The structure-property relationships of Ultra-High Performance Concrete (UHPC) were quantified using imaging techniques to characterize the multiscale hierarchical heterogeneities and the mechanical properties. Through image analysis the average size, percent area, nearest neighbor distance, and relative number density of each inclusion type was determined and then used to create Representative Volume Element (RVE) cubes for use in Finite Element (FE) analysis. Three different size scale RVEs at the mesoscale were found to best represent the material: the largest length scale (35 mm side length) included steel fibers, the middle length scale (0.54 mm side length) included large voids and silica sand grains, and the smallest length scale (0.04 mm side length) included small voids and unhydrated cement grains. By using three length scales of mesoscale FE modeling, the bridge of information to the macroscale cementitious material model is more physically based.

The concrete technology has during the last century changed dramatically where the concept of high strength concrete has gone from 30MPa to well over 100MPa. UHPC has many areas of application and is used more and more frequently in all manner of structures. It is also a suitable concrete in areas and environments that are demanding and harsh for the concrete due to its overall excellent durability properties which suggest lifespans of reinforced structure elements that far exceeds what is economically available to design for today with a low cost of maintenance. The aim of this research was to produce Ultra High-Performance Concrete using mostly locally available materials. Tests were made on the binary materials where the particle packing was optimized through the Punkte method. A series of smaller mortar mixes was made where the effect of different proportions of i.e. silica fume, flyash, superplasticizer had on the concrete mix. The fresh and hardened properties of the mix as mini cone flow, slump flow, density, compressive strength and flexural strength was evaluated to obtain a mix which exhibited the properties sought for, high strength and good workability. The results showed that it is difficult to find an optimum mixture since the design of a recipe always has compromises and rarely all criteria’s can be met fully. The concrete produced had a W to C ratio between 0,21, 20wt% of silica fume, 4,5wt% of superplasticizer and max filler size of 1mm. Some mixes of the concrete were also produced with flyash replacement and with steel fibers. This resulted in concretes exhibiting compressive strength over 140MPa, flexural strength of 18MPa without fiber reinforcement and with self-consolidating properties. The replacement of cement with 30wt% of flyash resulted in better workability and long term(1year) compressive strength almost equaled the concrete without flyash replacement.

Yes / This paper evaluates the influence of mineral admixtures partially replacing cement, sea sand replacing quartz, sea water replacing fresh water on ultra-high performance concrete (UHPC). The fluidity and mechanical properties were studied. Besides, the impermeability, chloride resistance and freeze-thaw resistance were investigated. Failure modes, scanning electron microscope (SEM) analysis, mass loss, relative dynamic modulus of elasticity and mechanical properties of UHPCs after freeze-thaw cycles were conducted. The results showed the fluidity of UHPC paste gradually increases with the improvement of water-binder ratio. It is recommended that the water-binder ratio of UHPC be set at 0.19. The fluidity also increases with the improvement of the content of slag, fly ash and water reducer, but decreases with the improvement of silica fume content. The flexural and compressive strengths of UHPC enhance with the improvement of the content of silica fume, but reduce with the improvement of the content of fly ash and slag. The UHPCs made of quartz sand, river sand and sea sand, all, achieve a high strength. UHPCs prepared at standard curing conditions, with or without steel fibers, mixed by artificial seawater and made of sea sand, exhibited excellent impermeability and chloride resistance. The frost resistant grade of all UHPC specimens prepared by standard curing are greater than F500 exhibiting excellent freeze-thaw resistance and sustainability.

No description available.

