Betonda sünme ve hasar analizi
Creep and damage analysis at concrete
- Tez No: 66790
- Danışmanlar: PROF. DR. SAİM AKYÜZ
- Tez Türü: Yüksek Lisans
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1997
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Yapı Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Yapı Malzemeleri Bilim Dalı
- Sayfa Sayısı: 83
Özet
ÖZET Betonda meydana gelen deformasyonlar çeşitli şekillerde sınıflandırılabilir. Bunlar ani deformasyon ve zaman içinde meydana gelen deformasyonlardır. Zaman içinde meydana gelen deformasyona sünme adı verilir. Beton sünme yapabilir bir yapı malzemesi olarak anılır. Sünme sonucunda cisimde gerilmeler doğar ve gerilme yayılış değişmesine yol açar. Betonarme ve öngerilmeli beton yapılarda bu olayla karşılaşıldığından dolayı,sünme olayının etkileri kesinlikle göz önünde bulundurulmalıdır. Sünme cisimdeki mekanik özellikleri zayıflattığı için hasara neden olur. Sünme sonucunda oluşan hasar çok çeşitli etkenlere bağlı olduğu için, hasar problernlerinin çözümünde matematik formülasyondan çok mühendislik yaklaşımı önemlidir. Yapılan çalışmalarda, portland çimentosu ile üretilmiş ve değişik gerilme seviyelerinde yüklenmiş numunelerde sünmeye bağlı hasar incelenmiştir. Numunelere mukavemetin % 75, % 60, % 40, % 20 'si oranında gerilme yüklenmiş ve rötre etkisini anlamak için de bir adet numune yüklenmeden incelenmiştir. Gerilmenin artmasıyla hasar ve hasar hızının arttığı saptanmıştır, a / R ' nin küçük değerlerinde non-lineer sünme ortaya çıkmaktadır. Hasarın hasar hızına etkisi, gerilmenin hasar hızına etkisinden önemlidir, a I R ' nin artmasıyla sünme numunelerinin kırılma zamanı azalmaktadır. ıx
Özet (Çeviri)
SUMMARY CREEP AND DAMAGE ANALYSIS AT CONCRETE In a general manner to introduce the topic we can say that creep is a slow continous deformation of a material under constant stress. A lot of materials display creep but there are some fundamental differences between concrete and many other common structural materials. Before to give 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 the 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 homogeneous and isotropic). Another essential 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 three 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 great variety of behaviour. However, by means of idealization deformations of concrete can be simplified and classified as in Table Al. Table 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 xrole 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 fundamental 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 largely irreversible character of long-term creep. There are a lot 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.. Generally 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 aproximately linearly with the applied stress up to stress-strength ratios of about 0,4 to 0,6.. There is no doubt that creep increases with an increase in the water-cement ratio. While investigating the effect of water-cement ratio on creep, one parameter can not be studied independent from other paremeters. Because a 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 in creep.. Humidity is one of the environmental influences on creep. In particular, it was observed that drying concrete creeps at a higher ultimate creep than concrete which remains wet or remains dry. xi. Temperature is the second major environmental factor in creep. It has been observed that increasing temperature increases the creep significantly.. Concrete kept continously wet creeps less than that cured in air. Finally we can say that a lot of factors influence on creep but there is also a relation between these influences. 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. But there is no agreement on the magnitude of the 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: e = So + Bt° (1) where s is the strain, t is the time, n is a constant independent of stress, 8o is the time independent strain and B is the coefficient of time-dependent term. If we take the logarithm of equation (1) : log( e-8o ) = logb + nlogt (2) Then if log(s-so) versus logt is plotted, it gives a straight line of slope n and intercept b. V is independent of stress and state of stress. Creep functions(=02=a3cpi(t) + CTsW) +2(t) +C43
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