Modelling of creep effect on moisture warping and stress developments in concrete pavement slabs

Abstract

One of the major reasons for the fatigue failure in concrete pavements attributes to the curling and warping deformations and the traffic loads, while creep effect can reduce such deformation and consequently the stress generated in slabs. The literature on the influence of creep effect on slab warping and stress generation is found rare. In this study, a test set-up was designed to measure the flexural creep of concrete beams exposed to sealed and drying conditions. The measured creep properties were then incorporated in finite element analysis to model the creep effect on warping deformation and stress generated in slabs under the conditions of bounded and unbounded with base. It is found that creep effect is significant in slab bonded with base, it reduces warping deformation and stress by 36 and 45%, respectively. The total stress is reduced by 34%. Therefore, it is of importance to take into account creep effect when conducting deformation and stress analysis in concrete pavement slabs.     

Tension-stiffening model for FRP-strenghened RC concrete two-way slabs

The main focus of this paper is to present a tension-stiffening model that is suitable for finite element analysis (FEA) aimed at investigating the effect of FRP strengthening on the tensile behaviour of concrete slabs. Available experimental results of the FRP-strengthened reinforced concrete slabs are used to calibrate the finite element model based on the ultimate load carrying capacity of the two-way slabs. The proposed tension-stiffening model is implemented into the constitutive concrete model defined in a general-purpose finite element code. Reinforced concrete behaviour in tension can signifcantly be changed due to strengthening. An overall increase in the post-peak stiffness based on the tensile stress-strain relationship is observed. A simplified bilinear model is introduced to define the behaviour of the FRP-strengthened concrete in tension. An expression of the fracture energy density is introduced to define the area under the concrete tensile stress-strain relationship. The tensile stress-strain relationship of concrete is referred to as the tension-stiffening model. It is shown numerically that the ultimate load capacity of two-way slab specimens is sensitive to the fracture energy density. Hence, a distinction has to be made between the definitions of the tension-stiffening model of FRP-strengthened and un-strengthened concrete. This distinction is the focus of this paper.

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Résumé L'objectif principal de cet article est de présenter un modèle approprié de raidissement en traction pour l'analyse d'éléments finis (AEF). L'analyse est destinée pour l'étude de l'effet du renforcement en polymère renforcé de fibres (PRF) sur le comportement en traction des dalles en béton armé. Les résultats expérimentaux des dalles renforcées sont employés pour calibrer le modèle d'éléments finis basés sur la capacité ultime des dalles bidirectionnelle. Le modèle de raidissement en traction proposé, est appliqué dans un code général d'élément finis. Le comportement du béton armé renforcé en traction peut être changé d'une manière significative due au renforcement. On observe une augmentation en tension de la rigidité du poteau crête basée sur la relation contrainte-déformation. Un modèle bilinéaire simplifié est présenté pour définir le comportement du béton renforcé de PRF en traction. Une expression de la densité d'énergie de rupture est présentée pour définir l'aire sous la courbe contrainte-déformation. La relation contrainte-déformation du béton en tension est appelée modèle raidissement en traction. On montre numériquement que la capacité de la charge ultime de spécimens bi-directionnels de dalle est affectée par densité d'énergie de rupture. Par conséquent, une distinction doit être faite entre les définitions du modèle raidissement en traction du béton renforcé de PRF est béton non renforcé. Cette distinction est l'objet de cet article.

     

Experimental study of retrofit solutions for damaged concrete bridge slabs

Accurate information on the actual performance of the structural system after retrofit is an essential part of a cost-effective bridge management program. This paper summarizes the results of a thorough experimental program concerning the reinforced concrete deck of a real 40 year-old viaduct. The structure exhibited severe damage at the extrados mainly due to environmental agents, chemical attack and action of asphalt milling machines. Samples of the deck were cut and carried to the laboratory in order to assess the possibility of retrofit. The design of retrofit was aimed at increasing the load carrying capacity through replacement of the deteriorated concrete with a new concrete overlay and strengthening in flexure for both negative (hogging) and positive (sagging) bending moments. Experimental testing on small specimens and nondestructive techniques were carried out to identify the material properties and to evaluate the level of damage. The bonding between external reinforcement and the original or new (standard or polymer-modified) concrete was assessed through single-shear push–pull tests on 33 prismatic specimens of 100 × 200 × 500 mm³ strengthened with CFRP strips. The efficiency of the retrofit techniques was checked at the structural level through four-point bending tests on eight slabs of 500 × 200 × 2000 mm³. This research can contribute to guidelines for concrete patch repair of FRP-retrofitted concrete bridge decks, to ensure better long-term performance under service loads and environmental effects.     

