A Review of Developments and Challenges for UHPC in Structural Engineering: Behavior, Analysis, and Design
Publication: Journal of Structural Engineering
Volume 147, Issue 9
Abstract
The number of studies on ultrahigh-performance concrete (UHPC) or ultrahigh-performance fiber-reinforced concrete (UHP-FRC) for more resilient and sustainable reinforced concrete (RC) structures has rapidly increased in recent years due to the superior mechanical and durability properties of UHPC. The application of UHPC as a replacement for conventional concrete materials in various RC elements necessitates a thorough understanding of the structural behavior of the reinforced UHPC (R/UHPC) components under various types of loadings. This paper presents a systematic review of the state-of-the-art developments in R/UHPC elements. It addresses various topics including beams, columns, beam–column joints, shear walls, bridges, structural retrofitting, and applications such as seismic and impact. Due to the ultrahigh tensile, compressive, and bond strengths, unique strain-hardening behavior, and ductility of UHPC, R/UHPC structures may exhibit substantially different behavior than conventional RC structures at both the component and system levels. This implies that particular attention must be paid when design and analysis approaches for conventional RC elements are applied to R/UHPC elements, and suggests that developing new design philosophies is warranted to take advantage of UHPC’s unique mechanical properties. The present paper summarizes the developments in analysis and design approaches for R/UHPC elements under axial forces, shear forces, and bending moments, and highlights the advantages and limitations of these approaches. Important design considerations for effectively utilizing UHPC in structural elements are also suggested. In addition to normal-strength steel reinforcing bars, the potential for reinforcing UHPC with high-strength steel bars, fiber-reinforced polymer components, and structural steel shapes is discussed based on the results of pioneering studies. Finally, this paper identifies challenges and suggests future directions for research on UHPC in structural engineering.
Introduction
Ultrahigh-performance concrete (UHPC), a structural material also known as ultrahigh-performance fiber-reinforced concrete (UHP-FRC), is an emerging field. UHPC is characterized by extreme durability and ultrahigh compressive strength due to the use of a low water-to-binder (w/b) ratio [less than 0.25 according to ACI 239R (ACI 2018)], high cementitious material content (usually more than ), high fineness admixture, and optimized gradation of granular materials (Richard and Cheyrezy 1995; Rossi et al. 2005; Farhat et al. 2007; Voo and Foster 2010; Wille et al. 2011a, c; Park et al. 2012b; Aghdasi et al. 2016; Alkaysi and El-Tawil 2016b; Hung et al. 2020a). UHPC can be recognized as a special class of high-performance fiber reinforced cementitious composites, which feature pseudo-tensile elastic-plastic, or strain-hardening behavior accompanied by closely spaced narrow cracks, commonly referred to as multiple cracking response [ACI 239R (ACI 2018); Naaman 2018)]. In addition to improving tensile response, the short discontinuous fibers in UHPC (usually more than 0.75% in volume fraction) have a confining effect, similar to that of conventional confinement steel, enabling UHPC to exhibit substantially higher compression ductility, strength retention, resistance to spalling, and energy-absorption capacity, compared with conventional high-strength concrete (HSC) materials.
Despite the superior mechanical properties of UHPC, there are concerns about its use. For example, due to its very low w/b ratio and high cementitious content, UHPC may have high autogenous shrinkage and rapid surface drying (Yoo et al. 2013), which require accurate evaluation and remedies for practical applications. Particularly, steel-reinforced UHPC (R/UHPC) can be more susceptible to shrinkage cracks than nonreinforced UHPC due the restraint of reinforcing bars on the volumetric change (Park et al. 2012a).
Researchers and engineers have been extensively appraising the use of UHPC in newly built structures, structural strengthening and rehabilitation, and civil infrastructure. For example, UHPC is extensively used in bridge systems (Voo et al. 2015; Graybeal et al. 2020), including decks (e.g., Hwang et al. 2009; Shao et al. 2017), cast in-situ connections (e.g., Graybeal 2015; Tazarv and Saiidi 2015), columns (Wang et al. 2019c), and lightweight and durable bridge girders (e.g., Graybeal 2008). Moreover, because of its low permeability, UHPC has been shown to be an effective solution to the detrimental effects associated with environmental exposure (Ahlborn et al. 2011; Alkaysi et al. 2016), having less requirement for maintenance and longer service life. The use of UHPC also enables designers to create innovative structural members that have less steel reinforcement, less reinforcement congestion, and reduced cross sections while providing superior mechanical performance (Graybeal 2006; Tadros and Voo 2016; Tadros et al. 2019; Graybeal et al. 2020; El-Tawil et al. 2020).
For structural applications, UHPC members are often reinforced with steel reinforcing bars (R/UHPC) to enhance their composite behavior. While R/UHPC members generally have enhanced structural performance, their structural mechanics and failure modes can be substantially different from those of regular RC members. Notably, the ultrahigh mechanical performance of UHPC can lead to detrimental effects on the behavior of R/UHPC structural members (e.g., Hasgul et al. 2018; Hung et al. 2016; Meda et al. 2012; Aghdasi and Ostertag 2018). Therefore, the design and analytical methods for conventional RC members might not be directly applicable for R/UHPC members, and must explicitly consider the unique characteristics of UHPC. However, studies on R/UHPC structural members are diverse and not yet comprehensive enough to yield unified design and analytical approaches with consistent accuracy. In the context of safe deployment of R/UHPC, it is essential to quantify the safety margin for the design provisions for R/UHPC members. It is also of great importance to identify the critical factors influencing the structural behavior of R/UHPC members. Thus, the objective of this study is to present a systematic and critical review of the state-of-the-art knowledge about R/UHPC structural members, with a focus on structural behavior and design and analytical approaches. This review is also positioned to promote the optimized use and design of R/UHPC and assist researchers in determining the necessity of potential future studies.
ACI 239R (ACI 2018) defines UHPC as a fiber-reinforced concrete that has a minimum compressive strength of 22 Ksi (150 MPa) with “specified durability tensile ductility and toughness requirements.” Given the range of properties observed in the literature, and to ensure sufficiently broad coverage of existing research efforts, the results reviewed herein pertain to concretes that, unless otherwise mentioned, had the following characteristics: (1) compressive strength in excess of 120 MPa [ASTM C1856/C1856M (ASTM 2017)]; (2) strain-hardening behavior under uniaxial tension [ACI 239R (ACI 2018)]; and (3) steel fibers that are 13–30-mm-long. The definitions of conventional, normal-strength, and high-strength concretes in this manuscript are based on ACI-318 (ACI 2019). Conventional and normal-strength concretes have a specified compressive strength of less than 55 MPa, while high-strength concrete has a specified compressive strength greater than 55 MPa.
Material Behavior
In the past, UHPC carried high material and construction costs, hindering wide-scale adoption in structural applications. Developments in the past decade have led to open-source formulations that are more economical than proprietary versions and perform just as well. One of the earliest open-source mixes in the United States was published by Wille et al. (2011a, b, c). This was subsequently followed by a number of related and other mixes documented in Wille et al. (2012), Wille and Boisvert-Cotulio (2013), Alkaysi et al. (2016), Berry et al. (2017), El-Tawil et al. (2018), Alsalman et al. (2018), Tai and El-Tawil (2020), Mendonca et al. (2020), and El-Tawil et al. (2020). These mixes are generally made of components readily available on the US open market and do not require any special mixing or placing equipment.
When properly formulated and reinforced with fibers, UHPC can display compressive and direct tensile strengths as high as 255 MPa and 37 MPa, respectively (Wille et al. 2011b, 2014). Changes in the type and quantity of fibers directly affect the ductility and strength of the material (Pyo and El-Tawil 2015; Pyo et al. 2016). UHPC also exhibits exceptional toughness prior to crack localization and self-consolidation properties (Pyo and El-Tawil 2015), and generally develops a high early strength in the range of 70–95 MPa within 24 h (Karmacharya and Chao 2019).
The compressive and tensile responses of UHPC are quite different from those of regular concrete, and must be adequately modeled for realistic analysis or design. In general, UHPC exhibits an almost linear compressive stress–strain response up to the strain at the peak stress. Strain softening usually commences right after the peak is reached and the descending slope is controlled by the amount and type of fiber reinforcement. Fig. 1(a) shows a compressive stress–strain curve proposed by Sritharan et al. (2003) for analysis and design purposes, compared with the experimentally measured response in Acker and Behloul (2004).
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As a fiber-reinforced cementitious material, UHPC resists tensile stress through composite action between the matrix and embedded fibers. The transmission of forces between these two components occurs through interfacial bond. After cracking, fibers bridge the cracks, providing resistance to crack opening and enhancing structural behavior and durability. As shown in Fig. 1(b), UHPC’s tensile response can be generally characterized by an elastic portion, followed by strain-hardening, a plateau, and then a long strain-softening phase. Aaleti et al. (2013) proposed the idealized tensile response shown in Fig. 1(b) for analysis and design purposes.
