Doctoral Degrees (Civil Engineering)

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Computational and Experimental Modelling of Masonry Walling towards Performance-Based Standardisation of Alternative Masonry Units for Low-Income Housing
(Stellenbosch : Stellenbosch University, 2019-12) De Villiers, Wibke Irmtraut; Van Zijl, G. P. A. G.; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: South Africa has a housing shortage estimated in excess of 2 million units. This backlog is being addressed predominantly with the construction of 40m2 low-cost, single storey, detached state subsidised houses built with conventional masonry units (CMU’s), namely concrete and burnt clay. The use of these materials has a significant negative impact on the environment and the thermal performance of conventional masonry walls is generally poor. These factors, and others, have led to the development of alternative masonry units (AMU’s) in South Africa, and internationally, with a lesser environmental impact and improved thermal performance. However, lack of standards presents a significant barrier to the uptake of AMU’s The regulatory framework governing the use of masonry in South Africa, and possible avenues through which AMU’s could gain access to the market, are explored. It is found that AMU’s could provide a reasonable and socially acceptable alternative to CMU’s in low-income housing (LIH) but the current regulatory framework does not accommodate AMU’s in a sufficiently practical manner to enable their widespread, off-the-shelf uptake. The ongoing process of the adoption of Eurocode 6 and the accompanying materials and testing standards by the South African masonry industry, facilitates the transition from prescriptive to performance-based (PB) regulation of masonry design. It is proposed that material non-specific, PB requirements for masonry units for structural application in LIH can be developed to assist the inclusion of AMU’s in the open market. To quantify PB criteria, two critical configurations of single-storey bonded masonry walls are generated, based on the deemed-to-satisfy provisions of the National Building Regulations (NBR). Subsequently, a simplified micro-scale finite element (FE) model is used to analyse these configurations under serviceability and ultimate limit state loading conditions, to serve as a performance prediction model from which PB criteria can be derived. Four masonry materials are selected for the study; conventional concrete (CON), alkali-activated material or geopolymer (GEO), compressed-stabilised earth (CSE) and adobe (ADB), representing a wide spectrum in terms of strength and stiffness. Characterisation tests of the four materials are used, together with numerical fitting to test data and data taken from literature, to generate the necessary parametric input for the FE model. The results of medium to large-scale in-plane and out-of-plane tests are used for validation of the FE model. The FE analyses revealed that for most of the load cases, the resistances of the walls failed to achieve the design load, even for the conventional CON blocks. A significant shortfall was found for the out-of-plane resistance against the wind load for all four materials, as well as structural vulnerability under seismic loading due to the geometric layout permitted by the deemed-to-satisfy rules in the NBR. These results preclude the immediate derivation of PB criteria for AMU’s but contribute significantly to the body of knowledge surrounding FE modelling of AMU’s. They also indicate that the NBR provisions for wall panel geometry require reconsideration, given the recent revision of the South African loading code. However, material non-specific PB regulation is still the recommended avenue for the standardised inclusion of AMU’s.
Contributions to structural mechanics and durability in structural engineering
(Stellenbosch : Stellenbosch University., 2016-12) Van Zijl, Gideon Pieter Adriaan Greeff; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Contributions to structural engineering have been made since 2001 from the basis of the Department of Civil Engineering of Stellenbosch University (SU). The inauguration of the Centre for Development of Sustained Infrastructure (CDSI) in 2002 has been instrumental in defining, directing and scoping the research and development in the categories Advanced cement-based construction materials, Crack formation and durability towards durability design, Renovation and retrofitting towards extended life span and Sustainable energy harvesting structures. The contributions are structured along these categories as chapters of this dissertation. Early career background in structural Engineering at the Institute for Structural Engineering (1987-1989) and Bureau for Mechanical Engineering (1989-1992) and higher education in computational and structural mechanics (PhD 1995-1999, Research Fellow 1999-2001, TU Delft), shaped the research interests in these fields. Continued affiliation with TU Delft (30%) and SU (70%) in the years 2001-2009 provided access to collegial expertise in related fields of experimental research, materials engineering, risk and reliability, and structural design at these institutions and beyond. In this way, national and international collaboration complemented structural and computational mechanics in well-rounded research programs in the mentioned categories. Clearly, the contributions are the result of collaboration in which the author to various degrees led, participated in and supervised research and development. Highlights of the contributions in the four categories are described at a relatively high level towards conveying the contributions in the national and international context. To a degree selective reporting is done, and the reader directed to detailed elaborations in roughly 200 dissertations, theses and technical papers supervised or co-supervised, authored and co-authored. Approach by the infrastructure pre-fabrication industry in South Africa towards development of accelerated and new product lines led to the development of advanced cement-based construction materials (ACM) with local ingredients, and appropriate adaption of the materials to industrial fabrication process of high-pressure extrusion. What started as fibre inclusion towards reduced traditional steel reinforcement in concrete pipes, led to development, characterisation, manufacturing and constitutive modelling of steel fibre concrete and strainhardening cement-based composites (SHCC). Roles of international leadership in co-chairing and chairing RILEM technical