K. Van Tittelboom

Methyl methacrylate as a healing agent for self-healing cementitious materials

Different types of healing agents have already been tested on their efficiency for use in self-healing cementitious materials. Generally, commercial healing agents are used while their properties are adjusted for manual crack repair and not for autonomous crack healing. Consequently, the amount of regain in properties due to self-healing of cracks is limited. In this research, a methyl methacrylate (MMA)-based healing agent was developed specifically for use in self-healing cementitious materials. Various parameters were optimized including the viscosity, curing time, strength, etc. After the desired properties were obtained, the healing agent was encapsulated and screened for its self-healing efficiency. The decrease in water permeability due to autonomous crack healing using MMA as a healing agent was similar to the results obtained for manually healed cracks. First results seem promising: however, further research needs to be undertaken in order to obtain an optimal healing agent ready for use in practice.

Detecting the Activation of a Self-Healing Mechanism in Concrete by Acoustic Emission and Digital Image Correlation

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process.

Performance monitoring of large-scale autonomously healed concrete beams under four-point bending through multiple non-destructive testing methods

Concrete is still the leading structural material due to its low production cost and great structural design flexibility. Although it is distinguished by such a high durability and compressive strength, it is vulnerable in a series of ambient and operational degradation factors which all too frequently result in crack formation that can adversely affect its mechanical performance. The autonomous healing system, using encapsulated polyurethane-based, expansive, healing agent embedded in concrete, is triggered by the crack formation and propagation and promises material repair and operational service life extension. As shown in our previous studies, the formed cracks on small-scale concrete beams are sealed and repaired by filling them with the healing agent. In the present study, the crack formation and propagation in autonomously healed, large-scale concrete beams are thoroughly monitored through a combination of non-destructive testing (NDT) methods. The ultrasonic pulse velocity (UPV), using embedded low-cost and aggregate-size piezoelectric transducers, the acoustic emission (AE) and the digital image correlation (DIC) are the NDT methods which are comprehensively used. The integrated ultrasonic, acoustic and optical monitoring system introduces an experimental configuration that detects and locates the four-point bending mode fracture on large-scale concrete beams, detects the healing activation process and evaluates the subsequent concrete repair.

Recovery against Mechanical Actions

Autogenic self-healing has been defined in chapter 1 as a self-healing process where the recovery process uses materials components that could also be present when not specifically designed for self-healing (own generic materials).

Recovery against Environmental Action

Autogenic self-healing has been defined in chapter 1 as a self-healing process where the recovery process uses materials components that could also be present when not specifically designed for self-healing (own generic materials).

Use of Acoustic Emission Analysis to Evaluate the Self-Healing Capability of Concrete

It has been estimated that, in Europe, 50% of the annual construction budget is spent on refurbishment and remediation of the existing structures [1]. Therefore, self-healing of concrete structures, which are very sensitive to cracking, would be highly desirable. In this research, encapsulated healing agents were embedded in the concrete matrix in order to obtain self-healing properties. Upon crack appearance, the capsules break and the healing agent is released, resulting in crack repair. The efficiency of this crack healing technique was evaluated by using acoustic emission (AE) analysis. Breakage of the capsules was proven as events with an energy higher than the energy related to concrete cracking were noticed. Upon reloading of beams with untreated cracks, fewer emissions were detected compared to beams with healed cracks. From this study it was shown that AE is a suitable technique to evaluate self-healing of cracks in concrete.

Visualization Of The Healing Process OnReinforced Concrete Beams By Application OfDigital Image Correlation (DIC)

Fabrication of concrete with self-healing capabilities has recently become a hot research topic. In general, material science is focused on the development of smart engineering concrete and cementitious composites with an extended service life. Indeed, materials that remain durable and keep their mechanical performance, damage mechanisms occurring should heal by themselves. In the case of this study, formation of damage and recovery of the mechanical properties is investigated by application of an encapsulated healing agent. On an experimental level, it is imperative to implement an optical, non-contact and online technique to visualize and compare the crack propagation at the loading and reloading (when the initial cracks are filled by the healing agent) stage. For that reason, optical measurements by application of Digital Image Correlation (DIC) are performed during the tests. Processing images captured by a 4-digital cameras system during all the loading stages of four-point bending tests give a fullfield view of the crack displacement and strain profiles. A step further, the visualization of the cracking phenomena by DIC offers a useful tool to apply fracture theories of concrete on healing systems.

Experimental techniques used to verify healing

There are many techniques to study materials. In this chapter the focus of experimental techniques to verify crack sealing (recovery against environmental actions) or crack healing (recovery against mechanical actions) has been limited to techniques that have been used and reported in self-sealing and self-healing research.

The Effect of Print Parameters on the (Micro)structure of 3D Printed Cementitious Materials

The extrusion-based 3D printing method is one of the main additive manufacturing techniques worldwide in construction industry. This method is capable to produce large scale components with complex geometries without the use of an expensive formwork. The main advantages of this technique are encountered by the fact that the end result is a layered structure. Within these elements, voids can form between the filaments and also the time gap between the different layers will be of great importance. These factors will not only affect the mechanical performance but will also have an influence on the durability of the components. In this research, a custom-made 3D printing apparatus was used to simulate the printing process. Layered specimens with 0, 10 and 60 min delay time (i.e. the time between printing of subsequent layers) have been printed with two different printing speeds (1.7 cm/s and 3 cm/s). Mechanical properties including compressive and inter-layer bonding strength have been measured and the effect on the pore size and pore size distribution was taken into account by performing Mercury Intrusion Porosimetry (MIP) tests. First results showed that the mechanical performance of high speed printed specimens is lower for every time gap due to a decreased surface roughness and the formation of bigger voids. The porosity of the elements shows an increasing trend when enlarging the time gap and a higher printing speed will create bigger voids and pores inside the printed material.

Design and testing of tubular polymeric capsules for self-healing of concrete

Polymeric healing agents have proven their efficiency to heal cracks in concrete in an autonomous way. However, the bottleneck for valorisation of self-healing concrete with polymeric healing agents is their encapsulation. In the present work, the suitability of polymeric materials such as poly(methyl methacrylate) (PMMA), polystyrene (PS) and poly(lactic acid) (PLA) as carriers for healing agents in self-healing concrete has been evaluated. The durability of the polymeric capsules in different environments (demineralized water, salt water and simulated concrete pore solution) and their compatibility with various healing agents have been assessed. Next, a numerical model was used to simulate capsule rupture when intersected by a crack in concrete and validated experimentally. Finally, two real-scale self-healing concrete beams were made, containing the selected polymeric capsules (with the best properties regarding resistance to concrete mixing and breakage upon crack formation) or glass capsules and a reference beam without capsules. The self-healing efficiency was determined after crack creation by 3-point-bending tests.