surface morphology significantly influenced the healing efficiency [12].
2.2.4.2 Hollow glass fibers
The use of hollow glass fibers (Figure 2.7 (A)) follows the similar concept as the microcapsules. Glass is a typical brittle material, once the glass fibers break, the healing agents flow into the matrix cracks and heal them so that the mechanical properties of concrete can be regained to a certain extent. The key advantage of hollow fibers approach is that the fibers can be placed at any location depending on the operational requirement to deal with specific failure threats (Figure 2.7 (B)). In order to quickly and easily see the internal damage in composite materials, a damage visual enhancement method was designed by Pang and Bond [12]. In their work, the fibers filled with healing agent were mixed with fluorescent dye to monitor the healing process (Figure 2.7 (C)).
(A)
(B)
2.2.4.3 Bacteria
On the other hand, it is found that bacteria incorporated in the concrete matrix as self ‐healing agent probably catalyzes the autonomous repair of cracks [21]. Basically, bacteria of the genus bacillus were used for the biological production of calcium carbonate‐based minerals. Such bacteria added in the cement matrix prior to casting should keep viable for prolonged periods. Once integrated in the concrete matrix, it should be able to produce amounts of minerals needed to plug or seal freshly formed cracks. In this sense, integrated bacteria would thus represent an internal self ‐healing agent which autonomously decreases matrix permeability upon cracks formation. The scenario is schematically shown in the following figure.
3 METHODOLOGY 3.1 Starting point The idea of this research comes from an “embedded capsules” approach to repair material itself. Two starting points were proposed in this study to realize the self ‐healing process in concrete repair system. As mentioned in the introduction, the first starting point is to use ECC material in studying the healing potential. Because ECC exhibits the high strain capacity and tight crack width control, those unique properties can promote the occurrence of self ‐healing. The second starting point is related to the saturated SAP, here SAP is considered as a water carrier enclosed in capsules since it is able to absorb a large amount of water, and the water released from SAP has the function of promoting the further hydration of the cement.
3.2 Approach The core question of this thesis can be simply stated as whether water can be released from the embedded capsules thereby promoting self ‐healing in ECC material. The first task is to find a proper way to seal the saturated SAP. To realize healed cracks in laboratory conditions, the capsules are ruptured by inducing artificial cracks for releasing water. Finally, mechanical test and
3.3 Challenge From the approach presented in the previous section, there are three main challenges involving sealing material and manufacture, cracking pattern during preloading and self ‐healing observation, which will be discussed as below, iv.
Sealing material and manufacture
The suitable sealing material should try to meet a set of requirement on physical, chemical features and mechanical properties at the same time. For instance, it is expected to has a stable capacity of water storage, and be sensitive to cracks whilst it is asked for a good bond strength between the sealing material and the matrix. On the other hand, the manufacture of capsules could be difficult without rolling machine. Since dry SAP powders used in this experiment has a small particle size of 300 μm in diameter, after swelling it becomes softer, such that it is difficult to be gathered to form a ball. v.
Cracking pattern during preloading
The second challenge is to determine the crack propagation through the capsules. In this research, the capsule consists of saturated SAP particles as the core and sealing shell as the outer surface. When the capsules are incorporated into ECC, the bond strength at interfacial transition zone (ITZ) between the sealing shell and the cement‐based matrix needs to be stronger than the strength of capsule itself, to ensure the artificial cracks propagate through
4 EXPERIMENTAL STUDY 4.1 Sealing material and manufacture 4.1.1 Introduction Since further hydration can only be realized in the presence of water or solution, SAP particles can be introduced as a water reservoir in cementitious composites. In order to cause the capsules to release the entrained water at the right time, the outer surface of saturated SAP needs to be sealed by a protected layer. In this research, the ideal sealing material can be defined as that which meets the following three requirements. The first is to appear impervious to leakage of water before inducing the microcracks. Second, this material should be sensitive to cracking, allowing the broken of capsules occurs at a certain level before arriving at ultimate strength of ECC material. Last requirement is the proper interfacial bond strength. This strength of interface between the capsule and the matrix requires being stronger than the strength of the capsule, to guarantee that cracks can propagate through the capsules rather than around them. Thus high bond strength at the interface is one of the important factors contributed to cracks passing through the capsules. Besides the intrinsic properties of sealing material, the diameter of capsule and the surface
small size could be more easily gathered and shaped into a ball when CEM I 52.5N was utilized to form a surface cover. To finish this process, the saturated SAP particles were sieved by 2.4mm size of sieve, to separate them from the excess cement. It is important to control the rate of shaking. If the amount of cement is less, the thin surface cover would not form. However, excess cement will absorb more water from saturated SAP particles. To maximize the contained water inside capsules, one method of avoiding water loss was to cure these balls in water at a temperature of 20 °C for 7 days, in order to achieve the hydration of cement and keep the SAP particles fully absorbing water. Afterwards, the out surface of ball was sealed by a shell of wax or epoxy‐cement, respectively. For the first case, paraffin wax was heated up to 105 °C and then kept a ball into this hot solution for 2 seconds. Finally, wax microsphere was obtained from rapid cooling of the suspension of molten wax droplets and it was cured under room condition. For the second case, 5 wt% epoxy and 100 wt% cement was mixed by hand and then rolled a ball in the epoxy‐cement paste until smooth. After this, the ball was cured in RH 100% at 20 °C for 7 days. In such a way, saturated SAP particles were made into two types of capsules (Figure 4.2).
