Analysis Of Behavior of Ultra-High- Performance Concrete (UHPC) Under Compressive and Flexural Loading

Analysis Of Behavior of Ultra-High- Performance Concrete (UHPC) Under Compressive and Flexural Loading

Submitted by

Shashikant Kumar Gupta

24050730208

(M.Tech), Structural Engineering

(Jharkhand University of Technology)

Under the guidance of

Prof. Debapriya Paul

(Department of Civil Engineering) 

C.I.T Ranchi 835103, Jharkhand (Jharkhand University Of Technology)

ABSTRACT

Ultra-high performance concrete (UHPC) combines a very dense cementitious matrix with, in most formulations, discontinuous steel fibers, giving compressive strengths above about 150 MPa and a tensile and flexural response that conventional and high-performance concretes cannot match. This article is a narrative critical review of published evidence on the behavior of UHPC and fiber-reinforced UHPC (UHPFRC) under compressive and flexural loading. It summarizes materials, mix-design principles and microstructure; the test methods and standards used to measure compressive strength, modulus, flexural strength and toughness; and the evidence on stress-strain response, flexural and post-cracking behavior, fiber content, geometry and orientation, curing, water-to-binder ratio, specimen size, sustainability and structural applications. Evidence is compared in tables that record material characteristics, fibers, curing, test method, findings and limitations of each study. The synthesis shows broad agreement that compressive strength is governed mainly by matrix packing and curing, and that flexural and post- cracking performance depends on fiber content, geometry, orientation and bond. It also shows disagreements, for example on whether fibers raise compressive strength and on which fiber length is most effective in flexure, and discusses methodological reasons for them. The review was not conducted as a systematic review: sources were located by targeted searches, and verification was mostly limited to abstracts and publisher records. The paper therefore reports no original experimental data, does not claim completeness, and identifies research gaps in test standardization, orientation control, size and rate effects, recent-literature coverage and life-cycle assessment.

KEYWORDS

Ultra-high performance concrete; UHPFRC; compressive behavior; flexural behavior; steel fibers; fiber orientation; curing; critical review

1. INTRODUCTION

   Concrete is the most widely used construction material, yet conventional concrete is limited by low tensile strength, brittle failure and, in aggressive environments, limited durability. Successive material developments, including high-strength concrete and high-performance concrete (HPC), have improved strength and durability but have generally increased brittleness. UHPC was developed to address this trade-off by combining very high compressive strength with significant tensile capacity and energy

   absorption.

   The modern concept of UHPC is commonly traced to reactive powder concrete (RPC), in which coarse aggregate is removed, the particle-size distribution is optimized, the water-to-binder ratio is made very low, and fibers and thermal treatment are used to obtain exceptional properties (Richard & Cheyrezy, 1995). Subsequent European and North American work, including guidelines, state-of-the-art reports and bridge applications, consolidated UHPC as a distinct class of material (Schmidt & Fehling, 2005; Fehling et al., 2014; Russell & Graybeal, 2013; ACI Committee 239, 2018). Definitions summarized in a recent review require a compressive strength above about 150 MPa and a sustained post-cracking tensile strength above about 5 MPa (Ullah et al., 2022). In this paper, UHPFRC denotes the fiber-reinforced form of UHPC, and UHPC is used for the material class in general.

   Compressive and flexural loading are the two most informative and most widely reported actions for characterizing UHPC. Compression reflects the quality of the matrix and its packing, whereas flexure integrates matrix cracking, fiber bridging and post-cracking resistance into a single test that is directly relevant to beams, girders and thin slabs. Reported values differ between studies because of differences in mixture, specimen size, curing, fiber orientation and test procedure, which complicates direct comparison and design use. A critical comparison of the evidence on these two actions is therefore useful.

   Review questions. This review addresses the following questions:

   1. What levels and mechanisms of compressive and flexural performance are reported for UHPC and UHPFRC?

   2. How do fiber content, geometry and orientation, curing, water-to-binder ratio and specimen size affect these properties, and where do studies agree or disagree?

   3. What methodological limitations restrict comparison between studies?

   4. What research gaps follow from the evidence?

   Scope and limitations. This is a narrative critical review. It does not include original experiments, and it was not conducted as a systematic review (Section 2). Numerical values are given only where traceable to a cited source, and statements that are the authors own synthesis are labelled as such.

2. REVIEW METHODOLOGY

   1. Type of review

      The paper is a narrative critical review with structured tabulation of evidence. It is not a systematic review or meta-analysis: no protocol was registered, no formal reporting framework such as PRISMA was followed, and no numbers of records identified, screened or excluded are reported because these were not systematically recorded. [AUTHOR INPUT REQUIRED: confirm with the supervisor that a narrative critical review is acceptable for the target journal.]

   2. Sources and search

      Literature was located during manuscript preparation (to October 2026) using targeted queries in a general web search engine, which returned publisher pages, open-access repositories (including PubMed Central and MDPI), institutional repositories, and the websites of ASTM International and the U.S.

      Federal Highway Administration (FHWA). Dedicated bibliographic databases (for example Scopus, Web of Science or ScienceDirect) were not searched. Queries combined the terms ultra-high performance concrete or UHPFRC with: compressive strength; stress-strain; flexural strength or toughness; steel fiber content; fiber orientation or distribution; curing or steam curing; life-cycle assessment; and ASTM C1856. Foundational papers already known to the author were looked up by title. Publication years were not restricted, with priority given to 2019-2026 work and to the foundational studies that define the field.

   3. Eligibility

      Included: peer-reviewed articles, official technical reports and applicable standards on UHPC or UHPFRC that report compressive, flexural, tensile or fiber-related behavior, specimen testing, curing, sustainability or structural application, and whose bibliographic details could be confirmed. Excluded: sources whose authors, journal or year could not be confirmed (these are listed in the accompanying editorial report rather than cited); patents; theses and conference abstracts without citable details; and studies on materials outside the UHPC class. Language was restricted to English. No formal quality scoring was applied.

   4. Screening and verification

      Candidate sources were screenedby title and abstract. For each cited source, bibliographic details and the findings used in the tables were checked against the abstract or record page where it was accessible. Full texts were not read for most sources, so the tables report abstract-level findings, and details that are not available at that level are marked [AUTHOR INPUT REQUIRED] or not stated. Sources are labelled in the reference list by verification level (see Section 12 note).

   5. Synthesis approach

      Evidence was organized by loading type and by influencing factor. Each source was summarized in the evidence tables (Tables 3 and 4) with its material, fiber, curing, test method, findings and limitations. Agreements and disagreements were identified by comparing findings for the same factor, and possible reasons for differences were proposed. These reasons are the authors interpretation and are labelled as such.

