CEB-FIP Model Code Research Articles

Effect of GGBFS and fly ash proportions on fresh, tensile and cracking features of alkali activated concrete with low NaOH concentrations

This study mainly investigated the tensile and cracking features of ash alkali activated concrete with different slag-fly ash proportions and low NaOH concentrations cured at ambient temperature. High-molarity NaOH leads to risk and is costly, whereas, in field conditions, heat curing is difficult. Therefore, in this study three mixes (mix-A, B and C) were developed using 0/100, 20/80, 40/60, 60/40, 80/20, and 100/0 slag-fly ash proportions in this study. Sodium hydroxide (SH) and sodium silicate (SS) were used as activators and concentrations of sodium hydroxide were used as 1M, 2M and 4M in mixes A, B and C respectively, but alkaline ratio (SS/SH) was fixed as 1.5 in all the mixes. Slump, and strength aspects (compressive, split tensile, and flexural) were evaluated. The tensile (tensile strength, tension stiffening) and cracking characteristics (crack spacing, crack width) were evaluated under uniaxial tensile loading on reinforced prismatic members. From test outcomes, workability in terms of slump of the composites decreased with increased percentage of slag or granulated blast furnace slag (GGBFS) in the total binder, but the tensile (tensile stress and tension stiffening effect) and compressive strengths increased with increasing percentage of GGBFS. Better cracking properties (i.e., minimum crack widths and reduced crack spacings) were observed when the mixes contained higher percentages of GGBFS. The obtained crack spacings were correlated with CEB-FIP model code, and existing research studies. The crack spacings obtained in this study are consistent with CEB-FIP model code. Finally, this study demonstrated that when slag-fly ash alkali activated concretes were prepared with solutions containing low NaOH concentrations and cured at room temperature (ambient), there was an increase in the strength and cracking properties with higher percentages GGBFS.

Influence of the Distribution and Level of Post-Tensioning Force on the Punching Shear of Flat Slabs

Punching shear is the most common failure mechanism of slabs that are supported directly on columns. The slab–column connection is always vulnerable to critical punching shear; thus, it is necessary to investigate the effect of various parameters on the punching shear behavior of the flat slabs. This work presents an experimental study to evaluate the effect of the level of prestressing force and layout of the strands on the punching shear behavior of the slab–column connection in terms of the maximum load, deflection, stiffness, ductility, and the absorbed energy. Five square post-tension flat slabs (2000 mm × 2000 mm) with 150 mm thickness were tested. Increasing the prestressing force increased the maximum load and the ductility with a delay in damage in the case of uniformly distributed strands. Additionally, the banded layout of the post-tensioning strands significantly increased the punching shear capacity of the post-tensioned flat slabs at all levels of prestressing. The influence of the layout of the strands on the flat slab ductility is clearly visible in cases of high prestressing force as the ductility decreases in cases with distributed strands when compared to the same flat slabs with banded strands. Finally, the predicted values of the ultimate load of the punching shear using different codes, including the Egyptian Code of Practice (ECP-203), the American Building code (ACI-318), the CEB-FIP Model code and the Euro code, are compared to the experimental values of the ultimate punching shear strength of the post-tensioned and non-post-tensioned flat slabs.

Effect of pigments on bond strength between coloured concrete and steel reinforcement

The effect of pigments on mechanical properties of coloured concrete intended for structural applications, including the bond stress-slip behaviour to embedded steel bars, is not well understood. Series of concrete mixtures containing different types and oncentrations of iron oxide (red and grey colour), carbon black, and titanium dioxide (TiO2) pigments are investigated in this study. Regardless of the colour, mixtures incorporating increased pigment additions exhibited higher compressive and splitting tensile strengths. This was attributed to the micro-filler effect that enhances the packing density of the cementitious matrix and leads to a denser microstructure. Also, the bond to steel bars increased with the pigment additions, revealing their beneficial role for improving the development of bond stresses in reinforced concrete members. The highest increase in bond strength was recorded for mixtures containing TiO2, which was ascribed to formation of nucleus sites that promote hydration reactions and strengthen the interfacial concrete-steel transition zone. The experimental data were compared to design bond strengths proposed by ACI 318-19, European Code EC2, and CEB-FIP Model Code.