Heated slabs under biaxial compressive loading: a test set-up for the assessment of concrete sensitivity to spalling

Explosive spalling of concrete members in fire is the violent expulsion of shards from the exposed face caused by the combined effect of pressure build-up in the pores due to water vaporization and of in-plane stress induced by both external loads and thermal gradients. Spalling progression leads to the reduction of bearing cross-section and often to the direct exposure of rebars to the flames. Since established predictive models are not available yet, experimental studies appear to be the most effective means of investigation on this phenomenon. To this purpose an experimental setup has been developed for the assessment of concrete sensitivity to spalling. The specimen is a concrete slab (800 × 800 mm) with a thickness comprised between 100 and 200 mm. The bottom face is heated via a horizontal furnace, in which a propane burner is actively controlled in order to follow the prescribed fire curve. During heating, a biaxial compressive load can be applied thanks to hydraulic jacks restrained by a steel frame. Load and slab thickness can be adjusted in order to represent the mechanical conditions achieved in the hottest region of thicker concrete members such as tunnel lining segments. The setup proved to be very effective in comparing spalling sensitivity among different concrete mixes, as is often required in initial material testing for strategic infrastructures such as tunnels.     

Non-destructive evaluation of concrete moisture by GPR: Experimental study and direct modeling

This paper reports research into the application of Ground Penetrating Radar (GPR) technology to the physical characterization of concrete and gives the details and results of an experimental study on the effect of concrete moisture on radar waves propagating through concrete laboratory slabs. Radar measurements were performed using a commercial radar system with 1.5 GHz ground-coupled antennas. The concrete slabs were conditioned so as to possess different degrees of saturation and a homogeneous moisture distribution. Electrical resistivity measurements were also carried out on each concrete sample to provide complementary information regarding the moisture content of the concrete. Interesting results were found concerning the ability of the radar technique to characterize the moisture state of concrete. Attention was especially focused on the amplitude, velocity and frequency spectrum of the waveforms recorded. In particular, the behavior of the transmitter-receiver direct wave was found to be greatly influenced by the moisture in the concrete. Finally, some experimental results were simulated using a simple dielectric model of heterogeneous mixtures.     

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.     

Numerical predictions of ballistic limits for concrete slabs using a modified version of the HJC concrete model

Some modifications to the Holmquist–Johnson–Cook (HJC) model (1993) for concrete under impact loading conditions are proposed. First, the pressure-shear behaviour is enhanced by including the influence of the third deviatoric stress invariant to take into account the substantial shear strength difference between the tensile and compressive meridians. Second, the modelling of strain-rate sensitivity is slightly changed so that the strain-rate enhancement factor goes to unity for zero strain rate. Third, three damage variables describing the tensile cracking, shear cracking and pore compaction mechanisms are introduced. A critical review of the constitutive model with alternative proposals for parameter identification is given. The model parameters are obtained for two concrete qualities, and perforation of concrete slabs is considered numerically and compared with experimental results from the literature. Ballistic limit assessments with deviations under 8% when compared to the experimental results are obtained, indicating that the modified version of the HJC concrete model represents a good compromise between simplicity and accuracy for large-scale computations of concrete plates impacted by projectiles.     

Optimal design for concrete pavement situated above box culvert: experimental and numerical study

This study examined the pavement distress data obtained from the field as well as the effects of the soil cover depth, joint position and reinforced slab length on the behaviour of concrete pavement using the finite element method. An ultimate strength model with a similarity of 1/2 was designed and constructed. Finite element analysis incorporating an implicit direct numerical integration scheme was conducted to obtain an optimal design for the pavement structure above a box culvert. An accelerated pavement test was conducted for validation, in which a realistic traffic load was imposed on the pavement model constructed in the laboratory. The strain histories were measured at each section and compared with those of numerical analysis. The optimal joint position and reinforced slab length that can minimise the damage to the pavement structure above a box culvert were also determined.     

An experimental and numerical study of reinforced ultra-high performance concrete slabs under blast loads

Ultra-high performance concrete (UHPC) which is characterized by high strength, high ductility and high toughness has been widely applied in modern structure construction. Outstanding mechanical feature of UHPC not only enables strong yet slim structure design but also highlights its potential in protective engineering against extreme loads like impact or explosion. In this research a series of reinforced concrete slabs are tested to determine their response under explosive loading conditions. Concrete materials used in the slab construction are ultra-high strength concrete (UHPC) and normal strength concrete (NSC). In total five slabs are tested including four UHPC slabs with varying reinforcement ratios and one control NSC slab with normal reinforcement. Explosive charges with TNT equivalent weights ranging from 1.0 to 14.0 kg at scaled distances ranging from 0.41 to 3.05 m/kg^1/3 are used in the current experiments. Test results verified the effectiveness of UHPC slabs against blast loads. Numerical models are established in LS-DYNA to reproduce the field blast tests on UHPC slabs. The numerical results are compared with the field test data, and the feasibility and validity of the numerical predictions of UHPC slab responses are demonstrated.     