   6. Limitations of the methodology

      The evidence base is a convenience sample and may be biased toward open-access and widely indexed sources. Abstract-level verification can miss qualifications stated in the full texts. Coverage of 2019-2026 work is limited and should be extended before submission. A future systematic update could use a documented search string in at least two bibliographic databases, for example (ultra-high performance concrete OR UHPC OR UHPFRC) AND (compressive OR flexural OR flexure OR stress-strain OR toughness) with publication years 2019-2026, followed by recorded screening; this string is a proposal and has not been run. [AUTHOR INPUT REQUIRED: whether to perform and report such a search.]

3. UHPC: DEVELOPMENT, MATERIALS AND MICROSTRUCTURE

   1. Development of UHPC

      The theoretical basis of UHPC lies in particle-packing optimization. Packing models allow the selection of aggregate and powder fractions that minimize voids and therefore the water and paste needed for

      workability (De Larrard & Sedran, 1994). Richard and Cheyrezy (1995) combined this principle with several measures: elimination of coarse aggregate to improve homogeneity, use of silica fume and fine quartz powder, a very low water-to-binder ratio enabled by high-range water reducers, optional thermal treatment, and steel fiber reinforcement. These principles remain the basis of most UHPC formulations.

      European research, notably in Germany and France, developed design recommendations and applications through the 1990s and 2000s (Schmidt & Fehling, 2005; AFGC/SETRA, 2013; Fehling et al., 2014). In North America, the FHWA carried out extensive characterization of a proprietary UHPC for bridge use, producing material property data, structural tests and design-oriented reports (Graybeal, 2006; Graybeal, 2007; Graybeal, 2008; Russell & Graybeal, 2013). More recently, studies have sought UHPC with locally available materials and without special heat treatment (Wille et al., 2011a; Wille et al., 2012; Lande & Thorstensen, 2021), with lower cost (Meng et al., 2017) and with reduced cement content (Yu et al., 2015). A state-of-the-art review published in 2022 collects work on composition, mechanical and durability properties and design considerations (Ullah et al., 2022).

   2. Constituents and mix design

      Published reviews describe UHPC as typically composed of Portland cement, silica fume, fine quartz sand and/or quartz powder, a polycarboxylate-type high-range water reducer, water and, in most cases, discontinuous steel fibers (Shi et al., 2015; Yu et al., 2014). The typical constituents and their functions are summarized in Table 1.

      Table 1. Major constituents of UHPC and their functions.

      Constituent material	Typical role in UHPC	Contribution to performance
      Portland cement	Principal binder; used in much larger	Forms the hydrated matrix; high content with low water-to-binder
      	quantity than in conventional concrete	ratio gives a dense, strong paste, and unhydrated grains may act as
      	(Shi et al., 2015)	fine filler (Wang et al., 2015)
      Silica fume	Ultrafine filler and pozzolan	Fills voids between cement grains and reacts pozzolanically,
      		densifying the matrix (Richard & Cheyrezy, 1995; Shi et al., 2015)
      Fine/quartz aggregate	Fine granular skeleton in place of coarse	Reduces heterogeneity and interfacial weak zones; grading is chosen
      	aggregate	by packing optimization (De Larrard & Sedran, 1994; Yu et al.,
      		2014)
      Superplasticizer	High-range water reducer, typically	Allows flowable placement at very low water-to-binder ratio, which
      	polycarboxylate-based	also supports fiber dispersion (Shi et al., 2015; Yu et al., 2014)
      Supplementary	Partial cement replacement and	May lower cement content and improve packing; effect depends on
      cementitious materials	additional fine filler	type and dosage (Yu et al., 2014; Yu et al., 2015)
      Steel fibers	Discontinuous reinforcement, most	Bridge cracks, giving tensile, flexural and post-cracking capacity;
      	commonly steel	length, content and orientation govern the response (Abbas et al.,
      		2015a; Russell & Graybeal, 2013; Yoo & Banthia, 2016)
      Water	Medium for hydration and workability,	Low water-to-binder ratio leaves less free water and fewer voids in
      	kept very low	the matrix (Graybeal & Hartmann, 2003)

      Note. The typical constituents are those reported in the reviews cited; proportions vary widely between studies and are not generalized here.

      Mix design proceeds by (i) selecting a target particle-packing curve or model, (ii) fixing a low water-to- binder ratio, (iii) tuning superplasticizer dosage for flowability, and (iv) choosing fiber type and volume fraction for the desired tensile and flexural performance. Yu et al. (2014) describe a mix-design approach based on optimized packing, and Wille et al. (2011a) show that UHPC with high compressive strength can be obtained with a simplified procedure and conventional materials. Numerical mix proportions vary

      widely between sources and are not generalized here. The review of Ullah et al. (2022) concluded that the best UHPFRC mixtures it surveyed combined steel fiber contents of 2-3% with water-to-cement ratios of 0.2-0.3; this is a conclusion of a review of other studies and not a design rule. Table 2 summarizes how the principal mix and processing variables affect compressive and flexural behavior; the underlying studies are discussed in Sections 5 and 6.

      Table 2. Factors affecting the mechanical behavior of UHPC.

      Factor	Effect on compressive behavior	Effect on flexural behavior	Engineering significance
      Water-to-	Lower ratios are associated with a denser	Indirect, through matrix strength and fiber-	Must be balanced
      binder ratio	matrix and higher strength (Shi et al., 2015;	mtrix bond (author interpretation; Yoo &	against flowability and
      	Yu et al., 2014)	Banthia, 2016)	fiber dispersion
      Fiber volume	Mechanical properties, including	Higher content gave stiffer post-cracking	Raises cost and can
      fraction	compressive, increased with dosage from	response and higher peak load (1% to 2%;	impair workability;
      	1% to 6% in one study (Abbas et al.,	Russell & Graybeal, 2013); flexural	dosage is chosen for
      	2015a); not studied separately by Graybeal	properties increased with dosage (Abbas et	target ductility and
      	(2007); no influence of fiber content in	al., 2015a)	capacity
      	another study (Lande & Thorstensen, 2021)
      Fiber geometry	Longer fibers (6, 10 and 15 mm at 2.5% by	Short fibers gave better flexural properties at	Results differ between
      	volume) improved compressive strength and	similar volume (Abbas et al., 2015a),	studies; optimum
      	modulus in one study (Wei et al., 2021)	whereas longer fibers raised flexural strength	depends on matrix,
      		and energy absorption in another study (Yoo	specimen size and
      		et al., 2016a); fiber length influenced	casting
      		orientation (Yoo et al., 2016a)
      Curing	Thermal treatment can raise strength	Affects matrix quality and thus cracking and	Curing must be reported
      condition	(Richard & Cheyrezy, 1995; Wang et al.,	bond (Wang et al., 2015; Yoo & Banthia,	and matched to precast
      	2015); strength-gain rate of a field UHPC	2016)	or field use
      	depended on curing temperature (Graybeal
      	& Stone, 2012)
      Particle	Denser packing reduces voids and supports	Indirect, through matrix quality and bond	Basis of mix design
      packing	strength (De Larrard & Sedran, 1994; Yu et	(author interpretation)
      	al., 2014); flow-table spread indicates
      	packing (Wille et al., 2011a)
      Matrix	Type of cement, silica fume, sand and	Affects cracking strength and bond (author	Trade-off between
      composition	HRWR influenced strength (Wille et al.,	interpretation)	cement reduction and
      	2011a); SCMs can reduce cement content (Yu et al., 2015)		early strength
      Aggregate	Fine aggregate replaces coarse aggregate,	[AUTHOR INPUT REQUIRED: verified	Maximum particle size
      characteristics	reducing heterogeneity (Richard &	data on aggregate size relative to fiber	influences homogeneity
      	Cheyrezy, 1995)	length]	and fiber dispersion (author interpretation)

      Note. HRWR = high-range water reducer; SCM = supplementary cementitious material. Entries marked author interpretation are not direct findings of the cited study.

