Reinforced Ultra High Performance Concrete Research Articles

Slenderness limit for reinforced ultra-high-performance concrete columns

The use of ultra-high-performance concrete (UHPC) allows for much smaller cross-sections compared to conventional reinforced concrete columns, which may make reinforced UHPC (R-UHPC) columns more susceptible to slenderness effects. Currently, there is no guideline in design standards for the slenderness limit of R-UHPC columns. This paper, therefore, attempts to develop a design provision for determining the slenderness limit of R-UHPC columns. Firstly, a numerical analytical model was proposed for predicting the load-deflection of R-UHPC columns under eccentric loading, which was validated by comparing its predictions with available experimental results from the available literature. Based on the validated model, a parametric study was then conducted to determine the key parameters affecting the slenderness limit of R-UHPC columns. It was found that the slenderness limit corresponding to the 5% strength reduction was sensitive to the ultimate compressive strain of UHPC, the tensile strength of UHPC, and the reinforcement ratio. On this basis, a design equation for the slenderness limit of R-UHPC columns in single curvature was statistically derived. Additionally, the slenderness limit for R-UHPC columns in non-sway frames was also proposed in a convenient form for design procedures.

Numerical investigation and design of UHPC-encased CFST stub columns under axial compression

Reinforced ultra-high-performance concrete (RUHPC)-encased concrete-filled steel tubular (CFST) columns in composite construction have exceptional fire and corrosion resistance, superior load-bearing capacity, and enhanced ductility compared to standalone RUHPC and CFST columns. However, research on their behavior is limited. This paper presents a 3D finite element (FE) simulation model for accurately predicting the behavior of axially loaded RUHPC-encased CFST short columns. The model accounts for the effects of the concrete confinement provided by the steel reinforcement and circular steel tube. The existing experimental data is used to validate the FE model. The validated model is then utilized to investigate the performance of RUHPC-encased CFST columns with various geometric and material properties. The range analysis technique is applied to an orthogonal design to assess the relative significance of the factors affecting the structural behavior. The findings reveal that RUHPC-encased CFST columns have high ultimate strength and ductility. The range analysis identifies the importance order of parameters as: the compressive strength of UHPC, the steel fiber concentration, tube diameter, stirrup spacing, tube thickness, and confined concrete’s compressive strength. Lastly, a simplified design model is developed to estimate the compressive capacity of RUHPC-encased CFST stub columns, providing accurate results that are verified by tests and numerical analysis.

Experimental study of interfacial bonding performance between section steel and ultra high-performance concrete

The key to fully exploiting the benefits of section steel and ultra high-performance concrete (UHPC) is to improve their bonding. The primary goal of this study was to investigate the interfacial bonding performance between these two materials. Section steel - reinforced ultra high-performance concrete (SSRUHPC) and section steel-reinforced concrete (SSRC) specimens were subjected to push-out tests. Failure modes, load-slip (PS) curves, and strain distributions were studied, and the experimental results showed that the PS curves of all specimens have a similar trend: a no-slip stage followed by a slip stage, a failure stage, and a residual stage. By increasing the cover thickness and stirrup ratio and lowering the embedded length, the bond strength between the section steel and UHPC may markedly increase. The results of this study show that the bonding between section steel and UHPC is superior to that between section steel and ordinary concrete. Finally, after carefully considering the experimental results, regression formulae for the SSRUHPC bond strength were developed, and the computed results were found to be accurate. Unlike other bond strength formulae, the formulae in this study consider the influence of the stirrup ratio on the bond strength, and the R2 values range from 0.763 to 0.961.

Ductility evaluation and flexural failure mode recognition of reinforced Ultra-High performance concrete flexural members

