Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Long term performance of a road pavement structure is significantly influenced by its potential to distribute traffic loading from the surface to the natural subgrade. The interlayer adhesion conditions play a substantial role in the induced stress-strain distribution across all layers of the entire structure. For layers constructed in stages like a granular base (GB) and a cement treated subbase (CTSB), the state of adhesion is questionable. Therefore a detailed investigation on the achievable adhesion and its influence on pavement performance is essential. In this study, the direct shear test was used to assess the interlayer adhesion strength in terms of resistance of the GB layer to slide on top of the CTSB. To evaluate the level of achieved shear strength, the interlayer shear results were compared to the inlayer strength for a granular base and cemented subbase. The shear test results were presented in terms of relationships between shear stress and displacement, shear stress and normal pressure and vertical and horizontal displacements. Based on frictional and dilatant approaches, shear test results demonstrated that the interlayer adhesion strength between the GB and CTSB is significantly influenced by the roughness conditions of the CTSB before placing the GB. Compacting materials of the base layer on top of the scarified CTSB produces a unified compound structure due to intimate interaction between the two layers. Moreover, the achievable adhesion depends on the maximum grain size available in the CTSB layer, confining pressure and moisture condition. The increase in maximum aggregate size deepens the interaction zone between the GB and scarified CTSB which results in high shear resistance. Ingress of water induces lubricant behaviour and weakens the shear resistance. In the design example, it was shown that the assumption of full adhesion between pavement layers, currently used in many design methods, over-estimates the pavement life. The routine construction process of placing the GB on top of quasi-smooth CTSB induces poor adhesion between the layers which therefore affects stress-strain distribution behaviour across all layers of the pavement structure and then reduces the life of every single layer. According to the design example, the granular base layer is the most susceptible to early failure due to its stress-dependent behaviour. The significant difference between pavement life when full adhesion is considered and when partial adhesion is allowed indicates that the achievable adhesion should be considered during the design of the structure rather than assuming full adhesion. Furthermore, the development of practical specifications and technical guidelines for improving the anticipated conditions in the field is recommended. / AFRIKAANSE OPSOMMING: Die langtermyngedrag van 'n plaveiselstruktuur word tot 'n groot mate beïnvloed deur die vemoë daarvan om om verkeerslaste vanaf af die oppervlakte na die natuurlike grondlaag te verprei. Die adhesie tussen die plaveisellae speel 'n belangrike rol in die verspreiding van spannings en vervormings deur al die lae van die struktuur. In lae wat in fases gebou word, soos 'n grofkorrelrige kroonlaag (GB) en 'n sementgestabiliseerde stutlaag (CTSB), is die adhesie onder verdenking. 'n Detailondersoek van die adhesie wat behaal kan word, en die invloed daarvan op plaveiselgedrag, is daarom noodsaaklik. In hierdie ondersoek is die direkte skuiftoets gebruik om die tussenlaag-adhesie vas te stel in terme van die weerstand van die GB-laag om oor die CTSB-laag te skuif. Om die vlak van skuifsterkte wat behaal kan word, te bepaal, is die tussenvlakskuifsterkte vergelyk met die interne skuifweerstand van die grofkorrelrige laag en van die gestabiliseerde laag. Die skuiftoetsresultate is uitgedruk in terme van die verbande tussen skuifspanning en skuifverplasing, tussen skuifspanning en normaalspanning en ook tussen vertikale en horisontale verplasings. Gebaseer op skuifweerstand en dilatansie het skuitoetsresultate gedemonstreer dat adhesie tussen die GB- en CTSB-lae baie beïnvloed word deur die ruheid van die CTSB voordat die GB gebou word. Indien die GB-laag bo-op 'n grofgemaakte CTSB-laag geplaas word, word 'n baie goeie verband en interaksie tussen die twee lae verkry. Die beskikbare adhesie hang ook af van die maksimum korrelgrootte in die CTSB-laag, die inperkspanning en die waterinhoud. Die toename in maksimum aggregaatgrootte maak die interaksiesone tussen die GB en die grofgemaakte CTSB dieper en dit lei tot hoër skuifweerstand. Infiltrasie van water dien as smeermiddel wat die weerstand verlaag. In die ontwerp-voorbeeld is gedemonstreer dat die aanname van volle adhesie tussen plaveisellae, soos wat tans in baie ontwerpmetodes gedoen word, tot oorskatting van die leeftyd van die plaveisel lei. Die normale konstruksiemetode waarin die GB-laag bo-op 'n semi-gladde CTSB-laag geplaas word, lei tot swak adhesie tussen die lae wat verspreiding van spannings en vervormings deur die plaveisel minder gunstig maak en die leeftyd van alle lae in die plaveisel verlaag. Volgens die ontwerp-voorbeeld is die grofkorrelrige kroonlaag die vatbaarste vir voortydige faling as gevolg van die sy spannings-vervormingsgedrag. Die beduidende verskil tussen plaveiselleeftyd wanneer volle adhesie aanvaar of slegs gedeeltelike adhesie toegelaat word, illustreer dat die werklike haalbare adhesie gebruik moet word eerder as om volle adhesie te aanvaar. Verder word die onwikkeling van praktiese spesifikasies en tegniese riglyne om die verwagte toestande in die plaveisel beter in ag te neem, voorgestel.

