Experiments and failure analysis of SHCC and reinforced concrete composite slabs

For all types of concrete structures, controlling of cracking, as well as the enhancement of serviceability and ultimate flexural capacity are important issues for deck slabs. This study presents an experimental campaign and accompanying nonlinear analysis of a series of Strain Hardening Cementitious Composite (SHCC) and reinforced concrete slab systems, simply-supported and subjected to four-point loading. In order to improve flexural performance both at the service and ultimate limit states, an SHCC layer with thickness of 150–400 mm was placed on the soffit of the composite slab; the SHCC was manufactured using two different processes, namely cast-in-situ SHCCs and extruded precast SHCC panel. Nonlinear analysis of SHCC and reinforced concrete slabs was also carried out to predict moment and curvature as well as deflections of the slab systems. The developed slab systems were found to have enhanced performance with regard to both at serviceability and flexural capacity, compared to the conventional reinforced concrete slab.     

两跨连续GFRP-混凝土空心组合板受力性能试验研究

为研究高延性混凝土(HDC)加固钢筋混凝土梁的受剪性能,该文对7根HDC加固梁及4根未加固梁进行静力试验,研究剪跨比、配箍率、加固层厚度和加固层附加箍筋对钢筋混凝土梁破坏形态、荷载-挠度曲线、受剪承载力以及裂缝的影响。结果表明：采用HDC面层对钢筋混凝土梁进行受剪加固,可以显著提高梁的受剪承载力;HDC面层可以代替部分箍筋的受剪作用,改善钢筋混凝土梁的剪切破坏形态;加固试件在达到极限位移之后,试件的完整性较好,剩余承载力较高。基于试验结果,利用桁架-拱模型,提出了HDC加固钢筋混凝土梁的受剪承载力计算公式,计算值与试验值吻合较好。     

Improving the blast resistance capacity of RC slabs with innovative composite materials

This paper examines the feasibility of using innovative composite materials to improve the blast resistance capacity of one-way reinforced concrete slabs. In order to achieve this objective, five slabs were tested under real blast loads. One of the slabs was used as the control unit to establish a baseline for comparison of the other four slabs. These four slabs were strengthened with carbon fiber and steel fiber reinforced polymers, comprising of two slabs retrofitted on a single side and two slabs retrofitted on both sides. Test results indicate that there was no significant increase in blast resistance when the slabs were retrofitted on a single side; however, slabs retrofitted on both sides displayed a significant increase in blast resistance. This result can be attributed to the negative moments that develop under the dynamics of blast loads. Another objective of this research program was to study the feasibility of using a modified displacement based methodology to predict the explosive charges weight and standoff distances required to impose a given damage level. Test results showed that for the most part the blast loads were effectively estimated using this method and the damage levels observed from the field tests correlated well with the predicted levels. This paper discusses the analytical steps used to predict the charges weight and standoff distances along with the relevant experimental results.     

Impact response of integrated hollow core sandwich composite panels

This paper deals with an innovative integrated hollow (space) E-glass/epoxy core sandwich composite construction that possesses several multi-functional benefits in addition to the providing lightweight and bending stiffness advantages. In comparison with traditional foam and honeycomb cores, the integrated space core provides a means to route wires/rods, embed electronic assemblies, and store fuel and fire-retardant foam, among other conceivable benefits. In the current work, the low-velocity impact (LVI) response of innovative integrated sandwich core composites was investigated. Three thicknesses of integrated and functionality-embedded E-glass/epoxy sandwich cores were considered in this study—including 6, 9 and 17 mm. The low-velocity impact results indicated that the hollow and functionality-embedded integrated core suffered a localized damage state limited to a system of core members in the vicinity of the impact. The peak forces attained under static compression and LVI were in accordance with Euler's column buckling equation. Stacking of the core was an effective way of improving functionality and limiting the LVI damage in the sandwich plate. The functionality-embedded cores provided enhanced LVI resistance due to energy additional energy absorption mechanisms.     

Fatigue behaviour of concrete bridge deck slabs reinforced with GFRP bars

GFRP bars are often used for the internal reinforcement of concrete bridge deck slabs as an alternative to traditional steel reinforcements with excellent results in terms of corrosion resistance. Several experiments on bridge decks were conducted to evaluate their structural behaviour but their fatigue performance still needs an adequate experimental investigation. This paper presents the results of an experimental campaign on four full scale concrete bridge deck specimens reinforced with GFRP bars that were designed, constructed and tested to resist cyclic moving loads. Two hydraulic jacks were used to simulate moving concentrated loads. After the cycles, the load was increased to the static failure. The slabs reinforced with GFRP bars showed a better fatigue performance compared to the requests of the European codes.     

