مجله تحقیقات بتن ایران
سال دهم شماره 1 (بهار 1396)

Plastic shrinkage occurs in fresh concrete usually within few hours after mixing the concrete and risk of its cracks endangered concrete structures especially in the elements with high surface to volume ratio such as slabs or highway pavement. An experimental investigation on capillary pore pressure, tensile strength and plastic shrinkage of concrete is presented here. The aim of research is to study the relationship between capillary pore pressure build up in concretes, early age tensile strength and plastic shrinkage strain. Capillary pore pressure apparatus was created for the first time in Iran for this research. Test was done in a climate chamber with the constant evaporation rate of 0.7 kg/m²/h .Eight types of concrete in two categories such as normal concrete and self-consolidating concrete were tested including mixture without any mineral admixtures and containing of pozzolanic materials such as silica fume and metakaolin. The results indicated that there is no strong relationship between capillary pore pressure and plastic shrinkage strain and it can be concluded that other parameters such as tensile strength development can play an important role in the plastic state of concrete. However, it should be mentioned that capillary pressure is the main cause and driving force of concrete shrinkage and the onset of capillary pressure is directly related to the onset of shrinkage. The results also showed that early age tensile strength can be effective factor in controlling cracks of concrete. No cracks appeared in the mixtures containing of silica fume and metakaolin because of tensile strength improvement up to 55 and 32 percent respectively, in comparison with the reference mixture. However capillary pore pressure in these mixtures was higher than pressure in the reference mixture.

Chloride penetration into concrete and consequently corrosion of reinforcing steel is one of the main causes of concrete deterioration in Persian Gulf and Sea of Oman. Concrete deterioration exposed to tidal condition is very severe when compared to the atmospheric exposure condition. Therefore, it is necessary to accurately model the simultaneous moisture and chloride ingress into concrete for durability-based design of reinforced concrete structures. In this study, a finite element model was developed to numerically solve the governing differential equations of moisture and chloride transfer. Comparison of the model output with the ones obtained from experimental works in Bandar Abbas and Qeshm research sites showed that the proposed model can predict the chloride profile with good accuracy. By using the test results, the model output showed that the maximum chloride concentration decreased with increasing in water to cement ratio (w/c) from 0.40 to 0.45 and then slowly increased with increasing in w/c up to 0.55. Although, the total chloride concentration, which ingress into concrete, increased up to 50 %.

The weak mechanical properties of lightweight aggregate concrete (LWAC) such as low strength and brittleness discourage its vast application in structural members. An effective method to improve the mechanical properties of concrete is the use of pozzolanic and reinforcing materials. The loading condition and the ingredients of mix compositions affect the concrete behavior, so more extensive and detailed research into the effects of type of concrete material on the stress-strain curve is required in order to predict the accurate response of structural members. In this paper, various amounts of Nano-Silica (1, 3 and 5 percent by weight of cement) and steel fiber with constant volume fraction of 1% were added to mixtures and the effects of these parameters on the compressive stress-strain curves were investigated. Furthermore, the key parameters that control the concrete behavior under compressive monotonic loading, including compressive strength, modulus of elasticity and strain at peak stress were evaluated. Results indicate that the optimized combined use of steel fibers and Nano-silica has significant effect on the improvement of compressive behavior of LWAC. The compressive strength and modulus of elasticity in the mix containing 3% Nano-silica and 1% steel fiber increase by about 46% and 26% compared to the plain LWAC mix.

Roller compacted concrete (RCC) pavement is one of the most economic and durable pavements which is used extensively in pavements with heavy traffic. The aim of this study is to investigate the feasibility of using wheat straw as fiber in order to change the behavior of RCC pavements and also to strengthen the RCC pavements. Cellulose fiber has proper effects on shrinkage cracks and toughness of RCC mixtures since it is of high tensile strength and low elastic modulus close to mortar. The Compatibility of fiber and concrete matrix and also the effects of straw on concrete components are discussed. The latter is to study if the straw can improve the features of the concrete or not. Natural fiber is a natural mixture with a cellulose structure. Different properties of cellulose, hemicelluloses and lignin, produce different layers. Polymer cellulose has glucose units and hemicelluloses are made up of different polysaccharide. Different types of fiber have different mixtures so it is expected that their behavior will be different in a cement matrix. Natural fiber is of low elastic modulus and high tensile strength and even their tensile strength is comparable to artificial types of fiber. Using special types of fiber such as steel fiber, which has a proper performance, is expensive, hence the basic aim of this study is to reinforce the concrete using cheap and accessible elements. Dry and saturated straw fiber with different aspects of ratio (30 to 150), different lengths and values, was added to RCC samples and VB test of fresh complex and compressive strength test were made out of 15 x 30 cm, 28 days samples and toughness testing of cylindrical 15*15 cm samples were applied. That set of these tests indicates the feasibility of using wheat straw at RCC as well as improved hardened concrete mixture toughness for 1 percentage of weight expressed in RCC mixture.

