A Comprehensive Review of the Advances, Manufacturing, Properties, Innovations, Environmental Impact and Applications of Ultra-High-Performance Concrete (UHPC)

: The article presents the progress and applications of ultra-high-performance concrete (UHPC), a revolutionary material in modern construction that offers unparalleled strength, durability, and sustainability. The overview includes the historical development of UHPC, covering its production and design aspects, including composition and design methodology. It describes the mechanical properties and durability of UHPC and highlights recent innovations and research breakthroughs. The potential integration of multifunctional properties such as self-heating, self-sensing, self-luminescence and superhydrophobicity, is explored. In addition, advances in nanotechnology related to UHPC are addressed. Beyond the actual material properties, the article presents an environmental impact assessment and a life-cycle cost analysis, providing an insight into the wider implications of using UHPC. To illustrate the environmental aspects, the determination of CO 2 emissions is explained using three numerical examples. Finally, various applications of UHPC are presented, focusing on the construction of buildings and bridges. By synthesizing the above-mentioned aspects, this review paper captures the dynamic landscape of UHPC and serves as a valuable resource for researchers and engineers in the field of construction materials.


Introduction
Ultra-high-performance concrete (UHPC) is defined as a type of concrete that exhibits compressive strength greater than 120 MPa, a nominal maximum aggregate size of less than 5 mm, and flowability between 200 mm and 250 mm [1].UHPC exhibits a tensile strength greater than 5 MPa.According to the Federal Highway Administration (FHWA), USA, UHPC is characterized by the use of a combination of Portland cement and supplementary cementitious materials, an optimised gradation of granular materials, a high volume of discontinuous internal fibres, and a low water-to-cementitious materials ratio of less than 0.25.The microstructure of UHPC is characterized irregular pores, which control liquid evaporation and enhance durability in comparison to conventional and high-performance concretes [2].
The development of UHPC can be traced back to the early 1980s [3].After decades of research, UHPC has emerged as a highly innovative product in the field of concrete technology, distinguished by its exceptional mechanical properties and superior durability.The service life of reinforced concrete structures is largely dependent on the durability of the concrete, and the excellent durability properties of UHPC can lead to a significant increase in service life, thus reducing maintenance costs.
Concrete is not really an invention of 20th or 21st century.Concrete, as a construction material, has been in use since ancient times, dating back to the Before Christ (BC) era.In 300 BC, Romans utilised pozzolana cement, and in 800 BC, Greeks employed lime cement.Even earlier, around 3000 BC, Egyptians made use of gypsum and lime mortars in their construction practices.The historical evolution of concrete includes various forms and nomenclatures across different civilization [27]; see Figure 1.By compacting granular materials and adjusting grain size distribution, a compressive strength of 280 MPa was achieved.This breakthrough indicated that low-porous materials had a tendency to develop ultra-high-performance Portland cement-based materials [9][10][11].
In the early 1980s, polymers were introduced to enhance the pore structure, aiming to eliminate defects in the cement paste.This innovation eventually led to the development of micro defect-free cement (MDF) [11,31].A complicated procedure was required to produce MDF concrete for achieving compressive strength of 200 MPa.Complex production technique, expansive raw material, brittleness and creep had confined its applications [11,31].
Bache [27] in Denmark developed dense silica particle cement (DSP).In contrast to the production process of MDF, the production of DSP did not involve critical steps.Instead, the particle packing density was improved using a high dose of superplasticizer (SP) and SF to overcome defects in DSP.However, heat and pressure curing were also part of the production of DSP.Despite having a compressive strength of 345 MPa, DSP concrete exhibited increased brittleness [11].
Brittleness was the major problem that needs to be addressed, therefore in 80s steel fibres were introduced to overcome the brittleness issue of DSP concretes.The introduction of steel fibres into the concrete marked a significant advancement in concrete technology.This new material featured a dense microstructure, high compressive strength, superior durability and ductility.Following DSP, the invention of compact reinforced composites (CRC) and slurry infiltrated fibre concrete (SIFCON) were considered even better creations with superior mechanical properties and durability.However, both CRC and SIFCON faced workability issues due to the non-availability of efficient superplasticizers, which posed obstacles in on-site applications [11,31,32].
The invention of reactive powder concrete (RPC) is considered a significant breakthrough in the development of UHPC.RPC was invented by Richard et al. [15] in the 1990s, who employed thermal treatment to enhance the fineness and reactivity of the components used in RPC.The utilization of a very high cement content, SF, fine quartz powder and quartz sand with optimum gradation, along with the induction of SP and steel fibres, all contributed to the production of a highly approachable UHPC.This mixture, characterized by a very low water-to-cement (W/C) ratio, was suitable for both laboratory and field applications.To enhance homogeneity, coarse aggregates were intentionally omitted.Reactive powder concrete (RPC) demonstrated compressive strengths ranging from 200 MPa to 800 MPa, coupled with excellent workability features [10,30,33].In 1997, the first-ever structure made entirely of Reactive Powder Concrete (RPC) was constructed in Sherbrooke, Canada, and it was a pedestrian bridge [9,11,[34][35][36].The successful application of RPC was hindered by the unavailability for common usage due to expensive material and production costs.
From the year 2000 onward, researchers have shifted their focus to the development of inexpensive and eco-friendly UHPC.Rather than solely pursuing higher strength, there has been an emphasis on minimizing carbon dioxide (CO 2 ) emissions by reducing cement contents [25].
Until now, various methods and procedures have been explored to reduce cement content to less than 850 kg/m 3 and silica fume content to less than 200 kg/m 3 .This reduction is achieved by incorporating supplementary cementitious materials (SCMs) and fillers into the concrete mixture [25,35,[37][38][39], like fly ash, ground granulated blast furnace slag (GGBFS), rice husk ash and Metakaolin [10,11].Also, researchers have tried to use standard curing methods instead of heat curing for energy saving [25,40,41].
In this new era, researchers are concentrating on creating UHPC using new materials and advanced technologies to address coupled issues and to expand the application of this state-of-the-art cementitious material.The goal is to maintain superior mechanical properties and durability [25].

Manufacturing and Design of UHPC 2.1. Composition of UHPC
Attaining excellent compressive strength and durability of UHPC, philosophy governed around four-points ideology [9], as noted hereunder: 1.
Low Water-Cement Ratio.Maintaining a very low water-cement ratio, typically between 0.20 to 0.25, is crucial.This helps in achieving a compact and robust structure through hydration.The low ratio reduces capillary pores, enhances ductility and prevents brittle collapse.

2.
High Particle Packing Density.Utilizing fine binder materials promotes high particle packing density, minimizing water demand in the fresh mixture.This not only improves compressive strength but also influences the brittleness of the concrete.

3.
Optimal Superplasticizer Dosage.Employing a high dose of superplasticizer is essential for regulating the workability of UHPC.This ensures that the concrete is sufficiently workable while maintaining the desired properties.4.
Use of Fibres.Incorporating fibres serves the purpose of inducing tension and bending tension in the UHPC.This, in turn, increases shear strength and imparts the necessary ductility to the concrete.
However, unlike conventional concrete, where coarse aggregates are an essential part, the concept of using coarse aggregates is disregarded in UHPC for the purpose of increasing the homogeneity of the mixture [11,30].
Therefore, we can categorize the typical ingredients in new development of UHPC as: • Binder materials; • Aggregates; • Fibres; • Chemical admixtures.

Binder Materials
Typically, from 800 to 1000 kg/m 3 of binder material is used in the production of UHPC [7,10,30].In the hydration process, 30-40% of the cement content contributes due to the very low water-to-binder (W/B) ratio in UHPC.Therefore, the remaining cement particles act as inactive filler [7,42].It is noted that a UHPC mixture with a binder content of 500 kg/m 3 exhibits a significantly lower slump value.In contrast, a UHPC design mix with 700 kg/m 3 of binder content can achieve a flowability of up to 130 mm [7,43].On the other hand, if the cement content in the UHPC mix design is replaced by 50-73%, the compressive strength may drop by 100 MPa or even lower.This 50-73% replacement of cement contents in the mixture corresponds to only 280 kg/m 3 [7,44].Therefore, there arose a necessity to introduce supplementary cementitious materials to decrease cement contents, subsequently reducing carbon emissions and enhancing flowability without compromising the mechanical properties of UHPC [7,[45][46][47][48].

Portland Cement
Depending on the environmental conditions and the specific applications, UHPC can be produced with different types of cement, including Type I-V and white cement.However, Type III and white cement are the most commonly used cements as they shorten the setting time of concrete and enhance strength due to their high C3S content and Blaine fineness.On the other hand, Type I cement can be considered due to its lower cost and reactivity, especially when exceptionally high early stage strength is not a mandatory criterion [11,25].

