Two-layer pre-stressed beams consisting of normal and steel fibered high strength concrete

The paper is focused on analysis of two-layer bending pre-stressed beams consisting of steel fibered (SF) high strength concrete (HSC) in compressed zone and normal strength concrete (NSC) in tensile zone. Investigation of such beams is important for RC structural design, because calculation of fibers volume ratio is significant, like that of reinforcing steel bars for usual RC elements. In other words, such elements are made of high performance concrete (HPC). There is a growing tendency that more effective HPC structures replace NSC ones, first of all in pre-stressed elements. Definition of the HSC class lower limit, to be used in the compressed zone of a two-layer pre-stressed beam, is given. It was demonstrated that SF have little effect on the beam elastic deflections. However, the ultimate deflections of the section increase because additional potential for plastic energy dissipation (PED) in the bending element. NSC, used in the section tensile zone, contributes additionally about 20% to the section’s PED potential compared to one-layer HSC beams. In order to guarantee sufficient section’s ductility of the pre-stressed beams, required to withstand dynamic loadings, a minimum SF ratio is proposed to be considered. The fibers take the tensile stresses, yielding cracks in the concrete matrix. A design method for calculation of the SF volume ratio, as a function of required ductility, is proposed. A numerical example, illustrating the efficiency of this method is presented.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Transport properties of self compacting concrete with limestone filler or fly ash

The durability of a cementitious material is greatly influenced by the permeability of the material for potentially aggressive substances. As the pore structure of self compacting concrete (SCC) might be different in comparison with traditional concrete (TC), some changes in durability behaviour may occur. At this moment however, it is unclear how significant these differences will be with regard to the concrete practice. In this paper, the gas and water transport in SCC with limestone filler or fly ash is investigated experimentally. Nine different concrete compositions are considered: one TC and eight SCC mixtures. Some important parameters like the water/cement (W/C) and cement/powder ratio (C/P), type of filler (limestone filler and fly ash), type of aggregate and type of cement are considered. The results of the gas and water transport are discussed and linked to experimental data concerning pore volume. Lower transport properties can be obtained by using fly ash instead of limestone as filler material, by lowering the W/C ratio, decreasing the C/P ratio at a constant W/C ratio or using blast furnace slag cement instead of portland cement. The effect of changing from gravel to crushed limestone is small. SCC is differing strongly of TC with respect to the apparent gas permeability. This difference is probably due to the differences in pore volume, as seen from MIP results.     

Investigation of stress-strain models for confined high strength concrete

The effects of confinement reinforcement on the behaviour of high strength concrete columns are investigated for which prismatic experimental specimens were prepared. In the experiment specimens, four longitude reinforcement and confinement reinforcement were used. For each experiment, stress-strain relationship of concrete was obtained and compared with models proposed earlier. The results show that confinement reinforcement improved the ductility of high strength concrete. The ascending branch of stress-strain curves depended on the ratio of confinement reinforcement was similar to the modified Kent-Park model and the descending branch similar to the Nagashima model.     

Size effect on failure of overreinforced concrete beams

The results of full-scale failure of singly reinforced four-point-bend beams of different sizes containing deformed longitudinal reinforcing bars are reported. The tests consisted of four groups with one, two and three different size combinations. The specimens were made of concrete with a maximum aggregate size of 10 mm. The beams were geometrically similar in one, two and three-dimensions, and even the bar diameter and cover thicknesses were scaled in proportion. The reinforcement ratio was 3%. The results revealed the existence of a significant size effect, which can approximately be described by the size effect law previously proposed by Bazant. The size effect is found to be stronger in two-dimensional similarities than for one and three-dimensional similarities.     

Flow behaviour of high strength high-performance concrete

The workability of flowable High-Performance Concrete (HPC) is nowadays mainly measured using conventional test methods such as the slump test or the slump-flow test. These single-point tests do not seem sensitive enough to characterize the high-workability of HPC. Due to the fluid consistency and uniformity of fresh HPC, it is possible to describe its flow properties by using a rheological test method. To evaluate the flowability based on rheology, fresh HPC is regarded as a two-phase material composed of a matrix phase and a particle phase. In the study, the effects of materials and proportioning on the rheological properties were investigated experimentally. A new rheometer was established by conducting a two-point test to investigate the flow behaviour of high strength HPC. Test results show that the high strength HPC with good uniformity and without tendency of segregation can possess the properties of rheology according to Bingham’s equation. An increase of the fraction of mortar in HPC can lead to a more distinct the rheological behaviour. Moreover, it is found that the application of a rheological method can provide more stable results than any other test method in describing the flowability of high strength HPC.     

