مجله تحقیقات بتن ایران
سال چهاردهم شماره 4 (زمستان 1400)

The main factor in integrated performance of repair overlay and substrate concrete is the bonding between them. The accurate measurement of bonding is dependent to factors such as strength and surface characteristics of substrate concrete, loading mechanism and curing condition of overlay. In this research, 8 mix designs are considered. For assessment of strength level and surface quality of substrate, two concrete grades in the ranges of 20-30 and 40-50 MPa, and also two surface conditions including simple finishing and ribbed finishing were considered. Repair overlays were considered from three different strength grades of self-compacting concretes contained 0.5% and 1% of steel fibers. After fabrication, for curing, three different conditions of curing in open air, curing in warm water, and standard curing were deemed. Compressive strength, flexural strength and splitting tensile strength tests were performed on samples, and Pull-Off method were used for evaluation of bonding strength. The results indicated that the impact of substrate strength on bonding is more significant than overlay strength, and also the effect of curing condition was more salient than finishing quality. The bonding strength of overlays contained 0.5% of steel fibers and with strength grades of C50, C60 and C70 cured in warm water were 105%, 108% and 104%, respectively, higher than same samples cured in open air.

In this experimental paper, the influence of using both longitudinal steel and GFRP bars on squat concrete shear wall has been investigated. The main purpose of this research is to study the effect of applying a hybrid system on the failure mechanism and seismic performance of specimens. For this purpose, three cantilever shear walls in full scale under cycle lateral loading with aspect ratio 1.0 have been tested. The S-SSW specimens with longitudinal and transverse steel bars, used as reference, and the G-SSW specimens reinforced with longitudinal and transverse GFRP bars as well as the SG-SSW specimens reinforced with both longitudinal steel and GFRP bars and GFRP transverse bars were examined. The results indicated the use of the hybrid system has changed the failure mode from concrete crushing in the compression zone to bar rupture. Also, the parameters such as residual deformation, secant stiffness, energy dissipation and ductility increased in the SG-SSW compared to the G-SSW specimen.

The reduction of natural resources makes researchers to revise the natural aggregates and find ways to reduce the negative effects of untreated wastes on the ecosystem of the region, so they have tried to increase the use of industrial waste material. In order to study the effect of untreated waste coal on mechanical performance of green concrete 9 mix designs with different contents of untreated coal waste (5%, 10%, 15%, 20%) as coarse and fine aggregates and constant water to cement ratio (w/c=0.4) were prepared. The results show that, replacing 10% of the aggregate with untreated coal waste instead of sand and gravel, compressive strength, elastic modulus and ultrasonic pulse velocity increase compared to plain concrete (without untreated coal waste). However, by increasing the percentage of coal waste, the tensile strength of the concrete specimens decrease compared to plain concrete. Thus selecting an appropriate amount of replacement untreated coal waste as a substitute for aggregate in concrete not only minimizes the negative impact of waste coal dumps, but also it can improve some properties of concrete, and be considered as a step toward sustainability.

According to increasing the use of PET products, the present study aimed to the use of PET fibers in the concrete industries. Moreover, the behavior of normal concrete contained recycled fibers is not studied and the guidelines aren’t published yet. The aim of this study was to determine the effect of polyethylene terephthalate fibers on the mechanical properties of hardened concrete. Fibers were used in three volumetric percentages of 0.15%, 0.30%, and 0.45% with the lengths of 1, 2, and 3 centimeters, separately. So, 10 combinations of specimens including the control specimen and 9 combinations of PET fiber reinforced concrete were made. All mixtures were examined for compressive strength, tensile strength, flexural strength, shrinkage, durability, and impact loading.The results showed that by adding the PET fibers, the compressive strength, the bending strength, and energy absorption increase 5 to 18 percent, 6 to 30 percent, and 30 to 156 percent, respectively. However, no noticeable change was observed in tensile strength , while even a reduction in tensile strength is monitored. The impact strength of the fiber concrete was increased two to seven times in comparison with the control concrete. By comparing the experimental results it was determined that the combinations that contained 2 cm fibers with 0.45% volume had the best performance compared with the other mixtures.

