Ortalama hafif agrega boyutunun yarı hafif betonların dona dayanıklılığı üzerindeki etkileri

Abstract

Bu çalışmada hafif agrega boyutunun,birim ağırlıkları yaklaşık 2000 kg/m3 olan yarı hafif betonların dona dayanıklılığı üzerindeki etkileri araştırılmıştır. Bu amaçla 5 adet karışım üretilmiştir. Bu karışımlarda hafif agrega boyutunun dışındaki tüm etkenler sabit tutul muştur. Hafif agrega boyutu ise bu karışımlarda sırasıyla 2/8 mm, 4/16 mm, 8/16 mm, 16/25 mm, 2/25 mm değerlerini almıştır. Her bir karışımdan 8 adet 15/30 cm boyutunda silindir numune üretilmiştir. Bunlara 28 gün süre ile standart kür (20ot2°C sıcaklığındaki kirece doygun suda saklama) uygulanmıştır. Bundan sonra 8 numuneden 4 ü şahit olarak saklanmış, diğer 4 ü ise 30 tekrarlı don deneyine tabi tutul muştur. En sonunda tüm numunelerde şekil değiştirmeler ölçülerek basınç deneyi uygulanmış ve bu deneylerin sonuçlarına dayanarak don hasarları hesaplanmıştır. Deneysel çalışmalara göre don etkilerine karşı en büyük dayanıklılığı hafif agrega boyutu 2/8 mm olan karışım göstermiştir. Don etkilerine dayanıklılık açısından 2. sırayı hafif agrega boyutu 8/16 mm olan karışım 3. sırayı hafif agrega boyutu 16/25 mm olan karışım almıştır. Don- çözülme etkilerine dayanıklılık bakımından en kötü durum da olan karışımlar 4/16 mm ve 2/25 mm hafif agrega boyutlu karışımlardır.

