Experimental and numerical modelling of the performance of self-healing concrete

Transcription

1 Experimental and numerical modelling of the performance of self-healing concrete Petros Giannaros Geotechnical and Environmental Research Group Civil Engineering Division Department of Engineering University of Cambridge Trumpington Street Cambridge, England CB2 1PZ, United Kingdom Supervisor: Prof. Abir Al-Tabbaa First year report towards the degree of Doctor of Philosophy Jesus College August 2014

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3 Abstract Microcracking in concrete creates durability issues that can eventually lead to structural failure. Autogenic, or natural, self-healing of these cracks occurs mainly if unhydrated cement remains in the concrete matrix due to insufficient mixing or initial lack of water. Autogenic self-healing capability of concrete can be enhanced through autonomic, or engineered, selfhealing. Here, microcapsules containing healing agent are added into the concrete mix which are subsequently ruptured when cracks propagate through them. This healing agent then flows into the crack volume, hardening and bonding to the crack face. This then prevents the ingress of harmful fluids into concrete. Autonomic healing of concrete has great potential to increase structure longevity and reduce maintenance costs. The broader aim of this work is to numerically model the performance of concrete incorporating microcapsules for autonomic healing. To achieve this, the first objective is to assess the effect that the addition of capsules has on the mechanical properties of concrete. This assessment is the first step in ensuring the technology is feasible both practically and economically. The addition of microcapsules must not have a significant adverse effect on the mechanical properties of the hardened concrete. If a reduction in mechanical properties is observed, this must be outweighed by the increase in performance arising following a healing event. The effect of capsule addition on the mechanical properties of cementitious materials was explored through a literature review as well as experiments. Two numerical models were used to quantify the reduction in stiffness caused by the addition of microcapsules. It was found that there is little agreement with data presented in the literature due to the variation in experimental and testing procedures as well as the difference in properties of microcapsules. Furthermore, the thickness of the microcapsule shell is a pivotal factor in determining whether capsule addition will have an increasing or decreasing effect on the composite mechanical properties. Further data is required to develop, and verify, a suitable numerical model that is capable of predicting the strength and stiffness of a cementitious material containing microcapsules.

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5 Contents Contents v 1 Introduction Background Aims and objectives Structure of the report Literature Review Self-healing definitions Self-healing cementitious materials Autogenic healing of concrete Autonomic healing of concrete Microencapsulation Microencapsulation in building construction materials Microencapsulation for self-healing cementitious materials Bacterial Encapsulation Requirements of microcapsules used in self-healing concrete Effect on mechanical properties Challenges with microencapsulation for self-healing concrete Modelling self-healing materials Mesoscale modelling Macroscale modelling Modelling autogenic self-healing Modelling autonomic self-healing via microencapsulation Materials and Methods Materials... 27

6 vi Contents Cement Sand Microcapsules Other materials Sample preparation Results Experimental and numerical work Microcapsule characterisation Determination of microcapsule size using image analysis Microcapsule weight and thickness Analogies of microcapsules in a concrete matrix Capsules modelled as entrapped air Capsules modelled as inclusions Capsules modelled as fibres or aggregates Compressive and flexural strengths Microscopic observations Model evaluation Conclusion and Future Work Conclusion Future work References 57

7 Chapter 1 Introduction 1.1 Background Damage and degradation of engineered materials with time is inevitable. For this reason, structures and associated materials are often designed to exceed their required specification. This is achieved through the use of safety factors as well as through the addition of redundant structural members. Materials are often combined to enhance overall performance; for example in reinforced concrete and metal alloys. Frequent inspection, maintenance and repair is thereafter necessary in order to maintain an acceptable level of performance. In contrast, self-healing materials are able to respond to inflicted damage and possess the ability to repair themselves. Self-healing materials, or often more appropriately self-healing material systems due to the interaction of various healing components, can be seen frequently in nature. For example, bone is a self-healing material that is able to completely heal following fracture. The principle behind the development of self-healing materials is therefore to introduce, or enhance, our engineered materials with self-healing capability. Due to its appealing concept, self-healing materials are receiving increasing attention from researchers in various fields as well as from industry. Measured by tonnage, concrete is the most widely-used man made material on earth and cement production itself is estimated to contribute between 5-7% of global CO 2 emissions [44]. Self-healing of concrete is therefore attractive from an environmental perspective. An increase in the service life of structures will result in a reduction in the use of materials for refurbishment as well as those required for re-building. It has been estimated that, in Europe, 50% of the annual construction budget is spent on rehabilitation and repair of existing structures [9]. Clearly, self-healing of materials may also be financially attractive. The main attraction to self-healing concrete is related to durability issues. Cracks of vari-