Silica Fume
Silica fume (SF) is usually added to the mixture in quantities between 5% and 25%.In quantities below 10%, it improves the concrete particle packing density and workability due to its fine particle size.However, if the SF content exceeds 10% of the UHPC, it can substantially diminish workability, as the large surface area of the SF tends to absorb free water [53].Moreover, owing to its high SiO 2 content, SF imparts a seeding effect and pozzolanic effect, enhancing cement hydration and refining the microstructure of UHPC [25].

Rice Husk
Rice husk is an agricultural waste which is used to replace SF, partially or completely, because of the amount of amorphous silica.The D 50 of rice husk ash is 50-100-times larger than SF, ranging from 5 µm to 20 µm.However, the surface area of rice husk ash is larger (i.e., 64,700 m 2 /kg) than SF (i.e., 18,500 m 2 /kg), thus increasing the water absorption and reducing the concrete workability [25].Moreover, the rice husk ash absorbs water during the mixing process and gradually releases it during the hydration process, delays the internal dehydration and hence mitigates the autogenous shrinkage [10,25].

Fly Ash
Fly ash, a byproduct obtained from coal power stations [10], is primarily used in the production of UHPC with main emphasis on type C and type F fly ash.Both hydraulic and pozzolanic reactions are experienced in type C fly ash, whereas type F only undergoes pozzolanic reactions due to its lack of CaO.Fly ash is employed to replace cement, typically with ratios ranging from 40% to 60% for type C and from 10% to 30% for type F. The more spherical shape of fly ash compared to cement mitigates inter-particle friction, consequently enhancing the workability of the concrete [25,54].

Granulated Blast Furnace Slag (GGBFS)
Ground Granulated Blast Furnace Slag (GGBFS) has been employed as a supplementary cementitious material in concrete for an extended period [55].The slag used in UHPC mixes can include copper slag, iron slag and barium slag.As an alternative SCM, slag typically replaces cement with ratios ranging from 30% to 60%.The angular shapes of slag particles elevate inter-particle friction, leading to decreased workability of UHPC in comparison to an equivalent quantity of fly ash [25].

Glass Powder
Glass powder is utilised as a substitute for cement or SF in UHPC mixtures with ratio from 10% to 50%.D 50 of the glass powder ranges from 1 µm to 20 µm with a smooth surface which mitigates the inter-particle friction, water absorption and, thus, increases the workability of the concrete [56,57].

Metakaolin
Metakaolin, derived from the calcination of natural clay, is employed to enhance the early strength and durability of concrete.Concurrently, the incorporation of metakaolin powder can ameliorate autogenous shrinkage and refine the pore structure [10,58].The use of metakaolin as a substitute for SF in the production of UHPC has been observed.It is noted that a high dose of metakaolin in UHPC may result in a reduction in compressive strength.Specifically, flexural strength increases by 2.6%, while compressive strength decreases by 6.7%.The availability, economical pricing and its white colour make metakaolin a preferred choice for UHPC production [10,58].

Aggregates
High-quality crushed quartz sand, typically with sizes ranging from 150 µm to 600 µm, is commonly used as an aggregate in the production of UHPC.However, the fine quartz sand can be substituted with alternative aggregates due to its high cost [25].
River sand can be used as an alternative to quartz sand [37,39].Nevertheless, the particles in river sand are 5-8-times larger than those in quartz sand, with sizes ranging from 0 mm to 4.75 mm.The use of river sand can downgrade the particle packing density compared to fine quartz sand.Therefore, masonry sand, with a size range of 0-2 mm, is employed to enhance particle packing [39].Masonry sand is generated by crushing coarse aggregates, leading to a particle shape that is more angular.This angular characteristic has the potential to negatively impact the workability of UHPC.
Limestone sand is characterized by its affordability, uniform composition, plentiful reserves and global accessibility [59][60][61].Additionally, it is considered as a more suitable replacement for fine quartz sand in the production of UHPC.Reports indicate that the cost per unit of volume can be reduced by less than 40% [62].
The inclusion of coarse aggregates (size > 4.75 mm) is typically avoided in UHPC mixes.This is attributed to several drawbacks: • The shape and angularity of coarse aggregates reduce the packing density of the concrete.• The concentration of stress at the contact points between aggregates has the potential to create vulnerabilities in the concrete matrix.• The interfacial transition zone (ITZ) between coarse aggregates and the UHPC matrix is weaker compared to the ITZ between sand and the UHPC matrix [25].

Fibres
Fibres are incorporated into UHPC to improve tensile properties by impeding the initiation and propagation of cracks.The attributes of fibres, such as their material (texture), aspect ratio, shape and surface conditions, play a significant role in influencing the properties of UHPC [25,[63][64][65].
Steel, carbon, wollastonite, basalt, polyvinyl alcohol (PVA), polypropylene-polyethylene (PP-PE) and biochip are types of hybrid fibres with different sizes [7,66] and shapes [7].Among the various types of fibres, steel fibres stand out as the most frequently employed in UHPC.It has been observed that as the steel fibre content increases from 0% to 2% (by mass of the mixture), it significantly affects the 28-day flexural strength and toughness of UHPC [67][68][69].On the other hand, the addition of steel fibres from 0.5% to 2.5% in the UHPC mix abruptly reduces the slump value from 4.29 to 1.10 [70].

Chemical Admixtures
Chemical admixtures are utilised to alter the fresh and/or hardened properties of UHPC.Specifically, high-range water reducer (HRWR) is employed to attain selfconsolidating properties.Among various types of HRWR, polycarboxylate ethers (PCE) exhibit high efficiency in dispersing cement particles [71].Nevertheless, the absorbed high-range water reducer (HRWR) on cementitious particles can disrupt the hydration reaction, leading to an extension of the setting time and a reduction in mechanical strength during the early stages [25].

Design Methodology
As mentioned earlier, high particle packing density, low void ratio and high mechanical properties form the basis for designing UHPC mixtures.Consequently, three models have been introduced based on the above concept: the dry particle packing method, wet particle packing method and performance-based method [25].

Dry Particle Packing Method
The dry particle packing model presented by Anderson and Andreasen to achieve the maximum packing density of UHPC mixtures is widely adopted [70].However, this model considers particles under dry conditions only.Dry particle packing method is further divided into two models: the discrete model [72][73][74][75] and the continuous model [76].
The first continuous model, developed by Fuller and Andersen by introducing the target particle size distribution P(D), is considered the most preferred choice for the design of UHPC mixtures [25,76].The modified Andreasen and Andersen model was introduced by Funk and Dinger [64], taking into consideration the minimum particle size effects on particle packing.P(D) is calculated as follows, see Equation (1).
In the above equation, D is the target particle size (µm), D min represents the minimum particle size (µm) in all materials and D max represents the maximum particle size (µm).The distribution modulus, q, is a parameter, and for fine particles, a q value of 0.23 (q < 0.25) is recommended [11,77].
The disadvantage of the dry particle packing methods is that they only consider particles under dry conditions, which may not accurately reflect the real/actual particle packing of UHPC mixtures due to the presence of water and other chemicals [78].

Wet Particle Packing Method
The wet particle packing model considers the effect of water and HRWR.The following steps must be considered to obtain the wet particle packing density [78]: • Weigh the water and binder materials and mix them.

•
Pour the paste into a cylindrical mould and weigh.

•
Calculate the solid concentration (Φ) and void ratio (u) form the Equations ( 2)-(4).• Achieve the maximum packing density by repetitions of above steps.
In Equation ( 2), V S is a solid volume of the particles; M and V refer to the mass and volume of the mixture in the mould, respectively; ρ w refers to the density of water; µ w refers to the volumetric ratio of water; ρ α , ρ β , and ρ γ refer to the solid density of different binder materials, respectively; and R α , R β , and R γ refer to the volumetric ratios of binder materials, respectively.
In Equation (3), u refers to the minimum void ratio, and in Equation ( 4), Φ represents the maximum solid concentration.However, the expected results are not achieved by solely optimizing the maximum particle packing density of UHPC [25].

Performance Based Method
A performance-based method was developed by Meng et al. [39] to enhance the nonproprietary UHPC [25].The step-by-step procedure is illustrated below, and the process is also demonstrated in the flow chart in Figure 2: • Select binder materials based on the flow properties of the paste.

•
Consider material combinations in terms of particle packing, flowability and mechanical properties.

•
Evaluate rheological properties, such as plastic viscosity (µ p ) and yield stress (τ 0 ), to confirm the composition of the mix.

•
Determine the workability and compressive strength as key properties to determine the w/b ratio for the mix.

•
Use the modified Andreasen and Andersen model to determine the sand gradation, calculate the ratio between the volume of the primary paste and the sand volume of the mortar, and compute the steel fibre content.

