Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by browse.metadata.advisor "Combrinck, Riaan"

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    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.
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    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.
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    Plastic shrinkage cracking and other evaporation-related impairments in 3D printed and cast concrete
    (Stellenbosch : Stellenbosch University, 2021-12) Moelich, Gerrit Marius; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The Fourth Industrial Revolution (Industry 4.0) aims to make manufacturing more agile, flexible and responsive to the customer through intelligent and autonomous solutions. One Industry 4.0 technology that shows great potential for the con- struction industry is 3D concrete printing as it eliminates the need for formwork and is so-called formfree. Early estimations show the formfree attribute can re- duce construction costs and time by 28% and 50%, respectively. Furthermore, 3D printing concrete (3DPC) increases design freedom, produces less waste material and improves workplace safety. The advantages of 3DPC's formfree attribute are noteworthy. However, the absence of formwork leaves 3DPC vulnerable to rapid pore water evaporation im- mediately after extrusion, causing plastic shrinkage, plastic shrinkage cracking (PSC) and an impaired long-term durability and mechanical performance. On- site printing in dry and windy climates, common in Southern Africa, Australia, Central America and the Middle East, is of concern since the evaporation rates tend to be more severe. However, for 3DPC, the more benign evaporation rate of indoor climates cannot be precluded from evaporation-related damage. This research aims to improve the resilience of concrete printed or cast in ad- verse climatic conditions by quantifying, understanding, modelling and mitigating the consequences of rapid early age pore water evaporation. Ultimately, this study contributed several novel findings that amounted to seven journal publications. First, a method was proposed for using weather data to characterise the ex- pected on-site evaporation rate for a specific location. Active PSC mitigation measures were recommended based on the result. Thereafter, the effect of solar radiation exposure on the accuracy of the available evaporation estimation equa- tions and severity of PSC were evaluated. The results showed that high levels of solar radiation exposure significantly increase the severity of PSC, and the Jansen- Haise model was recommended to estimate the pore water evaporation rate in these conditions. A test method was proposed to induce and measure plastic shrinkage and early age cracking in 3DPC. 3DPC exhibited severe plastic shrinkage that started immediately after extrusion. Early age cracks appeared and increased in severity within the first one to two hours at a moderate evaporation rate. This behaviour was attributed to the low bleeding rate, absence of coarse aggregates, the high quantities of fines, and high surface area to volume ratio of 3DPC. It is estimated that the risk of PSC in 3DPC is three times as high as cast concrete. Decreasing the magnitude or delaying the evaporation rate decreased the plastic shrinkage and reduced the risk of cracking. Based on this result, an empirical model for the risk of PSC was proposed for 3DPC. To minimise the risk, the evaporation rate should be reduced to below 0.1 kg/m2/h until the final setting time. Evidently, 3DPC is also vulnerable to PSC at lower evaporation rates, typical in indoor climates. The eficacy of several mitigation measures for early age cracking in 3DPC was evaluated with the proposed test method. At a low dosage, short polypropylene microfibres prevented (100% reduction) the formation of PSC without adversely affecting the printability or buildability. Alternatively, increasing the structuration rate, with additives, can reduce early age cracking while improving buildability. Superabsorbent polymers also improved buildability by increasing the stiffening rate through continued pore water absorption. Discontinuities in the transfer of plastic shrinkage from one layer to the next were noticed and coined as interlayer slip. Interlayer slip and microcracking, caused by pore water evaporation from a restrained 3DPC specimen, did not reduce the mechanical and durability performance significantly. Therefore, visibly uncracked 3DPC shows satisfactory durability performance for a short pass time. Finally, internal curing, with superabsorbent polymers, increased the long- term flexural strength (19%) and interlayer adhesion (10%) of printed concrete by promoting the hydration of anhydrous cementitious particles near the interlayer. Evaporation of the interlayer moisture has the direct opposite effect. An analytical model for the interlayer adhesion was proposed based on the amount of interlayer surface moisture which was estimated from the initial surface moisture, the pass time, bleeding rate and evaporation rate. The model was validated for an interlayer bond strength reduction from 30% to 50%. However, applying the model to the experimental results of other researchers suggest that this range is extendable.
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    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.