Concrete subjected to repeated cycles of freezing and thawing may deteriorate rapidly, or it may remain in service for many years without showing signs of distress. Failure of the material may take the form of loss of strength, crumbling or some combination of the two. Concrete in a wet environment - in bridge piers, pavement near oceans, wharves, and offshore structures, for example - is especially vulnerable. Yet, appropriate mix designs and good engineering practice' can produce concrete that is durable under severe climatic conditions. Standardized tests and tests designed to simulate the natural environment are an important part of the design process, and they provide some indication of the durability of a given concrete. These tests however, may give empirical information that will not be valid under actual exposure conditions. To arrive at a sound conclusion, one must use informed engineering judgment to interpret the test results. As the temperature of saturated hardened concrete is lowered, the water held in the capillary por es in the cement paste freez. in a manner similar to the freezing in the capillaries in rock, and expansion of the concrete takes place. On re-freezing, further expansion takes place so that repeated cycles of freezing and thawing have a cumulative effectJThe larger pores in concrete, arising from incomplete compaction, are usually air-filled and therefore not appreciably subject to the action of frost. Freezing isaqradual process, partly because of the rate of heat transfer through concrete, partly because of a progressive increase in the concentration of dissolved alkalis in the still unfrozen water, and partly because the freezing point varies with the size of the cavity. Since the surface tension of the bodies of. ice in the capillaries puts them under pressure that is higher the smaller the body. Freezing starts in the largest cavities and gradually extends to smaller ones. Gel pores are too small to permit the formation of nuclei of ice above - 78°C, so that in pratice no ice is formed in them. However, with a fall in temperature, because of the difference in entropy of gel water and ice, the gel water acquires an energy potential enabling.it to move into the capillary cavities containing ice. The diffusion ix of gel water which takes place leads to a growth of the ice body and to expansion. We have thus two sources of dilating pressure. a) First freezing of water results in an increase in volume of approximately 9 percent, so that the excess water in the cavity is expelled. The rate of freezing will determine the velocity with which water disdaced by the advancing ice front must flow out, and the hydraulic pressure developed will depend on the resistance to flow. b) Second dilating force in concrete is caused by diffusion of water leading to a growth of a relatively small number of bodies of ice. The effect of concrete properties on the durability of concrete can be given as follows: The use of mixes with water/cement ratios sufficiently low for the paste to have only small capillaries and only little freezable water. It is essential though that substantial hydration takes places before exposure to frost. Such concrete has a low permeability and does not imbibe water in wet weather. Drying before exposure slightly raises the resistance of concretes with hith water/cement ratios, but in the case of mixes with a water/cement ratio below about 0.45 the effect of inade quate hydrations is predominant : the concrete with the shorter period of wet curing has a lower resistance to frost. The effect of greater hydration is to reduce the amount of freezable water in the paste* the freezing temperature decreases with age because of an increase in the concentration of alkalis in the still remaining freezable water. The chemical compositions of cement and its fineness have no effect on the frost resistance of concrete except at very early ages when these caharacteristics would affect the degree of hydratione and thus influence both the strength of the paste and the quantity of freezable water in it. A possible interpretation that the relative resistance to frost of cements of different types varies with the water/cement ratio of the mix, is not supported by our knowledge of the phenomena involved. To reduce the danger of frost attack good compaction of concrete is essential, and for this reason aggregates and techniques that lead to segregation and honeycombing must be avoided. The use of aggregate with a large maximum size ora large proportion of flat particles is inadvisable as pockets of water may collect on the underside of the coarse aggregate. Tests of frost resistance of concrete: There are four ASTM tentative methods, but two have become accepted. In both of these, rapid freezing is applied but in one both freezing and thawing take place in water, while in the other freezing takes place in air and thawing in water. Freezing saturated concrete in water is several times more severe than in air and the degree of saturation of the specimen at the beginning of the test also affects the rate of deterioration. The effects of frost can also be assesed from measurements of loss of compressive or flexural strength or from observations on the change in length or weight of the specimen. The last method is applicable when frost damage takes place mainly at the surface of the sepecimen, but it is not reliable in cases of internal failure; the results depend also on the size of the specimen. It may be noted that if failure is primarily due to unsound aggregate it is more rapid and more severe than when the cement paste is disrupted first. We can see that a number of tests and means of assesing the results is available, and the interpretation of test data is difficult. If the tests are to yield information indicative of the behaviour of concrete in practice, the test conditions must not be fundamentally different from the field conditions.one diffuculty lies in the fact that a test must be accelerated in comparison with the conditions of outdoor. Freezing, and it is not known at what stage acceleration affects the significance of the test results. There is no douht however, that some accelarated freezing and thawing tests result in the destruction of concrete that in practice would be satisfactorily durable. One difference between the conditions in the laboratory and actual exposure lies in the fact that in the latter case there is seasonal drying during the summer months, but with permanent saturation imposed in some of the laboratory tests all the air butües çan eventually become saturated with a consequent failure of the concrete. The use of lightweight concretes especially in residential structures is of great practical importance owing to their superiority over normal concretes in respect of thermal insulating properties in the face of today's energy crisis. The own weight of structural concrete represents a very large proportion of the total load in structures and therefore it is advantageous to reduce the density of normal concrete, to reduce the loads and the thermal conductivity. If lightweight concrete is used in concrete XI construction, a decrease of about 25% of the total weight in the structural system is obtained. Which causes a considerable decrease in gravitational and seismic forces. Hence, it is expected that the use of structural lightweight concrete is a good solution for the construction of buildings specially in earthquake regions. Following methods are widely used for the production of lightweight concrete : - Natural lightweight aggregates of low density are used instead of normal aggregate. Hence, the concrete obtained is know as lightweight aggregate concrete. - Lightweight concrete is obtained by introducing large voids within the mortar phase. This kind of concrete is know as aerated, cellular, foamed or gas concrete. - Lightweight concrete is produced by omitting the fine aggregate fractions from the mix, this is described by the name of no-fines concrete. Most of these lightweight concretes are not edequate for structural purposes. Artificial lightweight aggregates such as expanded clay and expanded shale are widely used for structural lightweight concrete, but such aggregates are, not yet available in some countries,»including Turkey on the other hand, there are abundant resources of natural pumice lightweight aggregate in Turkey, which has lower strength as compared to artificial ones, in spite of this, it is possible to produce a moderate strength,, semi lightweight concrete, when the pumice lightweight aggregate is combined with normal limestone agregates. 3h the present work, such a combination of pumice and limestone has been used. In this work, the maximum particle size_the water/cement ratio and the volumes of agregates in lm concrete were kept constant, various fractions of agregate grading were substituted with lightweight aggregate and unit weight of the concretes were kept constant. Five different fractions of the aggregate grading were substituted with lightweight aggregate. The work consists of five parts : In the first part, an introduction is made to the subjected matter of the investigation, definitions of some relevant terms and a classification of lightweight concretes are given. The second part is devoted to the experimental studies. The materials used, the principles assumed, the mix compositions, the methods of mixing and curing, the equipment and the methods employed in the testing and measurements made are described. xii In the third part, the experimental results are presented. This results are discussed and evaluated in the fourth part. In the fifth part the conclusions are summarized. Five different fractions of the aggregate grading were substituted with lightweight aggregate and concretes were produced using one constant effective water/cement ratio. While substituting lightweight aggregate for a selected fraction of normal concrete aggregate, the mix composition was not altered and the substituted volumes were kept constant and equal. On the basis of the above mentioned assumptions five different mixes were prepared and 8 standart 6 x 12 cylinders were cast from each mix. They were cured for 4 weeks in water and then tested. In this work the sepicemens is stored in saturated lime water until the time freezing and thawing tests are started. Then the specimens are brought to a temperature -20°C in 4 hour and kept in this temperature for 4 hour* This cycle was repeated for 30 times. The main conclusious obtained from this study can be summarized as follows* 1- The mixtures, having a light aggregate fraction of 2/8 mm, have shown the maximum durability against freezing and thawing. After the thirty cycles of freezing and thawing no significant change has been recorded in stress- strain curves, and modulus of elasticity. The compressive strength has decresed only by 12.3%. The second most durable mixture against freezing and thawing is the one having a light agregate fraction of 8/16 mm. For these concretes, some changes, imply ina damage^ have occured in the stress- strain curves. A 11.8% decrease in modulus of elasticity and a 17.8% decrease in the strain at compressive strength have been observed. No decrease in the compressive strength has been seen. The third most drable mixture is the one having a light aggregate dimension of 16/25 mm. In the stress- strain curves, the changes implying a degree of damage equal to the one recorded for the 8/16 mm mixtures, have been seen. There occured a decrease of 21.9 % in the modulus of elasticity and a 44.2 % decrease in the strain at the compressive strength. No change has occured in the compressive strength. xiii The least durable mixtures against freezing and thawing are those with the light weight aggregate fractions at 4/16 nun and 2/25 mm. For the 4/16 mm mixtures, there obtained a decrease of 8.4 % in the compressive strength, 53.4 % in the modulus of elasticity and 62.8 % in the strain at the compressive strength. For the 2/25 mm mixtures, no decrease has been recorded in the modulus of elasticity and in the strain of compressive strength while a 14.7 % decrease was observed in the compressive strength. 2- Compressive strength decreases as the average light weight agregate size increases. 3- Strain at the compressive strength decreases as the average lightweight aggregate size increases.