Experimental investigation of steel fiber reinforced concrete box beams under bending

Reduction of dead weight of a reinforced-concrete (RC) structure without too much concession in its load carrying capacity has always been an attractive study subject because it engenders (1) a decrease in dimensions of the members, (2) a decrease in the reinforcement steel, and (3) a decrease in lateral inertia forces during severe earthquakes. In this study, nine RC beams of outer dimensions of 300 × 300 × 2000 mm, six of which are box beams, designed and produced using a C20 class steel fiber concrete, (SFRC) with the commonly used steel fiber type of Dramix-RC-80/0.60-BN at a dosage of 30 kg/m³, are tested under bending. The mechanical behaviours of all these nine beams under bending are recorded from the beginning of the test till the ultimate failure of the tensile reinforcement in a two-point beam-loading setup. The proportions of (1) loss in ultimate load versus reduction in dead weight and (2) (ultimate experimental load)/(ultimate theoretical load) of the SFRC box beams are determined for two different box thicknesses. Dimensionless behaviour relationships of all the SFRC beams are determined, and the experimentally obtained relationship between the ratio of (actual ultimate load)/(theoretical ultimate load) and the ratio of (wall thickness)/(beam height) for the SFRC box beams is expressed diagrammatically.     

Snap-back softening instability in high-strength concrete beams

Three-point bending tests on pre-cracked slabs of high-strength concrete are interpreted on the basis of a virtual crack propagation model. As theoretically shown by the model, a snap-back softening instability appears only for initial crack lengths smaller than 0.3 times the beam depth. As a limit-case, when the material is sufficiently brittle or the specimen size is sufficiently large, such an instability can be predicted by the LEFM condition K₁=K_IC.     

Strengthening of concrete beams using fiber-reinforced platics

One application of composite materials in civil engineering is examined: the strengthening of a reinforced concrete beamin situ by externally-bonded fiber reinforced plastic (FRP). Studies of the mechanical properties of the interface and the rheological behaviour of composite materials are very important to design. For the experimental determination of the mechanical properties of the concrete/glue/plate interface, a new test is suggested. An iterative analytical model capable of simulating the bond-slop and the material non-linearity, based on the compatibility of deformations and the equilibrium of forces, is developed in order to predict the ultimate forces and deflections. A new equation is proposed to anticipate the maximal shear and normal stresses at the interface goal to anticipate the failure mode due to the debonding of the plate. Finally, a series of large-scale beams strengthened with fiber reinforced plastic is tested up to failure; load-deflection curves are measured and compared with the predicted values to study the efficiency of the externally-bonded plate and to verify the theoretical method.

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Résumé Nous examinons un cas d'application des matériaux composites pour le génie civil: le renforcement par placage de tissus composites de poutres en béton armé. La connaissance des propriétés d'adhérence béton-composites et du comportement rhéologique de ces matériaux est indispensable. Pour cela, un nouvel essai a été proposé pour déterminer les propriétés mécaniques de l'interface béton/colle/plaque. Pour dimensionner les ouvrages, nous proposons une méthode d'analyse itérative capable de simuler la non-linéarité des matériaux et le glissement des plaques afin d'évaluer les niveaux de portance de ces structures. Une nouvelle equation a été développée pour prévoir la contrainte maximum de l'interface et finalement une série de poutres renforcées par placage de tissus composites a été testée jusqu' à rupture pour étudier l'efficacité de la méthode de placage et vérifier la méthode de calcul.

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Editorial note Prof. Patrice Hamelin is a RILEM Senior Member.     

Plastic rotation capacity of high-strength concrete beams

This work describes an experimental study on the plastic rotation capacity of high strength beams. Nine simply supported isotatic beams were tested, by applying comprising two symmetrical concentrated loads at approximately one-third and two-third's span. A method of analysis is defined that leads to the development of a parameter that characterizes the plastic rotation capacity at the failure section by means of a plastic analysis of the tested beams. The influence of concrete strength and the longitudinal tensile reinforcement ratio on the capacity for plastic rotation is examined and discussed. The results are discussed and compared with previous studies.