The linear portions of the tensile and compressive regimes are characterized by an elastic modulus, . Several equations have been proposed to link the elastic modulus to compressive stress. For example, Sritharan et al. (2003) proposed [], while Garcia and Graybeal (2007) used a similar equation but with a slightly reduced coefficient (3,835 instead of 4,150 or 46,200 instead of 50,000). Rather than providing an explicit equation, ACI 239R (ACI 2018) just lists a range of values from 6,000–7,200 Ksi (40–50 MPa).
The ACI 318 (ACI 2019) and AASHTO codes (AASHTO 2020) use a strain of 0.003 as the crushing strain (or maximum design compressive strain), , at a postpeak compressive stress of for plain concrete. Chao et al. (2019) observed of 0.015 and 0.003 for UHPC with 3% microsteel fiber by volume and plain concrete, respectively, in large-scale beam testing, where the strains were measured by a digital image correlation (DIC) system. The high compressive strain capacity of UHPC is not unusual for a high-performance fiber-reinforced cementitious material, as noted by Naaman (2018). The discrepancy between the response depicted in Fig. 1(a) and values noted in Chao et al. (2019) is attributed to the differences in UHPC fiber content. Further research is needed to specify values suitable for design.
Longitudinal reinforcement is typically used in UHPC structural members subjected to bending. Previous research shows that, because of their tension-stiffening effect, reinforcing bars or prestressing strands used in structural members enhance the cracking distribution and tensile ductility of fiber-reinforced concrete. Fig. 2 illustrates the results of an investigation by Aghdasi et al. (2016), which provides the total (UHPC + #10M rebar) and pure (UHPC only) tensile stress–strain curves, as well as the tensile stress–strain curve of UHPC specimen with no rebar. The results indicate that, while the tensile strength remained nearly the same (7.7 MPa), the presence of rebar considerably enhanced the tensile ductility of the UHPC. In the UHPC specimen, tensile strain-hardening was maintained up to a strain of 1.3%, nearly 7.5 times larger than the specimen with no rebar.
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To date, most of the studies on UHPC material response have focused on monotonic behavior. There is a marked scarcity of data on high- and low-cycle fatigue loading. Although initial indications are that the high-cycle fatigue resistance of UHPC is extremely high (Ocel and Graybeal 2007; Fitik et al. 2008, 2010; Carlesso et al. 2019), future research studies are needed to confirm this finding and fully characterize the tensile and compressive response of fatigue-loaded UHPC, especially in high-demand applications such as wind towers and bridges. To the knowledge of the authors, there are no studies that have developed models of the low-cycle response of UHPC, and therefore research into this area is urgently needed.
In addition, unlike plain concrete, because UHPC’s tensile capacity may be largely utilized in the strength design of a UHPC structural member, its long-term behavior can have an impact on the performance of the member. It has been shown that UHPC can experience tensile creep under long-term loading; however, tensile creep of UHPC can be decreased approximately 65% when thermal treatment of 60°C (140°F) for 72 h or 90°C (194°F) for 48 h is applied, respectively, prior to loading (Garas et al. 2010). Further research is warranted to investigate this effect on the long-term performance of UHPC members. In compression, UHPC is known to have much less creep than conventional concrete. As in tension, the use of heat treatment appears to decrease creep even further (Russell and Graybeal 2013).
Steel-Reinforced UHPC Structural Members
Bond between UHPC and Steel Reinforcing Bars
Sufficient bond strength between reinforcing bars and concrete is required to ensure full composite action in reinforced-concrete members. The high bond strength between UHPC and reinforcing bar represents an effective solution to reduce the required development length (Harajli 2009; Graybeal and Yuan 2014; Bandelt and Billington 2016). Due to its simplicity, the direct tension pullout test has generally been adopted to evaluate the bond strength of UHPC. Only a limited number of studies performed an evaluation of beams with lap splices, which are commonly believed to provide more realistic assessments of the bond strength in actual structural members [ACI 408R (ACI 2003)].
Similar to the bond strength of conventional reinforced concrete, the use of a longer embedment length, larger side cover, and reinforcing bars with a smaller diameter lead to a greater bond strength between the reinforcing bar and UHPC (Graybeal 2010; Graybeal and Yuan 2014; Fehling et al. 2012; Ronanki et al. 2016). Graybeal (2010) and Graybeal and Yuan (2014) performed direct tension pullout tests to investigate the bond performance of deformed reinforcing bars embedded in UHPC. The test setup was developed to simulate the tension–tension lap splice configuration that may be encountered in a field-deployed closure connection system. The test results indicated that noncontact lap splice specimens had higher bond strength than contact lap splice specimens because the tight spacing limited the ability of the fiber reinforcement. In addition, the influence of the mechanical properties of UHPC on the bond strength cannot be accurately represented by relying solely on the compressive strength or the square root of the compressive strength. Other mechanical properties of UHPC can contribute to the bond strength of reinforcing bars in UHPC, such as casting position, embedment length, side cover, and bar size, spacing, and strength.
Fehling et al. (2012) verified that the bridging effect of the fibers across the splitting cracks provided confinement that enhanced the postpeak pullout behavior of reinforcing bars. Lagier et al. (2015) performed direct tensile tests on lap splice specimens to investigate the influence of fiber content on the bond strength of UHPC; the study found that the tensile strain-hardening behavior of UHPC considerably improved the bond performance and splitting crack control. Longer splice length led to greater bar stress regardless of the fiber content. For a given splice length of , an increase in the fiber content () from 1% to 2% and 4% considerably increased the ultimate bond stress by 29% and 53%, respectively. UHPC with had an average bond strength of 10 MPa for 25- and 35-mm deformed steel bars with a splice length, where is the diameter of the longitudinal reinforcing bar being developed or spliced. In addition, it allowed 25-mm deformed bars to fully develop the yielding strength of 420 MPa when the splice length was .
Alkaysi and El-Tawil (2016a) studied the bond between UHPC and steel reinforcing bars using the bar pullout setup developed by Chao et al. (2009). Their test results showed that reducing the fiber content from 2% to 1% resulted in a reduction of approximately 24% in the bond strength. Alkaysi and El-Tawil (2016a) also indicated that the assumption of uniform bond stress distribution along the bonded length of the reinforcing bar within UHPC may not be valid.
Based on beam and pullout tests, Ronanki et al. (2016) suggested a minimum embedment length of in conjunction with a minimum concrete cover of , which is required to develop the yield strength of #13M to #22M 420-MPa reinforcing bars embedded in UHPC. Čítek et al. (2016) explored the potential of the high bond strength of UHPC for reducing the required concrete cover for steel reinforcing bars embedded in UHPC. The test results demonstrated that a small concrete cover of allowed the embedded 12-mm diameter steel bar to yield without premature bond failure.
R/UHPC Tensile Members
The results of uniaxial tensile tests on R/UHPC members conducted by Hung et al. (2019) showed that the high tensile strength of UHPC can contribute substantially to the tensile strength of R/UHPC. The inclusion of a 2% of hook-ended steel fibers (30-mm-long) enhanced the tensile strength of the R/UHPC members with a longitudinal reinforcing ratio by 50%–100%. It also increased the tensile stiffness of R/UHPC in the range of 100%–500%. This was attributed to the bridging effect provided by the steel fibers that helped the embedded reinforcing bar transfer tension across the cracks. Notably, although the inclusion of steel fibers in conventional RC also enhances the tensile resistance, the enhancement was not as substantial as that for R/UHPC due to the better bond mechanism between the reinforcing bar and the UHPC (Lee et al. 2013).
Hung et al. (2019) indicated that the bond interaction between UHPC and the embedded reinforcing bar influenced the tensile performance of R/UHPC. The enhanced bond strength (due to the inclusion of steel fibers) transformed the tensile failure pattern of R/UHPC from multiple localized cracks into a single localized crack; this aggravated the strain localization in the embedded steel bar and, consequently, reduced the yield strain and ultimate strain of R/UHPC but did not affect the ultimate strength. Notably, the strain localization became detrimental and caused premature rupture of steel bars within R/UHPC members, especially for smaller-diameter reinforcing bars that possessed a higher bond strength. The test results indicated that the rupturing strains of the #5(D16), #6(D19), #7(D22), and #8(D25) steel bars embedded in a UHPC tension member could be as low as 0.3%, 4%, 7%, and 8%, respectively. Similar observations were reported by Aghdasi and Ostertag (2018), who recommended a minimum longitudinal reinforcement ratio of 2.0%–4.0% for UHPC to prevent a brittle structural behavior due to localized rebar elongation, early strain-hardening, and eventually premature fracture of the reinforcing bar at the location of the macrocrack. Hung et al. (2016) suggested a method to alleviate such premature failure for flexural members by moderately reducing the bond strength of the rebars in the plastic hinge region. However, this method must be carefully performed after deliberate design to ensure that it does not overly aggravate other performance degradation mechanisms, such as significant bond slip and insufficient strength development in reinforcing bars, which could sabotage the structural performance of R/UHPC.
