Steel Fiber Volume Fraction Research Articles

Structural Behavior of Reactive Powder Concrete under Harmonic Loading

Industrial buildings usually are designed to sustain several types of load systems, such as dead, live, and dynamic loads (especially the harmonic load produced by rotary motors). In general, these buildings require high-strength structural elements to carry the applied loads. Moreover, Reactive Powder Concrete (RPC) has been used for this purpose because of its excellent mechanical strength and endurance. Therefore, this study provides an experimental analysis of the structural behaviors of reinforced RPC beams under harmonic loads. The experimental program consisted of testing six simply supported RPC beams with lengths of 1500 mm, widths of 150 mm, and thicknesses of 200 mm under harmonic loading with varied frequencies between 10 and 20 Hz. Different steel fiber ratios of 0%, 0.5%, 0.75%, 1.0%, 1.5%, and 1.75% were provided in the concrete mixes to explore the effect of steel fibers on the dynamic behavior of these beams. Except for the steel fiber volume fraction, all of the examined specimens shared the same material attributes and reinforcing details. The outcomes proved the positive effect of adding steel fibers on the dynamic response under the effect of harmonic loading. The optimum volume fraction of steel fibers was characterized by a percentage of 1.5%. Moreover, the vibration amplitude was more affected by the steel fibers than the support reactions. The inertial force increased as the harmonic loading duration increased. This increase in the inertial force by the load duration was enhanced after adding the steel fibers. However, this enhancement started to decline after increasing the steel fiber content to 1.75%.

Serviceability evaluation model of SFRC beams reinforced with FRP bars based on deformation control

This research investigated the flexural behavior of steel fiber-reinforced concrete (SFRC) beams reinforced with fiber-reinforced polymer (FRP) bars to promote its application in engineering. Ten beams with widths of 150 mm, heights of 250 mm and lengths of 1800 mm were tested under four-point bending. The experimental parameters included the reinforcement type, steel fiber volume fraction, concrete strength, and longitudinal reinforcement ratio of the FRP bars. Based on the test results, a prediction model was proposed for the balanced reinforcement ratio (ρfb), considering the influence of steel fibers. Deformation prediction equations in the current design codes in the US, China, and Canada were compared based on the experimental results. The increase in the reinforcement ratio (ρf) can improve both the service moment (Ms) and the ultimate moment (Mu). With an increase in ρf/ρfb, the Ms/Mu value decreased for under-reinforcement beams but increased for over-reinforcement beams. Based on the linear regression theory, a new calculation model for the flexural moment utilization efficiency of SFRC beams reinforced with FRP bars in serviceability limit states is established. The ductility indices of all beams are above the minimum limit of 4, as proposed by CSA S6-14.

Experimental Study on the Flexural Behaviors of Prestressed Segmental Ultra–High–Performance Concrete Channels and Reinforced Conventional Concrete Deck Composite Girders

Flexural testing on two prestressed segmental ultra–high–performance concrete channels and reinforced conventional concrete deck composite girders (PSUC–RCCD) was carried out. One was made up of four segments with dry joints, and the other was formed of one channel beam without a dry joint. Both of them poured a conventional concrete deck slab on site. The mechanical behaviors of the girders, including the whole loading process, the crack pattern, and the failure mode were investigated and compared. The effect of the number of segments and the steel fiber volume fraction of UHPC on the bending behavior of the PSUC–RCCD girder was explored using the finite element method. This study showed that the loading process of semi-segmental and integral girders is similar; the whole loading process of the girders can be divided into the elastic phase, crack development, and the failure phase. The only notable difference between the two girders was the stage of crack development; specifically, after cracking, the stiffness of the semi-segmental girder reduced rapidly, while the “bridging effect” of the steel fibers in the integrated girder caused a slow reduction in rigidity. The flexural cracks in the semi-segmental girder were significantly less than those in the integral girder in terms of the number of cracks, and were present only at the joints. The finite element analysis showed that the number of segments had little influence on the flexural capacity of the girders, but the girders with even numbers of segments cracked earlier than those with odd segments. Increasing the steel fiber volume fraction in UHPC (ultra–high–performance concrete) had a small effect on the cracking load of the semi-segmental girders but enhanced its ultimate flexural capacity. Based on this experiment, a calculated method for estimating the flexural capacity of semi-sectional girders was proposed. The calculated values were in good agreement with the experimental and finite element values. In the preliminary design, the flexural capacity of the semi-segmental section could be estimated by multiplying the flexural capacity of the integral section by a resistance factor of 0.95.

