Betonda su/ çimento oranının ve yükleme yaşının sünme üzerine etkileri
The Effects of water/ cement ratio and loading age on creep at concrete
- Tez No: 127019
- Danışmanlar: PROF. DR. SAİM AKYÜZ
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2002
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: İnşaat Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Yapı Malzemeleri Bilim Dalı
- Sayfa Sayısı: 106
Özet
ÖZET BETONDA SU/ÇİMENTO ORANININ VE YÜKLEME YAŞININ SÜNME ÜZERİNE ETKİLERİ Betonda meydana gelen deformasyonlar çeşitli şekillerde sınıflandırılabilir: Bunlar kısa sürede oluşan deformasyonlar ve zamana bağlı meydana gelen deformasyonlar. Değişmez gerilme altında zamana bağlı deformasyona sünme adı verilir. Beton zamana bağlı deformasyon yapabilir, dolayısıyla sünme yapabilen yapı malzemesi olarak anılır. Sünme sonucunda kompozit bir malzeme olan betonda ve donatılı betonlarda gerilmeler doğar ve gerilme yayılış değişmesine yol açar. Betonarme ve öngerilmeli beton yapılarda bu olayla karşılaşıldığından dolayı sünme olayının etkileri kesinlikle göz önünde bulundurulmalıdır. Beton zamanla mekanik özellikleri değişen bir malzemedir. Zamanın yanında birtakım yapısal parametreler de bu özelliklerini etkiler. Bu çalışmada zaman ve yapısal parametrelerden su/çimento oranının betonun viskoelastik davranışına etkileri ele alınmıştır. Üç farklı su/çimento oranına sahip betonlar, herbiri doğrusal viskoelastik bölgede kalacak şekilde 14., 28., ve 42. günlerde değişmez eksenel gerilme altında tutularak rötresiz sünme deformasyonları okunmuş ve buradan sünme diyagramları elde edilmiştir. Bu deneyde yapılan çalışmalar agrega konsantrasyonu ve diğer tüm değişkenler sabit tutularak su / çimento oranın (%35, %55, %75) ve yükleme yaşının (14, 28, 42) sünme olayı üzerine etkileri incelenmiştir. Bu parametreler ışığında çizilen sünme diyagramlarıyla bu yukarıda adı geçen parametrelerin sünme olayına etkileri incelenmiştir. Bu incelemeler yapılırken bu diyagramların katı cisim modeline ıxuyduğu tespit edilmiş ve bu model ışığında sünme fonksiyonları ve Laplace-Carson dönüşümlerinden yararlanılarak gevşeme fonksiyonları elde edilmiştir. Bu yukarıda sözü geçen diyagramlar ve fonksiyonlar elde edilirken bu değerler gerilmeler bağımsız hale getirildi. Bunun amacı değişken olan mukavemeti sabit o/R oranının içindeki R nin değişken olmasından dolayı uygulanan gedmenin farklı olmasıdır. Bu fonksiyonlar elde edildikten sonra ve çizilmiş olan diyagramlardan su / çimento oranın artmasıyla ve yükleme yaşının artmasıyla sünmenin yine arttığı gözlenmiştir.
Özet (Çeviri)
SUMMMARY THE EFFECTS OF WATER / CEMENT RATIO AND LOADING AGE ON CREEP AT CONCRETE In a general manner to introduce the topic we can say that creep is a slow continuos deformation of a material dependent to time under constant stress. Lots of materials have creep property but there are some important differences between concrete and many other common structural materials. Before giving knowledge about creep of concrete, we should know what sort of material is concrete. Concrete is a multi-phase material, consisting of particles of aggregate embedded in a matrix of cement paste. In time-dependent deformation cement paste plays dominant role and aggregate can be considered as a modifier of creep event. First of all, concrete is a heterogeneous material at virtually any level of observation. Because it consists of cement gel, hydrated cement, unhydrated cement, aggregate and water. Finally concrete differs from some other materials, for instance steel ( At a macroscopic level steel is considered homogenous and isotropic). Another important point is that, concrete is manufactured on site which may effect the variability of concrete and the stability of its properties. Creep in general may be described in terms of different stages. In the primary creep range, the rate of creep decreases with time. If the material exhibits a minimum creep rate, the secondary creep range (sometimes called stationary creep) designates the range of steady state creep. The straight line relation of secondary creep may be a convenient approximation when the magnitude of this creep is large compared with primary creep. Actual materials exhibit a general variety of behaviour. However, by means of idealisation deformations of concrete can be simplified and classified as in Table Al. XITable Al. Classification of deformation at concrete On a phenomenological basis, several mechanisms of creep can be distinguished. They are: mechanical deformation theory, viscous flow, plastic flow, seepage of gel water, delayed elasticity and microcracking. But none of them is capable of explaining phenomena. It is possible that the actual creep involves two or more mechanisms. The only statement that can made is that the presence of some evaporable water is essential to creep. However, the changes in the creep behaviour of concrete at high temperatures suggest that at that stage the water ceases to play a role and the gel itself becomes subjected to creep deformation. It would appear that creep is a function of the relative amount of the unfilled space, and it can be speculated that it is the voids in the gel that govern both strength and creep. The volume of voids, of course, a function of the water-cement ratio and by the degree of hydration. But we can say that the capillary voids do not remain full even against full hydrostatic pressure of the ambient medium. Thus internal seepage is possible under any storage conditions. The fact that creep of non-shrinking