External reinforcement of high strength concrete columns

With the technology development on the compressive strength of concrete over the years, the use of high strength concrete has proved most popular in terms of economy, superior strength, stiffness and durability due to many advantages it could offer. However, strength and ductility are inversely proportional [J. Mater. Civil Eng. 11 (1999) 21]. High strength concrete is a brittle material causing failure to be quite sudden and ‘explosive' under loads. It is also known that structural concrete columns axially compressed rarely occur in practice. The stress concentrations caused by the eccentric loading further reduce the strength and ductility of high strength concrete. Therefore, studies for high strength concrete columns under eccentric loading are essential for the practical use.

This paper experimentally investigates a number of high strength concrete columns that are externally reinforced with galvanised steel straps and fibre-reinforced polymers subjected to concentric and eccentric loading. The experimental results show that external reinforcement can enhance the properties of high strength concrete columns.     

On the orientation of fibres in structural members fabricated with self compacting fibre reinforced concrete

The incorporation of fibres into concrete produces important benefits, mainly on the residual load-bearing capacity. These improvements depend on the type, content and orientation of the fibres, being a strong relationship between the number of fibres in the fracture surfaces and the post peak parameters. Although the fibres could be homogeneously distributed after mixing, the casting and compaction processes can significantly affect the fibre distribution and orientation, and consequently the mechanical performance of the material. In the case of Fibre Reinforced Self Compacting Concrete (FR-SCC) the existence of significant flow and wall effects may influence fibre orientation. This paper analyzes the fibre orientation in thin structural elements cast with FR-SCC and its effects on the residual mechanical properties. A slab of 0.90 × 1.80 × 0.09 m, a wall of 0.50 × 2.00 × 0.08 m, and a beam of 0.15 × 0.15 × 2.50 m were selected as representative elements where different concrete flow conditions take place. A strong heterogeneity in the orientation of the fibres was found. The fibre orientation varied with the flow rate and with the wall effect; the thickness of the elements or the proximity to the bottom of the moulds appeared as important variables. It was demonstrated that in thin elements the residual mechanical properties can be quite different when diverse zones and/or directions of the structural elements are considered.     

Failure modes and fatigue strength of improved HSS welds

Post-weld improvement methods can significantly improve the fatigue strength of a structure. In some cases, the degree of improvement is limited by alternate failure modes. The material strength and type of loading also influence the observed fatigue crack behaviour. This study reports on crack patterns and strength for both constant and variable amplitude fatigue tests of high strength steel (HSS) welds. Some specimens were in the as-welded state, while others were post-weld treated, using methods generally categorized as residual stress modification processes. Failure modes were significantly different for CA and VA loading and VA loading showed less improvement.     

Shock properties of conventional and high strength concrete: Experimental and mesomechanical analysis

Large scale heterogeneity is a major problem when shock properties of concrete materials shall be derived efficiently. A mesomechanical method proposed earlier [Riedel W. Beton unter dynamischen Lasten: Meso- und makromechanische Modelle und ihre Parameter, Ed.: Fraunhofer-Institut für Kurzzeitdynamik, Ernst-Mach-Institut EMI. Freiburg/Brsg.: Fraunhofer IRB Verlag; 2004. p. 117–43. ISBN:3-8167-6340-5 〈http://www.irbdirekt.de/irbbuch/〉; Thoma K, Riedel W, Hiermaier S. Mesomechanical modeling of concrete shock response, experiments and linking to macromechanics by numerical analysis. In: Proceedings of European conference on computational mechanics, München, Germany, September 1999 (CD-ROM).], combining plate impact experiments on the constituents ‘cement’ and ‘aggregate’ together with simulations of the concrete mesostructure, is extended in this work. Hereby, the parameters describing macroscopic stress wave propagation are analysed in direct simulations of the discontinuous composite of aggregate, mortar and pores. This allows the replacement of an important number of experiments on large samples with inevitable scattering by a reduced set of smaller standard tests together with simulations. The basics and the validation of this methodology are demonstrated in the acoustic regime and compared to available shock experiments for a wide pressure range. The present paper describes the enlargement of the pressure range for the derived equation of state properties. Therefore, the concrete mortar has been impact tested in shock reverberation configuration leading to reflected pressures up to 18 GPa. The shock and equation of state properties are derived for two 35 MPa conventional strength mixtures and a 135 MPa high strength concrete. The results are compared to available literature sources.     