(A) Saturated SAP particles
(B) Shaped by cement cover
105 104
) 103 % ( 102 r e t 101 a w g100 n i n 99 i a m 98 e R 97 96 95 0
20
40
60
80
Time (h)
(A) Capsules sealed by sealed by paraffin paraffin wax, curing at 23 at 23 °C RH 70% 105
100
) % ( r 95 e t a w g 90 n i n
4.2 Functional performance of ECC of ECC 4.2.1 Introduction This chapter focused on proposing the experimental program and set‐ups used in this research. To study the functional performance of healed ECC, the overview of the experimental program can be designed as following steps. After preparation of ECC specimens embedded with the capsules, the principal task is to introduce cracks to these specimens. For this end, the ECC specimens are under three‐point bend to form cracks inside. After healing for 28 days, the specimens will be tested again in three‐point bending. Meanwhile, the reference without capsules is parallel tested for comparison. The crack pattern and the healing products can be observed by Nano‐CT, CT, environment scanning electron microscopy (ESEM) and light microscope techniques to verify whether the self ‐healing phenomenon takes place.
4.2.2 Materials and mix proportion As mentioned above, various modified ECC incorporating local waste materials have been developed to optimize ECC mix proportion and investigate the self ‐healing properties. BFS as the main cement replacement material in the Netherlands shows a potential of pozzolanic reaction but these reactions need to be activated by the hydration products of Portland cement. In mix
Table 4.1 Mix proportion proportion of ECC of ECC (by (by weight) weight) Mix
CEM I
Limestone
Number
42.5N
Powder
b
1
2
1.2
c
1
2
1.2
1
2
1.2
M1
M2
d
M3
Remark:
a
BFS
Saturated
Water
0 0.02 (0.46% by volume) 0.02 (0.23% by volume)
a
Super‐
PVA fiber
ratio
plasticizer
(by volume)
1.092
0.26
0.030
2%
1.092
0.26
0.030
2%
1.092
0.26
0.030
2%
SAP
Water/powder
Powder includes cement, BFS and limestone powder
b
Mixture without capsules (reference)
c
Mixture with capsule size of 8mm of 8mm (coarse capsule)
d
Mixture with capsule size of 5mm of 5mm (fine capsule)
4.2.3 Specimen preparation The ECC specimen preparation followed the procedure described in [3]. In the first place, the solid materials, CEM I 42.5, BFS and limestone powder were mixed with a HOBART mixer for 2 minutes at low speed. Then water and superplasticizer were added at low speed mixing for 1
4.2.4 Three‐point bending test In this research, the three‐point bending test was the main method to induce artificial cracks and also characterize the mechanical properties of the modified ECC material. 4.2.4.1 Experimental set‐up
As seen in Figure 4.5, the support span of three‐point bending test set‐up is 110mm and the load is located in the middle of the specimen. The configuration of three‐point bending test set‐up is explained more in Appendix D. Two linear variable differential transducers (LVDTs) are fixed on both sides of the set‐up to measure the vertical deformation at mid cross‐section of the specimen. The test was conducted under deformation control at a constant speed of 0.01 mm/s. At least two measures were done for each mixture, and the flexural strength and deflection were calculated based on the average results of these measures.
Remark: X stands for M1, M2, and M3 respectively. Therefore, 3 mixtures and 3 schemes for each mixture, resulting in 9 combinations in total.
Figure 4.6 Bending test program of ECC material
4.2.5 Nano‐computer tomography (nano‐CT) 4.2.5.1 Experimental set up
Image J (free license) was applied to generate 3D image and further analyze the cracking pattern.