Makita and Brühwiler (2014) investigated the tensile fatigue behavior of R/UHPC through uniaxial constant amplitude tests. The results indicated a fatigue endurance limit at 10 million cycles when the maximum fatigue force was about half of the ultimate strength. In the early stage of the test, the fatigue behavior was mainly dominated by the UHPC. In the middle and final stages of the fatigue test, it was governed by the steel bars because the steel bars improved the fatigue force-bearing capacity of UHPC by distributing the fatigue stress. The study emphasized the potential of R/UHPC for strengthening the fatigue behavior of RC members.
Hung et al. (2019) proposed a simple tetra-linear model for reasonably representing the tension-stiffening behavior of UHPC up to failure. The required parameters for the model are the tensile strength of the UHPC material and the yield strength of the bare reinforcing bar. The model can be implemented in conjunction with fiber-based sectional analyses (Hung et al. 2016) and smeared crack modeling (Hung and El-Tawil 2010, 2011; Hung and Li 2013) for predicting the tensile and flexural behavior of R/UHPC structural members.
R/UHPC Beams
Flexural Behavior and Design
Reinforcement Ratio
A number of studies experimentally investigated the effect of longitudinal reinforcement ratio (, up to 7%) on the flexural behavior of R/UHPC rectangular beams (Yang et al. 2010; Yoo and Yoon 2015; Yoo et al. 2016; Hung and Chueh 2016; Chen et al. 2018a; Hasgul et al. 2018; Shao and Billington 2019; Turker et al. 2019). These test results concluded that the performance indicators of R/UHPC beams—including ductility, postcracking stiffness, cracking pattern, and flexural strength—all improved with a higher longitudinal reinforcement ratio up to about 5% (Li 2016; Turker et al. 2019), which was significantly higher than the maximum allowable reinforcement ratio in current design codes. A higher reinforcing ratio, however, was found to reduce the cracking moment of R/UHPC beams (Yoo et al. 2016). According to the results obtained by Yang et al. (2010) and Chen et al. (2018a), R/UHPC beams exhibited a ductility index greater than 3.4 when the reinforcement ratio was higher than 1.1%. Shao and Billington (2019) experimentally demonstrated that increasing the longitudinal reinforcing ratio from 0.96% to 2.10% prevented the fracture of reinforcing bars in R/UHPC flexural members and improved the drift at the peak load from 1.1% to 4.2%.
Fiber Content and Type
Existing studies have consistently indicated that the flexural strength of R/UHPC beams increases along with the fiber content () (Khalil and Tayfur 2013; Yoo and Yoon 2015; Kahanji et al. 2017; Hasgul et al. 2018). However, enhancement in the flexural strength due to inclusion of fibers gradually decreased from 50% to 23% as the longitudinal reinforcement ratio increased from 0.9% to 4.3% (Hasgul et al. 2018). Compared with the nonfiber R/UHPC beams, Yoo and Yoon (2015) showed that, although adding 2% of steel fibers enhanced the flexural strength by 27%–54%, it reduced the ductility by 13%–73% due to crack localization, as discussed earlier.
Khalil and Tayfur (2013) demonstrated that crimped and hooked steel fibers ( and aspect ) at a fiber content of 1% performed similarly well in enhancing the flexural strength by approximately 25% compared with beams without fibers. Compared with smooth steel fibers, while the use of twisted steel fibers led to similar enhancement in the flexural strength, it was more effective in enhancing the postpeak response and ductility (Yoo and Yoon 2015).
Yoo et al. (2014, 2016) investigated the effect of straight steel fibers with different lengths (, 16.3, 19.5, and 30 mm) on the flexural behavior of R/UHPC beams with a constant fiber volume of 2%. The results showed that the flexural strength, deflection capacity, and energy absorption capacity of R/UHPC beams increased along with the fiber length up to 19.5 mm; increasing the fiber length from 19.5 to 30 mm reduced the flexural performance of R/UHPC beams due to the considerably lower amount of fibers across the crack planes. In terms of the cracking strength of R/UHPC beams, the difference in length of fibers did not have any noticeable influence.
Placing Methods
Yang et al. (2010) and Yoo et al. (2014) investigated the effect of placement method on the flexural behavior of simply supported UHPC beams with three- and four-point bending test setups. The UHPC material contained 2% of smooth fibers and had self-consolidating characteristics. The beam specimens were made by pouring UHPC at the center or the end of the beam mold and allowing it to flow until the mold was filled with UHPC. The results demonstrated that the beams with UHPC placed in the center (maximum moment region) exhibited higher cracking and ultimate strengths than those with UHPC placed in the corner, regardless of the fiber length (, 16.3, 19.5, and 30 mm). The image analyses from these studies confirmed that the beams with UHPC placed in the corner had poorer fiber dispersion and fewer fibers across crack planes than those beams with UHPC placed in the center (Yoo et al. 2014).
Flexural Analysis and Design
The flexural failure modes of R/UHPC members can be controlled by the crushing of UHPC and excessively localized flexural cracks (Shao and Billington 2019). Among these two distinct failure modes, the crushing in the extreme fiber of the beam section (due to the full utilization of the ultrahigh compression strength of UHPC) has only been observed in a limited number of studies (Li 2016; Stürwald 2018; Turker et al. 2019; Shao and Billington 2019). Notably, the four-point bending tests on R/UHPC flexural members performed by Li (2016) demonstrated that, even though the longitudinal reinforcing ratio was increased to as high as 4.6%, substantially localized flexural cracks developed and extended upward to the flexural compression zone prior to significant concrete crushing, as shown in Fig. 3. This occurred because the high compressive strength and peak strain of UHPC effectively delayed and restrained the crushing of UHPC in the flexural compression zone.
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R/UHPC flexural members in the literature were often reported to fail due to localized cracks (usually one or two) with the absence of crushing in the compression zone of the section (Meade and Graybeal 2010; Li 2016; Yang et al. 2010; Yoo et al. 2017a; Chen et al. 2018a; Hasgul et al. 2018; Stürwald 2018). For this failure mode, the loss in load-carrying capacity was attributed to the rupture of reinforcing bars and/or the reduced effective beam depth due to the excessively extended localized cracks. For such a failure mode, the maximum compressive strain was reported to be as low as 0.001 (Yoo et al. 2017a), implying that the ultrahigh compressive strength of UHPC was not fully utilized. In addition, this failure pattern usually resulted in a low deformation capacity (Meade and Graybeal 2010; Yang et al. 2010; Yoo et al. 2017a). Fig. 4 presents the difference in flexural cracking between reinforced HSC and UHPC beams with a longitudinal reinforcing ratio of 1.2% (Yang et al. 2020). It shows the microcracking under service loads, and then localization in one crack in UHPC versus several localized cracks in HSC.
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In the flexural strength model for R/UHPC members, the constitutive relationship of UHPC in compression can be represented using a parabolic function suggested by Hognestad (1951) and JSCE (2008) or simplified as a linear stress–strain response (Li 2016; Haber et al. 2018). The stress–strain response of UHPC in tension is often modeled using the elastic-perfectly-plastic behavior with the tensile strength being the localization stress, as shown in Fig. 1(b) (JSCE 2008; AFNOR 2016; Baby et al. 2017; Yoo et al. 2017a; Shao and Billington 2019). Yoo and Yoon (2015) implemented sectional analyses to predict the flexural capacity of R/UHPC beams that had a failure mode controlled by the rupture of steel reinforcing bars. The sectional analysis incorporated a proposed fiber orientation coefficient and the material models obtained by the inverse analyses of experimental results. The error of the predicted strength was less than 20% for the tested R/UHPC beams.
Shao and Billington (2019) showed that, while the maximum compressive strain measured on the deflected R/UHPC beam () was 0.0014 when the reinforcing ratio was 0.96%, it was as high as 0.0065 when the reinforcing ratio was increased to 2.1%, which was considerably greater than the peak strain values obtained by the UHPC cylinder tests [usually between 0.0027 and 0.0052 for UHPC-class materials (Haber et al. 2018)]. Therefore, the traditional strength-based prediction methods for RC members do not directly apply to R/UHPC members because of the different limit states, e.g., the maximum compressive concrete strain at the peak load.
Shao and Billington (2019) proposed a flexural failure path prediction method and a minimum reinforcing ratio equation for R/UHPC beams, aimed to prevent the use of R/UHPC beams with low ductility. In their model, when the failure mode was governed by crack localization, the tensile capacity of UHPC was assumed to act over the entire tension zone of the section, and the nominal moment capacity was assumed to occur when the tensile reinforcement strain reached the crack localization strain of UHPC. This approach is distinguished from other strength prediction methods where the strength was calculated assuming that the UHPC in the extreme compression fiber of the section has reached the specified ultimate strain (JSCE 2008; AFNOR 2016; Hasgul et al. 2018). For the failure mode controlled by the crushing of UHPC, Shao and Billington (2019) assumed that the nominal moment capacity occurred when the strain in the extreme compression fiber of the beam section reached the crushing strain of the UHPC. In such case, the tensile capacity of UHPC was neglected in the tension zone of the section. This approach also differed from other strength prediction methods (JSCE 2008; AFNOR 2016; Hasgul et al. 2018) where the tensile resistance of UHPC is active in the tension zone. Shao and Billington (2019) tested a model that underestimated the flexural strength of the R/UHPC member by 6% and 14% when the failure mode was governed by localized cracks and UHPC crushing, respectively.
