Fib Model Research Articles

The role of additives in estimating service life of self-compacting concrete mix design using FIB modelling

The purpose of this study is to examine the influence of additives on estimating the useful service life of self-compacting concrete (SCC). For that, durability tests were performed on 43 SCC mix designs to estimating the service life of SCC based on the FIB model. The investigated supplementary materials included xanthan gum at concentrations of 0.2% and 0.25% by weight of cement, microsilica at concentrations of 5%, 7% and 10% by weight of cement, and nanosilica at concentrations of 2%, 3% and 4% by weight of cement. All 43 designs met the reliability index of the model, thereby obtaining the estimated service life for each design. The findings revealed that in chloride-rich environments, the control sample exhibited greater penetration depth compared to the concretes incorporating xanthan gum, microsilica and nanosilica additives. This observation suggests that the inclusion of xanthan gum extends the service life of SCC due to the formation of a denser and more cohesive structure in the hydration products of xanthan gum-modified concrete. Additionally, the investigations conducted indicate that the presence of nano and micro additives effectively reduces voids, while xanthan gum plays a significant role in minimising cracking and enhancing the thickening properties of the concrete.

Effects of chloride salt erosion and freeze–thaw cycle on interface shear behavior between ordinary concrete and self-compacting concrete

Self-compacting concrete (SCC) has been widely used for concrete jacketing to strengthen RC structures due to its high workability, fluidity, and segregation resistance. In this study, a test was carried out to study the effects of substrate curing including chloride salt erosion and the freeze–thaw cycle on the interface shear strength between ordinary concrete and the SCC. Eighty-four Z-type specimens were designed and tested through direct shear experiments, and the influence of interface treatment (implanting steel bar and cutting groove), concrete type, and substrate curing on the interface shear strength were analyzed. Four calculation models of interface shear strength based on different codes are compared, the applicability of these models is also analyzed. The results show that: For close the substrate concrete and the SCC strength, the effect of chloride salt erosion and freeze–thaw cycle will cause a slight decrease in the interface shear strength compared with the specimen that substrate is not cured, the influence of chloride ion erosion and freeze–thaw cycle on the degeneration of interface shear strength for specimen with implanting steel bar is not more obviously than specimen with cutting groove. Any single code model can not accurately predict the interface shear strength of all specimens with different interface treatments and substrate curing. The fib model can accurately predict interface shear strength of specimens which the average compressive strength of old and new concrete is less than 40 MPa and the substrate is not cured. Whether the substrate is cured or not, EC2 and CSA A23.3–14 models can quite accurately predict the interface shear strength of specimens with an implanting steel bar, but for specimens with cutting groove, the modified AASHTO model can accurately predict the interface shear strength. Except modified AASHTO model, the factor considering the effect of interface treatment should be taken into account when using fib, EC2 and CSA A23.3–14 models to calculate the interface shear strength.

Design of slender and squat reinforced concrete members with shear reinforcement

AbstractAs proposed in several design standards for structural concrete, including Eurocode 2 (EN:1992‐1‐1:2004) and fib's Model Code 2010, the shear design of slender members with shear reinforcement can be performed by means of consistent models based on suitable stress fields. This is for instance the case of the variable‐angle truss models, where the acting shear force is equilibrated by an inclined compression field and the shear reinforcement in tension. The application of such design expressions to squat members or when gravity loads are applied on the upper chord near to the supports, has however been observed to lead to overly conservative results. This is due to the fact that a direct strut can develop in the concrete between the load and the support without the need of being suspended through shear reinforcement. In addition, this leads to a more favorable cracking state and thus to enhanced shear resistance. Despite the fact that such phenomena can be also formulated in a consistent manner with the stress field method, empirical corrections of the resistance of slender members have traditionally been adopted to design beams when significant loads act close to supports. In this paper, the benefits of designing such cases on the basis of tailored stress fields are demonstrated. Such an approach provides a smooth transition between slender and squat members and, when compared to tests, leads to a significantly lower Coefficient of Variation than empirical corrections. On that basis, simple design expressions are formulated in an effort to serve as a revision for the next generation of design codes (Model Code 2020 and 2nd generation of Eurocode 2).

