Ultra-high Performance Concrete Matrix Research Articles

Multi-scale characterization of lightweight aggregate and superabsorbent polymers influence on autogenous shrinkage and microstructure of ultra-high performance concrete

This study investigates the effects of internal curing agents on the autogenous shrinkage, hydration, and microstructure of ultra-high-performance concrete (UHPC). Lightweight aggregate (LWA) and superabsorbent polymers (SAP) were examined across various dosages as representative internal curing agents. Measurements were taken for the mechanical properties, autogenous shrinkage, and internal relative humidity of the UHPC. The mechanisms of internal curing were analyzed using thermogravimetry, pore structure, and microstructural evaluations. Additionally, porosity and microhardness in the matrix around the fibers and internal curing agents were tested to quantitatively assess their impact on the properties of the UHPC matrix. Results indicate that high dosages of LWA or SAP negatively affect the compressive strength of UHPC. A dosage of 15 % LWA increased flexural strength by 11 %, whereas SAP showed no significant improvement in flexural strength. LWA effectively reduced autogenous shrinkage by 49.4 %-88.8 %, while a SAP dosage of 0.3 % reduced autogenous shrinkage by 82 %. Both LWA and SAP increased porosity by 33.9 %-55.9 %. SAP released water within the matrix, forming voids filled with calcium hydroxide. The interface between LWA and the matrix was dense, with porosity near the LWA and SAP interfaces being 23.5 %-53.9 % and 26.0 %-38.2 % lower, respectively, compared to other areas. The microhardness in the LWA and SAP interface regions was 20 % and 16.2 % higher than in different places. The LWA pre-saturated method can effectively reduce autogenous shrinkage, and 15 % LWA has no adverse effect on mechanical strength.

A Study on the Mechanism by Which Graphene Oxide Affects the Macroscopic Properties and Microstructure of Abrasion-Resistant Ultra-High-Performance Concrete (UHPC)

To further enhance the abrasion resistance of UHPC in demanding abrasion environments, this study investigated the effects of graphene oxide (GO) on the workability, mechanical properties, and abrasion resistance of UHPC. Utilizing 27Al Nuclear Magnetic Resonance (NMR), 29Si NMR, microhardness, and BET analysis, the study analyzed the mechanisms through which GO influences UHPC’s microstructure in terms of abrasion resistance. Additionally, molecular dynamics simulations were employed to examine the mechanisms by which GO enhances UHPC’s abrasion resistance at the nano and micron scale. The findings show that an optimal amount of GO can improve the mechanical properties and abrasion resistance of UHPC. When 0.03% of GO (by cementitious material mass) was incorporated, the impact on workability was minimal, yet compressive strength increased by approximately 1.80%, flexural strength by 3.02%, impact wear resistance by 1.78%, the abrasion loss rate decreased by 10.01%, ultimate impact energy increased by 1.76%, and the toughness index improved by 10.10%. GO enhances abrasion-resistant UHPC primarily by increasing hydration, refining pore structure, and improving the microstructure of the interfacial transition zone. While GO increases the hydration degree of the UHPC matrix, it does not alter the silicate chain in C-A-S-H gels within the paste. Additionally, the incorporation of graphene oxide can refine the pore structure of the UHPC cement paste and improve the microstructure of the interfacial transition zone (ITZ) between the aggregate and the cement paste. The molecular dynamics simulation reveals that, under abrasive forces, GO forms strong, stable chemical bonds with the C-A-S-H base atoms, significantly enhancing the abrasion resistance of C-A-S-H.

