Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

Properties of high-strength steel fiber-reinforced concrete beams in bending

This paper presents research results of ten high-strength reinforced concrete beams and steel fiber-reinforced high strength concrete beams, with steel fiber content of 1% by volume. The enlarged ends of mild carbon steel fibers with three different dimensions were selected. This research shows that the flexural rigidity before yield stage and the displacement at 80% ultimate load in the descending curve are improved, and crack number and length at comparable loads is reduced after the addition of steel fibers. The descending part of the load-displacement curve of the concrete beams without steel fibers is much steeper than that with steel fibers, which shows that the addition of steel fibers makes the high strength concrete beams more ductile.     

Cracking characteristics of reinforced steel fiber concrete beams under short- and long-term loadings

In this paper, an analytical method for the prediction of maximum crack width in reinforced steel fiber concrete (SFC) beams under short-term loading is first presented. The method accounts for the enhanced cracking strength, restraint against crack growth, and reduced tensile steel strains due to the presence of steel fibers. Based on a correlation analysis, a semiempirical formula for the long-term crack widths in reinforced SFC beams under sustained loads is also proposed. Tests were carried out on 10 beams to investigate the effect of steel fiber content on the cracking characteristics in both the short- and long-term. The results indicated that the use of steel fibers greatly reduced the maximum crack widths in reinforced concrete beams. Good agreement was generally obtained between the analytical predictions and test results.     

A finite element assessment of flexural strength of prestressed concrete beams with fiber reinforcement

This paper presents an assessment of the flexural behavior of 15 fully/partially prestressed high strength concrete beams containing steel fibers investigated using three-dimensional nonlinear finite elemental analysis. The experimental results consisted of eight fully and seven partially prestressed beams, which were designed to be flexure dominant in the absence of fibers. The main parameters varied in the tests were: the levels of prestressing force (i.e, in partially prestressed beams 50% of the prestress was reduced with the introduction of two high strength deformed bars instead), fiber volume fractions (0%, 0.5%, 1.0% and 1.5%), fiber location (full depth and partial depth over full length and half the depth over the shear span only). A three-dimensional nonlinear finite element analysis was conducted using ANSYS 5.5 [Theory Reference Manual. In: Kohnke P, editor. Elements Reference Manual. 8th ed. September 1998] general purpose finite element software to study the flexural behavior of both fully and partially prestressed fiber reinforced concrete beams. Influence of fibers on the concrete failure surface and stress–strain response of high strength concrete and the nonlinear stress–strain curves of prestressing wire and deformed bar were considered in the present analysis. In the finite element model, tension stiffening and bond slip between concrete and reinforcement (fibers, prestressing wire, and conventional reinforcing steel bar) have also been considered explicitly. The fraction of the entire volume of the fiber present along the longitudinal axis of the prestressed beams alone has been modeled explicitly as it is expected that these fibers would contribute to the mobilization of forces required to sustain the applied loads across the crack interfaces through their bridging action. A comparison of results from both tests and analysis on all 15 specimens confirm that, inclusion of fibers over a partial depth in the tensile side of the prestressed flexural structural members was economical and led to considerable cost saving without sacrificing on the desired performance. However, beams having fibers over half the depth in only the shear span, did not show any increase in the ultimate load or deformational characteristics when compared to plain concrete beams.     

Finite element analysis of partially prestressed steel fiber concrete beams in shear

This paper presents a finite element formulation for the modeling of the behavior of partially prestressed steel fiber concrete beams in shear. Based on a secant modulus approach, the formulation treats steel fiber concrete as an orthotropic material, characterized by appropriate constitutive relations in the principal compressive and tensile directions. An experimental program with the partial prestressing ratio, the shear span:effective depth ratio, and the volume fraction of steel fibers as test variables was carried out and the deflections of the beam, concrete, and steel strains were monitored and compared with the results of the finite element analysis. The finite element formulation was found to predict the deformational characteristics and the ultimate load of the test beams well. Steel fibres were observed to improve the beam stiffness after the occurrence of first shear crack and to enhance the shear strength of partially prestressed concrete beams significantly.     

Strain compatibility between HPFRCC and steel reinforcement

To build reinforced concrete structures able to mitigate steel corrosion produced by environmental attack, a reduced crack width should appear in tensile concrete. At least in the serviceability stage, fibers added to ordinary concrete could be a way to satisfy this requirement. Depending on the type, on the volume content and on the aspect ratio of fibers, FRC (fiber reinforced concrete) can show a higher ductility and sometimes a higher tensile strength than ordinary concrete. However, with or without fibers, concrete cannot produce tensile strains totally compatible with those of the steel rebars. To overcome this problem, new FRCs, called High Performance Fiber-Reinforced Cementitious Composites (HPFRCC), have been recently tailored to develop an ultra-high ductility. In these composites, since the strain at maximum stress is higher than the steel strain at yielding, strain incompatibility vanishes. In the present paper, in order to prove the existence of compatible strains between steel and HPFRCC, numerical results and experimental measurements are compared. This is possible by introducing a mechanical model of tension-stiffening, and by referring to tests to reinforced HPFRCC elements in tension. The good agreement between theoretical and experimental results is also found for reinforced HPFRCC beams in bending.     