Deflection behaviour of FRP reinforced concrete beams and slabs: An experimental investigation

The flexural response of FRP RC elements is investigated through load–deflection tests on 24 RC beams and slabs with glass FRP (GFRP) and carbon FRP (CFRP) reinforcement covering a wide range of reinforcement ratios. Rebar and concrete strains around a crack inducer are used to establish moment–curvature relationships and evaluate the shear and flexural components of mid-span deflections. It is concluded that the contribution of shear and bond induced deformations can be of major significance in FRP RC elements having moderate to high reinforcement ratios. Existing equations to calculate short-term deflection of FRP RC elements are discussed and compared to experimental values.     

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.     

Stress distribution in bonded joints: An exploration within a mathematical model

The introduction of “toughened” adhesives with their enhanced capacity to cope with strain has placed increasing emphasis on the importance of mathematical models to be able to cope with the plastic as well as elastic characteristics of adhesives. The changes in stress distribution caused by a variety of modifications to a collar and pin joint have been explored within the framework of an elastic/plastic model. The results indicate quite clearly how important it is to match the adhesive with the adherends and their proposed geometry.     

Membrane action,deflections and cracking of two-way reinforced concrete slabs

The paper presents a brief and critical review of some major studies on two-way reinforced concrete slabs concerning the application of membrane analysis for the prediction of ultimate loads, determination of deflections and crack widths.

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Résumé On procède à un examen critique de quelques études les plus importantes sur les dalles armées dans deux directions croisées en relation avec l'application de l'effet de membrane pour la prédiction des charges de rupture, et la détermination des fléchissements et de la fissuration.

     

Dynamic response and behavior of reinforced concrete slabs under impact loading

Reinforced concrete slabs are among the most common structural elements. In spite of the large number of slabs designed and built, the effect of their details on their behavior under impact loads are not always appreciated or properly taken into account. This experimental study was aimed at understanding the dynamic behavior of structural concrete slabs under impact loading to improve the state of the art of protective design. This study investigated the effects of different types of slab reinforcements and the applied impact loads on the dynamic response and behavior of reinforced concrete slabs.     

Modelling basalt fibre reinforced glass concrete slabs at ambient and elevated temperatures

The analysis of tests conducted on small-scale slabs at ambient and elevated temperatures is presented in this paper. The slabs were produced from a new type of concrete containing different levels of glass sand and basalt fibre. Two methods were used for this purpose: a simplified method developed previously and a finite element method, using the software package ABAQUS. For the slabs at ambient temperature, the results showed a good correlation for the load–displacement relationship between the test and the two models up to the failure loads. For the slabs at elevated temperature, the ABAQUS model gave a reasonable prediction for the temperature–displacement relationship while the simplified method gave a conservative prediction for the maximum allowable vertical displacement. As a result, the simplified method underestimated the temperature at which the reinforcement fracture occurs for this type of concrete slab, incorporating glass sand and basalt fibres. Further work is required to remove this conservatism from the simplified design method for this type of concrete.     

An experimental study on the effect of chloride penetration on moisture diffusion in concrete

Chloride penetration and moisture diffusion are two important transport processes for studying the long-term durability of concrete. The chloride penetration and moisture transfer in concrete are considered as two coupled transport processes. An experimental study was conducted for determining the related material parameters in the coupled transport processes. For the coupled moisture diffusion, we consider that the total moisture fluxJ _H ^t is resulted from two driving forces: the gradient of moistureH, and the gradient of free chlorideC _f , that is,J _H ^t =?D _H-H grad(H)?D _H-Cl grad(C _f′ ). In this formulation, two transport parameters are involved: the moisture diffusion coefficientD _H-H and the coupled moisture diffusion coefficient due to chloride diffusionD _H-Cl . A seepage test was designed specifically for determining the two transport parameters. The test data showed that the chloride penetration accelerates the moisture diffusion significantly. The two transport parameters were determined using the present test data. The result showed thatD _H-Cl is not a constant, but depends linearly on the free chloride concentration.     

Strengthening slabs using externally-bonded strip composites: Analysis of concrete covers on the strengthening

This study pertains to the experimental and theoretical behaviour of slabs strengthened by fibre reinforced polymer (FRP). The experimental results show that FRP significantly increases punching failure stress, resulting in a reduction of slab rotation around the loading column. The theoretical investigation presents a finite element model for the bending of strengthened slabs. The developed model considers the concrete as a 3D multi-layered non-linear material and explicitly takes into account the steel reinforcement and the FRP strips. The proposed model is then used to analyse the effects of a concrete cover on the reinforcement and repairs. In the analysed cases, the results show that an average reduction in the concrete shear modulus, between steel rod and FRP, of more than 30% leads to significant reductions of stress and slab stiffness.