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Résumé Ce travail décrit une étude expérimentale sur la capacité de rotation plastique de poutres en béton à haute résistance. Neuf poutres isostatiques ont été testées avec une charge constituée par deux forces concentrées et symétriques situées environ au tiers et aux deux tiers de la portée. Une méthode d'analyse est définie, conduisant au développement d'un paramètre qui caractérise la capacité de rotation plastique de la section de rupture, utilisant une analyse plastique des poutres testées. L'influence de la résistance du béton et du taux d'armatures longitudinales de traction sur la capacité de rotation plastique est examinée et discutée. Les résultats sont analysés, discutés et comparés avec des études antérieures.

     

Recent trends in steel fibered high-strength concrete

Steel fibered high-strength concrete (SFHSC) became in the recent decades a very popular material in structural engineering. High strength attracts designers and architects as it allows improving the durability as well as the esthetics of a construction. As a result of increased application of SFHSC, many experimental studies are conducted to investigate its properties and to develop new rules for proper design. One of the trends in SFHSC structures is to provide their ductile behavior that is desired for proper structural response to dynamic loadings. An additional goal is to limit development and propagation of macro-cracks in the body of SFHSC elements. SFHSC is tough and demonstrates high residual strengths after appearance of the first crack. Experimental studies were carried out to select effective fiber contents as well as suitable fiber types, to study most efficient combination of fiber and regular steel bar reinforcement. Proper selection of other materials like silica fume, fly ash and super plasticizer has also high importance because of the influence on the fresh and hardened concrete properties. Combination of normal-strength concrete with SFHSC composite two-layer beams leads to effective and low cost solutions that may be used in new structures as well as well as for retrofitting existing ones. Using modern nondestructive testing techniques like acoustic emission and nonlinear ultrasound allows verification of most design parameters and control of SFHSC properties during casting and after hardening. This paper presents recent experimental results, obtained in the field SFHSC and non-destructive testing. It reviews the experimental data and provisions of existing codes and standards. Possible ways for developing modern design techniques for SFHSC structures are emphasized.     

Experience with high-strength concrete in combination with high-strength steel in precast reinforced and prestressed concrete

This paper deals with the experience gained before the war with centrifugally moulded (spun) concrete masts, tubular and half-tubular beams, reinforced with high strength steel and after the war with prestressed, vibrated high strength precast concrete, including lightweight concrete, with the advantage of gap-grading, particularly when employing phased frequencies at vibration.     

Relationship between impact energy and compression toughness energy of high-strength fiber-reinforced concrete

A simple, economical, and practical drop-weight impact testing machine was developed to determine the impact resistance for high-strength fiber-reinforced concrete (HSFRC) composite. Impact and compression tests were carried out on concrete cylinders reinforced with three different aspect ratios of hooked-end steel fibers l/d (length/diameter): 60, 75, and 83 (30/0.50, 60/0.80, and 50/0.60 mm/mm), and four different percentages of steel fibers 0.5%, 1.0%, 1.5% and 2.0% by volume of concrete. For each aspect ratio and volume of fibers, complete stress–strain curves of HSFRC were generated in order to determine the total energy absorbed for each cylindrical specimen in compression. The addition of steel fibres to concrete has improved impact resistance and also the compression toughness. The test results showed that a logarithmic relation exists between compression toughness energy (E_Ct) by means of the generated stress–stress curves from the compressive tests and the impact energy (E_I) by means of the modified impact machine for HSFRC at different l/d ratio of 60, 75, and 83.     

Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.     

Effects of stirrup,steel fiber,and beam size on shear behavior of high-strength concrete beams