Experimental Assessment and Numerical Modeling of the Bond-Slip Correlation for Steel Rebars in r.c. Members.

Refined non-linear static or dynamic analyses are increasingly used to assess the behavior of new and existing reinforced concrete structures. To perform these analyses, an adequate knowledge of the force–displacement, bending moment–curvature, and bending moment–rotation curves of relevant parts of structural members is needed, and modeling the bond–slip correlation for steel rebars becomes fundamental. The paper presents the results of an experimental campaign on r.c. specimens under tension, aiming, differently from previous studies, to better reproduce the bond–slip relationship accounting for the local confinement and anchorage conditions of real structural members. Resorting to an original numerical procedure allowing us to predict the relative displacement between steel reinforcement and the surrounding concrete in a reinforced concrete element, once assigned the stress in the naked steel bar and the bond–slip law, the experimental results are compared with the numerical outcomes obtained by adopting codified bond–slip laws. The comparison highlights that experimental values of sliding are well below those that are commonly given in existing bond slip laws, such as that adopted by the CEB-FIP Model Code. A new bond–slip model, which is able to satisfactorily predict actual strain fields and slips along the investigated r.c. elements, is thus proposed with the final aim of extending its implementation into non-linear analyses of r.c. structures.

Analytical prediction of nonlinear behaviors of beams post-tensioned by unbonded tendons considering shear deformation and tension stiffening effect

ABSTRACT This is a proposal for a nonlinear analytical model that was developed to predict the structural response of post-tensioned beam with unbonded tendon. The structural response includes deflections under the service load before or after cracking, as well as the ultimate load capacity. A comprehensive procedure for the deflection calculation was implemented, considering the additional shear deformation and tension stiffening effect. The shear deformation calculation is based on the Timoshenko beam theory, considering the shear modulus reduction upon cracking. Furthermore, a modification of the tension stiffening model from CEB–FIP Model Code 1990 is proposed to account for the significant strain concentration in the post-yielding region. A fixed-point iterative procedure is introduced in the prediction model to compute the incremental strain in the unbonded tendon. Each iteration brings the estimated strain increase in the tendon closer to the actual value until it is reasonably accurate. Predictive accuracy of the proposed model in both load–deflection relationships and strain rates, as a function of deflections, is successfully validated against experimental beam tests and a nonlinear finite element analysis considering concrete plasticity, respectively.

Time-Dependent Analysis of Precast Segmental Bridges

Prestressed segmentally constructed balanced cantilever bridges are often subjected to larger deflections than those predicted by calculations, especially for long-term effects. In this paper, the case of modular balanced cantilever bridges, which are prestressed segmental bridges obtained through a repetition of the same double cantilever, is investigated. The considered bridges are two typical cases of modular balanced cantilever both subjected to large deformations during their lifetime. In this case, due to the unusual employed static scheme, creep deflections indefinitely evolve over time particularly at the end of the cantilevers and in correspondence with the central joint. These remarkable deflections cause discomfort for vehicular traffic and in certain cases can lead to the bridge collapse. Important extraordinary maintenance interventions were necessary to restore the viability of the bridges and to replace the viaduct design configuration. To this aim, the static schemes of the structures were varied, introducing new constraints, new tendons, and carbon fiber reinforcements. In the present work, time analysis was performed to compare the time-dependent behavior of the bridge according to two different creep models, the CEB-FIP Model Code 2010 and the RILEM Model B3, with the real-time-dependent behavior of the bridge observed during its lifetime. The two different employed models exhibit different behaviors in terms of displacements and bending moments acting on the bridge. Interesting considerations are made on their reliability in simulating the long-term creep effects that evolve indefinitely over time. Moreover, retrofitting techniques have been proposed and modeled to predict their effectiveness in reducing time-dependent deflections.

