Isıl işlem görmüş hafif betonların değişken yükler altında zamana bağlı davranışları

Abstract

Bu çalışmada ponza taşı hafif agregası kullanılarak üretilen iki farklı birim ağırlıktaki hafif betonun değiş ken yükler altında ve yüksüz durumdaki zamana bağlı dav ranışı incelenmiştir. Yalnız işlem sıcaklığı değişken tutulan bu betonlardan l.Grup birim ağırlıkta 40 oc, 60 °C, 70 °C ve 95 OC, 2. Grup birim ağırlıkta ise 50 °C ve 80 °C de ısıl işlem uygulanmış ve ayrıca her iki grup betondan x20 °C de normal ortam sıcaklığında olmak üzere toplam 8 seriden oluşan beton üretilmiştir. ^Çalışmada önce farklı işlem sıcaklığı uygulanmış^ hafif betonların sünmesi değişken gerilmeler altında de neysel olarak bulunmuştur. Daha sonra seçilen bir loga ritmik sünme fonksiyonuna bu deney sonuçları uygulanarak denklemdeki "a" ve "b" katsayılarının yükleme yaşı ile olan değişimi saptanmıştır. Sünme deneylerinde her yaş taki beton basınç dayanımının 1/3 ünün altında kalınarak gerilme uygulanmış ve lineer viskoelastik bir varsayımla süperpozisyon ilkesinden yararlanılmıştır. Bulunan bu katsayıların logaritmaları ile yükleme yaşının logaritma sı arasında iyi bir korelasyon olduğu görülmüştür. Logaritmik sünme fonksiyonu yükleme yaşından bağım sız hale getirilerek "k" katsayıları ile ifade edilmiş tir. Bu katsayılar tüm seriler için hesaplanmış ve be ton kür koşullarıyla olan ilişkisi grafik olarak göste rilmiştir. Kurulan bu ilişkiden yola çıkılarak, herhan gi bir işlem sıcaklığında tutulan betonun sünmesi yükleme yaşına bağlı olarak iyi bir yaklaşımla tahmin edilebile ceği gösterilmiştir. Değişken gerilmeler altındaki ha fif betonun uzun süreli yüklemelerdeki davranışı üzerine ısıl işlemin etkisi, kısa süreli yüklemelerdeki aynı be ton özelliklerine olan etkisi ile benzerlik gösterdiği sonucuna varılmıştır. Ayrıca bu betonların yükleme yaşında/ basınç daya nımları ile elastisite modüllerinin işlem sıcaklığı ile bilinen ilişkisi vurgulanmış ve bu betonların rötre deformasyonları deneysel olarak bulunarak irdelenmiştir. Çalışmada varılan sonuçlar tablo ve şekiller halin de verilerek değerlendirilmiş ve irdelenmesi yapılmıştır. Deney sonuçları ile kullanılan bağıntılar arasında iyi bir ilişkinin olduğu saptanmıştır. Diğer deney sonuçla rı da değerlendirilmiş olup Bölüm 5 de bulunan sonuçla rın kısa bir özeti verilmiştir.

