بررسی مقاومت مکانیکی بتن غلتکی حاوی نانوکربنات کلسیم در برابر پدیده فرونشست

یکی از پارامترهای تاثیرگذار در میزان عمر مفید روسازی های بتن غلتکی، مقاومت آن ها در برابر تنش های ناشی از نشست زمین وابسته به زمان است که در این رابطه اعمال تنش ناشی از نشست های زمین به علل زمین شناختی باعث خرابی شریان های حیاتی می شود که از جمله می توان به ایجاد شکست هایی در سطح روسازی راه ها می شود. در این راستا به بررسی تاثیر فناوری نانو در بتن غلتکی و تحمل تنش های ناشی از این گونه نشست ها می پردازیم. در این پژوهش سعی شده است که با استفاده از مهندسی نانو، درصدهای مختلف نانو کربنات کلسیم، جای گزین سیمان در بتن غلتکی شده و آزمایش های مقاومت فشاری و کششی بتن غلتکی در سنین مختلف انجام شود. نتایج حاصل از این پژوهش نشان می دهد که با افزایش بهینه میزان پودر نانو کربنات کلسیم حرارت اولیه بتن غلتکی و مقاومت مکانیکی نمونه ها در سنین بالا به علت ریزدانه بودن و خاصیت پرکنندگی و سطح ویژه بالا افزایش چشم گیری می یابد. نتایج این تحقیق نشان می دهد استفاده بهینه از این مواد باعث بهبود برخی خواص بتن غلتکی از جمله مقاومت کششی و فشاری می شود که در برابر تنش های ناشی از پدیده فرو نشست مقاومت چشم گیری از خود نشان می دهد.

Effect of Nano Calcium Carbonate on Mechanical Strength and Permeability of Roller-Compacted Concrete

