Although ultra high-performance concrete (UHPC) has great performance in strength and durability, it has a disadvantage in the environmental aspect; it contains a large amount of cement that is ...responsible for a high amount of CO
emissions from UHPC. Supplementary cementitious materials (SCMs), industrial by-products or naturally occurring materials can help relieve the environmental burden by reducing the amount of cement in UHPC. This paper reviews the effect of SCMs on the properties of UHPC in the aspects of material properties and environmental impacts. It was found that various kinds of SCMs have been used in UHPC in the literature and they can be classified as slag, fly ash, limestone powder, metakaolin, and others. The effects of each SCM are discussed mainly on the early age compressive strength, the late age compressive strength, the workability, and the shrinkage of UHPC. It can be concluded that various forms of SCMs were successfully applied to UHPC possessing the material requirement of UHPC such as compressive strength. Finally, the analysis on the environmental impact of the UHPC mix designs with the SCMs is provided using embodied CO
generated during the material production.
The incorporation of carbon nanotubes (CNTs) in Ultra-High Performance Concrete (UHPC) was investigated with regard to the dispersion, mechanical properties, and electromagnetic shielding ...effectiveness (SE). Dispersed CNT solution (0–2.0 wt%) was prepared by sonication and subsequent shear mixing with superplasticizer to achieve the high flowability for UHPC slurry. The dispersion method was effective to manufacture CNT-UHPC composite up to the critical incorporation concentration (CIC). Conducted experiments revealed that CNTs below the CIC improve the mechanical properties of UHPC through pore filling, bridging, and a denser C–S–H structure. CNTs substantially improve the electrical conductivity and resulting SE up to the percolation threshold. Two methods of ASTM D4935-18 and IEEE-STD-299 with electromagnetic compatibility (EMC) theory verified the result and suggested that larger specimen with sufficient incident area should be fabricated to accurately evaluate the SE of actual structures. Otherwise, the ability of SE of UHPC-CNT composites could be biased.
Despite the growing interest in 3D concrete printing, its current progress is limited by reinforcing methods. Inclusion of steel fibers is a potential reinforcing solution; however, the effect of ...printing process on orientation of the fibers is still unknown. This study aims to quantitatively investigate the orientation distribution of steel fibers in 3D printed ultra-high performance concrete. The effects of extrusion nozzle size, Cartesian print speed, and fiber volume fraction on the orientation of fibers were evaluated using digital image analysis. The consequent effects of the fiber orientation on the mechanical properties of the 3D-printed specimens were also determined. The results were compared with those of the conventionally mold-cast specimens. The results revealed that the smaller nozzle size and higher fiber volume fraction significantly enhanced the fiber alignment parallel to the printing direction. This preferential fiber alignment led to superior mechanical performance of the printed specimens to the mold-cast specimens.
This study presents an optimized design method in developing ultra-high performance concrete (UHPC) with high wet packing density, in which the multiply effects of solid and liquid phases on UHPC ...packing mode are considered. Specifically, a model based on D-optimal method is firstly established to assess the influence of solid granular particles and superplasticizer (SP) on the UHPC packing density. Based on the theoretical analysis, the mixture proportions can be optimized, and UHPC with high wet packing density could be produced. Then, to evaluate the reliability of the math-physical method in developing UHPC, its compressive strength and pore structure are tested. The obtained experimental results show that the designed UHPC contributes high packing density, leading to optimized pore structure and extraordinary compressive strength. The experimental verification further highlights that D-optimal method is a promising and effective approach to design UHPC, and eventually for the development of sustainable UHPC.
•An optimized design approach for UHPC is proposed and demonstrated.•The effect of liquid phases on UHPC particle packing is investigated.•D-optimal method is employed for the UHPC theoretical analysis.•The meso/micro pore structure of the designed UHPC is studied.•A sustainable UHPC could be produced based on the proposed method.