   3. Microstructure

      The microstructure of UHPC is characterized by low porosity, a dense calcium-silicate-hydrate-rich matrix, a high proportion of unhydrated cement particles that act as micro-aggregates, and an improved interfacial transition zone around fine aggregate and fibers, as discussed in reviews of hydration and microstructure (Wang et al., 2015; Shi et al., 2015). Thermal treatment accelerates hydration and

      pozzolanic reaction and can refine the pore structure, which is one reason curing regime strongly influences strength (Richard & Cheyrezy, 1995; Wang et al., 2015). The dense matrix increases the matrix tensile strength and bond with fibers, which in turn increases the stress that fibers can carry (Yoo & Banthia, 2016). Figure 1 contrasts the conceptual microstructures of conventional concrete and UHPC.

      Figure 1. Conceptual composition and microstructure of (a) conventional concrete and (b) UHPC, showing coarse aggregate and larger voids in (a) and cement grains, silica fume, fine/quartz aggregate, steel fibers and a dense, low- porosity matrix in (b). Schematic representation based on reported behavior; not experimental data; not to scale.

4. TEST METHODS, STANDARDS AND QUANTITIES USED IN THE LITERATURE

   This section describes how the properties discussed in later sections are defined and measured. It is a description of established methods and of the quantities reported in the reviewed studies; no tests were performed for this paper.

   ASTM C1856/C1856M is the standard practice for fabricating and testing specimens of UHPC. Its scope covers UHPC with a specified compressive strength of at least 120 MPa, a nominal maximum aggregate size below 5 mm and a flow between 200 and 250 mm on the modified flow table, and it includes procedures for fresh and hardened specimens (ASTM International, 2024). Conventional methods, modified for UHPC by that practice, are widely used for the individual properties: ASTM C39/C39M for compressive strength of cylinders, ASTM C469/C469M for modulus of elasticity, ASTM C78/C78M for flexural strength by third-point loading, and ASTM C1609/C1609M for flexural performance of fiber- reinforced concrete including post-peak response. Specimen size and shape influence reported strengths (Graybeal & Davis, 2008; Riedel et al., 2019), and the load rate used in the FHWA compression studies was about 1 MPa/s (Graybeal, 2007; Graybeal & Stone, 2012).

   Compressive strength is the maximum load divided by the cross-sectional area:

   f = Pmax

   C

   (1)

   where fC is compressive strength (MPa), Pmax is the maximum load (N) and is the cross-sectional area of the specimen (mm²).

   Flexural strength (modulus of rupture) for a beam failing within the middle third of the span under third- point loading is

   P L

   R = b d2 (2)

   where R is the modulus of rupture (MPa), P is the maximum load (N), L is the span length (mm), b is the average beam width (mm) and d is the average beam depth (mm) (as defined in ASTM C78/C78M).

   The chordal modulus of elasticity from a compression test follows ASTM C469/C469M:

   2 1

   = (3)

   2 0.000050

   where E is the chordal modulus (MPa), S2 is the stress at 40% of ultimate strength, S1 is the stress corresponding to a longitudinal strain of 50 microstrain, and E2 is the longitudinal strain at S2.

   For fiber-reinforced beams, ASTM C1609/C1609M defines the residual strength at a net deflection of

   L/150 and the toughness as the area under the load-deflection curve up to that deflection:

   150

   = 150

   2

   (4)

   where fD is the residual strength at net deflection L/150 (MPa), PD is the corresponding residual load

   150

   (N), and L, b,d are as in Eq. (2).

   Engineering stress and strain in compression are

   P ,1L

   a = , E =

   L0

   150

   (5)

   where a is stress (MPa), P is applied load (N), is cross-sectional area (mm²), E is axial strain (dimensionless), ,1L is the measured change in gauge length (mm) and L0 is the original gauge length (mm).

   These formulations are the established forms in the cited standards; their applicability to UHPC specimen sizes, fiber lengths and strength levels should be checked against the current standard text. Different studies use different toughness indices and notched or unnotched beams, which limits quantitative comparison (Section 8.3).

5. COMPRESSIVE BEHAVIOR: EVIDENCE AND SYNTHESIS

   1. Strength levels, curing and matrix composition

      UHPC reaches compressive strengths far above those of conventional concrete. Graybeal and Hartmann (2003) reported strengths that could exceed about 200 MPa for the FHWA-studied material, and Graybeal (2008) used a UHPC of 193 MPa in a prestressed girder. Wille et al. (2011a) produced a UHPC exceeding 200 MPa from commercially available U.S. materials without heat treatment, pressure or special mixing equipment, and proposed the flow-table spread as a quick guide to packing density and strength. Lande and Thorstensen (2021) obtained a compressive strength of 166 MPa and a modulus of 46 GPa with constituents found in southern Norway.

      Curing influences strength development. Thermal treatment can raise strength (Richard & Cheyrezy, 1995; Wang et al., 2015), and the strength gain of a rapid-strengthening UHPC formulation depended on the curing environment (Graybeal & Stone, 2012). A 2022 review of microwave, autoclave, carbon, steam, electric, ambient and water curing found that all methods gave satisfactory compressive strengths and that a single optimum regime cannot be specified, because curing temperature, duration and materials all matter (Hamada et al., 2022). A lower water-to-binder ratio is associated with a denser matrix and higher strength provided workability and fiber dispersion are maintained (Shi et al., 2015; Yu et al., 2014).

   2. Stress-strain behavior, modulus and the role of fibers

      Graybeal (2007) established the compressive stress-strain behavior and modulus of a UHPFRC reinforced with 13 mm steel fibers by cylinder tests and compared the results with equations developed for normal and high-strength concrete; the effect of the fibers on compressive properties was not studied separately. Evidence on the fiber effect elsewhere is not uniform. Abbas et al. (2015a) reported that mechanical properties increased with steel fiber dosage over 1% to 6% by volume. Wei et al. (2021), using fibers of 6, 10 and 15 mm at a fixed 2.5% dosage, found that longer fibers improved both compressive strength and elastic modulus. By contrast, Lande and Thorstensen (2021) found that compressive strength and modulus were not influenced by fiber content in their mixes. Section 8.2 discusses possible reasons. The typical response is sketched conceptually in Figure 2; the sketch is not drawn from any dataset.

