The determine of the performance classes of steel fiber reinforced concrete

Abstract

Traditional concrete has weak characteristics regarding tensile strength and load bearing capacity following cracks. These weak characteristics of concrete cause sudden collapse on the concrete following rupture from dynamic effects such as impact, stroke and earthquake. The studies were performed to improve these characteristics by using different types of fibers in recent years. In these studies, performed in order to improve ductility and mechnical properties of concrete, it is observed that various types of steel fibers are used more than the other types of fibers. Tensile strength, aspect ratio and volume ratio of steel fiber have important effects on performance of Steel Fiber-Reinforced Concrete (SFRC). For the performance classes ofSFRCs, their equivalent flexural tensile strength values were determined according to both Serviceability Limit State (SLS) and Ultimate Limit State (ULS). For this purpose, water-cement ratios of 0.55 were used in the production of concretes. For same water/cement ratio volume fractions ofdiffrent type steel fibers were varied from 0.26 to 0.64%. This study used three different steel fiber type (L/d=80, 65, 55). The complete load versus deflection curves were obtained for all un-notched beams of 150x150x750 mm. During the testing, the load and signals for deflections were recorded and stored by a computerized data acquisition system. Hence, five concrete batches were made for same water/cement ratio. In each series, the same aggregates (sand: 0 to 4 mm, limestone fines: 0 to 5 mm, crushed limestones: 4 to 11 mm and 8 to 22 mm) were used. Ordinary Portland cement .(CEMI 42.5) contents in the mixtures with "water/cement" ratio of 0.55 were 350 kg/m3. The amount of water reducing admixture varied between 1.35% and 1.65% by weight of cement for different concrete mixtures to maintain approximately the nominal slump between 100 and 130 mm. In mixing, cement, sand, limestone fines, and crushed limestones were blended first in dry condition. Half of the water reducing admixture and of the water were mixed in a pan and added to the mixture. The remaining of the water reducing admixture and the water were added to the mixture gradually to provide homogeneity in the mixture. Steel fibers were scattered in the mixture and carefully mixed to achieve a uniform distribution. The specimens were cast in steel moulds and compacted on a vibration table. All the specimens were de-moulded after about 24 hours, stored under wet burlap at 20°C until 28 days of age, then laboratory air-cured until testing days at 56 days. The dimensions of the beams, prepared for four point bending tests, were 150x150x750 mm. At least four beam specimens from each concrete mixture were used. For each mixture, three cylinders, 150 mm in diameter and 300 mm in height, were used for compressive strength and modulus of elasticity tests. Six disc specimens, 150 mm in diameter and 60 mm in height, were prepared for the splitting test. For a certain volume fraction of hooked end steel fibers, the fracture energy and the equivalent flexural tensile strengths for both SLS and ULS increase significantly. It is shown that the ability of the beam to absorb energy was substantial, even if the cut-off points were taken at the specified deflections for SLC and ULC. According to the experiment results, it is observed that steel fiber volume percentage and aspect ratio in each matrix strength do not affect compressive strength and elasticity module of steel fiber reinforced concrete. On the contrary, as steel fiber volume percentage and aspect ratio in each matrix strength increased, energy absorption capacity and equivalent flexural strength that are utilized in designs based on performance increased. It is examined that steel fiber aspect ratio and toughness increment along with volume percentage increment are more effective in same fiber aspect ratio and volume rated steel fiber reinforced concretes with higher matrix strength. The necessity of high energy to separate the fiber-matrix bond while fibers are breaking off and getting scraped from matrix may interpret the increase in toughness. As for the enhancement in toughness by increasing steel fiber volume, it can be explained by the increment in fiber's crack bridging effect with the increase of steel fiber quantity in unit area. One of the reasons why semi-brittle concrete gains ductile behavior characteristic can be explained with steel fiber's load bearing from the splitting of matrix to the crack formation in fiber-matrix interface.

Özet

Çelik Lif Donatılı Betonların (ÇLDB) performans sınıflarını belirlemek için performansa dayalı tasarım parametrelerinden olan eşdeğer eğilme-çekme dayanımı Kullanılabilirlik Sınır Durumu 'na (KSD) ve Taşıma Gücü Sınır Durumu 'na (TSD) göre belirlenmelidir. Bu amaçla yalın ve çelik lifli tüm karışımlarda su/çimento oranı 0.55 alınmış ve sabit tutulmuştur. Beton üretimlerinde 80, 65 ve 55 olmak üzere üç farklı narinlikteki (L/d) uçları kancalı çelik lifler, her bir narinlikte üç farklı hacim oranlarında kullanılmıştır. Karışımlarda çimento (CEM 142.5 R) miktarı 350 kg/m3 olarak sabit tutulmuştur. Kimyasal katkı, belirli işlebilirlikte (çökme değerimi00-130 mm) karışımlar üretebilmek için çimento ağırlığının % 1.35-% 1.65 arasında değişen oranlarda kullanılmıştır. ÇLDB 'ler üzerinde basınç, elastisite modülü, yarmada çekme ve kiriş eğilme deneyleri yapılmıştır. 150x150x750 mm boyutundaki çentiksiz kiriş numuneler üzerinde yapılan kiriş eğilme deneyleri sonucunda yük-sehim eğrileri elde edilmiştir. Yükrsehim eğrilerinin altında kalan alanlar esas alınarak Almanya Beton Birliği Yöntemi'ne (ABBY) göre eşdeğer eğilme dayanımları (KSD ve TSD) belirlenmiştir. Deneyler sonucunda, çelik lif hacim oranı artışının betonun basınç ve elastisite modülüne etkisi olmadığı görülmüştür. ÇLDB"nin yarma çekme ve eğilme dayanımının arttığı ve lif narinliği büyük olan betonlarda bu artışların daha fazla olduğu belirlenmiştir. Belirli bir çelik lif narinliği için çelik lif hacmi arttıkça, KSD ve TSD için eşdeğer eğilme çekme dayanımları belirgin bir biçimde artmıştır. Çelik lif donatılı betonların performans sınıflarının elde edilmesi yapısal tasarım ve uygulamalarda büyük yararlar sağlayacağı düşünülmektedir.