Although ultra-high performance concrete (UHPC) exhibits material tensile ductility far in excess of that expected from conventional or high-strength concretes, structural UHPC members typically fail in flexure due to reinforcement rupture after crack localization, which may result in a low structural ductility. In the flexural design of reinforced UHPC (R-UHPC) members, attention should be paid to addressing the issue related to the low structural ductility due to crack localization. This study firstly develops a two-dimensional numerical model to simulate the flexural response of R-UHPC beams, and the numerical results agree well with the experimental results of the load–deflection curves as well as the failure modes. Parametric analyses are then carried out to examine the effect of the flexural reinforcement ratio, tensile strength of steel reinforcement, tensile behavior of UHPC and compressive strength of UHPC on the flexural failure mode and ductility behavior of R-UHPC members. The results indicate that when the failure is controlled by fracture of tensile steel bars after crack localization, higher reinforcing ratio, higher steel post-yield hardening strength, and lower steel fiber volume lead to more ductile behavior. When the failure is controlled by crushing of UHPC, however, higher reinforcing ratio and lower UHPC compressive strength lead to a lower ductility. In addition, a flexural failure mode recognition method is proposed for R-UHPC flexural members, whose accuracy is verified based on the available experimental results of 105 R-UHPC flexural members. Finally, a minimum flexural reinforcement ratio is proposed to avoid low structural ductility in R-UHPC flexural members.

Flexural behavior of damaged RC beams strengthened with prestressed UHPC layer

Based on ultra-high-performance concrete (UHPC) and prestressed technique, a new flexural strengthening technology using a prestressed UHPC (PUHPC) layer for the damaged reinforced concrete (RC) structure was proposed in this study. To investigate the flexural strengthening efficiency of PUHPC layer, the bending tests of six beams were carried out in this study, including four PUHPC strengthened RC beams with different pre-damaged degrees (PUB), one reinforced UHPC (RUHPC) strengthened control beam (RUB) and one unstrengthened RC control beam (CB). The experimental results showed that the PUB was typical bending failure. The cracking load and ultimate load of PUB increased by 145% to 190% and 90% to 94% compared with that of CB respectively. Similarly, 98% and 38% of improvement of the characteristic loads comparing with RUB were observed. Moreover, the post cracking stiffness of PUB were significantly improved compared with CB and RUB, while the ductility indexes were decreased. However, with the increase of the pre-damaged degree of RC beam, the cracking resistance, flexural stiffness and ductility of PUB decreased correspondingly. It is worth mentioning that the interface between PUHPC layer and RC beam has a high bond strength, and the slight slip at the interface did not lead to debonding failure during the bending test. Also, the calculation model for predicting the ultimate flexural capacity of PUB was developed, which yielded promising predictions with reasonable accuracy.

Cracking and yield behavior of reinforced UHPC beams containing steel slag under flexural test

In this paper, the flexural performance of reinforced ultra-high performance concrete (R-UHPC) beams containing steel slag is investigated and the effect of various variables on cracking and yield load are analyzed. First, the cracking, deformation and strain behavior of steel slag R-UHPC beams were investigated by flexural tests, and the role of steel slag powder was analyzed. Then, a characteristic load identification method was proposed and a database of characteristic bending moments was established. The random forest model was used for feature selection to extract the critical variables affecting the characteristic bending moments. Finally, the prediction method of characteristic bending moments was established by cross-section analysis, and the influence of each factor on characteristic bending moments was analyzed. The results show that the addition of steel slag powder decreases the mechanical properties of UHPC, which in turn decreases the flexural behavior of the beam, but the effect on the ultimate load is limited. At the same time, the addition of steel slag powder significantly reduces energy consumption, so the application of steel slag UHPC in beams is feasible. Compared to other variables, the tensile strength of UHPC has a greater effect on the cracking bending moment and the reinforcement ratio has a greater effect on the yield bending moment of the beam.

TDOF model for UHPC members under lateral low-velocity impact

The dynamic behaviors of ultra-high performance concrete (UHPC) members under lateral low-velocity impact loading have been intensively studied by experiment and numerical simulation, while the corresponding structural design approach is still limited and mainly concerned in this study. Firstly, through comprehensively considering the compression arch action, strain rate effect, and unloading stiffness, the dynamic resistance function of UHPC members, e.g., reinforced UHPC (R-UHPC) and UHPC-filled steel tube (UHPC-FST) beams/columns, are built and verified. Then, a two degrees of freedom (TDOF) model of the drop hammer test on UHPC members is improved by considering the P-δ effect and stress wave propagation effect. Furthermore, the impact force- and mid-span deflection-time histories of the existing five series of drop hammer tests on UHPC members are applied to verify the validity and applicability of the established TDOF model. The influence of considering the stress wave propagation effect, strain rate effect, and steel fibers in UHPC for predictions of the TDOF model are discussed by parametric analyses. Finally, based on the energy balance, the explicit formulae are proposed to predict the maximum and residual mid-span deflections of the flexural critical UHPC members under the lateral low-velocity impact, which can be helpful for the design of UHPC structures under lateral impact loadings.