      Figure 2. Typical compressive stress-strain response of UHPC without and with fibers, showing the near-linear ascending region, peak stress and post-peak behavior. Schematic representation based on reported behavior; not experimental data.

      Axes are intentionally unscaled.

   3. Specimen size, shape and test method

      Graybeal and Davis (2008) tested 51, 76 and 102 mm cylinders alongside 51, 70.7 and 100 mm cubes of UHPFRC with strengths of 80 to 200 MPa, and concluded that the 76 mm cylinder and the 70.7 mm and 102 mm cubes could substitute for the standard 102 mm cylinder, favoring the 70.7 mm cube where machine capacity or cylinder end preparation constrains testing. Riedel et al. (2019) examined mixtures without fibers and reported that, compared with ordinary-performance and high-performance concrete, the effect of specimen geometry on compressive strength is very small for UHPC. These findings suggest that the choice of specimen can matter less for fiber-free UHPC but should still be reported, and that the conclusions for fiber-reinforced material need separate confirmation.

   4. Summary of compressive evidence

      Table 3 summarizes selected studies. Findings are taken from abstracts or reports, and limitations are the authors

      assessment from the information available.

      Table 3. Compressive behavior of UHPC reported in selected studies.

      Author(s) and year	Material / mix	Fiber type and content	Curing	Specimen / test	Key findings	Limitations	Source
      Graybeal &	Proprietary FHWA	Steel; content	[AUTHOR INPUT	Material property	Strength could exceed	Read from an	TRB 82nd
      Hartmann	UHPC; very low w/b	[AUTHOR INPUT	REQUIRED]	testing	about 200 MPa; tensile	FHWA-hosted	Annual
      (2003)		REQUIRED]			strength about 7-14 MPa;	summary; details	Meeting
      					durability exceeded HPC	not available
      Graybeal	UHPFRC, high	Steel, 13 mm;	[AUTHOR INPUT	Cylinders, about 1	Stress-strain behavior and	Fiber effect not	ACI
      (2007)	cement and silica fume content	content not stated	REQUIRED]	MPa/s, axial gauges	modulus established; compared with equations	studied separately; single proprietary	Mater. J. 104(2)
      					for normal and high-	mix
      					strength concrete
      Graybeal &	UHPFRC, 80-200	Fiber-reinforced	[AUTHOR INPUT	51, 76, 102 mm	76 mm cylinder and 70.7	Details of ratios	ACI
      Davis (2008)	MPa	(type not stated in	REQUIRED]	cylinders; 51, 70.7,	and 102 mm cubes	not available at	Mater. J.
      		abstract)		100 mm cubes	judged acceptable	abstract level	105(6)
      					substitutes for the 102
      					mm cylinder
      Wille et	Commercially	Steel fibers (per	No heat treatment,	Compression;	Strength above 200 MPa;	Fiber effect not the	ACI
      al. (2011a)	available U.S. materials; cement, silica fume, sand,	keywords)	pressure or special mixer	flow-table spread	spread proposed as proxy for packing density and strength	focus; maerials- specific	Mater. J. 108(1)
      	HRWR varied
      Abbas et	UHPC with varied	Steel, 8, 12, 16 mm	[AUTHOR INPUT	Compressive,	Properties increased with	Curing and	Constr.
      al. (2015a)	fiber length and	at 1%, 3%, 6% by	REQUIRED]	splitting tensile,	dosage; no degradation	specimen size not	Build.
      	dosage	volume		flexural; chloride exposure	after chloride exposure	available at abstract level	Mater. 75
      Graybeal &	Rapid-strengthening	Discrete steel fibers	Strength gain	Compressive	Rate of property	Specific	FHWA-
      Stone (2012)	UHPC for field		depended on curing	strength and	development depended	proprietary	HRT-12-
      	connections		temperature	modulus gain at	on curing environment	formulation	064
      				about 1 MPa/s
      Riedel et	OPC, HPC and fiber-	None	Standardized	Specimens of	Geometry effect on	Fiber-free	ACI
      al. (2019)	free UHPC; mean		(details in source)	different shapes	strength very small for	mixtures only	Mater. J.
      	cylinder strengths 30- 200 MPa			and sizes	UHPC compared with OPC and HPC		116(6)
      Lande &	Locally produced	Steel fibres;	[AUTHOR INPUT	231 specimens;	166 MPa and 46 GPa, not	Single region;	Nordic
      Thorstensen	UHPC, southern	content, type and	REQUIRED]	digital image	influenced by fibre	fiber ranges not	Concr.
      (2021)	Norway; nine mixes	hybrid		correlation	content	stated at abstract	Res.
      		combinations				level	(2021)
      Wei et	UHPC, four mixes	Straight steel, 6, 10,	[AUTHOR INPUT	Quasi-static	Longer fibers improved	Single dosage;	Eng.
      al. (2021)		15 mm, 0.2 mm	REQUIRED]	compression and	compressive strength and	authors list to	Struct. 245
      		diameter, 2.5% by		flexure; impact	modulus	verify
      		volume		beams
      Hamada et	Review of curing	Not applicable	Microwave,	Review of	All methods satisfactory;	Compressive	Case Stud.
      al. (2022)	methods		autoclave, carbon,	compressive	no single peak regime can	strength only;	Constr.
      			steam, electric,	strength studies	be specified	secondary	Mater. 17
      			ambient, water			evidence

      Note. HPC = high-performance concrete; OPC = ordinary-performance concrete; HRWR = high-range water reducer; w/b = water-to-binder ratio; FHWA = Federal Highway Administration; TRB = Transportation Research Board. Findings are at abstract or report level. [AUTHOR INPUT REQUIRED] marks details to be taken from the original papers.

6. FLEXURAL AND POST-CRACKING BEHAVIOR: EVIDENCE AND SYNTHESIS

   1. Flexural strength, toughness and fiber content

      Flexural tests capture both the cracking strength and the post-cracking response. Graybeal (2008) tested a full-scale prestressed UHPC I-girder without mild steel reinforcement, examined cracking, stiffness and moment capacity, developed a relationship between tensile strain and crack spacing, and proposed a flexural design philosophy. The FHWA state-of-the-art report notes that increasing fiber content from 1% to 2% produced a stiffer post-cracking response and higher peak loads in the beam tests it reviews (Russell & Graybeal, 2013). Abbas et al. (2015a) found that flexural properties increased with fiber

      dosage over 1% to 6%, and in tunnel-lining segments (Abbas et al., 2015b) load capacity increased linearly with dosage. Yoo and Yoon (2015) reported that 2% by volume of steel fibers raised the load capacity of reinforced UHPC beams but reduced ductility, an important trade-off for design.

