Farklı bileşimli betonlarda tekrarlı yükün betonun elastiklik modülüne etkisi /
Yazar: VOLKAN CAN TİRYAKİ
Danışman: DOÇ. DR. HASAN YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Deprem Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Deprem Mühendisliği = Earthquake Engineering ; İnşaat Mühendisliği = Civil Engineering

Özet:
İnşaat mühendisliği açısından betonarme yapılar ülkemizde en çok tercih edilen taşıyıcı sistem olarak kabul görmektedir. Betonarme taşıyıcı sistemler kendi ağırlıklarından ve haraketli yüklerden dolayı oluşan düşey yüklerin dışında, deprem esnasında tesir eden yatay olduğu kabul edilen yüklere de maruz kalmaktadır. Betonarme sistemlerde depreme karşı dayanıklı tasarım yapılabilmesi için, taşıyıcı sistemi oluşturan malzemenin deprem yükleri karşısında davranışı  oldukça iyi bilinmelidir. Beton yapı malzemesi olarak deprem esnasında tekrarlı yüklere maruz kalmakta, betonun mekanik özellikleri bu tekrarlı yükler karşısında değişime uğramaktadır. Günümüzde yapı sistemlerinde birden farklı bileşimli beton tipleri kullanılmaktadır. Betonun yük karşısında belirli bir zaman içerisinde şekil değiştirebilme kapasitesi olan elastisite modülü tekrarlı yükler karşısında etkilenmektedir. Bu çalışmada aynı koşullarda art arda üretilmiş altı farklı tipte betona aynı genlikte tekrarlı yükler, uygulanmıştır.
Bu çalışma neticesinde, farklı maksimum dane çaplı ve farklı su/çimento oranlı betonlarda deprem yükleri gibi tekrarlı yük kabul edilen yüklerin betonun mekanik özelliklerinin başında gelen elastisite modülünün değişimi incelenmiştir. Betonun dayanımının arttıkça tekrarlı yükler karşısındaki şekil değiştirme işi de artmış, yüksek dayanımlı betonun daha gevrek bir kırılma sergilemesinin nedeni gözlenmiştir. Yüksek su oranına sahip numunelerin elastisite modülü ve karakteristik basınç dayanımları düşük su oranına sahip numunelere göre daha düşük çıkmıştır. Maksimum dane boyutu en büyük olan numunelerin elastisite modülü ve karakteristik basınç dayanımları maksimum dane boyutu daha küçük olan numuneler göre daha yüksek çıkmıştır. Beton numunelerin tekrarlı yüklemeler sonrasında  ses geçiş süreleri artmış beton numunelerin tekrarlı yüklemeler sonrasında iç yapılarında gözenekler oluşmuştur. Deneyler süresince tekrarlı yükleme sonucu elastisite modülünün en çok etkilendiği karışımda su oranı diğerlerine göre daha yüksek, maksimum dane boyutu ise daha küçüktür. 3 farklı yük oranında yükleme yapıldığında 6 tekrarlı yük uygulanması durumuna göre 0.85 oranında yüklemeler sonucu elastisite modüllerinde belirgin düşüşler gözlenmiştir.

Abstract:
EFFECTS OF REPEAT LOADING TO THE ELASTIC MODULUS OF  DIFFERENT COMPOSITES OF CONCRETE
SUMMARY
The reinforced concrete structures are most commonly preffered stuructures in our country. Besides of the vertical dead and live loads, these reinforced concrete structures has, laterally accepted loads which caused from earthquakes. Due to the these loads the material response of the static system should be known against earthquake. The concrete material has repeat loads during the earthquakes.The mechanical properties of the concrete would change against to the these repeated loads. Moreover desings of the concrete with different composites are used for the concrete stuructures. It is a well known fack that, concrete structures are form time to time subject to dynamic and repeated loads.
Variability of grain sizes causes diffences at the concrete properties, these differences could change the fatigue life of the concrete because of the repeated loads.
The capacity of the concrete's strain against to the stress has been defined elasticity modulus. This property of the concrete has been effected againist to the repeat loads. One another importance of the repeat loads is that some materials has excessive stresses even during the service loads. The materials which will have repeat loads should have some repeat loads before the fatigue damage, this fact will be more realistic approachment for researches about properties of the concrete. Repeat loads causes progressive and permanent process of the cahnge of the internal structure of the material. Concrete when subjected to the repeated loads, may exhibit excessive cracking and may eventually fail after a sufficient number of load repetitions, even if the maximum stress applied is less than the static strenght of a similar specimen.In concrete subjected to compression stress, stress concentrations occur because of the above mentioned causes and microscopic flaws invariably grow and combine with each other and propagate in cement aggregate interface.
After reaching several cycle of the loads there will be permenant defects inside of the concrete structure. These defects could change the value of the elastic modulus of the concrete which has been  considered during the desing phase. And finally durability problems could start after the change of the elastic modulus of the desing.
