Minimum Flexural Reinforcement Research Articles

Evaluation of minimum flexural reinforcement for precast prestressed concrete NEXT beam bridges

The minimum flexural reinforcement requirement has been used in the current bridge design specifications to protect the member from brittle failure after the formation of the first flexural cracks. Several variables have been reported to affect this requirement, such as concrete strength, amount of prestressing in the member, and type of cross section. Recently, the Precast/Prestressed Concrete Institute (PCI) developed a new type of beam section (NEXT beam) to accelerate bridge construction and enhance the sustainability of bridges. As a newly developed beam section, no research on the minimum flexural reinforcement has been reported for NEXT beam bridges. This paper aimed to examine the minimum flexural reinforcement requirements in the current AASHTO LRFD Bridge Design Specifications for NEXT beam bridges. A comprehensive parametric study was analytically conducted with various parameters, including bridge section, beam section, concrete strength, and span length. The results from this study showed that the current minimum flexural reinforcement requirements were met for all bridges examined herein; the concrete strength, beam cross section, and span length could affect the levels of safety against brittle failure after first flexural cracks for NEXT beam bridges.

Minimum flexural reinforcement of fly ash and GGBS based geopolymer concrete members

Geopolymer concrete is a relatively new binder material that can replace cement in concrete. Fly ash is activated in the presence of alkali activator (combination of alkali Silicate and alkali hydroxide) that binds the fine aggregate and coarse aggregate in geopolymer concrete. These concretes need temperature curing to the extent of 60 °C for about 24 h. The temperature curing to this extent is difficult in conventional construction practices. Many researchers have reported that fly ash and ground granulated blast furnace slag (GGBFS) based geopolymer concrete can be cured in the room temperature and can develop desired strength. With this advent, research in the direction of geopolymer concrete in respect of mechanical strength characteristics like tensile strength, compressive strength and modulus of elasticity, mix design and confinement aspects of geopolymer concrete, is increasing. This paper presents the behavioural aspects of reinforced geopolymer concrete members subjected to flexural loading. Three different proportions of geopolymer concrete with target 28-day compressive strength of 30 MPa, 45 MPa and 60 MPa were developed. The test results indicated that minimum flexural reinforcement is needed in geopolymer concrete beams to avoid brittle failure and controls the cracking. Minimum flexural reinforcement is needed to maintain the integrity of the reinforced concrete members even after the tensile resistance of concrete is exhausted. Thus, an expression for the minimum flexural reinforcement needed in beams is proposed.

Ductility evaluation and flexural failure mode recognition of reinforced Ultra-High performance concrete flexural members

Although ultra-high performance concrete (UHPC) exhibits material tensile ductility far in excess of that expected from conventional or high-strength concretes, structural UHPC members typically fail in flexure due to reinforcement rupture after crack localization, which may result in a low structural ductility. In the flexural design of reinforced UHPC (R-UHPC) members, attention should be paid to addressing the issue related to the low structural ductility due to crack localization. This study firstly develops a two-dimensional numerical model to simulate the flexural response of R-UHPC beams, and the numerical results agree well with the experimental results of the load–deflection curves as well as the failure modes. Parametric analyses are then carried out to examine the effect of the flexural reinforcement ratio, tensile strength of steel reinforcement, tensile behavior of UHPC and compressive strength of UHPC on the flexural failure mode and ductility behavior of R-UHPC members. The results indicate that when the failure is controlled by fracture of tensile steel bars after crack localization, higher reinforcing ratio, higher steel post-yield hardening strength, and lower steel fiber volume lead to more ductile behavior. When the failure is controlled by crushing of UHPC, however, higher reinforcing ratio and lower UHPC compressive strength lead to a lower ductility. In addition, a flexural failure mode recognition method is proposed for R-UHPC flexural members, whose accuracy is verified based on the available experimental results of 105 R-UHPC flexural members. Finally, a minimum flexural reinforcement ratio is proposed to avoid low structural ductility in R-UHPC flexural members.

Minimum Flexural Reinforcement Steel Ratios of High‐Strength Concrete Beams

Considering that most studies depend on theoretical equations to determine the minimum reinforcement ratio, only a few studies on this ratio are available. Therefore, a more defined limit should be suggested to design codes by performing additional investigations and experimental studies on this limit. This study examines the behavior of high‐strength concrete (HSC) beams with low reinforcing steel ratios to establish a limit for the lowest flexural reinforcement ratio that will ensure ductility. Experiments were performed on 12 reinforced HSC beams with a concrete compressive strength of 99 MPa, which were divided into three categories depending on their size. Each category comprised four beam reinforcement ratios (0%, 0.13%, 0.33%, and 0.65%), and two main parameters (beam size and reinforcement ratio) were investigated. Furthermore, to ensure flexural failure at the middle‐span, adequate web reinforcing was used in all the beams and tested under a four‐point load until they exhibited failure. Based on regression analysis, an equation was proposed for the rupture modulus of reinforced beams. The findings suggest that, in addition to the yielding strength of the reinforcements and the compressive strength of the concrete, the depth of the beams should be considered when computing the minimum flexural reinforcement of beams.

