Tahar El-Korchi

The influence of silica fume on the compressive strength of cement paste and mortar

Abstract The compressive strengths of silica fume cement paste and mortar were evaluated at various water-cementitious ratios. Five different water-cementitious ratios were used including, 0.22, 0.25, 0.28, 0.31, and 0.34 and two contents of silica fume, 16% and 25% by weight of cement. Superplasiticizer content was adjusted for each mix to ensure that no segregation would occur. The results show that the increase in compressive strength of mortar containing silica fume, as a partial replacement for cement, greatly contributes to strengthening the bond between the cement paste and aggregate. Partial replacement of cement by silica fume and the addition of superplasticizer increases the strength of mortar but has no influence on the strength of cement paste. Results were verified by statistical analysis using hypothesis testing at a 95% confidence level. It was also demonstrated that superplasticizer in combination with silica fume plays a more effective role in mortar mixes than in paste mixes. This can be attributed to a more efficient utilization of superplasticizer in the mortar mixes due to the better dispersion of the silica fume particles. The paper also reviews some of the available literature on the influence of silica fume on cementitious composites and unsettled questions associated with this topic.

Strength and reliability of carbon-fiber-reinforced cement composites

Abstract The effect of carbon fibers on the mechanical properties of cement paste composites is studied. The addition of polyacrylonitrite-based carbon fiber to a cementitious paste matrix results in a significant improvement in the tensile and flexural properties of the composites. The uniaxial tensile strength results are obtained using the novel cementitious composites axial tensile technique. The addition of 1,2 and 3 %vol. of carbon fiber to a cement matrix results in an increase in the uniaxial tensile strength of 32, 48 and 56%, respectively. The enhancement of the composite flexural strength was more significant, as compared to the uniaxial tensile strength. The flexural strength of cement matrix increased by 72, 95 and 138% with the addition of 1,2 and 3 %vol. of carbon fiber, respectively. Weibull statistics indicate that reliability in flexure was not enhanced by fiber addition and there was no correlation between the percent fiber loading and the reliability of the composites. However, the reliability of the carbon-fiber-reinforced composite in tension was greatly improved and there is a positive correlation between fiber loading and the Weibull modulus, m.

Behaviour of carbon fiber reinforced cement composites in direct tension

Abstract The effect of carbon fibers on the tensile strength of cement paste matrix was experimentally and analytically investigated. The tensile strength values were obtained using the cementitious composites axial tensile technique (CCATT) [1,2]. The addition of 1, 1.5, 2, and 3 volume percent of polyacrylonitrite-based (PAN) carbon fiber to a cement matrix results in an increase in the uniaxial tensile strength of 32, 42, 48 and 56 percent, respectively. A predictive model based on the rule of mixtures is developed. The model accounts for the tensile strength of composites with short and randomly dispersed carbon fibers. In addition, the developed equation of the tensile strength accounts for the orientation and the length of fibers, the specimen geometry and the effectiveness of the reinforcement with increasing fiber content. The developed constitutive model compares satisfactorily with experimental results and it can be used as a practical tool for approximating the tensile strength of CFRC composites.

System identification of smart structures using a wavelet neuro-fuzzy model

This paper proposes a complex model of smart structures equipped with magnetorheological (MR) dampers. Nonlinear behavior of the structure–MR damper systems is represented by the use of a wavelet-based adaptive neuro-fuzzy inference system (WANFIS). The WANFIS is developed through the integration of wavelet transforms, artificial neural networks, and fuzzy logic theory. To evaluate the effectiveness of the WANFIS model, a three-story building employing an MR damper under a variety of natural hazards is investigated. An artificial earthquake is used for training the input–output mapping of the WANFIS model. The artificial earthquake is generated such that the characteristics of a variety of real recorded earthquakes are included. It is demonstrated that this new WANFIS approach is effective in modeling nonlinear behavior of the structure–MR damper system subjected to a variety of disturbances while resulting in shorter training times in comparison with an adaptive neuro-fuzzy inference system (ANFIS) model. Comparison with high fidelity data proves the viability of the proposed approach in a structural health monitoring setting, and it is validated using known earthquake signals such as El-Centro, Kobe, Northridge, and Hachinohe.

Nonlinear system identification of smart structures under high impact loads

The main purpose of this paper is to develop numerical models for the prediction and analysis of the highly nonlinear behavior of integrated structure control systems subjected to high impact loading. A time-delayed adaptive neuro-fuzzy inference system (TANFIS) is proposed for modeling of the complex nonlinear behavior of smart structures equipped with magnetorheological (MR) dampers under high impact forces. Experimental studies are performed to generate sets of input and output data for training and validation of the TANFIS models. The high impact load and current signals are used as the input disturbance and control signals while the displacement and acceleration responses from the structure–MR damper system are used as the output signals. The benchmark adaptive neuro-fuzzy inference system (ANFIS) is used as a baseline. Comparisons of the trained TANFIS models with experimental results demonstrate that the TANFIS modeling framework is an effective way to capture nonlinear behavior of integrated structure–MR damper systems under high impact loading. In addition, the performance of the TANFIS model is much better than that of ANFIS in both the training and the validation processes.

