بررسي تنش جاري شدن و ويسكوزيته پلاستيك مخلوط‌هاي بتن خود تراكم حاوي پودر سنگ

Evaluation of yield stress and plastic viscosity of self-consolidating concrete mixtures containing stone powder using new concrete rheometer made for the first time in Iran

استفاده از بتن خود تراكم در پروژه هاي عمراني بدليل مزاياي آن شامل كيفيت بالا و جاري شدن تحت وزن خود بدون نياز به لرزش ، در حال افزايش است. ريولوژي اين بتن يكي از خواص بسيار مهم آن است كه بر روي خصوصيات مكانيكي و دوام آن تاثير چشمگيري دارد. در اين تحقيق با استفاده از دستگاه ريومتر ويژه بتن كه براي اولين بار در ايران ساخته شده ، پارامتر هاي ريولوژي بتن خود تراكم تعيين شده است. براي اعتبار سنجي عملكرد دستگاه و كاليبراسيون آن از روغن با ويسكوزيته مشخص استفاده شده است. مقدار ويسكوزيته پلاستيك بدست آمده توسط دستگاه برابر 208/2 پاسكال ثانيه و مقدار ارايه شده ويسكوزيته روغن توسط كارخانه برابر 219/2 شده است. بنابراين نتايج نشان مي دهد كه دستگاه ريومتر ساخته شده بخوبي منطبق با طرح بوده و پارامتر هاي ريولوژي توسط دستگاه با دقت مناسبي قابل تعيين است. همچنين چند مخلوط آزمايشي بتن خود تراكم حاوي دو نوع پودر سنگ به عنوان پر كننده ساخته شده است. مقادير محاسبه شده ويسكوزيته پلاستيك و تنش تسليم اين مخلوط ها نشان مي دهد نمونه حاوي پودرسنگ آهك تا 200 كيلوگرم در متر مكعب بهترين عملكرد را نشان مي دهد و با افزايش اين مقدار به 300 كيلوگرم در متر مكعب مقدار تنش جاري شدن بتن افزايش مي يابد.

Self-consolidating concrete(SCC) application is using in construction projects more and more because of its advantages including high quality and flowing under its own weight without any vibration. Rheology of self-compacting concrete is effective not only on the properties of fresh concrete, but also on the mechanical properties and durability of concrete. Fresh concrete can be considered suspended particles aggregate in the cement paste. Usually, the concrete flow behavior is expressed by the Bingham model. The rheological properties of the fluids and concrete can be measured by the rheometers that have been developed to study the behavior of yield stress and plastic viscosity. It can be stated that the first rheometre was invented in 1973 by Tattersal and Banfill and it was called MKI rheometer. MKI could be measured g and h parameters that they were converted to the rheology parameters by specific calibrations and factors. The rheometer vane starts to rotate in the concrete at first with the constant speed. Then the rotation speed is increased step by step (each 5 seconds). This is necessary for the measuring of thixotropy properties. In the late 1980s, the BML viscometer was developed and later, In 1996 in France BT-Rheom was developed. But research by Tattersal and co-authors compared the results of various rheometrs and shows that these devices measurements (yield stress and plastic viscosity) are different. ICAR is the latest generation of rheometers that was developed by Koehler in 2004.The advantages of ICAR device is being portable, low-cost and suitable for the self-consolidating concrete especially. The advantages and limitations of each of the rheometer and their history report is described in ACI 238.1R. Lascar and colleagues have shown in their study that there is the relationship between geometry and torque rheometer and yield stress and plastic viscosity of concrete.In this study, using a special concrete rheometer that has made for the first time in Iran, rheological parameters of SCC are determined. Devices made in Iran is similar to the ICAR rheometer that was developed by Koehler in 2004. In other words, this device is from coaxial rheometer type that is the most accurate in determining the plastic viscosity and yield stress of concrete. The device consists of a generator engine torque, four-blade vane. For performance evaluation and calibration of device, standard oil with determined viscosity by factory is used. Plastic viscosity value obtained by rheometer was equal 2.208 Pa.S and the oil viscosity provided by the factory was 2.219. Therefore results show that Rheometer performance is according with design plan and rheological parameters can be determined well by this device. Also some experimental SCC mixture contains two kinds of powder as filler is made. The calculated values of plastic viscosity and yield stress of these mixtures show that SCC containing limestone powder to 200 kg/m3 indicating the best performance and with increasing the amount of filler to 300 kg/m3 cause to increase yield stress.