Elastodynamic behaviour of fibre-reinforced cement

The elastodynamic properties of anisotropic, short fibre-reinforced, thin cement sheets have been determined by frequency measurements in the vicinity of longitudinal, flexural and torsional resonance frequencies of rod samples in their different modes of vibration. The experimental techniques described in the present paper provide results of good accuracy. In-plane and transverse tensile and shear moduli are given for one type of material with plastic and cellulose fibre reinforcement and the influence of humidity, age and temperature on damping and stiffness are discussed.     

Influence of yield stress and compressive strength on direct shear behaviour of steel fibre-reinforced concrete

This study aims in examining the influence of the paste yield stress and compressive strength on the behaviour of fibre-reinforced concrete (FRC) versus direct shear. The parameters studied are the steel fibre contents, the aspect ratio of fibres and the concrete strength. Prismatic specimens of dimensions 10 × 10 × 35 cm made of concrete of various yield stress reinforced with steel fibres hooked at the ends with three fibre volume fractions (i.e. 0%, 0.5% and 1%) and two aspects ratio (65 and 80) were tested to direct shear. Three types of concretes with various compressive strength and yield stress were tested, an ordinary concrete (OC), a self-compacting concrete (SCC) and a high strength concrete (HSC). The concrete strengths investigated include 30 MPa for OC, 60 MPa for SCC and 80 MPa for HSC.The results show that the shear strength and ductility are affected and have been improved very significantly by the fibre contents, fibre aspect ratio and concrete strength. As the compressive strength and the volume fraction of fibres increase, the shear strength increases. However, yield stress of concrete has an important influence on the orientation and distribution of the fibres in the matrix. The ductility was much higher for ordinary and self-compacting concretes (concrete with good workability). The ductility in direct shear depends on the fibre orientation and is significantly improved when the fibres are perpendicular to the shear plane. On the contrary, for concrete with poor workability, an inadequate distribution and orientation of fibres occurred, leading to a weak contribution of the fibres to the direct shear behaviour.     

Bond behaviour of reinforcement in self-compacting concretes

This experimental work examines the bond strength between reinforcement steel and concrete, and the top-bar effect in self-compacting concretes. Eight different concretes were used, four self-compacting (SCC) and four normally-vibrated (NVC). Tests were conducted on 200 mm cube specimens and 1500 mm high columns. It was found that, at moderate load levels, SCC performed with more stiffness, which resulted in greater mean bond stresses. The ultimate bond stresses are also somewhat greater although, due probably to the negative effects of the bleeding having less impact on failure, the differences between SCC and NVC are reduced considerably, and even disappear completely for concretes of more than 50 MPa. On the other hand, the top-bar effect is much less marked in SCC, and therefore a change in the factor that takes into account this effect in the formulas used for calculating the anchorage length of the reinforcement is proposed for these concretes.     

Properties of polymer modified steel fiber-reinforced cement concretes

Polymer modified steel fiber-reinforced concretes were produced with addition of both steel fibers and a styrene butadiene rubber emulsion (SBR). Both flexural and compressive strength of the composites after 28 days curing were tested. Microstructures of the composites were analyzed by using scanning electron microscope and mercury intrusion porosimetry. Results show that the addition of steel fibers increases both flexural and compressive strength of the composites. The flexural strength increases significantly when containing 3–10 wt.% SBR. The optimal use of SBR is 5 wt.%. However, the compressive strength may decrease with the addition of SBR. When the addition arrives 10 wt.%, a 16% reduction is observed. The overall porosity and pore size distribution of the composites vary with SBR content. The addition of 3 or 5 wt.% SBR can refine the pore size distribution. Interweaving polymer films were observed in the composites.     