Although a number of investigations have been carried out on the short term behavior of concrete, in practice, concrete structure is subjected to long term loading rather than short term loading. Under long term loading, strain increases with time. Hence, concrete exhibits time dependent behaviors which are called creep and shrinkage. The calculations of creep and shrinkage are especially important for prestressed concrete structures. There is no sufficient information about the long term creep behavior of concrete under variable stresses. Also, very little information is available on the effect of the acceleration of hardening on the long term loading of concrete. Since concrete is a widely used material in structural engineering, we need sufficient information about the creep of concrete for different curing conditions under variable stresses. This information is especially needed for both the further analysis of concrete and better understanding of the material behavior. Today's construction industry is making use of prefabrication to speed up construction of residential structures. To be able to provide fast production, acceleation of hardening in concrete is required. Creep experiments are also time consuming under normal curing conditions. For this purpose, the widely used method is heat treatment. The application of heat treatment to concrete provides an early resistance, the construction elements thus obtained begin to carry service loads. Accelerated tests exhibit the fast increase in concrete resistance during early ages, at later stages heat treatment causes a reduction in the compressive strength compared to that of normal curing condition at 20 °C. Normal concrete is used mainly for structural purposes. High unit weight of normal concretes pose some problems in their production and application. - xi - The use of lightweight concretes especially in residential strustures is of great practical importance owing to their superiority over normal concretes in respect of thermal insulating properties. The structural lightweight aggregate concretes are obtained by substituting the normal concrete aggregate with lightweight aggregate. If lightweight concrete is used, the decrease which is about 25 % of the total weight of structural system causes a decrease in the forces induced by earthquake. Hence, it is understood that the use of lightweight concrete isagood solution in the construction of buildings which are resistant to earthquake. Lightweight concretes are prefered for many advantages compared to normal concrete and used in structures except in construction elements where high strength is required. Natural pumice lightweight aggregates brought from Nevşehir-Göreme region were used. Pumice, volcanic tuff, and volcanic cinders are the most abundant natural lightweight aggregates in Turkey. Eastern and central Anatolia, especially, have great pumice reserves. It is know that natural lightweight aggregates have lower strengths and endurances compared with artificial ones. In spite of this, it is possible to produce moderate strength semi lightweight concrete when lightweight aggregate is combined with normal aggregates. Moreover, structural lightweight and insulating concrete (walls and panels) can be produced if necessary precautions against moisture effects are taken. The use of pumice aggregate in residential constructions at cold weather regions provides good results in termal insulation and in the reduction of air pollution. In this study the maximum particle size, the grading and the cement content of concrete were kept costant. Concretes cast were in two diffeent.e,... densities, hence effective water content and unit weights were varied. In both mixes, the composition of concrete was kept constant. Concretes that have a certain curing cycle consist of eight series. For all series, the application of the heat treatment, the initial waiting time, heating rate and the cooling rate are two hours; 25 °C/hour and 10 °C/hour, respectively. In the first group of' concretes the heat treatment temperatures were kept at 20 °C, 40 °C, 60 C, 70 °C and 90 °C. On the other hand, in the second - xii - group concretes the heat treatment temperatures were 20 °C, 50 °C and 80 °C. In this study after a certain heat treatment is applied, the creep of lightweight concrete has been experimentally measured under variable stresses. A logarithmic creep function is described to represent the creep of concrete under constant stress. The creep behavior of concrete with time varying mechanical properties under variable stresses has been calculated assuming a linear viscoelastic material. Then, the variation of the coeficients of this function with the age of loading are experimentally determined. The study consists of five parts. In the first part, an introduction is made to the subject of the investigation, definitions of some relevant terms and properties of lightweight aggregate are given. The influence of the heat treatment on the properties of concrete are explained. Literature review dealing with the creep function of concrete are given, especially the creep behavior concrete under variable stresses is elaborated and a general evaluation of these works are made. The second part is devoted to experimental work. The tests carried out on the materials used, the principles assumed, the concrete produced are listed and the main lines of the long term tests carried out on the hardened lightweight concretes are explained. In the third part, experimental results are given. The results obtained from the short term and long term loadings are tabulated in the tables and are shown in the figures. In the fourth part, the experimental results are discussed and evaluated. In this part, the variations of both compressive strength and modulus of elasticity with the age of loading are examined. Results obtained revail. that the heat treatment increases the strength: of concrete paralel to the applied temperature in the early age of concrete, whereas, strength loss related to control specimens - xin - is observed in the later ages. The same results are obtained between the modulus of elasticity and the age of loading of concrete. Creep experimental results are deeply examined in this part. Than, the shrinkage deformations are discussed and evaluated. In fifth part, the conclusions are summarized. In this part, an overall evaluation of results obtained is made and suggestions are given for future research. The conclusions which may be drawn from the present study as follows; 1) There was no segregation in fresh lightweight concrete, 2) Compressive strength and modulus of elasticity increase at early ages when heat treatment is applied, decrease in potantial resistance values are seen in comparison to cured control specimens, in normal environmental conditions. This derease varies with the unit weigh of concrete. 3) A logaritmic expression is chosen as the creep function to express the time dependent behavior of concrete under variable stresses. In this creep function as showbelow coefficient of (a) shows instantaneous strains, and (b) represents time dependent strains. &(t) - or0. (t) >(t)= a + b loge (t + 1) 4) In the creep function, the coefficients (a) and (b) both decrease with the age of loading: (b) showing a higher rate of decrease than (a). There is a good correlation between the logarithm of the age of loading and the logarithms of (a) and (b). Thus, the creep function may be expressed in terms of teh age of loading and temperature of heat treatment as below. 0(tff»,T)=a(&,T)+b(TjrT).log (t-S +1) - xiv - 5) Time dependent coefficients (a) and {b) of the logaritmic creep function can be expressed in terms of (k)'s which do not depend on the age of loading. It is also observed that there is a good relation between (k) *s and the temperature of the heat treatment process. a= k !.?& bak- kjssk^T), k2=k2(T), k3^k. (T), k4=k4(T) ^4=^4 (Figs. 4. 18 T 4. 19, 4. 20? 4.21). 6) Using these coefficient, (a) and (b) were predicted fojr a certain heat treatment. To be able to check the heat treatment at 60 °C which are not included in the calculations of the parameters, both experimentax and theoretical results show good agreement at this temperature. 7) At a certain temperature, there is no need to carry out tests for the intermediate ages of loading for the proposed method. 'Therefore, once the creep function for a certain lightweight concrete is determined its creep under variable stresses can be computed. The theoretical predictions obtained using the proposed method were found to be in good agreement with the experimental results. Hence, it can be said that, assuming concrete to be linear viscoelastic material under service loads, this will not lead to a significant error.