Tensile behavior analysis and prediction of steel fiber recycled fine aggregate concrete

This paper investigated the splitting tensile and axial tensile behaviors of steel fiber recycled fine aggregate concrete (SFRFAC). The effects of recycled fine aggregate (RFA) replacement ratio, steel fiber volume fraction, steel fiber type and silica fume content on the tensile properties of SFRFAC were studied. The digital image correlation (DIC) technique was utilized to observe the surface crack development of SFRFAC specimens. The results show that the splitting tensile strength of SFRFAC was greater than the axial tensile strength. Furthermore, the use of RFA decreased the splitting tensile strength, axial tensile strength and axial tensile elastic modulus of SFRFAC, and increased its axial tensile peak strain, while the addition of steel fiber and silica fume enhanced the splitting tensile strength, axial tensile strength, axial tensile peak strain and axial tensile elastic modulus of SFRFAC. Finally, the relationship between splitting tensile strength and axial tensile strength and the relationship for the axial tensile stress–strain of SFRFAC were proposed, respectively. Moreover, the modified damage constitutive model was also proposed by considering the damage threshold and the damage variable correction parameter to make the damage constitutive model more reasonable.

Analysis of Shear Model for Steel-Fiber-Reinforced High-Strength Concrete Corbels with Welded-Anchorage Longitudinal Reinforcement.

According to the shear capacity test results of six steel-fiber-reinforced high-strength concrete (SFHSC) corbels with welded-anchorage longitudinal reinforcement under concentrated load, the effects of shear span ratio and steel fiber volume fraction on the failure mode, cracking load and ultimate load of corbel specimens were analyzed. On the basis of experimental research, the shear transfer mechanism of corbel structure was discussed. Then, a modified softened strut-and-tie model (MSSTM), composed of the diagonal and horizontal mechanisms, was proposed, for steel-fiber-reinforced high-strength concrete corbels. The contributions of concrete, steel fiber and horizontal stirrups to the shear bearing capacity of the corbels were clarified. A calculation method for the shear bearing capacity of steel-fiber-reinforced high-strength concrete corbels was established and was simplified on this basis. The calculation results of the model were compared with the test values and calculation results of the GB50010-2010 code, the ACI318-19 code, the EN 1992-1-1 code and the CSA A23.3-19 code. The results showed that the concrete corbel with small shear span ratio mainly has two typical failure modes: shear failure and diagonal compression failure. With the increase in shear span ratio, the shear capacity of corbels decreases. Steel fiber can improve the ductility of a reinforced concrete corbel, but has little effect on the failure mode of the diagonal section. The calculated values of the national codes were lower than the experimental values, and the results were conservative. The theoretical calculation values of the shear capacity calculation model of the corbels were close to the experimental results. In addition, the model has a clear mechanical concept considering the tensile properties of steel-fiber-reinforced high-strength concrete and the influence of horizontal stirrups, which can reasonably reflect the shear transfer mechanism of corbels.

Cracking Pattern and Bearing Capacity of Steel Fiber-Reinforced Concrete Single-Layer Tunnel Lining

