Reinforcement In Ultra-high Performance Concrete Research Articles

Effect of endogenous chloride ion on the corrosion behavior of reinforcement embedded in sea-sand UHPC matrix in different immersion environments

Ultra-high performance concrete (UHPC) is an innovative cement-based composite material characterized by extremely high durability and mechanical properties, which provides a potential possibility of using non-desalted sea-sand as aggregate. However, the long-term steel corrosion behavior in UHPC prepared by simulated sea-sand under different immersion environments remains unclear, which is of great significance for the application of sea-sand UHPC in practical engineering. In this study, the pore structure of sea-sand UHPC matrix was measured using nitrogen adsorption/desorption method. The corrosion behavior of rebar in sea-sand UHPC with different endogenous chloride ion contents soaking in pure water and NaCl solution was characterized by linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and cyclic polarization (CP). The experimental results show that the initial UHPC pore structure is refined by incorporation of sea-sand, and is similar to that of ordinary UHPC at the later hydration. Regardless of the endogenous chloride ion content and immersion environment, the corrosion rate evaluated by corrosion current density (icorr) and polarization resistance (Rp) of reinforcement in UHPC is extremely low. The measurement of EIS shows that UHPC group has higher matrix resistance (Rc) and charge transfer resistance (Rct), while lower electric double layer capacitance (CPEdl) of reinforcement, which indicates that the steel in UHPC made of sea-sand is still in a state of passivation. However, the steel bar in comparison specimen OPC soaking in NaCl solution is in the corrosion stage. Furthermore, sea-sand UHPC possesses of excellent ability of anti-pitting corrosion of reinforcement after 180 days of immersion in NaCl solution.

Fibre pullout behaviour of fibre-reinforced UHPC with TPE-coated fibres

The use of micro fibres as reinforcement in Ultra-High Performance Concrete (UHPC) enhances its mechanical properties, especially the flexural and tensile strengths and a ductile post-failure behaviour is achieved. To guarantee this positive influence, an optimal bonding between fibre and cementitious matrix is necessary. To achieve this, a coating of fibres with a thermoplastic elastomer is proposed in this study. Pullout tests with brass coated steel and stainless steel fibres with and without a TPE-coating (thermoplastic elastomer) were carried out as well as flexural strength tests of UHPC reinforced with the same fibres. For better comparability between pullout and flexural strength tests, a modified pullout setup, allowing inclined fibre pullout, was developed and used in this study. The coating increases the adhesive bond between fibre and UHPC matrix indicated by straight fibre pullout. However, pullout tests with inclined fibres reveal contrary results, displayed in the flexural bending tests.

Growth of C-S-H phases on different metallic surfaces.

The influence of reinforcement, especially fibre reinforcement in ultra-high performance concrete is strongly dependent on the bonding (adhesive, shear and friction bond) between metallic surface and cementitious matrix. As usually straight fibres are used for fibre reinforcement and, thus, no significant mechanical bonding is existent, the adhesive bond is particularly important. Previous studies stated that the adhesive bonding behaviour between metallic materials and cementitious matrix strongly depends on the chemical composition of metallic alloys. Therefore, in order to address this topic, the present study investigates the growth of C-S-H phases on stainless steel and on cold drawn steel. This growth process was realised by a surface treatment of the metallic alloys using a synthetically manufactured Tricalciumsilicate (C3 S) powder diluted in water. After defined times of the C3 S treatment the process is stopped to get a time dependent growing behaviour of the cementitious phases. Light microscopy as well as scanning electron microscopy was used in order to investigate the surfaces following the application of the C3 S. The results reveal that the growth of C-S-H phases is more dependent on the metallic surface and its topography than on the alloy composition.

