Sentetik fiber ve çimento katkılı kumların mukavemet özellikleri
Strength properties of cemented and fiber reinforced sands
Yazar: BAHATTİN BERK AYRAÇMA
Danışman: YRD. DOÇ. DR. BERRAK TEYMÜR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Fiber çarpımları = Fibre products ; Zemin iyileştirme = Soil improvement ; Zemin mekaniği = Soil mechanics ; İnşaat mühendisliği = Civil engineering

Özet:
Tez kapsamında, bir zemin iyileştirme yöntemi olarak, kumlu zeminlerde fiber kullanımı araştırılmıştır. Fiber katkılı zeminlerin mukavemet parametrelerin bulunması ve gerilme şekil değiştirme ilişkilerinin incelenmesi için, İstanbul Teknik Üniversitesi, Hamdi Peynircioğlu Zemin Mekaniği ve Geoteknik laboratuvarlarında bir dizi laboratuvar deneyi yapılmıştır. Deneylerde, öncelikler kullanılacak malzemelerin fiziksel özellikleri tayin edilmiş, sonrasında kayma mukavemeti özellikleriyle ilgili mekanik deneylerden kesme kutusu ve serbest basınç deneyleri yapılmıştır.
Deneylerde kullanılan ana malzemeler kum, fiber ve çimentodan oluşmaktadır. Bu malzemeler İstanbul Teknik Üniversitesi Yapı Malzemeleri laboratuvarlarından elde edilmiştir. Yapılan çalışmalarda iki farklı kum ve fiber tipi denenmiştir. İlk setlerde temiz silika kum ve cam fiber kullanılmış olup, ardından Akpınar kumu ve BASF firmasının ürünü olan Masterfiber 15 MF kodlu polypropylene fiber seçilmiştir. Sözü edilen malzemeler kullanılarak yapılan kesme kutusu deneylerinin ardından, Akpınar kumu ve PP fiberle hazırlanan karışımlar üzerinde serbest basınç deneyleri gerçekleştirilmiştir. Serbest basınç deneyi numunelerine, fiber lifleri ve kum daneleri arasında aderans sağlanması bakımından çimento da eklenmiştir. Çalışmalarda CEM II tipindeki çimento kullanılmıştır.
Kesme kutusu deneylerinde, numuneler, gevşek ve sıkı durumları temsil etmesi açısından, %20 ve %60 rölatif sıkılıkta hazırlanmıştır. Kullanılan fiber katkı, kuru kumun ağırlığına oranla karışımlara katılmıştır. Fiber oranı %0,25 - %1,00 arasında değişmektedir.
Serbest basınç deneylerinde, kesme kutusu deney sonuçlarına göre belirlenen iki fiber oranının kullanımı tercih edilmiştir. Çimento oranı ise, literatürde mevcut bulunan çalışmaların ve uygulama projelerinin ışığında belirlenmiştir. Fiber oranı ağırlıkça %0.50 ve 1.00 iken, çimento oranları yine kuru kum ağırlığınca %5,00, 10,00 ve 15,00 olarak seçilmiştir. Tüm karışımların su/çimento oranı sabit tutulmuş ve yeterli işlenebilirlik, kum ve fiber yüzey alanının büyüklüğü düşünülerek %65 olarak belirlenmiştir. Numuneler, kalıptan çıkarıldıktan sonra, nemlendirilmiş ve oda sıcaklığında sabit nemde bekletilmiştir. Deneyler 7. Ve 28. günlerde gerçekleştirilmiştir.
Tezin ilk bölümünde, fiber malzemelere ait genel bilgiler verilmiş, kullanım amaçları ve kullanım alanlarına değinilmiştir. İkinci bölümde, kısaca, sıkça kullanılan zemin iyileştirme yöntemlerinden bahsedilmiştir. Üçüncü bölümde, literatür araştırması sırasında elde edilen bilgilere yer verilmiş, zeminlerin kayma gerilmeleri altındaki davranışı açıklanmış, tez konusuyla ilgili yapılan benzer çalışmalar incelenmiştir. Sonraki kısımda, deneylerde kullanılan malzemeler hakkında bilgi verilmiş ve yapılan deneysel sonuçlardan, uygulama aşamalarından ve karşılaşılan güçlüklerden bahsedilmiştir. Deney sonuçlarından elde edilen veriler de bu bölümde paylaşılmıştır. Sonuç bölümünde, deney sonrası alınan veriler incelenmiş ve karşılaştırmalar yapılmıştır. Fiber ve çimento katkının, bir zemin iyileştirme yöntemi olarak kullanımı irdelenmiş, sonraki çalışmalar ve olası uygulamalar için önermelerde bulunulmuştur.
