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Carbon nanotubes (CNTs) and carbon nanofirbers (CNFs) have excellent properties (mechanical, electrical, magnetic, etc.), which can make them effective nanoreinforcements for improving the properties of materials. The incorporation of CNT/Fs in a wide variety of materials has been researched extensively in the past decade. However, the past study on the reinforcement of cementitious materials with these nanofilaments has been limited. The findings from those studies indicate that CNT/Fs did not significantly improve the mechanical properties of cementitious materials. Two major parameters influence the effectiveness of any discrete inclusion in composite material: The dispersion quality of the inclusions and the interfacial bond between the inclusions and matrix. The main focus of this dissertation is on the dispersion factor, and consists of three main tasks: First a novel thermodynamic-based method for dispersion quantification was developed. Second, a new method, incorporating the utilization of silica fume, was devised to improve and stabilize the dispersion of CNFs in cement paste. And third, the dispersion quantification method and mechanical testing were employed to measure, compare, and correlate the dispersion and mechanical properties of CNF-incorporated cement paste produced with the conventional and new methods. Finally, the main benefits, including the increase in strength and resistance to shrinkage cracking, obtained from the utilization of CNFs in cement paste will be presented. The investigations and the corresponding results show that the novel dispersion quantification method can be implemented easily to perform a wide variety of tasks ranging from measuring dispersion of nanofilaments in composites using their optical/SEM micrographs as input, to measuring the effect of cement particle/clump size on the dispersion of nano inclusions in cement paste. It was found that cement particles do not affect the dispersion of nano inclusions in cement paste significantly while the dispersion of nano inclusions can notably degenerates if the cement particles are agglomerated. The novel dispersion quantification method shows that, the dispersion of CNFs in cement paste significantly improves by utilizing silica fume. However, it was found that the dispersion of silica fume particles is an important parameter and poorly dispersed silica fume cannot enhance the overall dispersion of nano inclusions in cementitious materials. Finally, the mechanical testing and experimentations showed that CNFs, in absence of moist curing, even if poorly dispersed, can provide important benefits in terms of strength and crack resistance.

Concrete, a composite material consisting of aggregates bound together with cement paste, is the most widely used construction material. Concrete is relatively cheap, very versatile and has excellent compressive strength. However, its tensile strength is limited and for this reason steel rebars are often added to create reinforced concrete (RC). Cracking inevitably occurs in all RC materials and associated structures due to a variety of mechanical and environmental actions. The generation of tiny microcracks within concrete facilitates the flow of potentially aggressive fluids that can corrode the embedded steel rebars and, in extreme cases, lead to premature structural failure. Concrete, along with all cement-based materials, does possess some inherent self-healing capacity and is able to heal certain-size cracks autogenously. This self-healing capability is very limited and therefore researchers have attempted to improve upon it by using a variety of techniques. In particular, the use of engineered additions for autonomic self-healing has gained significant interest in the past two decades. An example is the addition of microcapsules that disperse throughout the hardened material subsequently providing reservoirs of healing agents. When cracks arise within the material, they rupture the embedded microcapsules causing a release of their contents into the crack volume. The released material then reacts to provide filling, sealing and healing of the crack. The primary aim of this research project was to investigate the autonomic self-healing performance of concrete containing microencapsulated sodium silicate. The effect of microcapsule addition on the fresh, hardened and self-healing properties of cement, mortar and concrete were all explored. Self-healing was monitored using a variety of techniques and results reveal the increased self-healing ability of microcapsule-containing cementitious materials as well as the efficacy of sodium silicate as a healing agent. Furthermore, the self-healing concrete field trial displays the great potential for microcapsules to be incorporated into large-scale self-healing concrete applications.