This study investigates the effectiveness of steel fibers and minimum amount of stirrups on the shear response of various sized reinforced high-strength concrete (HSC) beams. For this, six large reinforced HSC beams with a shear span-to-depth ratio (a/d) of 3.2 were manufactured. Three of them contained 0.75% (by volume) steel fibers without stirrups as per ACI Committee 318, while the rest were reinforced with the minimum amount of stirrups without fibers. Test results indicate that, with increasing beam size, significantly lower shear strength was obtained for steel fiber-reinforced high-strength concrete (SFR-HSC) beams without stirrups, than for the plain HSC beams with stirrups. The inclusion of steel fibers effectively limited crack propagation, produced more diffused initial flexural cracks, and led to higher post-cracking stiffness, compared to plain HSC. On the other hand, the use of minimum stirrups gave better shear cracking behaviors than that of steel fibers, and effectively mitigated the size effect on shear strength. Therefore, a large decrease in shear strength, with an increase in the beam size, was only obtained for SFR-HSC beams without stirrups. A shear strength decrease of 129% was obtained by increasing the effective depth from 181 mm to 887 mm. The shear strengths of reinforced steel fiber-reinforced concrete beams were not accurately predicted by most previous prediction models. Therefore, a new shear strength formula, based on a larger dataset, that considers the size effect, is required.     

Ductile tearing assessment of high-strength steel X-joints under in-plane bending

This study reports an experimental investigation of a fatigue-cracked, pre-notched circular hollow section X-joints fabricated from high strength steels (with the yield strength higher than 800 MPa) subjected to brace in-plane bending. The circular hollow section X-joint entails a prefabricated V-notch near the weld toe at the crown position. The experimental procedure applies a fatigue pre-cracking cyclic load followed by a monotonic brace in-plane bending, which leads to brittle through-thickness crack propagation after some amount of ductile tearing. The ductile tearing assessment, integrating the fracture resistance curve obtained from the small-scale fracture specimens and the crack extension in the large-scale tubular joint, predicts closely the load level at which unstable crack extension takes place. The generic level 2A curve outlined in the BS7910 provides an un-conservative estimate on the failure load of the X-joint specimen. The parametric numerical investigation reveals that the strength definition for the cracked joints imposes a significant effect on the shape of the failure assessment curve.     

Fretting fatigue behavior of high-strength steel monostrands under bending load

In this paper, the fretting fatigue behavior of pretensioned high-strength steel monostrands is investigated. To measure the local deformations on the strands, a novel method based on the digital image correlation (DIC) technique was used to quantify the relative movement between individual wires along the length of the monostrand. Information about the monostrand bending stiffness and the extent of relative displacement between core and outer wires of a monostrand undergoing flexural deformations is provided. From the series of dynamic fatigue tests, a fretting fatigue spectrum is derived and compared with the localized bending fatigue spectrum. The presented spectra can be used for the estimation of monostrand bending fatigue life. The results presented herein form the basis for the development of a fretting failure criterion for monostrand cables experiencing transverse displacements and are of special interest for the fatigue analysis of modern stay cable assemblies where fretting constitutes a major mechanism of the fatigue life reduction.     

Mixed-mode fracture toughness testing of concrete beams in three-point bending

Results obtained for mixed-mode fracture toughness parameters K _c , G _c , J _c , G _F (plane strain mixed-mode stress-intensity factor, energy release rate, J-integral and fracture energy, respectively) for small notched concrete beams in bending indicate that all these parameters decrease with x/S (x is the distance from support, S is the span) in general to values near midspan consistent with Mode I results. All the parameters except J _c vary with notch depth in a similar manner for each notch location.     

Analysis of metallic fibre-reinforced concrete beams submitted to bending

Increasing the use of metallic-fibre reinforced concrete (MFRC) in structures is related to the definition of analytic methods. In choosing a pertinent kinematic mechanism, the principles deduced from reinforced concrete analysis may be applied to these materials, provided that the material behaviour has been determined. Thus, a model of the bending behaviour of beams is proposed based on the equilibrium of forces in a cracked section. This approach explicitly takes into account the crack opening. To provide an adequate material behaviour, a uniaxial tensile test is presented. It gives a Force-Crack Opening relationship. Samples are cored in the beams to represent thein-situ material. Calculations are compared with experimental data. These tests were performed using three types of fibres and two beam sizes. An extension of the approach to combined flexure and axial load is compared with data appearing from the literature.