不同方法加固预应力混凝土空心板受力性能试验研究

为了对预应力混凝土空心板梁端部剪跨区内出现的腹剪斜裂缝进行加固,提出了对空心板梁端部注浆加固的措施,以达到增加空心板受剪截面积的目的。进行了3片足尺20 m先张法预应力钢筋混凝土空心板梁注浆加固后抗剪性能试验,以验证加固后空心板梁的受力性能及加固效果,三片梁端部注浆长度分别为1.5 m、2.0 m、2.5 m,分析了不同注浆长度下试验梁的应变、挠度、裂缝分布、刚度、承载力变化规律,通过与未加固前梁体试验结果对比,分析端部注浆加固措施对梁体抗剪承载能力的改善程度。试验表明,采取端部注浆加固措施后,梁体整体刚度变化较小,梁体在正常使用荷载作用下,端部剪跨区内没有出现腹剪斜裂缝,梁体的抗剪能力有较大提高,加固后梁体的破坏模式为加固段与未加固段交界处发生的弯剪破坏。     

Different CFRP strengthening techniques for prestressed hollow core concrete slabs: Experimental study and analytical investigation

This study presents an experimental–analytical investigation on the structural behavior of precast prestressed hollow core RC slabs strengthened in flexure by CFRP laminates. Externally bonded and near surface mounted (NSM) laminates were used. The CFRP area and using transverse anchorage were also investigated. Results demonstrated that NSM technique resulted in optimum strengthening efficiency. The increased bond strength also resulted in full activation of the NSM laminates at failure. However, the NSM flexural strengthening level should be carefully designed to avoid unfavorable shear-tension failure mode. Moderate efficiency was associated with the externally bonded technique due to the premature de-bonding. However, this efficiency was optimized by using transverse CFRP laminates as anchorage, which re-directed de-bonding further away from the laminates’ ends and delayed failure, but at much lower deformations than those of the control slab. A rational analytical study was conducted. Comparisons between the experimental and analytical results demonstrated satisfactory agreements.     

Performance of reinforced concrete slabs strengthened with different types and configurations of CFRP

A total of eight reinforced concrete slabs, 2440 × 600 × 125 mm strengthened with different layers and configurations of CFRP sheets were fabricated and tested. In addition, nonlinear finite element analysis (NLFEA) using ANSYS package was used to simulate the behavior of the test specimens. After reasonable validation of NLFEA with the experimental test results of companion slabs, NLFEA was expanded to provide a parametric study of eighteen slabs. The load–deflection, load strain, and failure modes obtained from the experimental test results and the NLFEA evidently confirmed that strengthening of under-reinforced concrete slabs with CFRP improves the flexural strength capacity and reduce the ductility. This was observed for both types of CFRP. The increase in the flexural strength and the reduction in the ductility increased with the increase in the number of CFRP layers. It was concluded that CFRP strengthening of slabs could be categorized as effective, economical, and successful only if substantial increase in the flexural strength capacity is achieved without changing the failure mode to a shear failure mode at the face of the supports or to a compression failure mode. Comparison between the two CFRP types, for almost equivalent applied area of CFRP, showed that the type of CFRP has significant influence on the behavior of the strengthened slabs. The difference is attributed to the difference in the mechanical properties and the bonding quality of the CFRP material.     

Holistic design of RC beams and slabs strengthened with externally bonded FRP laminates

Structural strengthening with externally bonded reinforcement is now recognized as a cost-effective, structurally sound and practically efficient method for rehabilitating deteriorated and damaged reinforced concrete structures. Although a variety of worldwide on-site applications using composite materials have been realized for the rehabilitation and reinforcement of structural elements, the technology is now at a stage where its future development and competitiveness with conventional methods will depend on the definition of valid design guidelines based on sound engineering principles rather than on the availability of new materials or production processes.The main objective of this paper is to present a general design philosophy for externally plated reinforced concrete beams and slabs, based on a holistic approach, in which appropriate strategies for achieving durable and safe strengthened structures are described.Essential to the design for safety, durability and ductility is the availability of structural models which are: (i) based on sound engineering principles; (ii) capable of reflecting the physical behaviour of strengthened members; (iii) of general applicability, irrespective of the type of external reinforcement material (steel or fiber-reinforced polymer), and the reinforcement configuration (web or tension plate); (iv) capable of describing all possible failure modes, in order to predict the weakest link chain of resistance of a structural member.It will be shown, with a series of numerical/experimental comparisons, that such requirements can be conveniently obtained with a unified approach in which materials and structures, calculation and experimental verification, modelling and analysis are integrated.     