Spalling behavior in ultra-high performance concrete: multi-technique insights and multi-scale fiber-rubber mitigation strategies

Explosive spalling of ultra-high performance concrete (UHPC) at elevated temperatures is one of the main problems limiting its application. While the intrinsic mechanism governing the spalling is determined by the UHPC matrix. Therefore, in this paper, multi-technique characterization approaches, i.e., acoustic signal characteristics, fractal characteristics of spalling fragments, and thermal cracks are employed to quantitatively characterize the spalling of the UHPC matrix. The effects of moisture content, heating rate, geometric characteristics of specimens, mineral admixtures, crumb rubber content, and fibers on concrete spalling are investigated to reveal the explosive spalling mechanism, reduce the risk of explosive spalling, and develop a spalling-resistant material. The waveform, cumulative energy, and spectrum of the spalling acoustic signal are used to assess the intensity of concrete spalling. The results show that the UHPC matrix with high moisture content, high heating rate, and large specimen size undergoes violent spalling events characterized by larger amplitude and smaller frequencies of acoustic signals. Incorporating crumb rubber increases the spalling events in the frequency range of 8000–10000 Hz, which plays a positive role in alleviating spalling. Meanwhile, the fractal characteristics of spalling fragments and thermal cracks can also quantitatively reflect the intensity of concrete spalling (i.e., the larger the fractal dimension, the more severe spalling). By minimizing the moisture content and heating rate, as well as increasing rubber content, the spalling fragments can be larger and the fractal dimension can be reduced. It is also found that saturated specimens spall with significant cumulative energy. While the specimens with a large size and low moisture content spall with lower cumulative energy and an increase in the proportion of spalling fragments in the small particle size range. A higher heating rate and larger specimens can induce a higher thermal stress, which can be more likely to spall explosively. While a lower moisture content can result in explosive spalling at a higher heating rate. Hence, it is believed that thermal stress can be the dominant factor affecting explosive spalling and the moisture content can be an accelerator. The addition of multi-scale fibers completely prevents explosive spalling by releasing vapor pressure from the channels created by fiber melting and improving the tensile strength of concrete, which also effectively restricts the thermal cracking of UHPC due to their hybrid bridging effects.

Influence of the graphene oxide-coated steel fiber on the microstructure optimization of UHPC

Benefiting from its superior mechanical properties, large specific surface area and abundant oxygen-containing functional groups, graphene oxide (GO) can generate nucleation and pore-infilling effects to optimize the microstructure of the cementitious composites, thus reinforcing the mechanical performance and durability of cement-based materials. However, due to the dispersion issues and relatively high cost, the widespread use of GO in the cement industry has not yet been realized. In the present study, we applied three different methods to coat industrial GO nanosheets on the steel fiber’s surface and reinforce the UHPC microstructure. The proposed coating method effectively disperses GO within the ultra-high-performance concrete (UHPC) matrix and targets its enhancement effects on the interfacial transition zone (ITZ), which has the weakest pore structure in UHPC. The result presents that the three-step dip method of GO coating on the steel fiber demonstrates higher effectiveness in reinforcing the microstructure of UHPC, the porosity of the matrix is 1.88 %, with a relative decrease of 37.1 ∼ 60.3 %. In addition, the average distance of ITZ after GO enhanced UHPC is about 3.7 µm, about 37.3 %-68.1 % shorter, and the ITZ porosity is 3.8 %, which is 15.6 %-65.5 % lower. With the assistance of the metal intrusion technique, backscattered electron characterization and deep learning-based analysis, the pore structure features of UHPC were extracted with a classification recognition accuracy of 88.5 %, and the pore intrusion volume inside the characteristic region of the UHPC matrix increased by 1.89–2.14 times. The extraction characteristics illustrated that the GO strengthening on steel fibers was more effective in ITZ, with an optimization efficiency of about 8.1 % higher than in other regions. We expect that the findings of this study could provide deep insights into GO modification for cementitious composite reinforcement applications and open new pathways for affordable alternatives to nano-enhanced composites.

