Numerical modeling on concrete structures and steel–concrete composite frame structures

A steel–concrete composite fiber beam-column model is developed in this study. The composite fiber beam-column model consists of a preprocessor program that is used to divide a composite section into fibers and a group of uniaxial hysteretic material constitutive models coded in the user defined subprogram UMAT in ABAQUS. The steel–concrete composite fiber beam-column model is suitable for global elasto-plastic analysis on composite frames with rigid connections subjected to the combined action of gravity and cyclic lateral loads. The model is verified by a large number of experiments and the results show that the developed composite fiber model possesses better accuracy and broader applicability compared with a traditional finite element model. Although the fiber beam-column model neglects the slip between the steel beam and concrete slab, there are essentially no effects on the global calculation results of steel–concrete composite frames. The proposed model has a simple modeling procedure, high calculation efficiency and great advantage when it is used to analyze composite frames subjected to cyclic loading due to earthquake.     

Testing and modeling of a novel FRP-encased steel–concrete composite column

A composite column consisting of steel, concrete and fiber reinforced polymer (FRP) is presented and assessed through experimental testing and analytical modeling. The composite column utilizes a glass FRP (GFRP) composite tube that surrounds a steel I-section, which is subsequently filled with concrete. The GFRP tube acts as a stay-in-place form in addition to providing confinement to the concrete. This study investigates the behavior of the proposed composite columns under axial loading. A total of seven specimens were tested. The influence of concrete shrinkage on the compressive behavior of the composite columns was also investigated. Significant confinement and composite action resulted in enhanced compressive behavior. The addition of a shrinkage reducing agent was found to further improve the compressive behavior of the composite columns. An analytical model was developed to predict the behavior of the composite columns under axial loading.     

Torsional effect on steel–concrete composite sections subjected to negative moment

This article summarizes the results of 13 push-out specimens under the effect of different loading combinations including shear, torsion and negative bending moment. The objective of the project is to record the reductions in the capacity of the stud shear connector and the change in the behaviour of the composite section when torsion is applied. An equation is presented to calculate the produced axial force in the stud shear connector for sections subject to any loading combination. Modifications are performed to the well-known exponential formula representing the load-slip relation of the studs. Finite element application is conducted to simulate the behaviour of the composite section and the stud shear connector as well. The simulation shows a very good agreement with the experimental results.     

Analysis of shear transfer and gap opening in timber–concrete composite members with notched connections

In timber–concrete composite members with notched connections, the notches act as the shear connections between the timber and the concrete part, and have to carry the shear flow necessary for composite action. The shear transfer through the notches generates shear and tensile stresses in both parts of the composite member, which may lead to brittle failure and to an abrupt collapse of the structure. Although simplified design formulas already exist, some structural aspects are still not clear, and a reliable design model is missing. This paper summarizes current design approaches and presents analytical models to understand the shear-carrying mechanism, to estimate the shear stresses acting in the timber and concrete, and to predict failure. The analysis concentrates on three problems: the shearing-off failure of the timber close to the notch, the shear failure of the concrete, and the influence of the shear flow on the gap opening between the timber and concrete. Parts of the model calculations could be compared to experimental observations. The conclusions of this paper contribute to improving current design approaches.     

Numerical simulation and validation of impact response of axially-restrained steel–concrete–steel sandwich panels

Steel–concrete–steel (SCS) sandwich panels are an effective means for protecting personnel and infrastructure facilities from the effects of external blast and high-speed vehicle impact. In conventional SCS construction, the external steel plates are connected to the concrete infill by welded shear stud connectors. This paper describes a programme of research in which the non-composite SCS panels with axially restrained connections were studied experimentally and numerically. High fidelity finite element models for axially restrained steel–concrete–steel panels subjected to impact loading conditions were developed using LS-DYNA. The simulation results were validated against the dynamic testing experimental results. The numerical models were able to predict the initial flexural response of the panels followed by the tensile membrane resistance at large deformation. It was found that the strain rate effects of the materials and the concrete material model could have significant effect on the numerically predicted flexural strength and tensile membrane resistance of the panels.     

Influence of carbon nanotubes on steel–concrete bond strength

In this study, the bond strength between steel and concrete reinforced with multi-walled carbon nanotubes (CNTs) is analysed. To this end, pull-out tests were carried out for concretes with incorporation of 0.05–0.1% of different types of functionalized and unfunctionalized CNTs with distinct aspect ratios and dispersion techniques. The results showed that CNTs can improve both compressive strength and steel–concrete bond up to 21% and 14% respectively, as compared to plain concrete. The highest compressive strength was found in concrete with higher amounts of lower aspect ratio CNTs, while the best steel–concrete bond performance was attained for concrete with lower amounts of higher aspect ratio CNTs. CNTs were effective to retain the crack propagation, increasing the bonding stiffness and reducing the deformation of concrete consoles between steel ribs. CNTs of higher aspect ratio could better contribute with their microcrack bridging effect. Microscopic analysis confirmed the adequate dispersion and microcrack bridging provided by CNTs, delaying the macrocrack propagation within the aggregate–paste and steel–concrete interfacial transition zones.     