Evaluation of crack width in FRC structures and application to tunnel linings

When steel bars are placed in a concrete structure, the evaluation of crack width and crack spacing is generally required in the serviceability stage. According to more or less aggressive conditions, crack width shall be limited in order to avoid, for instance, the corrosion of steel reinforcement. The presence of fibers in the concrete cast may help to achieve this goal, since fibers remarkably increase the bridging actions across a crack. However, new mechanical models are needed to evaluate these effects, which are generally neglected by classical approaches. Code requirements are based on semi-empirical formulae, in which the average structural performances are analyzed by referring to a single cross-section, instead of a wide portion of an R/FRC or RC element in bending. To evaluate crack patterns more accurately, a suitable block model is therefore introduced in this paper. With the new approach, the bridging effects of fibers, as well as the bond-slip mechanism between steel bars and FRC in tension, are taken into account. By means of such model, it is possible ble to predict at one time the values of crack width, crack spacing, and crack depth, and compare them to data obtained by bending tests on concrete beams. Moreover, to evaluate the possible crack patterns in R/FRC tunnel linings, the proposed block model has been extended to the serviceability stage of massive structures subjected to combined compressive and bending actions. This paper follows a previous work by the same authors (Chiaia et al. Mater Struct 40(6):593–694, 2007) and completes the design procedures for FRC cast-in-place tunnel linings.     

Probabilistic analysis of the cracking of RC beams

Three beams of rectangular cross-section and having same cross-sectional dimensions were tested in two-point bending, over an effective span of 4.2 m. All three beams contained steel only in the tension zone and the tension steel was distributed in three different ways. A deterministic analysis of strains, crack spacings and crack widths for all three beams is carried out and the results are compared with the respective experimental values. A probabilistic analysis of strains, crack spacings and crack widths is performed for all the three beams at different stages of loading.     

Flexural strength of reinforced concrete T-beams with steel fibers

The ultimate strength of reinforced concrete T-beams reinforced with conventional steel bars and short discontinuous steel fibers are studied. It is found that the presence of steel fibers reduced effectively the deflection, width of cracks and also improved the ductility and flexural rigidity of the concrete beams. Hence, an appreciable increase to the ultimate compressive strain is observed as well as the increase in the ultimate compressive strength. These are reflected by an increase in the value of the compressive block parameters. In addition, an increase in tensile strength is achieved and a rectangular tensile stress distribution is proposed. It was found that a negligible difference in moment capacity between overreinforced and underreinforced concrete beams. Therefore, it may be economical to use more amount of tension reinforcement than that allowed by the codes. Theoretical equations are developed to calculate the ultimate strength of reinforced concrete T-beams taking into account the effect of amount of compression reinforcement and amount of steel fibers. Theoretical equations show good agreement when compared with experimental results.     

Use of permanent ferrocement forms for concrete beam construction

The results of an experimental investigation to examine the feasibility and effectiveness of using precast U-shaped ferrocement laminates as permanent forms for construction of reinforced concrete beams are presented in this paper. The precast permanent ferrocement forms are proposed as a viable alternative to the commonly used wooden and/or steel temporary forms. The experimental program comprised casting and testing of three control reinforced concrete beams of dimensions 300?×?150?×?2000?mm and eighteen beams of total dimensions of 300?×?150?×?2000?mm consisting of a reinforced concrete core cast in a precast U-shaped permanent ferrocement form of thickness 25?mm. Each control beam was reinforced with two steel bars of 12?mm diameter at the top and bottom of the beam and stirrups of 10?mm diameter placed at 200?mm intervals. The concrete core of the beams incorporating permanent ferrocement forms was reinforced with two steel bars of 12?mm diameter placed at the tension side of the beam without any stirrups. Three types of steel mesh were used to reinforce the ferrocement laminate. These types are: woven wire mesh, ×8 expanded wire mesh, and EX156 expanded wire mesh. Single layer and double layers of each type of the steel mesh were employed. All specimens were tested under three-point flexural loadings. The performance of the test beams in terms of strength, stiffness, cracking behavior and energy absorption was investigated. The results showed that high serviceability and ultimate loads, crack resistance control, and good energy absorption properties could be achieved by using the proposed ferrocement forms.     