committees followed, as well as co-editing of books on the state-of-the-art of Durability of SHCC and a Framework of durability design with SHCC respectively. Particular contributions of significant potential towards the ability to design for durable, sustainable infrastructure, were made in chloride-induced corrosion and alkali silica reaction. In both cases crack formation and durability, i.e. structural durability in service conditions are the points of departure in order to assess actual structural performance in presence of such deteriorating processes. The work in ACM was extended to ultra-high strength concrete, and recently to lightweight aerated concrete (LWAC) and lightweight foam concrete (LWFC). The thermal, acoustic and potential mechanical advantages of LWAC and LWFC are subjects of a current significant research effort in the CDSI towards developing these lightweight materials for structural application in residential infrastructure. Constitutive models developed for traditional construction materials, as well as several of the ACM, enabled the iterative computationalexperimental development and validation of retrofitting strategies for both unreinforced load-bearing masonry and reinforced concrete structures for new functionality or extended structural life span. Finally, a role of leadership and collaboration was fulfilled in research of the solar chimney power plant concept with national and international partners, bringing the concept for harvesting of sustainable energy to a pre-feasibility level. The contributions have laid the link between construction material properties, structural behaviour and durability. Through the fundamental experimental research, structural mechanics and computational mechanics, it has been made possible to utilise the advanced properties of ACM to advance structural performance and durability. Human capital well-versed in the fundamental principles of this multi-level structural engineering approach has been developed in the process of research supervision by the author.
Cracking of Plastic Concrete in Slab-Like Elements
(Stellenbosch : Stellenbosch University, 2016-03) Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept of Civil Engineering.
ENGLISH ABSTRACT: The cracking of plastic concrete involves two cracking types namely: plastic settlement cracking which is caused by differential settlement of the concrete and plastic shrinkage cracking which is caused by evaporation of free concrete pore water. These cracks are mainly a problem for slab-like elements exposed to conditions with high evaporation rates and typically occur within the first few hours after the concrete has been cast. The early occurrence of these cracks greatly reduces the durability and service life of a concrete structure. These cracks remain a problem in the construction industry even though there are several effective, but mostly neglected, precautionary measures. The reasons these cracks remain a problem are due to the complex nature of the cracking as well as the lack of a unified theory or model that can account for all the complexities involved. With this in mind, this study aims to fundamentally understand both plastic settlement and plastic shrinkage cracking in slab-like elements individually and combined as well as to determine the tensile material properties of plastic concrete. Once the cracking is fundamentally understood the final objective is to develop a model that can simulate the cracking of plastic concrete using a finite element method approach. The fundamental understanding of these cracks was obtained by conducting various tests on different mixes at various climates and in various moulds. The tests showed that both crack types can occur separately, where plastic settlement cracking occurs first in the form of multiple cracks at the surface as well as shear induced cracks beneath the surface, followed by plastic shrinkage cracking in the form of a singular, well defined crack. In addition, a significant deviation from the individual cracking behaviour was observed when combining these cracks, highlighting the shortfall of most available literature where these cracks are seldom researched in tandem. From all the tests, six different cracking behaviours were identified depending on the potential severity for each cracking type. The test also showed worryingly that both these cracks can be present internally without being visible at the concrete surface where they act as weak spots for future crack growth. The practically challenging tensile testing of plastic concrete was conducted with a newly built direct tensile test setup, which provided stress-strain curves that were used to determine the tensile material properties of plastic concrete such as: Young’s modulus, tensile strength, strain capacity and fracture energy. This included tests at different temperatures as well as cyclic tests. The results showed that the tensile material properties develop significantly faster, the greater the ambient temperature surrounding the concrete as well as the resilient nature of a still plastic concrete which proved to be capable of withstanding cyclic loading without failure, while a solid but still weak concrete could not. The tensile material properties together with the measured strains of plastic concrete were combined to provide both an analytical and numerical estimation of the cracking behaviour of plastic concrete. The analytical estimation was more simplistic and required a few crude assumptions, while the numerical estimation used finite element methods to create a model that accounted for the major complexities involved such as time-dependency of material properties and anisotropic volume change of plastic concrete. Both the analytical and finite element model gives adequate representation of the cracking behaviour for extreme climates but not for normal climates, with the size discrepancy between the interior and surface cracks during experiments as well as the relaxation of stresses in plastic concrete being provided as the main reasons for the poor correlation. The finite element model was further used to conduct a parameter study, where the settlement and shrinkage strains were shown to govern the size of the final crack, while the material properties only influence the time of crack onset and rate of crack widening. Finally, the finite element model was successfully applied to a large scale example of a concrete slab, indicating that the model can be a helpful tool to simulate the cracking of plastic concrete without the need to perform timely experiments.