Hybrid fibre reinforced ultra-high performance concrete beams under static and impact loads

High mechanical strength and ductility of fibre reinforced ultra-high performance concrete (UHPC) are beneficial to its loading capacity under static and dynamic loads. To further improve the material performance, especially the crack restraint capability and flexural capacity, hybrid fibre reinforcement in UHPC is investigated in the present study. This study first investigated the influence of hybrid straight steel fibres on the static mechanical performance of UHPC. Four mixtures with single and hybrid fibre reinforcement were developed. Fibres with three different lengths (6, 10, 15 mm) but identical diameter (0.2 mm) and volume dosage (2.5%) were incorporated. Quasi-static compression and flexural tests were conducted on these mixtures. The findings in this study indicated that while longer fibre provided an improvement of both compressive strength and elastic modulus, UHPC with hybrid fibres demonstrated better flexural performance. With notched three-point bending test results, tensile softening curves and fracture energy of UHPC were obtained and discussed. The flexural strength and fracture energy of mixture with hybrid long and short fibre was 21.84% and 20.13% greater than those with single long fibre reinforcement. Subsequently, the dynamic behaviour of UHPC beams (2000 × 168 × 168 mm) with a single and hybrid fibre reinforcement was investigated against low-velocity impact. The impact scenario was modelled with 641 kg drop hammer falling freely to the mid-span of the UHPC beams. The test results showed all specimens exhibited flexural response with minimal damage. The impact resistance of UHPC beams reinforced with hybrid fibres was found better than that with single long fibre reinforcement. The maximum and residual displacement of the mixture with hybrid long and medium fibre was 16.08% and 23.95% lower than that with long fibre reinforcement.

Effect of Elevated Temperature on the Bond Strength of Prestressing Reinforcement in UHPC.

The study explores the effect of elevated temperatures on the bond strength between prestressing reinforcement and ultra-high performance concrete (UHPC). Laboratory investigations reveal that the changes in bond strength correspond well with the changes in compressive strength of UHPC and their correlation can be mathematically described. Exposition of specimens to temperatures up to 200 °C does not reduce bond strength as a negative effect of increasing temperature is outweighed by the positive effect of thermal increase on the reactivity of silica fume in UHPC mixture. Above 200 °C, bond strength significantly reduces; for instance, a decrease by about 70% is observed at 800 °C. The decreases in compressive and bond strengths for temperatures above 400 °C are related to the changes of phase composition of UHPC matrix (as revealed by X-ray powder diffraction) and the changes in microstructure including the increase of porosity (verified by mercury intrusion porosimetry and observation of confocal microscopy) and development cracks detected by scanning electron microscopy. Future research should investigate the effect of relaxation of prestressing reinforcement with increasing temperature on bond strength reduction by numerical modelling.

Analysis of seismic performance of gravity dam based on ultra-high performance concrete reinforcement

Damage and crack of gravity dams under earthquakes directly affect the dam’s safety. Ultra-high performance concrete has high strength and durability. Whether to replace steel bars for seismic reinforcement becomes the focus of consideration. In the paper, the plastic damage model of ultra-high-performance concrete is established. Taking the gravity dam section as an example, referring to the design concept of the “gold coated silver” gravity dam, three different forms of seismic-reinforced schemes of ultra-high-performance concrete in weak areas of the dam are presented and analyze damage time histories. The calculation results show that the application of ultra-high performance concrete for reinforcement can greatly reduce the damage of the dam and improve the seismic performance of the dam. The study suggests that proper reinforcement measure can be taken according to the conditions of dams.

Functional microfibre reinforced ultra-high performance concrete (FMF-UHPC)

The present paper proposes a novel class of fibre reinforced concrete, realized by using shape memory alloy (SMA) fibres as reinforcement in ultra-high performance concrete (UHPC). SMAs can change their geometry imposed by a solid to solid phase transformation triggered by thermal activation or stress. This effect gives the possibility to use one geometry of fibres giving a minimum negative impact on the rheological properties and workability of concrete and a second geometry of the fibres for maximum positive impact on final mechanical properties. The present work highlights the influence of fibres with various shapes on the workability and the mechanical properties. The impact of SMA fibres in fresh and hardened UHPC compared to UHPC prepared with standard steel fibres is discussed. In light of the advances related to the unique properties of the SMA fibres, this novel concrete is referred to as “Functional Microfibre reinforced Ultra-High Performance Concrete” (FMF-UHPC).