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.     

Shear strength of concrete beams reinforced with steel bars milled from scrap metals

The paper reports a study on the shear resistance of concrete beams reinforced with mild steel bars that are milled from scrap metal such as old vehicle parts and obsolete machinery. It has been previously reported that because the chemical compositions of carbon, sulphur and phosphorus in these reinforcing steel bars exceed the maximum allowable limits, the characteristic tensile strengths are too high and ductility too low for standard mild steel. Concrete beams reinforced with such bars to resist flexural tensile and shear stresses were tested under a two-point loading system to provide a central constant moment region and outer shear spans. Tested beams exhibited little deflection and very low ductility prior to collapse. Experimental failure loads for the beams averaged 123% of the theoretical failure load, which was generally governed by either shear or yielding of the tension steel. Shear failure was mostly initiated by diagonal tension cracks, followed by either crushing of the concrete, or splitting of the concrete over the longitudinal tensile bars near the supports. Failure of the beams was brittle and the post-cracking strain energy absorption averaged 357.9 Nm. At failure the maximum crack width in the beams ranged from 1.12 to 5.0 mm, the largest sizes forming in the diagonal shear cracks.     

The diagonal tension behavior of fiber reinforced concrete beams

This research studied the diagonal tension behavior of 16 beams reinforced with longitudinal bars and steel fibers. The variable parameters included the concrete compressive strength and the percentage of fibers (0%, 0.5%, 1.0% and 1.5% by volume). The beams were tested under static loads resulting in high diagonal tension stresses. The shear reinforcement was composed of stirrups instrumented with strain gages to detect the effect of the fibers on the strains. Research results indicate that as the fiber volume increases, the shear strength and the ductility of the beams increased, providing significantly higher shear strength than specified by the ACI-318 Code.     

Experimental and analytical study of SIMCON tension members

The strength and ductility of slurry infiltrated mat concrete (SIMCON) tension members were investigated both experimentally and analytically to construct a mechanical model for simulating tensile force–displacement relationships. In addition to standard strength testing, special tests were conducted on tension specimens with preset cracks to determine the interaction between steel fibers and the cement matrix near an opening crack. These tests were conducted on two sets of preset-crack specimens: (i) with symmetrically inclined fibers and (ii) with aligned fibers having variable debonded lengths on each side of the crack. Using measured bridging forces of inclined fibers, an efficiency factor of plane random fibers, compared to aligned fibers, was determined to be approximately 0.58. It was found that the ductility of SIMCON mainly stems from plastic deformation of steel fibers rather than fiber pull-out. SIMCON tensile response was characterized by elastic, nonlinear hardening and softening regimes. The hardening response was notch insensitive without multiple crack formation. In the elastic regime, only minute stiffness reduction was observed. The nonlinear hardening regime was characterized by internal damage growth without visible crack formation and ended with the appearance of a co-linear set of partial cracks. The softening regime was described by a localized failure of fibers with variable failure strains at the co-linear cracks. Based upon the experimental observation that a co-linear set of partial cracks form at the ultimate composite stress, upper and lower bounds of the SIMCON stress–strain relation in the hardening regimes were obtained.     

Analytical and experimental investigations on the fracture behavior of hybrid fiber reinforced concrete

In the present study, Mode-I fracture tests of hybrid fiber reinforced concrete (HFRC) composite beams were conducted and the fracture properties and other post peak strength characteristics of the HFRC composites were evaluated and analyzed. The HFRC composite was produced using three types of fibers namely steel, Kevlar and polypropylene. A total of 27 HFRC composite beam specimens were cast and tested using the RILEM recommended three point bending test. The main variables were the fiber volume content and combinations of different fibers. The load versus crack mouth opening displacement (CMOD) curves of HFRC composite beams were obtained. Inverse analysis was carried out to determine the tensile strength and crack opening relationship. Analytical models based on comprehensive reinforcing index were developed for determining the influence of the fibers on fracture energy, flexural tensile strength, equivalent tensile strengths and residual tensile strengths of HFRC composites. Based on the experimental results and inverse analysis, a model for predicting the tensile softening diagram of HFRC composite mixes was also developed. The analytical models show conformity with the experimental results.     

Influence of concrete strength on crack development in SFRC members

Tension stiffening is still a matter of discussion into the scientific community; the study of this phenomenon is even more relevant in structural members where the total reinforcement consists of a proper combination of traditional rebars and steel fibers. In fact, fiber reinforced concrete is now a worldwide-used material characterized by an enhanced behavior at ultimate limit states as well as at serviceability limit states, thanks to its ability in providing a better crack control.This paper aims at investigating tension stiffening by discussing pure-tension tests on reinforced concrete prisms having different sizes, reinforcement ratios, amount of steel fibers and concrete strength. The latter two parameters are deeply studied in order to determine the influence of fibers on crack patterns as well as the significant effect of the concrete strength; both parameters determine narrower cracks characterized by a smaller crack width.     