In recent years, steel fiber-reinforced concrete (SFRC) single-layer linings have been used in tunnel engineering. Compared to plain concrete single-layer linings, SFRC single-layer linings demonstrate enhanced bearing capacity, durability, and sustainability. Existing studies primarily focused on the mechanical properties of SFRC; however, limited investigations have been conducted on the cracking pattern of SFRC linings. This study uses laboratory tests to examine the influence of steel fiber content and aspect ratio on the mechanical properties of concrete, such as compressive strength and elastic modulus. After the recommended content and aspect ratio of steel fiber are proposed through tests, the cracking pattern and safety performance of plain concrete and SFRC linings under surrounding rock pressure are studied using a similar model test. The test results indicate that the recommended steel fiber volume fraction and aspect ratio for CF35 SFRC are 0.58% and 70, respectively. Due to the effect of loose load, cracks initially develop on the inside of arch crowns in both plain concrete and SFRC single-layer linings. Subsequently, new cracks appear on the inside of the lining floor and the outside of the two wall feet. Numerous narrow cracks with rugged and winding expansion paths can be found on SFRC single-layer linings. Conversely, plain concrete single-layer linings exhibit fewer cracks with larger widths along a straighter path. The initial cracking load of a single-layer lining made of plain concrete is 0.027 MPa, whereas for a single-layer lining made of SFRC, it is 0.04 MPa. This indicates that SFRC can effectively enhance the initial cracking load of lining structures. In the event of damage to the lining, the most critical area for the plain concrete single-layer lining is at the two wall feet, where the minimum safety factor is 1.66. However, for the SFRC lining in the same location, the safety factor is 2.7, resulting in a 62.7% increase in safety.

Experimental and Numerical Study on the Mechanical Performance of Ultra-High-Performance Concrete T-Section Beams

Aiming to investigate the mechanical performance of UHPC T-section beams, five specimens are fabricated and tested, considering the variable steel fiber volume fraction (SFVF). The code of the Association Francaise de Génie Civil (AFGC) is evaluated by test data. Additionally, based on Abaqus (2020), refined finite element analysis (FEA) models of specimens are established and validated by experimental data. Moreover, the parametric sensitivity analysis is carried out, which aims to further investigate the effect of shear span ratio, longitude reinforcement ratio, and stirrup ratio on the bending-shear behavior of T-section beams. The test results indicated that the ultimate load of the specimen improves with the increase of SFVF, and the use of steel fibers can greatly improve the shear capacity instead of the bending capacity. Furthermore, SFVF can change the failure mode; the specimens fail in shear failure when SFVF < 2%, while they fail in bending failure when SFVF ≥ 2%. From the evaluation of codes, the AFGC code is conservative in the prediction of ultimate capacity, which can guide the design of UHPC structures well. Additionally, from the parametric analysis of FEM, the failure mode transformed from shear failure to bending failure as the shear span ratio increased, particularly in specimens with SFVF ≥ 2.5%. Moreover, the stirrup ratio ρsv has a significant effect on the shear performance of structures with SFVF ≤ 1%, while it has less effect with SFVF ≥ 2%.

The Shear Capacity Estimation of Reinforced Concrete Short Beams with Steel Fiber by Digital Image Correlation Approach

This research aimed to estimate the shear capacity of reinforced concrete short beams with steel fiber (RSF short beams) using DIC (digital image correlation). Four-point bending tests of RSF short beams with shear span ratio (a/d) of 1.0, 1.5, and 2.0 were conducted. 0.5% or 1.0% volume fraction of steel fiber was used. The distribution of minimum principal strain (Ɛ2) was obtained from DIC then the width of compressive strut was evaluated. The effects of volume fraction of steel fiber and shear span ratio were investigated. Finally, the shear capacity of RSF short beams was estimated with the help of DIC.

Experimental study on the properties of internal cured concrete reinforced with steel fibre

Internal cured concrete modified with superabsorbent polymer (SAP) has received extensive attention due to its excellent crack resistance and durability, but the decrease of mechanical property caused by the macro pores left after SAP water release has limited its application to a certain extent. In this study, steel fibre was used for improving the strength and toughness of internal cured concrete modified with SAP (IC) to satisfy the development trend towards high strength concrete nowadays. Four steel fibre volume fraction dosages (0.5%, 1%, 1.5% and 2%) were adopted. Water absorption test, freeze–thaw test and dynamic compressive test were conducted to investigate the influences of steel fibre on dense degree, frost resistant, and mechanical property of IC respectively. The results showed that, 1) under standard curing conditions (20 °C and 95% RH), with the addition of SAP, the dense degree of concrete was weakened, but the frost resistant was strengthened; 2) the dense degree, frost resistant, and dynamic compressive properties of IC were considerably improved by adding steel fibre; 3) the optimal volume fraction dosage of steel fibre is 1.5%. Furthermore, a dynamic compressive constitutive model was developed, which can reflect the effects of steel fibre dosage and strain rate simultaneously.