This study evaluates the effects of two types of graphite nanoplatelet (GNP-C and GNP-M) and one type of carbon nanofiber (CNF) on rheological properties, hydration kinetics, autogenous shrinkage, ...and pore structure of ultra-high-performance concrete (UHPC). The dispersion method was optimized to secure uniform dispersion of the nanomaterials in the UHPC. The plastic viscosity decreased with the nanomaterials content as the content was increased from 0 to 0.05%. As the nanomaterials content increased from 0 to 0.3%, the duration of induction period was extended by the addition of CNF, but shortened by use of GNP-C or GNP-M; cumulative hydration heat release was increased by introduction of nanomaterials; the autogenous shrinkage of UHPC with CNF, GNP-C, and GNP-M was increased by 30%, 20%, and 20%, respectively. The use of 0.3% CNFs reduced the total porosity of the UHPC by approximately 35%, indicating that the presence of CNFs refined the microstructure of UHPC.
•Nine large-sized reinforced UHPC beams are fabricated and tested under flexure.•Steel rebar and fiber play a significant role in limiting the crack development of UHPC beam.•A high-precision model ...is developed to predict the average crack spacing of UHPC beam.•Flexural capacity of UHPC beam increases almost linearly with the reinforcement ratio.•Contribution of steel fibers to the flexural capacity decreases with the increase of reinforcement ratio.•UHPC tensile capacity needs to be considered when a reinforcement ratio is less than 2.9%.
This research is aimed at investigating the flexural and cracking behaviors of ultra-high-performance concrete (UHPC) beams. Nine UHPC beams with different reinforcement ratios (0, 1.0%, 2.9%, 4.8%, and 7.1%) and fiber volume fractions (2.0% and 3.0%) were considered under flexure. A section analysis was also performed to predict the flexural and cracking behaviors of UHPC beams and was verified by experimental results. The test results showed that the reinforcement and steel fibers can play a significant role in limiting crack development. Considering the performance of the steel fibers, a high-precision equation for predicting the average crack spacing of UHPC beams was developed. The formulas in the French standard NF P 18–710 overestimated the maximum crack width by a significant margin for the UHPC beams with a high reinforcement ratio (>4%). The flexural stiffness of the reinforced UHPC beams increased as the reinforcement ratio increased, whereas their initial stiffness without reinforcement was larger than that with a low reinforcement ratio of 1.0% because of the weak bond interface between the UHPC and the reinforcement. The flexural capacity of the UHPC beams basically increased linearly with the reinforcement ratios. The result of the sectional analysis indicated that the contribution of steel fibers to the flexural capacity decreased significantly with the increase in the reinforcement ratio. In the ultimate limit states design, the contribution of UHPC tensile capacity could be considered only as a safety reserve when the reinforcement ratio is greater than 4.0%, whereas its contribution needs to be considered for the UHPC beams with a reinforcement ratio of less than 2.9%.
•The state of research on flexural strengthening of RC beams or slabs with UHPC was reviewed.•Different failure modes of UHPC-RC composite members under flexure were identified.•The main parameters ...affecting the flexural strength were discussed.•Analytical and numerical models were summarized for strengthened beams or slabs with UHPC.
The use of ultra-high performance concrete (UHPC) to strength existing reinforced concrete (RC) structures in flexure has been explored in recent decades. As UHPC developed in different countries performed different properties, the effectiveness of RC structures strengthened with UHPC varies. Moreover, the lacking of code provisions restricts the wide application of this novel strengthening technology. It is necessary to review experimental studies for the guidance of code elaboration. In this research, the state of research on the flexural strengthening of RC beams or slabs with UHPC was presented. From the technical literature review, an experimental database was established. In order to examine the effectiveness of strengthening schemes with UHPC, size effect, and mechanical properties of RC beams or slabs, pre-damage degree for RC, strengthening configuration, characteristics of UHPC layer, curing conditions for UHPC and interfacial preparation for concrete substrate were discussed. In the literature review presented, different failure modes of UHPC-RC composite members under flexure were also identified. Then, analytical and numerical models developed in the literature to reproduce structural response and to predict cracking and ultimate capacity of strengthened beams or slabs with UHPC were summarized, and a comparison between these models was presented. The experimental evidence showed that UHPC could be used to increase the flexural strength of RC beams or slabs. Also, a cost analysis comparing with other strengthening techniques (such as CFRP) was presented. Finally, some future work is recommended to complement this promising strengthening technique for existing RC structures under flexure.