Experimental, Numerical, and Theoretical Studies of Bond Behavior of Reinforced Fly Ash-Based Geopolymer Concrete

This paper presents the results of an experiment study and suggests a theoretical formulation for the bond behavior of reinforced fly ash-based geopolymer concrete. Three grades (20 MPa, 30 MPa and 40 MPa) of a geopolymer concrete along with three reinforcement diameters (12, 16, and 20 mm) were selected for experimental work. The bond behavior of the reinforced geopolymer concrete is determined using pullout test, finite element analysis (FEA), and theoretical work. The test data indicated that the bond strength of reinforced fly ash-based geopolymer concrete increases about 1.97 to 2.56 times with the increase of compressive strength from 20.33 MPa to 41.12 MPa. For grade 30 MPa and 40 MPa specimens, the concrete cover to diameter ratio (c/db) increased up to 4.19 resulted in the increase of bond strength. Then, the bond strength decreases with the increase of the c/db ratio from 4.19 to 5.75, while grade 20 MPa specimens is vice versa. The bond-slip relations between the reinforcement and geopolymer concrete determined from FEA is in good agreement with experimental results. The coefficient of variation (CoV) is only 0.01. However, this behavior is quite different from the data calculated by the fib model.

Development and Verification of Prognostic Prediction Models for Patients with Brain Trauma Based on Coagulation Function Indexes.

Objective To assess the effect of adding coagulation indices to the currently existing prognostic prediction models of traumatic brain injury (TBI) in the prediction of outcome. Methods A total of 210 TBI patients from 2017 to 2019 and 131 TBI patients in 2020 were selected for development and internal verification of the new model. The primary outcomes include death at 14 days and Glasgow Outcome Score (GOS) at 6 months. The performance of each model is evaluated by means of discrimination (area under the curve (AUC)), calibration (Hosmer-Lemeshow (H-L) goodness-of-fit test), and precision (Brier score). Results The IMPACT Core model showed better prediction ability than the CRASH Basic model. Adding one coagulation index at a time to the IMPACT Core model, the new combined models IMPACT Core+FIB and IMPACT Core+APTT are optimal for the 6-month unfavorable outcome and 6-month mortality, respectively (AUC, 0.830 and 0.878). The new models were built based on the regression coefficients of the models. Internal verification indicated that for the prediction of 6-month unfavorable outcome and 6-month mortality, both the IMPACT Core+FIB model and the IMPACT Core+APTT model show better discrimination (AUC, 0.823 vs. 0.818 and 0.853 vs. 0.837), better calibration (HL, p = 0.114 and p = 0.317) and higher precision (Brier score, 0.148 vs. 0.141 and 0.147 vs. 0.164), respectively, than the original models. Conclusion Our research shows that the combination of the traumatic brain injury prognostic models and coagulation indices can improve the 6-month outcome prediction of patients with TBI.

Optimum proportion of fly ash or slag for resisting concrete deterioration due to carbonation and chloride ingress

Chloride ingress is considered as the primary reason causing corrosion of steel inside reinforced concrete (RC). Carbonation can also result in significant corrosion of RC structures in locations where CO2 concentration is high due to atmospheric condition or environmental hazards. Consequently, RC structure could experience considerable degradation if carbonation occurs in conjunction with chloride induced corrosion. However, from the available literature, inconsistent outcome has been found regarding the effect of amount of supplementary cementing materials (SCMs) such as fly ash and slag on concrete’s carbonation. In this study, an attempt has been made to establish interaction zones from the contrasting results observed between carbonation and chloride penetrability of concrete with respect to fly ash or slag addition. The findings of the study were based on results obtained from Rapid Chloride Permeability Test (RCPT) and carbonation tests of 108 mixes with a range of water to binder (w/b) ratios, binder contents, aggregate gradations and fly ash/slag percentages. An apparent optimum proportion of 20% replacement of cement by fly ash or slag has been proposed considering combined action of chloride ingress and carbonation. Moreover, tentative service life has been estimated using fib model with the observed carbonation depth data. In addition, replacing cement with the optimized amount of fly ash or slag will reduce cost and provide a sustainable solution to concrete construction.

Chloride profiles with a peak – why and what are the consequences for predictions?

Chloride ingress profiles do almost always have a peak at some depth but most prediction models are missing this peak. Some prediction models, such as the fib model, simply “cut off” a slice of the concrete up to the peak in further predictions. Other prediction models use data only from the profiles beyond the peak but include the concrete up to the peak as if it has the same properties as the rest of the concrete. A physical model has been developed to quantify the local changes because of leaching and the consequences of these changes with time. The model uses Fick’s 1st law for chloride diffusion and linear chloride binding. The depth of leaching with time is modelled with a simple square-root equation. The consequences of leaching are assumed to be linear from the surface into the maximum depth of leaching. The consequences of leaching are modelled as depth-dependent changes of porosity, chloride binding and the diffusion coefficient in Fick’s first law.