Tests and Simulation of the Bond-Slip between Steel and Concrete with Recycled Aggregates from CDW

This works intends to analyze, experimentally and numerically, the bond-slip behavior between steel and concrete made with recycled aggregates (RA) from construction and demolition waste (CDW) from several recycling plants in Portugal. Pull-out tests performed in concrete mixes with RA from CDW are described and the main results (bond strength, bond-slip curves and failure modes) are shown and discussed. Additionally, a comparison between experimental and analytical (using equations from the literature) results is made. Afterwards, finite element (FE) models using Abaqus are developed and compared with the experimental results. The interface behavior between steel and concrete follows that prescribed by the CEB-FIP Model Code and uses as input experimental results. After validation, the models show good results when extended to predict the bond-slip behavior of the remaining concrete mixes studied. It is generally concluded that, per each 10% of natural aggregates replaced with RA from CDW, the bond strength decreases in circa 3% (numerically) to 5% (experimentally). CEB-FIP Model Code equations are shown to be able to predict bond strength and, when used in FE models, to lead to accurate simulation of the bond-slip response of steel and concrete with RA from CDW: the average ratio between numerical and experimental bond strengths is 0.95. In the scope of the FE models developed, a simple equation to be used along with those prescribed by CEB-FIP, which accounts for fraction and ratio of aggregates replaced, is put forward, as a first approach, showing good results.

Analysis of damage-plasticity model of concrete under uniaxial compression loading

Concrete is a quasi-brittle material and shows different behavior in compression and tension. It shows elastic behavior at initial stage and damage-plasticity behavior beyond elastic limit. Therefore, development of material behavior model of concrete is a complex phenomenon. In this study, concrete damage plasticity theory has been described under experiment on concrete cylinder considering uni-axial compression loading and interpreted with analytical data calculated using CEB-FIP model code equation. The code has divided the stress-strain curve for concrete compression into three sections according to concrete’s elastic and non-elastic behaviors. Those three sections have been considered to calculate analytical data. In experiment, concrete behavior has been observed in two phases. The damage value for different stresses at the various points on the stress strain curve has been calculated. According to analytical data, the concrete shows elastic behavior up to 8.3MPa stress point and no damage occur in the concrete within the limit. However, in experimental data, concrete shows elastic behavior up to only 2.28MPa and damage occurred beyond the stress. Finally, the percentage of damage of concrete due to compression obtained from analysis and experiment has been assessed and compared. Above 32 percent of concrete damage is found for 22.5 MPa in both cases.

Development of strength and elastic modulus of concrete sealed in steel tube under sustained load at early age

Concrete-Filled Steel Tubular Structures (CFSTSs) have become popular among the structural engineering community due to significantly higher load carrying capacity compared to conventional reinforced-concrete structures. Much research has been conducted on understanding the behavior of CFSTSs under various loading conditions and design theories have been established to predict the load carrying capacities of such structures. However, existing models do not consider the effects of sustained early loads on concrete strength and elastic modulus development of CFSTSs. With the need for rapid construction, CFSTSs may be subjected to loading at an early stage before concrete is fully cured. Such early loading may incur negative effects on strength and elastic modulus development of concrete within the confined environment. This paper propose theoretical models based on the compressive packing model (CPM) to simulate strength and elastic modulus development of early-age concrete under sustained stress. Development of concrete properties at early age is described using Hydration kinetics, and maximum paste thickness in the CPM model is modified using energy conservation to simulate sustained loads. Early concrete strength and the elastic modulus development rules were investigated experimentally for sustained loads. Predictions from the proposed models are compared with conventional models from CEB-FIP Model Code. Results showed that when loaded at a very early stage, a relatively high stress to strength ratio will result in causing damage in concrete. Such damage significantly affects the strength and elastic modulus development. Compared with concrete loaded at 28 days, concrete loaded at early stages showed significant reduction in concrete strength and elastic modulus.

Analysis and modeling of uniaxial compressive creep of MMA-modified unsaturated polyester polymer concrete

Creep and shrinkage are the major time-dependent phenomena occurring in concrete. Because they significantly affect the long-term dimensional changes and relaxation of restrained stresses in concrete, it is of great importance to accurately consider these characteristics in the structural analysis and design process. In this study, laboratory experiments were conducted to examine the creep under uniaxial compression and linear setting shrinkage of unsaturated polyester (UP) polymer concrete modified with three different levels of methyl methacrylic (MMA) monomer (i.e., 10, 20, and 30 wt.%). The creep of UP-MMA polymer concrete was measured as per ASTM C512 under a sustained compressive stress of 20% of the compressive strength for 90 days, while the linear setting shrinkage was monitored based on the JSCE method for 7 days. Results indicated that as the MMA content increased, the 90-day creep increased although the differences were non-significant. On the contrary, the linear setting shrinkage tended to decrease with an increase in MMA content. The compressive strength was found to decrease by 7.2% as the MMA content increased from 10 wt.% to 30 wt.%. Additionally, the present study compared the measured creep coefficients with those estimated by ACI 209 and CEB-FIP Model Code 1990. It was revealed that the current creep models for Portland cement concrete are inappropriate in predicting the creep of UP-MMA concrete. A new predictive model for creep applicable to the UP-MMA concrete is proposed.

