بررسی آزمایشگاهی با رویکرد آماری مقاومت ضربه ای بتن های توانمند کامپوزیتی

در این تحقیق، یک مطالعه آزمایشگاهی با رویکرد آماری، بر روی مقاومت فشاری و خمشی و مقاومت ضربه‏ای بتن های توانمند انجام شده است. بر روی داده‏های به دست آمده از آزمایشات آنالیز آماری صورت گرفت. سه سری بتن که به ترتیب دارای 0.5، 0.75 و 1درصد الیاف پلی پروپیلن بودند، ساخته شد. در مجموع 240 نمونه بتنی در سه گروه ساخته شد. از هر طرح اختلاط، 20 نمونه مکعبی 100 در 100 میلیمتر، 20 نمونه تیر کوچک به ابعاد 320 در 80 در 60 میلیمتر، 40 نمونه دیسک کوچک به ارتفاع64 میلیمتر و قطر 150 میلیمتر ساخته شد. از نمونه‏های مکعبی به منظور تعیین مقاومت فشاری، از تیرهای کوچک به منظور تعیین مقاومت خمشی و از دیسک های کوچک جهت تعیین مقاومت ضربه‏ای استفاده شد. آزمایش ضربه با استفاده از روش وزنه افتان که به وسیله آیین نامه544 aci پیشنهاد شده، انجام گرفت. آنالیز آماری صورت گرفته نشان داد، داده‏های مربوط به خصوصیات مکانیکی دارای پراکندگی کمتری نسبت به داده‏های مربوط به مقاومت ضربه‏ای می‏باشند. همچنین پراکندگی بین داده‏ها با افزایش درصد الیاف، افزایش می یافت.

Experimental study on impact behaviour of high performance concrete with statistical approach

Highperformance concrete (HPC) exceeds the properties and constructability of normal concrete. Normal and special materials are used to make these specially designed concretes that must meet a combination of performance requirements. Special mixing, placing, and curing practices may be needed to produce and handle highperformance concrete. Extensive performance tests are usually required to demonstrate compliance with specific project needs (ASCE 1993, Russell 1999, and Bickley and Mitchell 2001). Highperformance concretes are made with carefully selected highquality ingredients and optimized mixture designs; these are batched, mixed, placed, compacted and cured to the highest industry standards. Typically, such concretes will have a low watercementing materials ratio of 0.20 to 0.45. Plasticizers are usually used to make these concretes fluid and workable. Highperformance concrete has been primarily used in tunnels, bridges, and tall buildings for its strength, durability, and high modulus of elasticity. High Performance concrete (HPC) are a class of fiber cement composites with fine aggregates that exhibit tensile strain hardening response under uniaxial loading. These materials are characterized by pseudoductile tensile strain hardening behavior and multiple cracking prior to failure. This figure emphasizes the transition from brittle concrete to quasibrittle FRC (strain softening behavior after first cracking) to ductile HPFRCC with strain hardening behavior after first cracking. In recent years, a new class of HPFRCC has emerged as ECC. Engineered Cementitious Composite (ECC) which was developed at University of Michigan had a typical moderate tensile strength of 46 MPa and ductility of 35%.Since there is not enough available information to give mechanical characteristics and also to calculate the mean, standard deviation and coefficient of variation, some statistical evaluations are necessary to obtain accurate results of the effect of inclusion of PP fiber on absorbed energy and impact resistance of concrete. Concrete is a heterogeneous material, and that is why results obtained from several tests are often significantly scattered. There is a few quantitative statistical data about the effect of PP fiber on compressive, flexural strength of HPC at the other research work; therefore it shows a necessity to study the effects of PP fiber on mentioned parameters.Gotten data were statistically analyzed. 240 concrete specimens were prepared in three series with different mix designs, containing 0.5, 0.75 and 1 percent of PP fibers. Twenty 100×100×100mm cubic specimens, twenty 320×80×60mm beam specimens andforty150×64mm discs were cast from each mixture. Cubic specimens were used to determine the compressive strength, beam specimens were tested to obtain flexural strength and cylindrical cutting specimens (discs) were subjected to the dropweight test following the ACI committee 544 to determine impact strength of mixed concretes. Statistical analysis done based on these experimental tests showed that in comparison with data of impact strength, data of mechanical properties have less dispersion. Also while increasing percentage of fibers, dispersion in data increases. According to results of compressive strength test on cubic specimens, adding fibers to specimens increased the coefficient of variations of compressive strength. The coefficient of variations of compressive strength for HPFRCC was increase from 4.96 % to 8.42 %. Also Statistical data for flexural strength are almost normally distributed. Mean flexural strength in HP1 group (1% fiber) was 6.24 MPa, which is 29 % and21 % more than HP0.5 group (0.5 % fiber) andHP0.75 group(0.75 % fiber), respectively. HP1 group's coefficient of variation is 9.88 % which is 11 % and 8 % more than the same parameter in HP0.5 and HP0.75 groups, respectively.