Ultra-high Performance Concrete Slabs Research Articles

Experimental Investigation on the Behaviour of Non-reinforced Ultra-High Performance Concrete Slabs

Ultra-high-performance concrete (UHPC) is expected to provide solutions for the development of lightweight, high-strength, and rapid construction of concrete bridges due to its excellent material properties. In order to study the influence of steel fiber on the bending performance of non-reinforced UHPC (NR-UHPC) slabs, the bending failure test of 8 NR-UHPC slabs was completed with the steel fiber content as a parameter, and the failure mode, load-deflection, crack widths and load-strain curves of the test slabs were analyzed. The test results show that the destruction process of the NR-UHPC slabs can be divided into three stages: elastic stage, cracking stage and failure stage. The load-deflection curve and load-strain curve of the specimen with steel fiber content of 0.5%-1% change linearly during the elastic phase. In the elastic phase, the load-deflection curve and the load-strain curve of the test piece with a steel fiber content of 2%-3% were not smooth after the linearity begins; the strain of the UHPC slab was almost the same within 0-10 kN. However, as the load increased, the larger the amount of steel fiber was, the smaller the strain was, that is, the UHPC slab had good crack resistance. According to the regression analysis of test results at home and abroad, recommended design formulas for the bending bearing capacity of NR-UHPC slab were put forward, which tally well with the test results.

Use of UHPC slab for continuous composite steel-concrete girders

The loss of composite action at the hogging moment zone for a continuous composite girder reduces the girder stiffness and strength. This paper presents an experimental investigation of the use of an ultra-high performance concrete (UHPC) slab at the hogging moment zone and a normal concrete (NC) slab at the sagging moment zone. The testing was conducted to verify the level of loading at which composite action is maintained at the hogging moment zone. Four two-span continuous composite girders were tested. The thickness of the UHPC varied between a half and a full depth of slab. The degree of shear connection at the hogging moment zone varied between full and partial. The experimental results confirmed the effectiveness of the UHPC slab to enhance the girder stiffness and maintain the composite action at the hogging moment zone at a load level much higher than the upper service load limit. To a lesser degree enhanced performance was also noted for the smaller thickness of the UHPC slab and partial shear connection at the hogging moment zone. Plastic analysis was conducted to evaluate the ultimate capacity of the girder which yielded a conservative estimation. Finite element (FE) modeling evaluated the girder performance numerically and yielded satisfactory results. The results indicated that composite action at the hogging moment zone is maintained for the degree of shear connection taken as 50% of the full composite action and use of UHPC as half depth of slab thickness.

Experimental and numerical study on static behavior of grouped large-headed studs embedded in UHPC

The static behavior of grouped large-headed studs (d = 30 mm) embedded in ultra-high performance concrete (UHPC) was investigated by conducting push-out tests and numerical analysis. In the push-out test, no splitting cracks were found in the UHPC slab, and the shank failure control the shear capacity, indicating the large-headed stud matches well with the mechanical properties of UHPC. Besides, it is found that the shear resistance of the stud embedded in UHPC is 11.4% higher than that embedded in normal strength concrete, indicating that the shear resistance was improved. Regarding the numerical analysis, the parametric study was conducted to investigate the influence of the concrete strength, aspect ratio of stud, stud diameter, and the spacing of stud in the direction of shear force on the shear performance of the large-headed stud. It is found that the stud diameter and stud spacing have an obvious influence on the shear resistance. Based on the test and numerical analysis results, a formula was established to predict the load-slip relationship. The comparison indicates that the predicted results agree well with the test results. To accurately predict the shear resistance of the stud embedded in UHPC, a design equation for shear strength is proposed. The ratio of the calculation results to the test results is 0.99.

Flexural behavior of an innovative dovetail ultra-high performance concrete joint using steel wire mesh interface treatment in composite bridges

An experimental program investigating the flexural behavior of an innovative dovetail ultra-high performance concrete joint for connecting precast ultra-high performance concrete slabs in composite bridges is reported in this study. Test parameters included interface treatment method, joint material, reinforcing bar overlapping form, and prestressing level. Specifically, a new steel wire mesh interface treatment was proposed, which could generate an additional fiber-bridging mechanism in the joint surface. An enhancement of 17.4%, 20.1%, and 50% on flexural cracking strength, ultimate strength, and ductility were obtained by using the steel wire mesh. Joint material showed no significant influence, while prestressing had obvious influence on the flexural behavior of ultra-high performance concrete joints. The post-cracking behavior, ductility performance, and stiffness degradation of the tested specimens were analyzed. Good deformability was obtained for the jointed ultra-high performance concrete slabs as the ratio of deflection to span was about 1/50 at failure. The post-cracking stiffness was retained at about 90% of the initial stiffness, while the ultimate stiffness was retained at about 10% of the initial stiffness. The research findings are useful in popularizing and applying the proposed innovative dovetail ultra-high performance concrete joints in composite bridges.