Crack width prediction of FRC beams in short and long term bending condition

The use of short fibers inside concrete matrix is an effective method for reducing the vulnerability of concrete constructions subjected to harsh environment. The action of the short fibers in reducing the crack opening is the main issue that needs a research effort in order to optimize the expected results. At the moment the analytical prediction of the crack width and spacing in fiber reinforced concrete (FRC) structural elements under bending loads is still an open problem. A crack width relationship for FRC/RC elements similar to those developed for plain concrete structural members would be desirable for designers and engineers involved in the design of FRC structural elements. The recent development of important technical design codes, such as RILEM TC 162 TDF and the new Model Code (MC) 2010, embrace this idea. However further validation of these models by experimental results is still needed. On the other hand the study of the influence of a sustained load on crack width in presence of a short fibers reinforcement is a topic almost unexplored and important at the same time. In this research the cracking behaviour of full-scale concrete beams reinforced with both traditional steel bars and short fibers has been analyzed under short and long term bending condition. A theoretical prediction of crack width and crack spacing was carried out according to international design provisions based on different analytical models. The theoretical results are discussed and compared in order to highlight the differences between the available models and to check the reliability of the theoretical predictions on the basis of the experimental data. A modified relationship to take into account of the presence of stirrups has been proposed on the basis of experimental results; furthermore, some critical aspects, such as the influence of the type of fibers and the effect of loading-time, have been underlined that should be addressed in future research work.     

CFRP-PCPs复合筋嵌入加固钢筋混凝土梁裂缝试验及计算方法研究

通过5根嵌入不同张拉控制应力的碳纤维增强塑料预应力混凝土棱柱体(CFRP-PCPs)复合筋加固钢筋混凝土梁受弯试验,对比分析试验梁的裂缝分布与发展,得到最大裂缝宽度与平均裂缝宽度在静力荷载作用下的变化特性。结果表明: 嵌入CFRP-PCPs复合筋能有效的减少被加固钢筋混凝土梁的裂缝宽度和高度。在试验基础上,根据国家现行混凝土规范,对平均裂缝间距和最大裂缝宽度计算公式进行参数修正,建立了CFRP-PCPs复合筋嵌入加固钢筋混凝土梁最大裂缝宽度计算公式,计算值与试验值吻合较好。     

Determination of the load-carrying capacity of steel fibre reinforced concrete beams

An earlier paper by Purkiss & Blagojevi (Composite Struct., 25, 45–9, 1993) detailed the results from some load-deflection tests on two sets of beams. The first set were reinforced both with two number 8 mm high yield bars and with steel fibres. The second set as a comparison had only high yield bars.

The results reported indicated that at a given load the beams with fibres produced lower deflections and did not give such an extensive ‘yield plateau’ as those beams without fibres. Using a method proposed by Hsu et al. (ACI Struct. J., 89, 650–7, 1992), albeit modified to allow for the tension stiffening of the concrete and the use of a continuous function for the reinforcing bar stress-strain relationship, the theoretical deformations and crack widths may be calculated. The predicted load-deformation results are in good agreement with the experimental values for both sets of beams. The predicted crack widths were determined using the methods in both BS 8110 and ENV 1992-1-1. ENV 1992-1-1 gives good agreement for the beams with bars only and for the beams with fibres up to the point of first visible crack. This is not unexpected as the model used for the tensile zone in the concrete assumes immediate failure after the achievement of the flexural tensile strength.     

Design of SFRC structural elements: post-cracking tensile strength measurement

Utilisation of steel fibre reinforced concrete (SFRC) for designing structural members requires knowledge of the post-cracking tensile response. This paper reviews the experimental characterisation tests and subsequent analysis commonly used for determining the post-cracking tensile properties of SFRC. The experimental program supporting this investigation comprised five different SFRC mixes with fibre volumes ranging from 0.75 to 1.25% used to fabricate a set of characterisation specimens for uniaxial tension tests, notched beam tests and round panel tests carried out in parallel with an extensive experimental program on large scale beams. Characterisation test results allowed a comparison between direct stress–crack opening measurements and the stress–crack openings retrieved from the inverse analysis of bending tests. Discrepancies in post-cracking tensile results obtained with the three types of tests are analyzed and related mainly to test configurations, the presence of a predefined crack, support conditions, fibre orientation, and cracked surface size. Results obtained using material characterisations are then applied to the reproduction of the structural behaviour of large scale beams, documented in a companion paper.