8 2 Introduction able sizes occur in concrete due to changes in temperature, hydration and loading. Cracks due to loading may compromise the structural integrity of the material whilst microcracks (those up to a width of 0.1mm) are almost inevitable and not themselves considered harmful. However, microcracks do create a durability issue and may eventually contribute to failure. The reason for this is that microcracks propagate and create channels in the concrete which increase its permeability and thus facilitate the flow of, potentially harmful, fluids. Sulphates, sea water and acid may all potentially attack the concrete. As well their damaging effects on concrete, these substances have the potential to corrode the steel reinforcement. Sound concrete provides a protective medium for reinforcement due to the formation of a thin protective film of iron oxide on the metal surface. However, if this layer is broken down and if water and oxygen are present then the reinforcement will rust. Rusting causes swelling of the reinforcement and thus causes cracking of the concrete which further aids the corrosion process. Additionally, in some applications, it is important that concrete is considered impermeable. For example, in the case of concrete gravity dams or high-density concrete used for the protection of radioactive waste. Autonomic self-healing using microcapsules has been developed for cementitious materials [44]. Here, microcapsules containing healing agent are embedded in the cementitious matrix which subsequently release their content once the shell is ruptured due to cracking. The majority of works to date have focused on verifying the self-healing principle and there has been little focused investigation on the effect of capsule addition on mechanical properties. This is the first step in analysing any self-healing system with embedded microcapsules. The variability in microcapsule materials and manufacture techniques has led to mixed report of the effect of microcapsule addition on cementitious materials. Some have reported a decrease in mechanical properties whilst others have reported an enhancement. It is necessary to assess the effect that different microcapsule parameters have on the macroscopic properties of cementitious materials. Difficulty in obtaining microcapsules of varying geometries and materials, as well as the inherent inhomogeneous nature of cementitious materials, makes the collection of clear data problematic. The ability to model these varying parameters would be optimal. Furthermore, simulation of the constantly changing macroscopic mechanical behaviour of a self-healing concrete structure incorporating multiple self-healing members is desired. Numerical modelling facilitates the exploration of long-term structural behaviour without the need to build physical structures and monitor them over decades.

9 1.2 Aims and objectives Aims and objectives The aim of this research project is to model the performance of concrete containing microencapsulated healing agent for autonomic healing. This requires the simulation of initial mechanical behaviour as well as those after damage and subsequent healing has occurred. The objectives are therefore to: Quantify the effect that the addition of microcapsules has on mortar specimens both experimentally and numerically Analyse the response of healed specimens to further cracking qualitatively through experiments and modelling Verify the parameters that determine whether a microcapsule is ruptured or debonded when cracking occurs and confirm through microscopic observations Simulate the recovery of compressive strength pre-failure due to the addition of microcapsules containing healing agent Affirm model suitability for concrete specimens 1.3 Structure of the report The report consists of five chapters. In Chapter 1, background information as well as the aims and objectives of this research are given. In Chapter 2, a literature review is given on research relevant to autonomic self-healing of cementitious materials via microencapsulation. Both experimental and numerical literature is reported. In Chapter 3, details of the experimental procedure involved for mortar sample preparation is given as well as a description of the materials used. Chapter 4 collates and discusses both the experimental and numerical results. Chapter 5 then concludes the report and includes a schedule of future work. References can be found at the end of the report.