Flexural behavior of an innovative dovetail UHPC joint in composite bridges under negative bending moment

The 5th Nanjing Yangtze River Bridge is a three-tower cable-stayed bridge with a main span of 600 m and a composite cross section consisting of a steel box girder- and precast ultra-high performance concrete (UHPC) deck slab. The precast UHPC slabs are connected using cast-in-place UHPC, and joints between the UHPC slabs are prone to cracking under negative bending moment. This paper investigates the flexural behavior of an innovative dovetail UHPC joint in seven UHPC slabs under negative bending moment. An innovative method using steel wire mesh is presented to enhance the interface between precast and cast-in-place UHPC at the joints. Test parameters of the UHPC slabs included interface treatment method, joint material, reinforcing bar overlapping form, and prestressing level. The steel wire mesh generated fibers that bridge the interface between the precast and cast-in-place UHPC, thus significantly enhancing the mechanical performance of the jointed UHPC slabs: (1) the nominal cracking strength was increased by 2.4 MPa; (2) the post-cracking stiffness was retained at about 80% of the initial stiffness; (3) the ultimate stiffness was retained at about 35% of the initial stiffness; and (4) closely-spaced multiple cracks occurred at the joints. A new ductility index defined as the ratio of the ultimate deflection and the flexural cracking deflection is proposed to characterize the post-cracking ductility. The research findings are useful in understanding and improving the flexural behaviors of UHPC bridge decks subjected to negative bending moment.

Behavior and strength of headed stud shear connectors in ultra-high performance concrete of composite bridges

This study presents an experimental and numerical investigation on the static behavior of headed stud shear connectors in ultra-high performance concrete (UHPC) of composite bridges. Four push-out specimens were tested. It was found that no cracking, crushing or splitting was observed on the concrete slab, indicating that UHPC slab exhibited good performance and could resist the high force transferred from the headed studs. The numerical and experimental results indicated that the shear capacity is supposed to be composed of two parts stud shank shear contribution and concrete wedge block shear contribution. The stiffness increment of a stud in UHPC was at least 60% higher than that in normal strength concrete. Even if the stud height was reduced from 6d to 2d, there was no reduction in the shear strength of a stud. Short stud shear connectors with an aspect ratio as small as 2 could develop full strength in UHPC slabs. An empirical load-slip equation taking into account stud diameter was proposed to predict the load-slip response of a stud. The reliability and accuracy of the proposed load-slip equation was verified by the experimental and numerical load-slip curves.

Ceramic balls protected ultra-high performance concrete structure against projectile impact–A numerical study

Ceramic materials have excellent mechanical properties such as light weight, great hardness and high compressive strength. In this paper, a numerical study is conducted to investigate the response of ceramic balls protected ultra-high performance concrete (UHPC) targets against the high-velocity rigid projectile impact using the coupled smoothed particle hydrodynamics-finite element (SPH-FE) method in LS-DYNA. Based on the validated numerical models, parametric studies are performed to explore the effect of diameter, spatial arrangement and material type of ceramic balls as well as the impact position on the dynamic performance of UHPC targets, and then perforation and ballistic limits of ceramic balls protected UHPC targets are obtained. Compared with other UHPC slabs at the striking velocities from 500 m/s to 850 m/s, UHPC slabs protected with 6-layer hex-pack arranged ceramic balls with the diameter of 20 mm is most effective in terms of reducing the depth of penetration (DOP). In addition, the utilization of ceramic balls is economical in protective structures since the damaged ceramic balls can be replaced and undamaged ceramic balls are reusable.

Static behavior of large stud shear connectors in steel-UHPC composite structures

This paper presents an experimental study on the static behavior of large stud shear connectors embedded in ultra-high performance concrete (UHPC). Test parameters included stud diameter, stud aspect ratio, concrete strength, and concrete slab thickness. Headed studs of two sizes, 22 mm and 30 mm in diameter, were used in this study. Extensive splitting cracks on concrete slab were detected for the normal strength concrete specimen whereas no cracks were found for the UHPC specimen with 30 mm studs, indicating that UHPC matches well with large studs. The shear strength, shear stiffness and ductility of a stud with 30 mm diameter were approximately 15%, 45% and 60% higher than those of a stud with 22 mm diameter, respectively. Stud aspect ratio and concrete slab thickness showed no obvious influence on the static behavior of the test specimens and short stud shear connectors with an aspect ratio of 2.3 could develop full strength in UHPC slabs. An empirical equation taking into account stud diameter was proposed to predict the shear load-slip curve of headed studs. Lastly, the current stud shear strength design provisions were evaluated using the experimental data.