Ultra High Performance Concrete (UHPC) is a class of cementitious materials that share similar characteristics including very large compressive strengths, tensile strength greater than conventional concrete and high durability. The material consists of finely graded cementitious particles and aggregates to develop a durable dense matrix. The addition of steel fibers increases ductility such that the material develops usable tensile strength. The durability and strength of UHPC makes it a desirable material for the production of highway bridge girders. However, UHPC's unique constitutive materials make it more expensive than conventional concrete. The cost and lack of appropriate design guidelines has limited its introduction into bridge products. The investigation presented in this thesis developed several optimization formulations to determine a suitable bridge girder shape for use with UHPC. The goal of this optimization was to develop a methodology of using UHPC in highway bridge designs that was cost competitive with conventional concrete solutions. Several surveys and field visits were performed to identify the important aspects of girder fabrication. Optimizations were formulated to develop optimized girder cross sections and full bridge design configurations that utilize UHPC. The results showed that for spans greater than 90 ft UHPC used in the proposed girder shape was more economical than conventional girders. The optimizations and surveys resulted in the development of a proposed method to utilize UHPC in highway bridges utilizing existing girder shapes and formwork. The proposed method consists of three simple calculations to transform an initial conventional design to an initial design using modified UHPC girders. / Master of Science

This thesis presents results of an experimental investigation to characterize single straight steel fiber pullout in Ultra-High Performance Concrete (UHPC). Several parameters were explored including the distance of fibers to the edge of specimen, distance between fibers, and fiber volume in the matrix. The pullout load versus slip curve was recorded, from which the pullout work and maximum pullout load for each series of parameters were obtained. The curves were fitted to an existing fiber pullout model considering bond-fracture energy, Gd, bond frictional stress, 𝛕0, and slip hardening-softening coefficient, 𝜷. The representative load-slip curve characterizing the fiber pullout behavior will be implemented into a computational modeling protocol, for concrete structures, based on Lattice Discrete Particle Modeling (LDPM). The parametric study showed that distances over 12.7 mm from the edge of the specimen have no significant effect on the maximum pullout load and work. Edge distances of 3.2 mm decreased the average pullout work by 26% and the maximum pullout load by 24% for mixes with 0% fiber volume. The distance between fibers did not have a significant effect on the pullout behavior within this study. Slight differences in pullout behavior between the 2% and 4% fiber volumes were observed including slight increase in the maximum pullout load when increasing fiber volume. The suggested fitted parameters for modeling with 2% and 4% fiber volumes are a bond-fracture energy value of zero, a bond friction coefficient of 2.6 N/mm² and 2.9 N/mm² and a slip-hardening coefficient of 0.21 and 0.18 respectively. / Master of Science

Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ASTRACT: The very high strength, enhanced ductility and long-term durability of ultra-high performance concrete (UHPC) makes it an ideal material to be used for building structures in the future. The non-heat treated UHPC requires less quality control than heat treated UHPC, which makes it more relevant to be applied in South Africa. This research focuses on developing non-heat treated UHPC with locally available materials, with the exception of short, straight, high strength steel fibre. While UHPC mix design guidelines have been proposed, ingredient materials available locally, but which do not necessarily comply with recommended property ranges, may be compensated for by particular strategies. The local ingredient materials are compared based on their mineralogy, specific surface area, particle size and grading by researchers who successful developed non-heat treat UHPC. The majority of local materials were found not that ideal for UHPC. Under such circumstances, following the general UHPC mix design, it is difficult to reach the same designated strength as those achieved by the other researchers. One of the problems for non-heat treated UHPC is its large shrinkage caused by very low water to cement ratio. A new mix design philosophy is developed for UHPC by making use of steel fibre to improve its compressive strength. Instead of avoiding the large shrinkage, this method uses shrinkage to improve the bond between steel fibre and matrix through the mechanism of shrinkage induced clamping pressure. Subsequently, the mechanism of bridging effect of steel fibre is used to confine shrinkage evolvement in UHPC. Through such a mix design philosophy, the steel fibres are pre-stressed inside UHPC so that it both improves the compressive strength and ductility. A UHPC strength of 168 MPa is achieve in this research. After the UHPC has successfully been developed, factors that can affect UHPC strength are tested. It is found that the environmental temperature of UHPC, cement composition and specimen cover during the moulded period significantly influence UHPC strength by approximately 24%. It is also found that after two days of de-moulding, the UHPC exposed to the air, achieved similar strength as that cured in water, which is helpful for future industrial application.