Figure 4.7 Schematic representation of nano‐CT system
4.2.5.2 Experimental program
As mentioned in Chapter 3.3, the important advantage of nano‐CT is that the non‐destructive technique
show the internal crack
meanwhile keep the
of the
4.2.6 Environmental scanning electron microscopy (ESEM) 4.2.6.1 Experimental set‐up
Environmental scanning electron microscopy (ESEM) was preferred in this study to analyze the quality of self ‐healing products formed inside the crack. This technique can provide insight into the chemical composition of healing products and therefore identify the self ‐healing behavior in ECC material. The ESEM retains all performance advantages of a conventional SEM (Figure 4.9), moreover eliminates the high vacuum constraint on the sample environment. The electron gun at the top of the column creates a electron beam, and then the electrons are accelerated and focused by a series of magnetic lenses and apertures. A set of scanning coils deflects the electron beam in a scanning pattern over the sample surface and the objective lens offers the final focusing. The interactions between the beam electrons and the sample atoms will generate a variety of signals in forms of secondary electrons (SE), backscattered electrons (BSE) and characteristic X‐rays, and emerging signals can be detected and reconstructed into a virtual image displayed on the monitor screens.
4.2.7 Light microscope As shown in Figure 4.10, a transmitted light microscope Leica MZ6 with cold light sources (CLS150) was used in this study to observe the typical crack pattern. This modular stereomicroscope can create brilliant three‐dimensional images of spatial object and Leica cold light sources provide strong light intensity even within small space. In addition, the CLS 150 has been specially adapted for automated control of the new transmitted light base via the powerful software of LAS (Leica Application Suite). Through the serial interface, brightness and the electronic shutter can be controlled using the computer.
5 RESULTS AND DISCUSSION 5.1 Recovered mechanical properties in ECC Several techniques have been used in examining self ‐healing behavior. In this section, the self ‐healing in ECC is evaluated from the point of view of mechanical properties.
5.1.1 Load‐displacement relation The displacement controlled three‐point bending test records the load‐displacement relationship. One example of load‐displacement curve at three different stages of loading is given in Figure 5.1 (A). As indicated, there is an initial linear‐elastic part up to the first crack strength. The following is of the propagation of cracks, more microcracks are formed and developed in the specimen but the loading continues to increase during this stage which is called hardening and the material is still capable of resisting higher levels of loading up to a maximum. After the peak load is reached, the applied load becomes to go down, a single macrocrack has appeared and the material has started to soften. Compared with the control samples bended until final failure (scheme A), the preloaded samples (schemes B and C) have different stages, as described in Figure 5.1 (B). When the desired
) N k ( d a o L Unloading Preloading
Curing for 28 days
Reloading
Displacement (mm)
(B) Preloaded samples (schemes B and C) Figure 5.1 Comparison of load ‐displacement curves for different schemes
5.1.2 General results of stress‐deflection curves To represent the typical feature of different mixtures and schemes, the general results of stress‐deflection curves from three‐point bending test are presented in Figures 5.2‐5.4. For
M2 series (coarse capsule)
16
Scheme A
14
) a P12 M (10 s s e r 8 t s g n 6 i d n e 4 B
Scheme B Scheme C
2 0 0
1
2
Deflection (mm)
3
4
Figure 5.3 Bending stress‐deflection curves of M2 series
16 14
) a P12 M (10 s s e r t 8
M3 series (fine capsule) Scheme A Scheme B Scheme C
Scheme A (bend to final failure)
16
M1
14
M2
) a P12 M ( 10 s s e r 8 t s g n 6 i d n 4 e B
M3
2 0 0
1
2
3
4
Deflection (mm)
Figure 5.5 Comparison of deflection hardening behavior from different mixtures
5.1.4 Deflection capacity and recovery In this study, deflection capacity is a concern to evaluate the self ‐healing behavior in ECC material. Deflection capacity is defined as the deflection which corresponds to the maximum bending stress (flexural strength). And the recovery of the deflection capacity can be computed according
4 Scheme A
Scheme B
Scheme C
) m3 m ( y t i c a p2 a c n o i t c 1 e l f e D 0 M1
M2
M3
Mixture designation
(A) Deflection capacity 140%
y120% t i c a p100% a c n 80% o i t
Scheme A
Scheme B
Scheme C
(fine capsule) under three schemes is higher than that of M2 (coarse capsule), especially for scheme C shown in Figure 5.7 (A). From the view of normalized flexural strength (Figure 5.7 (B)), the values from M3 (fine capsule) nearly remain at a level of 100% of control value, which almost arrives at the same recovery level of M1 (no capsule). These seem to indicate that small capsule size attains beneficial influence on flexural strength and its recovery compared with large capsule size. On the other hand, it should be note that the flexural strength of M2 (with capsules) under scheme C (bend to 1.3mm deflection) exhibits a sharp reduction, and is recovered only about 60% of the control specimen. This may be explained that the flexural strength was already reached during the preloading stage for the case of 1.3mm deflection level. A single macrocrack was likely to be generated and this can be further confirmed by the microscopic observation in the later section.