Static behavior of grouped large headed stud-UHPC shear connectors in composite structures

Accelerated bridge construction (ABC) utilizing precast systems offers many benefits such as faster onsite construction and lower traffic impacts. Ultra-high performance concrete (UHPC) becomes an innovative solution that facilitates some types of ABC. This paper proposes a novel steel-UHPC composite system and large studs with diameter of 30 mm. Push-out tests are conducted to investigate the shear behavior of large studs embedded in UHPC and compared with normal strength concrete (NSC). A total of 6 single stud and 18 grouped studs specimens are used to investigate the effects of casting method, precast deck strength, infilling material strength and transverse reinforcement in the shear pocket. Test results showed that grouped stud effect in the UHPC specimen was insignificant. Ultimate strength of grouped stud in precast UHPC deck was 10% higher than that of NSC precast slab while interfacial slip of UHPC specimen was 17% lower. Noting that grouped large studs better match with UHPC since no visible crack was observed at failure in UHPC slab while NSC slab displayed a number of cracks. Finally, load-slip relationship for large grouped studs embedded in UHPC was proposed based on experimental results and comparisons between the test results of strength and codes predictions were made.

Flexural Design and Analysis of Composite Beams with Inverted‐T Steel Girder with Ultrahigh Performance Concrete Slab

This study intends to improve the efficiency of the composite beam combining a slab made of steel fiber‐reinforced ultrahigh performance concrete (UHPC) and a steel girder without top flange. To that goal, the experiment is conducted on 24 composite beams fabricated with varying compressive strength of UHPC, steel fiber content, stud spacing, and slab thickness to evaluate the behavior of the studs and the flexural behavior of the composite beam combining the UHPC slab and the inverted‐T steel girder. The experimental results show the test members developed sufficient ductile behavior with respect to the slip limit of 6 mm stipulated in Eurocode‐4 and regardless of the considered test variables. The experimental ultimate horizontal shear force is seen to be clearly larger than the static strengths of the stud predicted by Eurocode‐4 and AASHTO‐LRFD. Improved design formulae for the composite beam shall be derived to reflect the UHPC slab thickness.

Anchorage of composite dowels in UHPC under fatigue loading

AbstractIn steel‐concrete composite structures, innovative composite dowels can be used for the connection of ultra‐high‐performance concrete (UHPC) slabs and high‐strength steel members. In addition to sufficient shear capacity, composite dowels have to ensure the transmission of tensile forces in the composite connection in order to prevent lifting of the concrete slab. This may lead to structural problems, particularly in the very slender concrete slabs of high‐strength composite beams, where composite dowels have very small embedment depths. Although findings concerning the structural anchorage behaviour of composite dowels under static loads are already available, studies on the fatigue of composite dowels under cyclic pull‐out loading are still lacking. As fatigue behaviour is crucial for applications in bridge construction, the present paper introduces cyclic pull‐out tests on composite dowels in UHPC slabs in which the influence of different load‐dependent parameters (upper load level and load range) as well as the use of transverse reinforcement has been investigated. Furthermore, an approach to assess the lifetime of composite dowels in UHPC under cyclic pull‐out loading is proposed.

Evaluation of the flexural behavior of composite beam with inverted-T steel girder and steel fiber reinforced ultra high performance concrete slab

Huge efforts have been made to develop ultra high performance concrete (UHPC) and exploit its remarkable properties. Among the achievements, various solutions were proposed to achieve optimal composite beams using steel fiber reinforced UHPC. In order to increase the economy in material, a composite beam combining a slab made of UHPC and a steel girder without top flange is proposed. In this composite beam, the inverted-T steel girder requires the studs be arranged on the web for the composition with the UHPC slab. Considering the absence of studies evaluating the flexural behavior of this new type of composite beam, this study examines experimentally the flexural behavior with respect to the stud spacing and slab thickness. To that goal, eight composite beams with varying stud spacing and UHPC slab thickness were fabricated together with two additional composite beams using slabs made of normal concrete without steel fiber reinforcement for comparison. In addition, an analysis method considering the tension-softening behavior of steel fiber reinforced UHPC in the material model and the corresponding strain compatibility conditions of the beam member is proposed. The comparison of the analytic and experimental results reveals the good accuracy of the predictions indicating that the empirical tension-softening curve reflects reasonably the actual behavior of steel reinforced UHPC composite member.

