Branko Šavija

Simulation-Aided Design of Tubular Polymeric Capsules for Self-Healing Concrete

Polymeric capsules can have an advantage over glass capsules used up to now as proof-of-concept carriers in self-healing concrete. They allow easier processing and afford the possibility to fine tune their mechanical properties. Out of the multiple requirements for capsules used in this context, the capability of rupturing when crossed by a crack in concrete of a typical size is one of the most relevant, as without it no healing agent is released into the crack. This study assessed the fitness of five types of polymeric capsules to fulfill this requirement by using a numerical model to screen the best performing ones and verifying their fitness with experimental methods. Capsules made of a specific type of poly(methyl methacrylate) (PMMA) were considered fit for the intended application, rupturing at average crack sizes of 69 and 128 μm, respectively for a wall thickness of ~0.3 and ~0.7 mm. Thicker walls were considered unfit, as they ruptured for crack sizes much higher than 100 μm. Other types of PMMA used and polylactic acid were equally unfit for the same reason. There was overall good fitting between model output and experimental results and an elongation at break of 1.5% is recommended regarding polymers for this application.

Microscale Testing and Modelling of Cement Paste as Basis for Multi-Scale Modelling

This work aims to provide a method for numerically and experimentally investigating the fracture mechanism of cement paste at the microscale. For this purpose, a new procedure was proposed to prepare micro cement paste cubes (100 × 100 × 100 µm3) and beams with a square cross section of 400 × 400 µm2. By loading the cubes to failure with a Berkovich indenter, the global mechanical properties of cement paste were obtained with the aid of a nano-indenter. Simultaneously the 3D images of cement paste with a resolution of 2 µm3/voxel were generated by applying X-ray microcomputed tomography to a micro beam. After image segmentation, a cubic volume with the same size as the experimental tested specimen was extracted from the segmented images and used as input in the lattice model to simulate the fracture process of this heterogeneous microstructure under indenter loading. The input parameters for lattice elements are local mechanical properties of different phases. These properties were calibrated from experimental measured load displacement diagrams and failure modes in which the same boundary condition as in simulation were applied. Finally, the modified lattice model was applied to predict the global performance of this microcube under uniaxial tension. The simulated Young’s modulus agrees well with the experimental data. With the method presented in this paper the framework for fitting and validation of the modelling at microscale was created, which forms a basis for multi-scale analysis of concrete.

Influence of Microencapsulated Phase Change Material (PCM) Addition on (Micro) Mechanical Properties of Cement Paste

Excessive cracking can be a serious durability problem for reinforced concrete structures. In recent years, addition of microencapsulated phase change materials (PCMs) to concrete has been proposed as a possible solution to crack formation related to temperature gradients. However, the addition of PCM microcapsules to cementitious materials can have some drawbacks, mainly related to strength reduction. In this work, a range of experimental techniques has been used to characterize the microcapsules and their effect on properties of composite cement pastes. On the capsule level, it was shown that they are spherical, enabling good distribution in the material during the mixing process. Force needed to break the microcapsules was shown to depend on the capsule diameter and the temperature, i.e., whether it is below or above the phase change temperature. On the cement paste level, a marked drop of compressive strength with increasing PCM inclusion level was observed. The indentation modulus has also shown to decrease, probably due to the capsules themselves, and to a lesser extent due to changes in porosity caused by their inclusion. Finally, a novel micro-cube splitting technique was used to characterize the tensile strength of the material on the micro-meter length scale. It was shown that the strength decreases with increasing PCM inclusion percentage, but this is accompanied by a decrease in measurement variability. This study will contribute to future developments of cementitious composites incorporating phase change materials for a variety of applications.

A 3D Lattice Modelling Study of Drying Shrinkage Damage in Concrete Repair Systems

Differential shrinkage between repair material and concrete substrate is considered to be the main cause of premature failure of repair systems. The magnitude of induced stresses depends on many factors, for example the degree of restraint, moisture gradients caused by curing and drying conditions, type of repair material, etc. Numerical simulations combined with experimental observations can be of great use when determining the influence of these parameters on the performance of repair systems. In this work, a lattice type model was used to simulate first the moisture transport inside a repair system and then the resulting damage as a function of time. 3D simulations were performed, and damage patterns were qualitatively verified with experimental results and cracking tendencies in different brittle and ductile materials. The influence of substrate surface preparation, bond strength between the two materials, and thickness of the repair material were investigated. Benefits of using a specially tailored fibre reinforced material, namely strain hardening cementitious composite (SHCC), for controlling the damage development due to drying shrinkage in concrete repairs was also examined.