•
Finalize making UHPC concrete mix as per the given instructions.
Buildings 2024, 14, x FOR PEER REVIEW 9 of 37 In Equation (2), VS is a solid volume of the particles; M and V refer to the mass and volume of the mixture in the mould, respectively; ρw refers to the density of water; µw refers to the volumetric ratio of water; ρα, ρβ, and ργ refer to the solid density of different binder materials, respectively; and Rα, Rβ, and Rγ refer to the volumetric ratios of binder materials, respectively.
In Equation (3), u refers to the minimum void ratio, and in Equation ( 4), Φ represents the maximum solid concentration.However, the expected results are not achieved by solely optimizing the maximum particle packing density of UHPC [25].

Performance Based Method
A performance-based method was developed by Meng et al. [39] to enhance the nonproprietary UHPC [25].The step-by-step procedure is illustrated below, and the process is also demonstrated in the flow chart in Figure 2

Production
Nowadays, the UHPC mixture is mixed, cast and vibrated like conventional concrete.The production procedural steps are as follows [10,18]

Production
Nowadays, the UHPC mixture is mixed, cast and vibrated like conventional concrete.The production procedural steps are as follows [10,18]:

•
Mixing of all dry materials for about 10 min.• Add water and superplasticizer into dry mixture and mix for approximately 5-10 min.

•
Fibres are added if required.

•
Finish the mixing when mixture shows enough flowability for good workability and sufficient viscosity.
The mixing procedure for UHPC appears to take a longer time than that for conventional concrete and consumes more energy.Due to the absence of coarse aggregates and the low W/B ratio, a specific sequence needs to be followed to prevent the UHPC mixture from overheating [79].Table 2 represents the different compositions of UHPC mixtures, including the quantity of cement utilised in producing one cubic meter of UHPC, along with the fibre type and percentage.PC is Portland cement, SF is silica fume, Q is quartz, FA is fly ash, FS is fine sand, GGBS is granulated blast furnace slag, QP is quartz powder, UFSP is ultra-fine silica powder, MK is Metakaolin, NMK is nano metakaolin, LS is limestone, GP is Geranium plant, WG is waste glass; * referred to the density of fibres.

Sampling
The preparation of UHPC samples for testing purposes is no different from that for conventional concrete.Both cubes and cylinders can be used for sampling or to perform compression tests.Nevertheless, advanced studies have shown that cubes are more suitable for compression tests than cylinders (Figure 3).

Curing
A suitable curing process is crucial for optimizing the performance of any concrete.UHPC requires less additional curing compared to other concretes because it is designed based on rheological behaviours and optimised gradation of granular materials.However, UHPCs still require proper hydration [79].Design mix ratios of each material and curing techniques affect the properties of UHPC [91][92][93].
Since the design of UHPC is based on a very low W/B ratio, a more precise curing method is essential to prevent water outflow before the hydration process.Therefore, the freshly cast exposed surface of UHPC needs to be carefully packed immediately to prevent surface dehydration.Materials such as metal, plastic or wood sheets are considered most suitable for packing the UHPC surface [79].
Below methods are most commonly used for curing:

Curing
A suitable curing process is crucial for optimizing the performance of any concrete.UHPC requires less additional curing compared to other concretes because it is designed based on rheological behaviours and optimised gradation of granular materials.However, UHPCs still require proper hydration [79].Design mix ratios of each material and curing techniques affect the properties of UHPC [91][92][93].
Since the design of UHPC is based on a very low W/B ratio, a more precise curing method is essential to prevent water outflow before the hydration process.Therefore, the freshly cast exposed surface of UHPC needs to be carefully packed immediately to prevent surface dehydration.Materials such as metal, plastic or wood sheets are considered most suitable for packing the UHPC surface [79].
Below methods are most commonly used for curing: The most economical and eco-friendly method is a standard room temperature curing.However, better compressive strength results can be achieved with a 24 h steam curing method than with a 28-day standard room temperature method [10,94].It was also found that the compressive strength of UHPC was higher when cured in an autoclave for 8 h compared to standard curing at room temperature and heat curing [95].The effects of different curing methos on the compressive strength, [37,43,48,49,52,55,86,, are shown in Figure 4.

Mechanical Properties of UHPC
UHPC has already demonstrated excellent mechanical properties.However, th mechanical properties of UHPC are still heavily influenced by the material's compositio mix proportions and the applied curing methods [118].In the following subsection various mechanical properties of UHPC are explained in the above context.

Compressive Strength
Compressive strength is a key property of any concrete.Standard testing method for compressive strength, such as ASTM C39 and ASTM C109, are also applicable UHPC [79,119,120].
Researchers have found that a compressive strength of 200 MPa may be achieved a UHPC mixture by replacing 30-35% of cement with SF [10,121].
The authors of [58] found that UHPC with metakaolin had the same or slightly low compressive strength than UHPC with SF.It is also noted that applying heat curing UHPC, incorporating metakaolin along with either crushed quartz or steel fibres, leads to further increase in compressive strength.In another study, the researchers found that th maximum compressive strength is achieved when Mmetakaolinmetakaolin replaces 15% the cement [84].
In a recent study by Chen et al. [122], a binder material incorporating SF with particle size ranging from 8 to 13 µm was utilised, complemented by fine aggrega

Mechanical Properties of UHPC
UHPC has already demonstrated excellent mechanical properties.However, the mechanical properties of UHPC are still heavily influenced by the material's composition, mix proportions and the applied curing methods [118].In the following subsections, various mechanical properties of UHPC are explained in the above context.

Compressive Strength
Compressive strength is a key property of any concrete.Standard testing methods for compressive strength, such as ASTM C39 and ASTM C109, are also applicable to UHPC [79,119,120].
Researchers have found that a compressive strength of 200 MPa may be achieved in a UHPC mixture by replacing 30-35% of cement with SF [10,121].
The authors of [58] found that UHPC with metakaolin had the same or slightly lower compressive strength than UHPC with SF.It is also noted that applying heat curing to UHPC, incorporating metakaolin along with either crushed quartz or steel fibres, leads to a further increase in compressive strength.In another study, the researchers found that the maximum compressive strength is achieved when Mmetakaolinmetakaolin replaces 15% of the cement [84].
In a recent study by Chen et al. [122], a binder material incorporating SF with a particle size ranging from 8 to 13 µm was utilised, complemented by fine aggregate sourced from quartz particles with a size from 0.15 to 0.84 mm.To further augment the bulk density of the mixture, a special type of quartz powder with an average particle size of 15 µm was incorporated.Polycarboxylate-based superplasticizer (SP) was also utilised.The study incorporated micro basalt fibre, macro basalt fibre and steel fibre both individually and in various combinations in the UHPC mixture.The results showed that the compressive strength of the UHPC mixture without any fibre and with micro and macro basalt fibres was almost the same, measuring 127.6 MPa and 128.5 MPa, respectively.However, the mixture containing only macro basalt and only steel fibres (3% by volume) exhibited compressive strengths that were 17.6% and 28.6% higher than the mixture without any fibre.

Flexural Strength
The researchers revealed that a UHPC mix containing SF or metakaolin had almost the same flexural strength regardless of the presence or absence of fibres and regardless of the curing method.However, the use of fibres in the mixture may enhance flexural strength, especially when the mixture includes fibre contents and undergoes heat treatment.In contrast, mixtures without fibres and without heat treatment exhibit higher flexural strength [58].However, the maximum flexural strength is achieved by substituting 15% of metakaolin [16].
A brittle behaviour is observed in UHPC when it does not contain any fibre content.

Flexural Strength
The researchers revealed that a UHPC mix containing SF or metakaolin had almost the same flexural strength regardless of the presence or absence of fibres and regardless of the curing method.However, the use of fibres in the mixture may enhance flexural strength, especially when the mixture includes fibre contents and undergoes heat treatment.In contrast, mixtures without fibres and without heat treatment exhibit higher flexural strength [58].However, the maximum flexural strength is achieved by substituting 15% of metakaolin [16].
A brittle behaviour is observed in UHPC when it does not contain any fibre content.However, the addition of macro basalt fibres or steel fibres (3% by volume) to the mix results in UHPCs with higher flexural strengths of 28.5 MPa and 31.8MPa, respectively [122].
The integration of steel fibres in UHPC plays an important role in achieving structural reliability through load transfer phenomena across cracks, increasing flexural strength in UHPC mixtures [124].The literature has also shown that increasing the fibre content in the UHPC mixture does not really improve the flexural strength [124].

Recent Innovations and Research in UHPC
The development of UHPC is progressing daily, consolidating its status as one of the most exceptional concrete materials currently available, characterized by its well-known properties; see Table 3.