By fully utilizing the high-strength and high compressive ductility of UHPC developed by Aghdasi et al. (2016), Chao et al. (2019) showed that a UHPC flexural member can demonstrate not only very high flexural strength, but also has extremely high curvature ductility, thus allowing a member to exhibit large displacement ductility even with a high longitudinal reinforcement ratio. A high amount of reinforcement ratio keeps rebar deformation small, thereby minimizing crack width, which in turn maintains the member’s stiffness, the compression zone, aggregate interlock, and dowel capacity for shear transfer, as well as bond strength. In addition, while the concrete’s cracking strength can hardly be considerably increased in a typical RC member (because the longitudinal reinforcement ratio is kept low to maintain tension-controlled behavior), experimental testing carried out by Chao et al. (2019) showed that a high reinforcement ratio can significantly increase the first cracking strength of concrete; the reason is that, when the reinforcement ratio reaches a certain critical threshold, reinforcing bars can effectively carry the force and prevent microcracks from growing and interconnecting to form a percolation crack.
Shear Behavior and Design
Many studies have been performed to evaluate the shear behavior of R/UHPC. The results commonly indicated that a higher fiber content increased the ultimate shear strength, ductility, and crack-width control ability of R/UHPC due to the improved tensile strength and ductility of UHPC at the material scale (Baby et al. 2014a, b; Yousef et al. 2018; Pourbaba et al. 2018; Wu et al. 2019). While steel fibers are a significant contribution to the ultimate shear strength of R/UHPC beams (Lim and Hong 2016; Hung and Wen 2020), the presence of stirrups is important to enhance the postpeak ductility of R/UHPC members, although it may cause blockage of fibers (Baby et al. 2010). Shear strength equations in ACI 318 (ACI 2014, 2019), Eurocode 2 (CEN 2004), and RILEM TC 162-TDF (RILEM 2003) were found to significantly underestimate the shear capacity of R/UHPC beams by more than 50% (Yousef et al. 2018; Pourbaba et al. 2018; Hung and Wen 2020).
Because the ultrahigh strength of UHPC enables considerable reduction of web thickness of beams, earlier investigations on the shear behavior of R/UHPC were focused on UHPC I-shaped beams or girders without stirrups. Graybeal (2006) experimentally demonstrated that the shear capacity for the AASHTO Type II prestressed UHPC girders without stirrups and draped strands could be examined by assuming that all shear forces were carried by diagonal tension and compression in the web. Other studies (Maguire et al. 2009; Russell and Graybeal 2013) also showed that the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specification (AASHTO 2008) for the shear design of I-girders were applicable to UHPC girders.
Baby et al. (2014a, b) investigated the shear performance of prestressed and reinforced UHPC I-shaped girders. The study indicated that structural softening behavior of shear-critical UHPC I-shaped girders occurred when the principal stress in the beam reached the tensile localization strain of UHPC. The shear capacity and shear-crack control ability of UHPC I-shaped girders were influenced by the uniaxial tensile behavior, fiber orientation, prestressing force, and shape of the beam cross section. It was also shown that the shear design recommendations of AFGC UHPFRC Design Guidelines (2002) were overly conservative for R/UHPC, with an actual-to-predicted strength ratio of 1.85 and 2.27 for the prestressed and reinforced R/UHPC beams, respectively.
Voo et al. (2010) experimentally showed that when prestressed UHPC I-shaped beams were subjected to shear, multiple narrow shear cracks occurred in the web prior to the formation of localized cracks. The test results also indicated that longitudinal shear failure may be the critical design condition for prestressed UHPC I-shaped beams with a significant moment gradient. Randl et al. (2018) studied the effect of fiber content (up to ) and stirrups on the shear failure behavior of I-shaped single-span UHPC beams. The addition of a 1%–2% of steel fibers led to finer distributed shear cracks and significantly higher shear strength, stiffness, and ductility. The obtained shear strengths of the beams were underestimated by the shear strength equation in the AFGC UHPFRC design (2013) by about 20%.
Lim and Hong (2016) showed that a higher stirrup ratio reduced the shear crack width and enhanced the shear deformation capacity of R/UHPC structural beams. Pourbaba et al. (2018) showed that R/UHPC members with an ratio of 0.9–1.8 and a reinforcement ratio of 2.2%–7.8% had normalized shear strength 70% higher than that of normal RC members ( is the shear span-to-depth ratio). The results also implied that the dowel effect of the longitudinal reinforcement had little effect on the shear capacity of short R/UHPC beams. Yousef et al. (2018) studied the shear strength of R/UHPC deep beams ( ratio less than 1) with a volume fraction of stirrups of 0.5%–1.7%. The study indicated that a higher amount of stirrups only had a slight effect on the diagonal cracking strength and the ultimate shear strength of R/UHPC deep beams. The ultimate shear strength of the tested R/UHPC deep beams was greatly underestimated by about 70% by ACI 318 (ACI 2014) and Eurocode 2 (AFNOR 2004), and by about 50% by KCI (2012) and AFGC (2013). Lim and Hong (2016) indicated that the stirrup spacing limit required by ACI 318 (ACI 2014) was conservative for R/UHPC beams with an ratio of 3.0, and suggested that it be relaxed to for R/UHPC beams.
Bermudez and Hung (2019) investigated the synergy of 30- and 60-mm-long hook-ended high-strength steel fibers in R/UHPC slender members without stirrups. The combination of short and long steel fibers was found to be more effective than monofibers in transferring the internal force through localized cracks and delaying the shear failure. In their study, the UHPC beam with 0.75% short fibers and 1.5% long fibers exhibited the highest synergy in enhancing the shear strength of the R/UHPC beam than other combinations of fibers at the same of fibers.
In AFGC (2013), UHPC is assumed as a homogeneous material, and the shear contribution of steel fibers is estimated by multiplying the uniaxial tensile strength of UHPC and the area of the critical diagonal crack surface. It is worth noting that, while the AFGC recommendation reasonably evaluated the shear capacity for the four R/UHPC beams having an ratio of 3.0 tested by Lim and Hong (2016) (with an average actual-to-predicted shear strength ratio of 1.15), it resulted in overconservative prediction for R/UHPC deep beams tested by Yousef et al. (2018), implying its inability to account for the effect of ratio on the shear strength of R/UHPC.
Voo et al. (2010) showed that the upper-bound plasticity modeling approach reasonably predicted the shear strength of R/UHPC beams. Qi et al. (2017) developed a mesoscale fiber-matrix discrete model for estimating the shear contribution of steel fibers and predicting the shear strength of R/UHPC beams. This model featured the effective fiber-distributed region along the critical diagonal shear crack, only where fibers were efficient at providing shear resistance. The model accounted for fiber orientation, embedment length, and bond strength between the fibers and matrix. The proposed model yielded reasonably accurate results for their tested R/UHPC beams with , but the error was increased to more than 35% when the was reduced to 2.5.
Past studies have demonstrated that UHPC structural members have a very high shear strength, which enables a large reduction in the use of shear reinforcement. Bermudez and Hung (2019) and Hung and Wen (2020) reported the shear strengths of 39 large-scale UHPC structural beams without stirrups. The test results indicated that the influence of the fiber content and coarse aggregate on the shear strength of UHPC beams were dependent on the ratio of the beam. Fig. 5 summarizes the normalized shear strengths of the UHPC beams reported by Hung and Wen (2020). Despite the absence of stirrups, the shear strength for the slender UHPC beam with simultaneous addition of coarse aggregate and a 1.5% of hook-ended steel fibers (30-mm-long) was as high as , which was about 10% higher than the maximum allowable design shear, , for RC members stipulated by ACI 318 (ACI 2019). For the long UHPC members that had no coarse aggregate (which is often used in practice and research studies), their shear strengths were and after the inclusion of 0.75% and 1.5% of fibers, respectively. These results were 3–5 times higher than the shear strength of plain concrete according to ACI 318 (ACI 2019). Overall, the test results demonstrated the great potential to eliminate or significantly reduce the shear reinforcement in UHPC members.
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Beams without stirrups were tested by Chao et al. (2019) using an ratio of 4.75. The test indicated that UHPC beams (3% microsteel fibers by volume and without coarse aggregate) had a shear strength of approximately 4.7 MPa or . The specimens failed due to shear-tension failure (bond splitting failure along the longitudinal rebars), which is commonly seen in slender beams without stirrups; these results generally follow the trend shown in Fig. 5. It should be noted that several major UHPC design guidelines, including AFGC (2013) and KCI (2012), express the shear strength of UHPC in terms of the square root of compressive strength to facilitate direct comparisons with the shear expression adopted by current RC design codes and integration of UHPC into today’s design practice. However, the adequacy of such expression of the UHPC shear strength requires further justification.