Mechanical properties of steel fibre–UHPC interface: Experimental study and numerical simulation

The aim of this study was to investigate the interfacial mechanical attributes between steel fibre (SF) and an ultra-high-performance concrete (UHPC) matrix. Using both experimental testing and numerical simulation, a comprehensive analysis on the pull-out process of SF from the UHPC matrix was conducted. The effects of fibre embedment depth, fibre diameter and fibre embedment angle on the mechanical properties of the SF–UHPC interface were examined. In the fibre pull-out tests, the maximum pull-out force of the group of specimens with different embedment depths showed a trend of rapid rise and then slow development, and the pull-out work increased with an increase in embedment depth. The maximum pull-out force and pull-out work of the group with variable fibre diameters increased with an increase in fibre diameter. The maximum pull-out force and pull-out work of the group with different embedment angles increased first and then decreased with an increase in embedment angle. The maximum pull-out force was 72.5 N when the embedment angle was 45° and the maximum pull-out work was 214.4 N.mm when the embedment angle was 30°. The results of finite-element simulations were found to be in good agreement with the experimental results.

Influence of further hydration on UHPC matrix properties

To investigate the evolution laws governing the macroscopic properties of ultra-high-performance concrete (UHPC) matrices under further hydration effect, a further hydration test was performed by varying the cement fineness and slag powder content. The influence mechanism of further hydration on strength was analyzed. The results revealed that the bound water content for further hydration and expansion strain for UHPC matrix with medium fineness cement achieved the minimum values, and the compressive strength reached the lowest value after 21 d of further hydration. Both the bound water content for further hydration and expansion strain showed a tendency to increase with increasing slag powder content, and the higher the slag powder content, the earlier was the peak strength achieved. The volume expansion of further hydration products caused the matrix to crack, and further hydration was manifested as damage effect, resulting in a decrease in strength.

Experimental study on bond behavior ofTRUHPCplate under freeze–thaw cycles

AbstractTextile‐reinforced concrete (TRC) has great potential to be used for strengthening concrete structures, and a combination of carbon fiber‐reinforced polymer (CFRP) grid and ultra‐high‐performance concrete (UHPC) could be more desirable. Although the UHPC is known for good durability, the bond degradation between UHPC and CFRP under freeze–thaw cycles (FTCs) remains unclear. Therefore, this paper conducted single fiber bundle and grid pull‐out tests on UHPC‐based TRC (TRUHPC) plates under FTCs to study the bond behavior between CFRP fiber and UHPC matrix. It was found that, after exposed to 700 FTCs, the pull‐out load of the single bundle of CFRP decreased by a maximum of 33.9%, while it dropped by 19.0% in case of the CFRP grid. Finally, a trilinear bond‐slip model was developed and verified against the experimental results. The findings of this study hold significant importance in assessing the tensile behavior of TRC plates under FTCs.

Tailoring of steel fiber surface by coating cellulose nanocrystal for enhanced flexural properties of UHPC

Ultra-high-performance concrete (UHPC) is an advanced generation of cementitious composites with excellent compressive strength and durability. However, the relatively poor interfacial transition zone (ITZ) between steel fibers and UHPC matrix leads to insufficient flexural properties and hinders its further applications. This study proposed a cost-effective, sustainable, and highly reactive coating material, cellulose nanocrystals (CNCs), to densify the ITZ of steel fiber surfaces, thus enhancing the flexural properties of UHPC. Utilizing the Herschel-Bulkley model, the critical concentration of 1.0 % is determined for enabling uniform dispersion of CNCs, enhancing the coating performance and reliability. The performance of CNCs as coating materials was evaluated by flexural test of UHPC with CNCs-coated steel fibers, pull-off test, scanning electron microscope (SEM), atomic force microscope (AFM), energy-dispersive spectroscopy (EDS), and Fourier-transform infrared spectroscopy (FTIR). Results showed that compared to UHPC with pristine steel fibers, the flexural strength and toughness of UHPC with CNCs-coated steel fibers were increased by up to 14 % and 18 %, because the single fiber pull-off energy was increased by 50 %. It can be attributed to the densified ITZ which is validated by the increased UHD C-S-H and HD C-S-H on ITZ. However, when the concentration of CNCs suspension (i.e., 1.5 %) exceeds the critical value, the CNCs coating film would stick the uniformly dispersed steel fibers together and then affect their distribution, thus reducing the mechanical performance of UHPC. This work provides a green and effective approach for promoting flexural properties and an in-depth understanding of CNCs coating mechanism.