Analytical Study on Moment-Curvature of Normal and High Strength Concrete Beams

In the present study, reinforced concrete beams areconsidered for the calculation of minimum flexuralreinforcement, Moment-Curvature relationship and ductilityindex using fracture energy. Normal strength concrete i.e. M20,M30, M40, M50 and high strength concrete i.e. M60, M70, M80, M90,M100 are considered in this experiment with the varying grade ofsteel from Fe250, Fe415, Fe500. In a particular grade ofconcrete, minimum flexural reinforcement is decreased with theincrease in grade of steel. In a particular grade of steel,minimum flexural reinforcement is increased with the increasein the grade of concrete. Ductility Index is decreased with theincrease in grade of concrete. Similar trend is observed whenfracture energy is not considered in calculation of minimumpercentage of reinforcement.

Experimental Study on the Structural Behavior of Cast in-situ Hollow Core Concrete Slabs

An experimental work has been carried out to study the flexural behavior of cast in-situ hollow core reinforced concrete (HCRC) slabs constructed by using easy, cost effective and implementable techniques in field. The precast elements made of different easily available affordable material i.e. concrete, polyvinyl chloride (PVC) and plaster of paris having voided cross- sections of circular, rectangular and triangular shapes were incorporated in one direction during pouring of concrete with minimum flexural reinforcement to construct HCRC slabs. A total of 14 slab specimens including 02 specimens per specification were tested with third point loading for the assessment of flexural behavior as per ASTM standards C78/C78M. The flexural behavior of HCRC slabs with polyvinyl and plaster of paris elements having hollow cross-sections was comparable with the control solid slabs, however, HCRC slab with concrete pipes showed 7 to 8 percent reduction in flexural strength with 19 to 20 percent reduction in self-weight. All the tested specimens performed well in shear as no shear failure was observed. This study reveals that HCRC slabs with locally available material having hollow cross section elements can be used for the construction of cast in-situ monolithic construction of one-way slabs with ordinary construction techniques. Doi: 10.28991/cej-2020-03091597 Full Text: PDF

Influence of Minimum Tension Steel Reinforcement on the Behavior of Singly Reinforced Concrete Beams in Flexure

The requirements of minimum flexural reinforcement in the last decades have been a reason for controversy. The structural behavior of beams in bending is the best way of investigating and evaluating the minimum reinforcement in flexure. For this purpose, twelve singly reinforced concrete beams with a rectangular cross-section of (125 mm) width by (250 mm) height and (1800 mm) length were cast and tested under two-point loads up to failure. These beams were divided into three groups with different compressive strengths (25, 50, and 80 MPa). Each group consists of four beams with different amounts of tension steel reinforcement approximately equal to (0% Asmin, 50% Asmin, 100% Asmin and 150% Asmin), two bar diameters (Ø6 mm and Ø8 mm) were used as the longitudinal tension reinforcement with different yield and ultimate strengths, the minimum amount of reinforcement required is calculated based on ACI 318M-2014 code. The results show that for the reinforced concrete beams, the flexural reinforcement in NSC beams increases the first cracking load and the increment increased with an increasing amount of reinforcement, while for HSC beams the increasing in first cracking load are very little when the quantity of reinforcement less than the minimum flexural reinforcement and increased with the increasing amount above the minimum flexural reinforcement. The equation of ACI 318M-14 code gives adequate minimum flexural reinforcement for NSC and overestimate value for HSC up to (83 MPa), A new formula is proposed for HSC rectangular beams up to (90 MPa) concrete compressive strength by reducing the equation of ACI 318M-14 code for minimum flexural reinforcement by a factor depending on concrete compressive strength.

Need for Fracture Behavior based Designer Friendly Expressions for Fracture Energy and Minimum Flexural Reinforcement

Need for Fracture Behavior based Designer Friendly Expressions for Fracture Energy and Minimum Flexural Reinforcement

Concrete Depth on Moment Capacity of Reinforced Concrete Slab: Safety Performance

Aimed to determine whether structural safety response of reinforced concrete slab will fall short of the code target safety value for flexural system and also determine its relation to moment capacity and minimum flexural reinforcement area requirement in consideration of the influence of un-certainties associated with the design parameters. The randomness of the design parameters makes the present deterministic design overly conservative, as such; it is becoming increasingly difficult to ignore the importance of using reliability approach in structural design application, because of its known advantage in accounting for the inherent variability in both loads and resistance variables. The study examined moment capacity violation and parametric sensitivity analysis with different concrete strengths class and span lengths, subjecting the deterministically designed required slab depth to further decrease and obtained the safety index value using First Order Reliability Method. The results indicate that concrete strength has little influence on minima reinforcement requirement with a value of about 3% surge for 500 mm change in span, which also results in increased ultimate moment of resistance of the concrete section with marginal decline in safety value. Interestingly, the safety value from the reliability analysis with optimized slab depth is well above the target safety limit specified for RC slab.