A large body of research conducted on the shear behavior of reinforced concrete and fiber-reinforced concrete slender beams with no stirrups has demonstrated the importance of size effect on ultimate shear strength (Chao 2020). Because R/UHPC members may be designed without shear reinforcement, the potential size effect in their ultimate shear stress capacity warrants future study.
Columns
Design codes require RC columns of special moment frames to have a dense array of transverse reinforcement to ensure a ductile failure mode under extreme load conditions [e.g., ACI 318 (ACI 2019)]. This requirement, however, results in steel congestion and construction difficulty. Due to the superior mechanical and high damage-resistant properties of UHPC, it is promising to use UHPC in structural columns to simplify the reinforcement design while sustaining the preferred column behavior.
Axial Compression
Prior studies (Shin et al. 2017, 2018; Hung et al. 2018; Hung and Yen 2021) have demonstrated that the replacement of conventional concrete with UHPC in short and slender reinforced columns allowed for reduction in the confinement steel without compromising the compressive behavior of the columns due to the confinement effect provided by hook-ended steel fibers (30-mm-long). The bridging effect provided by the steel fibers at a low volumetric fraction of 0.75% was shown to effectively alleviate spalling and crushing and prevent buckling of longitudinal reinforcing bars for R/UHPC columns (Hung and Yen 2021). This improved damage pattern led to significantly higher peak axial strength of the columns (about 50% higher on average). Hung and Yen (2021) indicated that the transverse reinforcement and fiber content had little influence on the axial stiffness of R/UHPC columns. In contrast, including coarse aggregate in UHPC columns substantially enhanced the axial stiffness by about 90% without compromising the peak compressive strength of R/UHPC columns. Shin et al. (2018) investigated the compressive behavior of R/UHPC circular columns confined by spiral reinforcement. The results demonstrated that the circular spirals led to considerably more ductile behavior of R/UHPC columns than the rectilinear hoops at the same volumetric ratio. The R/UHPC columns confined by closely spaced spiral reinforcement maintained about 80% of the axial capacity up to a high axial strain beyond 0.02.
Similar to the procedure for evaluating the compressive strength of conventional RC columns, the equivalent stress block factor for R/UHPC columns was suggested in different studies (Shin et al. 2017; Hung and Yen 2021). Shin et al. (2017) showed that the equivalent stress block factor was about 0.85 for R/UHPC columns, whereas the equivalent factor obtained by Hung and Yen (2021) was considerably lower than 0.85. This difference might be attributed to the size effect associated with the small cross section of the columns tested by Shin et al. (2017). Hung and Yen (2021) assessed the applicability of the strength equation in ACI ITG-4.3R (ACI 2007) for evaluating the compressive strength of R/UHPC columns. Their results demonstrated that the ACI ITG-4.3R equation was able to reasonably estimate the peak compressive strength for the R/UHPC columns in the literature, with an average underestimation of less than 10%. Shin et al. (2017, 2018) suggested design equations for determining the amount of seismic confinement for R/UHPC columns. Hung and Yen (2021) assumed that the cross-sectional area of the effectively confined UHPC core was dependent on the steel fibers and transverse reinforcement, and proposed a model for estimating the confined UHPC compressive strength. The developed model was capable of reasonably estimating the confined UHPC strengths of 26 UHPC columns, with an average error of 1%.
Interaction of Axial Compression, Moment, and Shear
Although using UHPC to replace conventional concrete in RC columns permits considerable reduction in the cross sections of columns, the increased slenderness effect could significantly magnify the P-Delta effect, which may result in reduced load-carrying capacity in the slender R/UHPC columns. Hung et al. (2018) conducted a series of tests on slender R/UHPC columns with both ends hinged and under eccentric loading to failure. The results showed that the inclusion of hook-ended steel fibers (30-mm-long) with in the slender R/UHPC columns led to an increase in crushing strain from 0.0025 to 0.0036. Moreover, although the fiber-reinforced UHPC column had 70% less confinement steel, its load-displacement relationship was comparable to that of the nonfiber UHPC column, with the reinforcement amount satisfying ACI 318 (ACI 2014). The ACI 318 magnified approach generated acceptable estimations for the total moment demand of the tested slender R/UHPC columns with an error of less than 6%.
Xu et al. (2017) studied the behavior of 14 R/UHPC columns under lateral displacement reversals. The columns had cross sections of , an ratio between 3.3 and 5.4, 2.0% hybrid microsteel fibers, and an axial load ratio between 0.20 and 0.36. The results showed that all R/UHPC columns exhibited satisfactory drift capacity between 3.6% and 6.1%, and even the transverse reinforcing ratio in some cases was as low as 0.6%. Ichikawa et al. (2016) explored the use of UHPC in the plastic hinge of RC columns. In this study, the plastic hinge region was fabricated with either an RC core encased in a UHPC jacket (RC-UHPC) or a UHPC hollow core combined with posttensioning (PC-UHPC). The results of bilateral tests on the columns showed that both behaved in an evident rocking mode. The RC-UHPC and PC-UHPC columns carried the axial load until the drifts of 6.5% and 3.5%, respectively. The lower drift capacity of the PC-UHPC column was mainly due to the significant torsional effect.
Chao et al. (2016) experimentally evaluated the seismic behavior of a full-scale RC column with its plastic hinge region cast using UHPC (the RC-R/UHPC column) and a reference ACI 318 compliant RC column that was entirely cast using regular concrete. The UHPC mixture consisted of straight microsteel fibers that were 12.5-mm-long with a tensile strength of 2,200 MPa and a fiber dosage of 3% by volume. They reported significant concrete crushing and bar buckling in the RC column, but no visible damage was detected in the RC-R/UHPC column up to 5.25% drift ratio. The ultimate failure of the RC-R/UHPC column was due to low-cycle fatigue of the longitudinal reinforcement at the interface between the footing and the column section. These test results show that columns made of UHPC materials can significantly reduce the need for postearthquake repair (i.e., great resilience capability).
Chan et al. (2020) proposed a UHPC connection for precast columns in regions of medium and high seismic activity. By taking advantage of the high bond strength of UHPC, the longitudinal reinforcing bars from the precast column were lapped with the footing dowel bars with a short splice length. Even though no shear reinforcement was deployed within the UHPC connection, the connection performed well, and the columns demonstrated lateral capacity and ductility that were similar to or better than cast-in-place RC columns.
Beam-Column Joints
The confinement effect and high shear and bond strengths of UHPC enable substantial simplifications in the design of reinforcement for beam-column joints. Yet, to date, only a limited number of studies have investigated the behavior and design of R/UHPC beam-column joints.
Wang et al. (2018a) tested 13 half-scaled exterior and interior beam-column subassemblages with R/UHPC joints, which were detailed to show a shear-dominant failure mode. Similar to the effect of axial load on regular RC joints, greater axial load enhanced the cracking strength and shear capacity of R/UHPC joints due to the larger column compression zone. It was also shown that the use of stirrups enhanced the shear-carrying capacity owing to the restraint of crack development. Wang et al. (2018a) proposed a shear strength model to evaluate the shear strength of R/UHPC beam-column joints. It was assumed that the shear strength of R/UHPC joints consists of three shear components: diagonal strut, fibers, and stirrups. The model yielded an actual-to-predicted strength ratio between 0.75 and 1.14 when evaluating the tested UHPC joints.
Shear Walls
RC shear walls have exceptional strength and stiffness for resisting lateral forces. However, low-rise RC walls are vulnerable to rapid decay in stiffness and strength under displacement reversals due to their shear-critical behavior, which significantly limits their energy dissipation capacity and ductility (Paulay et al. 1982; Hidalgo et al. 2002; Hung et al. 2017).
Hung et al. (2017) and Hung and Hsieh (2020) investigated the cyclic behavior of squat R/UHPC shear walls with flexure- and shear-dominant responses. The designs and results are selectively presented in Table 1 and Fig. 6. The results of the flexure-dominated squat R/UHPC shear wall showed that the addition of hook-ended steel fibers (30-mm-long) not only enhanced wall shear capacity by 70% but also reduced the strain demands in the longitudinal and transverse reinforcement by more than 20% and 40%, respectively. The strength of the shear-dominated R/UHPC declined by more than 30% at 1.5% drift due to the substantially localized diagonal tensile cracks [Fig. 6(b)], which considerably impaired the strut mechanism of the wall for resisting shears. Hung and Hsieh (2020) applied a strut-and-tie model to evaluate the shear capacity of squat R/UHPC walls; they suggested a compressive softening coefficient of UHPC to account for the reduced compressive strength of the UHPC strut due to orthogonal tension. The model reasonably predicted the shear strength of the squat R/UHPC walls, with an error of less than 5%.
| Squat walls | (%) | Designs | Experimental results | |||
|---|---|---|---|---|---|---|
| Drift capacity (%) | Failure mode | |||||
| UHPC-HS- | 0 | 0.52 | 0.75 | 0.43 | 1.5 | Flexure-shear |
| UHPFRC-NS- | 2 | 0.54 | 0.81 | 0.61 | 3.0 | Flexure-shear |
| UHPC-HS- | 0 | 0.9 | 0.87 | 0.69 | 1.0 | Shear |
| UHPFRC-HS- | 2 | 1.01 | 0.86 | 1.01 | 3.0 | Flexure-shear |
| UHPC/150-NS | 1.5 | 0.54 | 1.82 | 0.99 | 1.5 | Shear |
| UHPC/150-HS | 1.5 | 0.75 | 1.39 | 0.99 | 1.5 | Shear |
Note: = shear span of the wall; and = nominal moment and shear strengths, respectively; = gross cross-sectional area of the wall; and = peak strength obtained by the test.