Wavelet-neuro-fuzzy control of hybrid building-active tuned mass damper system under seismic excitations

This paper proposes a wavelet-based fuzzy neuro control algorithm for the hazard mitigation of seismically excited buildings equipped with a hybrid control system. The wavelet-based fuzzy neuro controller is developed through the integration of discrete wavelet transform, artificial neural network, and a Takagi-Sugeno fuzzy controller. The hybrid control system is an integrated model of an actuator, a tuned mass damper, and viscous liquid dampers: an active tuned mass damper (ATMD) is located on the top floor of the structure and viscous liquid dampers are located on each floor. To demonstrate the effectiveness of the proposed wavelet-based adaptive neuro-fuzzy inference system (WANFIS) controller, an eight-story building employing passive viscous liquid dampers as well as an ATMD is investigated. A variety of earthquakes such as an artificial earthquake, the 1940 El-Centro, Kobe, Northridge, and Hachinohe earthquakes are used as disturbance signals. It is demonstrated that the WANFIS controller is effective in reducing the structural responses of the hybrid structure system subjected to a variety of disturbances.

Room temperature tensile and flexural strength of ceramics in AlN-SiC system

Abstract Tensile and bend strength were obtained for four compositions in the aluminum nitride-silicon carbide system: AlN, 75%AlN-25%SiC particulate, 50%AlN-50%SiC solid solution and SiC. Room temperature tensile strength, four-point bend strength, and the Weibull parameters for each of the tested materials are presented. Optical and SEM fractography were employed to identify failure initiating flaws for the fractured tensile specimens. The four-point bend strength is predicted using the Weibull tensile parameters and compared to measured values. Accuracy of the predicted results are rationalized based on observed microstructures and failure initiating flaws.

Design and Application of Concrete Tiles Enhanced with Microencapsulated Phase-Change Material

AbstractPhase-change materials (PCMs) have a high heat of fusion compared to that of traditional material, and for this reason, they are able to store and release larger amounts of energy at their transition temperature. The inclusion of PCMs in buildings has attracted much interest worldwide because of their ability to reduce building energy demand and increase indoor comfort. This paper presents the development and testing results of a concrete tile system with microencapsulated PCMs. The concrete tiles were cast for use in a high-performance house built for the Solar Decathlon China 2013 competition. The paper shows that the addition of PCMs reduced the overall compressive and flexural strength properties of the concrete. A more than 25% decrease in compressive strength was observed with the addition of 20% PCM per volume of concrete. However, a significant improvement in the thermal properties of the concrete tile PCMs was measured. The thermal energy storage capability of the PCM-enhanced concrete ti...

Investigation of Moisture Susceptibility of Warm-Mix Asphalt Mixes Through Laboratory Mechanical Testing

Moisture can lead to serious damage and failures in hot-mix asphalt concrete pavements. This is an even greater concern for warm-mix asphalt because the much lower production temperatures may not completely dry the aggregates. In this Maine Department of Transportation study, the use of fracture energy parameters was evaluated to determine the influence of incomplete drying of mixes on their mechanical properties. Fracture energy–based parameters [energy ratio (ER); ratio of energy ratio (RER)] were determined from the following testing of mixes with fully and partially dried aggregates, some of which were subjected to moisture conditioning: resilient modulus, creep compliance, and indirect tensile strength (ITS) at 5°C. The results indicate that (a) resilient modulus, creep compliance, and ITS were all affected by the presence of moisture in mixes; (b) the trend and the degree of influence of moisture for different mechanical parameters were different; (c) the moisture conditioning process caused larger decreases in modulus and ITS values than did incomplete drying of aggregates; however, the same moisture conditioning process caused much larger decreases in modulus and ITS in mixes prepared with incompletely dried aggregates than did the counterparts prepared with fully dried aggregates; and (d) fracture energy–based parameters (ER and RER) appeared to be more-distinctive moisture effect and damage indicators than are the other parameters.

Comparative study of ride quality measuring devices

State highway agencies (SHAs) in New England have an ongoing interest in providing the public with smoother rides. As an incentive to contractors, some SHAs include a bonus and penalty clause for new construction based on profile measurements and smoothness indices. These states are considering the use of high-speed profilers for profile measurements and quality control/quality assurance implementation. Therefore, an assessment of the accuracy and repeatability of these devices is essential. A comparative study of high-speed profilers owned by the New England SHAs was conducted in Worcester, Massachusetts. Two sites were profiled using the dipstick and the high-speed profiling devices. Statistical analysis of International Roughness Index measurements were used for evaluation of precision and bias, profiler speed, and sensor type. The repeatability for the profilers was good, especially for combined wheel paths (standard deviation between 0.016 and 0.079 m/km). The accuracy for the profilers varied depending on profiler type.