Mladena Luković

Effect of Moisture Exchange on Interface Formation in the Repair System Studied by X-ray Absorption

In concrete repair systems, material properties of the repair material and the interface are greatly influenced by the moisture exchange between the repair material and the substrate. If the substrate is dry, it can absorb water from the repair material and reduce its effective water-to-cement ratio (w/c). This further affects the hydration rate of cement based material. In addition to the change in hydration rate, void content at the interface between the two materials is also affected. In this research, the influence of moisture exchange on the void content in the repair system as a function of initial saturation level of the substrate is investigated. Repair systems with varying level of substrate saturation are made. Moisture exchange in these repair systems as a function of time is monitored by the X-ray absorption technique. After a specified curing age (3 d), the internal microstructure of the repair systems was captured by micro-computed X-ray tomography (CT-scanning). From reconstructed images, different phases in the repair system (repair material, substrate, voids) can be distinguished. In order to quantify the void content, voids were thresholded and their percentage was calculated. It was found that significantly more voids form when the substrate is dry prior to application of the repair material. Air, initially filling voids and pores of the dry substrate, is being released due to the moisture exchange. As a result, air voids remain entrapped in the repair material close to the interface. These voids are found to form as a continuation of pre-existing surface voids in the substrate. Knowledge about moisture exchange and its effects provides engineers with the basis for recommendations about substrate preconditioning in practice.

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.

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.

Elastic Modulus of the Alkali-Silica Reaction Rim in a Simplified Calcium-Alkali-Silicate System Determined by Nano-Indentation

This work aims at providing a better understanding of the mechanical properties of the reaction rim in the alkali-silica reaction. The elastic modulus of the calcium alkali silicate constituting the reaction rim, which is formed at the interface between alkali silicate and Ca(OH)2 in a chemically-idealized system of the alkali-silica reaction, was studied using nano-indentation. In addition, the corresponding calcium to silica mole ratio of the calcium alkali silicate was investigated. The results show that the elastic modulus of the calcium alkali silicate formed at the interface increased with the increase of the calcium to silica mole ratio and vice versa. Furthermore, the more calcium that was available for interaction with alkali silicate to form calcium alkali silicate, the higher the calcium to silica mole ratio and, consequently, the higher the elastic modulus of the formed calcium alkali silicate. This work provides illustrative evidence from a mechanical point of view on how the occurrence of cracks due to the alkali-silica reaction (ASR) is linked to the formation of the reaction rim. It has to be highlighted, however, that the simplified calcium-alkali-silicate system in this study is far from the real condition in concrete.

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.

On the Potential of Lattice Type Model for Predicting Shear Capacity of Reinforced Concrete and SHCC Structures

Due to the brittle nature of shear failure, shear capacity is considered to be one of the most critical and relevant issues in the design of concrete structures. In this research, the possibility of using a discrete lattice model to predict the shear capacity of reinforced concrete beams is investigated. Beams without shear reinforcement and with different beam geometry and varying reinforcement ratios were studied and fracture behaviour from the simulations is compared to experimental results.

Behaviour of Bonded SHCC Overlay Systems

Strain-hardening cement-based composite (SHCC) has proved to be a suitable material for repair layers on concrete substrate. Bonded SHCC overlays may bridge cracks in the substrate, and normally exhibit an enhanced resistance to drying shrinkage. In addition to the SHCC material properties, the interface behaviour has a significant influence on the failure process. By varying the interface roughness and bond strength, it is possible to attain a balance between the debonding of the interface and SHCC cracking, and to ensure both monolithic mechanical behaviour of the structure and sufficiently small crack widths in the SHCC overlay. The smaller these crack widths are, the lower the permeability and the greater the self-healing potential of the cracks. In the case of mechanical loading, a rather weak bond may be beneficial, as the crack widths in the SHCC tend to be smaller under such conditions. In the case of drying shrinkage of the SHCC overlay, however, weak bonding may lead to large debonded interface areas, making the use of SHCC inadvisable in such instances.

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.

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

Differential shrinkage between repair material and concrete substrate is considered to be the main cause of premature failure of repair systems. 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 a repair system. In this work, a lattice type model is used first to simulate moisture transport inside a repair system and then to model time dependent damage. three-dimensional (3D) simulations are performed, and damage patterns and crack widths are qualitatively verified with some experimental observations.