Construction and Infrastructures
Construction, Infrastructures, high elevated temperature zone, harsh marine environment and high security zone construction UHPC is recognized for its remarkable strength and durability, coupled with a notably low porosity, high packing density and minimal permeability [64,127].To achieve UHPC with exceptional strength, durability and increased sustainability, several considerations come into play.These strategies involve the exclusion of coarse aggregates exceeding 5 mm, the reduction of W/C ratio facilitated by superplasticizers, and the substitution of Ordinary Portland Cement with various SCMs such as GGBS, fly ash, metakaolin and rice husk ash [52,117,128,129].
Research findings indicate that Ultra-High-Performance Fibre-Reinforced Concrete (UHPFRC) possesses remarkable tensile and flexural strengths compared to ordinary concrete.The addition of steel fibres plays a crucial role in enhancing the mixture's mechanical properties.Specifically, the incorporation of 2% volume of steel fibres results in a significant increase of up to 228% in tensile splitting strength and 180% in flexural strength.
The use of long fibres further elevates the load-carrying capacity and strain hardening performance of UHPFRC.Notably, UHPFRC exhibits outstanding energy absorption and dissipation capabilities, surpassing those of ordinary concrete.In addition, its resistance to crack formation and transmission makes it a more favourable building material [13].The introduction of steel fibres changes the failure pattern of UHPFRC from brittle to ductile failure [130].While there is no doubt that steel fibres play an important role in enhancing the tensile strength of UHPC, their influence on the compressive strength appears to be uncertain.Other factors, such as variations in test specimen geometry and additional parameters, could also contribute to influence the results [26].
Despite the acknowledged environmental advantages of incorporating SCMs such as fly ash, rice husk ash and GGBS as replacements for cement in UHPC, many researchers believe that the diverse chemical compositions pose a challenge, leading to unreliable performance [131].The purity and efficiency of raw materials in production plants is directly proportional to the chemical compositions [132].As a consequence, the commercial application of UHPCs with SCMs as cement replacements still remains quite limited [25].
UHPC encounters challenges in establishing widespread usage.While it demonstrates success on a laboratory scale, difficulties arise when attempting large-scale operations, potentially leading to mixer failures [133].The volume production of batches is consequently limited.Another important factor standing in the way of wider acceptance is the high cost associated with the components used in UHPC, particularly the expensive cement, silica fume and steel fibres.For instance, a typical strain-hardening UHPC [134,135] comprises 1100-1300 kg/m 3 of cement and 200-350 kg/m 3 of silica fume, constituting 18-26% of the mass of the binder material [115].Table 4 reflects the difference between mechanical properties and effect of different methods of UHPC with SF and with metakaolin.The findings indicate that metakaolin exhibits a higher level of reactivity compared to SF and demonstrates comparable or superior effects in enhancing both mechanical properties and durability.Several studies have dealt with the incorporation of metakaolin in UHPC [9,86].UHPC with metakaolin exhibited almost equivalent mechanical properties, albeit with a slightly longer mixing time, and was considered as a substitute for SF.Notably, metakaolin contributed to a 13% increase in flexural strength [58,136].
A wet packing method considers the water absorption of metakaolin, to elevate the mass ratio of metakaolin to cement.Subsequently, engineers designed a mixture of UHPC where metakaolin was completely replaced by SF and observed that no significant changes in mechanical properties occurred [115].Metakaolin was swapped with two-thirds of SF in a UHPC design mix.Advanced autogenous shrinkage was noticed with respect to the reference mix design, which was purely based on SF [115].Furthermore, the addition of metakaolin in the UHPC mix significantly reduced the drying shrinkage in contrast to UHPC with only SF [99].Metakaolin in UHPC also exhibited better permeability resistance [86].
Basalt fibres are considered superior due to their equivalent mechanical strength, their enhanced durability compared to glass fibres, their lower cost compared to carbon fibres, their sustainability resulting from abundant raw materials and their environmentally friendly production process [137].The outstanding mechanical properties of basalt, particularly within broad temperature ranges, contribute to the preservation of concrete integrity [89].
The thermally treated sugar cane bagasse ash (SCBA) exhibited measured compressive, tensile and flexural strengths at 162.5 MPa, 17.78 MPa and 24.05 MPa, respectively, after treatment at 700 • C [138].Additionally, in Ultra-High-Performance Fibre-Reinforced Concrete (UHPBFC) mixes, a thermally treated rice husk ash was incorporated, replacing two-thirds of the cement by volume.This resulted in a significant improvement in strength performance, with a respective increase in compressive strength of 9.7%, 14.5% and 10.2% at 7, 28 and 120 days in the mixtures, respectively [38].
The performance of UHPC has undergone extensive investigation through the incorporation of various industrial and agricultural wastes.However, a notable gap exists in research regarding the utilization of waste from the perfume or geranium industry in concrete.This gap is particularly significant in light of prevailing climate and environmental concerns related to CO 2 emissions.Exploring these specific waste materials in concrete can offer valuable insights into their potential impact on UHPBFC performance and contribute to sustainable and environmentally conscious concrete practices [90].
The impact of integrating basalt fibre into the concrete mixes was noticeable in terms of increased toughness and elongation of the time between the initial appearance of the first crack and the ultimate failure [139].The addition of basalt fibre can significantly improve bonding properties [140].
Basalt fibre possesses the capability to shift the failure mode of Basalt Fibre-Reinforced Polymer Composites (BFRPC) from tensile failure to tensile shear failure.Additionally, it effectively increases the stress level during cracking, consequently minimizing damage in BFRPC [141].This observation implies that UHPBFC demonstrates enhanced thermal resistance when subjected to elevated temperatures [142].
Recent global climate changes have led to a demand for materials that are able to meet the structural requirements imposed by these changes.UHPC has emerged as a material that can fulfil these demands.In contrast to conventional concrete, UHPC offers a unique combination of properties that make it particularly suitable for modern structures/buildings that have to withstand extreme weather conditions.In addition to its fundamental superior mechanical and durability properties, UHPC has the potential to act as a smart and multifunctional concrete.Such smart and multifunctional building materials can exhibit additional properties, such as superhydrophobicity, self-healing, self-sensing, self-cleaning, self-luminescence, high-water repellence and conductivity, which increase their overall benefits (Figure 6) [143].
Buildings 2024, 14, x FOR PEER REVIEW 16 of 37 concrete.This gap is particularly significant in light of prevailing climate and environmental concerns related to CO2 emissions.Exploring these specific waste materials in concrete can offer valuable insights into their potential impact on UHPBFC performance and contribute to sustainable and environmentally conscious concrete practices [90].
The impact of integrating basalt fibre into the concrete mixes was noticeable in terms of increased toughness and elongation of the time between the initial appearance of the first crack and the ultimate failure [139].The addition of basalt fibre can significantly improve bonding properties [140].
Basalt fibre possesses the capability to shift the failure mode of Basalt Fibre-Reinforced Polymer Composites (BFRPC) from tensile failure to tensile shear failure.Additionally, it effectively increases the stress level during cracking, consequently minimizing damage in BFRPC [141].This observation implies that UHPBFC demonstrates enhanced thermal resistance when subjected to elevated temperatures [142].
Recent global climate changes have led to a demand for materials that are able to meet the structural requirements imposed by these changes.UHPC has emerged as a material that can fulfil these demands.In contrast to conventional concrete, UHPC offers a unique combination of properties that make it particularly suitable for modern structures/buildings that have to withstand extreme weather conditions.In addition to its fundamental superior mechanical and durability properties, UHPC has the potential to act as a smart and multifunctional concrete.Such smart and multifunctional building materials can exhibit additional properties, such as superhydrophobicity, self-healing, self-sensing, self-cleaning, self-luminescence, high-water repellence and conductivity, which increase their overall benefits (Figure 6) [143].

Superhydrophobic Concrete
The natural hydrophilicity of concrete poses a potential risk to its mechanical durability, and it can contribute to structural failure.This vulnerability arises from the ease with which water can infiltrate the porous structure of concrete, leading to damage through freeze-thaw cycles, chemical erosion and other weathering factors.In response to this challenge, researchers have dedicated efforts to developing techniques aimed at improving the water-resistance of concrete [144][145][146][147]. Diminishing the water-to-cement ratio and incorporating extra cementitious materials to the concrete mix are widely regarded as common practices to mitigate the deterioration process of concrete structures [148][149][150].