There are two different damage phases at concrete which has expose to the repeat loadings. There is very complex failure mechanism. At the first step there are micro defects occurs like tearing between the link of the cement and aggregate. At the second step  micro defects develops more with the increasing number of load repeats. At the conclution of this load combinations creates failure and disintegrations. The damage of the concrete which has been caused by repeat loads are related to the number of the cycle and value of the load.When implemented value of the load gets lower the number of the repeats are increases which causes failure, if the load ratio increases more, the number of cycle decreases on the contrary. The failure mechanism of the repeat loading is fatigue damage. But there is no certain calculated equation to get fatigue life and fatigue limit at old researches. Fatigue failure occurs when a concrete structure fails catastrophically at less than desing load after being exposed to a large number of stress repeats. The fatigue strenght of concrete is defined as a fraction of the static strenght that it can support repeatedly for a given number of repeats. Fatigue strength is influced by concrete composition,enviromental conditions,loading conditions and mechanical properties.
Fatigue is a process of progressive and permanent internal damage in a material subjected to repeated loading. This is attributed to the propagation of internal microcracks which results in a significant increase of irrecoverable strain.
When materials are subjected to the repeat loading they are generally seem to undergo a progressive deterioraion of their mechanical properties. Some old researches have wittnessed an increase in the static compressive strenght of conrete subjected to a limited number of low intensity compressive cycles. There has been also observed an increase in concrete static strenght compared to specimens not previously subjected loading, as well as an increase in fatigue strenght.
In this study the number of six different types of composites has been produced. Three different grain diameter with the maximum size of 31,5mm, 16mm and 4mm produced with two different water content like 0,4 and 0,7. All speciment were cast at labratory site.Standart 150mm x 300mm cylinders were used for the procurements. The speciments were demolded after 24 hours from casting time. After reaching maturity with the proper curing conditions as 28days has been waited for the all mixes. During the procurements prober vibration system with vibro platform at ITU material labratory has been implemented for the proper placement of the speciments. During castings slump, air content, density tests were done for the fresh concrete properties. Also In this study, one of the main mechanical properties of the concrete, the reason of elastic modulus changings by repeat loads like earthquake loads, analyzed and researched with the different sizes of aggregates and with the different water cement contents of the concrete speciments. For the all mixes fresh and dry concrete properties has been compansated.
 On the other hand the effects of aggregate size to the air content, the effect of water ratio to the elastic modulus, the relations of elasticity modulus of the concrete with the ultrasonic pulse velocity values, density and compressive strenght has also been investigated. For the strain meauserement extensometer has been use. For the axial loadig the special hydraulic press machine has been used. After the repeated loads for the high compressive strenght mixes strain increases by these loading.By the way the reason of the brittle fracture for the high compressive strenght concrete,has been monitored. 
The main study of this thesis, the effect of the repeat load to the elasticity modulus of the concrete, has been anlyzed after the each repats respectively. Evaluating the conclusions three different load value has been implemented. These are %65, %75  and %85 of compressive strenght. %10 value of compressive strenght has been implemented for the unload limit.When the load level of %85 compressive strenght implemented,especially for the low grain diameter like 4mm, large amount of elastic modulus decreasings occured. There was no significant decrease for the elastic modulus of the specimens when the load ratio of  the %65 and %75  implemented. Some of mixes elastic modulus decreased with the load raito of %75, for 31,5mm the grain diameter mixes there is no reduction after 6 repeat loading with the load ratios of 0,65 and 0,75. 
For the grain size number of 16mm there was no significant decrease at elasticity modulus, with the  ratio of 0,65. There was certain increment at the value of elastic modulus for the all mixes when water content changes 0.7 to 0.4 compressive strength of the speciments also increased with the decreased water content. 
For the grain size number of 4mm there is no significant decrease at elasticity modulus after load ratio of 0.65. But there was some reduction observed when the load ratio of 0.75 performed. There was significant decrease monitored when the load ratio of 0.85 performed
Besides for the all same grain size, diffrent water content specimens like compansating, maximum grain size 31,5mm water content 0,4 specimen and maximum grain size 31,5 water content 0,7 specimen, there is also value increasing of elastic modulus with the decrease of  water content. On the other hand  the micro defects and increases at the ultra sonic measurement for all speciments monitored which has been exposed to the repeat loading. Air content of the mixes also investigated for the porosity formation of the different grain size and different water content specimens. 
There has been concluded air content increasing with the decreasing of the water content, air content increasing with the decreasing of the grain size. 
The relationship between elastic modulus and compressive strenght has been analyzed for the all mixes. There has been nearly lineer relationship of elastic modulus between compressive strenght for the all mix designs. There is certainly brittle failure for the low water content specimens as againist high water content ones. Future studies could be done with high number of load repeating. This fact could clarify about the behaviour of the several types of mix designs with the low load raitos like 0.65 and 0.75. In this thesis with low number of repeats there was nearly no change at elastic modulus of concrete with the load ratio of 0.65 and 0.75.