      Strain-hardening UHPC can exhibit deflection hardening and multiple cracking in flexure (Naaman & Reinhardt, 2006; Wille et al., 2011b), and the tensile response of hybrid steel and non-metallic fiber systems has been examined for toughness and cost benefits (Li & Deng, 2021). A review of 2016 covers flexural behavior in relation to fiber type, content and curing (Yoo & Banthia, 2016).

   2. Fiber length, geometry and hybridization

      Evidence on fiber length is mixed. Abbas et al. (2015a) found that, at similar volume, short fibers gave better flexural properties, and Abbas et al. (2015b) found that shorter fibers gave higher cracking and peak loads in segments while longer fibers gave more ductile post-peak softening. Yoo et al. (2016a) reported that longer fibers raised flexural strength and energy absorption with little effect on first-cracking properties. Yoo et al. (2018) found that long straight and twisted fibers improved quasi-static flexural performance, especially at large deflections. Hybrid systems are reported to help: in the work of Wei et al. (2021), a mixture with hybrid long and short fibers reached a flexural strength and fracture energy 21.84% and 20.13% greater than a single-long-fiber mixture, and Lande and Thorstensen (2021) found the highest flexural tensile strength with micro straight fibers alone or in a 50% combination with macro hooked-end fibers, with a potential cost reduction of about 30%.

   3. Fiber distribution and orientation

      Fiber orientation emerges as a dominant source of variability. The FHWA review stresses that post- cracking response is highly sensitive to fiber dispersion and orientation (Russell & Graybeal, 2013). Barnett et al. (2010) showed that casting method changed fiber alignment in round panels, with fibers aligning perpendicular to the flow, and that panels poured from the centre were significantly stronger because more fibers bridged the radial cracks. Yoo et al. (2016b), who quantified orientation by image analysis, found that fibers better aligned with the tensile direction gave higher flexural strength, deflection capacity and toughness under quasi-static loading, with no obvious difference in first-cracking properties. Kang et al. (2011) examined fiber distribution characteristics and flexural strength. Li et al. (2020) used a magnetic probe on UHPFRC plates and found a mean orientation angle of 45.60 degrees and a correlation between peak flexural strength and the product of fiber volume content and the cosine of the orientation angle. Maryamh et al.

   4. Summary of flexural evidence

      Table 4 summarizes selected studies. Findings are at abstract or report level, and limitations are the authors assessment.

      Table 4. Flexural behavior of UHPC reported in selected studies.

      Author(s) and year	Material / mix	Fiber type and content	Curing	Specimen / test	Key findings	Limitations	Source
      Graybeal (2008)	Steel-fiber	Steel; content	[AUTHOR	24.4 m prestressed I-	Tensile strain-crack spacing	Single girder;	J. Bridge
      	UHPC, 193	[AUTHOR INPUT	INPUT	girder, 26 strands, no	relation; flexural design	proprietary	Eng. 13(6)
      	MPa	REQUIRED]	REQUIRED]	mild steel	philosophy	UHPC

      Meade & Graybeal tests as reported by Russell & Graybeal (2013)	Steel-fiber UHPC	Steel; 0%, 1%, 2% by volume	[AUTHOR INPUT REQUIRED]	Sixteen 152 x 381 mm beams, four-point bending, 4.88 m span; reinforcement 0-1.00%	1% to 2% fibers: stiffer post-cracking response, higher peak load; no crushing noted	Secondary report; scale of specimens	FHWA- HRT-13- 060
      Abbas et al. (2015a)	UHPC,	Steel, 8, 12, 16	[AUTHOR	Flexural strength tests	Short fibers better at similar	Details at	Constr.
      	varied fiber	mm; 1%, 3%, 6%	INPUT		volume; properties rose with	abstract level	Build.
      	length and		REQUIRED]		dosage		Mater. 75
      	dosage
      Abbas et al. (2015b)	UHPC	Steel, various	[AUTHOR	1/3-scale tunnel-lining	Short fibers: higher cracking	Segment scale;	Eng. Struct.
      	segments	lengths; dosages	INPUT	segments; flexural and	and peak loads; long fibers:	volume and	(2015)
      		not stated	REQUIRED]	edge-point load	more ductile post-peak;	pages to verify
      					capacity linear with dosage
      Yoo & Yoon (2015)	UHPC beams	Smooth (13, 19.5,	[AUTHOR	Ten beams; rebar ratios	About 27-54% higher	Reinforced	Eng. Struct.
      	with rebars	30 mm), twisted	INPUT	0.94% and 1.50%	capacity, 13-73% lower	beams only	102
      		(30 mm); 2%	REQUIRED]		ductility with 2% fibers
      Yoo et al. (2016a)	UHPC	Steel, three lengths,	[AUTHOR	Flexure; image analysis;	Longer fibers raised strength	Content not	Compos.
      		same diameter;	INPUT	finite elements	and energy absorption;	available at	Struct. 147
      		content not stated	REQUIRED]		orientation affected by	abstract level
      					length
      Yoo et al. (2016b)	UHPFRC	Steel [AUTHOR	[AUTHOR	Two placement methods	Better alignment: higher	Details at	Compos.
      		INPUT	INPUT	and sizes; quasi-static	strength, deflection	abstract level	Struct. 157
      		REQUIRED]	REQUIRED]	and impact; image	capacity, toughness
      				analysis
      Yoo et al. (2018)	UHPC beams	Straight (aspect	[AUTHOR	Quasi-static and impact	Long straight and twisted	Content not	Cem.
      		ratios 65, 97.5, 100) and twisted	INPUT REQUIRED]	beams	fibers improved large- deflection behavior	stated	Concr. Compos.
      		(100)					86
      Barnett et al. (2010)	UHPFRC	Steel; content	[AUTHOR	Panels cast in different	Centre-poured panels	Pre-2019; panel	Mater.
      	round panels	[AUTHOR INPUT	INPUT	ways; resistivity; CT;	significantly stronger; fibers	geometry	Struct.
      		REQUIRED]	REQUIRED]	flexure	align perpendicular to flow		43(7)
      Li et al. (2020)	UHPFRC	Straight steel fibers	[AUTHOR	Plates; magnetic probe;	Peak flexural strength	Plate-specific;	Materials
      	plates		INPUT	small flexural specimens	correlates with fiber volume	calibration	13(22)
      			REQUIRED]		x cosine of orientation angle
      Maryamh et	UHPFRC	Steel; varied	[AUTHOR	Casting from side or	Results explained by fiber-	Details at	Struct.
      al. (2021)		diameter and	INPUT	middle; micro CT; four-	systemcharacteristics from	abstract level	Concr.
      		volume fraction	REQUIRED]	point bending	images		22(1)
      Wei et al. (2021)	UHPC	Straight steel 6, 10,	[AUTHOR	Notched three-point	Hybrid long and short	Single dosage;	Eng. Struct.
      		15 mm; 2.5%	INPUT	bending; impact beams	fibers: +21.84% strength,	authors list to	245
      			REQUIRED]	(2000 x 168 x 168 mm)	+20.13% fracture energy	verify
      					over single long
      Lande &	Locally	Steel fibres,	[AUTHOR	231 specimens; DIC	Highest flexural tensile	Single region;	Nordic
      Thorstensen (2021)	produced	content, type,	INPUT		strength: micro straight	ranges not stated	Concr. Res.
      	UHPC	hybrid	REQUIRED]		alone or 50% with macro		(2021)
      					hooked

      Note. UHPFRC = ultra-high performance fiber-reinforced concrete; DIC = digital image correlation; CT = computed tomography. Percent values are reproduced only where stated in the cited abstracts or reports. [AUTHOR INPUT REQUIRED] marks details to be taken from the original papers.