Long-term loss of tension stiffening of concrete containing recycled aggregate and steel fibres

This paper presents experimental results on the long-term loss of tension stiffening of concrete containing recycled aggregate and steel fibres. Washed construction and demolition wastes (CDW) size 20 mm were used as a replacement for the natural coarse aggregate, and hooked-end steel fibres (DRAMIX 3D 65/35BG) were added to the mixes. Square section prisms of concrete (120x120x1200 mm) reinforced with a central steel bar were prepared and cast with different replacement percentages of recycled aggregate and volume fractions of steel fibres. A direct tensile load was applied to the steel bar and the measurements of the strain on the concrete surfaces were then recorded for a period of 35 days to assess the long-term loss of tension stiffening. The results indicated that the substitution of NA by RA increased the loss of tension stiffening over time up to 12.9%. However, the addition of steel fibres proved to be highly beneficial; the inclusion of 0.5% and 1.0% steel fibre content to the NC and RAC reduced the loss up to 9.8% and 20.4%, respectively. According to the obtained results, modifications to the Eurocode2 method for predicting the long-term deflection of reinforced concrete beams were proposed to be implicated in the calculation steps for this type of concrete.

Study on Dynamic and Static Mechanical Properties of Copper-Plated Steel-Fiber-Reinforced Self-Compacting Concrete.

The mechanical properties and impact resistance of conventional self-compacting concrete (SCC) need to be further improved. In order to explore the dynamic and static mechanical properties of copper-plated steel-fiber-reinforced self-compacting concrete (CPSFRSCC), the static mechanical properties and dynamic mechanical properties of CPSFRSCC with a different volume fraction of copper-plated steel fiber (CPSF) are tested, and a numerical experiment is carried out to analyze the experimental results. The results show that the mechanical properties of self-compacting concrete (SCC) can be effectively improved by adding CPSF, especially for the tensile mechanical properties. The static tensile strength of CPSFRSCC shows a trend that increases with the increase in the volume fraction of CPSF and then reaches the maximum when the volume fraction of CPSF is 3%. The dynamic tensile strength of CPSFRSCC shows a trend that increases first and then decrease with the increase in the volume fraction of CPSF, and then reaches the maximum when the volume fraction of CPSF is 2%. The results of the numerical simulation show that the failure morphology of CPSFRSCC is closely related to the content of CPSF; with the increase in the volume fraction of CPSF, the fracture morphology of the specimen gradually evolves from complete fracture to incomplete fracture.

The Effects of Steel Fiber Types and Volume Fraction on the Physical and Mechanical Properties of Concrete

Different types and amounts of steel fibers have varying effects on the improvement of concrete’s mechanical properties. In order to identify the most suitable steel fiber types for the practical production of prefabricated pavements and derive a formula to predict and evaluate the mechanical properties based on steel fiber volume fraction, this study conducted experimental research on the physical mechanical properties of concrete using the method of equal volume substitution of coarse aggregate. The influence of steel fiber type and volume fraction on the microstructure and failure mechanism of steel fiber reinforced concrete (SFRC) was analyzed through electron microscopy scanning. The mechanical properties of plain concrete were used as benchmark. The results showed that when the steel fiber volume fractions were 0.6% and 1.5%, the bending and split tensile strengths of milled steel fibers were 3.1% higher than those of hooked-end steel fibers and the compressive strength of SFRC was significantly increased by 13.5%. The comprehensive mechanical properties of wave-shaped steel fibers were inferior to the first two types. Considering the requirements of SFRC in engineering, milled steel fibers are more suitable to be concrete components. This is because the tangling property of the hooked-end steel fibers makes them difficult to be shaped. In contrast, the smooth surface of milled steel fibers exerts a smaller impact on the forming of specimen and they are more economical. The mechanical properties of SFRC improve as the volume fraction of steel fibers increases from 0% to 2%, but start to decline when the volume fraction exceeds 2%. A volume fraction of 0.6% for steel fibers is sufficient to meet the standard for bending and tensile strength in heavy-duty concrete pavements. Finally, the relationship expressions between the compressive strength, flexural strength, and split tensile strength of SFRC and the steel fiber volume fraction were obtained through fitting the experimental data using Origin software.