Nonlinear finite element modeling of concrete deep beams with openings strengthened with externally-bonded composites

This paper aims to develop 3D nonlinear finite element (FE) models for reinforced concrete (RC) deep beams containing web openings and strengthened in shear with carbon fiber reinforced polymer (CFRP) composite sheets. The web openings interrupted the natural load path either fully or partially. The FE models adopted realistic materials constitutive laws that account for the nonlinear behavior of materials. In the FE models, solid elements for concrete, multi-layer shell elements for CFRP and link elements for steel reinforcement were used to simulate the physical models. Special interface elements were implemented in the FE models to simulate the interfacial bond behavior between the concrete and CFRP composites. A comparison between the FE results and experimental data published in the literature demonstrated the validity of the computational models in capturing the structural response for both unstrengthened and CFRP-strengthened deep beams with openings. The developed FE models can serve as a numerical platform for performance prediction of RC deep beams with openings strengthened in shear with CFRP composites.     

Strength and failure modes in resistance welded thermoplastic composite joints: Effect of fibre–matrix adhesion and fibre orientation

The strength and failure modes of resistance welded thermoplastic composites were investigated. Special attention was paid to the effect of basic characteristics of the adherends such as fibre–matrix adhesion and fibre orientation. 8HS woven GF/PEI composites were resistance welded. Intralaminar failure was found to be the major failure mechanism for the well welded joints, consisting of either fibre–matrix debonding or laminate tearing. An improved fibre–matrix adhesion was found to result in significantly higher lap shear strength. Besides, the main apparent orientation of the fibres on the welding surfaces was found to have an effect on the strength of the joints.     

Experimental and computational investigations on fire resistance of GFRP composite for building façade

Composite materials such as glass fibre reinforced polymers (GFRPs) possess the advantages of high strength and stiffness, as well as low density and highly flexible tailoring; therefore, their potential in replacing conventional materials (such as concrete, aluminium and steel) in building façade has become attractive. This paper addresses one of the major issues that hinder the extensive use of composite structures in the high-rise building industry, which is the fire resistance. In this study, a fire performance enhancement strategy for multilayer composite sandwich panels, which are comprised of GFRP composite facets and polyethylene foam core, is proposed with the addition of environmentally friendly, fire retardant unsaturated polyester resins and gel-coats. A series of burning experimental studies including thermo-gravimetric analysis (TGA) and single burning item (SBI) are carried out on the full scale composite sandwich as well as on single constituents, providing information regarding heat release rate, total heat release, fire growth rate, and smoke production. Experimental results are compared with fire safety codes for building materials to identify the key areas for improvements. A fire dynamic numerical model has been developed in this work using the Fire Dynamics Simulator (FDS) to simulate the burning process of composite structures in the SBI test. Numerical results of heat production and growth rate are presented in comparison with experimental observations validating the computational model and provide further insights into the fire resisting process. Parametric studies are conducted to investigate the effect of fire retardant additives on the fire performance of the composite sandwich panel leading to optimum designs for the sandwich panel.     

Dynamic analysis of walls strengthened with composite materials

The bidirectional dynamic behavior of walls strengthened with composite materials is studied. For that purpose, a multi-layered high order finite element is developed. The finite element accounts for the bidirectional (plate-type) dynamic behavior and for the interfacial interaction between the adhesively bonded components. The formulation uses a viscoelastic first order shear deformation orthotropic plate theory for the independent modeling of the existing wall and the composite layers and a high order theory for the displacement fields of the adhesive layers. The Finite element framework simplifies the coupling with adjacent structural elements and the use of standard computational procedures. The convergence of the formulation and two numerical examples are studied. The first case studies the response of a strengthened wall to a step base acceleration. The second case studies a wall built in a surrounding frame and strengthened on the outer face. The numerical study examines the capabilities of the model and reveals some of the unique aspect of the dynamic response, including the effects of the orthotropy and orientation of the strengthening system. It also highlights the potential of the high order finite element to become a platform for the modeling and dynamic analysis of the strengthened wall.     