Fatigue behaviour of steel fibre reinforced concrete
(Stellenbosch : Stellenbosch University, 2022-04) Fataar, Humaira; Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Concrete is a heterogeneous material that is known to have a weak tensile capacity. The construction industry has successfully utilised steel reinforcement in concrete to overcome its brittle tensile behaviour, however, steel reinforcement is generally not sufficient to resist the formation and propagation of tensile cracks. As a result, discrete fibres are added to the concrete mixing stage to produce fibre reinforced concrete (FRC). The use of steel fibres has been known to improve the post-cracking behaviour of steel fibre reinforced concrete (SFRC), and is one of the most readily available fibres in industry. Due to concrete’s popularity as a construction material, many of its applications may experience flexural fatigue loading at some point during its lifespan. A significant amount of research has been conducted on the fatigue behaviour of plain concrete and FRC in the past century. However, the research focused primarily on the uncracked behaviour, with few researchers considering the cracked behaviour. In FRC, the fibres are activated only at crack initiation and therefore, this work aimed to investigate the fatigue life and failure mechanisms of pre-cracked SFRC subjected to fatigue loading. Experiments were conducted at a single fibre and macroscopic level, using hooked-end steel fibres. The pre-cracks ranged from 0.6 mm to 2.5 mm, at fatigue load levels of 50%, 70% and 85% of the maximum static load. Various methods were used to attempt to predict the fatigue life and failure mechanisms. A single fibre pull-out model was developed to categorise the various fibre pull-out phases and the level of deformation associated with each phase. When subjected to a static pre-slip of the fibre, followed by fatigue loading, the failure mechanisms were both fibre pull-out and fibre rupture. The fatigue capacity and failure mechanisms of the single fibre specimens vary depending on the combination of pre-slip and load level. The pull-out failures are generally unable to resist many load cycles due to a diminished fibre anchorage. The rupture failures tend to occur after a significant number of cycles have already passed, since the fibres rupture due to fatigue failure. The macroscopic behaviour subjected to fatigue loading shows fibre rupture to be the dominant failure mechanism, which differs from the static behaviour, where fibre pull-out occurs. The fatigue resistance decreases with an increase in pre-crack and load level. The single fibre pull-out model is used to classify the fibres into the different phases along the height of the crack for the various pre-cracks. A fatigue life prediction approach developed from the macroscopic fatigue results in the form of a modified S-N curve, was used to predict the fatigue behaviour. Experimental results from similar work performed was compared with the modified S-N curve and the results show that for deflection softening behaviour of SFRC, the model overestimates the fatigue capacity. Therefore, post-cracking behaviour influences the fatigue capacity of SFRC. The framework for an analytical model was developed to predict the fatigue failure mechanisms of pre-cracked SFRC. Fibre pull-out is likely to occur when large pre-cracks are present, whereas fibre rupture is more likely at low pre-cracks.