Experimental investigation of ultra-high performance concrete slabs under contact explosions

Unlike ductile behaviour under static loads, a reinforced concrete structure can respond in a brittle manner with highly localised damage like concrete spalling, cratering and reinforcement rupturing under close-in or contact explosions. High speed fragmentation resulting from concrete spall may cause severe casualties and injuries. It is therefore important to have a better understanding of the concrete spall phenomena and fragments distribution. In the present study, contact explosion tests were carried out on concrete slabs to observe the concrete crater and spall damage. Seven slabs including two control specimens made of normal strength concrete (NRC) and five ultra-high performance concrete (UHPC) slabs are tested. The superior blast resistance capacity of UHPC slabs is verified through comparison against NRC slabs. The influence of longitudinal reinforcement spacing and slab depth on the spall resistance of UHPC slabs is investigated. Predictions through available empirical methods are made and compared with the test observations. The accuracy of these empirical methods is discussed. All fragments resulting from the contact blast tests are collected and analysed through sieve analysis. It is found that Weibull distribution can be used to model the fragments size distribution of NRC slabs while Log-normal distribution better models the fragments size distribution of UHPC slabs.

압축강도 수준을 고려한 강섬유 보강 UHPC와 역T형 강재 합성보의 휨거동 실험 연구

In a will to subdue the brittleness as well as the low tensile and flexural strengths of ordinary concrete, researches are being actively watched worldwide on steel fiber-reinforced Ultra High Performance Concrete (UHPC) obtained by admixing steel fibers in ultra high strength concrete. For the purpose of maximizing advantage of UHPC, this study removes the upper flange of the steel girder to apply an inverted T-shape girder for the formation of the composite beam. This paper intends to evaluate the behavior of the shear connectors and the flexural characteristics of the composite beam made of the inverted T-shape girder and UHPC slab using 16 specimens considering the compressive strength of concrete, the mixing ratio of steel fiber, the spacing of shear connectors and the thickness of the slab as variables. In view of the test results, it seemed that the appropriate stud spacing should range between 100 mm and 2 or 4 times the thickness of the slab. Moreover, the relative displacement observed in the specimens showed that ductile behavior was secured to a certain extent with reference to the criteria for ductile behavior suggested in Eurocode-4. The specimens with large stud spacing exhibited larger values than given by the design formula and revealed that the shear connectors developed larger ultimate strength than predicted owing to the action of UHPC and steel after non-composite behavior. Besides, the specimens with narrow stud spacing failed suddenly through compression at the upper chord of UHPC before reaching the full capacity of the shear connectors.

Investigation of ultra-high performance concrete slab and normal strength concrete slab under contact explosion

Dynamic performance of concrete structures under blast loading conditions is a topic of importance as such load generates severe structural damage including flexural damage, shear damage and concrete spall damage which may impose threats to the personnel and instruments shielded by the reinforced concrete structure. To mitigate blast effects on civil structures, a new kind of concrete material named Ultra-High-Performance-Concrete (UHPC) is now widely studied and applied. UHPC material is known for its high compressive and tensile strength, large energy absorption capacity as well as good workability and anti-abrasion ability. In a previous study, the performance of UHPC slab under blast loads had been investigated through free air explosion tests. The blast resistance capacity of UHPC had been demonstrated through comparison with normal strength concrete. In the present study, the dynamic performance of UHPC slab under contact charge explosion is experimentally studied and compared with normal strength concrete slab under the same loading scenario. Numerical models are established to reproduce both the previous free air explosion tests and the current contact explosion tests. In particular, finite element model is established to simulate the free air explosion test, and coupled smoothed particle hydrodynamics (SPH) method and finite element method is utilized to simulate the contact blast tests. Numerical results are compared with the experimental observations, and the feasibility and accuracy of the numerical model are validated.

Cracking Behavior of UHPC Slabs

The combination of fibers with traditional reinforcement may be a very interesting design solution to achieve more durable and economical structures. In this study, a total of seven slabs; six ultra high performance concrete (UHPC) slabs and one normal strength concrete slab were tested previously by the Author to observe the crack spacing and number of cracks in tension area, crack width and absorbed energy. Four slabs of UHPC with steel fiber of 0.5%, one UHPC slab with 1.1% steel fiber and one slab of UHPC without steel fiber were used in the analysis. For UHPC, the contribution of fibers to cracking in terms of crack width and spacing is significant when the amount of fibers increased. Also, normal strength concrete slabs have longer crack spacing as compared with UHPC members.

