Atık Çelik Liflerin Betonun Mekanik Özelliklerine Etkisi

Abstract

Bu çalışmanın amacı atık araç lastiklerinden elde edilen çelik liflerin betonun mekanik özelliklerine etkisini incelemektir. Bu kapsamda, bir tanesi lifsiz (yalın) olmak üzere atık çelik lif türü ve lif içerikleri değiştirilerek farklı karışımlar hazırlandı. Çalışmada boylulukları 164, 81 ve 41 olan ve geometrileri farklı üç çeşit atık çelik lif kullanıldı. B türü liflerle lif içerikleri 5 kg/m³, 10 kg/m³, 15 kg/m³ olmak üzere 3 çeşit; C türü liflerle lif içerikleri 10 kg/m³, 20 kg/m³, 30 kg/m³ ve 40 kg/m³ olmak üzere 4 çeşit; D türü liflerle ise lif içerikleri 20 kg/m³, 40 kg/m³ ve 60 kg/m³ olmak üzere 3 çeşit beton üretildi. Çökme ve birim kütle deneyleri yapıldıktan sonra 100×100×500 mm boyutlarında kalıplara yerleştirildi ve vibrasyonla sıkıştırıldı. Üretilen tüm karışımlarda etkin su/çimento oranı 0.50, bağlayıcı içeriği 360 kg/m3 olarak sabit tutuldu. Tüm numuneler, 24 saat sonra kalıptan çıkartılarak 28 gün boyunca sürekli olarak 20°C±2°C’de kirece doygun su içerisinde bekletildi. 28. günde sudan çıkartılan numuneler deney gününe kadar laboratuvar şartlarında muhafaza edildi. Sertleşmiş beton deneyleri 56ncı günde yapıldı. Kırılma enerjisinin ve eğilme dayanımının belirlenmesi için RILEM TC 50-FMC’nin önerisine göre çentikli kiriş numuneler üzerinde üç noktalı eğilme deneyi yapıldı. Eğilme deneyinden çıkan parçalar 100×100×100 mm boyutlarında kesilerek üzerlerinde basınç dayanımı ve yarmada çekme dayanımı deneyleri yapıldı. İşlenebilirlik lif içeriğinin ve boyluluk oranının arttırılmasıyla azaldı. Kullanılan atık çelik liflerin betonun basınç dayanımına belirgin bir katkı sağlamadığı, ancak yarma dayanımını önemli düzeyde arttırdığı görüldü. Lif içerikleri ve boyluluk oranları yük-sehim eğrilerinde çatlak sonrası şekil değiştirme davranışlarını etkiledi. Lifsiz beton numune çatlak sonrası gevrek bir davranış sergilerken, lifli numunelerde ise lif oranlarına bağlı olarak şekil değiştirme sertleşmesi gözlendi. Her üç lif türünde de betonlarda lif içerikleri arttırıldıkça ilk çatlak yükü, tepe yükü ve ilk çatlak yüküne karşılık gelen sehim değeri arttı. Ayrıca artan lif içeriği ile birlikte ilk çatlak gerilmesindeki sehim değeri ile maksimum çatlak gerilmesindeki sehim değeri arasındaki fark büyüdü. Atık çelik liflerin içerikleri ve boyluluk oranları arttırıldıkça üretilen betonların kırılma enerjileri sürekli bir artış gösterdi. Atık çelik lif içeriğinin arttırılması ve atık çelik liflerin boyluluğunun artması eğilme dayanımını belirgin bir şekilde arttırdı.