Performance of connections for prefabricated timber–concrete composite floors

Timber–concrete composite beams and slabs require interlayer connectors, which provide composite action in the cross-section. A range of mechanical connectors is available on the market with an extensive variety of stiffness and strength properties, which are fundamental design parameters for the composite structure. Another crucial parameter is the cost of the connector, including the labour cost, that if too high may prevent the use of the composite system. In order to reduce the construction cost and make timber–concrete structures more widespread on the market, it is believed that a high degree of prefabrication should be achieved. For a simple and cost effective construction process, the use of “dry” connections, which do not require the pouring and curing of concrete on site, may represent a possible solution. This paper reports the outcomes of an experimental programme aimed to investigate a number of different mechanical “dry–dry” connectors previously embedded into a prefabricated concrete slab. Direct shear tests on small blocks made of a glulam segment connected with a prefabricated concrete slab were performed. The shear force-relative slip relationships were measured and all the relevant mechanical properties such as slip moduli and shear strengths were calculated. It was found that some of the new developed connection systems for prefabricated concrete slab can perform as satisfactorily as those for cast-in-situ slabs, with the additional benefit of being relatively inexpensive.     

An elastic–plastic and residual stress analysis steel reinforced thermoplastic composite cantilever beam

In the present study an analytical elastic–plastic stress analysis is carried out for a low-density homogeneous polyethylene thermoplastic cantilever beam reinforced by steel fibers. The beam is loaded by a constant single force at its free end. The expansion of the region and the residual stress component of σ_x are determined for 0°, 15°, 30°, 45°, 60°, 75° and 90° orientation angles. Yielding begins for 0° and 90° orientation angles at the upper and lower surfaces of the beam at the same distances from the free end. Although it starts first at the upper surface for 15°, 30° and 45°, it starts first at the lower surface for 60° and 75° orientation angles. The elastic–plastic analysis is carried out for both the plastic region which spreads only at the upper surface and the plastic region which spreads at the upper and lower surfaces together. The residual stress components of σ_x and τ_xy are also determined. The intensity of the residual stress component is maximum at the upper and lower surfaces of the beam, but the residual stress component of τ_xy is maximum on or around the х-axis. The beam can be strengthened by using the residual stresses. The distance between the plastically collapsed point and the free end is calculated for the same load in the beam for 0°, 15°, 30°, 45°, 60°, 75° and 90° orientation angles.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Push-out tests on J-hook connectors in steel–concrete–steel sandwich structure

New form of J-hook connectors and ultra-lightweight cementitious material have been developed by the authors in the previous research to produce steel–concrete–steel sandwich slim decks which have superior performance to resist blast and impact loads. This paper investigates the shear strength behavior of the J-hook connectors embedded in ultra-lightweight cement composite core and compares the behavioral differences with those in normal strength concrete. A total of 102 push-out tests were carried out on standard test specimens with varying parameters including concrete types (normal weight, lightweight and ultra-lightweight), concrete strengths, and types of J-hook connectors. Design guides are proposed to predict the shear strength and load–slip behavior of the J-hook connectors embedded in ultra-lightweight cement composite. The predicted results are compared with the test results together with those predicted by modern codes which were primarily developed for headed shear studs. Through the comparisons and verifications, it is observed that the proposed formulae offers better and more reliable predictions on shear strength as well as load–slip behaviors compared with the available methods in the literature.     

Fracture properties of concrete reinforced with steel–polypropylene hybrid fibres

This research discusses polypropylene fibres and three sizes of steel fibres reinforced concrete. The total fibre content ranges from 0% to 0.95% by volume of concrete. A four-point bending test is adopted on the notched prisms with the size of 100×100×500 mm³ to investigate the effect of hybrid fibres on crack arresting. The research results show that there is a positive synergy effect between large steel fibres and polypropylene fibres on the load-bearing capacity and fracture toughness in the small displacement range. But this synergy effect disappears in the large displacement range. The large and strong steel fibre is better than soft polypropylene fibre and small steel fibre in the aspect of energy absorption capacity in the large displacement range. The static service limitation for the hybrid fibres concrete, with “a wide peak” or “multi-peaks” load–CMOD patterns, should be carefully selected. The ultimate load bearing capacity and the crack width or CMOD at this load level should be jointly considered in this case. The K_IC and fracture toughness of proper hybrid fibre system can be higher than that of mono-fibre system.     