Interfacial bond behavior of steel fibers embedded in manufactured sand-based ultra-high performance concrete

The interfacial bond region between matrix and fibers is a non-negligible weak link that can significantly affect the mechanical properties of manufactured sand-based ultra-high-performance concrete (UHPMC). This study investigates the interfacial bond characteristics between steel fiber-UHPMC matrix using single-fiber pullout tests. Four variable parameters were considered: manufactured sand (MS) replacement ratio, stone powder content, fiber embedment length, and steel fiber geometry. Additionally, the microstructure of the steel fiber-UHPMC matrix interface was evaluated using backscattered electron microscopy (BSEM) and scanning electron microscopy (SEM). The results indicate that two typical failure modes including fiber pullout slip failure (in straight and hooked-end fibers) and fiber rupture failure (in corrugated fibers) were observed. The analysis of interfacial evaluation indicators shows that increasing the MS replacement ratio from 25 % to 100 % results in a decreasing trend in peak load, peak tensile stress, and energy consumption. Peak bond strength decreases with increasing embedment length. With the increase in stone powder content, the peak load, peak bond stress, and energy consumption increase initially and then decrease. Additionally, the interfacial failure mechanisms between straight/hooked-end steel fibers and the UHPMC matrix were further elucidated through bond-slip behavior characterization and microstructure analysis. Finally, a design-oriented bond-slip model was developed to predict the interfacial pull-out behavior of steel fiber -UHPMC matrix, showing favorable agreement between predicted and test results. These findings contribute to understanding the interfacial pullout behavior of steel fibers-UHPMC matrix, providing data reference for the performance optimization of UHPMC.

Review of factors affecting bond between ultra high performance concrete and steel fibers

Novel materials such as ultrahigh-performance concrete (UHPC) have advanced over conventional concrete but suffer from brittleness owing to their high strength. The addition of steel fibers significantly overcomes this limitation by acting as a stress-transferring bridge between the matrix and the fibers, which enhances the durability and mechanical performance of the mix. This study compiles data from several previous research studies, offering a comprehensive understanding of the varying factors influencing the pullout behavior and bonding characteristics between the UHPC matrix and steel fibers. The findings of this study reveal that deformed steel fibers embedded at 30° and 45° exhibited significantly greater loading rate sensitivity on the pull-out energy under quasistatic and seismic loading rates when compared to straight steel fibers. Steel fibers with large aspect ratio (i.e., 125) exhibit 77%, 40%, and 10% higher strain capacity, energy absorption capacity, and post-cracking strength, respectively than fibers with a low aspect ratio (i.e., 62.5) under impact loading. The possible factors in the domain of fiber–matrix interactions of fiber-reinforced concrete are covered and discussed.

Investigation of the macro performance and mechanism of biochar modified ultra-high performance concrete

Ultra-high performance concrete (UHPC), an extraordinary category of cementitious composites, showcases unparalleled mechanical and durability properties. Given the current disparity in carbon levels and the environmental hazards associated with cement production, various types of biochar materials have been incorporated into cement composites. This approach offers a mutually advantageous solution, effectively reducing CO2 emissions while simultaneously preserving natural resources. This study aims to evaluate the use of palm shells, a waste biomass, as a substitute for aggregate in the UHPC matrix. The investigation focused on replacing 1–40 % of sand, specifically considering apricot, date, and peach shells biochar at an 8 % replacement. A comprehensive analysis was carried out on a total of 12 combinations, encompassing a control group. For biochar-mortar admixtures, a decrease in workability and density were observed. Replacing 1 % of sand with biochar in UHPC mortar improved compressive strength, but increasing biochar dosage weakened it. Biochar-mortar was demonstrated a dual impact on drying shrinkage. Mixtures with higher biochar dosages showed a slight increase in permeability. Incorporating 1 % biochar into UHPC led to a slightly lower electrical flux value. SEM images were obtained, highlighting the compact morphological structure seen in both the control and the 1 % biochar mortar. MIP test showed nearly 1.6 times increase in the 2-dimensional porosity, from 10.49 % (control) to 16.83 % for sample which had 30 % biochar. Based on the results obtained, it could be concluded that biochar possesses the capability, which enhances the properties of UHPC mortar when used as a partial replacement for conventional fine aggregate in minor fractions.