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Résumé Le développement de l'utilisation des bétons renforcés de fibres métalliques dans les structures est lié à la définition de méthodes de calcul. à partir d'un mécanisme cinématique pertinent, les principes utilisés pour le calcul de structures en béton armé peuvent être adaptés à ces matériaux, à condition de déterminer correctement leur comportement. On propose ainsi une modélisation de poutres, soumises à une flexion, basée sur l'équilibre d'une section fissurée. Ce modèle prend explicitement en compte l'ouverture de fissure. Un essai de traction uniaxiale est utilisé pour déterminer le comportement du matériau. Il fournit une relation effort-ouverture de fissure. Les éprouvettes sont carottées dans les poutres afin de représenter le matériauin situ. Le modèle est comparé à des résultats expérimentaux. Les essais ont été réalisés en utilisant trois types de fibres et deux géométries de poutres. Une extension de l'approche à la flexion composée est comparée à des résultats de la littérature.

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Editorial note: Pierre Rossi is working at the Laboratoire Central des Ponts et Chaussées, a French RILEM Titular Member. Pierre Rossi was a member of RILEM Technical Committee 90 FMC on Fracture Mechanics of Concrete, he is still active as a member of the Editorial Group of this committee. He is also participating in the work of the recently set up Technical Committee on Test and Design Methods for Steel Fibre Reinforced Concrete.     

Bond behavior of GFRP and steel bars in ultra-high-performance fiber-reinforced concrete

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data.     

Nonlinear finite element analysis of reinforced concrete beams strengthened by fiber-reinforced plastics

Numerical analyses are performed using the ABAQUS finite element program to predict the ultimate loading capacity of rectangular reinforced concrete beams strengthened by fiber-reinforced plastics applied at the bottom or on both sides of these beams. Nonlinear material behavior, as it relates to steel reinforcing bars, plain concrete, and fiber-reinforced plastics is simulated using appropriate constitutive models. The influences of fiber orientation, beam length and reinforcement ratios on the ultimate strength of the beams are investigated. It has been shown that the use of fiber-reinforced plastics can significantly increase the stiffnesses as well as the ultimate strengths of reinforced concrete beams. In addition, with the same fiber-reinforced plastics layer numbers, the ultimate strengths of beams strengthened by fiber-reinforced plastics at the bottom of the beams are much higher than those strengthened by fiber-reinforced plastics on both sides of the beams.     

BFRP筋钢纤维高强混凝土梁受弯承载力试验与理论

通过11根玄武岩纤维增强聚合物复合材料（BFRP）筋钢纤维高强混凝土梁的受弯性能试验,研究了钢纤维混凝土层厚度、钢纤维体积分数和BFRP筋配筋率对BFRP筋钢纤维高强混凝土梁受弯破坏形态及其承载力的影响。结果表明,BFRP筋钢纤维高强混凝土梁的破坏模式可分为受压破坏、受拉破坏和平衡破坏3种;钢纤维混凝土层厚度和钢纤维体积分数的变化对于BFRP筋钢纤维高强混凝土梁受弯承载力具有一定程度的影响,当BFRP筋配筋率为0.77%时,掺加体积分数为1.0%钢纤维的梁受弯承载力较无钢纤维梁提高了22.7%,在受拉区0.57倍截面高度内掺加1.0vol%钢纤维的梁受弯承载力达到全截面钢纤维混凝土梁受弯承载力的86.7%;增大BFRP筋配筋量可显著提高BFRP筋钢纤维高强混凝土梁的受弯承载力,BFRP筋配筋率为1.65%的试验梁受弯承载力较配筋率为0.56%的试验梁提高了39.4%。针对不同的破坏模式,提出了BFRP筋钢纤维高强混凝土梁受弯承载力和平衡配筋率的计算方法,并结合安全配筋率的概念对试验梁的破坏模式进行了预测,试验结果与分析结果吻合良好。     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Mechanical behaviour of thermally damaged high-strength steel fibre reinforced concrete