Filament-based material extrusion (MatEx) additive manufacturing has garnered huge interest in both academic and industrial communities. Moreover, there is an increasing need to expand the material catalog for MatEx to produce end use parts for a wide variety of functional applications. Current approaches towards MatEx of semicrystalline thermoplastics are in their nascent stage with fiber reinforcements being one of the most common techniques. MatEx of commodity semicrystalline thermoplastics has been investigated but most of the current methods are extremely material and machine specific. The goal of this dissertation is to enable MatEx of semicrystalline polymers with mechanical properties approaching that of injection molded parts. Tailored molecular architectures of blends that can control the crystallization kinetics from the melt state are investigated. By modifying the crystallization time window, the time during which chain diffusion can occur across the deposited layers is prolonged, which allows for a stronger bond between layers. Such differences in the crystallization process impact the z-axis adhesion and residual stress state, which directly affect mechanical properties and warpage in the printed parts. The impact of blend composition on polymer chain diffusion, crystallization profiles, and print properties resulting from the repeated non-uniform thermal history in filament based MatEx is studied. The melt flow behaviour is characterized using rheology and its effect on the interlayer adhesion of printed parts and print precision is explored. The extent of polymer chain re-entanglement post deposition on the printer bed is quantitatively determined using interrupted shear rheology protocols. Tensile bars are printed and mechanically characterized to analyze the tensile performance of the printed parts. Correlating the rheological findings with the mechanical performance of the printed parts provides valuable insights into the complex interlayer welding process during MatEx and is critical to improving existing machine designs and feedstocks in order to achieve printed parts with properties approaching their injection molded counterparts. The results will be essential in identifying optimal processing conditions to maximize material specific printed part performance as well as highlight the associated limitations to enable MatEx of next generation materials. / Doctor of Philosophy / Compared to traditional subtractive manufacturing techniques, additive manufacturing (AM) has the potential to transform modern manufacturing capabilities due to its unique advantages including design flexibility, mass customization, energy efficiency, and economic viability. The filament-based material extrusion (MatEx), also referred to as fused filament fabrication (FFF), employing thermoplastic polymers (and composites) has emerged as one of the most common AM modality for industrial adoption due to its operational simplicity. However, the widespread application of MatEx has been limited due to the lack of compatible materials, anisotropic mechanical properties, and lack of quality assurance. Most of the research on FFF has been performed on amorphous polymers with almost negligible levels of crystalline content such as polylactic acid (PLA) and acrylonitrile-butadiene-styrene (ABS). Semicrystalline polymers are an attractive choice for FFF feedstocks compared to the amorphous ones due to their improved thermal resistance, toughness, and deformability. However, processing semicrystalline polymers using FFF is challenging due to the volumetric shrinkage encountered during crystallization from the melt state. This results in the buildup of significant levels of residual stresses at temperatures lower than the crystallization temperature of the polymer resulting in warpage of the printed parts. The research presented in this dissertation aims to address the aforementioned challenges by characterizing semicrystalline polymer feedstocks under conditions representative of the multiphysics encountered during a typical FFF process. Several strategies to limit shrinkage and warpage are discussed that involve tuning the thermal profile and crystallization kinetics during printing. The former is achieved by addition of thermally conductive carbon fiber reinforcements while the latter is realized by blending amorphous resins or low crystallinity polymers to the semicrystalline polymer matrix. The fibers results in a more homogenous temperature distribution during printing while the incorporation of the resins modify the rate of crystallization; both of which play a pivotal role in reducing the residual stress build-up and hence minimizing the warpage during printing. The printability of the materials is investigated based on the shear- and temperature dependent viscous response of the polymers. The printed parts with fiber reinforcements exhibit high levels of mechanical anisotropy compared to the blends with the resins, likely due to differences in polymer chain mobility at the interface. The tensile properties of the printed polymer blends are slightly inferior to those obtained using traditional manufacturing techniques; however, properties close to 90-95% of injection molded properties are recovered through a simple post-processing thermal annealing step. The obtained results will assist in optimizing the processing parameters and feedstock formulation in order to consistently produce printed parts with minimal defects and tailored mechanical properties for functional applications.