20 18
)16 a P 14 M ( h t12 g n10 e r t s 8
Scheme A
Scheme B
Scheme C
5.1.6 Flexural stiffness and recovery Stiffness measurement was used to monitor the extent of self ‐healing within preloaded ECC specimens. In this research, the flexural stiffness is the equivalent slope of initial linear‐elastic stage of flexural stress‐deflection curve as shown in Figure 5.8, stiffness and its recovery can be calculated by the following formulas respectively:
Stiffness = tanθ =
σ ΔL
Normalized value = (
[MPa / mm]
scheme Y − control control
(3)
)flexural stiffness
(4)
where scheme Y stands for scheme A, scheme B and scheme C, control = scheme A
16 14
) a P12 M ( 10 s s e r 8 t s g n 6 i dσ n e 4 B
Flexural strength
Linear‐elastic stage
30 Scheme A
Scheme B
Scheme C
) 25 m m / a20 p M ( s s 15 e n f f 10 i t s l a r u 5 x e l F 0 M1
M2
M3
Mixture designation
(A) Flexural stiffness 140% 120% s s e n f f100% i t s l 80% a r u x e 60% l f
Scheme A
Scheme B
Scheme C
5.2 Nano‐CT observation 5.2.1 Identification of microcracks in nano‐ CT image As mentioned before, in order to check how the cracks develop in the interface zone between the capsule and the cement‐based matrix, the crack pattern before and after curing was investigated with nano‐CT scan. First of all, Figure 5.10 illustrates the components of the ECC material represent in the nano‐CT 2D images according to the grey‐level histogram. The pore and void inside the capsule are presented by the darkest, while the lightest corresponds to cement‐based matrix and the medium gray is an indication of the capsule. Due to the similarity of density between the paraffin wax and the SAP, the wax shell and the saturated SAP particles are very difficult to be distinguished in this gray‐level image.
Capsule Void inside
Matrix Crack
8 mm
“Opened”
“Closed”
(A) Before curing_28 days old
8 mm
After curing_56 days old
“Opened”
“Opened” “Opened”
“Closed” “Closed”
(A)
(B) Figure 5.12 3D view of the cracking pattern
5.2.2 Opening of capsules In this section, the opening possibility of capsules will be discussed. All capsules which might be opened by the cracks from the preloaded specimens (schemes B and C) of M2, M3 series (with capsules) were checked based on the principle of “opened” and “closed”. Figure 5.13 gives a summary of opened ratio of capsules, and point out that there are more than 50% capsules being
5.3 Light microscope and ESEM observations 5.3.1 Multiple cracking behavior The multiple cracking behavior of ECC is one of the distinct differences from normal concrete. Because of the fiber bridging effect, the cracks can progressively open. When the cement‐based matrix starts to crack, the fibers will slip out the matrix. With the matrix crack extends, the fibers can be completely pulled out from the matrix. As a result of this process, the ECC is able to take the increasing load and forms new cracks at other sites. Figure 5.14 obtained by the light microscope shows the typical crack pattern of multiple cracking behavior. The bridging effect of fibers observed under ESEM is also shown in Figure 5.15.
200 μm
5.3.2 Observation of interface zone Besides, the crack path at the interface between the capsule and the cement‐based matrix was also studied by ESEM technique. One thing is clear from the Figure 5.16: when the crack propagates across the wax shell, the crack width appears smaller than that of developing in the matrix. This is most probably associated with the characteristic of paraffin wax with a dense laminated structure. In some cases, the cracks go around instead of passing through the capsules, see Figure 5.17. This is caused by the weaker bond strength at the interface zone compared with that of the capsule itself.