      Figure 3 sketches the typical flexural load-deflection response conceptually; it is not drawn from any dataset.

      Figure 3. Typical flexural load-deflection response of UHPC beams, showing initial elastic response, first cracking, post- cracking response with fiber bridging and ultimate load for deflection-hardening and deflection-softening behavior.

      Conceptual representation; schematic representation based on reported behavior; not experimental data.

7. FAILURE MECHANISMS

   The mechanisms below are the authors synthesis of the behavior described in the cited sources and of general understanding of fiber-reinforced cementitious composites. Figure 4 shows the fiber-bridging mechanism that underlies the tensile and flexural mechanisms, and Table 5 summarizes the mechanisms by loading condition.

   Figure 4. Crack propagation and fiber bridging in UHPC: a matrix crack of opening w is crossed by steel fibers that transfer tensile stress through bond and pull-out. Schematic representation; not to scale; not experimental data.

   Compressive failure. In UHPC without fibers, the dense homogeneous matrix stores significant elastic energy, and once lateral tensile strains cause axial splitting the failure is sudden. Fibers restrain lateral dilation and crack opening, leading to a more gradual post-peak response (author synthesis).

   Tensile cracking. Cracking begins when the matrix tensile strength is reached at a flaw or at the tension face of a beam. The high matrix strength and high bond mean that cracks form at higher stress than in conventional concrete (Yoo & Banthia, 2016).

   Fiber bridging. After a crack forms, fibers crossing it carry tensile stress through bond and pull-out, limiting crack opening. Whether the composite can carry more load than at first cracking depends on whether the bridging stress exceeds the cracking stress, which is the criterion distinguishing strain- hardening from strain-softening behavior (Naaman & Reinhardt, 2006).

   Crack propagation. In strain-hardening material, bridging allows new cracks to form before any single crack localizes, producing multiple fine cracks (Habel et al., 2006; Wille et al., 2014). In strain-softening material, one crack localizes and opens.

   Post-cracking behavior. The post-cracking response is governed by fiber content, orientation, length and bond, and is highly sensitive to fiber dispersion and orientation (Russell & Graybeal, 2013; Yoo et al., 2016b).

   Flexural failure. Flexural failure occurs by localization of a dominant crack followed by fiber pull-out or fiber rupture, or, in members with reinforcement, by yielding or rupture of reinforcement, depending on the section design (Graybeal, 2008; Russell & Graybeal, 2013).

   Table 5. Failure mechanisms of UHPC under compressive and flexural loading.

   beams (Yoo & Yoon, 2015)

   Loading condition	Failure mechanism	Crack development	Role of fibers	Typical observation
   Compression	Axial splitting driven by	Vertical splitting cracks near	Restrain lateral dilation and	Sudden failure without
   	lateral tensile strain,	peak load; without fibers,	crack opening, leading to more	fibers; more gradual
   	followed by crushing	development is rapid (author	gradual post-peak response	softening with fibers
   	(author synthesis)	synthesis)	(author synthesis; not studied	[AUTHOR INPUT
   			separately by Graybeal, 2007)	REQUIRED: own test
   				observations]
   Flexure	Tensile cracking at the	Single localizing crack (strain-	Carry stress across cracks;	No concrete crushing
   	tension face when the	softening) or multiple cracks	fiber length had little effect on	noted in rectangular
   	matrix tensile strength is	(strain-hardening) (Naaman &	first-cracking properties but	beam tests reviewed by
   	reached	Reinhardt, 2006; Wille et al.,	raised strength and energy	Russell & Graybeal
   		2014); crack spacing related to	absorption (Yoo et al., 2016a)	(2013)
   		tensile strain in girders
   		(Graybeal, 2008)
   Post-cracking	Localization of a	Crack widening after	Transfer tension by bond and	Longer fibers gave more
   flexure	dominant crack, then	localization, preceded by	pull-out; response sensitive to	ductile post-peak
   	fiber pull-out or rupture;	multiple cracking in strain-	dispersion and orientation	softening in tunnel-lining
   	reinforcement yielding	hardening material	(Russell & Graybeal, 2013);	segments (Abbas et al.,
   	in reinforced members		fibers raised capacity but	2015b)
   			reduced ductility in reinforced

   Note. Entries marked author synthesis combine general understanding of fiber-reinforced cementitious composites and are not direct findings of a single cited study.

8. CRITICAL SYNTHESIS

   1. Points of agreement

      The reviewed studies agree on several points. First, compressive strengths above about 150 MPa, and in several cases about 200 MPa, are attainable, including with locally available materials and without heat treatment (Wille et al., 2011a; Lande & Thorstensen, 2021). Second, fibers govern tensile, flexural and post-cracking behavior, and higher fiber content generally improves flexural capacity and post-cracking stiffness within the ranges studied (Russell & Graybeal, 2013; Abbas et al., 2015a; Yoo & Yoon, 2015). Third, fiber orientation and distribution have a large influence on flexural response, as shown with several different measurement techniques (Barnett et al., 2010; Yoo et al., 2016b; Li et al., 2020; Maryamh et al., 2021). Fourth, specimen geometry has only a small influence on the compressive strength of fiber-free UHPC (Riedel et al., 2019), while curing has a strong but mix-dependent effect (Hamada et al., 2022).

   2. Points of disagreement and possible reasons

      Fiber effect on compressive behavior. Abbas et al. (2015a) report increasing mechanical properties with fiber dosage, Wei et al. (2021) report improvement with fiber length, whereas Lande and Thorstensen (2021) report no influence of fiber content on compressive strength and modulus. Possible reasons, which are author interpretation, include differences in fiber dosage range, matrix packing and water-to-binder ratio, curing, specimen size, and whether fibers alter air content or compaction. The studies are not directly comparable, and the full texts should be consulted before drawing quantitative conclusions.