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Bridge Systems
Accelerated bridge construction (ABC) has gained popularity in recent years. In ABC, precast RC elements are manufactured offsite and are later connected in the field using cementitious grouts. ABC offers many advantages over traditional onsite construction methods in terms of substantially shorter onsite construction time, lower traffic impact, and higher-quality structural members. However, field-cast cementitious grouts are susceptible to premature bond failure at the interface between the precast deck panel and the connection grout. This failure results in cracking and water infiltration in the joint connecting prefabricated deck panels, and thereby causes durability issues such as reinforcing bar corrosion, leakage, and degradation in bridge performance (Tayeh et al. 2012). UHPC has emerged as a promising solution to address many of these challenges. The advanced mechanical properties of UHPC—in terms of cracking resistance, strength, ductility, durability, and creep—allow for simple reinforcement details within the connection region, leading to enhanced element constructability and simplified onsite assembly that is particularly well-suited for ABC applications (Graybeal 2006, 2010; Graybeal and Tanesi 2007; Magureanu et al. 2012; Graybeal and Baby 2013; Haber and Graybeal 2018).
Another common application of UHPC in bridge systems is to overlay a thin UHPC layer on a normal-strength concrete (NC) deck (Aaleti and Sritharan 2017; Wibowo and Sritharan 2018). More recently, Wang et al. (2019a) investigated the flexural behavior of steel box girders with UHPC decks. Zhu et al. (2020) proposed a UHPC-steel waffle bridge deck system. Conventional bridge decks are particularly vulnerable in environments that are subjected to freeze–thaw cycles, deicing salts, and dynamic loads. Applying a thin UHPC overlay to bridge decks can improve the durability of these structures by increasing crack resistance and providing protection against moisture penetration and chloride ingress. For precast deck panels with UHPC connections and UHPC deck overlays, bonding characteristics between UHPC and the substrate is an important factor.
UHPC-Concrete and UHPC-Steel Bond Behaviors
According to the LRFD procedure specified by AASHTO (2008), the strength of connections is dependent on the adhesion and friction between the connected materials. Sarkar (2010) performed slant shear tests on composite cylinder specimens to investigate the bond strength between UHPC and NC. It was demonstrated that the interface texture of the substrate played a major role in the bonding of UHPC and NC. Hussein et al. (2016) performed uniaxial tensile tests to identify the adhesion between UHPC and HSC with varying degrees of roughness on the connecting interface; they found that the bond between UHPC and the roughened HSC substrate was strong enough to ensure failure occurring in the HSC rather than at the interface.
To assess mechanical behavior for UHPC bridge deck connections, Graybeal (2010) carried out flexural tests on full-depth precast deck panel specimens with cast-in-place UHPC connections. The connecting strength was enhanced with diamond-shaped shear keys, and the precast panels had exposed aggregate finish. The specimens were subjected to cyclic loading of at least 2 million cycles, after which static load was applied until failure of the specimen. The study found that the structural performance of the cast-in-place UHPC connections met or exceeded that of conventional connections. Carbonell et al. (2014) experimentally showed that the bond performance between UHPC and NC was sufficient for bridge overlay applications regardless of the roughness degree of the concrete substrate, the age of the composite specimens, the exposure to freeze–thaw cycles, and the different loading configurations.
De La Varga et al. (2018) attempted to enhance the shrinkage properties and bond performance of grouted connections. The adopted strategies included using UHPC to replace conventional grouts, connection surface treatment for precast members, internal curing of connection grouts, and interface premoistening. The results of component-level tests showed that the use of UHPC materials improved cracking resistance and shrinkage and bond properties compared with conventional cementitious grouts. Aaleti and Sritharan (2019) and Valikhani et al. (2020) showed that proper surface treatment with sufficient roughness yielded greater bond strength in shear or compression. Haber et al. (2018) characterized the bond behavior and performance of thixotropic UHPC overlays to concrete substrates. The top surface of the concrete substrate was prepared using two different practice-ready techniques: scarification and hydrodemolition using ultrahigh-pressure water jetting. The test results showed that the thixotropic UHPC overlay developed sufficient bond strength to the concrete substrate if it was consolidated and, in such a case, concrete surface roughness became less critical for adequate direct tension bond strength to develop. It was also found that hydrodemolition led to a higher degree of macrotexture roughness and provided enhanced mechanical interlock than scarification.
Compared with the wealth of studies on UHPC–concrete bond behavior, the investigations on the UHPC–steel interface are scarce. Wang et al. (2019b) performed experimental and numerical studies on the interfacial properties of steel beams with a UHPC overlay. The applied epoxy-based adhesive between the UHPC–steel interface substantially improved the bond strength, allowing the composite beam to fail in flexural after the steel beam yielded.
Noncontact Lap-Splice Connections with UHPC
Haber and Graybeal (2018) investigated the bond strength between UHPC and the embedded steel reinforcing bar using direct tension pullout tests. It was found that the lap-splice strength increased with a higher fiber content from 1% to 3%. Based on the test results of Graybeal and Yuan (2014), the US Federal Highway Administration (FHWA) published guidelines for the design and detailing of noncontact lap-splice connections for reinforcing bars embedded in UHPC. Table 2 presents a summary of the minimum cover and embedment lengths of deformed steel reinforcing bars embedded in UHPC (Haber and Graybeal 2018). These recommendations assumed that the UHPC had 2% high-strength steel fibers by volume and a minimum compressive strength of 97 MPa.
| Yield strength of reinforcement (MPa) | Minimum cover () | Embedment length () |
|---|---|---|
Source: Data from Haber and Graybeal (2018).
Structural Retrofitting and Rehabilitation
Retrofitting for Insufficient Splice Length
Dagenais and Massicotte (2015) carried out an experimental program involving slender RC beams with reinforcing bars having insufficient splice length in the flexure-critical region. The beams were retrofitted by replacing the concrete surrounding the spliced bars with the UHPC material containing 3% of 10-mm-long steel fibers. Dagenais and Massicotte (2015) concluded that replacing the original concrete surrounding the lapped bars with UHPC beyond the depth of the spliced region by a distance of allowed #25M and #35M bars with splice lengths of and to develop the yield strength, respectively.
UHPC Overlays and Jacketing
Concrete overlays and jacketing are common approaches for structural retrofitting and rehabilitation. To satisfy the targeted structural stiffness and strength, conventional approaches often require considerable enlargement in the cross section of the strengthened structural components, which can greatly increase the gravity load of the components and reduce the available space. Moreover, the required additional transverse reinforcement makes the construction of retrofitting more time-consuming and labor-intensive. Given the high strength, confining effect, antispalling ability, and crack-width control capacity of UHPC, it is possible to eliminate these concerns in retrofitting techniques by replacing conventional concrete with UHPC (Brühwiler and Denarié 2013; Noshiravani and Brühwiler 2013).
Beams and Slabs
Habel et al. (2007) demonstrated that RC beams with a UHPC layer in the tension side exhibited improved flexural behavior in terms of strength, stiffness, and crack width. The improvement was further enhanced when the UHPC layer was reinforced with steel reinforcement. It was shown that the composite UHPC-RC element behaved essentially monolithically until flexural failure occurred. Only minor interfacial cracks developed when localized flexural cracks propagated through the UHPC-RC interface in the ultimate state.
Yin et al. (2017) investigated the behavior of RC slabs strengthened with UHPC. Their study consisted of two series of experimental tests: (1) a rehabilitation series, in which UHPC was applied as a patch material for replacing deteriorated concrete parts; and (2) an overlay series, in which UHPC was used to retrofit soffits of RC slabs. In the former series, the application of a thick UHPC patch transformed the failure mode of a slab from brittle diagonal shear failure to ductile flexure failure; however, the ultimate strength was not improved. In the overlay series, all retrofitted slabs failed due to debonding at the UHPC–concrete interface after diagonal shear cracks occurred in the original RC slab. Similarly to the results of retrofitted beams reported by Habel et al. (2007), the use of a UHPC overlay improved the stiffness of slabs and delayed the formation of cracks. The inclusion of steel reinforcing bars in the UHPC overlay enhanced the strength of the retrofitted slab only when sufficient concrete cover was provided to ensure the development of sufficient bond strength.