Wax shell Crack path on the wax shell Crack path Matrix
Figure 5.16 Crack pattern on the wax shell
Crack
Under ESEM
Crack inside:
Cut
Cut
Under ESEM
Crack surface:
Figure 5.18 Illustration of crack inside and crack surface
3 1
2
4
Figure 5.20 ESEM images of crack surface
5
6
5.4 Discussion of potential of self ‐healing in ECC 5.4.1 Influencing factors In this research, six mechanical indicators are involved: deflection capacity, flexural strength, stiffness and their normalized values. Two factors (capsule content and capsule size) were designed to explore the self ‐healing potential of ECC material, and they are discussed as below,
vii. Capsule content (no capsule & with capsules)
As mentioned in Chapter 4.2.2, the ECC specimens without capsule (M1) and with capsules (M2 and M3) were taken into account to investigate how the capsules influence the healing properties. When comparing the magnitude of deflection capacity between M1 (without capsule) and M2 and M3 (with capsules) at each parallel scheme, the reference M1 (without capsule) always exceeds the performance of M2 and M3. The same trend also occurs in most cases of the flexural strength and stiffness. This can be explained that, the capsule itself behaves as the normal aggregate, and the presence of aggregates disturbs the matrix properties especially the fiber distribution of the ECC composite and has negative influences on the mechanical properties. However, from the results of recovered mechanical properties, it is noticed that the deflection (M2
M3)
more even distribution of multiple cracks. Thus small capsule size has more benefits on mechanical properties and mechanical recovery and cracking pattern since it changes matrix and fiber‐matrix properties to a less extent.
Macrocrack
Coarse_1.0mm
Fine_1.0mm
(A)
Coarse_1.3mm
28 days old (before reloading)
Fine_1.3mm
which is driven by the humidity gradient. According to Huang [24], the numerical simulation was established to show the relationship between the distance of moisture transport into the mortar and the time, see Figure 5.23. It indicates that the moisture will move far away from the crack surfaces with the increase of curing time. Similar to that, in this study it was confirmed that moisture had moved to the crack surface, but when the water supply is limited, the moisture content left on the crack surfaces is too little to produce more healing products in short curing period. Thus the sufficient water supplement is essential for desirable healing efficiency. The amount of water for further hydration of unhydrated cement will be investigated in the further research. Since the moisture transportation can be considered as the function of time, the curing time becomes another factor influencing the healing efficiency. However, when the amount of water is sufficient, the curing time is not a decisive factor for self ‐healing. 50 45 40
8h
8h
35
)30 m 25 m ( 20 Z 15 10
2h
2h 1h 0.5h
1h 0.5h
6 GENERAL CONCLUSIONS AND RECOMMENDATIONS 6.1 General conclusions In this thesis, the self ‐healing potential of the ECC material by means of available water released from the capsules containing the water saturated SAP has been investigated. Based on the experimental results of mechanical test and microscopic observation, the following conclusions can be drawn, The mass of water is almost not influenced by the curing time when the paraffin wax is used as sealing material. Due to its better capacity of water storage compared to that of epoxy‐cement paste, the paraffin wax was preferred as the sealing material in this research. Recovery of mechanical properties are regarded as the indicators of the self ‐healing efficiency. The recovery of deflection capacity, flexural strength and stiffness were examined in this research. The recovered deflection capacity was enhanced while the improvement on flexural strength and stiffness were rarely shown. It could be considered that the self ‐healing efficiency was not remarkable, since the mechanical properties were not significantly improved. The ECC specimens with the small capsule size of 5mm in diameter have preferable performance
6.2 Recommendations Several feasible improvements and further research to realize the self ‐healing of cementitious materials in concrete repair system can be given as following,
i.
Alternative sealing material
As discussed before, a suitable sealing material is not only capable of storing water, but also has high bond strength at the interface between the sealing material and the cementitious matrix. The capsule is considered as the weakest element in the composite and the interface bond of sealing material directly determines on the crack pattern, which influences the release of healing agent. Thus selection of a proper sealing material is crucial as a basis for further study.
ii.
Alternative water reservoir
Liapor particle is a promising candidate for carrying water instead of SAP. It has a high water absorption capacity (30%‐40% by weight) and the particle is roughly spherical with a diameter of 1‐10mm. Therefore, it can be made into a small capsule. As known, the capsule size significantly influences the mechanical properties of the composite. Moreover, control of capsule size is essential to the uniform distribution of capsules and the probability of capsule opening. The application of liapor needs to be studied further.
REFERENCES [1] M. G. Grantham, Diagnosis, inspection, testing and repair of reinforced concrete structures, M. G. Associates, 1999, p.3‐5 [2] Li V.C., Kanda T., Engineered cementitious composites for structural application,
ASCE J.