•Typical three-segment axial stress-strain curves of FRP-confined UHPC are defined.•Stress reduction and the actual confinement ratio threshold reduce with the content of steel fibers.•Addition of ...steel fibers compromises effects of specimen size.•Performances of existing axial stress-strain models for FRP-confined concrete are verified.
Ultra-high performance concrete (UHPC) usually exhibits a weak ductility under axial compression. However, the ductility can be enhanced by implementing confining devices such as fiber-reinforced polymer (FRP) wraps. In order to gain in-depth understandings on the compressive behavior of FRP-confined UHPC, axial compression tests on 52 FRP-confined and 12 unconfined UHPC cylinders were carried out in this study. The failure modes, stress-strain behaviors, confinement efficiency, and the influences of crucial variables (steel fiber contents, specimen sizes, FRP fiber types, and FRP thickness) on the compressive behavior of FRP-confined UHPC were investigated and discussed. Results show that the increase in steel fiber content could increase the first peak stress and its corresponding strain, hoop strain efficiency, avoid the abrupt stress reduction after the first peak stress, as well as alleviating the effect of specimen sizes. Addition of steel fibers also alters the actual confinement ratio threshold for sufficient confinement. Comparisons suggest that FRP confinement fiber types have limited influence on the normalized stress-strain behavior of specimens with similar level of actual confinement ratio. Additionally, performances of four existing axial stress-strain models for FRP-confined concrete are verified and the results reveal that Teng et al.’s model performs the best although this model is incapable in capturing the strain softening behavior after the first peak stress.
•A machine learning framework is developed to predict failure mode of UHPC beams.•Genetic programing algorithms is applied for predicting shear capacity of UHPC beams.•The proposed expressions can be ...utilized in practice for design of UHPC beams.
This paper presents a data-driven machine learning (ML) framework for predicting failure mode and shear capacity of Ultra High Performance Concrete (UHPC) beams. To this end, a comprehensive database on 360 reported tests on UHPC beams with different geometric, fiber properties, loading and material characteristics was collected. This database was then analyzed utilizing different ML algorithms including, support vector machine (SVM), artificial neural networks (ANN), k-nearest neighbor (k-NN), and genetic programing (GP), to identify key parameters governing failure pattern and shear capacity of UHPC beams. The outcome of this analysis is a computational-based ML framework that is capable of identifying failure mode of UHPC beams and simplified expressions for predicting shear capacity of UHPC beams. Predictions obtained from the proposed framework was compared against the values obtained from design equations in codes, and also results from full-scale tests to show the reliability of the proposed approach. The results clearly infer that the proposed data-driven ML framework can effectively predict failure mode and shear capacity of prestressed and non-prestressed UHPC beams with varying reinforcement detailing and configurations.
This paper presents experimental data on the effect of carbon nano-fiber (CNF) dispersion on mechanical behavior, workability, and permeability of ultra-high-performance concrete containing CNF ...(UHPC-CNF). The addition of CNFs in concrete has been shown to provide improved performance. However, an inhomogeneous distribution of CNFs in the cement matrix can result in no impact or decreased mechanical performance. Due to the van der Waal's forces and hydrophobic surface properties of the CNFs, good dispersion in mix fluids and cement paste is challenging. Experimental studies of UHPC-CNF with different dispersion methods are provided. Further mixing with optimized UHPC paste allowed for the investigation of the optimal dosage of CNFs, impact on porosity, compressive strength, flexural strength, hydration rate, and slump. Scanning electron microscopy (SEM) analysis visually revealed the effect of dispersion. The results indicate that shear mixing and subsequent ultrasonic dispersion with chemical surfactants can provide a well-dispersion liquid admixture of CNFs resulting in high mechanical performance of UHPC-CNFs composites. The water permeability and chloride resistance of optimized UHPC-CNFs composites were further evaluated with the wicking test and ponding test to reveal improved performance for these properties as well.
•An optimized UHPC mix design is selected, which provided the highest compressive strength and the largest slump.•The good dispersion of CNFs in UHPC improved flexural behavior and porosity; and verified by SEM analysis.•A sensitivity analysis is provided to determine the optimal dispersion parameters, such as the dispersion energy delivered.•The impact of adding CNFs to UHPC are comprehensively studied from mechanical properties, workability, and permeability.
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