Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Strain Hardening Cement Based Composite (SHCC) is a type of High Performance Fibre Reinforced Cement-based Composite (HPFRCC). SHCC contains randomly distributed short fibres which improve the ductility of the material and can resist the full tensile load at strains up to 5 %. When SHCC is subjected to tensile loading, fine multiple cracking occurs that portrays a pseudo strain hardening effect as a result. The multiple cracking is what sets SHCC aside from conventional Reinforced Concrete (RC). Conventional RC forms one large crack that results in durability problems. The multiple cracks of SHCC typically have an average crack width of less than 80 μm (Adendorff, 2009), resulting in an improved durability compared to conventional RC. The aim of this research project is to quantify the cracking behaviour of SHCC which can be used to quantify the durability of SHCC. The cracking behaviour is described using a statistical distribution model, which represents the crack widths distribution and a mathematical expression that describes the crack pattern. The cracking behaviour was determined by measuring the cracks during quasi-static uni-axial tensile tests. The cracking data was collected with the aid of a non-contact surface strain measuring system, namely the ARAMIS system. An investigation was performed on the crack measuring setup (ARAMIS) to define a crack definition that was used during the determination of the cracking behaviour of SHCC. Several different statistical distributions were considered to describe the distribution of the crack widths of SHCC. A mathematical expression named the Crack Proximity Index (CPI) which represents the distances of the cracks to each other was used to describe the crack pattern of SHCC. The Gamma distribution was found to best represent the crack widths of SHCC. It was observed that different crack patterns can be found at the same tensile strain and that the CPI would differ even though the same crack width distribution was found. A statistical distribution model was therefore found to describe the CPI distribution of SHCC at different tensile strains and it was established that the Log-normal distribution best describes the CPI distribution of SHCC. After the cracking behaviour of SHCC was determined for quasi-static tensile loading, an investigation was performed to compare it to the cracking behaviour under flexural loading. A difference in the crack widths, number of cracks and crack pattern was found between bending and tension. Therefore it was concluded that the cracking behaviour for SHCC is different under flexural loading than in tension. / AFRIKAANSE OPSOMMING: “Strain Hardening Cement-based Composite” (SHCC) is ‘n tipe “High Performance Fibre Reinforced Cement-based Composite” (HPFRCC). SHCC bevat kort vesels wat ewekansig verspreid is, wat die duktiliteit van die material verbeter en dit kan die maksimum trekkrag weerstaan tot en met ‘n vervorming van 5 %. Wanneer SHCC belas word met ‘n trekkrag, vorm verskeie fyn krake wat ‘n sogenaamde vervormingsverharding voorstel. Die verskeie krake onderskei SHCC van normale bewapende beton. Normale bewapende beton vorm een groot kraak met die gevolg dat duursaamheidsprobleme ontstaan. Die gemiddelde kraakwydte van SHCC is minder as 80 μm (Adendorff, 2009) en het dus ‘n beter duursaamheid as normale bewapende beton. Die doel van die navorsingsprojek is om die kraak gedrag van SHCC te kwantifiseer en wat dan gebruik kan word om die duursaamheid van SHCC te kwantifiseer. Die kraak gedrag is beskryf deur ‘n statistiese verspreiding model wat die kraak wydtes se verspreiding voorstel en ‘n wiskundige uitdrukking wat die kraak patroon beskryf. Die kraak gedrag was bepaal deur die krake te meet tydens die semi-statiese een-asige trek toetse. Die kraak data was met behulp van ‘n optiese vervormings toestel, naamlik die ARAMIS, versamel. ‘n Ondersoek is gedoen op die kraak meetings opstelling (ARAMIS), om ‘n kraak definisie te definieer wat gebruik is om die kraak gedrag te bepaal. Daar is gekyk na verskeie statistiese verdelings om die kraak wydtes van SHCC te beskryf. Die kraak patroon van SHCC is beskryf met ‘n wiskundige uitdrukking genoem die “Crack Proximity Index” (CPI) wat die krake se afstande van mekaar voorstel. Dit is bevind dat die Gamma verdeling die kraak wydtes van SHCC die beste beskryf. Daar is waargeneem dat verskillende kraak patrone by dieselfde vervorming verkry kan word en dat die CPI kan verskil al is die kraak wydte verdeling dieselfde. ‘n Statistiese verdelingsmodel is dus gevind om die CPI verdeling van SHCC te beskryf by verskillende vervormings, en daar is vasgestel dat die Log-normaal verdeling die CPI verdeling van SHCC die beste beskryf. Nadat die kraak gedrag van SHCC bepaal is vir semi-statiese trek-belasting, is ‘n ondersoek gedoen waar die trek-kraak gedrag vergelyk is met buig-kraak gedrag. ‘n Verskil in die kraak wydtes, aantal krake en kraak patroon is gevind tussen buiging en trek. Dus is die gevolgtrekking gemaak dat die kraak gedrag van SHCC verskillend is in buiging as in trek.