specimens is independent of the ambient humidity would indicate that the basic cause of creep“in air”and“in water”is the same. It is probable that the slow, long-term part of creep is due to causes other than seepage but the deformation can develop only in the presence of some evaporable water. This would suggest viscous flow or sliding between gel particles. Such mechanisms are compatible with the influence of temperature on creep and can explain also the big amount of irreversible character of long-term creep. There are lots of factors influencing creep. Some of these properties are minor, many are indirect and can be accounted for by other properties of concrete, but it is important to determine all factors for understanding the creep event. They are as follows. XUGenerally the type of cement is not very important for creep. However creep is related to the stress-strength ratio for the concrete and as we know strength is related to cement type. It is accepted that aggregates are materials which completed their deformation. Aggregates act as a restraint to reduce the potential deformations of the paste. The aggregate content and modulus of elasticity are the most important parameters affecting creep of concrete. Aggregate size, grading and surface texture have little influence. The effective factor on creep is not the stress itself but the value of stress-strength ratio act as a dominant role in creep event. It is relevant to note that, creep is found to increase approximately linearly with the applied stress up to stress-strength ratios of about 0,3 to 0,8. There is no doubt that creep increases an the water-cement ratio. While investigating the effect of water-cement ratio on creep, one parameter cannot be studied independent from other parameters because the change in water-cement ratio would cause a change in strength of concrete. The rate of creep decreases when concrete age increases. Creep decreases with an increase in the size of the specimen. The influence of shape of the concrete member is very small on creep. Humidity is one of the environmental influences on creep. In particular, it was observed that drying concrete creeps at higher ultimate creep than concrete which remains wet or dry. XIII. Temperature is the second major environmental factor on creep. It has been observed that increasing temperature increases the creep significantly.. Concrete kept continuously wet creeps less than that cured in air. Finally we can say that a lot of factor influence an creep but there is also a relation between these factors. Under uniform compression creep occurs not only in the axial direction but also in the normal directions. As in elastic strain, the ratio of lateral deformation to the former deformation can be termed as creep Poisson's ratio. From the experimental data, it can be 0, a value about 0,05 or the values at elastic deformation situation. The factors influence on creep are valid for shrinkage too. Although the deformation- time curves and magnitude of deformations of creep and shrinkage look like each other, there are strong indications that their mechanisms are different. Various mathematical methods have been suggested to represent the time dependence behaviour of materials. It was observed the following empirical power function of time describes creep of many different materials with good accuracy over a wide span of time: fCO^o+^ooXl-e-") (1) where s is the strain, t is the time, a is a constant, 8o is the time independent strain and e (oo) is the time-dependent finally strain. If we take the logarithm of equation (1) XIV£ - S Then if ln(l ) versus t plotted, it gives a straight line of slope a In this study, it is aimed to explain the effects of water-cement ratio and loading age on creep. In the first chapter, the general view about phenomena is given. In the second chapter the structure of concrete, the effect of creep on concrete, mechanism of creep, the factors influencing creep, creep Poisson's ratio, the relation between creep and shrinkage, the non-linear viscoelastic behaviour of concrete under uniaxial stress, time-dependent functions at non-linear creep situation are explained. In the third chapter experimental work and the materials that are used in the concrete mixture are given. In the fourth chapter experimental results and their conclusions are given. In creep experiment prismatic concrete specimens of 10 x 10 x 50 cm were used. Three water-cement ratio are used to determine the effects of water-cement ratio:%35, %55, %75 and three loading ages are used for this experiment: 14, 28, 42. Three of specimens are not loaded for determine the shrinkage of three different water-cement ratio. Totally we used 12 specimens: 3 for shrinkage, 3 for 14th loading age, 3 specimen for 28th loading age and 3 specimen for 42* loading age. We used a constant stress level : a/R=0,30. Longitudinal creep strains are measured from the specimens by using demec strain meter. Strain-time functions were obtained from experimental results and non-linear creep functions were also calculated. The results obtained in the experimental work can be outlined as follows :. For the same a/R, when the loading ages increase, the creep functions which are independent of stress, decrease.. For the same oVR, when the water/cement ratio increases, the creep functions which are independent of stress, increase. In this study, we see that concrete shows linear viscoelastic type of behavior ^ at the range of o/R=0.3,v
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