New Energy Dissipation Mechanisms for Steel Fiber Reinforcement in Ultra High-Performance Concrete

Abstract This research focused on improving ultra high-performance concrete (UHPC) toughness through the addition of annealed plain carbon steel fibers and stainless steel fibers that both exhibit increased ductility and strain hardening compared with conventional steel fibers used for concrete reinforcement. Implementing optimized heat treatments and selecting proper alternative alloys can improve the postyield carrying capacity of UHPCs through plastic deformations, phase transformations, and fiber pullout. This research focused on the flexural response and dynamic penetration resistance of a UHPC known as Cor-Tuf with three different steel fiber types: plain carbon steel, stainless steel that can exhibit phase transformation-induced plasticity, and annealed carbon steel with reduced tensile strength but increased ductility and strain hardening. Annealed carbon steel fibers were able to reduce mass loss by 0.8 % for 5-cm-thick dynamic impact panels. By using a phase-transformable stainless steel, the ultimate flexural strength increased from 32.0 to 42.5 MPa (33 % increase) and the postimpact velocity decreased an average of 31.5 m/s for 2.5 and 5-cm-thick dynamic impact panels. Phase transformations (austenitic to martensitic) were quantified in the stainless steel fibers of postyielded UHPC specimens using a vibrating sample magnetometer. Stainless steel fibers sampled from the postyielded tensile face of a flexural beam increased from 75 to 107 emu/g. The results of the study evidence improvements in tensile properties and toughness that can be accomplished by modifying the stress versus strain response of steel fiber reinforcement and including new energy dissipation mechanisms such as phase transformation.

Transverse bending behavior of the steel-UHPC lightweight composite deck: Orthogonal test and analysis

The lightweight composite deck (LWCD) is a new countermeasure to address issues of fatigue cracking in orthotropic steel decks (OSDs) and premature damage in asphalt overlays, which is composed of an OSD and a thin ultra-high performance concrete (UHPC) layer. The objective of this paper is to reveal the transversal bending behavior of the LWCD under negative bending moments. An orthogonal experimental test was accomplished on 40 steel-UHPC composite specimens. The specimens covered four parameters, i.e., the stud spacing, the UHPC thickness, the cover thickness, and the number of steel reinforcement in UHPC. The responses of the specimens were analyzed and discussed from different aspects, including the failure mode, the load-deflection curve, the strain distribution, the cracking status, and the interfacial slip. To facilitate the design of the LWCD, the influence of the four parameters on the average crack spacing, the cracking stress in UHPC and the ultimate load was discussed. In addition, equations for calculating the ultimate load of the composite specimens were proposed and were validated through the test results.

Experimental investigation of bond behavior of mild steel reinforcement in UHPC

Ultra-High Performance Concrete (UHPC) is a relatively new class of advanced cementitious composite material, which exhibits high compressive and tensile strengths. The discrete steel fiber reinforcement included in UHPC allows the material to maintain a tensile capacity beyond cracking of the cementitious matrix. The bond behavior of deformed reinforcing bar in UHPC is investigated in this study by conducting pull out tests and beam tests with lap splices. The rebar used for these tests are #4 (M13), #5 (M16) A615 grade 60 bars and # 6(M19), #7 (M22) A615 grade 80 bars. The embedment length and side cover for the pull out tests were varied from 6db to 8db (db = diameter of the rebar) and 1db to 3.5db, respectively. The beam tests were conducted under four-point loading and the clear cover for these tests was varied from 1db to 3db. The rebar for all beam specimens were spliced at mid-span for a length of 8db. The beam specimens were instrumented to capture the strains at different points along the splice length. 16 pull out and 12 beam tests were completed and the effect of embedment length, concrete cover, bar size and bar type on the bond strength were investigated. This paper presents the details of this experimental investigation and adds significant new data on the bond stress distribution along the development length. It was found that the development length of embedded reinforcement in UHPC can be significantly reduced when compared to normal concrete.