A simplified design approach for predicting the flexural behavior of TRM-strengthened RC beams under cyclic loads

This paper proposes a simplified design approach to predict the flexural behavior of reinforced concrete (RC) beams strengthened by high-performance textile reinforced mortar (TRM) under cyclic loads. Based on the experimental results, the effect of main factors on the beams as well as the effectiveness and reliability of TRM strengthening layer under cyclic loads were examined as per ACI 437 method. The reinforcement ratio of the original RC beams, the textile reinforcement ratio of TRM layer, and the interface bond between the original beam and TRM layer, all have significant influence on the flexural behavior and failure modes of TRM-strengthened beams. The simplified models based on the fib model and the Architectural Institute of Japan (AIJ) standard were suggested to predict the flexural capacity of the strengthened beams. These were also used to evaluate the flexural curve of the beams while employing simplified calculation models of the deflection capacities of the RC beams. The results verified that the simplified capacity curve model presented good accuracy to evaluate the TRM-strengthened RC beams.

Prediction model of carbonation depth for recycled aggregate concrete

The prediction of carbonation depth for recycled aggregate concrete (RAC) is investigated in this paper. The existing prediction models were evaluated, and it showed that the coefficient of variation (COV) of model error for the existing models is high. By introducing the weighed water absorption of aggregates, the COV of model error can be effectively decreased. Compared with the existing models, the proposed model can predict more accurate carbonation depths. For RAC specimens, compared with the fib model and Xiao and Lei's model-a, the COV of model error of the proposed model is 0.36 which is decreased by 33.3%, and when compared with Xiao and Lei's model-b and Silva et al.’s model, the corresponding decreases are 55.2% and 16.2%. Finally, the proposed model is validated by a 10-year-old carbonation experiment, which indicates that the proposed model is reasonable and can be applied to predict the carbonation depth of RAC.

The challenge of the performance-based approach for the design of reinforced concrete structures in chloride bearing environment

The performance-based approach, published by the International Federation for Structural Concrete (fib), was applied for the design of a RC element in a marine environment, with corrosion resistant reinforcement, to analyse the potentiality of the model as well as the possible reasons which limit its use. Results showed that the fib model allows to compare different solutions and to consider the benefits connected with the use of preventative measures. However the definition of reliable values for some input parameters, as the critical chloride threshold for corrosion resistant reinforcement, is demanded to the designer and this aspect clearly limits a widespread use.

Rational Approach to Prediction of Shear Capacity of RC Beam-Column Elements

This paper presents a new predictive formula for the shear capacity evaluation of reinforced concrete members subjected to combined axial, bending, and shear forces. The formula is based on a tensorial approach to concrete shear resistance that is very similar to the one used in the shear-enhanced fiber beam element. The total shear resistance is broken down into the contribution of concrete and contribution of shear reinforcement. The concrete contribution to the shear resistance is calculated using a normal-shear stress failure envelope. Normal (longitudinal) stresses are calculated from axial and bending forces acting on the concrete member. In the formulation, a number of simplifications are made to keep the formula as simple as possible but still sufficiently accurate. The resulting formulation, although capable of accounting for all of the major variables that influence the shear strength, including size effect, remains particularly simple and with a compact notation. The predictions of the proposed formula are compared with those used by the American Concrete Institute (ACI), the European Committee for Standardization (CEN) Eurocode, and the International Federation for Structural Concrete (fib) model code, and its accuracy is checked against a vast experimental database available in the literature. Results and comparisons are very encouraging and confirm the soundness of the underlying mechanical model. The capability of this model to provide a unified approach for reinforced and unreinforced members opens up the possibility to extend the application of the proposed formula to engineered cementitious composites, such as fiber-reinforced concrete.

Justification of Fédération International de Béton, fib, 2000 model for elastic modulus of normal and high-performance concrete, HPC

This article outlines an experimental and theoretical comparison between the fib 2000 model for elastic modulus of normal concrete, NC, and HPC and 144 laboratory tests. Two dimensions of specimen were studied (diameters 56 and 100 mm) and one climate (20 °C, sealed or with ambient relative humidity, RH, 60%). The age dependence did not seem to be well correlated between the derived and the measured elastic modulus. The fib 2000 model overestimated the elastic modulus of mature concrete and in contrast slightly underestimated the elastic modulus of young NC. As a whole, the derived modulus when using the fib model was about 110% of the elastic modulus measured at loading. A relationship to strength was found for this diversity with an increasing overestimation of the E-modulus at lower strength.

Practical design in structural concrete: American and European practice

In the past several decades, there has been considerable progress toward harmonising, if not unifying, practical design in structural concrete between American and European practice. This article attempts an assessment of these trends using the 1991 IABSE Stuttgart Colloquium Summarizing Statement as a framework for the assessment. Besides the well-known ACI Building Code, several notable North American bridge codes and an emerging ISO Simplified Design Standard are compared with the CEB-FIP (now fib) Model Codes and Eurocode developments. It is shown that European and American practices are gradually converging and important concepts are largely in harmony, although there are still many differences in details.