      Optimal fiber length and combination. Short fibers performed better at similar volume in one study (Abbas et al., 2015a), whereas longer fibers raised flexural strength and energy absorption in another (Yoo et al., 2016a), and hybrid combinations performed best in two further studies (Wei et al., 2021;

      Lande & Thorstensen, 2021). Plausible reasons (author interpretation) are that the optimum depends on the matrix and bond, on the size of the specimen relative to fiber length, on fiber orientation produced by casting, on whether beams or notched specimens are used, and on whether the stage evaluated is first cracking, peak load or large-deflection toughness. The consistency of the finding that first-cracking properties are little affected by fiber length (Yoo et al., 2016a; Yoo et al., 2016b) supports the view that differences arise mainly after cracking.

      Ductility versus capacity. Fibers raised the capacity of reinforced beams but lowered their ductility in one study (Yoo & Yoon, 2015), which contrasts with the general view of fibers as ductility enhancers; the difference is plausibly related to how fibers change the crack pattern in reinforced members (author interpretation).

   3. Methodological limitations of the evidence

      Several limitations restrict comparison between studies. Specimen geometries, beam sizes, notched versus unnotched tests and toughness indices differ widely. Fiber orientation is reported in only some studies, although it can dominate flexural response. Many studies are single-laboratory programmes on a single matrix, so interactions between matrix and fiber effects cannot be separated. Curing regimes vary and are sometimes incompletely reported. Loading rates are often not stated, and reliable data on rate effects in compression and flexure were not identified in this review. The evidence tables in this paper are limited to abstract-level information, which itself is a limitation of this review.

   4. Comparison with conventional concrete and HPC

      Table 6 gives a qualitative comparison of conventional concrete, HPC and UHPC, and Figure 5 shows the same comparison graphically. Strength values are included only where a source is cited; entries for conventional concrete and HPC are the authors synthesis.

      Table 6. Qualitative comparison of conventional concrete, HPC and UHPC.

      Property	Conventional concrete	High-performance concrete (HPC)	UHPC
      Matrix density	Lowest of the three; coarse	Denser than conventional	Highest; optimized packing and very low water-to-binder
      	aggregate and larger voids	concrete	ratio (De Larrard & Sedran, 1994; Richard & Cheyrezy,
      	(see Figure 1)		1995)
      Compressive strength	Lowest of the three	Intermediate	Highest; strengths of about 200 MPa or more reported (Graybeal & Hartmann, 2003; Wille et al., 2011a)
      Flexural/tensile	Low tensile capacity; brittle	Improved tensile capacity	Highest; tensile strength of about 7-14 MPa reported for
      behavior	after cracking	but generally limited	the FHWA-studied UHPC (Graybeal & Hartmann, 2003);
      		ductility	strain-hardening possible (Naaman & Reinhardt, 2006;
      			Wille et al., 2014)
      Crack control	Limited	Moderate	Fibers bridge cracks; multiple fine cracking in strain- hardening mixtures (Habel et al., 2006; Wille et al., 2014)
      Durability	Baseline	Improved relative to	Durability properties reported to significantly exceed
      		conventional concrete	HPC (Graybeal & Hartmann, 2003); no loss of
      			mechanical properties after chloride exposure in one
      			study (Abbas et al., 2015a)
      Permeability	Highest of the three	Lower	Lowest; dense, low-porosity matrix (Shi et al., 2015;
      			Wang et al., 2015)
      Fiber	Not typically used	Not typically used	Steel fibers typical (Russell & Graybeal, 2013)
      reinforcement

      Property

      High-performance Conventional concrete concrete (HPC)

      UHPC

      Typical applications

      General building and civil construction

      Structures requiring higher strength or durability (author synthesis)

      Prestressed bridge girders (Graybeal, 2008); field-cast connections between prefabricated elements (Graybeal & Stone, 2012); precast tunnel-lining segments (Abbas et al., 2015b)

      Note. HPC = high-performance concrete; UHPC = ultra-high performance concrete; FHWA = Federal Highway Administration. The comparison is qualitative; strength values are given only where a source is cited, and entries for conventional concrete and HPC are author synthesis.

      Figure 5. Conceptual comparison of conventional concrete, high-performance concrete (HPC) and UHPC for compressive strength, tensile/flexural performance, durability, crack control and matrix density. Qualitative relative ranking only, based on general trends reported in the literature; not experimental data.

9. STRUCTURL APPLICATIONS AND SUSTAINABILITY

   1. Structural applications

      Statements below combine verified findings and author interpretation, and application-specific claims should be supported by project-specific references to be added.

      • Bridges: Graybeal (2008) demonstrated that a prestressed UHPC girder could be built without mild steel reinforcement and tested to capacity, and the FHWA report reviews bridge applications (Russell & Graybeal, 2013).

      • Field-cast connections: a rapid-strengthening UHPC formulation was developed for connections between prefabricated elements, with strength gain depending on curing temperature (Graybeal & Stone, 2012).

      • Tunnel linings: UHPFRC lining segments were tested with varying fiber lengths and dosages (Abbas et al., 2015b).

      • Precast components and thin elements: high flowability and tensile capacity allow slender sections, but fiber orientation control is critical (Russell & Graybeal, 2013).

      • Repair, rehabilitation and high-rise construction: [AUTHOR INPUT REQUIRED: verified sources].

   2. Sustainability considerations

      UHPC contains much more cement per cubic meter than conventional concrete, so its per-volume embodied impact is high, but structural efficiency and durability may offset this at the structure level, which can only be shown through life-cycle assessment. Partial replacement of cement with supplementary cementitious materials and optimized packing has been investigated for reducing cement content (Yu et al., 2014; Yu et al., 2015), and a cost-effective optimization has been reported (Meng et al., 2017). Durability and potential service-life extension are widely cited advantages (Shi et al., 2015). Evidence at structure level is study-specific: in one recent cradle-to-grave analysis of twelve UHPC beams and four conventional concrete beams tested in bending, the UHPC beams showed life-cycle cost and carbon emission reductions of up to 55% and 58%, respectively, and omitting shear reinforcement lowered both further (Ghahsareh et al., 2025). These figures depend on that studys beam designs, monitoring data and maintenance assumptions and cannot be generalized. [AUTHOR INPUT REQUIRED: further verified life-cycle assessment studies.]

      The advantages and limitations discussed in this review are summarized in Table 7.

      Table 7. Advantages and limitations of UHPC.