Materials in Civil Engineering, 1998, Vol.10, No.2, p.66‐69 [3] Zhou,J. Qian, S. Ye, G. Breguel, K van and Li, V.C., Development of Engineered Cementitious Composites with Limestone Powder and Blast Furnace Slag, submitted to Materials and Stuctures, 2009 [4] Huan He, Zhangqi Guo, Piet stroeven, Martijn Stroeven, Lambertus Johannes Sluys, Self ‐healing capacity of concrete – computer simulation study of unhydrated cement structure, Image Anal Stereol 2007, p.137‐143 [5] Tsuji M., Okuyama A., Enoki K. and Suksawang S., Development of new concrete admixture preventing from leakage of water through cracks, JCA Proc. Of Cement & Concrete 52, 1998, p.418‐423 [6] Li V.C., Engineered Cementitious Composites (ECC) – Tailored Composites Micromechanical Modeling, Fiber Reinforced Concrete: Present and the
though
Future, Canadian
Society of Civil Engineers, 1998, p.64‐97 [7] Yingzi Yang, Michael D. Lepech, En‐Hua Yang, Victor C. Li, Autogenous healing of engineered cementitious composites under wet‐dry cycles, Cement and Concrete Research 39, 2009, p.
application, Citus Books, 1999 [20] Victor C. Li, Yun Mook Lim, Yin‐Wen Chan, Feasibility study of a passive smart self ‐healing cementitious composite, Compisites part B, 1998, p.819‐827 [21] Henk M. Jonkers, Arjan Thijssen, Gerard Muyzer, Oguzhan Copuroglu, Erik Schlangen, Application of bacteria as self ‐healing agent for the development of sustainable concrete, Ecological Engineering, 2009 [22] Haoliang Huang, Guang Ye, Klaas van Breugel, Numerical simulation on moisture transport in cracked cement‐basis materials, 2010 [23] McMASTER ‐CARR, http://www.mcmaster.com/#barrel ‐tumblers/=7yvslm [24] Susan Wilson, Cost effective self healing concrete developed at URI, 2010
APPENDIX A Mix proportion of epoxy‐cement material
Mix proportion CEM I
Epoxy
52.5N [g]
[g]
20
0.6135
Hardener [g] 0.7730
Epoxy/hardener
Water
Water/cement
Epoxy/cement
ratio
[g]
ratio
ratio
1:1.26
5.6135
0.300
0.05
46
APPENDIX B Mix design of ECC
Material property and mix proportion of M1 Component
Density 3
Mix proportion
Weight
Material property and mix proportion of M2
Volume 3
[g/cm ]
(by weight)
[g]
[cm ]
Portland cement
3.15
1
550
175
BFS
2.85
1.2
660
232
2.70
2
1100
407
Water
1.00
1.092
601
601
Super‐plasticizer
1.17
0.030
16.5
PVA fiber
1.30
Limestone powder
0.02 (by volume)
Saturated SAP
0
inside capsule
(by CEM weight)
Number of capsules
ratio Total
Component
[mm]
Density 3
Mix proportion
Weight
Volume 3
[g/cm ]
(by weight)
[g]
[cm ]
Portland cement
3.15
1
550
175
BFS
2.85
1.2
660
232
2.70
2
1100
407
Water
1.00
1.092
601
601
14
Super‐plasticizer
1.17
0.030
16.5
14
37.1
29
PVA fiber
1.30
37.1
29
0
0
11.0
6.7
Limestone powder
0.02 (by volume)
Saturated SAP
2%
inside capsule
(by CEM weight)
0
Number of
25
(0% by volume)
capsules
(0.46% by volume)
Capsule size Water/powder
Diameter
0
Capsule size Water/powder
0.26
ratio 2964
1457
Total
47
Diameter [mm]
8 0.26 2975
1465
Material property and mix proportion of M3 Mix proportion
Weight
Volume
[g/cm ]
(by weight)
[g]
[cm ]
Portland cement
3.15
1
550
175
BFS
2.85
1.2
660
232
2.70
2
1100
407
Water
1.00
1.092
601
601
Super‐plasticizer
1.17
0.030
16.5
14
PVA fiber
1.30
37.1
29
11.0
3.3
Component
Limestone powder
Density 3
0.02 (by volume)
Saturated SAP