The Effect of Volcanic Stone and Metakaolin on the Compressive Properties of Ultrahigh-Performance Concrete Cubes

Over the past few decades, ultrahigh-performance concrete (UHPC) has been widely studied and applied because of its outstanding mechanical properties, such as its high strength and notable durability. However, because of its high cost and easy shrinkage cracking during early pouring in mass concrete construction, to reduce the cost of UHPC and reduce the cracks caused by early pouring, volcanic stone was used as a new type of UHPC coarse aggregate, while metakaolin (MK) was added to the system at the same time, and then two parameters, namely the volcanic rock particle size group and the MK dispersion ratio, were set. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetric (TG) microanalysis methods were used to reveal the influence of changes in the material microstructure, phase composition, material composition and crystallinity of the mineral composition on the compressive properties of the UHPC cubes. The results show that the mechanical “lock-in effect” of the structure formed by the volcanic rock holes and mortar can effectively improve the shear resistance of the UHPC–volcanic rock interface, and the compressive strength of the UHPC cubes increases with the volcanic stone’s particle size. When the MK dispersion ratio is less than 4%, the cube compressive strength of the UHPC and the contents of CaCO3 crystals, C-S-H gel and travertine in the UHPC increase with an increasing MK dispersion ratio. At an age of 28 days, compared with the addition of 1% MK, the addition of 4% MK increases the production of C-S-H gel and travertine in the UHPC matrix by 24.82%. When the MK dispersion ratio is 4%, the crystallinity values of the C-S-H gel, travertine and limestone in the UHPC are greater. Adding MK at a 4% dispersion ratio can promote the crystallization of limestone into a large amount of calcite, which can increase the strength of UHPC. On the one hand, the addition of volcanic coarse aggregate results in the retention of more free water and bound water; on the other hand, it also makes it difficult to crystallize CaCO3. The combined action of MK at a 4% dispersion ratio and volcanic rock significantly inhibits CaCO3 crystallization.

Fractal and Multifractal Analysis of Microscopic Pore Structure of UHPC Matrix Modified with Nano Silica

Nano silica (NS) has been found to have a positive impact on enhancing the microporous structure of Ultra-High-Performance Concrete (UHPC). However, there is a lack of effective methods to accurately characterize the regulatory improvement mechanism of NS on the pore structure of UHPC. In this study, our objective is to investigate the influence of NS on various characteristic parameters of the pore structure in UHPC, including porosity, average pore size, box fractal dimension, and multifractal spectral parameters. To analyze these effects, we employ a combination of X- CT image processing techniques and fractal theory. Furthermore, we conducted regression analysis using linear functions to explore the correlation between these parameters and the 28d compressive strength of UHPC. The experimental results demonstrate that NS promotes the refinement of matrix pore size, leading to a denser microstructure of the matrix. Fractal analysis revealed that the pore structure of NS-modified UHPC exhibited favorable fractal characteristics. The fractal dimension and multiple fractal parameters provided complementary insights into the pore structure of NS-modified UHPC from different perspectives. The fractal dimension described the global information, indicating that NS improved matrix defects and reduced the complexity of the pore structure. On the other hand, the multiple fractal parameters supplemented local information, highlighting how the increase in micropores contributed to the heterogeneity of the pore structure. The results of the correlation analysis indicate that the developed mathematical model has a good fit with the 28d compressive strength of UHPC.