Mechanical and elastic behaviour of concretes made of recycled-concrete coarse aggregates

In this paper an investigation of mechanical behaviour and elastic properties of recycled-aggregate concretes is presented. These concretes were prepared by alternatively using two different (coarse and finer coarse) recycled-aggregate fractions both made of recycled concrete coming from a recycling plant in which rubble from demolition is collected and suitably treated. Several concrete mixtures were prepared by using only virgin aggregates (as reference), 30% finer coarse recycled aggregate replacing fine gravel and 30% coarse recycled aggregate replacing gravel. Five different water to cement ratios were adopted as: 0.40, 0.45, 0.50, 0.55 and 0.60. Concrete workability was in the slump range of 190–200 mm. Compression tests were carried out after 28 days of wet curing. In addition, concrete elastic modulus and drying shrinkage were evaluated. Results obtained showed that structural concrete up to C32/40 strength class can be manufactured by replacing 30% virgin aggregate with recycled-concrete aggregate. Moreover, a correlation between elastic modulus and compressive strength of recycled-aggregate concrete was found and compared to those reported in the literature. Finally, on the basis of drying shrinkage results, particularly if finer coarse recycled-concrete aggregate is added to the mixture, lower strains could be detected especially for earlier curing time.     

Impact behaviour and damage tolerance of woven carbon fibre-reinforced thermoplastic composites

Instrumented drop weight impact and compression after impact test methods were used to assess the impact performance and damage tolerance of firm-stacked, woven carbon fibre-reinforced PEEK and woven carbon fibre-reinforced PPS composite systems. The tests were performed at three energy levels with a constant velocity, and at three velocity levels with a constant energy. Extreme energy of the levels were also determined. While the effect of the impact velocity was found to be significant within the range of the velocities used, the impact energy significantly affected the impact performance of the panels. Carbon fibre PPS panels evinced a high resistance to perforation by way of extensive delamination. On the other hand, carbon fibre PEEK panels showed an ability to confine the damage zone and hence, to markedly increase the damage tolerance of the panels. A set of material selection criteria for impact applications, based on the results of this study, has bee proposed     

Minimum cost design of fibre-reinforced concrete-filled steel tubular columns

This paper studies the minimum cost design of fibre-reinforced concrete-filled steel tube columns. It takes into account the effects of confinement of concrete and its steel fibre reinforcement. The constraints of the optimum design, prescribed by the strength and stability requirements, are adopted from the provisions of modern structural codes. A computer program for the analysis has been developed, which emphasizes the dimensioning of the composite column and identification of the optimum quantity of fibres in matrix material. The paper concludes with two representative numerical examples, which show that in-filled composite columns can be designed both economically and effectively using the proposed methodology.     

Comparison of properties of steel slag and crushed limestone aggregate concretes

Steel slag is produced as a by-product during the oxidation of steel pellets in an electric arc furnace. This by-product that mainly consists of calcium carbonate is broken down to smaller sizes to be used as aggregates in asphalt and concrete. They are particularly useful in areas where good-quality aggregate is scarce. This research study was conducted to evaluate the mechanical properties and durability characteristics of steel slag aggregate concrete in comparison with the crushed limestone stone aggregate concrete. The durability performance of both steel slag and crushed limestone aggregate concretes was evaluated by assessing water permeability, pulse velocity, dimensional stability and reinforcement corrosion. The results indicated that the durability characteristics of steel slag cement concretes were better than those of crushed limestone aggregate concrete. Similarly, some of the physical properties of steel slag aggregate concrete were better than those of crushed limestone aggregate concrete, though the unit weight of the former was more than that of the latter.     

聚合物乳胶对钢纤维增强混凝土力学性能的影响

通过对聚合物乳胶/钢纤维增强混凝土材料的抗折、抗压、劈裂抗拉强度等试验,研究了此类复合材料的力学行为和乳胶及钢纤维的增强机理.结果表明,乳胶的掺入,能够改善纤维表面与水泥砂浆基体性能,增加纤维与基体之间的界面粘结强度,从而可以显著提高混凝土的抗折、劈裂抗拉强度等性能.     