It is assumed that concrete structures are resistant to fire. Past research in this field has shown that in these structures, numerous problems occur during a fire. During the fire in concrete structures, fire fighters mainly use water to put out the fire. Upon contact with water, a great thermal difference occurs in the heated organ which affects most properties. No independent experimental study, to the researcher’s knowledge, has been conducted in order to investigate the effect of increased duration of high temperature on compressive and tensile strength of concrete containing glass fiber exposed to fire. However, the present study aims to investigate the effect of increased duration of high temperature on compressive and tensile strength of concrete containing glass fiber at a rate of 1%, 2%, and 3% exposed to high temperature (600 °C) in three different target times including 30 minutes, one hour, and two hours. Samples were cooled in two ways: slow cooling (exposure to air) and fast cooling (water spray immediately after exposure to heat). A total of 84 cubic samples (size: 150×150×150 mm and cylindrical samples sized 150×300 mm were prepared for studying compressive strength and tensile strength, respectively. After 28 days of processing and gaining the required strength, samples were put in annealing furnace and the experiments were conducted. The results showed that heat reduces the strength considerably. The slowly-cooled samples showed better performance compared to those cooled quickly. Also, heat caused many cracks in concrete samples.

Glass powder and Metakaolin are suitable alternative for part of cement in concrete. The use of waste glass powder in concrete is economic and reduces environmental impacts resulting from the release of recycled glass in nature. On the other side, Metakaolin because of pozzolanic features can increase the resistance of concrete. In this study, by simultaneous use of these two substances in the production of self-compacting concrete, fresh and hardened concrete properties are evaluated. For this purpose, Seventeen mixtures with various percentages of glass powder and Metakaolin (5%, 10%, 15% and 20%) and constant Water/cement ratio equal to 0.32 were made. Fresh concrete tests, including slump flow, T50, L Box and V-funnel and also hardened concrete tests, including compressive, tensile and water absorption for all of SCC mixtures was performed. Results show that by increasing the amount of glass powder, slump flow and V-funnel time increces but compressive and tensile strenght and initial water absorption decreases. By adding metakaolin and increase it to 15%, compressive and tensile strength increases.

The objective of the present research approach based on soft computing (neural network) in the evaluation of seismic fragility of the highway bridge. In addition to the empirical methods and expert’s judgmental, seismic fragility curves are often determined by using analytical method in Structures, recently. The derivation of seismic fragility curves of the horizontal curved bridge based on the neural network with a focus on concrete column ductility measure by using 129 seismic records is performed. Earthquake records have been chosen from the PEER strong motion database and scaled on 0.1g to 1.3g. By using 1677 nonlinear dynamic analysis, incremental dynamic analysis (IDA) curves was drawn. Characteristics of earthquake ground motion as input and extraction of nonlinear dynamic analysis of columns ductility as output, are variables in building the neural network. Feature Extraction different records in different seismic intensity represents a seismic record neural network. Obviously, these characteristics are properly associated with structural damage. By transforming collection features different seismic record (matrix n×m) to the data with the same characteristics of seismic input (matrix p×m, p

Beam-column connections are key members in moment frames resisting seismic loads which their failure may lead to total collapse of structure. Application of fiber reinforced polymer (FRP) composites has been received much attention in seismic rehabilitation of these members. However, the main challenge facing this technique is the premature debonding of FRP composites off the concrete substrate posed. The present study was conducted to investigate the capability of grooving method (GM) coupled with FRP fan at termination point of FRP composites in postponing or completely elimination of undesirable debonding failure mode in shear strengthened reinforced concrete (RC) joints. To do so, three half-scale RC beam-column subassemblies were tested under reversed cyclic lateral load. Results revealed that GM, coupled with FRP fan at termination point of FRP composites, was able to eliminate altogether the debonding failure mode and remarkable improvement in terms of load-carrying capacity, ductility, and energy dissipation was achieved.

This paper aims to evaluate the properties of concrete containing waste glass at a low water/binder ratio. Totally, seven concrete mixtures made with water/binder ratio of 0.27 in which recycled glass was partially replaced either as cement or fine aggregate. The properties of waste glass concrete were evaluated by compressive strength, ultrasonic pulse velocity, splitting tensile strength, initial and final absorption and electrical resistivity. Results showed that although, glass powder inclusion decreases the compressive strength of high-strength concrete (HSC), the use of waste glass as fine aggregate increased the 90-days compressive strength of reference HSC from 102.3 MPa to 106 MPa. Furthermore, good correlations can be found between UPV and compressive strength, with R2 values equal to 0.87 and 0.79 for HSC with glass powder and HSC containing glass sand, respectively. Splitting tensile strength of HSC with waste glass was lower than those obtained for reference HSC. A low absorption (below 3% at 30 min) can be achieved for waste glass mixes classified as ‘‘good’’ concrete quality. The electrical resistivity of HSC containing waste glass is determined to be higher than 20 kΩ-cm at the age of 28-days. In general, waste glass can be used either as cement or fine aggregate in the production of HSC with acceptable mechanical and durability properties.

Plastic concrete is a kind of concrete that contains montmorillonite, which is the soil containing clay in addition to ordinary concrete materials. This concrete has lower compressive strength in compare with normal one, but has higher plasticity as a specification of ability to give higher ductility. In this study, the plastic concrete strength was increased by adding polypropylene fiber while maintaining ductility constant, and for this purpose, a concrete mixture was made, in which ratio of water to cement was considered 1.17 and ratio of bentonite to cement was considered 0.12. The plastic concrete was made by adding 0, 0.25 and 0.5 percent of polypropylene fiber, on which tensile strength, compressive strength and permeability tests were done. The results show that, the strength at 28 day increased significantly while the strength is about ordinary concrete at early age to 7 day and the effect of adding fiber on tensile strength is more than the compressive strength of concrete. The ductility and toughness strength have improved to a significant level based on stress-strain diagrams. Permeability of plastic concrete was increased by adding fiber in spite of the usual behavior for fiber reinforced concrete.