مجله تحقیقات بتن ایران
سال یازدهم شماره 1 (بهار 1397)

Fiber reinforced polymers (FRP) are one of the most commonly used materials for rehabilitation and retrofit of structures, mainly concrete structures. There are many reported studies on the behavior of these materials under different types of loading and structures. The aim of this study is to declare the challenges and investigate the efficiency of existing models for samples with rectangular and square sections. Moreover, a new empirical model is developed to predict the strength of FRP confined samples using gene expression programming (GEP). The best model is selected after establishing and controlling several models with different combinations of the influencing parameters. An analysis is carried out to check the performance of the introduced model based on the common criteria such as correlation of coefficient (R). This parameter is 0.923 and 0.922 for training and testing datasets, respectively which reveals a good agreement between the predicted and measured data in the developed GEP model.

Here, the efficiency of the non-destructive Break-Off (BO) and Pull-off tests was investigated for assessing the in-place compressive strength of steel fiber reinforced concrete (SFRC). SFRC was studied due to its advantages in increasing tensile and flexural strength in particular. To provide a through and comprehensive database, 24 mix designs were selected with cement contents of 400, 450 and 500 kg/m3 with constant water/cement ratio of 0.4 for all mixes, two maximum aggregate sizes of 12.5 and 25 mm along with steel fiber volume fractions of 0%, 0.33%, 0.67% and 1% for ages of 14, 28 and 90 days. A total of 360 Break-Off and Pull-off tests and 216 standard cube tests were carried out in this investigation.Then, effective parameters of SFRC and non-destructive tests results were evaluated. The studies showed that volumetric percentage and features of steel fibers had a significant influence on concrete properties as well as Break-Off and Pull-off tests results. According to the experimental results it could be generally concluded that the influencing factors, namely, SFRC properties due to presence of steel fibers and non-destructive tests significantly affect the results as follows:Generally, for a constant W/C ratio, it can be concluded that raising the cement content increase the mean values of Break-Off and Pull-off strengths.
In adition to, conventional numerical regression model was developed in this study. Statistical indices were used to compare the efficiency and accuracy of model. The result of this study has confirmed the accuracy of linear regression model for non-destructive tests.

High performance fiber reinforced cement composites (HPFRCC) are a class of fiber cement composites that exhibit post-cracking strain hardening response when subjected to direct tension. While undergoing this hardening behavior, the composite experiences the formation of multiple cracks prior to damage localization.
In the present study, This paper investigates the effects of HPFRCC on the behavior of RC exterior beam–column joint under reversed cyclic loading. HPFRCC using tow different types of fiber namely hooked end steel fiber and polypropylene fiber are explored in this study. The main parameters considered include the load–deflection relationship, crack propagation, moment–rotation relationship at the joint, and energy absorption capacity. half-scale external RC beam–column joints were subjected to lateral cyclic loading of increasing amplitudes.. The specimen was subjected to reverse cyclic loading under controlled deformation at the tip of the beam until failure. At post cracking stages, the HPFRCC joint showed significant improvement in the ultimate shear and moment capacities, as well as in the deformation behavior and damage tolerance, compared with the NC specimen at ultimate and failure stages.

There are many plain concrete arch bridges in Iran for more than seventy years. These bridges not only have not been designed for earthquake loading, but also high-speed trains would be used in railways of Iran. Therefore, calculation of fundamental frequencies of the bridges against earthquake and high-speed train vibrations is necessary for considering resonance effects. To evaluate complex behavior of these bridges, results of field tests are useful. Since it is not possible to perform field tests for all arch bridges, these structures should be simulated correctly by computers for structural assessment. Several parameters are employed to describe the bridges, such as number of spans, length of spans, geometrical and material properties. In this study, results of field tests are used and adapted for 64 three dimensional finite element models with various physical parameters. Computer simulations show modal deformations of bridges at longitudinal and transverse directions. Also, these results demonstrate that fundamental frequencies of bridges decrease after increasing span length and number of spans. Plus, some relations are proposed for calculation of fundamental frequencies of plain concrete arch bridge.

Concrete is heterogeneous at all length scales and its microstructure evolves continuously over decades. Through the use of nanoparticles, it is possible to alter the microstructure of cementitious materials from within the first microsecond. This study is aimed to carry out a comprehensive investigation on the effects caused by the addition of nanoclays on properties of cement based composites. Therefore, a replacement of cement by with different percentages of montmorillonite nanoclay (0%, 0.5%, 1%, 1.5%, 2% and 3% by weight of cement) in cement pastes and mortars was planned and The effects of adding nanoclay on hydration process, rheological properties (adhesive properties, water demand and setting time), mechanical properties (compressive strength) and durability properties (water absorption and drying shrinkage) of mixes were investigated. As well, The cement matrix microstructure was examined using scanning electron microscopy. The results indicates that the addition of nanoclays causes positive effects on cement composites properties.