This study describes the strength, toughness and strain-rate sensitivity of fibre-reinforced cement-based foams subjected to variable loading rates. Drop-weight impact tests were conducted on beams with cast density between 475 - 1200 kg/cu.m. The study shows that under quasi-static loading, the compressive strength, elastic modulus and the modulus of rupture of plain mixes scale with the square of the relative density. On the other hand, the flexural toughness factor scaled linearly with it. Fibres were seen to increase the flexural strength at all rates of loading, regardless of cast density. Further, cement based foams were seen to be strain-rate sensitive. The resistance of cement-based foams to sulphate exposure was also investigated. Heavier cement-based foams are more susceptible to sulphate attack and perform poorly with an increase in the duration of exposure when compared to the lightest mix which showed improved responses up to 30 days of exposure due to self-healing. / Structural Engineering

Deterioration of reinforced concrete structures due to reinforcement corrosion is a serious problem that faces concrete infrastructure worldwide. Effect of the rebar corrosion in the shear span on the structural behaviour is not fully addressed in the published literature. This study examined the effects of corrosion of the longitudinal reinforcement in the shear span on the structural behaviour of RC beams and the effectiveness of three rehabilitation schemes on the structural performance of such beams. The experimental program consisted of testing fifteen medium-scale reinforced concrete beams (150mm wide x 350mm deep x 2400mm long) under static load. Test variables included: span to depth ratio, the degree of corrosion and the anchorage end condition and repair schemes. Two span to depth (a/d) ratios were considered: a/d=3.4 with one-point loading and a/d=2.4 with two-point loading. Two anchorage end-conditions were used: bonded or un-bonded reinforcement in the an-chorage zone. Four degrees of corrosion were chosen to simulate minor (2.5% to 5% mass loss), medium (7.5% mass loss), and severe (15% mass loss) degrees of corrosion. Corrosion was induced in the longitudinal reinforcement in the shear-span using accelerated corrosion techniques based on Faradays’ law. Three different repair scenarios were applied. The first scenario included removing the deteriorated concrete, cleaning the corroded steel and patching with a new self-compacting concrete. The second scenario included U-wrapping the beams cross-section using Glass fiber reinforced cement-based composite (GFRCM), and Carbon fiber reinforced cement-based composite (CFRCM) without removing the deteriorated concrete. The third scenario included patch repair and confinement by wrapping with GFRCM or CFRCM. Following corrosion and repair, all specimens were loaded statically to failure. Test results showed no major effect of shear-span corrosion on the flexural behaviour for the beams with end anchorage whereas a noticeable effect on the flexural behaviour was observed for beams with no end anchorage regions. The corrosion degree and the shear span to depth ratio affected the mode of failure for the specimens with no end anchorages. The type of repair significantly affected the overall behaviour of the corroded specimens. An analytical model was proposed and used to predict the load-deflection response of the tested specimens. The program calculated the mid-span deflection for a given load as an integration of the deflection of a series of elements, with the deflection being based on the elongation of the steel reinforcement in each element. A modified bond stress-slip model was incorporated into the calculations to account for the change in bond strength caused by the corrosion and/or confinement that are provided by repairs. The predicted results were in reasonable agreement with the experimental results.

This multi-scale investigation aimed to provide new knowledge and understanding of creep and shrinkage of high performance lightweight concrete (HPLC) by assessing prestress losses in HPLC prestressed members in a large-scale study; by quantifying the effect of the constituent materials and external conditions on creep and shrinkage in a medium-scale study; and by improving the fundamental understanding of creep and shrinkage in a small-scale study. Creep plus shrinkage prestress losses were between two and eight times lower than those estimated for the design standards and approximately 50% of those measured in similar strength normal weight high performance concrete girders. The lower creep and shrinkage exhibited by HPLC was found to be caused by a synergy between the pre-soaked lightweight aggregate and the low water-to-cementitious material ratio matrix. That is, the water contained in the lightweight aggregate contributes to enhance hydration by providing an internal moist curing. The water in the aggregate also contributes to maintain a high internal relative humidity which reduces or eliminates autogenous shrinkage. This higher internal relative humidity also reduces creep by preventing load-induced water migration. Finally, lightweight aggregate exhibits a better elastic compatibility with the paste than normal weight aggregate. This improved elastic matching and the enhanced hydration are believed to reduce peak deformations at the ITZ which further decreases creep and shrinkage.