The influence of rheology on the cracking of plastic concrete
(Stellenbosch : Stellenbosch University, 2020-03) Kolawole, John Temitope; Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Plastic cracking occurs during concrete’s early hours and impairs its serviceability and durability. The early hours refer to the time of mixing to the time around the final set. Concrete possesses rheological properties during the plastic phase while settlement and shrinkage leading to plastic cracking also occur during this phase. Therefore, there is bound to be an interaction between the concurrent concrete’s rheological behaviour and cracking behaviour. During this same plastic phase, concrete possesses pronounced rheo‐viscoelastic properties that influence the plastic cracking behaviour. The heterogeneous nature of concrete coupled with its time‐dependent behaviour (due to hydration) at early hours makes investigations into the above‐identified properties in relation to plastic cracking complicated and lacking in the literature. With this in mind, the goal of this study was to establish links between the rheo‐related properties of concrete and its plastic cracking. Experimental investigations started with concrete mixes designed for varied rheological properties but similar hardened properties. Rotational and dynamic shear rheometries were employed to characterise the plastic phase of the concrete in order to establish the shear rheo‐physical and rheo‐viscoelastic properties. The plastic cracking behaviour of the concrete mixes were also investigated. Analytical methods were, thereafter, used to predict the occurrence of plastic cracking of the concrete mixes. Results show that concrete can both be thixotropic and rheopectic due to rheology modifiers and condition pre‐history. Various thixotropy evaluation methods adopted for the study showed similar trends except for the hysteresis loop area that was dependent on the concrete’s initial set condition and was, therefore, pegged as merely suitable for qualitative measurement. Plastic concrete’s viscoelastic behaviour is majorly influenced by hydration, coarse solid volume fraction and constituent materials such as rheology modifiers. Furthermore, it was discovered that plastic concrete possesses pseudo‐strain hardening ability under the application of linear (microscopic) shear strain. This was similar to the physical (macroscopic) response earlier detected as shear thickening and rheopexy. The study further shows that self‐settlement is the major dominating process during the early plastic phase of concrete’s cracking behaviour and takes the form of a gravitational shearing process. This self‐settlement is directly linked to the yield stress and thixotropy of the concrete. This same process influences the plastic shrinkage and rate of capillary pressure during the period of the self‐settlement, thereby, linking the plastic shrinkage and rate of capillary pressure to the yield stress and thixotropy. Plastic concrete generally possesses the inherent ability to relax early restrained cracking stress, but its ability for strain dissipation heavily depends on the material constituents such as rheology modifiers. The analytical methods for prediction revealed that the plastic concrete damage tends to be strain‐oriented with a pressure‐insensitive form of ductile failure. In addition, the cracking strains during the self‐settlement period are mainly shear‐related. Finally, it was proposed that it is possible to control the concrete mix design to avoid damage/microcracking associated with the plastic phase.
The Mechanical Behaviour of High-Performance Concrete with Superabsorbent Polymers (SAP)
(Stellenbosch : University of Stellenbosch, 2016-03) Olawuyi, Babatunde James; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: High performance concrete (HPC) is known to be of low water-binder ratio (W/B) and exhibits high strength, durability and elastic modulus amongst many other properties. HPC is susceptible to autogenous-shrinkage-caused-cracking under restraints while previous research efforts directed at mitigating autogenous-shrinkage in HPC by the introduction of IC agents have reported superabsorbent polymers (SAP) to be the most promising. This study seeks to fill the existing gap on proper understanding of the effect of SAP addition on the mechanical behaviour of HPC. The work studied the mechanical properties of HPC containing SAP as internal curing agent (IC-agent) using two grain sizes of SAP (˂ 300 μm and ˂ 600 μm) at varied SAP contents (0%; 0.2%; 0.3%; and 0.4% bwob) in four reference HPC mixtures (M1F, M1S, M2 and M3) after 7, 28, 56 and 90 days of curing in water. SAP absorption in cement pore solution (CPS) was determined using the tea-bag test and the 25 g/g absorption in CPS obtained after 10 minutes of immersion was used for provision of additional water in the HPC mixtures. Experimental works were carried to study the impact of SAP addition on the rheology of the HPCs, as well as identify and establish the effect of varying sizes and volume of SAP on rate of cement hydration and strength development. The work involved quantifying and modelling the mechanical behaviour (strength in compression, tension, elastic and fracture properties) of the low W/B (0.2 – 0.3) HPC (C55/67 – C100/115) with SAP. Microstructure and molecular interaction of the internal constituent of the HPC were also investigated using the X-ray computed tomography (CT) scanning and scanning electron microscopy (SEM). The study observed a slight decrease in the compressive strength of HPC as SAP content increases but there is no such effect on the elastic and fracture properties of the concrete. The 25 g/g SAP absorption result of the teabag test over-estimates the actual amount of water used up by SAP in the internal curing of HPC. The 3D void analysis of the HPC via CT scanning revealed that SAP created voids in the HPC is only about half (i.e.12.5 g/g) of the teabag test result of 25 g/g and affirms that the required additional water for SAP’s effective internal curing of the low W/B HPC is 12.5 g/g. The study concludes that the optimum additional water for SAP addition in the low W/B HPCs at no negative effect on mechanical properties is 12.5 g/g.