IntroductionNanostructured materials have gained increasing attention of industry and the academia in recent decades, due to their prominent behaviors. In this regard, the building industry is considered to be the major consumer of nanostructured materials in terms of its needs, including strength, resistance, durability and high performance. Studies on nanoscale behavior of cement and concrete to develop new building materials and their applications are of high importance. A typical method for the development of high performance concrete (HPC) often contains various parameters, including the mix of conventional concrete with different types of additives. NanoCalcium carbonate (NanoPrecipitated Calcium Carbonate) is a nanosized filler which is used in this research. The results indicate that the higher the optimal content of nanoprecipitated calcium carbonate powder, the higher the initial heat of the rollercompacted concrete; also, the resistance of the samples significantly increases over time. However, the level of permeability of rollercompacted concrete decreases by optimal increase of nanocalcium carbonate powder due to its fine grains, filling properties, and high specific level. The results of this study show that the adequate use of this material improves some properties of rollercompacted concrete.Material and methodsIn this study, the content of Nanocalcium carbonate used was selected at 0, 1, 2, 3 and 4 percent replacing a volume of cement consumed in concrete. Type II Portland cement, crushed fluvial sand, and crushed coarse aggregates with a maximum size of 19 mm were used. The aggregates rsquo; grading range in the mix has been selected according to the ACI32510R. The chemical formula of Nanocalcium carbonate powder is CaCo3 and the average particle size is between 1540 nmAccording to the rollercompacted concrete specifications, 5 mix designs have been used with different proportions of stone materials in preparing of concrete. The samples were made on a vibrating table and in the cylindrical molds of 15 times; 30 cm according to ASTM C1176 standard.By increasing the cement grade, the slope of the Vebe curve increases, which means an increase in speed and reduction in efficiency over time in higher grades. Increasing the cement grade from 275 to 300 kg/m3 leads to increased Vebe time. In other words, it can be said that the efficiency is reduced at a lower rate in lower grades of new rollercompacted concrete mix. The Vebe time of the rollercompacted concrete pavement should be between 3040 s to achieve optimal efficiency. According to the results of Vebe time, the efficiency of the rollercompacted concrete with the grade of 300 kg/m3 has a better functionality than other mixtures and lasted more than others in the 30 to 40 second range. Accordingly, concrete with a grade of 300 kg/m3, is the compressive strength according to this design.Determining the compressive strength of cylindrical concrete samples of different ages is done according to the ASTM C39/C39M standard. For permeability test, the BS EN 123908: 2009 was used in which the sample should be put under pressure of (0.5 plusmn;)5 for 72 hours immediately after molding. Determining the tensile strength of concrete cylindrical samples at different ages is done according to the ASTM C496 standard. The peak is obtained using the XRD analysis of the crystallite size by determining the width of the peaks. In interpreting the XRD data, a list of peak resolution and their intensities is observed. To determine the elemental composition of materials, a nondestructive analytical technique is used by Xray which is socalled XRF (Xray fluorescence). A scanning electron microscope is a powerful magnification tool and is used to distinguish elements.Results and discussionThe results indicate that the increased Vebe time occurs by an increase in the percentage of nanocalcium carbonate. In terms of the compressive strength of cylindrical rollercompacted concrete samples, 2% of nanocalcium carbonate at the ages of 7, 28, and 90 days has been effective in increasing compressive strength in higher ages. Such that, at the ages of 28 and 90 days, it is increased by 12% and 15 % compared to the control sample, respectively. The nano content increases over 15% causes decreased compressive strength and thus had negative effects on the rheological properties of the rollercompacted concrete. In terms of tensile strength of the cylindrical rollercompacted concrete samples, 2% of nanocalcium carbonate at the ages of 7, 28 and 90 days has been effective in increasing compressive strength in higher ages, such that at the ages of 7, 28 and 90 days, it has been increased by 25%, 30% and 30 % compared to the control sample, respectively. However, it can also be concluded that the excessive increase has partly reduced the tensile strength.The variation of the permeability coefficient is a function of concrete porosity and water penetration in the rollercompacted concrete. Also, there are significant changes in the concrete permeability coefficient by adding different percentages of nanocalcium carbonate to concrete.Adding nanocalcium carbonate up to 2% of cement weight to the rollercompacted concrete reduces the permeability coefficient of the rollercompacted concrete. One of the reasons for this phenomenon is capillary interstice filling in the rollercompacted concrete. Moreover, the nanocalcium carbonate increase of over 2% of cement weight raises the permeability of the rollercompacted concrete.Adding 4% of nanocalcium carbonate to the rollercompacted concrete pattern increases the intensity of the peaks in the XRD test. Given that the average crystallite size is obtained from full width at half height of the peaks, by increasing the peaks rsquo; intensity and their width at half height of the peaks, we get smaller crystallite size. Also, by adding 4% of nanocalcium carbonate, the widths of the peaks are increased, which means smaller crystals and increased crystallite inner tension.ConclusionNanocalcium carbonate, due to its special features, including a high specific surface area, has a good performance in improving the mechanical properties and durability of the rollercompacted concrete, if it is used at a certain and optimal amount. The rollercompacted concrete with the grade of 300 kg / m3 has better functionality than other mixtures, and lasted more in the 30 to 40 second range.The mix design containing 2% of nanocalcium carbonate replacing cement, has the highest compressive strength at the age of 7 days and shows 4% increase in resistance compared to a control sample at the age of 7 days. The mix design containing 2% nanocalcium carbonate has the highest compressive strength at the age of 28 days and shows 12% increase in resistance compared to a control sample at this age and improved the compressive strength. The mix design containing 2% nanocalcium carbonate has the highest compressive strength at the age of 90 days and shows 15% increase in resistance compared to a control sample at this age. The mix design including 3% of nanocalcium carbonate replacing cement, has the highest tensile strength at the age of 7 days, and shows 25% increase in resistance compared to a control sample at the same age. The mix design containing 2% of nanocalcium carbonate replacing cement, has the highest tensile strength at the age of 28 days and shows 30% increase in resistance compared to a control sample at the same age. The mix design containing 2% of nanocalcium carbonate replacing cement, has the highest tensile strength at the age of 90 days and shows 30% increase in resistance compared to a control sample at the same age. ./files/site1/files/131/3Extended_Abstract.pdf