Ultra-High-Performance Concretes (UHPCs) are a promising class of cementitious materials possessing mechanical properties superior to those of Normal Strength Concretes (NSCs). However, UHPCs have been slow to transition from laboratory testing to insertion in new applications, partly due to an intuitive trial-and-error materials development process. This research seeks to addresses this problem by implementing a materials design process for the design of UHPC materials and structures subject to blast loads with specific impulses between 1.25- and 1.5-MPa-ms and impact loads resulting from the impact of a 0.50-caliber bullet travelling between 900 and 1,000 m/s. The implemented materials design process consists of simultaneous bottom-up deductive mappings and top-down inductive decision paths through a set of process-structure-property-performance (PSPP) relations identified for this purpose. The bottom-up deductive mappings are constructed from a combination of analytical models adopted from the literature and two hierarchical multiscale models developed to simulate the blast performance of a 1,626-mm tall by 864-mm wide UHPC panel and the impact performance of a 305-mm tall by 305-mm wide UHPC panel. Both multiscale models employ models at three length scales – single fiber, multiple fiber, and structural – to quantify deductive relations in terms of fiber pitch (6-36 mm/revolution), fiber volume fraction (0-2%), uniaxial tensile strength of matrix (5-12 MPa), quasi-static tensile strength of fiber-reinforced matrix (10-20 MPa), and dissipated energy density (20-100 kJ/m²). The inductive decision path is formulated within the Inductive Design Exploration Method (IDEM), which determines robust combinations of properties, structures, and processing steps that satisfy the performance requirements. Subsequently, the preferred material and structural designs are determined by rank order of results of objective functions, defined in terms of mass and costs of the UHPC panel.

When compared to traditional concrete, steel fibre reinforced concrete (SFRC) shows several enhancements in performance, including improved tensile resistance, toughness and ductility. One potential application for SFRC is in columns where the provision of steel fibres can improve performance under axial and lateral loads. The use of SFRC can also allow for partial replacement of transverse reinforcement required by modern seismic codes. To improve workability, self-consolidating concrete (SCC) can be combined with steel fibres, leading to highly workable SFRC suitable for structural applications. Recent advances in material science have also led to the development of ultra-high performance fibre reinforced concretes (UHPFRC), a material which exhibits very high compressive strength, enhanced post-cracking resistance and high damage tolerance. In heavily loaded ground-story columns, the use of UHPFRC can allow for reduced column sections. This thesis presents the results from a comprehensive research program conducted to study the axial behaviour of columns constructed with highly workable SFRC and UHPFRC. As part of the experimental program, twenty-three full-scale columns were tested under pure axial compressive loading. In the case of the SFRC columns, columns having rectangular section and constructed with SCC and steel fibres were tested, with variables including fibre content and spacing of transverse reinforcement. The results confirm that use of fibres results in improved column behaviour due to enhancements in core confinement and cover behaviour. Furthermore, the results demonstrate that the provision of steel fibres in columns can allow for partial replacement of transverse reinforcement required by modern codes. The analytical investigation indicates that confinement models proposed by other researchers for traditional RC and SFRC can predict the response of columns constructed with SCC and highly workable SFRC. In the case of the UHPFRC columns, variables included configuration and spacing of transverse reinforcement. The results demonstrate that the use of appropriate detailing in UHPFRC columns can result in suitable ductility. Furthermore, the results demonstrate the improved damage tolerance of UHPFRC when compared to traditional high-strength concrete. The analytical investigation demonstrates the need for development of confinement models specific for UHPFRC.

This thesis investigates the behaviour of UHPC as a fill material for precast deck panels subjected to negative bending. Two full-scale test specimens were constructed. The transverse joints between the panels, the shear pockets, and the deck haunches were all filled with UHPC. A total of four tests were performed including two static tests to failure and two fatigue tests, one of which was performed to failure. Testing consisted of a loading apparatus acting upwards on the deck soffit in an attempt to impose tensile stresses across the transverse joints, representing the conditions that a transverse joint in the negative moment region of a continuous bridge deck would experience. It was concluded that the transverse UHPC joint performed satisfactorily by transferring bending stresses and shear stresses across the joint from one panel to the adjacent panel. Overall, the test specimens displayed performance levels expected from conventional cast-in-place concrete deck alternatives. / February 2016