Today, with increasing global awareness and regulation of air pollution, interest in the smog-abating property of photocatalytic materials is increasing. Titanium dioxide (TiO2) is the most well known photocatalytic semiconductor and is often considered as one way of solving pollution by a passive but an effective way, particularly to reduce atmospheric nitrogen oxides (NOx=NO+NO2). This relatively new technology is already being used in some of the countries as a construction material, commercially sold as photocatalytic cement, photocatalytic pavement, self-cleaning tiles, and self-cleaning glass. Prior research has examined the photocatalytic properties of TiO2 itself, as well as TiO2-containing cement-based materials. The majority of this effort has been on characterizing and enhancing the photocatalytic efficiency. However, relatively little research was performed to assess the potential impact of the photocatalytic reaction on the "parent" or "host" material. In this research, the focus is on the effect of photocatalysis on the composition, structure, and properties of cementitious materials, which contain titania nanoparticles at early and late ages. Fundamental examinations on the addition of these chemically non-reactive nanoparticles to cement-based materials are performed. The high surface area of nanoparticles could alter early age properties of cementitious materials, such as setting time, dimensional stability, and hydration rate. Various experimental techniques as well as mathematical modeling were used to examine and explain the early age hydration of cementitious materials when TiO2 nanoparticles are present. Further, the effects of the TiO2 on the long term durability of cement-based materials are investigated to demonstrate their suitability for long-term use in the field. The photocatalytic NOx oxidation efficiency and NOx binding capability of TiO2 containing cementitious materials are experimentally investigated. The durability of TiO2-cement is examined by various techniques on samples that went through extensive photocatalysis and environmental exposures. These investigations have led to tentative conclusions on the use of TiO2 nanoparticles in cementitious materials, and suggest avenues for future study.

No description available.

No description available.

Deterioration of reinforced concrete structures due to reinforcement corrosion is a serious problem that faces concrete infrastructure worldwide. Effect of the rebar corrosion in the shear span on the structural behaviour is not fully addressed in the published literature. This study examined the effects of corrosion of the longitudinal reinforcement in the shear span on the structural behaviour of RC beams and the effectiveness of three rehabilitation schemes on the structural performance of such beams. The experimental program consisted of testing fifteen medium-scale reinforced concrete beams (150mm wide x 350mm deep x 2400mm long) under static load. Test variables included: span to depth ratio, the degree of corrosion and the anchorage end condition and repair schemes. Two span to depth (a/d) ratios were considered: a/d=3.4 with one-point loading and a/d=2.4 with two-point loading. Two anchorage end-conditions were used: bonded or un-bonded reinforcement in the an-chorage zone. Four degrees of corrosion were chosen to simulate minor (2.5% to 5% mass loss), medium (7.5% mass loss), and severe (15% mass loss) degrees of corrosion. Corrosion was induced in the longitudinal reinforcement in the shear-span using accelerated corrosion techniques based on Faradays’ law. Three different repair scenarios were applied. The first scenario included removing the deteriorated concrete, cleaning the corroded steel and patching with a new self-compacting concrete. The second scenario included U-wrapping the beams cross-section using Glass fiber reinforced cement-based composite (GFRCM), and Carbon fiber reinforced cement-based composite (CFRCM) without removing the deteriorated concrete. The third scenario included patch repair and confinement by wrapping with GFRCM or CFRCM. Following corrosion and repair, all specimens were loaded statically to failure. Test results showed no major effect of shear-span corrosion on the flexural behaviour for the beams with end anchorage whereas a noticeable effect on the flexural behaviour was observed for beams with no end anchorage regions. The corrosion degree and the shear span to depth ratio affected the mode of failure for the specimens with no end anchorages. The type of repair significantly affected the overall behaviour of the corroded specimens. An analytical model was proposed and used to predict the load-deflection response of the tested specimens. The program calculated the mid-span deflection for a given load as an integration of the deflection of a series of elements, with the deflection being based on the elongation of the steel reinforcement in each element. A modified bond stress-slip model was incorporated into the calculations to account for the change in bond strength caused by the corrosion and/or confinement that are provided by repairs. The predicted results were in reasonable agreement with the experimental results.