Superhydrophobic Concrete
The natural hydrophilicity of concrete poses a potential risk to its mechanical durability, and it can contribute to structural failure.This vulnerability arises from the ease with which water can infiltrate the porous structure of concrete, leading to damage through freezethaw cycles, chemical erosion and other weathering factors.In response to this challenge, researchers have dedicated efforts to developing techniques aimed at improving the waterresistance of concrete [144][145][146][147]. Diminishing the water-to-cement ratio and incorporating extra cementitious materials to the concrete mix are widely regarded as common practices to mitigate the deterioration process of concrete structures [148][149][150].
In recent times, significant attention has been directed towards minimizing water infiltration and enhancing the durability of concrete [144,147].Current strategies involve refining pore characteristics and increasing density by adjusting the water-to-cement ratio while incorporating supplementary cementitious materials [148,150].
The idea of superhydrophobic concrete has proven to be a promising strategy to enhance the resistance of concrete.According to the Laplace-Washburn equation [151], the capillary negative pressure plays a vital role, and elevating the water contact angle (WCA) can generate a more potent repellent force.This force actively expels water from pores, enhancing the concrete's water-resistant properties.Two primary techniques, surface and bulk modifications, are utilised to create superhydrophobic concrete.
Surface modification using hydrophobic agents has been practiced since 1986 [152].For instance, in the Netherlands, the Ministry of Transport applied a hydrophobic coating to concrete bridges, resulting in an 80% decrease in chloride infiltration during de-icing salt cycles compared to the control sample [153].
Regarding bulk modification, the adding of a hydrophobic admixture effectively inhibited the corrosion of the reinforcement in concrete [154].Moreover, the induction of bacterial biofilm during casting produced spike-like crystal structures, resulting in a hydrophobic hybrid mortar with exceptional resistance to various water sources [22].
Achieving superhydrophobicity usually involves applying hydrophobic substances on the concrete surface or within shallow surface pores, creating continuous protective coatings for enhanced long-term durability upon solidification.In contrast, bulk modification entails the direct mixing of hydrophobic modifiers with cement, which leads to their accumulation on the cementitious hydration products during the moulding process [22].

Surface Modification Method
Nanocomposite coatings, incorporating inorganic nanoparticles (NPs), typically outperform pure polymers due to improved mechanical stability, high temperature resistance and water-repellent properties.The sol-gel method, a straightforward approach, is used to create superhydrophobic nanocomposite coatings.SiO 2 NPs are commonly used in this method due to their chemical stability, straightforward synthesis and adjustable size.For example, Gao et al. [155] investigated the effectiveness of superhydrophobic asphalt concrete by blending hydrophobic fluoro silanes with SiO 2 NPs.
The 'mix + paint' technique has been employed, utilizing stable and functional polymers as adhesives to securely anchor particles.Typically, polydimethylsiloxane (PDMS) is used to create superhydrophobic surfaces due to its exceptional attributes, including chemical inertness, non-toxicity, cost-effectiveness, biocompatibility and eco-friendliness [127,156].
Despite established methods for producing superhydrophobic concrete materials, challenges persist in their practical application.These materials must maintain both structural integrity and durability, especially under prolonged UV exposure.Additionally, achieving superoleophobicity is crucial, as contaminated superhydrophobic surfaces lose their selfcleaning abilities with oil-based substances.It is noteworthy that SiO 2 nanoparticles might not meet these criteria, prompting exploration into alternative nanoparticles [22].
Achieving superhydrophobicity usually involves applying hydrophobic substances on the concrete surface or within shallow surface pores, creating continuous protective coatings for enhanced long-term durability upon solidification.In contrast, bulk modification involves directly mixing hydrophobic modifiers with cement and their accumulation on cementitious hydration products during moulding [22].

Bulk Modification Method
Silane and siloxane, known for their compact molecular structures, can permeate concrete's dense composition and chemically interact with the cementitious substrate.Both silane and siloxane, individually or combined, have demonstrated effectiveness in altering concrete [154,157].For example, self-cleaning concrete is achieved by incorporating silane into the mixture [158].The introduction of low-surface-energy agents during moulding can yield superhydrophobic concrete.However, this approach alters the original structure, potentially impacting performance, leading to reduced compressive strength due to internal hydrophobization [22].
Siloxane-based admixtures, such as polyethylhydrosiloxane (PEHSO) and polymethylhydrosiloxane (PMHS), as mentioned in the literature [135], exhibit robust reactivity.These additives react with -OH groups of Ca(OH) 2 , forming a durable hydrophobic cross-linked structure that bonds with cement hydration products via siloxane oligomers' polymerization.
In contrast to the surface modification, the bulk modification has a significantly effect on the concrete strength.Incorporating hydrophobic chemicals, including silane-based compounds and polydimethylsiloxane PDMS, typically results in substantial reductions in both compressive and flexural strength [121,159].For instance, Song et al. [159] observed a 40.5% decrease in compressive strength and a 33.3% decrease in flexural strength with the addition of fluoroalkyl silane.Additionally, Subbiah et al. [83] noted a significant 60% decrease in compressive strength with fluoroalkyl silane and SiO 2 /TiO 2 nanoparticles, rendering such concrete unsuitable for construction purposes.

Self-Sensing UHPC
The concept of "smart concrete structures" was initially introduced by American researchers, particularly Chen P-W, in conjunction with the development of self-sensing concrete.Subsequent research and practical experiments have demonstrated that the combination of concrete and carbon fibres can serve as a strain or stress sensor [161].
The manufacturing process involved impregnating precast graphene nanoplate/cementbased sensors with silane/isopropanol solutions.The cement-based sensors treated with silane exhibited satisfactory performance in sensing stress and strain, boasting an average gauge factor of 141.8.Additionally, these sensors indicated outstanding hydrophobic properties, as evidenced by the greatest water contact angle of 163 • on the intact surface [162].
The study investigated the mechanical, electrical and self-sensing characteristics of UHPC with stainless steel fibres and copper-coated steel fibres (CCSFs).The inclusion of 1.2 vol% CCSFs was found to significantly enhance the flexibility of UHPC, resulting in a 33.7% increase in peak compressive strain.UHPC reinforced with stainless steel fibres and CCSFs demonstrated excellent synchronization and repeatability of its self-sensing feature when subjected to three different cyclic loading amplitudes within the elastic range.
The strain sensitivity of UHPC reinforced with stainless steel fibres reached a maximum value of 26.6 under a cyclic compressive load of 37.5 MPa.In addition, UHPC with 1.2 volume percent of CCSFs achieved a strain sensitivity of 28.2 under a monotonic peak load, where the compressive stress was monitored in the range from 0 to 125.7 MPa.The electrical resistance of UHPC with stainless steel fibres/CCSFs can be used for the real-time monitoring of stress, strain and damage evolution in structures [12].

Self-Heating UHPC
The self-heating capability, active de-icing and snow-melting performances take the UHPC to the next level.Self-heating composites were produced and assessed.The UHPC slab, reinforced with stainless steel wires (SSWs), exhibited consistent electrical conductivity and robust self-heating capability throughout repeated cycles.Moreover, it demonstrated effective active de-icing and snow-melting properties under various environmental conditions [163].

Other Concepts
According to statistics, the electricity consumption of street lighting accounts for around 20-30% of the total electricity consumption of national lighting.Nevertheless, the power utilization rate is below 65%, which indicates a considerable waste of energy [164].One possible solution to this problem is to provide road surfaces or associated concrete components with self-illuminating properties by integrating built-in illumination devices or luminescent materials.This approach not only has the potential to save energy and reduce environmental light pollution but also to solve problems such as uneven illumination and glare associated with conventional streetlights [165].Developed countries, including Germany, the United States and France, have already begun researching and developing self-luminous pavements [166].The integration of self-luminescent properties into concrete materials is a novel concept in civil engineering and has been the subject of recent research [131].
German artist Konrad Lueg introduced one of the first light-emitting projects by inducing phosphorescent paints as smart materials like the "Schattenwand mit Blitzelektronik" [167].Some successful engineering applications of luminous pavements have been accomplished.In 2012, the Dutch studio Roosegaarde presented the idea of a luminous highway as part of a smart highway project [168].Thus, UHPC, known for its excellent mechanical properties, durability and versatility, can serve as an ideal material for the integration of smart functions, including self-luminescence.
Intelligent manufacturing of UHPC, specifically through 3D printing, represents a recent and innovative approach that combines optimal design through computer-aided technology with the use of particle packing models and machine learning techniques.This approach aims to achieve high efficiency and accuracy in predicting the properties of the resulting UHPC [15].

Multifunctionality
Our latest experimental work has focused on the development of an innovative basaltreinforced UHPC that offers multifunctionality through the simultaneous integration of self-cleaning and self-illuminating properties.The produced samples, characterized by high particle packing density, show adequate workability and achieve high compressive strength.The incorporation of basalt fibres significantly reduces brittleness.To ensure the desired long-lasting visual appearance of decorative UHPC and minimize future maintenance costs, we introduced a time-effective strategy to create a light-emitting biomimetic surface design [126].
The prospects for UHPC, enhanced with superhydrophobic and self-luminescent features, represent a transformative path for civil engineering and infrastructure development.Incorporating superhydrophobic properties not only increases durability but also reduces maintenance costs and improves resistance to environmental pollutants.At the same time, the integration of self-luminescent properties creates a breakthrough solution for efficient lighting at night, demonstrating the potential for a concept shift in construction technology.