The incorporation of steel fibres can compensate the inherently brittle behaviour of high strength concrete. This paper studies the residual mechanical behaviour of thermally damaged high strength steel Fibre Reinforced Concrete (FRC). The type and content of fibres were included as variables, a mortar and a normal strength fibre concrete were also tested. Two exposure conditions werre selected, I hour at 500°C and 24 hours at 150°C. FRC follow similar residual compressive behaviour as the plain concrete, but the presence of fibres lead to slight increase in strength and in the stress at which cracks initiate. Flexural tests on notched beams were performed (RILEM TC 162-TDF recommendation). It was found that the shape of the load-deflection curves in FRC exposed to 150°C was similar to the undamaged concrete. The reductions in flexural strength were lower in FRC than in plain concrete, and the equivalent post-peak strength was less affected than first-crack strength, showing the effect of fibre reinforcement. For the most severe exposure condition the degradation of the material is reflected by an increased non-linearity, nevertheless some FRC still exhibited a strengthening type behaviour and kept an almost constant load capacity during the post-peak.

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Résumé L'incorporation des fibres d'acier dans le béton peut compenser le comportement fragile du béton de haute résistance. Est étudié dans ce travail le comportement mécanique résiduel du béton de haute résistance renforcé de fibre d'acier (BRF) endommagé par la température. On a considéré comme variables le type et le dosage de fibres; et on a aussi étudié un mortier et un béton de résistance normale. On a adopté deux conditions d'exposition à haute température, 1 heure à 500°C et 24 heures à 150°C. Les bétons renforcés de fibres d'acier (BRF) présentent un comportement résiduel en compression identique à celui du béton normal, mais la présence de fibres produit un léger accroissement de la résistance à la compression et de la contrainte où la formation de fissures commence. On a réalisé des essais de traction en flexion dans des éprouvettes prismatiques entaillées (recommandation de la commission technique RILEM TC 162-TDF). Les courbes charge-déformation du BRF exposé à 150°C présentent la même forme que celles du béton de référence. Par rapport au béton normal, les BRF ont une réduction inférieure de la résistance en flexion; de plus, la résistance équivalente après pic a·été moins affectée que la résistance des premières fissures, mettant en évidence l'effet de la présence des fibres. On a trouvé un accroissement de la non-linéarité dans les échantillons plus endommagés, alors que les BRF présentaient toujours un comportement de renforcement et maintenaient une capacité de charge pratiquement constante pendant l'après-pic.

     

Stability of sulphur concrete beams with steel reinforcement

Tests are reported on the structural behaviour of unmodified and modified sulphur concrete beams with steel reinforcement and subjected to air and water curing. The sulphur concrete contained fly ash as a filler and both dicyclopentadiene and dipentene were used as modifiers. The tests were conducted at three months and one year. Unmodified concrete beams showed enhanced strength, stiffness and ductility with ageing when cured in a dry environment. Modified concrete beams showed improved behaviour compared to unmodified beams, but even when dry, they showed strength loss with ageing and their long term stability is open to question. Ageing in a wet environment has a destructive effect on the structural behaviour of all sulphur concrete beams. Loss of strength and softening of sulphur concrete lead to shear failures, and severe corrosion and cracking occur. Stability of sulphur concrete beams can be guaranteed only when they are unmodified and kept dry. In a wet regime sulphur concrete cannot have stability and durability.

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Résumé On rend compte d’essais du comportement structural de poutres de béton de soufre avec armatures d’acier, les unes traitées les autres non, conservées dans l’air et dans l’eau. Des cendres volantes ont été utilisées comme charge et les dicyclopentadiène et dipentène comme agents modificateurs. Les essais ont été conduits sur 3 mois et 1 an. On a constaté une amélioration des propriétés de résistance, de rigidité et de ductilité pour des poutres de béton non traitées dans le vieillissement en atmosphère sèche. Le comportement des poutres de béton traitées a été supérieur à celui des poutres non traitées, mais, même en milieu sec, elles ont accusé une perte de résistance avec le vieillissement et on peut s’interroger sur leur stabilité à long terme. Le vieillissement en milieu humide a un effet destructif sur le comportement structural de toutes les poutres de béton de soufre. La perte de résistance et l’adoucissement du béton soufré conduisent à la rupture par cisaillement; d’autre part une corrosion et une fissuration importantes se produisent. On ne peut garantir la stabilité des poutres de béton de soufre que lorsqu’elles sont non traitées et conservées à sec. En régime humide, le béton de soufre ne peut être ni stable ni durable.