Modeling Local Bond Stress-Slip Relationships of Reinforcing Bars Embedded in Concrete with Different Strengths.

Although many different analytical models of local bond stress–slip have been proposed, considering the possible differences between materials in different countries, their applicability needs to be further explored. In this paper, the local bond stress–slip characteristics of reinforcing bars embedded in concrete with different strengths were experimentally studied. The experimental variables included the concrete strength (20, 40, and 60 MPa) and deformed rebar size (#4, #6, and #8). The experimental results of the bond stress–slip relationship were compared with the Euro-International Concrete Committee (CEB-Comité Euro-International du Béton)-International Federation for Prestressing (FIP-Fédération Internationale de la Précontrainte) Model Code and prediction models found in the literature. In addition, based on the test results, an empirical model of the bond stress–slip relationship was proposed. The evaluation and comparison results show that, regardless of the concrete strength grades, the predicted value calculated using the CEB-FIP Model Code will underestimate the bond strength of the specimens with different steel bar diameters. From this perspective, it is more conservative. In contrast, the proposed model can predict the test results with a reasonable accuracy.

Numerical Investigation of the Strengthening Method by Circumferential Prestressing to Improve the Fatigue Life of Embedded-Ring Concrete Foundation for Onshore Wind Turbine Tower

An embedded-ring foundation connected to the steel tower above it by inserting the steel ring into the concrete foundation is a traditional and widely used form for wind turbine towers. An insufficiently embedded depth of the steel ring leads to stress concentration on the corner of the concrete above the windward-side T-shaped plate. A damage zone of concrete develops, leading to gaps between the steel ring and the foundation concrete and a decline in the restrain stiffness of the foundation pier, which induces a larger horizontal displacement of the steel tower and a decrease in the natural frequency for the wind turbine system. To improve the fatigue life of the concrete around the steel ring under the precondition of not destroying the original foundation, a strengthening method using a circumferential prestressing technique is proposed in this paper. A series of numerical analyses were carried out to analyze the stress state change in the foundation concrete before and after strengthening. The fatigue life of the concrete above the T-shaped plate was evaluated according to CEB-FIP model code (fib Model Code for Concrete Structures 2010). The results show that the strengthening method can effectively decrease the fatigue stress amplitude and improve the fatigue life of the concrete above the T-shaped plate.

Experimental and numerical study on creep and shrinkage effects of ultra high-performance concrete beam

Compared with normal concrete, ultra high-performance concrete (UHPC) is of increasing interest for bridge engineering because of its superior tensile strength, compressive strength, and significantly enhanced durability. The creep and shrinkage effect have a significant influence on the long-term behavior of UHPC. However, the existing research on the time-dependent behavior of UHPC is still limited. In this research, based on numerical integration method, the ABAQUS user subroutine for simulating creep and shrinkage of both normal concrete (NC) and UHPC were developed by the recursive algorithm of adjacent stress increment in the time history. The accuracy of developed user subroutine was validated by creep and shrinkage test of reinforced NC beams. Subsequently, the long-term creep and shrinkage test of UHPC specimens was conducted with stress to strength ratio of 20%, 30%, and 40%, respectively. Based on the test results and the CEB-FIP Model code 1990, the simplified design formulas for predicting creep and shrinkage of UHPC were proposed respectively based on the developed UHPC material. In the proposed design formulas, the final shrinkage strain was 145 με, and the final creep coefficient was 0.46 for the developed UHPC specimens based on the test environment condition. The comparison between the test result and proposed formulas indicated good applicability and accuracy of the proposed formulas. Based on the developed ABAQUS user subroutine and the proposed design formula for creep and shrinkage, the finite element (FE) models of RC beam and UHPC beam were developed. The influence of creep and shrinkage on the long-term mechanical behavior of simply supported prestressed beams were investigated.