The process of designing cementitious layers (weakly and strongly cemented) against fatigue distress in road structures is well accepted. Research and field investigations with the aid of the Heavy Vehicle Simulator (HVS) revealed, however, that almost all weakly cemented subbase layers undergo non-traffic and traffic¬associated cracking and eventually degradation of the cemented material into a granular state (post-cracked phase). It is therefore very important to analyse these layers in the post-cracked phase and to incorporate the results of this analysis in the design, for both new and rehabilitation designs. The investigations revealed that the rate of degradation of these materials is largely dependent on traffic loading and the moisture conditions within the pavement layers. The purpose of this study is to investigate the behaviour of weakly cemented subbase layers in road structures mainly under a bitumen base between 90 mm and 140 mm thick. This behaviour includes both pre-cracked and post-cracked phases. It is shown that the fatigue life of bitumen base layers is mainly governed by the condition of the weakly cemented subbase layers. In Chapter 1 a brief historical review is given of the development of fatigue distress criteria of the cementitious layers. It is shown that the maximum horizontal tensile strain at the bottom of these layers is the main distress criterion in the pre-cracked phase. Unconfined compressive strength and durability requirements are also discussed. Some aspects of the current design methods are outlined in Chapter 2. The concept of equivalent granular states in the post-¬cracked phase of cementitious layers was derived from HVS test findings. However, before this document no behavioural prediction models were available to quantify accurately the post-cracked state of these layers. The actual mechanisms of distress were also not clear. In Chapter 3, a detailed investigations and analysis of ten dif¬ferent HVS tests at four different sites in Natal are discussed. The purpose of the analysis, is firstly to illustrate the powerful method of full-scale accelerated HVS-type testing and secondly to indicate the importance of the upper subbase layer, the initial condition of the in-situ structure, the importance of water condi¬tions within the pavement structure, and finally the different states of behaviour of this type of road structure, including predictions of future behaviour based on linear elastic theory. The characteristics of the weakly cemented upper subbase layer are shown to be of paramount importance in the final behaviour of these structures. In Chapter 4 a method of analysing the behaviour of mainly weakly cemented layers in the post-cracked phase is proposed. This method arises from the HVS testing discussed in Chapter 3, and may be regarded as the most important improvement on the current method discussed in Chapter 2. The analysis incorporates the determination of the effective elastic moduli of weakly cemented subbase layers, including both the wet and the dry periods during the structural design period of these layers. In Chapter 5 the effect of relatively weak interlayers within asphalt base structures is discussed and evaluated. The analysis incorporates the relative position and thickness of the inter layer during both wet (low modulus) and dry (high modulus) conditions. A summary and detailed discussion, together with recommendations for future research, are given in Chapter 6. The need for the incorpo¬ration of durability (erodibility) criteria for weakly cemented materials is also discussed. More research should be done on the effects of accelerated curing compared with normal curing methods. This investigation includes aspects of soil-lime-cement reactions together with delayed compaction techniques to reduce shrinkage cracking. The need for better quality control as well as improved construction techniques for weakly cemented materials is also discussed. This thesis also contains two appendices. In the first of these detailed photographic records of the different HVS tests and performances are given. In the second appendix an example of an input computer program to plot the three dimensional behavioural model is given. / Dissertation (MEng)--University of Pretoria, 2009. / Civil Engineering / unrestricted

Yes / Cementitious composites, the most abundant human-made materials in the world, are challenged to be more sustainable, durable and cost-effective to adapt to the development of structural engineering, economy and environment. Owing to their excellent strength, toughness and durability, nano-fillers reinforced cementitious materials have attracted broad attention in civil engineering researches and applications. However, it is worth noting that nano-fillers reinforced cementitious materials achieve their proprieties by using of different industrial nano-fillers, i.e., graphenes, carbon nanotubes, carbon nanofibers, etc. Although the properties of conventional cementitious materials are improved, the incorporation of the above nano-fillers are high cost and environmental footprint. Different from high-energy consuming carbon nanofillers, nanocellulose is one of the biomass-derived nanofillers with excellent nanometer properties, biological performances and composite effects, and it has proved to be a promising green filler to enhance the mechanical properties, durability and functional properties and lower the carbon footprint of cementitious composites. Therefore, this paper provides an overview on biomass-derived nanocellulose modified cementitious composites, mainly focusing on their fabrication, properties (early performance, mechanical performance, durability, and functional performance) and applications. It also concludes with an outline of some future opportunities and challenges in the development of biomass-derived nanocellulose modified cementitious composites.