2%
inside capsule
(by CEM weight)
Number of
50
capsules
(0.23% by volume)
3
Capsule size Water/powder ratio Total
Diameter [mm]
5 0.26 2975
1461
48
APPENDIX C Converting the weight of SAP into the number of capsules Basically, capsules in the same series were controlled to be the similar size according to different requirements of capsule size (M2: diameter of 8mm, M3: diameter of 5mm). During the making process of capsules, saturated SAP particles were shaped into a ball by a thin cover of cement, and less cement was remained especially after curing them in water for 7 days. On the other hand, dry SAP powder can absorb amounts of free water many times than its own weight. Based on above two points, the weight of cement cover and SAP particles can be assumed to be neglected, the weight of ball is therefore considered as the weight of water only. As a result, the calculation of “saturated SAP” with 2% weight ratio of cement in Appendix B, means the amount of free water needed for healing, which is represented by Wwater. This section interprets that how to convert the amount of free water into the number of capsules. First, the saturated SAP particles after shaping by cement cover and 7 days’ curing in water is called a” ball”, and the weight of a ball actually is the weight of water as explained above. In order to investigate the weight of a ball, 100 balls were divided into 10 groups and each group included 10 balls. Each group of balls was weighed by electric balance, and then the average weight of one ball (Wball) can be evaluated. Due to accidental errors, it was proposed that the number of capsules for different mixtures was multiplied by a magnification factor 1.1, then it follows that:
W
APPENDIX D Three‐point bending test configuration and bending stress calculation F Specimen
A
m m 0 4 = h
A
L=55mm
110mm 160mm
b=40mm
Test set‐up
Section A‐A
M=F/2*L Moment diagram
Bending stress at mid‐span: σ =
M W
=
(F/2)*L 2
(1 / 6) *b * h
=
(F /2)* 55 2
(1 / 6) * 40 * 40
= 0.002578125F[kN ⋅mm
50
2
] = 2.578125F[MPa]
APPENDIX E Bending stress‐deflection curves M1b (preloaded to 1.0mm_28days old)
M1a (no capsule_28days old)
M1c (preloaded to 1.3mm_28days old)
16
16
M1b ‐1
14
M1a ‐1
) 14 a P 12 M ( s s 10 e r t s 8 g n 6 i d n 4 e B
16
) a P 12 M ( s 10 s e r t 8 s g n 6 i d n 4 e B
M1a ‐2
M1b ‐2
2
2
M1c‐1
14
M1c‐2
) a P12 M ( s10 s e r t 8 s g n 6 i d n e 4 B
M1c‐3 M1c‐4
2
0
0
0
0
1
2
3
0
4
1
2
3
4 0
1
Deflection (mm)
Deflection (mm)
2
Deflection (mm)
M1b (no capsule_56days old)
M1c (no capsule_56days old)
16
16 M1b ‐1
14
) a P 12 M ( s 10 s e r t 8 s g n 6 i d n e 4 B
M1c‐1
14
M1c‐2
) a P12 M ( s10 s e r t 8 s g n 6 i d n e 4 B
M1b ‐2
2
M1c‐3 M1c‐4
2
0
0 0
1
2
3
4
0
Deflection (mm)
1
2
Deflection (mm)
51
3
4
3
4
M2a (Capsule size of 8mm_28days old)
16
M2a ‐1
14
) a12 P M ( s 10 s e r 8 t s g n i 6 d n e 4 B
M2a ‐3
M ( s10 s e r t 8 s g n 6 i d n e 4 B 2
2
0
0 0
1
2
3
M2c‐2
2 0 1
Deflection (mm)
2
3
4
0
1
Deflection (mm)
M2b (Capsule size of 8mm_56days old)
16
M2c‐1
14
) a12 P M ( s10 s e r 8 t s g n i 6 d n e 4 B
M2b‐ 3 M2b‐ 4
0
4
M2c (preloaded to 1.3mm_28days old)
16
M2b‐ 1 M2b‐ 2
14
M2a ‐2
) a P12
M2b (preloaded to 1.0mm_28days old)
16
14
M2c‐1
14
M2b ‐ 2
) a12 P M ( 10 s s e r t 8 s g n 6 i d n e 4 B