Al-Osta et al. (2017) and Paschalis et al. (2018) investigated the performance of UHPC jacketing for enhancing the flexural behavior of RC beams. Two attachment techniques were evaluated: (1) sandblasting RC beam surfaces and then casting UHPC in situ around the beam; and (2) bonding prefabricated UHPC strips to the RC beam using epoxy adhesive. Both techniques performed similarly in enhancing the flexural response of the beam in terms of crack propagation, stiffness, and strength. However, Al-Osta et al. (2017) pointed out a critical concern about loss in beam ductility when using UHPC jacketing as a part of the tensile retrofitting. Zanuy and Ulzurrun (2019) proposed a flexural model for composite UHPC-RC elements. The model included a new tension chord approach to account for the tension stiffening and the interactions at the steel–concrete and concrete–UHPC interfaces.
Columns
Koo and Hong (2016) used UHPC jacketing to strengthen the cyclic behavior of RC columns with dimensions of . It was demonstrated that installing a 50-mm-thick UHPC jacket to the as-built RC column transformed the original shear failure to a flexural-shear failure. In addition, it improved the lateral strength of the column by over 125% and the drift capacity from 1.2% to 2.7% even without additional transverse reinforcement. Hung et al. (2020b) investigated the effectiveness of UHPC jacketing with steel meshes for strengthening the seismic performance of shear deficient RC columns with a high axial load demand. The retrofitted columns had the same dimensions as the original column, with only the concrete cover being replaced with UHPC. The results indicated that the strength of the column was enhanced by about 50%, and drift capacity greatly improved from 1.5% to 3%, as shown in Fig. 7(a). In the case when the jacket did not have steel meshes, the use of a 40-mm-thick UHPC jacket still successfully delayed the initiation of strength degradation from 1% to 1.5% drift.
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Beam-Column Joints
Khan et al. (2018) applied UHPC jacketing to strengthen scaled exterior beam-column joints with dimensions of . Concrete substrate was sandblasted prior to the 30-mm-thick UHPC jacketing. When the UHPC jacket was cast in place, retrofitting improved the peak strength of the joint by about 150% due to the additional confinement provided by the UHPC jacket and the delay in the formation of localized cracks. UHPC jacketing also significantly improved the initial stiffness and deformation capacity of the joint. When the UHPC jacket was prefabricated and attached to the joint using epoxy resins, although the strength improved similarly to the cast-in-place UHPC jacket, the displacement ductility was not enhanced, and the retrofitted specimen ultimately showed a brittle failure mode due to the detachment of UHPC jackets. Further studies are required to evaluate whether the use of high-modulus epoxy or mechanical and chemical anchors can ensure that critical damage is shifted from the substrate surface to the prefabricated UHPC jackets.
Sharma and Bansal (2019) evaluated the effect of initial damage on the performance of scaled exterior RC joints repaired using UHPC. The beam and column had identical cross sections of . The dimensions of the joint remained identical after repairing. The results indicated that a higher initial damage level reduced the ductility, stiffness, strength retention, and energy dissipation of the repaired joint. It was concluded that UHPC jacketing was suitable for repairing beam-column joints with slight and moderate damage levels. The drift ductility was improved from 3.4 to 4.8. Hsiao (2020) applied steel-mesh–reinforced UHPC jackets to repair a 1/2-scaled RC beam-column joint that had no seismic details and was significantly damaged after experiencing 5% beam drift demand. In the retrofitting design, the damaged cover was replaced with a steel-mesh–reinforced UHPC jacket, which only slightly increased the thickness of the joint by about 10%. The repair significantly enhanced the strength about two-fold, and transformed the failure pattern from the joint shear failure to the preferred plastic hinge in the beam, as shown in Fig. 7(b).
UHPC Pavements
Using an approach similar to ABC for bridges, Karmacharya and Chao (2019) developed a fast repair method for airfield pavements, in which prefabricated UHPC pavement panels are connected by cast-in-place UHPC closure joints onsite. The interface shear resistance is provided by the roughened surface of the existing concrete and the UHPC precast panels. The roughness proved to have sufficient resistance without using any dowel bars. A proof-of-concept implementation was carried out for pavement replacement at the Dallas-Fort Worth International Airport, and the replacement work of a taxiway pavement was completed within 6 h from excavation of existing concrete pavement to final surface finish of UHPC joints.
Impact Resistance
Studies (Mao et al. 2015; Yu et al. 2016; Verma et al. 2016; Liu et al. 2018) indicated that UHPC showed superior ductility and damage-control ability under impact and blast loads due to its high tensile strength and fracture toughness.
Mao et al. (2015) applied blast loading by the detonation of spherical PE4 explosive charges placed directly above a nonreinforced UHPC slab at a 500-mm standoff distance. No spalling was observed from slab rear face, and cracks on slab front face were only observed at critical charge size. Use of a higher fiber content was found to improve the resistance to blast loading. Yu et al. (2016) indicated that UHPC with hybrid fibers, especially using a higher content of long fibers, had greater energy absorption capacity and cracking control under impact loading than that of monofibers. Liu et al. (2018) demonstrated that UHPC absorbed a large amount of impact energy when subjected to ogive-nosed projectiles. Pyo et al. (2016) pointed out that the fibers in UHPC members had a high stress demand under impact loads, and therefore it was necessary to use high-strength fibers in UHPC to ensure high impact-resistant performance.
At the material scale, Habel and Gauvreau (2008) and Tran et al. (2016) indicated that the advantageous strain-hardening behavior of UHPC also existed under high strain rates. The fracture energy of UHPC reached as high as even at high strain rates of (Tran et al. 2016). Park et al. (2016) showed that UHPC under a uniaxial tension at a high strain rate of showed a postcracking strength three times, strain capacity two times, and peak toughness value four times higher than the case with a static strain rate. Yoo and Kim (2019) found that the bond strength at the interface of the matrix and fibers was less sensitive to strain rate for hook-ended steel fibers than smooth steel fibers, due to the consistent fiber fracture mode under both static and impact loads.
To date, few studies have been conducted on the impact- and blast-resistance of R/UHPC structural elements. Riedel et al. (2010) reported that replacing conventional concrete in RC panels with UHPC significantly improved the damage of the panels subjected to scaled aircraft engine impact, and the absorbed kinetic energy was enhanced by 110%. Notably, the high compressive strength of UHPC reduced the front-side missile penetration to a “footprint” without significant spalling. On the rear side of the R/UHPC panel, the bridging effect of fibers effectively restrained the crack width to half the fiber length and successfully prevented concrete spalling. Their test results showed that when the impact velocity was increased to , the fibers on the rear side of the panel were pulled out due to the significant inelastic shear deformation. The rear-side steel reinforcing bars acted as a membrane preventing the missile from perforating the R/UHPC panel.
Yoo et al. (2017b) evaluated the behavior of R/UHPC beams under drop-weight impacts. Including steel fibers in UHPC with prevented local failure at the contact surface and reduced maximum and residual deflections by redistributing tensile stress. Compared to short smooth steel fibers, the use of long steel fibers led to greater residual moment capacity, and the use of twisted steel fibers enhanced the microcracking pattern more obviously. Aoude et al. (2015) and Xu et al. (2016) experimentally demonstrated that R/UHPC columns had significantly improved blast performance compared with regular reinforced concrete and high-strength concrete columns in terms of damage tolerance, blast fragments, strength capacity, and maximum and residual displacements. Aoude et al. (2015) concluded that a higher fiber content (up to 4%), higher aspect ratio and tensile strength of fibers, higher longitudinal reinforcing ratio, and use of seismic detailing led to enhanced blast behavior of R/UHPC columns.
Innovative UHPC Composites Reinforced with High-Performance Materials
Reinforcement with FRP
Chen and El-Hacha (2011) and El-Hacha and Chen (2012) investigated the feasibility of an innovative hybrid bridge deck system composed of UHPC and fiber reinforced polymer (FRP) materials. They studied the flexural and fatigue properties of hybrid FRP-UHPC beams, which incorporated a pultruded glass FRP (GFRP) hollow box, a top UHPC layer to resist compressive forces, and a bottom carbon FRP (CFRP) or steel FRP (SFRP) sheet to resist tensile forces. The experimental results indicated that the addition of UHPC and CFRP/SFRP layers to the GFRP hollow box section beam improved the flexural stiffness and strength of the beam. The fatigue stress limit of the hybrid beams was approximately 40% of the tensile strength of the GFRP material, which was higher than the limit required by the code standards for GFRP material.
Wang et al. (2018b) experimentally studied the compressive behavior of FRP-confined UHPC. The compressive failure of the composite occurred due to the development of localized cracks within the UHPC that caused the hoop rupture of the FRP. The confinement efficiency was found to be lower for FRP-confined UHPC compared with FRP-confined NSC and HSC, owing to the ultrahigh strength of UHPC. Increasing the amount of FRP confinement enhanced the postpeak behavior of the FRP-confined UHPC.
The use of FRP reinforcing bars to replace conventional steel bars has been recognized as a feasible technique for strengthening and rehabilitation of RC members. The high compressive and bond strengths of UHPC can facilitate full strength development in FRP reinforcing bars (usually greater than 1,500 MPa) and full composite action. Ferrier et al. (2016) evaluated the shear failure behavior of an innovative lightweight UHPC beam reinforced with FRP bars. The FRP-reinforced UHPC beam exhibited stable postcracking hardening behavior until brittle shear failure occurred, and had peak strength more than two times higher than the reference RC beam. In addition, the high Young’s modulus of the CFRP reinforcing bars enhanced the bending stiffness of the beam.