Shear strength of reinforced concrete beams with stirrups

This study presents alternative shear strength prediction equations for reinforced concrete (RC) beams with stirrups. The shear strength is composed of the contribution of the nominal shear strength provided by stirrups and the nominal shear strength provided by concrete. For the concrete contribution, cracking shear strength values estimated by Arslan’s equations are almost same those obtained with ACI 318 simplified equation in terms of coefficient of variation (COV). However, mean values estimated by ACI 318 tend to be more conservative comparing to the mean values obtained with Arslan’s equations. Thus, for the consideration of concrete contribution to shear strength, Arslan’s equations are used. To obtain the shear strength of RC beams, shear strength provided by stirrups is added to the concrete shear strength estimated by Arslan’s equations. Results of existing 339 beam shear tests are used to investigate how accurate proposed equation estimates the shear strength of RC beams. Furthermore, ACI 318 and TS500 provisions are also compared to the aforementioned test results. It is found that proposed equations for beams with shear span to depth ratios (a/d) between 1.5 and 2.5 are also conservative with a lower COV than ACI 318 and TS500. However, when a/d ratios exceed 2.5 (both normal and high strength concrete beams), ACI 318, TS500 and proposed equations give similar COV value.     

Fracture stability and residual strength assessment of reinforced concrete beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, fracture properties, and on the tension-softening behavior of concrete. In this work, a method is proposed to assess the residual strength of reinforced concrete beams subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of percentage reinforcement and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.     

Prediction of moment redistribution in continuous prestressed concrete beams

Abstract

The inelastic behavior of statically indeterminate non‐prestressed concrete structures can be predicted by the trilinear theory proposed recently by the author [17]. In this paper, the trilinear theory is further developed to include prestressed concrete by using concepts defined by α, β, and τ. This generalized theory excellently predicts the moment redistribution of two 50 ft‐long continuous prestressed concrete test beams described in [7].

The research reported in this paper shows that the trilinear momentcurvature relationship for prestressed concrete sections may well be employed in predicting the behavior of continuous beams. The analytical method presented in this paper allows a structural engineer to correctly evaluate the response of continuous prestressed concrete beams throughout the loading history.     

Optimum content of copper slag as a fine aggregate in high strength concrete

This study investigated the mechanical properties of high strength concrete incorporating copper slag as a fine aggregate and concluded that less than 40% copper slag as sand substitution can achieve a high strength concrete that comparable or better to the control mix, beyond which however its behaviors decreased significantly. The workability and strength characteristics were assessed through a series of tests on six different mixing proportions at 20% incremental copper slag by weight replacement of sand from 0% to 100%. The results indicated that the strength of the concrete with less than 40% copper slag replacement was higher than or equal to that of the control specimen and the workability even had a dramatic growth. The microscopic view demonstrated that there were limited differences between the control concrete and the concrete with less than 40% copper slag content. It also suggested that the determination of the copper slag replacement level should consider with the desired compressive strength of concrete.     

Effects of steel fiber addition on the behaviour of biaxially loaded high strength concrete columns

This paper presents the effects of various amounts of steel fibers on the behaviour of eccentrically loaded high strength reinforced concrete columns. A total of 14 both short and slender square section steel fiber and plain high strength reinforced concrete column specimens were constructed and tested to investigate the addition of steel fibers on load–deflection behaviour, ultimate strength capacity, ductility and confinement. The complete nonlinear experimental stress–strain relationships of steel fiber and plain high strength concrete were obtained for different concrete strengths. In the study, a theoretical procedure considering the nonlinear behaviour of the materials is proposed for ultimate strength analysis and load–deflection behaviour of eccentrically loaded columns including slenderness effect. The complete experimental and theoretical biaxial load–deflection curves of the column specimens have been obtained and reported in the paper. The column specimens and some steel fiber columns available in the literature have been analysed for the ultimate strength capacities. Good agreement has been achieved between the analysis and the test results.     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

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.     

Shear design of reinforced concrete beams, slabs and walls

The paper presents the background to the shear design provisions for reinforced concrete beams and slabs used in the Australian practice. Correlation of design equations with experimental results are given. The design provisions are illustrated by examples. The importance of shear strength in the design of structural walls is discussed. A new expression to calculate the shear strength of walls is presented.