An experimental and numerical study of reinforced ultra-high performance concrete slabs under blast loads

Abstract Ultra-high performance concrete (UHPC) which is characterized by high strength, high ductility and high toughness has been widely applied in modern structure construction. Outstanding mechanical feature of UHPC not only enables strong yet slim structure design but also highlights its potential in protective engineering against extreme loads like impact or explosion. In this research a series of reinforced concrete slabs are tested to determine their response under explosive loading conditions. Concrete materials used in the slab construction are ultra-high strength concrete (UHPC) and normal strength concrete (NSC). In total five slabs are tested including four UHPC slabs with varying reinforcement ratios and one control NSC slab with normal reinforcement. Explosive charges with TNT equivalent weights ranging from 1.0 to 14.0 kg at scaled distances ranging from 0.41 to 3.05 m/kg1/3 are used in the current experiments. Test results verified the effectiveness of UHPC slabs against blast loads. Numerical models are established in LS-DYNA to reproduce the field blast tests on UHPC slabs. The numerical results are compared with the field test data, and the feasibility and validity of the numerical predictions of UHPC slab responses are demonstrated.

Enhancing cracking resistance of ultra-high-performance concrete slabs using steel fibres

This study investigates the effect of steel fibres on early-age shrinkage cracking behaviour of ultra-high-performance concrete (UHPC) slabs. For this, four large UHPC slabs, 2600 mm long and 80 mm deep, are fabricated. Two different UHPC mixtures with and without steel fibres (Vf = 0% and 2%) are considered. Test results show that all UHPC slabs without steel fibres exhibit shrinkage cracks at a very early age (before 1 d), whereas no shrinkage cracks are found in the UHPC slabs with steel fibres. The two principal reasons for these observations are the higher shrinkage strain and lower tensile strength of UHPC without steel fibres compared with its counterpart with steel fibres incl. Restrained shrinkage strain is found to be much lower (approximately 75% or more) than the free shrinkage strain, and exhibits a completely different behaviour to that of strain under free conditions.

인장연화거동을 고려한 강섬유 보강 초고성능 콘크리트 바닥판과 역T형 강재 합성보의 휨거동 해석

ABSTRACT Ultra high performance concrete (UHPC) has been developed to overcome the low tensile strengths and brittleness of conventional concrete. Considering that UHPC, owing to its composition and the use of steel fibers, develops a compressive stre ngth of 180 MPa as well as high stiffness, the top flange of the steel girder may be superfluous in the composite beam combining a slab made of UHPC and the steel girder. In such composite beam, the steel girder takes the form of an inverted-T shaped structure wi thout top flange in which the studs needed for the composition of the steel girder with the UHPC slab are disposed in the web of the steel girder. This study investigates experimentally and analytically the flexural behavior of this new type of composite beam to propose details like stud spacing and slab thickness for further design recommendations. To that goal, eight composite beams with varying stud spacing and slab thickness were fabricated and tested. The test results indicated that stud spacing running from 100 mm to 2 to 3 times the slab thickness can be recommended. In view of the relative characteristic slip limit of Eurocode-4, the results showed that the composite beam developed ductile behavior. Moreover, except for the members with thin slab and large stud spacing, most of the specimens exhibited results different to those predicted by AASHTO LRFD and Eurocode-4 because of the high performance developed by UHPC.

Headed stud shear connector for thin ultrahigh-performance concrete bridge deck

Ultrahigh-performance concrete (UHPC) provides much higher compressive and tensile strength than conventional concrete. UHPC is advantageous for use as a bridge slab deck owing to its higher strength, stiffness, and durability. One drawback, however, is the fact that the joint region connecting the deck and girder generally has a thicker cross-section to ensure proper shear connection, which hinders making the overall UHPC deck thinner and lighter. In addition, the shear strength of stud shear connectors embedded in UHPC slab has not been verified to be the same as that in a conventional concrete deck. This study investigates a stud shear connector embedded in a UHPC deck through 15 push-out tests. The ultimate strength of the stud and relative slips are measured. The test parameters were chosen to prove the feasibility of a thinner slab. The stud aspect ratio, overall height-to-diameter, and cover thickness on top of the stud head requirement are also examined to verify the existing geometrical constraints specified in the AASHTO LRFD and Eurocode-4 design codes for UHPC decks. It was shown that the aspect ratio can be reduced from 4 to 3.1 without loss of shear strength of the stud, and the cover could be reduced from 50mm to 25mm without causing a splitting crack at the UHPC slab. However, the required ductility demand, 6mm, was not realized in all cases. Therefore, the stud shear connectors in a UHPC deck should be designed according to the elastic criterion.