      Aspect	Advantages	Limitations/challenges
      Mechanical	Very high compressive strength (over 200 MPa reported;	Testing is demanding because of machine capacity and end
      strength	Wille et al., 2011a) and tensile strength of about 7-14	preparation, and specimen type affects results (Graybeal &
      	MPa (Graybeal & Hartmann, 2003)	Davis, 2008)
      Durability	Durability properties reported to exceed HPC (Graybeal & Hartmann, 2003); no loss of mechanical properties	Long-term in-service data are limited [AUTHOR INPUT REQUIRED: verified long-term studies]
      	after chloride exposure in one study (Abbas et al., 2015a)
      Structural	Prestressed girder built without mild steel reinforcement	Design assumptions can be conservative for lightly
      efficiency	(Graybeal, 2008)	reinforced members (Russell & Graybeal, 2013); design
      		provisions for some applications are lacking (Abbas et al.,
      		2015b)
      Crack control	Fibers bridge cracks and can give multiple fine cracks	Post-cracking response is sensitive to fiber dispersion and
      	(Habel et al., 2006; Wille et al., 2014)	orientation (Russell & Graybeal, 2013); fibers can reduce
      		ductility of reinforced beams (Yoo & Yoon, 2015)
      Construction	High flowability and near self-placing character reported for the FHWA-studied UHPC (Graybeal & Hartmann,	Mixing and placing must be controlled to achieve fiber dispersion and orientation (Russell & Graybeal, 2013);
      	2003); rapid-strengthening formulations for field-cast	strength gain depends on curing temperature (Graybeal &
      	connections (Graybeal & Stone, 2012)	Stone, 2012)
      Cost	Possible reduction in section size and maintenance (author interpretation; requires life-cycle analysis)	High binder and fiber content generally raise material cost (author interpretation) [AUTHOR INPUT REQUIRED:
      		verified cost data]
      Sustainability	Packing optimization and mineral admixtures can reduce	High cement content per unit volume; no verified life-cycle
      	cement content (Yu et al., 2015); durability may extend	assessment is included [AUTHOR INPUT REQUIRED]
      	service life (Shi et al., 2015)

      Applications Bridge girders (Graybeal, 2008), field-cast connections (Graybeal & Stone, 2012), tunnel-lining segments (Abbas et al., 2015b)

      Guidance exists (AFGC/SETRA, 2013; ACI Committee 239, 2018) but harmonization between documents is incomplete (author interpretation)

      Note. HPC = high-performance concrete; FHWA = Federal Highway Administration.

10. RESEARCH GAPS AND FUTURE DIRECTIONS

    The following gaps follow from the evidence reviewed (author analysis):

    1. Harmonized testing and reporting protocols for post-cracking flexure, including specimen size, notch use, toughness index and fiber orientation.

    2. Quantitative links between casting-induced fiber orientation and flexural response, using validated orientation measurement.

    3. Resolution of the conflicting evidence on the effect of fibers on compressive behavior and on the optimum fiber length, through factorial programmes with matched matrices and casting.

    4. Size effects and loading-rate effects in compression and flexure, for which reliable data were not identified.

    5. UHPC with high SCM content, local materials and alternative fibers, with full mechanical and durability characterization.

    6. Life-cycle and cost assessment at structure level, and long-term durability data.

    7. Constitutive models calibrated for design, including orientation, size and rate effects.

    8. A systematic review of 2019-2026 literature, which this narrative review did not provide.

    Table 8 summarizes existing knowledge, gaps and recommended research.

    Table 8. Research gaps and future research directions.

    behavior

    Research area	Existing knowledge	Identified gap	Recommended future research
    Compressive	Conflicting reports on fiber effect (Abbas et al., 2015a;	Conversion and size effects	Inter-laboratory tests on
    Lande & Thorstensen, 2021; Wei et al., 2021); cylinder	for fiber-reinforced UHPC;	fiber-reinforced
    	and cube relations for 80-200 MPa UHPFRC (Graybeal &	reliable compressive loading-	specimens; rate-dependent
    	Davis, 2008); small geometry effect for fiber-free UHPC	rate data were not verified in	compression tests
    	(Riedel et al., 2019); over 200 MPa without heat treatment	this review
    	(Wille et al., 2011a)
    Flexural	Girder behavior (Graybeal, 2008); fiber length effects	Harmonized specimen size,	Beam tests with
    behavior	(Yoo et al., 2016a); fiber orientation effects (Yoo et al.,	test setup and orientation	orientation mapping and
    	2016b; Li et al., 2020; Maryamh et al., 2021); quasi-static	reporting	standardized reporting
    	and impact response (Yoo et al., 2018); tensile strain-rate
    	response (Pyo et al., 2015)
    Fiber	Conflicting length effects (Abbas et al., 2015a; Yoo et al.,	Systematic optimization of	Factorial programmes
    optimization	2016a); hybrid fiber systems (Wei et al., 2021; Lande & Thorstensen, 2021; Li & Deng, 2021); strength-ductility trade-off in reinforced beams (Yoo & Yoon, 2015)	length, content, geometry and orientation	with matched casting and orientation control
    Sustainable	Cement reduction through packing and mineral admixtures	Validated life-cycle data and	Combined mechanical,
    UHPC	(Yu et al., 2015); cradle-to-grave comparison of UHPC	mechanical-durability trade-	durability and life-cycle
    	and conventional beams (Ghahsareh et al., 2025)	offs	studies

    Long-term durability	No loss of mechanical properties after chloride exposure in one study (Abbas et al., 2015a); curing methods reviewed for compressive strength only (Hamada et al., 2022)	Long-term, fatigue and field- exposure data	Long-duration exposure and monitoring programmes
    Numerical	Finite element analysis with existing material models	Constitutive models including	Calibrated and validated
    modeling	compared with flexural tests (Yoo et al., 2016a)	orientation, size and rate effects	models for design
    Research area	Existing knowledge	Identified gap	Recommended future research
    Field	Girder (Graybeal, 2008), field-cast connections (Graybeal	Design provisions and long-	Instrumented field
    applications	& Stone, 2012), tunnel-lining segments (Abbas et al., 2015b)	term performance monitoring	projects and design- oriented guidance

    Note. UHPFRC = ultra-high performance fiber-reinforced concrete.

11. CONCLUSIONS

    1. UHPC compressive strength is governed primarily by matrix packing, low water-to-binder ratio and curing; strengths of about 200 MPa or more are reported (Graybeal & Hartmann, 2003; Wille et al., 2011a), and no single curing regime is optimal for all materials (Hamada et al., 2022).

    2. Evidence on whether fibers raise compressive strength is inconsistent between studies, and further matched comparisons are needed.

    3. Flexural and post-cracking behavior are governed mainly by fiber content, geometry, orientation and bond, with higher fiber content giving stiffer post-cracking response and higher peak load over the ranges reviewed (Russell & Graybeal, 2013); fiber orientation can dominate the response.

    4. Evidence on the best fiber length conflicts, and hybrid fiber systems are reported to be effective in two studies; differences are plausibly related to matrix, orientation, specimen size and test type.

    5. Comparison across studies is limited by differences in specimens, test methods, toughness indices and reporting of orientation and curing.

    6. This review is narrative, abstract-level and not systematic; its conclusions should be confirmed against full texts and extended with a systematic search before submission.

    DECLARATIONS

    Author contributions, funding, conflict of interest, data availability, acknowledgements and use of AI- assisted writing tools: [AUTHOR INPUT REQUIRED: complete according to the target journals policy; disclosure of any AI-assisted drafting or editing is required by many journals and must be made accurately]. This review reports no original experimental data.

12. REFERENCES

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