تاثير الياف پلي‌پروپيلن در چسبندگي ميان بتن و آرماتور در بتن خودتراكم

The Effect of Polypropylene Fibers on Bond Strength between Concrete and Steel in Self-Compacting Concrete

بتن خودتراكم، بتني بسيار سيال و مخلوطي همگن است. به دليل تردبودن و شكنندگي بتن، استفاده از الياف‌هاي مختلف در بتن رايج شده است تا با ايجاد شرايط يكنواخت بر شكل‌پذيري آن بيفزايد. در اين طرح به بررسي چسبندگي ميان بتن و آرماتور و تاثير الياف پلي‌پروپيلن در اين چسبندگي پرداخته شده است. پارامتر متغير در طرح‌هاي اختلاط، مقدار الياف پلي‌پروپيلن است كه داراي مقادير ‌0، 6/0، 2/1 و 8/1 كيلوگرم در متر‌مكعب است. جهت بررسي خودتراكمي آزمايش‌هاي بتن تازه و براي بتن سخت‌شده، آزمايش مقاومت فشاري، كششي و چسبندگي انجام شده است. نمونه‌ها براي آزمايش مقاومت چسبندگي به صورت مكعبي با آرماتوري عمودي در مركز آن تهيه و در دو شرايط خشك و مرطوب نگهداري شدند. حضور الياف پلي‌پروپيلن موجب بهبودي خاصيت كششي و چسبندگي ميان بتن و آرماتور در بتن خودتراكم شده است، در حالي‌كه كارايي بتن تازه را كم كرده است.

Self-Compacting Concrete (SCC) is a highly fluid yet stable concrete that can flow consistently under its own weight, pass between bars, and fill in formwork without the need for compaction. Fiber reinforcement can extend the technical benefits of SCC by also providing a crack bridging ability, higher toughness, and long-term durability. The use of polypropylene fibers, however, is known to alter the flow properties of fresh concrete. To maintain a self-consolidating ability, the fiber length and total volume must be controlled, which can restrict the gain in toughness imparted by the fibers. Thus, fibers intended for use in SCC need to be carefully optimized. The study analyzes the impact of polypropylene fibers on SCC performance under fresh conditions, as well as its mechanical properties under hardened conditions. For the mechanical properties of SCC, the study has been conducted as follows: compressive strength with elapsed age, tensile strength, bond strength, all of which were measured after the sample being cured for 28 days. The bonding strengths of reinforcing bars were measured using cubic specimens of SCC. The SCC specimens were cast without applying compaction. A total of 5 SCC mixtures were made with combinations of fibers in varying proportions from 0 to 1.8 kg/m3. All specimens from these five mixtures are cured under two different conditions: (1) in a water tank of 25±2?c, and (2) in air in the laboratory. At the end of each curing period, three specimens out of each concrete combination and curing condition were tested for compressive, tensile and bond strengths, and the average of the three values was taken. Applying 1.8 kg/m3 polypropylene fiber to the SCC resulted in 23% reduction in the slump flow (from 690 mm to 530 mm). Polypropylene fibers did not influence the compression of SCC. However, applying these fibers at their maximum percentage volume, determined through this study, increased the tensile strength by 25% in the splitting tensile strength test, and 7% in the bond strength test.