طراحي و ساخت كامپوزيت ECC با الياف پلي وينيل الكل و برآورد آزمايشگاهي و تحليلي ويژگي‌هاي مهندسي كامپوزيت جديد

Experimental and Analytical Estimation of Mechanical Properties of Engineered Cementitious Composites (ECC) with Polyvinyl Alcohol Fibers

خصوصيات كششي، فشاري و خمشي يك كامپوزيت سيماني توانمند با نام كامپوزيت سيماني مهندسي يا ECC كه مسلح به الياف پلي وينيل الكل است، با استفاده از چيدمان هاي دقيق آزمايشگاهي برآورد شده و نمودارهاي تنش- كرنش كششي، فشاري و رفتار سخت شوندگي كرنشي آن ارزيابي شده است. نتايج نشان‌دهنده‌ي رفتار منحصر به فرد اين ماده در كشش و ظرفيت كرنشي بسيار زياد ناشي از ترك خوردگي هاي متعدد است. همنچنين رفتار خمشي اين ماده با آزمايش خمش چهار نقطه‌يي برآورد و با عملكرد خمشي بتن اليافي معمولي مقايسه شده است، كه ظرفيت تغييرشكلي بسيار بالا به همراه رفتار سخت شوندگي خمشي ECC را در مقايسه با ظرفيت تغييرشكلي كم و رفتار نرم شوندگي خمشي بتن اليافي معمولي تاييد مي كند. نتايج نشان‌دهنده‌ي قابليت استفاده از اين ماده در مقاوم سازي سازه هاست. همچنين روشي براي مدل سازي تحليلي اين ماده بر مبناي مدل ترك پخش‌شده با كرنش تجزيه‌شده و رفتار چند خطي سخت شوندگي در كشش و خميرساني دراگر- پراگر پيشنهاد شده است. نتايج تحليلي اجزا محدود نشان مي دهد كه روش مدل سازي پيشنهادي مي تواند با دقت قابل قبولي رفتار اين ماده را در بارگذاري هاي مختلف پيش بيني كند.

A number of experimental tests were conducted to better understand of the mechanical performance of a special class of high performance fiber reinforced cementitious composites, namely engineered cementitious composites (ECC) which is reinforced with high modulus polymeric fibers. The design approach of the ECC materials is based on fracture mechanics principles, which will be described in the first part of the paper. Unlike concrete or regular fiber reinforced concrete (FRC), ECC materials are characterized by their ability to sustain higher level of loading after first cracking undergoing additional straining. This strain hardening gives ECCs a significant advantage under flexural loading. The application of ECC materials and other strain hardening cementitious composites require adequate information on their basic material properties for structural purpose. The ECC examined in this investigation is a Portland-cement based mortar matrix with a low volume fraction (typically 2%) of high modulus PVA fibers. The ECC properties measured by use of accurate experimental test setups include uniaxial tensile, compressive, and flexural behaviors as well as multiple cracking development. The experimental results showed that the ECC material exhibits a unique strain hardening behavior with high tensile strain capacity due to multiple cracking. The results of uniaxial tensile tests presented in terms of first crack strength, ultimate strength and ultimate strain at the peak stress. The average first crack strength was found to be 3.25 MPa and all ECC dogbone specimens showed strain hardening behavior with strain capacity from 3 to nearly 3.7%. The compressive behavior of ECC cylinders exhibited an average of ECC beams compressive strength of 47 MPa at a strain level of 0.55%. The flexural characteristics tested under four-point bending revealed high deformability and strain hardening behavior in comparison to low deformation capacity and strain softening behavior of steel fiber reinforced concrete beams. The significant enhancement of energy absorption capacity and tight crack width control in ECC beams under bending indicate that ECCs can serve as a basic structural material or use in repair works.