Utilizing sugar factory lime waste and crumb rubber for sustainable Ultra-High-Performance Concrete

Ultra-High-performance Concrete (UHPC) offers significant advancements in construction due to its superior strength and durability. However, its traditional formulation demands extensive cement use, leading to high costs and adverse environmental impacts. This research explores the feasibility of repurposing lime waste, a by-product of sugar production, as a substitute for cementitious materials in UHPC, while also examining the use of crumb rubber powder as a complementary filler within the UHPC matrix. Glass, steel, and synthetic macro fibers are also added to improve the ductility of the concrete mix. A series of mechanical tests are conducted to evaluate the performance of the developed concrete. The results are compared with those of conventional cement and other cement substitutes. It is found that replacing 10 % of cement with lime waste and rubber powder produces the optimal UHPC mix, with 104 MPa compressive strength, 7.1 MPa bending strength, 5.8 MPa tensile strength, and 32.8 GPa modulus of elasticity. Moreover, steel fibers are the best reinforcement option, as they significantly improve the ductility and malleability of the concrete, achieving 13,456 J rupture energy and 17.4 MPa bending strength. However, a microstructure analysis reveals that the cement substitutes cause a slight deterioration in the concrete quality, but this is compensated by the benefits of the fibers. It is also shown that the use of lime waste and rubber powder reduces the carbon footprint and the cost of UHPC, making it a more sustainable and economical choice for construction materials. Therefore, it is concluded that the use of lime waste and rubber powder as cement substitutes in UHPC is a feasible and beneficial strategy for developing environmentally friendly UHPC with enhanced mechanical properties. This study contributes to the advancement of eco-conscious UHPC and opens new avenues for future research.

Synergistic effect of microfibers and oriented steel fibers on mechanical properties of UHPC

The study introduces a novel approach to enhance the properties of Ultra-High Performance Concrete (UHPC) through multi-scale synergistic reinforcement using microfibers (0.66 mm) and oriented steel fibers (13 mm). Various aspects were investigated, including rheological properties, fiber orientation, and fiber pull-out characteristics. Besides, quasi-static mechanical properties and dynamic compressive properties of UHPC were tested and the bending failure was monitored by acoustic emission technique. Experimental results showed that microfiber improved mechanical properties of UHPC matrix and enhanced the fiber-matrix interface. Compressive strength, flexural strength, and toughness of specimens were increased by as high as 25 %, 78 %, and 55 %, respectively, when microfiber was mixed with oriented steel fibers. In terms of dynamic impact performance, UHPC exhibited significant strain rate sensitivity, and microfibers blended with oriented steel fibers resulted in complete damage pattern when subjected to impact loading, and dynamic compressive strength and energy absorption increased by 48.5 % and 54.8 %, respectively. Therefore, oriented steel fibers coupled with microfibers provide an effective method to enhance the mechanical performance of UHPC.

Effects of medium-burnt quicklime on autogenous shrinkage of ultra-high performance concrete cured at room temperature

Ultra-High Performance Concrete (UHPC), with its excellent tensile properties, can be used as an alternative to decorative materials that are heavy in cross-section, enabling architectural decoration field to develop in the direction of light weight and changeable shape. However, the extremely low water-binder ratio of UHPC produces obvious autogenous shrinkage, which leads to the development of many micro-cracks. When the micro-cracks develop to a certain extent, they can have a very negative impact on the finishing of the architecture. This paper studies the influence of medium-burnt quicklime dosing on the hydration process and mechanical properties of UHPC cured at room temperature and elucidates the mechanism by which medium-burnt quicklime reduces autogenous shrinkage in UHPC. The results show that the crystal growth pressure generated by the hydration of f-CaO (free calcium oxide) can significantly compensate the autogenous shrinkage of UHPC. The dosage of 3 % medium-burnt quicklime can make the mechanical properties and shrinkage properties of UHPC reach the relative optimum, and the autogenous shrinkage of UHPC can be reduced to 181.36 με. Although the medium-burnt quicklime reduces the hydration products by delaying the hydration of UHPC, it also can reduce the porosity and the percentage of the multi-harmful pores, and refine the pore aperture. In addition, the interfacial transition zone between the UHPC matrix and the steel fibres can be reduced due to the extrusion of Ca(OH)2 crystal growth pressure on the surrounding crystals, resulting in a tighter lapping between the crystals.