Analytical solutions for flexural design of hybrid steel fiber reinforced concrete beams

Hybrid reinforced concrete (HRC) is referred to as a structural member that combines continuous reinforcement with randomly distributed chopped fibers in the matrix. An analytical model for predicting flexural behavior of HRC which is applicable to conventional and fiber reinforced concrete (FRC) is presented. Equations to determine the moment–curvature relationship, ultimate moment capacity, and minimum flexural reinforcement ratio are explicitly derived. Parametric studies of the effect of residual tensile strength and reinforcement ratio are conducted and results confirm that the use of discrete fibers increases residual tensile strength and enhances moment capacity marginally. However improvements in post-crack stiffness and deformation under load is substantial in comparison to conventional steel reinforcement. Quantitative measures of the effect of fiber reinforcement on the stiffness retention and reduction of curvature at a given applied moment are obtained. The approach can also be presented in a form of a design chart, representing normalized moment capacity as a function of residual tensile strength and reinforcement ratio. Numerical simulations are conducted on the steel fiber reinforced concrete (SFRC) and HRC beam tests from published literature and the analytical solutions predict the experimental flexural responses quite favorably.

Minimum flexural reinforcement in rectangular and T‐section concrete beams

AbstractThe prescriptions provided by codes of practice for assessing the minimum reinforcement amount for strength purposes in reinforced concrete beams usually disregard the non‐linear contribution of concrete in tension and size‐scale effects. In the present paper, these phenomena are taken into account correctly in the description of the flexural failure in lightly reinforced concrete beams by means of a numerical algorithm based on non‐linear fracture mechanics. In this context, the application of dimensional analysis permits a reduction in the number of governing parameters. In particular, it is demonstrated analytically that only two dimensionless parameters, referred to as reinforcement brittleness number and stress brittleness number, are responsible for the brittle‐to‐ductile transition in the mechanical response. According to this approach, new formulae suitable for evaluating the minimum reinforcement in practical applications is proposed for both rectangular and T‐sections. A comparison with experimental results demonstrates the effectiveness of the proposed model for different reinforcement percentages and beam depths.

유로코드에 규정된 최소철근비를 갖는 보의 휨거동에 대한 실험적 연구

유로코드에 규정된 최소철근비를 갖는 보의 휨거동에 대한 실험적 연구

Performance evaluation of HSC beams with low flexural reinforcement

The main objective of the current research is to establish experimental data for minimum flexural reinforcement, ρmin, of high strength concrete (HSC) rectangular beams. Nine full-scale singly reinforced beams with flexural reinforcement ratios varying from 50% to 100% of the minimum limit specified by the ACI 363R-35were tested in flexure. Concrete compressive strengths of 52, 73 and 96.5MPa were used. The test results including crack patterns, deflections and strains in the tensile flexural steel bars show that a 25% reduction of the ACI 363R-35 limit for the ρmin would result in a satisfactory flexural beam behavior with a reserve flexural parameter (Py,/Pcr)⩾1.29 and a displacement ductility index λΔ>5 for all concrete grades which may lead to good savings in the amount of the flexural reinforcement. Also, it was noted that the displacement ductility index λΔ increased as the concrete compressive strength increased for the same ratio (ρ/ρmin) up to 75MPa and then decreases as fcu increases. For the same concrete compressive strength with low values of flexural reinforcement ratio, ρ, the displacement ductility index λΔ increased as ρ increased. The experimental results of this study were compared with the limits specified by available codes and researches.

Minimum shear reinforcement for thick plates and two-way slabs

Thick reinforced concrete plates of over 250mm thickness usually exhibit brittle shear failure under concentrated loads. Potential punching failures in the vicinity of concentrated loads have already been taken into consideration by conventional design methods. The current research is a continuation to previous work published by the authors on minimum flexural reinforcement for thick plates. The main focus of this research is to design adequate shear reinforcement for thick concrete plates. In this paper, new models are developed to calculate minimum shear reinforcements for thick plates, concrete plates and two-way slabs. Design guidelines as well as a design example for minimum shear reinforcement that the design engineers can follow are given. A comparison between the proposed formulae with experimental tests carried by different researchers is conducted. A comprehensive review of different design codes formulae is discussed and compared with the proposed formulae. The proposed formulae are simple and account for member size effect through fracture mechanics concepts. For all equations, (MPa) is used for stresses and (mm) for lengths unless otherwise noted.