Durability of UHPC
UHPC is a material designed to withstand harsh environmental conditions.Consequently, the assessment of its durability is considered essential.The evaluation of UHPC durability primarily focuses on water and chloride-ion permeability, corrosion of steel reinforcement, freeze-thaw resistance and fire resistance [150].A brief assessment of the durability of UHPC is therefore provided below.

Water Permeability
In the previous section, it was discussed that the hydrophilic nature of concrete can adversely affect its mechanical durability and potentially lead to failures due to the corrosion of steel rebars and/or fibres [150].However, UHPC exhibits lower porosity and a denser microstructure, providing assurances of excellent permeability resistance [36,169].UHPC is designed at a low W/B and low porosity, which is the origin of its high permeability resistance quality [148,170].

Chloride-Ion Permeability
Two testing methods are defined to assess chloride-ion permeability.One is the Rapid Chloride Permeability Test (RCPT), and the other is the Nordtest method NT Build 443 [150].In the Nordtest method, NT Build 443 coefficient of chloride ion diffusion is the main determining factor to evaluate the chloride-ion permeability; for UHPC, it ranges from 0.2 × 10 −13 to 4.1 × 10 −13 m 2 /s, based on the W/B, curing regime, medium solution concentration, steel fibre volume and testing age [150].It is noteworthy that the chloride ion diffusion coefficient of UHPC is one point of magnitude lower than that of conventional concrete (CC) [171,172].The chloride ion diffusion coefficients of C30, C80 and UHPC were also mentioned by [173], which are 1.1 × 10 −13 , 6.0 × 10 −13 and 0.2 × 10 −13 m 2 /s, respectively.

Corrosion of Steel Reinforcement
It is obvious that UHPC has a very low chloride ion permeability and therefore a high corrosion resistance [150].An accelerated corrosion test was conducted to assess the corrosion rate of steel bars in high-performance concrete (HPC) and UHPC.The results indicated that the time to crack formation in UHPC was more than half the time observed in HPC [174].Also, the rate of corrosion for steel bars in UHPC was described as 0.01 lm/year, which is quite lower than the permissible value of 1 lm/year [173].In 2022, Kravanja et al. [160] presented the concept of a green superhydrophobic hybridization concrete that has anticorrosive properties.

Freeze-Thaw Resistance
Typically, freeze-thaw action in concrete leads to random cracking, surface scaling and joint deterioration due to D-cracking [150,175,176], leaving the structure with very low mechanical properties and permeability resistance.HPC possesses excellent freeze-thaw resistance due to its highly impermeable matrix with reduced capillary porosity [150,[177][178][179].Many researchers have discovered that after 300 or 600 freeze-thaw cycles, UHPC exhibited no deterioration [93,150].Kravanja et al. also revealed that by applying superhydrophobic concrete, freeze-thaw damages may be reasonably decreased, and concrete shows high resistance against the freeze-thaw action [160].

Fire Resistance
Due to its low thermal conductivity and high thermal capacity, high fire resistance is one of the most important properties of concrete as a building material.If the free moisture content in the concrete is below 3-4%, explosive spalling in CC is unlikely to occur [150].
It is reported that UHPC samples exhibit a lower temperature difference between inner and outer layers than CC and HPC samples.However, spalling and loss of lower mass occur when the temperature reaches 800-900 • C [180,181].
Recently, one study revealed that UHPC with Macro Basalt Fibre (MBF) exhibited enhanced spalling resistance and higher mechanical properties at 400 • C and 500 • C compared to UHPC with steel fibres [122].

Nanotechnology
The significant progress in nanotechnology has opened up avenues for the effective use of various nanoscale materials, each endowed with distinctive characteristics.The substantial advancements in nanotechnology and nanomaterials have led to the integration of new properties directly into the fundamental structure of materials, thereby resulting in further enhancements [14].
The performance of modified concrete composites is influenced by the integration of nanomaterials.Strength and durability can be improved using nanomaterials in conjunction with modified cement binders [182].Therefore, a significant reduction in service and maintenance costs is expected [183].
Generally, nanoparticles are classified into two categories; Figure 7 depicts nanoparticles classifications.
The significant progress in nanotechnology has opened up avenues for the effective use of various nanoscale materials, each endowed with distinctive characteristics.The substantial advancements in nanotechnology and nanomaterials have led to the integration of new properties directly into the fundamental structure of materials, thereby resulting in further enhancements [14].
The performance of modified concrete composites is influenced by the integration of nanomaterials.Strength and durability can be improved using nanomaterials in conjunction with modified cement binders [182].Therefore, a significant reduction in service and maintenance costs is expected [183].
Generally, nanoparticles are classified into two categories; Figure 7 depicts nanoparticles classifications.Several studies have been conducted to investigate the impact of different nanoparticles on modified cement materials.In a recent study, nano-silica (NS) and C-S-H seeds were incorporated into the UHPC mix.The results indicated a substantial increase in compressive strength, with a 457% improvement when 0.3% of NS was used and a 25% enhancement when C-S-H seeds were employed, compared to the reference sample [184].
Different nanoparticles affect the mixture in different ways; some nanomaterials enhance the mechanical properties of the UHPC mix, while at the same time affecting the freshness properties.For example, nano-silica is a widely used nanomaterial in modifiedcement mixtures, typically ranging from 1% to 6%.Nano-silica can also reduce the flowability and setting time of the mixture, with maximum a reduction observed at 58% and 60%, respectively [14,[185][186][187][188][189][190][191][192][193][194][195][196][197][198][199][200][201].Several studies have been conducted to investigate the impact of different nanoparticles on modified cement materials.In a recent study, nano-silica (NS) and C-S-H seeds were incorporated into the UHPC mix.The results indicated a substantial increase in compressive strength, with a 457% improvement when 0.3% of NS was used and a 25% enhancement when C-S-H seeds were employed, compared to the reference sample [184].
Nanoparticles not only enhance the mechanical and fresh properties of concrete but also contribute to making UHPC a multifunctional material.Silicon dioxide, for example, has the property of preventing the carbonization of concrete.In addition, the addition of titanium or titanium dioxide (TiO 2 ) to the mixture imparts self-cleaning properties to the concrete and enables it to absorb pollution.Silica and SF help to transform concrete into a self-compacting and rapidly hardening material.Carbon nanotubes play a role in minimizing the shrinkage of concrete [215].

Environmental Impact of UHPC: Life-Cycle Assessment and Sustainable Practices
Concrete, as a widely used construction material, has a substantial global environmental impact.Cement production, a key component of concrete, is responsible for approximately 7% of the total CO 2 emissions worldwide [216].
The fabrication of UHPC is also coupled with increased CO 2 emissions, which therefore have a negative impact on the environment.For this reason, the assessment procedure should integrate life-cycle assessment (LCA), a vast and standard method for evaluating the environmental impacts of assets and life-cycle cost, etc. [217].
LCA is a methodology that calculates the environmental impact of any building/structure throughout its entire life cycle (from its creation to its end of its life) [216,218].In general, operation and maintenance (O & M) costs of a building are directly related to the initial costs invested.Lower initial costs usually lead to higher O & M costs and higher initial costs can lead to lower O & M costs.Figure 8 explains the structure of LCA.Typically, we can divide the entire procedure into two parts as shown in Figure 9 below.minimizing the shrinkage of concrete [215].

Environmental Impact of UHPC: Life-Cycle Assessment and Sustainable Practices
Concrete, as a widely used construction material, has a substantial global environmental impact.Cement production, a key component of concrete, is responsible for approximately 7% of the total CO2 emissions worldwide [216].
The fabrication of UHPC is also coupled with increased CO2 emissions, which therefore have a negative impact on the environment.For this reason, the assessment procedure should integrate life-cycle assessment (LCA), a vast and standard method for evaluating the environmental impacts of assets and life-cycle cost, etc. [217].
LCA is a methodology that calculates the environmental impact of any building/structure throughout its entire life cycle (from its creation to its end of its life) [216,218].In general, operation and maintenance (O & M) costs of a building are directly related to the initial costs invested.Lower initial costs usually lead to higher O & M costs and higher initial costs can lead to lower O & M costs.Figure 8 explains the structure of LCA.Typically, we can divide the entire procedure into two parts as shown in Figure 9 below.

Environmental Impact Assessment (EIA)
The environmental impact assessment procedure is based on various indicators, which are shown in Figure 10.

Life cycle assessment
Environmental impact assessment Life cycle cost analysis

Global warming potential
Acidification of land and water sources Depletion of nonrenewable

Environmental Impact Assessment (EIA)
The environmental impact assessment procedure is based on various indicators, which are shown in Figure 10.
The accumulative environmental impact, measured in terms of CO 2 emissions, represents the global warming potential of the infrastructure throughout its service life.The initial phase involves the material formulation process, starting from raw material mining, transportation, etc. CO 2 emissions may be computed by multiplying the impact coupled with every raw material, which keeping in view the quantity of each [219].