Mechanical behavior of steel fiber reinforced recycled aggregate concrete under dynamic triaxial compression

An experimental study of cylindrical steel fiber reinforced recycled aggregate concrete (SFRAC) specimens subjected to hydrostatic pressure and dynamic axial loading in a triaxial cell was performed. A confining stress range of 0–40 MPa and strain rate from 10−5 s−1 to 10−2 s−1 were employed using a servo-hydraulic testing system. In addition, two other control variables – volume fraction of steel fiber and replacement ratio of recycled coarse aggregate were also considered. A series of complete stress-strain curves were obtained, and the results were discussed in the aspects of the failure modes, peak triaxial strength, toughness, octahedral stress and failure criterion. The results indicated that an increase in the confining pressure resulted in a change in the failure modes, a significant increase in the peak strength and ductility. In particular, with an increase in confining pressure, the enhancement of strain rate on the peak strength decreased, indicating that there was a coupling effect between these two. The steel fibers had little influence on the failure modes of SFRAC, but significantly improved its ductility and toughness. The addition of recycled coarse aggregates markedly reduced the peak strength and the reduction reached nearly 20%, which was larger than other reports in uniaxial and true triaxial loading. Finally, a failure criterion for SFRAC specimens subjected to dynamic loading was used for validating the experimental results.

Behavior and constitutive model of ultra-high-performance concrete under monotonic and cyclic tensile loading

Ultra-high-performance concrete (UHPC) holds great promise for constructing lightweight and high-performance concrete structures. Hence, the proportion of live load increases, in turn, leads to a higher live-to-dead load ratio. UHPC structures suffer from cyclic stress amplitude in higher possibility. Therefore, the behavior and constitutive model of UHPC under cyclic tensile loading is of significance for the understanding of bearing capacity and deformation property for structures subjected to cyclic loading, which, nevertheless, is seldomly reported. In this regard, the present study conducted direct tensile tests on 35 UHPC dog-bones with four steel fiber volume fractions (i.e., 0%, 1%, 2%, and 3%) and two fiber shapes (i.e., straight and hooked-end). A constitutive model for UHPC under tensile loading was proposed, implemented into OpenSees, and employed in numerical simulations of UHPC beams subjected to bending in cyclic loading. The numerical results were compared with test results on UHPC beams for model verification. The experimental and numerical results show that the UHPC with straight fiber volume fractions less than 1% demonstrated a brittle failure, whereas the UHPC with straight fiber volume fractions higher than 1% or with a hooked-end fiber volume fraction of 2% exhibited a ductile failure. The increase in fiber volume fraction resulted in great enhancements of the initial crack and peak stresses, and the peak and ultimate strains, and the energy absorption capacity. Besides, the use of hooded-end fibers also led to a slight improvement in tensile strain behavior of UHPC but negatively affected the initial crack stress and peak stress. It also provided sufficient bonding between fiber and matrix. The envelope curves and failure patterns for UHPC under monotonic and cyclic loading are close. The proposed constitutive model for UHPC under tensile loading is composed of an envelope curve that is assumed bi-linear for strain softening UHPC or tri-linear for strain hardening UHPC, an unloading curve and a reloading curve described by an exponential function, and a damage index defined as the reduction of unloading modulus. The proposed constitutive model for UHPC under tensile loading can provide acceptable results for the prediction of the bearing capacity and deformation property of UHPC beams without reinforcements subjected to bending in cyclic loading.

