DESIGNING STRUCTURES WITCH NEW SELF-HEALING MATERIALS
Thomas J Henderson
3rd Year MEng Civil Engineering
Student ID: 1534812
Project Supervisor: Tony Jefferson
I hereby declare:
that except where reference has clearly been made to work by others, all the work presented in this report is my own work;
that it has not previously been submitted for assessment; and
that I have not knowingly allowed any of it to be copied by another student.
I understand that deceiving or attempting to deceive examiners by passing off the work of another as my own is plagiarism. I also understand that plagiarising the work of another or knowingly allowing another student to plagiarise from my work is against the University regulations and that doing so will result in loss of marks and possible disciplinary proceedings against me.
Table of Contents
1. Introduction 6
2. Literature Review 8
2.1. Introduction 8
2.2. Crack formation 9
2.3. Self-Healing 10
2.3.1. Passive & Active modes 10
2.3.2. Autonomic healing 10
2.3.3. Autogenous healing 10
2.4. Autonomic Healing in Concrete 11
2.4.1. Encapsulated 11
2.4.2. Vascular Network 14
2.5. Autogenous healing in concrete 15
2.5.1. Calcium Carbonate 15
2.5.2. Enhancement of Autogenous healing 16
2.6. Bond 18
3. Methodology 21
3.1. Introduction 21
3.2. Material Composition 21
3.2.1. Mortar 21
3.2.2. Mould Setup 22
3.2.3. PETg 23
3.3. Preliminary measures 24
3.3.1. 3D Printing 24
3.3.2. Filament 25
3.4. Specimen Casting 25
3.4.1. Procedure 25
3.5. Structural Design 27
3.5.1. Design 1 27
3.5.2. Design 2 27
3.5.3. Design 3 28
3.5.4. Design 4* 28
3.6. Testing 28
3.6.1. Beam Bending 28
188.8.131.52. Flexure specifications 29
3.6.2. Cube & Cylinders 29
3.6.3. Tensile tests 29
4. Theory 29
4.1. Under-reinforced moment calculations 29
4.2. Crack width calculations 30
4.3. Bond stress 32
4.4. Tensile tests 33
5. Theoretical Analysis 34
5.1.1. Cube & Cylinders 34
5.2. Tensile 35
5.3. Bond 36
5.4. Beam Calculations 36
5.4.1. Ribbed 5mm 37
5.4.2. Spiral 3mm 39
5.4.3. Spiral 5mm 40
6. Experimental Results & Discussion 42
6.1. Cube & Cylinder test results 43
6.1.1. Cube 44
6.1.2. Cylinder 44
6.2. PETg 44
6.2.1. Tensile Tests 44
6.2.2. 3D Printing 46
6.3. Bond 47
6.4. Design of new structures for self-healing 48
7. Conclusion 54
Concrete, one of the most commonly used building materials in the world is used for a vast array of infrastructure and is well known for its high compressive strength but relatively low tensile strength, with such properties it is easy for crack formation to appear when a force is applied, for e.g. a reinforced concrete beam in structures across the world. The crack formation allows the ingress of harmful substances and chemicals that can cause corrosion from the inside out as well as greatly effecting non-resistant materials such as steel, reducing the mechanical properties of the beam. Once crack formation has occurred repair works must take place to the ensure safety of the structure, such repairs cause issues that are: time-consuming, labour intensive and sometimes cannot be repaired due to inaccessibility. With an increasing amount of research being carried out over the last several decades, evidence suggests that the self-healing process can fully or partially enclose cracks that have formed.
Knowledge has significantly increased in the two main areas of self-healing: 1) Autogenous healing which mainly focuses on ongoing hydration of un-hydrated cells within the concrete matrix or carbonation of calcium hydroxide, 2) Autonomic healing that focuses on the implementation of a healing agent through various techniques such as encapsulation and vascular networks.
Include a section where you give a brief overview of the literature and previous work that has been done on self-healing for concrete and mention when self-healing was first introduced, the progression it has made in the last several decades.
