Coupled effect of fiber-aggregate characteristics and mortar rheology on heterogeneous performance of fiber reinforced self consolidating concrete (FR-SCC)

Abstract

Introduire des fibres dans un béton fluide à rhéologie adaptée (FCAR) permet d'améliorer ses performances mécaniques. Cependant, cela peut augmenter le risque d'hétérogénéité des mélanges SCC renforcés de fibres (FR-SCC), notamment en présence de barres de renforcement. Pour assurer une performance d'écoulement adéquate des FR-SCC, il est nécessaire d'optimiser les caractéristiques physiques et rhéologiques de ses différentes phases, y compris le squelette solide (fibre-agrégat) et la matrice du mortier. La présente recherche vise à évaluer l'effet couplé des caractéristiques des systèmes granulats-fibres et des propriétés rhéologiques du mortier sur les performances d'écoulement homogène du béton autoplaçant fibré (SCC-F). Dans la première phase, une enquête exhaustive a été entreprise pour identifier les caractéristiques des fibres et des granulats grossiers afin de concevoir 19 mélanges SCC-F. Ensuite, un nouveau dispositif de test empirique, Square-Box, a été utilisé pour évaluer les performances d'écoulement régulier de divers mélanges SCC-F en termes de passabilité et de stabilité dynamique. Enfin, dans la dernière phase, les mélanges FR-SCC étudiés dans les phases précédentes ont été coulés dans le dispositif d'essai développé pour évaluer l'effet combiné des caractéristiques des composants du mélange, et la présence de barres de renforcement sur la distribution et l'orientation des fibres (FOD). Les résultats montrent que l'optimisation des mélanges FR-SCC en termes d'effet couplé du squelette solide et de la rhéologie du mortier, en plus de l'effet des barres d'armature, est une approche prometteuse pour obtenir un FOD correct et des FR-SCC maniables.