M2c (Capsule size of 8mm_56days old)
16 M2b ‐ 1
) a P12 M ( s10 s e r 8 t s g n 6 i d n e 4 B
M2b ‐ 3 M2b ‐ 4
2
Deflection (mm)
M2c‐2
2
2
0
0 0
1
2
3
0
4
Deflection (mm)
1
2
Deflection (mm)
52
3
4
3
4
M3a (Capsule size of 5mm_28days old)
16
M3b (preloaded to 1.0mm_28days old)
M3c (preloaded to 1.3mm_28days old)
16 M3a ‐ 1
14
) a P 12 M ( 10 s s e r t 8 s g n 6 i d n e 4 B
M3a ‐ 3 M3a ‐ 4
2
2
0
0 0
1
2
3
M3b‐1
14
M3a ‐ 2
) a P 12 M ( s 10 s e r t 8 s g n 6 i d n e 4 B
16
4
M3c‐1
14
) a P 12 M ( s 10 s e r t 8 s g n 6 i d n e B 4
M3b‐2 M3b‐3
M3c‐2 M3c‐3
2 0 0
1
Deflection (mm)
2
3
0
4
1
Deflection (mm)
M3b (Capsule size of 5mm_56days old)
2
Deflection (mm)
M3c (Capsule size of 5mm_56days old)
16
16 M3b‐1
14
) a P12 M ( 10 s s e r t 8 s g n 6 i d n e 4 B
14
M3c‐1
) a P12 M ( 10 s s e r t 8 s g n 6 i d n e 4 B
M3b‐2 M3b‐3
2
M3c‐2 M3c‐3
2
0
0 0
1
2
3
4
0
Deflection (mm)
1
2
Deflection (mm)
53
3
4
3
4
APPENDIX F Results of opened ratio of capsules under nano‐CT observation
Sample M2b‐1 M2b‐2 M2b‐3 M2c‐1 M2c‐2 M2c‐3 M3b‐1 M3b‐2 M3b‐3 M3c‐1 M3c‐2 M3c‐3
Number of opened capsules Front 6 8 4 5 4 5 5 6 8 12 9 9
Right 6 7 4 5 4 5 5 6 7 11 7 9
Average 6 8 4 5 4 5 5 6 8 12 8 9
Number of closed capsules Front 3 4 6 3 4 4 6 5 7 7 5 8
Right 4 4 6 3 4 3 6 5 6 6 7 9
54
Average 4 4 6 3 4 4 6 5 7 7 6 9
Total capsules Opened ratio 10 12 10 8 8 9 11 11 14 18 14 18
63% 65% 40% 63% 50% 59% 45% 55% 54% 64% 57% 51%
APPENDIX G Results of EDX of the preloaded ECC specimens Crack surface‐‐‐Location 1 Element
Wt %
At %
K‐ Ratio
Crack surface‐‐‐Location 3
Z
A
F
Element
Wt %
At %
K‐ Ratio
Z
A
F
C
5.14
9.53
0.0156
1.0500
0.2884
1.0010
C
4.85
9.51
0.0153
1.0546
0.2989
1.0011
O
42.74
59.53
0.0625
1.0323
0.1417
1.0001
O
37.07
54.56
0.0488
1.0368
0.1268
1.0001
Mg
1.25
1.15
0.0056
0.9902
0.4497
1.0028
Mg
1.13
1.10
0.0050
0.9944
0.4456
1.0029
Al
1.64
1.35
0.0093
0.9610
0.5870
1.0050
Al
1.47
1.29
0.0083
0.9651
0.5836
1.0052
Si
4.82
3.82
0.0337
0.9890
0.7031
1.0064
Si
4.17
3.50
0.0293
0.9932
0.7015
1.0074
S
0.63
0.44
0.0053
0.9767
0.8478
1.0206
S
0.50
0.37
0.0043
0.9820
0.8513
1.0241
K
0.66
0.37
0.0066
0.9394
0.9724
1.0991
K
0.87
0.53
0.0090
0.9440
0.9743
1.1156
Ca
42.05
23.38
0.3994
0.9615
0.9873
1.0005
Ca
48.87
28.71
0.4665
0.9660
0.9877
1.0004
Fe
1.07
0.43
0.0090
0.8742
0.9643
1.0000
Fe
1.06
0.45
0.0089
0.8787
0.9564
1.0000
Total
100.00
100.00
Total
100.00
100.00
Element
Wt %
At %
K‐ Ratio
C
6.24
11.58
Crack surface‐‐‐Location 2
Crack surface‐‐‐Location 4
Z
A
F
Element
Wt %
At %
K‐ Ratio
0.0186
1.0501
0.2831
1.0010
C
7.12
12.80
0.0209
Z
A
F
1.0476
0.2794
1.0009
O
40.61
56.61
0.0581
1.0324
0.1385
1.0001
O
42.87
57.88
0.0649
1.0300
0.1470
1.0001
Mg
1.34
1.23
0.0061
0.9903
0.4554
1.0030
Mg
1.60
1.42
0.0073
0.9880
0.4600
1.0030
Al
1.82
1.50
0.0104
0.9612
0.5920
1.0052
Al
2.01
1.61
0.0115
0.9589
0.5944
1.0050
Si
5.35
4.25
0.0376
0.9891
0.7058
1.0065
Si
5.75
4.42
0.0403
0.9868
0.7061
1.0059
S
0.67
0.47
0.0057
0.9768
0.8454
1.0206
S
0.54
0.37
0.0045
0.9739
0.8427
1.0187
K
0.70
0.40
0.0070
0.9396
0.9710
1.0988
K
0.67
0.37
0.0066
0.9371
0.9704
1.0898
Ca
42.51
23.65
0.4032
0.9616
0.9860
1.0003
Ca
38.66
20.83
0.3656
0.9591
0.9858
1.0003
Fe
0.77
0.31
0.0064
0.8744
0.9636
1.0000
Fe
0.77
0.30
0.0065
0.8719
0.9681
1.0000
Total
100.00
100.00
Total
100.00
100.00
55