The aim of this project is to not delve into the field of how to enhance or improve self-healing processes but rather take a look at the materials used to encapsulate the healing-agents and how they behave. Through the use of 3D printing reinforcement with PETg to gain a better understanding of the material and the techniques used to produce the reinforcement, giving an insight into their strengths and weaknesses. The main aims of the project are as follows: give a quick over of aims
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2. Literature Review
Before the discussion of results in this paper, it is important to understand and have knowledge of the self-healing processes in cementitious material, providing brief background knowledge of articles published on self-healing and the techniques found in articles, journal books etc. Studies that have reviewed self-healing will focus on the appearance of microcracks in concrete structures and how these cracks affect the mechanical properties, these cracks can form due to a variety of factors: plastic shrinkage, thermal stresses, settlement, drying shrinkage, weathering, corrosion of reinforcement or due to an applied load (Fernandez, 2012)
There are 3 main types of self-healing approaches in concrete which include autogenous, vascular, and capsule-based self-healing as described by (Van Tittelboom, 2013) In this literature review, a broad amount of information will be conveyed to try and gain an understanding of the key elements that effect self-healing efficiency, for instance: admixtures, structural bodies, temperatures, water/air cured, crack width and many more. Each self-healing method has been tested with multiple substances to see how the efficiently the self-healing acts when in a different structural form. It is of great importance that these structural bodies are studied with varying materials and dimensions to understand how and when they will break to release the healing agent through capillary action.
2.2. Crack formation
The formation of cracks in cementitious materials can be associated with volumetric changes such as shrinkage, thermal stress, creep and direct stress caused by fatigue, environmental effects as well as flexural stress caused by bending can cause damage to the structure integrity (Nawy, 1991). It is important to understand these behaviours affect the rate of crack width as well as the crack width itself.
Concrete is known to be strong in compression however weak in tension therefore after a period of time a crack opening can appear in the tension zone (Concrete-Society, n.d.), better visualized as a sagging moment in a concrete prism/beam. It is often common practice to pair concrete with a material member such as rebar which is strong in tension to create a strong and reliable body to support the factors that may want to hinder its mechanical properties.
Because of the weakness of concrete in tension, these cracks are prone to form and there is little that can be done to stop this process from happening. Therefore, it has been found that substances in liquid or gas form can easily access the rebar and cementitious matrix, causing deterioration of the tension support and will undergo a process called alkali-silica reaction
2.3.1. Passive & Active modes
The two types of healing that can take place to heal materials are stated as passive & active modes. Passive which is a time-released process where chemicals are released due to a rupture or fibre breakage within the material and therefore passive healing takes places. The active mode of self-healing is done with human interaction through activation of a healing agent that flows through a vascular network. (Dry, 1994)
2.3.2. Autonomic healing
Autonomic healing is where the healing process is non-intrinsic and heavily relies on the additives and manufactured bodies which have been implemented into the material itself (C. Joseph, 2008). This process is for a more instantaneous healing method where small cracks can be healed before propagating to a state that would greatly affect the mechanical properties of the concrete specimen.
2.3.3. Autogenous healing
Autogenous healing was first discovered in 1836 by the French Academy of Science however it wasnt until much later, that autogenous healing in concrete was studied. Autogenous healing in concrete can be referred to as the independent work of the chemicals and materials of the specimen itself, it is intrinsic to ones self and does not require the additives of healing agents to recover its mechanical properties (M. Rooij, 2013). It has been found that autogenous healing is most prominent in early age cracking of concrete due to the high volume of un-hydrated cells which encounter water, therefore, causing a self-sealing effect (SSE).
2.4. Autonomic Healing in Concrete
Encapsulation of healing agents is where a structural body holds a healing agent and is released upon crack formation, the crack causes failure to the capsule and ruptures the shell. Causing a release for the agent to flow out and fill the crack through capillary action, which can be a problem within itself. Having triggered the rupture of the capsule, the agent is still to be activated via an embedded catalyst causing the bonding between the crack faces, therefore, healing the crack and potentially regaining the mechanical properties lost through initial propagation. (M.R. Kessler, 2003)