Since more than a decade, TU Delft has been working on the development of bacteria
based self-healing concrete. The self-healing ability of the material is based on a
biological mechanism in which a limestone producing bacteria is added to the material
to repair cracks.
The purpose of this study was to understand and compare the efficiency of different
methods and materials used for incorporating bacteria into mortar. The aim of the study
was to create a self-healing mortar mix for practical applications.
Concrete is a widely used construction material. Most structural elements are made
using concrete and covered by a protective layer of mortar, called plaster. Cracks
usually propagate from the surface to the inside, which means that the plaster is the
first to crack. If the mortar is made self-healing, then the concrete underneath can be
better protected from the elements, and this could effectively increase the lifespan of
the structure.
For this project, fibre reinforced mortar is healed using bacteria (Bacillus Cohnii) which
is applied to the concrete using internal (different types of embedded capsules) and
external methods (paste applied to the cracks).
This is done to check the effectiveness of the bacteria in repairing damaged concrete
and to observe which method of application works best. The effectiveness is analysed
using optical and electron (BSE) microscopy and a permeability test to observe the
water tightness of the sample after cracking.
Additionally, characterization tests are performed on the capsules and performance
tests are carried on the mortar samples, to better understand their behaviour.
This study would help in making concrete structures more durable which would make
them more sustainable and cheaper in the long term.
This study found that crack healing is dependent on the crack width and the number of
capsules present in the material near the crack. The compressive and flexural strength
at 28 and 84 days was found to be higher in samples embedded with alginate capsules
and PLA capsules. External healing was found to be a good method for healing existing
materials but the healing was found to be dependent on the amount of paste that
adhered to the crack wall

The demand to build with wood and other bio-based materials has increased rapidly the last decade, due to the ambition to build more with sustainable building materials. One of the major challenges in building with wood, is to account for the lower strength and stiffness perpendicular to the grain. Wood is an anisotropic material, meaning the material has different properties in different directions. Loading in the perpendicular to the grain direction is sometimes unavoidable and fracture often occurs in a brittle manner. Unfortunately, our understanding and knowledge of the perpendicular to the grain fracture is limited. This is especially the case for hardwoods, for which the material properties and the parameters important for the fracture process are often unknown.

The goal of this thesis is to reduce this knowledge gap on the material properties and the relevant parameters in mode-I (pure tension) fracture process of (tropical) hardwood. The knowledge gained in this research provides the answer to the main research question of this thesis: Which parameters should be considered when describing the constitutive model of a (tropical) hardwood in mode-I tension?

A single-end notch three-point bending (SEN-TPB) test demonstrated a strong influence of the orientation of the growth rings on the perpendicular to the grain fracture process of azobé. The value for the peak load and post-peak behaviour change significantly depending on the orientation in which the sample is placed. Furthermore, visual observations and results from a profilometer revealed that there is an inconsistency in the roughness of the cracked surface area between the different series. From the microscopic analysis is concluded that this inconsistency in general mechanical behaviour and the roughness of the cracked surface area is linked to the composition of ray and parenchyma cells. Moreover, the direction of the ray and parenchyma cells with respect to the fracture plane is different depending on the orientation of the sample. When the ray cells are orientated perpendicular to the crack plane, a higher strength is obtained. The thin-walled parenchyma cells significantly reduce the post-elastic stiffness if these cells are orientated perpendicular to the crack plane. The influence of the composition and orientation of both cells is reflected in the numerical value for the modulus of elasticity, the tensile strength and the fracture energy. This research showed that the composition of the cells in a wood specie and the orientation in which the growth rings are placed significantly influence the mode-I fracture process of a hardwood. This influence is reflected in the modulus of elasticity, the tensile strength, the fracture energy and the shape of the tension softening curve perpendicular to the grain. The results of this research provide new insight into the fracture modelling of hardwood. Both in numerical modelling and design considerations the variation in the material properties because of the orientation of the growth rings, should not be neglected.