جستجوی مقالات مرتبط با کلیدواژه "چرخه یخ زدن و آب شدن" در نشریات گروه "عمران"

Adding fresh concrete to old concrete is a common method for repairing or strengthening structures. In this research, in order to evaluate the shear and tensile strength of the joint of old and new concrete under successive cycles of freezing and thawing of new concrete with cement grades of 300, 350 and 400 kg/m3 and three water-to-cement ratios of 0.4, 0.45, 0.5 and bubble-making materials with amounts of 0.0, 0.1, 0.2, 0.3 and 0.4 of the weight of cement used. Then, 300 consecutive cycles of freezing and thawing were performed on the samples after 3, 7 and 28 days of processing period. Freezing and thawing periods include lowering the temperature of the samples from 4°C to -18°C and raising it from -18°C to 4°C, which is done alternately and in a period of 4 hours for each thawing-freezing cycle. The samples were frozen for 3 hours and placed in water for 1 hour for the thawing process. The results of this research show that the effect of freezing and thawing cycles on the shear strength is more than the tensile strength of the bond and the increase in the weight percentage of the bubble-making material has the greatest effect on the shear stress during the 28-day processing period, and with the increase in the weight percentage of the bubble-making material from zero to 0.4, the difference in the amount of shear stress in the conditions with and without the freezing and thawing cycle decreases. The maximum decrease in the shear strength of the joint bond after the application of the temperature cycle is zero in the amount of bubble-making material, so that for a 28-day concrete sample, the shear strength decreases by 93% on average in the ratio of water to cement and different grades of cement. According to the results of this research, with the increase in the weight percentage of bubble-making materials from zero to 0.4, for concrete with 300, 350 and 400 kg/m3 grade, the amount of shear stress for different water-cement ratios and different processing periods decreases on average by 15%, 14% and 11%, respectively. But for laboratory conditions with freezing and thawing cycles, the amount of shear stress increases significantly with the increase in the weight percentage of bubble-making materials, so that for concrete with 300, 350 and 400 kg/m3 grade in the 28-day processing period and the ratio of water to cement 0.45, with the increase in the weight percentage of bubble-making materials from zero to 0.4, the amount of shear stress reaches from a very small value of 0.42, 0.45 and 0.47 to 2.59, 2.91 and 2.99 MPa. With the increase of water-cement ratio in conditions without freezing and thawing cycle, the amount of shear strength decreases, but in conditions with freezing and thawing cycles, the shear strength first increases and then decreases, so that the highest value of shear strength occurs in the water-cement ratio of 0.45.  Also, the highest bond strength after applying freezing and thawing cycles in the samples, after 3 days of processing in water to cement ratio of 0.4, grade of 400 kg/m3 and using 0.4% of cement weight used as bubble material occurs.

In this research, the effect of soil stabilization with cement at the same time as its reinforcement with fibers has been studied on the shear strength of sandy soil exposed to freeze-thaw cycles. In order to achieve this goal, laboratory studies were carried out with the help of unconfined compressive strength tests (UCS tests) on different compounds obtained from mixing cement, fibers, and sandy soil. More than 336 cylindrical laboratory models with dimensions including 3.6cm in diameter and 8cm in length have been made. Various modes have been observed during the failure of the samples, including shear, tensile, plastic yielding, and composite failure modes. The fibers used in the present research are waste products of tire factories known as DTY. Percentages of 2, 4, and 6 for cement and 0, 0.5, and 1 for fibers with lengths of 0.5, 1, and 1.5 cm were used relative to the weight of dry sandy soil in making the samples. Uniaxial cylindrical samples were tested for unconfined compressive strength after 7 and 28 days of curing time and under 0, 1, 2, and 3 freeze-thaw cycles. The results show that the act of stabilizing the soil with cement, along with reinforcing it to a certain amount of fibers, improves the uniaxial compressive strength before and after freezing and thawing cycles. This amount depends on the percentage of cement and the curing period. Also, adding cement in a certain curing time increases the unconfined compressive strength before and after applying the cycle, increases stiffness, reduces the ductility and toughness of the sample, and brittle failure when breaking occurs in the soil. Also, the addition of fibers, to some extent, improves the weaknesses caused by soil stabilization, such as reducing the failure axial strain, decreasing the residual strength, and the toughness of the materials in the conditions before and after freezing and thawing.