Tensile creep of cracked macro synthetic fibre reinforced concrete
(Stellenbosch : Stellenbosch University, 2015-03) Babafemi, Adewumi John; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Macro synthetic fibres are known to significantly improve the toughness and energy absorption capacity of conventional concrete in the short term. However, since macro synthetic fibre are flexible and have relatively low modulus of elastic compared to steel fibres, it is uncertain if the improved toughness and energy absorption could be sustained over a long time, particularly under sustained tensile loadings. The main goal of this study is to investigate the time-dependent crack mouth opening response of macro synthetic fibre reinforced concrete (FRC) under sustained uniaxial tensile loadings, and to simulate the flexural creep behaviour. For the purpose of simulating the in-service time-dependent condition, all specimens were pre-cracked. Experimental investigations were carried out at three levels (macro, single fibre and structural) to investigate the time-dependent behaviour and the mechanisms causing it. At the macro level, compressive strength, uniaxial tensile strength and uniaxial tensile creep test at 30 % to 70 % stress levels of the average residual tensile strength were performed. To understand the mechanism causing the time-dependent response, fibre tensile test, single fibre pullout rate test, time-dependent fibre pullout test and fibre creep test were done. Flexural test and flexural creep test were done to simulate the structural level performance. The results of this investigation have shown significant drop in stress and increase in crack width of uniaxial tensile specimens after the first crack. The post cracking response has shown significant toughness and energy absorption capacity. Under sustained load at different stress levels, significant crack opening has been recorded for a period of 8 month even at a low stress level of 30 %. Creep fracture of specimens occurred at 60 % and 70 % indicating that these stress levels are not sustainable for cracked macro synthetic FRC. The single fibre level investigations have revealed two mechanisms responsible for the time-dependent crack widening of cracked macro synthetic FRC under sustained loading: time-dependent fibre pullout and fibre creep. In all cases of investigation, fibre failure was by complete pullout without rupture. Flexural creep results have shown that the crack opening increases over time. After 8 months of investigation, the total crack opening was 0.2 mm and 0.5 mm at 30 % and 50 % stress levels respectively. Since the crack opening of tensile creep and flexural creep specimens cannot be compared due to differences in geometry, specimen size, load transfer mechanisms and stress distribution in the cracked plane, a finite element analysis (FEA) was conducted. Material model parameters obtained from the uniaxial tensile test and viscoelastic parameters from curve fitting to experimental uniaxial creep results have been implemented to successfully predict the time-dependent crack opening of specimens subjected to sustained flexural loading. Analyses results correspond well with experimental result at both 30 % and 50 % stress levels.
Time-dependent Behaviour of Cracked Steel Fibre Reinforced Concrete: from Single Fibre Level to Macroscopic Level
(Stellenbosch : Stellenbosch University, 2016-03) Nieuwoudt, Pieter Daniel; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The addition of steel fibres to a concrete matrix is known to improve the material’s post crack mechanical behaviour under short term loading conditions. However, limited information is available on the material’s post-crack behaviour under long-term loading, particularly under sustained uni-axial tensile loading. The purpose of this research study is to investigate and quantify the time-dependent Crack Mouth Opening Displacement (CMOD) behaviour of cracked Steel Fibre Reinforced Concrete (SFRC) under sustained uni-axial tensile loading, and to develop a mathematical model which is able to simulate the time-dependent crack width opening behaviour of cracked SFRC under sustained uni-axial tensile loading. To reach this goal, experimental investigations were performed at two levels, namely macroscopic level and single fibre level. At the macroscopic level, the short term mechanical properties of SFRC were investigated by performing compressive and uni-axial tensile strength tests. To investigate the long term mechanical properties of cracked SFRC, sustained uni-axial tensile load tests were performed at stress levels ranging from 30 % to 85 % of the residual tensile strength. To understand the mechanisms causing the time-dependent CMOD as well as the factors that can influence the behaviour, single fibre pull-out rate tests and single fibre sustained load tests were performed on hooked-end steel fibres. The SFRC showed significant toughness and energy absorption capacity after cracking, both under compression and uni-axial tensile loading. The sustained uni-axial tensile load results showed that the time-dependent CMOD increases with the applied sustained stress level. Over the measured time period of 240 days none of the tested specimens fractured even for stress levels as high as 85 % of the residual tensile strength. Significant variability was found in the results at each load level and it was concluded that the variation in the crack plane fibre count for each specimen is one possible reason for the