Experimental Validation of Minimum Flexural Reinforcement for Thick High-Strength Concrete Plates

First Name is required invalid characters Last Name is required invalid characters Email Address is required Invalid Email Address Invalid Email Address

Making sense of minimum flexural reinforcement requirements for reinforced concrete members

The main aims of this research were to summarize the apparent origin of current minimum reinforcement provisions, examine the margin of safety provided by existing provisions for reinforced concrete members of different sizes and shapes, and propose more consistent results than those from existing provisions. Five existing or proposed methods were included in the study. Parametric analyses show that the proposed method provides the most reasonable margins of safety among the methods examined. The study focuses on determinate reinforced concrete beams, which include only mild tensile reinforcement and no prestressing. High-strength steel and concrete were included. The study also found that, in many cases,, flexural failure at minimum reinforcement levels can be initiated by crushing of the concrete rather than the fracture of the reinforcing steel.

Progressive Collapse of Reinforced Concrete Structures: A Multihazard Perspective

Through accident or act of terrorism, structures may be subject to conditions that could lead to progressive collapse. Redistribution of loads following an imposed local damage to a structure depends on strength, continuity, redundancy, and deformation and energy dissipation capacities of the structure. For reinforced concrete frame structures, these characteristics depend on the seismic and wind design loads to a great extent. In this paper, using the response of multi-degree-of-freedom and equivalent single-degree-of-freedom systems, it is demonstrated that the vulnerability of frame structures against progressive collapse caused by man-made hazards depends heavily on their resistance to natural hazards. It is also shown that following the loss of a column and in spite of satisfying the current structural integrity requirements, premature beam bottom bar fracture can occur. Such bar fracture can be avoided if the minimum beam bottom continuous bars are set equal to the minimum flexural reinforcement.

Studies on ductility and evaluation of minimum flexural reinforcement in RC beams

Some experimental investigations on ductility and prediction of minimum flexural reinforcement in reinforced concrete (RC) beams are reported. The minimum flexural reinforcement was evaluated using optimum ductility in RC beams. Beams of size 100 mm, 200 mm and 400 mm were tested, which were designed with varying percentages of flexural reinforcement i.e. 0.15, 0.30, 0.60 and 1.0. The beams were tested under four-point loading to study the flexural behaviour under uniform bending moment. The experimentally obtained average compressive strength of concrete was 30 MPa. The influence of beam size (depth) on cracking and normalised ultimate flexural strength, ductility and overall average rotation has been studied. The cracking in RC beams is complex phenomenon in small size beams, while the cracking strength decreases as the depth increases beyond 200 mm. The flexural strength of RC beams, from the present study, appears to decrease as the depth increases. The ductility of RC beams increases as the percentage of flexural reinforcement increases. The ductility number has been derived from dimensional analysis using fracture mechanics principles. The ductility of RC beams decreases as the depth of beams increases. An optimum percentage of flexural reinforcement has been established using optimum ductility number, Np, which is equal to 0.20. The minimum flexural reinforcement was found to decrease as the beam depth increases, and decreases as the yield strength of reinforcement increases.

Prediction of Modulus of Rupture of Concrete

Tensile strength of concrete is an important parameter for determining deflection and minimum flexural reinforcement. Many design codes use the modulus of rupture as the cracking strength. Relations giving the modulus of rupture as a function of concrete compressive strength are based on tests on concrete with a compressive strength lower than 40 MPa. This study presents a statistical study on 395 data points from a large number of research programs with concrete compressive strength ranging from 20 MPa to 130 MPa. It further reveals that the data are largely scattered and that minimum, mean, and maximum values for the modulus of rupture should be defined. New relations are proposed by giving the modulus of rupture as a function of the square root of the compressive cylinder strength of concrete and the compressive cylinder strength of concrete raised to the 2/3 power. The use of the new relations in design codes is illustrated.

Minimum flexural reinforcement for T-beams made of higher strength concrete

Design codes specify minimum flexural reinforcement for reinforced concrete beams. With the extensive use of higher strength concrete, the empirical expressions of the past for minimum flexural reinforcement, in which the concrete strength is not considered, had to be revised. Six reinforced concrete T-beams, having small ratios of flexural reinforcement, were tested to study the behaviour at the positive moment region and to evaluate the code requirements on minimum flexural reinforcement. A criterion was set and evaluations of different minimum reinforcement requirements were made using this criterion and the test data.Key words: beams, crack control, ductility, flexural strength, high strength concrete, minimum flexural reinforcement, reserve strength.