Smart Crack Control in Concrete through Use of Phase Change Materials (PCMs) : A Review

Cracks in concrete structures present a threat to their durability. Therefore, numerous research studies have been devoted to reducing concrete cracking. In recent years, a new approach has been proposed for controlling temperature related cracking—utilization of phase change materials (PCMs) in concrete. Through their ability to capture heat, PCMs can offset temperature changes and reduce gradients in concrete structures. Nevertheless, they can also influence concrete properties. This paper presents a comprehensive overview of the literature devoted to using PCMs to control temperature related cracking in concrete. First, types of PCMs and ways of incorporation in concrete are discussed. Then, possible uses of PCMs in concrete technology are discussed. Further, the influences of PCMs on concrete properties (fresh, hardened, durability) are discussed in detail. This is followed by a discussion of modelling techniques for PCM-concrete composites and their performance. Finally, a summary and the possible research directions for future work are given. This overview aims to assure the researchers and asset owners of the potential of this maturing technology and bring it one step closer to practical application.

Cracking of SHCC due to reinforcement corrosion

Reinforcement corrosion is the most important deterioration mechanism affecting reinforced concrete infrastructures. After corrosion starts, expansive pressures are exerted onto the surrounding concrete, causing cracking and spalling of the cover concrete. The amount of cover cracking can possibly be reduced by using strain hardening cementitious composites (SHCCs). In this work, the ability of strain hardening cementitious composite to resist cracking due to corrosion of reinforcement is studied. An accelerated corrosion experiment is performed to speed up the corrosion process. Micro-computed X-ray tomography technique (CT-scanning) was used for monitoring rust formation during accelerated corrosion of reinforcement and subsequent cover cracking. Development of cracks in SHCC specimen was compared to a reference specimen. While the SHCC specimen developed a large number of small cracks, the reference specimen exhibited extensive spalling due to corrosion. The SHCC specimen showed superior performance compared to the reference specimen due to its multiple microcracking ability. SHCC proved to be an excellent alternative to brittle cementitious materials when corrosion induced cracking of the cover is a concern.

Failure Modes in Concrete Repair Systems due to Ongoing Corrosion

Corrosion of steel reinforcement is the main cause of deterioration in reinforced concrete structures. It can result in cracking and spalling of the concrete cover. After the damaged cover is repaired, reinforcement corrosion might continue and even accelerate. While the development of the corrosion cell is difficult to control, the damage can be possibly delayed and controlled by use of a suitable repair material. The lattice fracture model is used in this paper to investigate the performance of strain hardening cementitious composite (SHCC) in concrete repair systems exposed to ongoing corrosion. Numerical results were verified by experimental tests when SHCC, nonreinforced material (repair mortar), and commercial repair mortar are used as repair materials. In experiments, reinforcement bars (surrounded by a repair material) were exposed to accelerated corrosion tests. The influence of the substrate surface preparation, the type of repair material, the interface, and the substrate strength on the resulting damage and failure mode of repair systems are discussed. In general, SHCC repair enables distributed cracking with small crack widths, up to several times smaller compared to repair mortar. Furthermore, more warning signs prior to the final failure are present in the SHCC repair system.

The Evaluation of Deformation and Fracture of Gilsocarbon Graphite Subject to Service Environments: Experimental and Modelling

Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of Advanced Gas Cooled Reactors (UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated material. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetrelength-scale. The results provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the service environment.

Piezoresistive Properties of Cementitious Composites Reinforced by PVA Fibres

The use of fibres to enhance the ductility of cementitious composites has been extensively studied for the past few years. The addition of polymeric or metalic fibres with random orientation to the composite or even natural long and aligned fibres demonstrated a very successful reinforcement capable to reach a high mechanical performance. Other property that has been studied is the use of those composites to work as strain sensors. To develop piezoresistive properties on cementitious composite, the addition of conductive materials is necessary. This research evaluated the incorporation of different volumes of multi-wall carbon nanotubes on the piezoresistive properties of strain hardening cementitious composites (SHCC) reinforced by PVA fibres. Through impedance measurements the opening of the cracks under tensile loading was studied. The characterization of this material can help on the understanding of self-sensing properties, adding value to the SHCC used by the repair industry and will contribute to the continuous infrastructure monitoring.

Mechanical Properties of Ductile Cementitious Composites Incorporating Microencapsulated Phase Change Materials

In the past two decades, much research has been devoted to overcoming the inherent brittleness of cementitious materials. To that end, several solutions have been proposed, mainly utilizing fibres. One of the most promising classes of materials is strain hardening cementitious composite (SHCC). It utilizes PVA fibres, and it relatively costly compared to regular concrete, so it is commonly used only in surface layers. This paper presents a multi-functional ductile cementitious composite based on SHCC. It uses microencapsulated phase change materials (PCMs), capable of reducing temperature fluctuations in the material due to their high heat of fusion. It is shown that, although addition of microencapsulated PCMs are detrimental to compressive strength, they have very little effect on the flexural strength and deflection capacity. This material has potential to reduce temperature effects on concrete surfaces, while at the same time being extremely ductile.