Çalışma içeriği incelendiğinde, yalnız fiber katkılı numuneler üzerinde gerçekleştirilen kesme kutusu sonuçları, fiber katkılı zeminlerin pik içsel sürtünme açısının yükseldiğini göstermektedir. Ayrıca, kırılma sonrası mukavemet düşüşü azalmaktadır. Gerilme – şekil değiştirme davranışı incelendiğinde, katkılı karışımların daha plastik bir davranış gösterdiği ve sünekliğin arttığı gözlemlenmiştir. Sözü edilen mukavemet özelliklerindeki gelişim, gevşek numunelerde daha fazla öne çıkmaktadır. Fakat, söz konusu değişimler, silika kum ve cam fiber kullanılarak hazırlanan karışımlarda, Akpınar kumu ve PP fiber içeren karışımlara göre daha az gözle görülür seviyelerdedir. Bu sebepten, çimento içeren karışımların hazırlanışında dane boyu daha büyük ve lif çapı daha küçük olan Akpınar kumu ile PP fiber kullanılmış olup, bu numuneler üzerinde serbest basınç deneyi yapılmıştır. Serbest basınç deneylerinden elde edilen sonuçlar incelendiğinde, fiber bulunmayan, yalnız çimento katkılı karışımların dayanımının, fiber ve çimentonun beraber kullanıldığı karışımların üzerinde kaldığı görülmektedir ve dayanım çimento dozajıyla artmaktadır. Bununla birlikte, fiber katkı önceden gevrek olarak nitelendirilebilen davranışı plastik davranışa dönüştürmüş ve numuneler, maksimum basınç değerinin ulaşılmasının ardından da dayanımlarının bir kısmını korumuşlardır.

Abstract:
Geotechnical aspects of any civil engineering application must be investigated for safe design and proper utilization. It's also a crucial necessity to find solutions to the possible engineering problems and for doing mandatory checks regarding to civil engineering, since geotechnical engineering consists one of the initial phases of any structural project.
Some of the common enquiries can be shown as site investigation, determination of soil profile, soil bearing against static and dynamic loadings, elastic and plastic settlements and liquefaction safety. Many aspects of a design like foundation dimensioning, determination of foundation type, solutions to slope stability problems are dependent on these analysis.
It's a fact, that with the increasing urbanization, especially in major cities, available sites open to construction are constantly decreasing. This fact, could lead up to the occurrence of the engineering projects which need to be built on soil profiles with poor engineering properties. It is obvious, that under these circumstances application of the job would be a challenge. Against this challenge, engineer might need to redesign the structural components. For example; foundation system might need to get switched to a deep foundation type like a piled raft foundation, instead of a shallow foundation type, which causes the costs to swing up.
In case of one or many of the controls regarding to geotechnical engineering fail, engineer has to redesign the project to satisfy the safety and economics. Soil improvement is an application to enhance the current soil properties of underlying soil profile. It consists of many stabilitation methods which can be more proper depending on the soil type, project and kind of the problem each of their own. Some of the methods can be named as compaction, drainage, soil alteration and soil reinforcement.
In the scope of the thesis, fiber reinforcement as a soil improvement technique is investigated for sandy soil types. Fiber reinforcement of soils can be defined as producing a soil matrix by inclusion of fibrous material. This application increases the shear strength, reduces the settlements, improves the hydraulic conductivity and drainage conditions with the help of tension bearing capabilities of mobilized fibers. Fibrous materials can be extracted from the nature or they can be produced synthetically.
Laboratory tests are performed to the better knowledge of soil strength and strength behavior as in shear stress – displacement curves. All of the tests are performed in Istanbul Technical University, Hamdi Peynircioglu Soil Mechanics laboratories. Materials used in the tests are gained by the help of the Istanbul Technical University, Building Materials laboratories. Majority of the materials used during the testing can be identified as sand, fiber and cement. Two kinds of sands and fibers are used during the studies, respectively, clean silica sand and Akpınar sand with glass fiber and
polypropylene fiber. The cement type used in the studies is CEM II type according to Turkish standards.
Performed laboratory tests can be defined in two categories. Firstly, physical property tests are done to decide on the physical properties of the material such as relative density, specific gravity and grain size distribution. After the determination of the physical properties, tests regarding mechanical properties of soil are performed. These consist of direct shear tests and unconfined compression tests.
In the first set of mechanical tests, direct shear tests are done by using both sand and fiber types. Different kinds of mixes are produced according to dry weight of the sand. These mixes include a range of fiber ratio of 0.25 to 1.00%. The samples are prepared in loose and dense states to represent different soil conditions, which have a relative density percentage of 20 and 60 respectively.
When glass fiber type reinforcement is used in clean silica sand, fiber ratio of 0.50% of dry sand resulted to be the optimum content for both dense and loose samples. It improved the shear strength of the sand by increasing the friction angle from 34o to 42o for loose samples.