variability at each load level. The single fibre pull-out rate results showed significant rate sensitivity for the slip at maximum pull-out force. This rate effect is induced by the interface between the hooked-end of the fibre and the surrounding matrix. The fibre embedment inclination angle and the geometry of the mechanical hooked-end of the fibre have been found to significantly affect the pull-out behaviour. The single fibre sustained load results showed that the pull-out due to a sustained load is dependent on the applied load level. The pull-out due to a sustained load is referred to as pull-out creep and is defined as the relative movement between the fibre and the matrix interface under sustained loading. It is found that the pull-out creep is induced by the localised compression of the surrounding matrix by the hooked-end of the fibre. The mechanisms responsible for the pull-out creep are therefore believed to be similar as the creep for bulk normal concrete under compression. A constitutive model was developed based on the theory of rheology to simulate the single fibre pull-out creep behaviour of an aligned hooked-end steel fibre. The model was then generalised to simulate the CMOD of cracked SFRC under sustained uni-axial tensile loading by assuming a uniform fibre distribution over the crack plane. It was found that the orientation of the fibres at the crack plane was a relevant factor that affects the time-dependent CMOD. The generalised model was able to simulate the CMOD under various applied sustained stress levels (30 % to 85 %) with relative high accuracy.
The time-dependent behaviour of cracked textile reinforced concrete (TRC)
(Stellenbosch : Stellenbosch University, 2023-03) Alexandre, Vital Jorge Fisch; Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The use of concrete faces drawbacks in the form of anthropogenic carbon dioxide emissions and a lower tensile strength threshold. The use of textile-reinforced concrete (TRC) is sought to tackle both these issues. Firstly, TRC elements provide an option for thinner structural elements and secondly, it improves the concrete’s post-crack behaviour when considering the short-term loading. However, there is limited research on the behaviour of TRC when subjected to long-term uni-axial loading, mainly due to the time-consuming nature of such related investigations. The purpose of this study is to investigate the behaviour of TRC composites when subjected to various sustained uniaxial loading levels for sustained time periods. This is accomplished by conducting both short- and long-term tests. The short-term investigations focus on the interaction between the textile and matrix and are assessed by delving into the pull-out of yarns from the matrix and uniaxial tensile strength tests. The long-term behaviour is investigated by performing tensile creep tests on predamaged and non-damaged specimens. The sustained loads applied ranged between 10 % and 75 % of the ultimate tensile static load test results. The single-yarn pull-out tests showed that the pull-out behaviour depends on the embedment length. Shorter lengths (25 and 35 mm) exhibited strain-softening behaviour with pull-out being the dominant failure mechanism. If the embedment length increases (30, 40, and 60 mm), then strain-hardening dictates the pull-out behaviour. Pull-out was found to be the failure mechanism for the 30 and 40 mm lengths and rupturing for the 60 mm embedment length. Additionally, the dynamic stage of the pull-out response was also identified to be associated with a bottleneck mechanism forming. This mechanism results from the imprint the warp yarn leaves in the matrix, constricting and dilating the extraction pathway. Particle fragments also cause congestion of the pathways, increasing the pull-out resistance. The uniaxial static pull-out tests were conducted with specimens containing two to six layers of textiles. It was discovered that the number of weft yarns in the observed section dictated the maximum number of cracks formed when considering the crack saturation. All samples also showed a ductile failure attributed to the telescopic failure mechanism. Moreover, the stiffness degradation showed that the samples failed when the secant modulus lowered to 2 GPa. The latter occurred regardless of the number of cracks and number of textiles, indicating that stiffness was related to the failure as opposed to the reinforcing area. The stiffness degradation is argued to be tied to the telescopic mechanism taking place. The sustained uniaxial load tests showed that the time-dependent strain increased with time and increasing sustained load level. The samples with a stress-level below 60 % did not fracture during the period over which the loads were sustained. However, the samples loaded at 75 % stress levels fractured within 10 minutes of loading. The residual strength tests also highlighted that the straightening of fibres during the sustained loading period, known as the training effect, enhanced the strength of the specimens compared to samples that were not subjected to sustained loads but with the same specimen age. Pre-damaged specimens also exhibited lower time-dependent strains compared to those with no predamage. This observation is also attributed to the training effect. The implications of the training effect was also noticed when considering the loading history of samples by increasing the stress level for a select few specimens.

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