Shear performances of reinforced ultra-high performance concrete short beams

This study aims to understand the shear performance of reinforced ultra-high performance concrete (R-UHPC) short beams and to propose a calculation method for their shear capacity. A total of 11 R-UHPC short beam specimens were tested to investigate the effects of key parameters on the shear capacity, including steel fiber volume fraction, longitudinal reinforcement ratio, stirrup ratio, and shear span-to-depth ratio. Experimental results indicated that the shear capacity increased with the increase of steel fiber volume fraction, longitudinal reinforcement ratio, and stirrup ratio, while it decreased with the increase of shear span-to-depth ratio. Results also showed that for R-UHPC beams, the effect of shear span-to-depth ratio can only be ignored for a ratio less than 1.2 rather than 1.5 that is required for reinforced concrete (RC) beams by the Chinese design code. Moreover, it was found that the calculation methods for the shear capacity of R-UHPC beams in the current codes underestimated the shear capacity of the specimens by 30 % ∼ 50 %, as these methods did not consider the aforementioned significant parameters. A calculation method was proposed based on the modified compression field theory (MCFT) to comprehensively consider all the aforementioned parameters. The proposed method exhibited a higher accuracy and less dispersion as compared to the methods specified by current codes.

Mechanical Properties of Polyethylene Fiber Reinforced Ultra High Performance Concrete (UHPC).

Ultra-high performance concrete (UHPC) is a kind of cement-based material with ultra-high strength, high toughness and excellent durability. However, the tensile strain capacity of UHPC is often below 0.5%, and the mode of single crack failure is the main failure pattern, which limits the development of UHPC. In order to overcome the weakness of the relatively low strain capacity of UHPC, five types of polyethylene (PE) fibers with different geometrical and mechanical parameters (length, diameter and elastic modulus) were added into the matrix, and the corresponding mechanical behavior was investigated. The experimental results showed that the high fiber length and fiber diameter of PE fibers are a benefit for the compressive strength and tensile strength of UHPC. The increase of the fiber diameter and elastic modulus remarkably attributed to the increase in the tensile strain capacity of UHPC. With the increase of the fiber diameter and elastic modulus, the overall energy absorption capacity G and the energy absorption capacity of the substrate prior to the softening section ga of UHPC were both enhanced. The diameter of PE fiber was the main factor affecting the energy consumption of UHPC. Among the five types of PE fiber, PF fiber (PF fiber is PF type polyethylene fiber; Fiber length: 15 mm; Fiber diameter: 27 μm; Elastic Modulus: 117 GPa) is the optimal fiber to increase the tensile mechanical behavior of UHPC.

Synergistic tensile response of reinforced ultra high performance concrete with low fiber contents

From the perspectives of ductility, economic and construction, reinforced ultra high performance concrete (UHPC) structures with low fiber contents have a significant potential in applications. This paper studied the tensile behaviors and damage mechanisms of reinforced UHPC with low fiber contents based on the direct tensile tests accompanied with acoustic emission (AE) detection system. The test results show that the high strength steel bar can prolong the tensile multiple-cracking stage of reinforced UHPC with low fiber contents in some extent at the serviceability stage and further improve its tensile ductility via inducing additional longitudinal cracks at the failure stage. It is found that the low fiber contents and steel bar grades have marginal effect on the tension stiffening at the first cracking of reinforced UHPC. Non-negligible tension stiffening effect still remains at yielding of steel bar and it is more obvious for normal strength steel bar and UHPC with a higher fiber content. The AE weighted peak frequency can identify the tensile damage model of reinforced UHPC with low fiber contents: no internal damage occurs during stage Ⅰ; matrix cracking occurs accompanied by marginal fiber pull-out during stage Ⅱ; matrix cracking and fiber pull-out are dominated during stage III and stage IV, respectively.