Concrete is one of the most widely used materials in the world for various types of structures due to its durability. This paper presents the results of an experimental study on various compressive strength and water absorption tests on concrete containing coal slag and nano silica as partial replacement of cement. For this research work, the tests were conducted for various proportions of coal slag replacement with cement of 0%, 10%, 15%, and 25%, nano silica of 0%, 1.5%, 2.5% and 3.5% and combination of both of them in concrete. The obtained results were compared with those of control concrete made with ordinary Portland cement. In this research, the effect of several amounts of coal slag and nano silica on some properties of concrete were considered in the production of test samples of 5*5*5 cm. After casting the cubes, they were tested for compressive strength and water absorption. Results showed that compressive strength of samples with 10% and 15% pozzolan were increased, but compressive strength of samples with 25% pozollan were decreased more than that of the control sample. Also, application of pozzolan resulted in less water sorption. The use of nano silica increased concrete compressive strength compared to samples without Nanosilica in most samples, and this increase was proportional to the amount of nano silica. Based on the these findings, it is concluded that Pozzolan coal slag is a suitable replacement for cement especially in presence of nano silica.

In recent years, with regards to the environmental and economic considerations and the importance of sustainable development, it is well understood that part of the cement or clinker must be replaced with other materials such as limestone powder, natural and artificial pozzolans, slag, etc. in this paper, some physical properties and durability of Portland, Portland-pozzolan, Portland-limestone and Portland-composite cements against alkali-silica reaction (ASR) are studied. In order to, experiments were carried out cement paste including normal consistency and time of setting and on cement mortar including accelerated expansion test and compressive strength change of mortar cubes in situations where conditions are favorable for ASR. The results show that durability of cements containing Trass is higher than others and Portland limestone cement (PLC) having up to 10% limestone provides competitive properties with PC concretes. Also, there is significant relation between results obtained by accelerated expansion test and new method used in this study.

In recent years, the confinement effect in high strength concrete (HSC) has been studied by many researchers. Confinement effect results in increasing the strength and ductility of concretes columns considerably . To determine the amount of these increases and to simulate the behavior of these columns a compressive stress strain model containing the confinement effect is essential . In this paper , a number of well known stress strain models are evaluated against experimental data obtained by other researchers and their shortages are determined . Considering compressive strength, the modulus of elasticity , the strain corresponding to maximum stress and confinement stress are considered as main parameters , a new stress-strain model is developed such that it takes into account the effects of confinement . Comparing the results predicted by the proposed model with experimental results for a wide range of compressive strength from 20 to 120 MPa, show a good agreement.

By 1988 in Japan,SCC(Self-Compacting Concrete) is developed and defined as a high capable flowability concrete without requirement of vibrator runs.Generally, SCC based on the materials used to modify its viscosity, can be divided into three categories;first:the powder type SCC, second:the VMA(Viscosity Modifying Admixtures ) type SCC and the third group that includes a mixture of SCC.In this research, 5 SCC projects were built using the VAM-type SCC and lime stone powder- type SCC,separately and also using the combination of both VMA-type SCC and lime stone powder-type SCC.In order to determine the possibility of viscosity in the projects, the VAM-type SCC has been used in the following percentages: 0.05 , 0.1, 0.15 and0.2 based on which the weight of cement are used as a substitute for lime stone powder-type SCC. The new mixtures were tested using different rheological states experiments such as the Slump, T 50, V- Funnel, V- Funnel at T: 5 Minutes, J-Ring and L-Box to set the standard range of self-compacting characteristics. After approval, in this section, the Projects were examined to determine the compressive strength at the age of 7 and 28 days. In addition, the effects of the VMA-type SCC and lime stone powder-type SCC with different proportions, were evaluated and compared in the fresh and hardened concrete experiments.The results showed an increase in the VMA- type SCC, in the subject of making bubbles and also a decrease in the specific gravity up to12.9percent, as well as the compressive strength has fallen up to60percent in maximum.

Introduction of self compacting concrete has brought huge technological advances. Use of SCC facilitated the concrete placing between the rebars, without need of external vibration, and just through the weight of concrete itself. This paper investigated the effects of high temperature on hardened properties of self compacting concrete (SCC) containing powdery admixture. For this purpose, self compacting concrete control (non-pozolan) specimen of ACI 237R-07 mixed design has been used in accordance with instructions. First, in order to ensure the satisfaction of tolerance related to quality control of SCC in fresh mode; slump flow, L box, T50 and V funnel were performed. Then the hardened specimens containing various minerals admixture (metakaolin, fly ash. silica fume and zeolite) were 91 days old heated to elevated temperatures (300, 500, 700 0C) to evaluate mechanical properties. Furthermore the ultrasonic pulse velocity test as non-destructive methods to estimate the compressive strength of specimen is presented.