abstract: Early-age cracks in fresh concrete occur mainly due to high rate of surface evaporation and restraint offered by the contracting solid phase. Available test methods that simulate severe drying conditions, however, were not originally designed to focus on evaporation and transport characteristics of the liquid-gas phases in a hydrating cementitious microstructure. Therefore, these tests lack accurate measurement of the drying rate and data interpretation based on the principles of transport properties is limited. A vacuum-based test method capable of simulating early-age cracks in 2-D cement paste is developed which continuously monitors the weight loss and changes to the surface characteristics. 2-D crack evolution is documented using time-lapse photography. Effects of sample size, w/c ratio, initial curing and fiber content are studied. In the subsequent analysis, the cement paste phase is considered as a porous medium and moisture transport is described based on surface mass transfer and internal moisture transport characteristics. Results indicate that drying occurs in two stages: constant drying rate period (stage I), followed by a falling drying rate period (stage II). Vapor diffusion in stage I and unsaturated flow within porous medium in stage II determine the overall rate of evaporation. The mass loss results are analyzed using diffusion-based models. Results show that moisture diffusivity in stage I is higher than its value in stage II by more than one order of magnitude. The drying model is used in conjunction with a shrinkage model to predict the development of capillary pressures. Similar approach is implemented in drying restrained ring specimens to predict 1-D crack width development. An analytical approach relates diffusion, shrinkage, creep, tensile and fracture properties to interpret the experimental data. Evaporation potential is introduced based on the boundary layer concept, mass transfer, and a driving force consisting of the concentration gradient. Effect of wind velocity is reflected on Reynolds number which affects the boundary layer on sample surface. This parameter along with Schmidt and Sherwood numbers are used for prediction of mass transfer coefficient. Concentration gradient is shown to be a strong function of temperature and relative humidity and used to predict the evaporation potential. Results of modeling efforts are compared with a variety of test results reported in the literature. Diffusivity data and results of 1-D and 2-D image analyses indicate significant effects of fibers on controlling early-age cracks. Presented models are capable of predicting evaporation rates and moisture flow through hydrating cement-based materials during early-age drying and shrinkage conditions. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2011