Experimental study on flexural fatigue behavior of composite T-beams in ultra-high performance concrete reinforced and normal-strength concrete

The purpose of this study is to investigate the flexural fatigue properties of reinforced ultra-high performance concrete (RUHPC) T-beams, and composite T-beams of normal-strength concrete-RUHPC (NC-RUHPC) and damaged reinforced concrete (RC) strengthened with RUHPC (RC-RUHPC) under constant-amplitude low-cycle fatigue tests. Firstly, the loading value in the fatigue test was obtained by static load tests. Then, the relationship between fatigue deflection, bending stiffness, fatigue crack width, fatigue crack width growth rate, and fatigue life of RUHPC, NC-RUHPC, and RC-RUHPC T-beams were studied with the fatigue tests. The test results show that one main crack is only formed near the midspan in the later stage of fatigue loading, other bending cracks are micro-cracks, and their propagation is slow and short. The fatigue fracture of steel bars embedded in tested beams occurs after more than 3.7 million cycles. When the steel bars are undergoing fatigue fracture, their stress is significantly lower than the yield strength, and two kinds of fracture surfaces are produced: intergranular fracture and transgranular fracture; the mechanical properties of steel bars are changed in the strength index and plasticity index after fatigue fracture. The cost of UHPC and other materials can be reduced by using NC-RUHPC or RC-RUHPC T-beams in practice.

Flexural cracking behavior of reinforced UHPC overlay in composite bridge deck with orthotropic steel deck under static and fatigue loads

Ultra-high performance concrete (UHPC) has been used to enhance the fatigue resistance of orthotropic steel decks, but the enhancement effect is compromised by UHPC cracks in negative moment regions. This research investigates the flexural cracking behavior of reinforced UHPC (R-UHPC) overlay in composite bridge deck under static and fatigue loads. The investigated behavior included the load-carrying capacity of composite deck, and the cracking process, pattern and resistance of the R-UHPC overlay. Ten composite specimens were tested, including five slabs for the transverse behavior and five beams for the longitudinal behavior. Static tests investigated the load-carrying capacity, strain distribution and nominal cracking strength. Fatigue tests investigated the cracking process and fatigue resistance of R-UHPC overlay. The fatigue process included an internal force redistribution stage (less than10% of fatigue life) and a stable stage (greater than90% of fatigue life). The fatigue resistance of R-UHPC overlay was evaluated with a modified Goodman diagram.

A modified space truss analogy model for ultimate torsional capacity of ultra-high-performance concrete solid and box beams

This paper presents an analytical model using a modified space truss analogy for predicting the ultimate torsional capacity of reinforced ultra-high-performance concrete (R-UHPC) beams. Both solid and box beams are considered. The credibility of the proposed model is evaluated by comparing the theoretical predictions of the ultimate torsional capacity and its corresponding twist ( Tu- αu) to experimental results of several R-UHPC beams (solid and box beams) available in the literature. The results are also compared with the calculation of the existing models. The comparisons show that the proposed model gives acceptable predictions of the ultimate limit torsional capacity of R-UHPC beams and the corresponding twist angle and offers better accuracy than the existing analytical models. Additionally, a parametric study was carried out to determine factors that influence the ultimate torsional capacity. The result shows that the ultimate torsional capacity is not just improved by the increase of the tensile strength of UHPC but also increased by the increase of the reinforcements (transversal and longitudinal).

Experimental validation of damaged reinforced concrete beam strengthened by pretensioned prestressed ultra-high-performance concrete layer