Environmental Impact Assessment (EIA)
The environmental impact assessment procedure is based on various indicators, which are shown in Figure 10.The accumulative environmental impact, measured in terms of CO2 emissions, represents the global warming potential of the infrastructure throughout its service life.The initial phase involves the material formulation process, starting from raw material mining, transportation, etc. CO2 emissions may be computed by multiplying the impact coupled with every raw material, which keeping in view the quantity of each [219].
In this way, CO2 emissions for any building/structure can be calculated by summing the unit CO2 emissions weighted by the volume of all the materials involved in the structure.Therefore, the CO2 emissions of all the materials at the manufacturing level can be calculated by Equation ( 5) [220].

𝐸𝑁 ∑ 𝑒𝑛 𝑉
In the above equation, nm represents the number of different materials used within the under-consideration structure, eni stands for the CO2 emissions of a unit of the i-th material (kg-CO2/m 3 ) in the manufacturing phase and Vi is the volume of each material, e.g., concrete, reinforcing steel, timber, etc.In this way, CO 2 emissions for any building/structure can be calculated by summing the unit CO 2 emissions weighted by the volume of all the materials involved in the structure.Therefore, the CO 2 emissions of all the materials at the manufacturing level can be calculated by Equation ( 5) [220].

Environmental impact assessment
In the above equation, n m represents the number of different materials used within the under-consideration structure, en i stands for the CO 2 emissions of a unit of the i-th material (kg-CO 2 /m 3 ) in the manufacturing phase and V i is the volume of each material, e.g., concrete, reinforcing steel, timber, etc.
The unit CO 2 emissions for different materials are influenced by several factors.Several industrial variables can influence the value of CO 2 emissions in the manufacturing phase [216].The unit carbon CO 2 emission of a unit of concrete (kg-CO 2 /m 3 ) in the production phase, en c,manu , can be calculated using Equation (6).
where ma i is the quantity of the basic materials utilised in the production of one cubic meter of concrete (kg/m 3 ), n cm is the number of batches for one cubic meter of concrete and cefm i is the CO 2 emission factor for the production of the i-th basic material (kg-CO 2 /kg).In this context, the basic ingredients of UHPC are cement, silica fume, fibres, water and HRWR [216].
The environmental impact assessment of the transportation phase includes the CO 2 emissions released during the transportation of materials or other things from the plant/unit/factory to the site, depending on the distance and type of transportation.It can be calculated as follows.
In Equation (7), bma i represents the unit weight of the i-th material (kg/m 3 ) used in a structure, V i is the volume of each i-th material (m 3 ), d i is the transportation distance of the i-th material (km), f e refers to the fuel efficiency (km/L) and ceft is the CO 2 emission factor associated with the transportation of fuel consumption (CO 2 kg/L-kg).
The construction phase, which is included in the environmental impact assessment, is generally related to the construction techniques.For example, different types of equipment are used in different construction techniques, resulting in different energy efficiency and environmental impacts.In general, electricity consumption is considered as an environmental impact in this phase [219].
The next phase of the environmental impact assessment is the maintenance, repair and rehabilitation (MR&R) phase.This phase is basically divided into two main categories: preventive maintenance and essential maintenance.Preventive maintenance actions can be carried out either at regular or irregular intervals to prevent the structure from damage or deterioration, while essential maintenance measures are carried out when the performance indicator of the structure reaches the specified target value.The CO 2 emissions at this stage for the environmental impact assessment are directly related to the process or technique used for the rehabilitation of the structure and can be calculated using the following formula [216].
In Equation (8), n refers to the number of maintenance actions within the life-cycle, E env,ini represents the initial CO 2 emissions associated with manufacturing and transportation, and r mi is the ratio of the CO 2 emissions of the i-th maintenance action to the CO 2 emissions of E env,ini .
The final phase is the "end of life phase", which leads either to demolition of the building/structure or to material treatment or recycling of the material process and coupled transportation.The advantage for the environment is always hidden in recycling or material recovery [219].

Life-Cycle Cost Analysis and Equivalent Annual Cost (LCCA)
Similar to the EIA mentioned above, the LCCA also considers the entire life cycle of the building/structure from planning to demolition in relation to the invested costs.Therefore, LCCA is a decision support method that allows a comparative study of the lifecycle costs of alternatives that fulfil a similar function but differ significantly in a life-cycle terms [221].The total cost during the lifetime of a building/structure can be calculated using Equation (9) [220].
In Equation ( 9), r is the monetary discount rate used to discount future cash flows to their present value; C cons are the initial construction costs, including material and labour costs; C ins are the inspection costs; C mt are the maintenance costs, which include both direct and indirect costs for maintenance activities; C fc are the failure costs, which include both direct and indirect costs due to structural failure; C dm are the demolition costs, including the costs of destruction, reclaim and recycling; R v is the residual monetary value of the structure at the end of its design service life, assuming it does not collapse; T is the investigated life cycle, such as the designed service life, n ins , n mt and n fe represent the times of inspection, maintenance, and failure events, respectively, during the investigated life cycle.
In order to calculate the life-cycle costs, we must first determine the "initial construction costs", which can be calculated using Equation (10).
In the above equation, n m is the number of different materials used in a building/structure, c mi is the unit price per ton of the i-th material, V i is the total volume of the i-th material and ρ i is the density of the i-th material.
To address this issue, the method of equivalent annual cost should be employed as an additional performance indicator for comparing assets with different service lives [222,223].The equivalent uniform annual costs LCC EUAC can be calculated using Equation (11) [216].
where LCC NPV is the net present value for life-cycle and cost analysis, r is the monetary discount rate and T is the investigated life cycle.

Numerical Examples for the Determination of CO 2 Emissions
Three numerical examples of three different mixtures for calculating CO 2 emissions in the production of one cubic meter of UHPC are shown in Table 5.Using Equation (6), the first UHPC mixture mixed for a bridge in China [216] has 877 kg emissions of CO 2 /m 3 ; the second mixture, U0.20F2.5, mixed with fly ash and aeolian sand [224], exhibits 852 kg emissions of CO 2 /m 3 ; and the third mixture with fly ash, and silica powder and sand, used for a bridge in Vietnam [225], shows 1146 kg-CO 2 /m 3 .In Table 5, each mixture is presented in two separate columns with the weights of the materials used and their unit CO 2 emissions.In terms of CO 2 emissions, cement is the most polluting material.If only the contribution of cement is calculated, it amounts to 692 kg-CO 2 /m 3 (79%) of the total CO 2 emissions for the first mixture, 538 kg-CO 2 /m 3 (63%) for the second mixture and 887 kg-CO 2 /m 3 (77%) for the third mixture.Note that the second mixture exhibits a lower percentage of CO 2 emissions from cement compared to the first and third mixtures, as a lower content of cement is mixed, and a higher unit CO 2 emission of steel fibre is used.

Applications of UHPC
The properties of UHPC such as strength, ductility, durability and aesthetic design flexibility, make it a smart material.Thanks to these unique properties of UHPC, various tasks can be achieved, such as load-bearing elements with thinner cross-sections (comparable to steel profiles), curved and more complex and sophisticated shapes [167].According to Grand View Research (GVR), the global market for UHPC was estimated at USD 892 million in 2016 and is expected to grow by 8.6% to USD 1867.3 million by 2025 [11].The following subsections present some very well-known structural applications of UHPC.

Buildings
In 2013, the building of the "Museum of European and Mediterranean Civilizations" was constructed in Marseille, France, with extensive use of UHPC [226].In 2014, the "foundation Louis Vuitton pour la Creation" was built in Paris, France, with complex geometric features.The roof of the "Jean Bouin Stadium" in Paris was also built with UHPC [226,227].In Switzerland, UHPC was used for the roof of the Olympic Museum in Lausanne [228].The façade of the National Museum of Qatar was built with UHPC and opened to the public in 2019 [226].In addition, some UHPC buildings are shown in Figure 11.
USD 892 million in 2016 and is expected to grow by 8.6% to USD 1867.3 million by 2025 [11].The following subsections present some very well-known structural applications of UHPC.

Buildings
In 2013, the building of the "Museum of European and Mediterranean Civilizations" was constructed in Marseille, France, with extensive use of UHPC [226].In 2014, the "foundation Louis Vuitton pour la Creation" was built in Paris, France, with complex geometric features.The roof of the "Jean Bouin Stadium" in Paris was also built with UHPC [226,227].In Switzerland, UHPC was used for the roof of the Olympic Museum in Lausanne [228].The façade of the National Museum of Qatar was built with UHPC and opened to the public in 2019 [226].In addition, some UHPC buildings are shown in Figure 11.