Calcification capacity of porous pHEMA–TiO2 composite hydrogels

Many investigations have been attempted to promote calcification of synthetic polymers for applications as orthopaedic and dental implants. In this study, novel titanium dioxide (TiO₂) reinforced porous poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels were synthesized. Calcification capacity of the composite polymers was examined using light microscopy, scanning electron microscopy and Fourier transform infrared spectroscopy after incubation of the materials in a simulated body fluid up to 53 days. Mechanical strength, porosity and in vitro cytotoxicity were also investigated. Calcification capacity of porous pHEMA was significantly enhanced by the addition of TiO₂ particulates. Infiltration of calcium phosphate, up to 1000 μm, was observed. The diffusion capacity of calcium ions was affected by the porosity and the interconnectivity of pores in the hydrogel polymers which were influenced by the presence of TiO₂ and the monomer concentration. Cell viability tests indicated that porous hydrogels containing 7.5% TiO₂ were not toxic to 3T3 fibroblast cells. These results demonstrate that incorporating TiO₂ nanoparticulates can promote enhanced formation of calcium phosphate whilst maintaining the porosity and interconnectivity of the hydrogel polymers and would be very useful for the development of orthopaedic tissue engineering scaffolds.     

Quantitative evaluation of sulfur–polymer matrix composite quality

In order to predict service life of the sulfur–polymer composite, the samples were subjected to the induced destruction using 10% hydrochloric acid solution. Control specimens were prepared using Portland cement binder. Sulfur–polymer composite showed limited mechanical strength and mass loss, while physico-mechanical properties of Portland cement composite regressed rapidly. The Image Pro Plus software was used for surface destruction monitoring. The simulations for composites were applied to the previously reported model for predicting the mechanical strength degradation during durability testing, based on the image analysis results. The results proved that the time gradient of structural change was useful for quantification of service life, therefore it can be accepted as a parameter that represents service life.     

Evaluation of non-linear behavior of timber–concrete composite structures using FE model

This article describes a numerical model that was developed for the analysis of composite timber–concrete beams. This model presents a simplified methodology for determining the effective bending stiffness of the timber–concrete composite structure. It is based on previous work done usually referred to in some non-normative literature by γ-method. The implemented methodology assumes some simplifications, as for instance, linear elastic behavior of all components, constant stiffness of the connection and sinusoidal loading. For comparison purposes, the work benefits from an experimental program in which full-scale beams were tested in bending and timber–concrete connections were tested in shear. The FE model has shown the ability to overcome the simplifications of the Eurocode, namely the variation of shear force along the beam axis. The numerical model is capable of detecting and quantifying the influence of the non-linear behavior of the connections on the composite structure. Different parameters are analyzed and, for instance, the ductility behavior of the timber–concrete connection could be more important than the maximum strength, which is an interesting result. By comparing theoretical predictions with test results, it is clear that the numerical model used in this work is a very interesting method when compared with the usual design models, such as that of Annex B of Eurocode 5 (EN 1995-1-1). The influence of the connections behavior on the ultimate load of the composite structure is very important and the described approach proved to give good predictions.     

Strengthening of non-seismically detailed reinforced concrete beam–column joints using SIFCON blocks

This article aims to propose a novel seismic strengthening technique for non-seismically detailed beam–column joints of existing reinforced concrete buildings, typical of the pre-1975 construction practice in Turkey. The technique is based on mounting pre-fabricated SIFCON composite corner and plate blocks on joints with anchorage rods. For the experimental part three 2/3 scale exterior beam–column joint specimens were tested under quasi-static cyclic loading. One of them was a control specimen with non-seismic details, and the remaining two with the same design properties were strengthened with composite blocks with different thickness and anchorage details. Results showed that the control specimen showed brittle shear failure at low drift levels, whereas in the strengthened specimens, plastic hinge formation moved away from column face allowing specimens to fail in flexure. The proposed technique greatly improved lateral strength, stiffness, energy dissipation, and ductility.     

Flexural toughness characteristics of steel–polypropylene hybrid fibre-reinforced oil palm shell concrete

Hybridization of steel–polypropylene leads to improvements of both the mechanical and ductility characteristics of concrete. In this investigation, the effect of steel, polypropylene (PP) and steel-PP hybrid fibres on the compressive strength, tensile strength, flexural toughness and ductility of oil palm shell fibre reinforced concrete (OPSFRC) was studied. The comparison on the above said properties between the specimens prepared with crushed and uncrushed oil palm shell (OPS) as lightweight coarse aggregate was also carried out. The experimental results showed that the highest compressive strength of about 50 MPa was produced by the mix with 0.9% steel and 0.1% PP hybrid fibres. The highest increments in the splitting tensile and the flexural strengths of the OPSFRC were found up to 83% and 34%, respectively. However, the mixes with 1% PP fibres produced negative effects on both the compressive and tensile strengths. The results on the toughness indices showed that the OPSC possess no post-cracking flexural toughness. Though, the flexural deflection and toughness of the OPSC was significantly enhanced by the addition of fibres; the dominance of the steel fibre on the first crack flexural deflection and toughness of OPSFRC was evident. The mixes with 0.9% steel and 0.1% PP hybrid fibres reported the highest improvement in toughness index and residual strength factor.