Strengthening reinforced concrete (RC) beams with ultra-high-performance concrete (UHPC) can improve mechanical properties and durability from previous studies. To further improve UHPC strengthening efficiency, a new technology using a pretensioned prestressed UHPC (P-UHPC) layer for flexural strengthening was proposed in this study. The flexural capacity and crack resistance of the damaged RC beams strengthened by the P-UHPC layer were investigated through a four-point bending test in order for the results to be used in the analysis and design of strengthening damaged structures. The flexural performance of two P-UHPC strengthened beams (PU), one reinforced UHPC (R-UHPC) layer strengthened beam (RU), and one unstrengthened RC control beam (CB) was tested. This study also determined the influence of UHPC shrinkage and prestressing force on the cracking load and ultimate flexural capacity of the P-UHPC strengthened beams by proposing a formal theory. Results showed that the failure modes of the PU beams were typically bending failure. The PU beams showed good composite performance, and there was no debonding failure at the UHPC-RC interface during the test. Moreover, the flexural performance of the damaged RC beams strengthened with the P-UHPC layer was significantly improved, with the average cracking load of the PU beams higher than that of the CB and RU beams by 87.4% and 28.1%, respectively. The average ultimate flexural capacity was 85.2% and 31.7% higher than the CB and RU beams, respectively. In addition, the P-UHPC layer had a more substantial inhibitory effect on the crack development of the damaged RC beams, and the re-expansion load of the cracks in the damaged RC of the PU beams was 111.5 % higher than the RU beam. Finally, the calculation model and theoretical formula for predicting the cracking load and ultimate flexural capacity of the P-UHPC strengthened beams were proposed, and the test results verified the applicability of the proposed calculation methods.

Tension-Stiffening Effect Consideration for Modeling Deflection of Cracked Reinforced UHPC Beams

Tension-stiffening effects can significantly influence the flexural performance of cracked reinforced concrete specimens. Such effect is amplified for fiber-reinforced concrete, given the fact that fibers can bridge the cracks. The objective of this study was to develop a model to predict the deflection of cracked reinforced ultra-high performance concrete (R-UHPC) beam elements. The modeling approach characterized the average bending moment of inertia by combining the existing model used for conventional reinforced concrete and the analytical model of stress distribution of UHPC along the cross-section. The finite element analysis (FEA) was employed to evaluate the flexural deflection based on the average bending moment of inertia. The calculated load-deflection relationships have been compared to experimental results. The results indicated that the relative errors of deflection between predicted and experimental results can be controlled within 15%, compared to values ranging from 5% to 50% calculated by neglecting the tensile properties of cracked UHPC and values ranging from 5% to 30% calculated by effective inertia of bending moment of ACI code. Therefore, the developed model can be used in practice because it can secure the accuracy of deflection prediction of the R-UHPC beams. Such a simplified model also has higher sustainability compared to FEA using solid elements since it is easier and time-saving to be established and calculated.

Coupled Experimental and Computational Investigation of the Interplay between Discrete and Continuous Reinforcement in Ultrahigh Performance Concrete Beams. II: Mesoscale Modeling

The first experimental campaign presented in the preceding Part I of this study is used to calibrate and validate a comprehensive computational framework called the lattice discrete particle model for fiber-reinforced concrete (LDPM-F). The model is then used to design the second experimental campaign that was also presented in the preceding Part I so that all beams fail in shear. Finally, the model is used to investigate and explain the observed failure modes, validate the fiber/reinforcement interplay effects postulated in Part I, and to analyze comprehensively the load-transfer mechanisms in the reinforced ultra-high performance concrete (R-UHPC) beams in both shear and flexural failure. This two-part study proves the effectiveness of coupling experimental analysis with comprehensive computational modeling to understand the behavior of structural members made from complex materials. Using this coupled understanding, detailed explanations of load-transfer mechanisms in shallow and deep beam shear failure as well as flexural failure are discussed and compared to simplified sectional analysis models showing the places of needed improvement in such models. These detailed discussions show the ability of the presented coupled approach to accurately predict these failure mechanisms and their dependence on fiber/reinforcement contents and their interplay. The presented accurate probing of different load-transfer mechanisms within the structural elements and how they vary during failure progression paves the road towards developing rigorous design formulations based on fundamental understanding of the complex mechanical behavior of these structural members.

Experimental study on circular steel tube-confined reinforced UHPC columns under axial loading

This paper investigates the compressive behavior of circular steel tube-confined reinforced ultrahigh-performance concrete (UHPC) columns. An experiment was conducted consisting of axial loading applied to 18 stub columns (λ = 4L/D = 13), including steel tube-confined reinforced UHPC (STCRU) columns, steel tube-confined UHPC (STCU) columns, UHPC-filled steel tube (UFST) columns, steel tube-confined reinforced concrete (STCRC) columns, and reinforced UHPC (RU) columns. The compressive behavior was also investigated by varying the addition of steel fibers. The failure mode of all the columns was shear failure except for the explosive failure of the RU columns without steel fibers and the drum-type failure of the STCRC columns. Interestingly, the addition of steel fibers greatly improved the strength and ductility of the RU columns. Finally, a reasonable formula for predicting the bearing capacity of STCRU columns was provided to overcome the application limits of existing specifications, and the formula can describe the axial bearing capacity of stub STCRU columns better than other approaches.