Bridges
In 1997, UHPC was used for the first time in the construction of a pedestrian bridge in Quebec, Canada.In 2000, UHPC became commercially available in the United States and the first UHPC girder bridge, known as the "Mars Hill Bridge", was built in Wapello County, Iowa [226].
In Malaysia, almost 113 UHPC bridges have been completed or are still under construction since 2010.One of the completed bridges in Malaysia, located in Perak, is shown in Figure 12.After the successful completion of these pedestrian bridges, UHPC pedestrian bridges were also built in Europe, North America, Asia and Australia [19].

Bridges
In 1997, UHPC was used for the first time in the construction of a pedestrian bridge in Quebec, Canada.In 2000, UHPC became commercially available in the United States and the first UHPC girder bridge, known as the "Mars Hill Bridge", was built in Wapello County, Iowa [226].
In Malaysia, almost 113 UHPC bridges have been completed or are still under construction since 2010.One of the completed bridges in Malaysia, located in Perak, is shown in Figure 12.After the successful completion of these pedestrian bridges, UHPC pedestrian bridges were also built in Europe, North America, Asia and Australia [19].
In 2005, the first four road bridges were built with UHPC: the "Shepherd's Gully Bridge" in Australia, the "Bourd-les-Valence Bridge" in France, the "Horikoshi C-ramp Bridge" in Japan, and the mentioned "Mars Hill Bridge" in Wapello Country Iowa USA [11].
in Quebec, Canada.In 2000, UHPC became commercially available in the United States and the first UHPC girder bridge, known as the "Mars Hill Bridge", was built in Wapello County, Iowa [226].
In Malaysia, almost 113 UHPC bridges have been completed or are still under construction since 2010.One of the completed bridges in Malaysia, located in Perak, is shown in Figure 12.After the successful completion of these pedestrian bridges, UHPC pedestrian bridges were also built in Europe, North America, Asia and Australia [19].In 2005, the first four road bridges were built with UHPC: the "Shepherd's Gully Bridge" in Australia, the "Bourd-les-Valence Bridge" in France, the "Horikoshi C-ramp Bridge" in Japan, and the mentioned "Mars Hill Bridge" in Wapello Country Iowa USA [11].
In 2013, the United States Federal Highway Administration (FHWA) published a report stating that 55 bridges were built with UHPC in the USA and Canada, 22 in Europe, and 27 in Asia and Australia [229].It is also reported that UHPC is actively used in the repair of bridge beams, bridge decks, bridge piles and wind turbine towers [230].
In addition to the successful applications of UHPC in buildings, bridges, and wind turbine towers, an experimental study investigated the durability of UHPC in a harsh marine environment over 21 years.The results showed that the durability of UHPC in harsh marine environments is significantly higher than that of conventional highperformance concrete (HPC) and normal concrete, regardless of the curing process.Chloride penetration was found to occur to a depth of about 10 mm regardless of the exposure time [230].Only recently, a successful experiment was conducted in India to produce a green approach for railroad sleepers made of ternary, blended prestressed cement concrete railway sleepers [231].

Conclusions and Future Perspectives
The review article reports on the progress and applications of ultra-highperformance concrete (UHPC).The article comprehensively addresses the historical development of UHPC, its production and the design aspects encompassing composition and methodology.It delves into the mechanical properties of UHPC, recent innovations and ongoing research.Topics such as multifunctionality, nanotechnology, durability In 2013, the United States Federal Highway Administration (FHWA) published a report stating that 55 bridges were built with UHPC in the USA and Canada, 22 in Europe, and 27 in Asia and Australia [229].It is also reported that UHPC is actively used in the repair of bridge beams, bridge decks, bridge piles and wind turbine towers [230].
In addition to the successful applications of UHPC in buildings, bridges, and wind turbine towers, an experimental study investigated the durability of UHPC in a harsh marine environment over 21 years.The results showed that the durability of UHPC in harsh marine environments is significantly higher than that of conventional high-performance concrete (HPC) and normal concrete, regardless of the curing process.Chloride penetration was found to occur to a depth of about 10 mm regardless of the exposure time [230].Only recently, a successful experiment was conducted in India to produce a green approach for railroad sleepers made of ternary, blended prestressed cement concrete railway sleepers [231].

Conclusions and Future Perspectives
The review article reports on the progress and applications of ultra-high-performance concrete (UHPC).The article comprehensively addresses the historical development of UHPC, its production and the design aspects encompassing composition and methodology.It delves into the mechanical properties of UHPC, recent innovations and ongoing research.Topics such as multifunctionality, nanotechnology, durability assessment, environmental impact, life-cycle cost analysis, a numerical example for determining CO 2 emissions and diverse applications in buildings and bridges are thoroughly discussed.
Research and innovation in the field of ultra-high-performance concrete (UHPC) have made it an innovative material with exceptional mechanical strength, durability and multifunctionality.Remarkable properties such as low porosity, high packing density and minimal permeability make UHPC stand out in comparison to conventional concrete.The incorporation of steel fibres enhances tensile and flexural strengths, shifting the failure patterns from brittle to ductile.However, challenges, including production costs and mixer failures in large-scale operations, remain.The use of supplementary cementitious materials such as fly ash, rice husk ash and metakaolin contributes to sustainability but faces challenges related to the different chemical compositions.Basalt fibres, thermally treated sugar cane bagasse ash, and rice husk ash emerge as promising alternatives with better mechanical properties.The integration of nanotechnology, such as nano-silica and other nanoparticles, has a significant impact on the properties of UHPC.While these nanoparticles improve the mechanical strength, they also affect the freshness properties, which requires careful consideration when developing the mix.In addition, UHPC shows promising progress as a smart and multifunctional material, offering capabilities such as superhydrophobicity, self-sensing, self-heating and self-luminescence.
The focus of further research and activities in UHPC should be on improving environmental impact and sustainability, implementing real cost reduction strategies, fostering innovations and exploring opportunities for global market expansion.To summarize:

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It is imperative to minimize the environmental footprint of UHPC production.Exploring sustainable alternatives, recycling methods and environmentally friendly components will improve the overall environmental profile.

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Overcoming the challenges of large-scale production is crucial for the wide acceptance of UHPC.Solving problems related to mixer failures and optimizing production processes can pave the way for wider application in construction projects.

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Developing cost-effective formulations without compromising performance is essential.Exploring alternative materials and production techniques may contribute to reducing the overall cost of UHPC, making it more accessible for various construction projects.

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Further research into the effects of different nanoparticles on the properties of UHPC is warranted.Understanding the interaction mechanisms and optimizing the nanomaterial content can lead to improved mechanical and fresh properties.

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Investigating the use of waste materials from different industries, such as the perfume or geranium industry, can contribute to sustainable and environmentally conscious UHPC practices.This can provide valuable insights into novel waste materials that can improve the performance of UHPC.

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Extending life-cycle assessments (LCAs) to comprehensively evaluate the environmental impact of UHPC throughout its entire life cycle is crucial.Incorporating LCA considerations into material selection and production processes can lead to sustainable practices.• With a high projected growth rate, the UHPC market is expected to expand globally.
Ongoing research, coupled with successful applications in real-world projects, is likely to increase adoption and have a greater impact on the construction industry.
Addressing challenges and exploring innovative solutions will contribute to the continued growth and sustainable integration of UHPC in various construction applications, including bridges, buildings, marine structures, military applications, wind turbine tow-

Figure 1 .
Figure 1.A brief overview of UHPC historical development.

Figure 1 .
Figure 1.A brief overview of UHPC historical development.
:  Select binder materials based on the flow properties of the paste. Consider material combinations in terms of particle packing, flowability and mechanical properties. Evaluate rheological properties, such as plastic viscosity (µp) and yield stress (τ0), to confirm the composition of the mix. Determine the workability and compressive strength as key properties to determine the w/b ratio for the mix. Use the modified Andreasen and Andersen model to determine the sand gradation, calculate the ratio between the volume of the primary paste and the sand volume of the mortar, and compute the steel fibre content. Finalize making UHPC concrete mix as per the given instructions.
:  Mixing of all dry materials for about 10 min. Add water and superplasticizer into dry mixture and mix for approximately 5-10 min. Fibres are added if required.

Figure 4 .
Figure 4. Effects of curing methods on compressive strength.

Figure 4 .
Figure 4. Effects of curing methods on compressive strength.

Figure 5 .
Figure 5.An overview of the UHPC strength trends.

Figure 9 .
Figure 9.The environmental assessment and life-cycle analysis divisions.

Figure 9 .
Figure 9.The environmental assessment and life-cycle analysis divisions.

Figure 9 .
Figure 9.The environmental assessment and life-cycle analysis divisions.