First-ply failure prediction of an unsymmetrical laminated ellipsoidal woven GFRP composite shell with incorporated surface-bounded sensors and internally pressurized

First-ply failure of an unsymmetrical laminated ellipsoidal woven Glass Fiber Reinforced Polymer (GFRP) composite shell internally pressurized was investigated analytically using the linear interpolation technique. The shell's boundary was fixed at its end. Tsai-Wu failure criterion was used as the composite failure design factor. The analytical results, including critical internal pressure and strains in global directions, were validated with the experimental results for some arbitrarily selected points on the shell surface along meridian axis. Manufacturing of laminated ellipsoidal composite shells was performed by using the Vacuum Infusion Process (VIP), a novel method commonly adopted for the fabrication of laminated composite shells. Surface-bounded sensors were installed on the shells' surface to measure the strain values after the internal pressure was applied. According to the analytical investigation findings, the failure factor was critical at the innermost ply. In addition, for each ply, the shell's edge was observed to be the region with the highest failure factor. The experimental findings confirmed that the failure occurred in the regions close to the shell's edge, as predicated by the analytical approach. The results from both approaches were in a close agreement. Subsequently, the effect of various parameters including thickness, aspect ratio, and stacking sequence on the first-ply failure of laminated ellipsoidal woven GFRP composite shell were investigated and the critical mechanical factors to avoid failure were determined.     

RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach

This paper deals with strengthening of reinforced concrete (RC) two-way slabs with carbon fibre reinforced plastic (CFRP) strips bonded to the tensile face. The first part deals with an experimental study. The fibre reinforced plastics (FRP) strengthened slab test presents a failure mode with debonding of the external FRP strips from the slab. The second part deals with a limit analysis modelling. The strengthened slab is designed as a three-layered plate. A simplified laminated plate model is used to describe the behaviour of three-layered plate supported in four sides, which is subjected to a load in the centre. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity and identify the different collapse mechanisms. Experimental results are compared with theoretical predictions.     

Flexural behaviour of RC slabs strengthened with prestressed CFRP strips using different anchorage systems

The Externally Bonded Reinforcement (EBR) technique using Carbon Fiber-Reinforced Polymers (CFRP) has been commonly used to strengthen concrete structures in flexure. The use of prestressed CFRP material offers several advantages well-reported in the literature. Regardless of such as benefits, several studies on different topics are missing. The present work intends to contribute to the knowledge of two commercially available systems that differ on the type of anchorage: (i) the Mechanical Anchorage (MA), and (ii) the Gradient Anchorage (GA). For that purpose, an experimental program was carried out with twelve slabs monotonically tested under displacement control up to failure by using a four-point bending test configuration. The effect of type of anchorage system (MA and GA), prestrain level (0 and 0.4%), width (50 mm and 80 mm) and thickness (1.2 mm and 1.4 mm) of the CFRP laminate, and the surface preparation (grinded and sandblasted) on the flexural response were the main studied parameters. Better performance was observed for the slabs: (i) with prestressed laminates, (ii) for the MA system, and (iii) with sandblasted surface preparation.     

Modeling of composite/concrete interface of RC beams strengthened with composite laminates

In this paper a finite element model for predicting shear and normal stresses in the adhesive layer of plated reinforced concrete beams has been developed. The numerical results carried out agree with those obtained in previous studies by other authors. It is found that shear stresses and high concentrations of peeling forces are present at the ends of the plates when the composite beam is loaded in flexure. These concentrations can produce premature failure of the strengthened beam because of debonding of the plate or cracking of the concrete cover along the level of internal steel reinforcement. The numerical simulation captures the actual interfacial stresses and, in particular, the maximum values of shear and normal stresses.     

Load and resistance factor design of fiber reinforced polymer composite bridge deck

The majority of our bridges were constructed with conventional civil engineering materials of steel and concrete in a typical slab on girder or truss construction. Reinforced concrete bridge decks have approximately 40% life of the steel girders that support these structures. In order to support the use of alternative materials to replace deteriorating concrete decks, this paper outlines the Load and Resistance Factor Design (LRFD) of Fiber Reinforced Polymer composite (FRP) panel highway bridge deck. The deck would be of a sandwich construction where 152.4 mm × 152.4 mm × 9.5 mm square pultruded glass FRP (GFRP) tubes are joined and sandwiched between two 9.5 mm GFRP plates. The deck would be designed by Allowable Stress Design (ASD) and LRFD to support AASHTO design truckload HL-93. There are currently no US standards and specifications for the design of FRP pultruded shapes including a deck panel therefore international codes and references related to FRP profiles will be examined and AASHTO-LRFD specifications will be used as the basis for the final design. Overall, years of research and laboratory and field tests have proven FRP decks to be a viable alternative to conventional concrete deck. Therefore, conceptualizing the design of FRP bridge decks using basic structural analysis and mechanics would increase awareness and engineering confidence in the use of this innovative material.