Chao et al. (2021) investigated a new highly sustainable and resilient UHPC column reinforced by corrosion-resistant high-strength FRP bars. The columns were designed based on a new design philosophy whereby the ductile component was the concrete and the elastic components were the reinforcing bars. Experimental testing on small-scale columns with basalt FRP bars showed that the ductility under cyclic loading was maintained up to a drift ratio of approximately 9% without strength degradation, and the postpeak strength was more than 80% of the peak strength at 10% drift ratio. The specimen exhibited small residual displacement and minor damage without the severe concrete spalling and crushing that occurs in conventional concrete or fiber-reinforced concrete at large drift ratios.
Reinforcement with High-Strength Steel Reinforcing Bars (with Nominal Yielding Strength above 690 MPa)
Hung and Chueh (2016) experimentally studied the flexural performance of UHPC structural beams longitudinally reinforced with high-strength steel bars under displacement reversals. The fibers were hook-ended steel fibers 30-mm-long. The high-strength steel reinforced UHPC beams exhibited ample ductility under displacement reversals. The failure mode was dominated by the rupture of steel reinforcing bars due to strain concentration at the localized concrete crack.
Hung et al. (2017) and Hung and Hsieh (2020) experimentally studied the cyclic behavior of squat UHPC shear walls with high-strength steel reinforcing bars and hook-ended steel fibers 30-mm-long. The high-strength steel reinforced UHPC wall failed in a preferable flexural-shear control mode at the 3% drift ratio. This drift capacity was 1.5 times greater than the acceptable drift ratio for the collapse prevention performance level suggested by ASCE/SEI 7 (ASCE 2016), suggesting that the high-strength steel-reinforced UHPC squat wall can maintain its superior load-carrying ability after experiencing the maximum credible earthquake event.
UHPC-Filled Steel Tubular Columns
HSC-filled steel tubular (HSC-FST) columns have been widely used in high-rise buildings to reduce the cross sections of columns. However, the core concrete tends to have a brittle shear failure mode, which results in reduced ductility of the column.
Chen et al. (2018b) studied the compressive behavior of UHPC-FST columns. The study showed that the enhancement in the compressive strength for UHPC due to the steel tube confinement was not as significant as that for ordinary concrete, mainly due to the high autogenous shrinkage and strength of UHPC. In addition to the experimental study, the authors developed a model for predicting the compressive strength of UHPC-FST columns based on regression analysis of the collected database results. Le Hoang et al. (2019) showed that circular UHPC-FST columns exhibited good ductility in axial compression until a shear failure plane occurred. A thicker steel tube led to higher compressive strength of UHPC, which was especially evident in the case with a lower fiber content. Le Hoang et al. (2019) concluded that the use of steel fibers in UHPC might be unnecessary when sufficiently thick steel tubes are used.
Xu et al. (2019) experimentally demonstrated that the behavior of HSC-FST columns under displacement reversals was significantly enhanced by replacing HSC with UHPC. The presence of steel fibers effectively improved stiffness retention and prevented premature local buckling of the steel tube, thereby enhancing the residual axial bearing capacity. Similar to the results of Le Hoang et al. (2019), increasing the fiber content from 1% to 2% had little influence in the ultimate strength and energy dissipation capacity of the UHPC-FST.
Conclusions and Suggestions
The superior material properties of UHPC under static and dynamic loads enable novel and more versatile structural members. Existing studies have highlighted the potential of UHPC in connections, repair, retrofitting, and protective applications against different types of loadings. The following conclusions can be drawn based on the extensive studies reviewed herein, which used UHPC materials that (1) had a compressive strength in excess of 120 MPa, (2) exhibited strain-hardening behavior under uniaxial tension, and (3) used 13–30-mm-long steel fibers.
1.
R/UHPC flexural members exhibit enhanced cracking pattern, stiffness, and strength compared to equivalent RC members. However, the strain-hardening nature of the material coupled with the high bond strength of reinforcing bars can lead to strain concentration in the bars when cracks localize, thus impairing structural ductility. Research studies have shown that ductility can be significantly enhanced by increasing the longitudinal reinforcing bars to mobilize the high compressive strength and strain capacity of UHPC. This, however, runs counter to well-established design philosophy for RC flexural design, which favors tension-controlled (underreinforced) flexural behavior. Therefore, further work is needed to establish consensus on a suitable design philosophy for R/UHPC flexural members.
2.
The use of UHPC is effective in restraining the compression-associated failure patterns in regular RC members due to the ultrahigh compressive performance of UHPC in terms of strength, failure strain, confinement, and resistance to spalling and crushing. Despite the simplified details and reduced amount of confinement steel, R/UHPC columns exhibit enhanced compressive performance compared with regular RC columns.
3.
Slender R/UHPC beams without stirrups can achieve shear strength that is substantially greater than the concrete design shear strength stipulated by current design codes for RC. This can have substantial benefits in structural design. For example, the typical shear-critical behavior in squat RC walls can be transformed into a flexural-dominant behavior, which leads to enhanced ductile and stable hysteretic responses. Because R/UHPC members may be designed without shear reinforcement, the potential size effect in shear capacity warrants future study.
4.
UHPC proves to be effective in applications such as overlays and jackets for retrofitting and repairing decks, slabs, beams, columns, beam–column joints, and pavements to enhance the bond, axial, shear, and flexural performance while simplifying the design and construction by reducing the amount of steel rebar. It also enables retrofitted members to exhibit the desired failure mode with only minimal or no enlargement in their dimensions. Retrofit using UHPC is a promising research area that warrants further attention.
5.
The combined use of high-strength steel reinforcing bars and UHPC is a feasible method to resolve the unfavorable behavior or failure modes that can occur when high-strength steel rebars are used in normal-strength concrete members. These unfavorable behavior or failure modes include excessive crack width, splitting failure, premature concrete crushing, and buckling of steel bars. For example, the strength capacity of the high-strength horizontal steel bars installed in squat UHPC walls can be fully utilized under large displacement reversals, even though their nominal steel yield strength is considerably larger than the maximum permissible value in current design codes. Research is needed so that future UHPC specifications can remove limits on the tensile strength of steel reinforcement.
6.
The use of UHPC enables simplification in the anchorage of embedded steel bars due to the high bond strength, which is particularly beneficial in the design of connections and high-strength reinforcing bars. However, as discussed earlier, significant caution must be exercised when R/UHPC members are designed using the tension-controlled (underreinforced) design criteria for conventional RC members in current codes.
7.
The experimental results of existing studies provide strong evidence that current seismic design provisions for RC can be relaxed for R/UHPC members with respect to the development length of reinforcing bars, amount of confining and shear reinforcement, maximum allowable design shear (for both UHPC and R/UHPC), maximum usable design strengths for concrete and steel reinforcing bars, and maximum longitudinal reinforcement ratio.
8.
Current R/UHPC models require further enhancement in accuracy and reliability, particularly for predicting the flexural and shear strengths of R/UHPC members. The establishment of a uniform code is critical to promote the use of R/UHPC on a practical level.
9.
Future research studies are needed to characterize the tensile and compressive response of fatigue-loaded UHPC, especially in high demand applications such as wind towers and bridges. Research on tensile creep of UHPC and the effect of different thermal treatment conditions on their viscoelastic response is also warranted.
Data Availability Statement
Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.
Acknowledgments
This study was sponsored in part by the Ministry of Science and Technology, Taiwan, under Grant No. 109-2636-E-006-015. The opinions, findings, and conclusions expressed in this paper are those of the authors, and do not necessarily reflect those of the sponsor.
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- Huihui Li, Lifeng Li, Chunjin Du, Meng Ye, Xudong Shao, Cong Zhou, Experimental Study on the Flexural Behavior of a Novel Nonprismatic Prestressed UHPC Composite Box Girder with Corrugated Steel Webs, Journal of Bridge Engineering, 10.1061/JBENF2.BEENG-6038, 28, 7, (2023).
- Ran Ding, Yong-Tao Sun, Xin Nie, Ding-Qiu Chen, Experimental study on seismic behaviour of an unreinforced precast wall-slab structure based on UHPC sandwich panels, Journal of Building Engineering, 10.1016/j.jobe.2023.106197, 68, (106197), (2023).
- Yue Huang, Junhui Wang, Qi'an Wei, Huaishuai Shang, Xiaoyang Liu, Creep behaviour of ultra-high-performance concrete (UHPC): A review, Journal of Building Engineering, 10.1016/j.jobe.2023.106187, 69, (106187), (2023).
- Chung-Chan Hung, Hsin-Jui Hsiao, Yi Shao, Cheng-Hao Yen, A comparative study on the seismic performance of RC beam-column joints retrofitted by ECC, FRP, and concrete jacketing methods, Journal of Building Engineering, 10.1016/j.jobe.2022.105691, 64, (105691), (2023).
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