Experimental Study on the Performance of Steel-Fiber-Reinforced Concrete for Remote-Pumping Construction.

Remote-pumped concrete for infrastructure construction is a key innovation of the mechanized and intelligent construction technology. This has brought steel-fiber-reinforced concrete (SFRC) into undergoing various developments, from conventional flowability to high pumpability with low-carbon features. In this regard, an experimental study on the mixing proportion design and the pumpability and mechanical properties of SFRC was conducted for remote pumping. Using the absolute volume method based on the steel-fiber-aggregate skeleton packing test, the water dosage and the sand ratio were adjusted with an experimental study on reference concrete with the premise of varying the volume fraction of steel fiber from 0.4% to 1.2%. The test results of the pumpability of fresh SFRC indicated that the pressure bleeding rate and the static segregation rate were not the controlling indices due to the fact that they were far below the limits of the specifications, and the slump flowability fitted for remote-pumping construction was verified by a lab pumping test. Although the rheological properties of the SFRC charactered by the yield stress and the plastic viscosity increased with the volume fraction of steel fiber, those of mortar used as a lubricating layer during the pumping was almost constant. The cubic compressive strength of the SFRC had a tendency to increase with the volume fraction of steel fiber. The reinforcement effect of steel fiber on the splitting tensile strength of the SFRC was similar to the specifications, while its effect on the flexural strength was higher than the specifications due to the special feature of steel fibers distributed along the longitudinal direction of the beam specimens. The SFRC had excellent impact resistance with an increased volume fraction of steel fiber and presented acceptable water impermeability.

Bond performance of carbon fiber reinforced polymer rebars in ultra-high-performance concrete

This paper investigated the bond performance between carbon fiber reinforced polymer (CFRP) bars and ultra-high-performance concrete (UHPC) by a hinged beam test investigation. A total of 11 sets of beam tests were carried out to investigate the effect of different parameters on the bond performance, including steel fiber volume fraction, bonded length, reinforcement diameter, and concrete cover. The tests showed that the bond damage modes between CFRP bars and UHPC were dominated by pull-out damage, with the surface of the bonded section of CFRP bars being peeled off from the internal core and no damage observed in the UHPCs. An increase in the cover and steel fiber volume fraction improves the bonding performance, while an increase in diameter reduces the bond performance. Based on this study, the formulae for calculating the ultimate bond strength and development length that integrated the effects of CFRP bar diameter, bonded length, and cover thickness were proposed. The bond-slip curve shows three stages, i.e., ascending section, descending section, and a residual section showing periodic fluctuations. The fluctuation period showed a certain degree of linear correlation with the rib spacing of CFRP bars. Based on the characteristics of tested curves, a bond-slip constitutive model was proposed to simulate the bond-slip behavior of CFRP bars in UHPCs with good agreement to test results.

Influence of adding steel – glass – polypropylene fibers on the strength and flexural behaviour of hybrid fiber reinforced concrete

This experimental research investigates the strength and load deflection characteristics of hybrid fiber reinforced concrete. Two or more types of fibers are combined to improve the overall properties of concrete. Grade of concrete chosen for study is M40 and the specimens were incorporated with 0.75% volume fraction of Steel, Glass and polypropylene fibers in equal proportions and in different combinations. This study primarily focused on compressive strength, split tensile strength and flexural tensile strength of composite with two types of fiber combinations. Results were compared with non-fibrous control concrete and strength effectiveness was observed to be more in Steel- Polypropylene fibers combination. Negative results were produced by Glass- polypropylene hybrid fiber composites. Based on the best strength results reinforced concrete beam specimen is made with Steel – polypropylene hybrid fiber combinations as a secondary reinforcement and it is tested under flexural loading conditions. Load deflection behaviour and crack studies were carried out on tested beam specimens. Finally experimental results were compared with Finite element software results and close agreement was observed between them.