La carbonatation est une pathologie du béton armé qui peut engendrer la corrosion des armatures et à terme de la fissuration. Dans le cadre de la gestion des déchets radioactifs, les structures et les conteneurs seraient soumis simultanément à un échauffement (exothermie des déchets), au CO2 atmosphérique, ainsi qu'à un séchage important. Afin de rendre compte de leur évolution à l'échelle séculaire, les données actuelles relatives à la carbonatation à température ambiante doivent être complétées, d'une part par une description de la phénoménologie en température, et d'autre part, par la prise en compte de l'impact des transferts hydriques en température (séchage) sur la carbonatation en insaturé. Le travail présenté se focalise sur l’étude de la durabilité de quatre pâtes de ciment différentes dont deux sont directement dérivées des formulations de référence sélectionnées par l’Andra (CEM I et CEM V) et un mélange baspH. Le premier volet est dédié à l'étude des transferts hydriques reposant sur la conduite d'essais de désorption. Il a notamment permis la caractérisation des isothermes de désorption en température (20, 50et 80°C). L'impact thermique modifie les isothermes : la teneur en eau à l’équilibre chute avec la température et le point d'amorçage de la condensation capillaire est déplacé. Une phase de modélisation a conjointement été conduite en support aux expérimentations. L'utilisation de l'équation de Clausius-Clapeyron a permis de décrire l'effet de la température sur les isothermes (par la détermination de la chaleur isostérique d’adsorption de chacun des matériaux). Bien que l'impact de la température sur la microstructure des pâtes de ciment soit avéré, la prise en compte du déplacement des équilibres thermodynamiques suffit à restituer cet effet thermique sur les isothermes. La détermination des perméabilités intrinsèques par exploitation des cinétiques de désorption (par analyse inverse) a montré la thermoactivation du transport d'eau. La contribution de l'évolution de la microstructure en température ne peut-être négligée sur la perméabilité des matériaux.Le deuxième volet exploratoire, est consacré à l'étude de la carbonatation en température. Il repose sur la mise en place d'un dispositif de carbonatation spécifique (fonctionnement en température) et la conduite d'essais de carbonatation à HR et température contrôlées. La campagne de caractérisation (DRX et ATG)a conduit à l'obtention de profils de carbonatation caractéristiques de chaque couple (HR, Température).Les évolutions minéralogiques (décomposition des hydrates, distribution polymorphique du carbonate de calcium précipité) mises en évidence en température se rapprochent de celles identifiées à température ambiante. En revanche, il ressort que les conditions environnementales influencent significativement les proportions polymorphiques : plus l'HR est faible, plus les teneurs en phases métastables (vatérite,aragonite) sont élevées. Les réactions de dissolution-précipitation mises en jeu dans la transformation polymorphique (des états métastables vers la calcite) sont inhibées à faible HR, par manque de milieu réactionnel. La cinétique de carbonatation, également impactée par les conditions environnementales, est régie par la concurrence de l'effet thermique sur les transferts hydriques et sur la solubilité rétrograde des réactifs. Les profondeurs carbonatées sont maximales aux points d'amorçage de la condensation capillaire propres aux différents matériaux et à chaque température. Les profondeurs carbonatées augmentent avec la température jusqu'à une température limite, caractéristique de la formulation, au-delà de laquelle la solubilité rétrograde des réactifs deviendrait le facteur limitant.Cette phase de compréhension des mécanismes mis en jeu dans la carbonatation en température et de leur niveau de couplage effectuée, les modèles prédictifs de carbonatation en insaturé pourront être étendus à l'application en température. Les résultats de ce travail fournissent les données d'entrées et de validation nécessaires à la validation des simulations numériques. / Carbonation is the major cause of degradation of reinforced concrete structures. It leads to rebar corrosion and cracking of the concrete cover. In the framework of radioactive waste management, cement-based materials used as building material for structures or containers would be simultaneously submitted to heating (due to the waste thermal output), subsequent drying and atmospheric carbon dioxide. Such environmental conditions are expected to modify the carbonation mechanisms (with respect to temperature). In order to describe their long-term evolution of material, a double approach was developed, combining the description of carbonation and drying for temperatures up to 80°C to complement available data at ambient temperature. The present work focuses on the durability study off our hardened cement pastes; two of them are derived from the reference formulations selected by Andra(CEM I and CEM V) and a low-pH mix. The first experimental campaign focuses on moisture transfer. The effect of temperature on drying is investigated through water vapour desorption experiments. The first desorption isotherms of four hardened cement pastes was characterized at 20, 50 and 80°C. The results show a significant influence of the temperature. For a given relative humidity (RH) the water content equilibrium is always reduced temperature is increased and the starting point of capillary condensation is shifted towards higher RHs. The experimental campaign is complemented through modelling activities. The impact of temperature on the first desorption isotherms is effectively described using the Clausius-Clapeyron equation(characterization of the isosteric heat of adsorption). The intrinsic permeability to water is evaluated through inverse analysis by reprocessing the experimental weight loss of initially saturated samples submitted to constant environmental conditions. The intrinsic permeability appears to increase with temperature in relation to the observed microstructure evolution (porosity coarsening).The environmental conditions impact is studied using preconditioned samples (12 different RHs and 20,50 and 80°C) and accelerated carbonation tests. The latter are performed in a new device allowing accurate control of the environmental conditions as well as the carbon dioxide concentration. The carbonated depths and the mineralogical modifications induced by carbonation are assessed using XRDand TGA for each temperature and RH. Most of the mineralogical modifications notified in temperature(hydrates consumption and nature of crystallographic phase of calcium carbonate) are similar with these identified at ambient temperature. Yet the results show a significant influence of the environmental conditions on calcium carbonate polymorphic abundance: the lower the RH, the more abundant the metastable phases (vaterite and aragonite).The rate of the polymorphic transformation (from the metastable states into calcite by dissolution precipitation)is believed to decrease with RH because of lack of liquid water. A significant influence of the environmental conditions on the carbonation rate is also observed. It depends of the competition between the temperature effect on moisture transfer and retrograde solubility of reactants. Carbonation depths appear to be maximal at the RH-starting point of capillary condensation of each material and temperature. Carbonation depths increase with temperature until a limit of temperature characteristic of the material. Above this temperature, reactants solubility might control the main process.