Evaluation of the Dynamic Behavior of Green Concrete under Impact Load by Using CEB-FIP Code

Growing environmental pollution, rapid depletion of natural resources (virgin), need for larger areas of landfill and the increased cost of landfill are the factors that led to focus on the recycling of concrete and the demolition of waste in the new concrete. The principle of recycling for sustainable construction was adopted during the present study by recycling plastic and glass waste as an alternative to aggregates in concrete. Eight concrete mixes were tested, and production of green concrete with waste content of 35% and residual strength about 83% was achieved. The dynamic properties of green concrete under impact load were calculated. Dynamic properties are very useful in the design of civil structures exposed to impact loads such as a gas explosion, runways, etc. CEB-FIP Model code (2010) show the most inclusive forms for expecting the strain rate development of concrete. The dynamic of compressive and flexural calculated by CEB-FIP models under strain rates range 10−2 and 100. This range used to predict dynamic loads in structures of civil engineering within the quasi-static strain level.

Time-dependent Behaviour Analysis of Long-span Concrete Arch Bridge

This paper continues the previous study on clarifying the time-dependent behaviour of Beipanjiang Bridge ‒ a reinforced concrete arch bridge with concrete-filled steel tubular stiffened skeleton. The obtained prediction models and the Finite Element Models were used to simulate the long-term behaviour and stress redistribution of the concrete arch bridge. Three-dimensional beam elements simulated the stiffened skeleton and surrounding concrete. Then, a parameters study was carried out to analyse the time-dependent behaviour of the arch bridge influenced by different concrete creep and shrinkage models. The simulation results demonstrate that concrete creep and shrinkage have a significant influence on the time-dependent behaviour of the concrete arch bridge. After the bridge completion, the Comite Euro-International du Beton mean deviation of displacements obtained by 1990 CEBFIP Model Code: Design Code model and fib Model Code for Concrete Structures 2010 model are 3.4%, 31.9% larger than the results predicted by the modified fib Model Code for Concrete Structures 2010 model. The stresses between the steel and the concrete redistribute with time because of the concrete long-term effect. The steel will yield if the fib Model Code for Concrete Structures 2010 model is used in the analysis. The stresses in a different part of the surrounding concrete are non-uniformly distributed.

Experimental Investigation of the Steel-Concrete Joint in a Hybrid Tower for a Wind Turbine under Fatigue Loading

This study investigated the fatigue behaviors at the joint of a hybrid steel–concrete wind turbine tower. A fatigue design approach was proposed for the joint based on CEB-FIP model code 1990 and Eurocode 2. In addition, to estimate the long-term fatigue performance related to continuous wind changes at the joint, two types of specimens were fabricated and tested. Each specimen consisted of steel and concrete sections, which were connected with anchor bolts, and they were designed to investigate the effect of the embedded length of the anchor bolt. Two million cycles of cyclic loading were applied to the specimens, after which a monotonic static loading was applied to investigate the residual performance of each specimen. The experimental results demonstrate that the joint of a hybrid steel–concrete wind turbine tower designed using the proposed approach retained sufficient fatigue performance. Thus, this approach can be effective for fatigue design of the joint.

Creep Characteristics Of Concrete Used In Long-Span Arch Bridge

The reinforced concrete arch bridge with concrete-filled steel tubular stiffened skeleton is extensively used in the mountainous area of southwest China due to their long span and high stability. Beibanjiang Bridge located in Shanghai- Kunming high-speed railway, which had a record span of 445 m, has recently been completed in 2016. However, concrete creep and shrinkage have pronounced effects on the long-term deflection and stress redistribution of this bridge. Several concrete creep and shrinkage specimens in the natural environment were made to predict the long-term behaviour of this bridge accurately. They were used to measure the concrete creep and shrinkage of the core concrete and surrounding concrete used in the arch bridge. The test results were compared to ACI209 R-92 Prediction of Creep, Shrinkage and Temperature Effects in Concrete Structures model, 1990 CEB-FIP Model Code 1990: Design Code model, fib Model Code for Concrete Structures 2010 model and Creep and Shrinkage Prediction Model for Analysis and Design of Concrete Structures-Model B3. Based on the numerical fitting method, the fib Model Code for Concrete Structures 2010 model was modified to suit the concrete creep and shrinkage experimental results. Then, the modified fib Model Code for Concrete Structures 2010 model was used to predict the timedependent behaviour of a concrete arch bridge.