Experimental Investigation on Flexural Behavior of Reinforced Ultra High Performance Concrete Low-Profile T-Beams

The flexural behavioral properties of ultra high performance concrete (UHPC) low-profile T-beams reinforced with a combination of steel fibers and steel reinforcing bars were investigated in this paper. Five large scale T-beams were tested and analyzed regarding their deflection, ductility, strain, curvature, load capacity and crack development. The experimental variables include the reinforcement ratio, the slenderness (length to diameter ratio) of the fiber reinforcements, and the fiber type. The experiments showed that all specimens exhibit flexural failure with the yielding of steel bars and excessive expansion of flexural crack, and the compression zone in the reinforced UHPC low-profile T-beam is not crushed because of the ultra high compressive strength and area of UHPC. In addition, it was concluded that using hooked-end fibers can effectively increase the specimen’s durability-based cracking load in comparison to straight fibers of same slenderness, whereas the reinforcement ratio and the slenderness of the fibers have little influence on this. Increasing the reinforcement ratio and using hooked-end instead of straight fibers increase the load capacity and bending stiffness of the specimen, as well as reduces the crack width at comparable applied load. A model was established to compute the ultimate capacity of UHPC low-profile T-beams and the prediction agrees well with the experimental results in the present and published investigations.

Mechanical and damage mechanisms of reinforced ultra high performance concrete under tensile loading

Understanding the tensile mechanism of reinforced ultra high performance concrete (RUHPC) is important for structural design and application of ultra high performance concrete (UHPC). In this study, the direct tensile test accompanied with crack width detection and acoustic emission (AE) source locating were conducted on RUHPC specimens with the longitudinal reinforcement ratios of 0%, 2.3% and 4.6%. Two types of RUHPCs were studied: high strain hardening RUHPC (HSH-RUHPC) and strain softening RUHPC (SS-RUHPC). Experimental results included the tensile stress (load)-strain curves, the crack width-strain curves and the AE source distribution maps under tensile loading. The tensile mechanisms were elaborated from two aspects. From the view of mechanical mechanism, during Stage I (the elastic stage) and Stage II (the elastic-plastic stage): (1) HSH-RUHPC had a globally uniform stress distribution while the steel rebar in SS-RUHPC changed from total elastic state to partial yielding and finally to partial strain hardening; (2) the reduction of the tensile strength of UHPC was observed in RUHPC, which was more obvious with the higher reinforcement ratio. From the view of damage evolution mechanism based on AE analysis method at the micro level: (1) HSH-RUHPC exhibited a homogeneous damage distribution owing to its multiple-micro-cracking mode before steel yielding; (2) SS-RUHPC showed several damage concentrations at crack positions after UHPC softening due to its several-macro-cracking mode. Meanwhile, Gini index was proved to be an effective parameter to evaluate the distribution nonuniformity of the internal damages of RUHPC.

Introduction to state‑of‑the‑art thin-walled GFRC

Thin-walled fibre reinforced concrete (FRC) elements are being adapted for large scale buildings with complex geometry envelopes. The current production methods, developed in the initial stages of glass fibre reinforced concrete (GFRC) elements in the 1970s, are limited when striving to produce more complex shaped FRC elements. The limitations of the FRC elements in terms of material properties and surface quality are described for these current state of the art production methods. New production methods and casting techniques are proposed that will advance the application of thin-walled FRC for buildings with complex geometry envelopes. Evaluation of the current state of the art production methods concluded that the sprayed GFRC methods are currently the most flexible solution which has the greatest potential for adapting the method to the requirements of complex geometry buildings. Further development of thin-walled GFRC elements would be possible by developing a mould system for complex geometry panels with an edge-return, which can utilise glass fibre reinforced ultra high performance concrete (GF-UHPC) with a vacuum technology, it would be possible to produce complex geometry GFRC elements with an increased material performance and yet still meet the aesthetic requirements of minimal visual defects in the surface of thin-walled elements.