Thèse sur les matériaux cimentaires en milieux non saturés / Thesis on unsaturated cement materials

At present, cementitious binders are used extensively in the construction industry and principally in concretes. They are also used in some applications like ground improvement. In these applications the cost of the binder, typically Portland cement, accounts for a considerable proportion of the total cost of the technique. In addition to the financial cost there is also the environmental impact of quarrying and processing of raw materials to produce Portland cements. Gypsum waste, by-pass dust and fly ash by-products have been identified as the alternative sources of cementitious binder. Using these materials has two advantages: they have little or no production cost; and the re-use of such material would negate the need for expensive disposal. This thesis describes a programme of laboratory testing and study on the possible field trials to investigate the possibility of using mentioned by-product materials as construction materials. Laboratory trials carried out to investigate the properties of waste materials in different combinations; binary and ternary using the same water content. Specimens were evaluated on the basis of Unconfined Compressive Strength at 3,7 and 28 days curing. It was found that pastes containing waste gypsums, Basic oxygen Slag and Run of station ash achieved the highest unconfined compressive strengths (up to 20 MPa) and five mixes of these groups were selected for further tests such as viscosity, permeability, expansion, XRD and freeze and thaw. Data obtained from the ternary combinations were analyzed using two different methods, i.e. Response Surface method and Artificial Neural Network. Two prediction models were created using MINITAB and MATLAB software and the predicted results were compared. It was concluded that the Artificial Neural Network had fewer errors than the response surface model. The feasibility of using by-product materials in two field trials was also studied and the possibility of 100% cement replacement in low strength concrete used in subway backfilling (using 80%BOS-15% Plasterboard Gypsum-5%bypass dust) and light weight blocks (60% run of station ash-20%plaster board gypsum-20% bypass dust) was investigated. It was found that waste gypsum could be used in both trials and the basic oxygen slag could be used for subway backfilling because it improved the flow. However it was not a good idea to use the steel slag in light weight products because of its density. The thesis concludes that there are several potential applications for the use of the waste gypsums in combination with other waste materials in the construction industry but further work is required before it can be used commercially. However the sources and differing chemical contents of the by-product materials may have significant impact on the cementitious behaviour of by product materials.

In this work, the chloride transport related properties of slag concrete have been studied. For this, traditional experiments and an innovative computational method were applied to novel mixes with a low carbon footprint. In part 1 combination of Ground Granulated Blast Furnace Slag and Steel Basic Oxygen Slag activated using ordinary Portland cement, waste cement industrial residues and recycled Plasterboard Gypsum were studied. In order to characterize these blended binders, the compressive strength, the volume stability, the mineralogical changes due to hydration and the setting times were measured. In part 2 the chloride penetration was simulated to study the transport properties using a multi-species model. In this, the ionic species flow is given by the Nernst– Planck equation; however, due to ion–ion interactions there are ionic fields that affect the final flux producing an additional voltage known as the membrane potential. In order to calculate the inputs of the model a neural network methodology was developed to find the fundamental properties of concrete including the diffusion coefficients In part 3 a range of traditional transport chloride related experimental tests were carried out on concrete mixtures developed in part 1. These tests were workability, compressive strength, open porosity, initial water absorption capacity (sorptivity), carbonation, chloride migration and self diffusion, electrical resistivity, water permeability, and corrosion. Additionally, the methods developed in part 2 were used to calculate the chloride transport related properties of those mixes.

This thesis presents a new bonded particle model that accurately predicts the wideranging behaviour of cementitious materials. There is an increasing use of the Discrete Element Method (DEM) to study the behaviour of cementitious materials such as concrete and rock; the chief advantage of the DEM over continuum-based techniques is that it does not predetermine where cracking and fragmentation initiate and propagate, since the system is naturally discontinuous. The DEM’s ability to produce realistic representations of cementitious materials depends largely on the implementation of an inter-particle bonded-contact model. A new bonded-contact model is proposed, based on the Timoshenko beam theory which considers axial, shear and bending behaviour of inter-particle bonds. The developed model was implemented in the commercial EDEM code, in which a thorough verification procedure was conducted. A full parametric study then considered the uni-axial loading of a concrete cylinder; the influence of the input parameters on the bulk response was used to produce a calibrated model that has been shown to be capable of producing realistic predictions of a wide range of behaviour seen in cementitious materials. The model provides useful insights into the microscopic phenomena that result in the bulk loading responses observed for cementitious materials such as concrete. The new model was used to simulate the loading of a number of deformable structural elements including beams, frames, plates and rings; the numerical results produced by the model provided a close match to theoretical solutions.