Mechanical Properties of Concrete with Various Water-Cement Ratio After High Temperature Exposure

Concrete building safety of fire is better than other building materials such as wood, plastic, and steel,because it is incombustible and emitting no toxic fumes during high temperature exposure. However,the deterioration of concrete because of high temperature exposure will reduce the concrete strength.Mechanical properties such as compressive strength and modulus of elasticity are absolutely corruptedduring and after the heating process. This paper aims to investigate mechanical properties of concrete(especially compressive strength and modulus of elasticity) with various water-cement ratio afterconcrete suffered by high temperature exposure of 500oC.This research conducted experimental method and analytical method. The experimental methodproduced concrete specimens with specifications: (1) specimen’s dimension is 150 mm x 300 mmconcrete cylinder; (2) compressive strength design, f’c = 22.5 MPa; (3) water-cement ratio variation =0.4, 0.5, and 0.6. All specimens are cured in water for 28 days. Some specimens were heated for 1hour with high temperature of 500oC in huge furnace, and the others that become specimen-controlwere unheated. All specimens, heated and unheated, were evaluated by compressive test.Experimental data was analyzed to get compressive strength and modulus of elasticity values. Theanalytical method aims to calculate modulus of elasticity of concrete from some codes and to verifythe experimental results. The modulus elasticity of concrete is calculated by 3 expressions: (1) SNI03-2847-1992 (which is the same as ACI 318-99 section 8.5.1), (2) ACI 318-95 section 8.5.1, and (3)CEB-FIP Model Code 1990 Section 2.1.4.2.The experimental and analytical results found that: (1) The unheated specimens with water-cementratio of 0.4 have the greatest value of compressive strength, while the unheated specimens with watercementratio of 0.5 gets the greatest value of modulus of elasticity. The greatest value of compressivestrength of heated specimens provided by specimens with water-cement ratio of 0.5, while the heatedspecimens with water-cement ratio of 0.4 gets the greatest value of modulus of elasticity, (2) Allheated specimens lose their strength at high temperature of 500oC, (3) The analytical result shows thatmodulus of elasticity calculated by expression III has greater values compares to expression I and II,but there is only little difference value among those expressions, and (4)The variation of water-cementratio of 0.5 becomes the optimum value.

Bond to reinforcement of polymeric lightweight flowable concrete for structural applications

Limited studies investigated the effect of styrene-butadiene rubber (SBR) latexes on bond properties of structural lightweight self-consolidating concrete (LWSCC). The mixtures tested in this investigation were prepared using expanded kaolinic clay lightweight aggregate (LWA), while the water-to-binder ratio was adjusted to secure compressive strength of 40 ± 3.5 MPa. Testing was realized using the beam-end specimen method, and the parameters under evaluation included the LWA content (up to 40% of coarse aggregate volume), SBR dosage (up to 15% of binder mass), and bar diameter. Test results have shown that the initial stiffness of load vs. slip curves and ultimate bond strength of LWSCC considerably improved with SBR inclusion. This was related to the coupled effect of the SBR polymers that help relaxing stresses during loading and presence of LWA that reduces bleeding and promotes creation of hydration compounds at the steel-concrete transition zone. The experimental data are compared with the design bond strengths determined by CEB-FIP Model Code 2010, ACI 318-14, and European Code EC-2.

Reinforcement design of concrete sections based on the arc-length method

Abstract The reinforcement design of concrete cross-sections with the parabola-rectangle diagram is a well-established model. A global limit analysis, considering geometrical and material nonlinear behavior, demands a constitutive relationship that better describes concrete behavior. The Sargin curve from the CEB-FIP model code, which is defined from the modulus of elasticity at the origin and the peak point, represents the descending branch of the stress-strain relationship. This research presents a numerical method for the reinforcement design of concrete cross-sections based on the arc length process. This method is numerically efficient in the descending branch of the Sargin curve, where other processes present convergence problems. The examples discuss the reinforcement design of concrete sections based on the parabola-rectangle diagram and the Sargin curve using the design parameters of the local and global models, respectively.