Fracture properties of steel fibre reinforced high-volume fly ash self-compacting concrete

This paper presents an experimental investigation on the fracture mechanical properties of steel fibre-reinforced high-volume fly ash self-compacting concrete (HFSCC). Specifically, self-compacting concrete (SCC) of various toughness was designed at desired steel fibre volume fractions of 0, 0.25 v%, 0.50 v%, 0.75 v% and 1.00 v% by volume and fly ash contents of 40 wt%, 50 wt%, 60 wt% and 70 wt% by mass. The fracture mechanical properties of HFSCC (cured for one year) were studied by a three-point bending beam test. The influence of fly ash and steel fibre content on fracture parameters was systematically discussed. The research indicated that the fracture mechanical properties of HFSCC with different fly ash percentage were improved by adding steel fibres. The fullness of the force (F)-crack mouth opening displacement (CMOD) curves gradually increased with the steel fibre volume fraction (Vf). The peak load (Fp), peak CMOD (CMODP), fracture toughness (KIC) and fracture energy (GF) of the fracture parameters linearly increased with Vf. Furthermore, the correlation between them was analysed numerically. The slope of the fitting line increased with increasing fly ash content, indicating that high-volume fly ash was more conducive to improving the fracture mechanics of SCC after long-term ageing. After the hydration reaction, residual calcium hydroxide was observed only in the cross-sectional image of HFSCC-70, it helped to improve the fracture mechanical properties of HFSCC.

Shear capacity of UHPFRC deep beams with web openings

The shear capacity of Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) deep beams with web openings located between the provided stirrups was investigated experimentally. All the tested beams were constructed from the same UHPFRC mix containing steel fibers volume fraction of 1.5 %. The test program included eight simply supported deep beams with the same cross-section and same longitudinal reinforcement. The principal studied parameters were opening height, width, location, and the stirrups ratio ρv %. The provided stirrups spacing sv was considerably more than the maximum stirrups spacing sv,max required by ACI 318 code for normal reinforced concrete, while sv,max required by the Egyptian code (ECP 203) were more or equal to sv. The equations used by French recommendations (AFGC) and Korea Concrete Institute (KCI) for the shear design of UHPFRC deep beams have been examined. The experimental results indicated that the shear capacity of UHPFRC deep beams is mainly controlled by opening height, width, and location. Square opening with a height of 20 % of the beam's overall height located at the natural load pass caused reduction of shear capacity by 43 % compared with the reference solid beam. Increasing the opening width by 75 % symmetrically around the natural load pass reduced the shear capacity of the UHPFRC beam by about 21 % while increasing the opening width by 125 % from one side out of the load pass caused a reduction by only 5 %. Increasing the stirrups ratio by 80 % for beams of similar opening size and location, increased the shear capacity by 17.1 %. Predicting the ultimate shear strength of the tested UHPFRC deep beams with openings of height 20 % and 40 % of the beam's total height using the equations of AFGC was just safe if these equations were calculated by deducting the height of opening from the beam effective depth. The equations used by KCI were also safe with a factor of safety about 6 % more than that of AFGC.

Research on eccentric-compressive behaviour of steel-fiber-reinforced recycled concrete-filled square steel tube short columns

This study, a total of 25 specimens, aims to assess the influence of the mass replacement percentage of recycled coarse aggregates (0%, 50%, 75%, and 100%), steel tube thickness (3 mm, 4 mm, and 5 mm), load eccentricity (0.10, 0.25, and 0.40), and volume fraction of steel fiber (0%, 0.6%, 1.2%, and 1.8%) on the eccentric-compressive behaviour of steel-fiber-reinforced recycled concrete-filled square steel tube short columns. The post-peak mechanical behaviour, deformation ability, and energy consumption degrade with increasing mass replacement of recycled coarse aggregate; the load eccentricity is detrimental to the ultimate compressive strength and aggravates the lateral deflection; thicker steel tubes and the incorporation of steel fiber improve the ultimate compressive strength, deformation ability, and energy consumption. Moreover, an incremental iteration model is proposed to calculate the ultimate compressive strength of steel-fiber-reinforced recycled concrete-filled square steel tube short columns under eccentric-compressive load. Compared to current codes, the predicted values using the proposed model are close to experimental values and are inclined toward the safe side.