Abstract: The use of highly-flowable cementitious mixtures, such as self-consolidating concrete (SCC), in concrete industry including repair application aims to facilitate the casting of highly congested reinforced elements. This is based on the various advantages of SCC mixtures in both fresh and hardened states. However, cementitious materials are generally considered as brittle, with low tensile and flexural strengths and weak strain capacity. Therefore, concrete is reinforced by rebar and fibers to improve strength-strain capacity and ductility of concrete structures. On the other hand, the addition of fiber to cementitious materials enhances their mechanical performance but can negatively impact their workability. This study deals with the homogenous performance of fiber-reinforced self-consolidating concrete (FR-SCC) as one of the most promising materials for repair applications. The performance of FR-SCC, as a diphasic suspension, depends on the characteristics of both fiber-coarse aggregate (suspended-solid skeleton) and mortar (suspending liquid) phases. The solid components play a key role in the overall performance of the concrete produced. Then, the optimization of the fiber-coarse aggregate (F-A) combination is necessary to enhance the workability design of FR-SCCs. In the first phase of this study, a comprehensive investigation was undertaken to identify the coupled effect of the characteristics of fibers and coarse aggregate on the packing density (PD) of F-A combination used without any cement paste/mortar. The F-A skeleton can be characterized in terms of particle-size distribution (PSD), volumetric content, and morphology of the coarse aggregate, as well as size, rigidity, and content of fibers. Various types of steel, polypropylene, and polyolefin fibers having different sizes and rigidities are investigated in this phase. Moreover, four combinations of three different classes of coarse aggregate were used to proportion F-A mixtures. Test results showed that shorter length, smaller diameter, and more flexible fibers can lead to higher PD of F-A systems. Moreover, the coarser aggregate skeleton with larger interparticle voids led to more available space for fibers to be deformed, hence improving the PD of F-A mixtures. To simulate the packing state of F-A skeleton in concrete matrix through the mixing process and low casting rates, the loosely-packed system (LPD) of F-A was considered without any compaction. On the other hand, to describe the packing state of F-A skeleton in the case of shotcrete and highly-pressure pumping, where a higher level of compaction applies on the F-A system, the densely-packed system (DPD) were defined. It was found that the LPD of F-A mixtures is mostly affected by the volumetric content and size of the fibers, however, the DPD was more controlled by the rigidity of fibers. New empirical models were proposed to predict the LPD and DPD of F-A combinations given the characteristics of coarse aggregate and fibers, as well as the level of compaction. The established models were employed to propose a new proportioning approach for FR-SCC mixtures to achieve the targeted workability. In the second phase, FR-SCC considered as a diphasic suspension of fiber and coarse aggregate (F-A ≥ 5 mm) skeleton in mortar suspension with solid particles finer than 5 mm. Accordingly, the coupled effect of the volumetric content of fibers and particle-size distribution of coarse aggregate, as well rheological properties of the mortar on the passing ability and dynamic stability of various FR-SCC mixtures were investigated. In total, 19 FR-SCC mixtures for conventional and high strength repair application were proportioned with water-to-binder ratios (W/B) of 0.42 and 0.35, respectively, and macro steel fibers of 0.1% to 0.5% volumetric contents. Flow performance of the investigated mixtures were evaluated in terms of flowability (slump-flow test), passing ability (J-Ring and L-Box set-ups), and dynamic stability (T-Box test). According to the established correlations, the main influencing parameters on homogeneous performance of FR-SCC include W/B, paste volume (Vp), volumetric content-to-packing density of F-A (φ/φmax), HRWR dosage, fiber content, mortar rheology, and volume of excess mortar. The robustness analysis results revealed that homogeneous flow performance of FR-SCC is more sensitive due to variations of the φ/φmax and paste volume rather than mortar rheology, W/B, and HRWR dosage. New blocking (BI) and dynamic segregation (DSI) indices were proposed in this phase. The proposed BI and DSI indices can be employed to evaluate the variation of φ/φmax of F-A portion flowing through restricted and non-restricted flow conditions, respectively. The proposed BI and DSI indices can enable proper assessment of the blocking- and shear-induced changes in the relative volume and particle-size distribution of coarse aggregate and fiber contents. A new workability-based classification is proposed based on the established trade-offs between flow performance under restricted and non-restricted conditions, including passing ability, blocking resistance, and dynamic stability. Characteristics of mixture constituents for FR-SCC mixtures with conventional- and high-strength levels were then recommended to ensure an acceptable homogeneous flow performance. However, the aforementioned empirical tests such as J-Ring, L-Box, and T-Box cannot be properly applied to evaluate the flow performance of FR-SCC under restricted flow conditions (repair applications). Therefore, in this study, a new empirical Square-Box test set-up was employed to evaluate the homogeneous flow performance of FR-SCC mixtures for repair applications (confined flow). The set-up consists of a close-circuit of four close-surface rectangular channels measuring 200 mm in height, 700 mm in length, and 100 mm in width, yielding a box with 700 × 700 mm outer dimensions, 500 × 500 mm inner dimensions, and a height of 200 mm. It was revealed that unlike the conventional L-Box and T-Box tests, the proposed Square-Box test could successfully simulate the flow conditions during casting of FR-SCC mixtures, including the flow distance and confinement, wall effect, and presence of highly congested reinforcing bars. The performance of FR-SCC mixtures was assessed in terms of dynamic stability and passing ability in non-reinforced and reinforced elements, respectively. According to the experimental results, the dynamic segregation and blocking indices of the investigated FR-SCC mixtures were found in good agreements with characteristics of F-A combination and rheological properties of mortar. According to the established correlations, fiber content and φ/φmax of F-A showed more significant effect on homogenous performance of the FR-SCC rather than the mortar rheology. The investigated mixtures exhibited significantly higher blocking indices through the proposed set-up compared to those obtained using the conventional L-Box test. Furthermore, the negative effect of the heterogeneous flow performance on mechanical properties of the investigated FR-SCC mixtures was assessed. The experimental results reflected that higher dynamic segregation led to more dissimilar compressive strength values at different flow distances. A new filling ability classification was established for FR-SCC mixtures. The specifications of the FR-SCC mixtures with high dynamic stability and passing ability properties were recommended. These include the appropriate ranges of volumetric content and rheology of cement paste, PSD of aggregate, macro-steel fiber content, and φ/φmax of fiber-coarse aggregate combination for confined and restricted flow conditions (e.g., repair application). In the last phase of this study, the FR-SCC mixtures investigated in Phase 3 were cast in the developed test set-up to evaluate the combined effect of mixture component characteristics, distance from casting point, wall effect, and the presence of reinforcement bars on fiber distribution and orientation. Using Image analysis method, the vertical and horizontal fiber distribution, as well as 3D fiber orientation was assessed. The results indicated that the presence and arrangement of bars caused fiber distribution and orientation heterogeneity in the region close to the bars. Moreover, the results indicated that the number of fibers decreased in longer flow distance, referring to dynamic segregation and blocking of fibers across the set-up. However, increasing flow distance positively affected fiber orientation (parallel to the flow direction) through the length of the set-up. According to the established empirical models, good correlations were found between characteristics of fiber-coarse aggregate (in terms of relative volumetric content to packing density of fiber-coarse aggregate (φ/φmax) and volume of fiber (Vf)) and mortar rheology with homogeneous performance of FR-SCC mixtures. According to the experimental results, the φ/φmax of coarse aggregates and plastic viscosity of mortar exhibited more remarkable effect on FOD than Vf and mortar yield stress (τ0). Therefore, optimizing the FR-SCC mixtures in terms of coupled effect of φ/φmax, and plastic viscosity, in addition to considering wall and rebar effect as well as the flow distance is a promising approach to achieve proper fiber distribution and orientation, for confined flow conditions (i.e., repair).