Time-dependent behaviour of concrete beams pretensioned by carbon fibre-reinforced polymers (CFRP) tendons

Due to its non-corrosive and high tensile strength properties, carbon fibre-reinforced polymers (CFRP) has attracted researchers’ interests worldwide to investigate the feasibility and effectiveness of using CFRP as reinforcements or prestressing tendons in concrete structures. However until now these studies have been mainly in the form of experimental tests and there is lack of theoretical framework provided. This paper first proposes an analytical method to predict the time-dependent behaviour, including concrete strains, curvature and deflection as well as loss of prestress, of concrete beams prestressed by CFRP tendons, under sustained service load condition. The paper then presents three illustrative examples of how to use this analytical method to calculate the time-dependent concrete strains, curvature and deflection of concrete beams pretensioned by CFRP tendons. It is concluded that the proposed theoretical framework is suitable for analysis of the time-dependent performance of the concrete beams pretensioned by CFRP tendons and the behaviour of such beams are comparable to those with steel and therefore CFRP can be effectively used as prestressing tendons in concrete beams.     

Flexural behaviour and strength of cold-formed steel L-headers

Cold-formed steel headers are structural components used over wall openings in cold-formed steel residential and light commercial construction. Recently, there has been an increased interest in cold-formed L-headers among homebuilders primarily due to their ease of installation and low material cost. The findings from an extensive laboratory testing program, of full-scale single and double cold-formed steel L-headers are presented in this paper. The objective of the research was to investigate the flexural behaviour and strength of L-headers under both gravity and uplift loads. Based on the results improved ultimate strength design expressions and new deflection expressions for a wide range of L-header assemblies have been proposed.     

Strength and behaviour of cold-formed steel offset trusses

Thirteen full-scale truss specimens fabricated with cold-formed steel C-sections were tested to study various practical strengthening techniques in order to achieve a desired behaviour and an increased capacity. Specimens were subjected to concentrated panel point loading simulating the realistic loading condition of a roof truss. Results revealed that local buckling of the top chord (LBTC) adjacent to the heel plate was the predominant failure mechanism. In instances where the heel plate was not adequately stiffened, the distortion or crippling of the plate occurred at failure. For the investigated pitches, an increase in specimen pitch resulted in an increase in the capacity. Strengthening both the top chord adjacent to the heel plate and the heel connection with both a shallow member and a heel plate stiffener resulted in the most significant increase in the capacity compared with the original truss configuration. Beam-column analysis of the top chords in the failure region was performed using both the conventional interaction design equations and the direct strength method as suggested in CSA S136-07.     

Analysis and behaviour of steel storage pallet racks

For steel storage pallet racks, lateral stiffness in down-aisle direction is usually provided by beam-to-column joints and base-plate connections, owing to the impracticability of using bracing systems in selected areas of pallet racks. Because of the high slenderness of members, second order effects influence remarkably the frame performance. The model of unbraced semi-continuous frame is hence adopted for design.

This paper presents the main results of a numerical study on the response of pallet racks commonly used in Europe. The influence of beam-to-column joint modelling on the overall frame response is singled out with reference to both service condition and ultimate limit states. The effect of an accurate definition of frame imperfections is considered. Finally, the influence of base-plate joints on the overall stability of the rack systems is discussed focussing attention on both stiffness and strength of the column bases.     

Accelerated engineering properties of high and low volume fly ash concretes reinforced with glued steel fibers

The present study focuses on the improvement of pozzolanic reaction of fly ash particles with the cement hydration products. Low and high volume fly ash concrete mixtures were studied systematically with the addition of accelerating admixtures and accelerated curing of the concrete specimens in a steam chamber for 18 h at 75°C. Also, the reinforcing effects of glued steel fibers addition on the compressive and flexural performance of fly ash concrete were investigated. The test results indicated that the addition of accelerator improved the rate of hardening and the inclusion of steel fibers provided higher flexural performance. Also, it can be noted that the high volume fly ash (50%) addition in concrete showed a reduction in strength; however, the addition of accelerator has compensated the deceleration in strength gain. The proper selection of concrete ingredients, addition of accelerator and initial steam curing for 18 h showed better improvement on the engineering properties in fly ash concrete. A maximum increase (41.7%) in compressive strength of fly ash concrete around 52.90 MPa was noticed for 25% fly ash substitution and 1.5% steel fibers addition. Dynamic elastic modulus was also calculated in loaded concrete specimen using ultrasonic pulse velocity test and showed a good agreement with the experimental value.     

