ارزیابی پارامترهای موثر بر نتایج آزمایش break-off برای تعیین مقاومت بتن حاوی الیاف فولادی

در این مطالعه آزمایش نیمه مخرب break-off برای ارزیابی مقاومت بتن حاوی الیاف فولادی در محل مورد بررسی قرار گرفته است. بتن حاوی الیاف فولادی بدلیل مزایای آن در افزایش چقرمگی و مقاومت بتن بویژه کششی و خمشی، در این مطالعه انتخاب شده است. بمنظور فراهم کردن یک پایگاه آماری کامل و جامع، 24 طرح اختلاط با مقادیر مختلف عیار سیمان 400، 450 و 500 کیلوگرم بر متر مکعب با نسبت آب به سیمان ثابت 4.0، حداکثر اندازه سنگدانه 5.12 و 25 میلیمتر و الیاف فولادی با نسبت های حجمی 0، 33.0، 67.0 و 1 درصد در سنین مختلف 14، 28 و 90 روز انتخاب گردید. سپس، عوامل تاثیرگذار بر خصوصیات بتن حاوی الیاف فولادی و همچنین نتایج آزمایش break-off مورد ارزیابی قرار گرفت. بررسی ها نشان می دهد علاوه بر مشخصات آزمایش، درصد و مشخصات الیاف فولادی تاثیر بسزایی بر خواص بتن و نتایج آزمایش break-off دارد. نتایج حاکی از آن است که ارزیابی مقاومت با روش فوق دارای قابلیت اعتماد مناسب می باشد، هر چند تخمین مقاومت بتن براساس یک منحنی کالیبراسیون کلی ممکن نیست. از اینرو منحنی های کالیبراسیون مستقل برای هر بتن با درصدهای متفاوت الیاف بمنظور تفسیر واقع گرایانه نتایج ارائه گردیده است.

Evaluation of effective parameters on Break-off test results for determining strength of steel fiber reinforced concrete

Here, the efficiency of the nondestructive BreakOff (BO) test was investigated for assessing the inplace compressive strength of steel fiber reinforced concrete (SFRC). SFRC was studied due to its advantages in increasing toughness and tensile and flexural strength in particular. To provide a through and comprehensive database, 24 mix designs were selected with cement contents of 400, 450 and 500 kg/m3 with constant water/cement ratio of 0.4 for all mixes, two maximum aggregate sizes of 12.5 and 25 mm along with steel fiber volume fractions of 0%, 0.33%, 0.67% and 1% for ages of 14, 28 and 90 days. A total of 360 BO tests and 216 standard cube tests were carried out in this investigation.Then, effective parameters of SFRC and BO test results were evaluated. In the BO method, the force required to break off an inplace concrete cylinder of 55 mm in diameter and 70 mm long, is related to the compressive strength of the concrete from a predetermined calibration curve developed for concrete mix.The studies showed that volumetric percentage and features of steel fibers had a significant influence on concrete properties as well as BO test results. According to the experimental results it could be generally concluded that the influencing factors, namely, SFRC properties due to presence of steel fibers and BO test significantly affect the results as follows: Generally, for a constant W/C ratio, it can be concluded that raising the cement content increase the mean values of BO strength. It can be stated that the maximum aggregate size within the range of 12.5 ndash;25 mm has a negligible effect on the BO test results for SFRC with 1% steel fiber volume fractions as the average value of BO strength for 25 mm aggregate concrete was significantly more than the corresponding value for 12.5 mm aggregate size for plain concrete. Moreover, the improving trend the BO strength with age is observed to be similar for all different concretes. Furthermore, increase of BO strength of SFR concretes respect to corresponding plain value is observed as concrete grows older. In addition, results showed that strength estimation based on a single general calibration curve is not feasible. Therefore, a specific calibration curve for each SFRC should be represented for a realistic assessment and interpretation of results. The coefficient of variation of the BO strength was in range of 4 ndash; 11%. The higher values are for concretes with higher amounts of fibers, especially at early ages. The reliability of the method seems to be good in which the coefficient of variation for each group of BO tests are below 11%. It should be noted that even under ideal conditions with a specific calibration it is unlikely that 95% confidence limits of better than plusmn;15% can be achieved for an absolute prediction of concrete compressive strength. The findings show that higher values are not only due to the lack of precision of the test procedure, but also due to the intrinsic heterogeneity of granular materials like concrete, particularly in presence of higher amounts of fibers.