When PP fiber type is used in Akpınar sand, optimum fiber content also turned out to be 0.50% for loose samples. On the other hand, no drastic change in regards to maximum shear stress is seen for dense samples. If the limitation of the shear stress loss after peak value is reached is considered, 1.00% fiber ratio gave better results for loose samples, while 0.50 and 1.00% of fiber content showed similar results for dense samples.
However, while the cost effectiveness and the ability of producing uniformly mixed samples are considered, further experiments are performed by using Akpınar sand and PP fiber.
After gathering the data from the direct shear test results, Akpınar sand and polypropylene fiber type are chosen to be used for the preparation of samples which are used in unconfined compression tests. During the unconfined compression tests, cement is also added into the mixture of sand and fiber to obtain an adhesion between soil particles and fiber threads. Inside the mixtures, two fiber ratios are used, which are 0.50 and 1.00%. Cement ratio is calculated according to dry weight of sand. Cement ratios are selected as 5, 10 and 15%. Fiber ratios are decided by investigating the results of the direct shear tests, while cement ratios are decided upon the literature studies and also examples from the site applications. Water/cement ratio for the mixtures used in the unconfined compression tests are kept constant. By considering the minimum water/cement ratio for hydration of cement, workability of the mixture and high surface area of the sand particles and fiber, water/cement ratio is selected as 65%. After the pouring of the samples, they are kept inside in molds about one day. After demolding, they are kept in a humid place to prevent water loss and tested in a period of 7 and 28 days. The molds used, which had a form of split-spoon, had a height of 100 mm and diameter of 50 mm.
During the experiments, initially, molds cut out from one meter long cylindirical PVC tubes are used. Diameter being 10 cm and their height being 20 cm. However, because of the problems such as; producing uniformly mixed samples, compaction of the samples and getting the samples out of the molds, smaller samples having a diameter of 5 cm with 10 cm height are used later on. Subsequent molds were made of metal and they were in a type of split spoon, which eleminated the previously mentioned problems.
In the first chapter of the thesis, an overall information about fibers and their usage fields with benefits are given. In the second part, some of the commonly used soil improvement methods are discussed and brief explanations are given. Literature studies are shown in the third chapter to take a look at the general shear behavior of the soils. Past studies done related to the study of the thesis are also stated in this chapter. Following chapter consists of the analysis of the laboratory work and their procedure. In the results section, data gathered from the laboratory tests are investigated and overall statements are made with the general results. The usage of the fiber and cement as a soil investigation method and its benefits are discussed with the recommendations for future work and possible site applications.
Within the great scope of studies, the laboratory test results including direct shear tests show an increase in the peak internal friction angle, when the fiber is used as the only reinforcement without cement. Moreover, decrease of post failure stress loss is reduced and limited with the inclusion of fibers. Also, if the shear stress – displacement plots are investigated, it can be seen, that the soil has became more plastic and has ductile behaviour. However, the improvements stated on the above are less effective for the samples consist of silica sand and glass fiber than samples consist of Akpınar sand and polypropylene fiber. It's acknowledged, that the reasoning behind this situation being, Akpınar sand having larger grain size and polypropylene fibers having a smaller diameter, which results a better and more evenly distributed sand-fiber matrix.
For this reason, Akpınar sand and polypropylene fibers are selected to be used for sample preparation for unconfined compressive strength tests. The results of the unconfined compression tests show, that the cement increases the soil's unconfined compressive strength. On the other hand, strength decreases with the addition of the fibers into the mix. However, while the strength decreases, behavior of the soil becomes more ductile as oppose to the brittle behavior of the cement only samples. It can also be seen, that the soil is kept some of its strength after the maximum value of stress is reached with the inclusion of fibers. When the fiber included samples are investigated, it is observed that the fiber threads were interconnecting the pieces of sample which are seperated by cracks, preventing it from collapsing, unlike only cement including samples.
It is also noted, that the samples having a cement ratio of 5%, showing a very brittle behaviour, because of the inadequate bonding of sand particles. Some of the samples even cracking and failing during the unmolding process.
The overall analysis of the data points out, that the optimum fiber content of fiber being about 5%, when glass fiber and silica sand are used. On the other hand, for samples consist of PP fiber and Akpınar sand, 1.00% of fiber gave better results considering post peak behaviour of dense soil samples. Direct shear box tests indicates that the fiber inclusion can lead to an increase in the peak internal friction angle while limiting the shear strength loss after the peak value.
When the unconfined compression test results of the study are investigated, it can be seen, that unconfined compression strength of the material is getting increased with the increased cement ratios. While the inclusion of polypropylene fibers decrease the maximum unconfined compression pressure, they also greatly improve the stress – strain behaviour of the mixtures by turning the brittle behaviour to plastic and producing a ductile curve. It is also observed, that the fiberious material are working as bridges inside the composite, preventing a total collapse failure and providing improvement against larger displacements of soil. Also ensuring the soil to bear more pressure after the peak value is reached.