Bridges are behaviorally special compared to conventional structures. The uncertainty of bridges is much less than that of buildings, so the failure of columns in a bridge can lead to complete failure. The use of fiber reinforced polymer (FRP) composites into wrapping as one of the methods of strengthening of reinforced concrete columns (RCCs) of bridges has received much attention in recent years. High stiffness and characteristic strength, long life, corrosion resistance and controllable thermal properties are important advantages of FRP over conventional materials. The use of glass-fiber reinforced plastic pipes (GRP) as casing for bridge piers, removes molds, reduces costs, increases energy absorption and ductility, and reduces water-washing around bridge piers. In this study, compressive strength tests were performed on six samples of oval RCCs with diameters of 200 and 120 and height of 600 mm with and without casings and the effect of carbon-FRP on them as an enclosing was investigated. The results showed that the wrapping and casing increases the strength and ductility of the columns. Addition of one-layer and two-layer wrapping averagely increased the strength by 19.7% and 28.7%, respectively; while the use of casing resulted in a 4.15-fold increase in the strength. Although the wrapping and the casing both create the enclosure, the casing has a much greater effect on increasing the strength and ductility of the columns due to its greater enclosure. Comparison of wrapping enclosure with casing in the final axial deformation of these columns showed that the casing is much more effective.

Today, concrete has a wide range of applications in various projects due to its desirable properties such as high compressive strength, fire resistance, high durability in corrosive environments and low permeability. Therefore, the use of effective methods for the principled compaction of concrete in order to improve the mechanical properties of concrete is of great importance. For this purpose, for the first time in this study, the use of high ultrasonic power waves with different powers, different times and 20 kHz frequency in concrete compaction and its effect on the water absorption rate of half an hour of concrete has been investigated. The results of this study show that the use of high ultrasonic power waves as a complementary process in the compaction of fresh concrete, leads to a reduction in the water absorption rate of half an hour of concrete about 12.5 to 15.5 percent, depending on the shape of the test mold.

This study was conducted to probable improvement of the performance of alkali activated slag mortar exposed to sodium sulfate solution. In this regard, the joint effect of silica fume replacing at levels of 0, 5 and 10% and curing at 23 ± 2, 40, 60 and 80 °C on the durability of alkali activated slag mortar against high concentration sulfate medium was tested. In order to investigate the behavior of the samples, length change and mass change tests were used and to identify the microstructure and products formed in their hardened paste, scanning electron microscopy imaging and X-ray diffraction analysis were performed. The results showed the positive effect of heat treatment and silica fume replacement on the durability of alkali activated slag mortar. Simultaneous application of 5% silica fume replacement and heat treatment with a temperature of 80℃ had the best effect on the performance of activated alkaline slag mortar in terms of sulfate resistance. Alkali activated mortar was completely superior to ordinary Portland cement in terms of resistance to sulfate medium. The results of length and mass change experiments were not enough to compare alkali activated slag mortar samples. This was made possible by adding the results of scanning electron microscopy imaging test and X-ray diffraction analysis.

In this study, the possibility of using recycled aggregates, prepared from recycling plant of Kermanshah, in the production of self-compacting concrete is investigated. Recycled aggregates were randomly collected from the whole Kermanshah province resulted from destruction of buildings or natural disasters. Here, self-compacting concrete samples including different percentages of the recycled coarse and fine aggregates and a combination of both were prepared in the form of 16 different mix designs. Fresh concrete tests including slump flow, T50 and L-BOX, and hardened concrete tests including compressive, tensile and flexural strength tests, as well as modulus of elasticity and Poisson ratio tests were performed. In this study, fly ash was used to retrieve for the probable reduction in the concrete strength due to the use of randomly collected recycled aggregates instead of natural aggregates. The results showed that the mixtures prepared using recycled coarse aggregates had higher mechanical properties than mixtures prepared using recycled fine aggregates. In all hardened concrete tests, the performance of the mixtures prepared using the combination of recycled fine and coarse aggregates was significantly lower than the state of using the coarse or fine aggregates separately. Also, in the case of fresh phase concrete tests, it was observed that with increasing the amount of recycled fine and coarse aggregates, the rheological properties of self-compacting concrete decrease compared to the mixtures using natural aggregates. Therefore, the recycled fine aggregates have a greater effect on the reduction of the properties of fresh concrete compared to the recycled coarse aggregates.