Application of sisal fiber powder in ultra-high performance concrete: A renewable material for mitigating autogenous shrinkage

Ultra-high performance concrete (UHPC) has high autogenous shrinkage, greatly raising the risk of cracking. This study innovatively proposed the use of sisal fiber powder (SFP) to mitigate the autogenous shrinkage of UHPC. The influence of SFP on the fresh properties, heat of hydration, autogenous shrinkage, internal relative humidity, mechanical properties, and microstructure of UHPC were investigated and discussed. The results showed that the yield stress and plastic viscosity of fresh UHPC were increased by 33.5∼291.1% and 17.7∼31.6% after adding 1–3 vol% SFP and adding SFP significantly reduced the hydration heat release rate of UHPC. The 7 d autogenous shrinkage was decreased by 59.0% after incorporating 3.0 vol% SFP. Moreover, adding SFP significantly increased the porosity of UHPC, hence the compressive and flexural strength was reduced by 4.2∼15.0% and 8.2∼17.0% respectively. However, the internal curing effect induced by SFP enhanced the late hydration and the average length of C-S-H. Finally, the nanoindentation results indicated that the elastic modulus of the UHPC matrix around SFP was improved. The results of this study verify the feasibility of SFP as an internal curing material to reduce the autogenous shrinkage of UHPC and provide a theoretical basis for the future practical engineering application of SFP in UHPC.

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.

Effect of Steel Fibers on Tensile Properties of Ultra-High-Performance Concrete: A Review.

Ultra-high-performance concrete (UHPC) is an advanced cement-based material with excellent mechanical properties and durability. However, with the improvement of UHPC's compressive properties, its insufficient tensile properties have gradually attracted attention. This paper reviews the tensile properties of steel fibers in UHPC. The purpose is to summarize the existing research and to provide guidance for future research. The relevant papers were retrieved through three commonly used experimental methods for UHPC tensile properties (the direct tensile test, flexural test, and splitting test), and classified according to the content, length, type, and combination of the steel fibers. The results show that the direct tensile test can better reflect the true tensile strength of UHPC materials. The tensile properties of UHPC are not only related to the content, shape, length, and hybrids of the steel fibers, but also to the composition of the UHPC matrix, the orientation of the fibers, and the geometric dimensions of the specimen. The improvement of the tensile properties of the steel fiber combinations depends on the effectiveness of the synergy between the fibers. Additionally, digital image correlation (DIC) technology is mainly used for crack propagation in UHPC. The analysis of the post-crack phase of UHPC is facilitated. Theoretical models and empirical formulas for tensile properties can further deepen the understanding of UHPC tensile properties and provide suggestions for future research.

Novel Ultra-High-Performance Concrete (UHPC) Enhanced by Superhydrophobic and Self-Luminescent Features

This study explores the potential of using basalt reinforced UHPC by incorporating simultaneously self-cleaning and self-luminescent features, paving the way for sustainable advancements in civil engineering. New green formulations of UHPC were developed by integrating supplementary cementitious materials and optimizing water to the binder ratio, followed by using basalt fibers to enhance strength and ductility. The fabricated samples with high particle-packing density exhibit sufficient workability and compressive strength up to 136 MPa, and, when incorporating basalt fibers, a notable reduction in brittleness. The inner microstructure of basalt fibers was observed to be smooth, homogeneously distributed, and well adhered to the UHPC matrix. To ensure the desired long-lasting visual appearance of decorative UHPC and reduce future maintenance costs, a time-effective strategy for creating a light-emitting biomimetic surface design was introduced. The samples exhibit high surface roughness, characterized by micro to nano-scale voids, displaying superhydrophobicity with contact angles reaching up to 155.45°. This is accompanied by roll-off angles decreasing to 7.1°, highlighting their self-cleaning features. The self-luminescence feature showcased intense initial light emission, offering a potential energy-efficient nighttime lighting solution.