Steel Fibre Reinforced Concretes (SFRCs) are cementitous materials reinforced with short steel fibers. Randomly distributed steel fibers in the matrix act as crack arresters. The real advantage of using of fibers in concrete can be seen after matrix cracking. These types of materials are usefull if a large amount of energy absorption capacity is required to prevent failure. SFRCs can also be used when high tensile strength and reduced cracking are required, as well as the cases, when conventional reinforcement can not be placed because of the complex shape of the structural member. SFRC has a wide-range of applications such as; pavements and overlays, industrial floors, precast products, hydraulic and marine structures, repairing and retrofitting of reinforced concrete structures, tunnel linings and slope stabilization works. There are many factors affecting the fracture energy of SFRC such as; fibre length, fibre diameter, fibre content, fibre orientation of fibers in matrix and matrix properties. Investigations on mechanical properties of SFRC indicate that an increase in the aspect ratio also increases the fracture energy. Every year more than 10 millions tons of waste tires are produced in the world and this amount is increasing rapidly. This number is 200 thousands tons for Turkey. The natural decomposition of the waste tires in the environment takes a very long time and these waste tires are a big threat for human health and the environment. Besides, regal regulations for waste tires in Turkey, EU and USA prohibit or limit the landfilling of these waste tires. Tires are 100% recyclable: the rubber, metals and textiles can all be recovered and used in many applications, as well as in consumer and industrial products and numerous examples are published in the literature. Numerous uses of scrap tires have been introduced, including use in landfills, fuel and energy recovery, septic drain fields, subgrade fill, various rubber and plastic products and asphalt binder. One of the possible areas of application is the realization of concrete elements, in particular the use of the steel contained in a tire as discontinuous reinforcing fibre in the concrete matrix. In this study, the influence of the aspect ratio and the content of steel fibers procured from waste tires on the mechanical properties of Waste Steel Fibre Reinforced Concretes (WSFRCs) under bending, compression and splitting forces was investigated. This study is divided into 6 parts. In the first chapter, waste steel fiber reinforced concretes are introduced and the purpose of the study is given. Secondly, the literature survey on the steel-fiber reinforced concrete, mechanical properties and steel fibers from waste tires are presented. In the third and fourth sections, the experimental study and results are presented, respectively. In the last section, conclusion and recommendations were given. A total of 11 mixtures (i.e. 10 WSFRCs and one plain concrete mixture) were cast for WSFRCs investigation. In all mixtures, water/cement ratio and cement content were kept constantat 0,5 and 360 kg/m³, respectively. Cement used was ordinary Portland cement with a density 3,12 Mg/m³. For the mixtures with the amount of high range water reducing admixture 1,33% by weight of cement. Sand (0-4 mm) and crushed sand (0-4 mm) were used as a fine aggregates; and crushed stone I (4-16 mm) and crushed stone II (16-31,5 mm) were used as a coarse aggregates. The maximum particle size of aggregate was 32 mm. Densities of fine aggregates are 2,65 g/cm³ and 2,76 g/cm³ whereas densities of coarse aggregates 2,74 g/cm³ and 2,73 g/cm³. The amount of aggregates were selected as 17%, 33%, 20% and 30% for sand, crushed sand, crushed stone I and crushed stone II. Three kinds of steel fibers were used: B, C, D,with aspect ratios of (L/d)B = 164, (L/d)C = 81, (L/d)D = 41. The waste steel fibre contents (Vf) were 5 kg/m³, 10 kg/m³, 15 kg/m³ for B, 10 kg/m³, 20 kg/m³, 30 kg/m³, 40 kg/m³ for C, 20 kg/m³, 40 kg/m³, 60 kg/m³ for D. After mixing, each concrete was cast into 100×100×500 mm prismatic molds, demolded approximately 24 hours after molding, and were cured in 20 °C ± 2 °C lime saturated water for 28 days. The specimens were removed from the curing tank at the age of 28 days and kept laboratory conditions until testing. Hardened concrete experiments were carried out on the 56-day samples. To measure the GF of WSFRCs for a notched beam under three-point bending, the procedures recommended by RILEM TC 50-FMC were followed. At least three beams of 500 mm length and 100×100 mm cross section were tested. The effective cross section, was reduced to 83×100 mm The load-deflection curves were used to evaluate the fracture energy. The area under the load versus deflection at mid span curve (W0) was taken as a measure of the fracture energy of the material. The results obtained here were based on the area under the complete load-deflection curve up to a specified deflection. The cut-off point was chosen as 4 mm deflection. Thus the specific fracture energy and net bending strength were obtained using the beam specimens. The specimens were tested for their flexural strength. The tested specimes were later cut into 100x100x100 mm pieces, to be tested for compressive and splitting tensile strengths. The test results revealed that the fiber volume fraction and the fiber aspect ratio had unfavourable effect on the workability of the concretes, whereas they had no significant effect on the compressive strengths. It was clearly seen that as the waste steel fiber volume fraction increases, the toughness and the first crack loads of WSFRCs increase. For aspect ratio of 164, the fracture energies of WSFRCs, of fibre content 5 kg/m3, 10 kg/m3 and 15 kg/m3 were determined as 463 J/m2, 630 J/m2 and 1132 J/m2, respectively. Similarly, for aspect ratio of 81, the fracture energies of WSFRCs; for fibre contents 10 kg/m3, 20 kg/m3, 30 kg/m3 and 40 kg/m3 are determined as 562 J/m2, 932 J/m2, 1375 J/m2 and 1856 J/m2, respectively. And for aspect ratio of 41, the fracture energies of WSFRCs, of fibre content 20 kg/m3, 40 kg/m3 and 60 kg/m3 are determined as 514 J/m2, 1151 J/m2 and 1394 J/m2. The net bending strength, splitting tensile strength and especially fracture energy and ductility of waste steel fiber reinforced concretes were significantly enhanced compared to those of plain concrete. The results obtained confirm the promising application of concrete reinforced with WSFRC; however, further research work is still necessary, extending also the investigations to medium and full-scale structural elements, representative of possible practical situations. Finally, theoretical models are needed to furnish reliable design guidelines for concrete reinforced with steel fibers recycled from waste tires (WSFRC). Waste steel fibers, because of their various productions processes, are having different diameters, aspect ratios, geometries. Profound investigations are required to be conducted on waste steel fibers characteristc properties. These investigations must be done by collaboration between both civil engineering and recycling technology researchers. As the result of these researches, lowering costs, promoting the industrial sector to recycle the waste materials will provide serious contributions to more protection of the environment.