超高性能混凝土与超高性能自密实混凝土的研制与应用

重点介绍超高性能混凝土与超高性能自密实混凝土的研制过程及工程应用,从原材料选用、配合比设计、各项试验过程及工程应用中的解决方案方面进行详细介绍,为在高性混凝土的研制或应用提供参考及借鉴.     

Fuzzy logic approach to predict stress–strain curves of steel fiber-reinforced concretes in compression

In this study, a new approach is developed for predicting stress–strain curve of steel fiber-reinforced concrete (SFRC) under compression, by use of fuzzy logic system (FLS). In the coverage of study, firstly, experimental studies were carried out. Cylindrical specimens were prepared in size of Ø150×300 mm with and without steel fiber. Two different steel fiber types (both are hooked end) were used as ratios 0 (control), 15, 30, 45 and 60 kg/m³. The stress–strain curves were defined for 28 ages of the cylindrical specimens. Secondly, the stress–strain curves for SFRC were modeled by use of fuzzy logic approach, and the results that were obtained from experiments and modeling were compared. As a result close relationship between both results was seen.     

In-plane shear behaviour of profiled steel sheeting

This paper will describe the behaviour of profiled steel sheeting under inplane shear and its application in building frames. Analytical models for shear strength and stiffness of the profiled steel sheeting are developed and validated by small scale model tests and finite element analyses. The strength, stiffness, failure modes and strain conditions are found to be greatly influenced by the boundary condition of the sheeting. The values of several factors related to the mode of attachment of the sheeting to practical building frames are studied to verify the suitability of design equations.     

Post-buckling behaviour of prestressed steel stayed columns

A steel column that is reinforced by prestressed stays generally has an increased strength in axial compression. A geometrically nonlinear model accounting for the post-buckling behaviour of the stayed column is formulated using the Rayleigh–Ritz method and then validated using the finite element method. It is found that the post-buckling behaviour is strongly linked to the level of the initial prestress. As the prestress is increased, the following different levels of the responses can be observed in sequence: initial Euler buckling that subsequently restabilizes strongly, the critical load increasing with a post-buckling path that is either stable or unstable, an upper limit for the critical load where the post-buckling is unstable after an initially rather flat response. These findings are important for designers aiming to achieve safer and more efficient designs for this structural component.     

Elastic buckling and second-order behaviour of pitched-roof steel frames

The aims of this work are two-fold: (i) to present the results of a study concerning the elastic in-plane stability and second-order behaviour of unbraced single-bay pitched-roof steel frames and (ii) to propose, validate and illustrate the application of an efficient methodology to design this type of commonly used frame. After (i) characterizing the relevant frame buckling modes and P-Δ second-order effects, and (ii) addressing the exact and approximate calculation of the associated bifurcation loads and secondary bending moments, the paper deals with the incorporation of these concepts in the definition of an efficient design procedure. In particular, it is clearly shown that, due to the rafter slope, the geometrically nonlinear behaviours of orthogonal beam-and-column and pitched-roof frames are qualitatively different. Finally, the proposed concepts and methodologies are illustrated through the presentation and discussion of numerical results involving fixed and pinned-base frames.     

Tests and behaviour of cold-formed stainless steel tubular X-joints

The paper presents a series of tests on cold-formed stainless steel tubular X-joints. The tubular X-joint specimens were tested without chord preload as well as with three different levels of preload applied to the chord members. The test specimens were fabricated from square and rectangular hollow sections as brace and chord members. A total of 32 tests was performed. High strength stainless steel (duplex and high strength austenitic) and normal strength stainless steel (AISI 304) specimens were tested. The test results were compared with the design strengths obtained using the CIDECT Guide and Eurocode for carbon steel structures. It is shown that the design strengths predicted by the current design specifications are very conservative for the test specimens calculated using the 0.1%, 0.2%, 0.5% and 1.0% proof stresses as the yield stresses. The 0.2% proof stress is comparatively more reasonable to predict the design strengths of stainless steel tubular X-joints for both ultimate limit state and serviceability limit state.