Isıl işlem parametrelerinin hafif beton özeliklerine etkisi

Abstract

ÖZET Bu çalışmada, ısıl işlem uygulamasında ön bekleme süresi, ısınma hızı ve işlem sıcaklığının hafif betonların erken ve ileri yaşlardaki elastik ve inelastik özeliklerine etkisi araştırılmıştır. Hafif betonların karışım granülometr ilerin de, hafif agregaların belirli fraksiyonları kırmataş agrega sı ile değiştirilerek birim ağırlığı hafif betonlar ve yarı hafif beton bölgesinde değişen betonlar üretilmiştir. En bü yük agrega boyutu, granülometri, çimento dozajı ve etkin su/ çimento oranı sabit tutulmuştur. Betonlara uygulanan ısıl iş lem çevrimlerinde ön bekleme süresi, ısınma hızı ve işlem sı caklığı değişken olarak seçilmiştir. Toplam çevrim süresi 22 saat ve soğutma hızı 10 °C/saat olarak sabittir. Değişken ısıl işlem parametrelerinden ön bekleme süresi 1, 4 ve 7 saat, ısınma hızı 10, 25 ve 40 °C/saat, işlem sıcaklığı ise 50, 70 ve 90 °C olarak uygulanmıştır. Beton özelikleri, her birim ağırlık için normal ortam şartlarında kürlenmiş olan kontrol betonlarına kıyasla incelenmiştir, üretilen silindirik ve prizmatik numuneler üzerinde ultrases hızı tayini, eğilme ve tek eksenli basınç deneyleri 1 ve 28. ci günlerde uygulanarak, elastik ve inelastik özelikler araştırılmıştır. Çalışmadan elde edilen en önemli sonuçlar şöyle özetle nebilir ; ön bekleme süresinin 7 saat olması durumunda 28. günde en yüksek basınç dayanımları elde edilmektedir. Buna karşılık, 4 saatlik ön bekleme süresi 28. ci günde basınç da yanımında ve ultrases hızında en az etkin olmasına rağmen eğilme dayanımlarını ve elastisite modülünü maksimum yapmak tadır. Genel olarak, 28 günlük betonlarda belli bir ısıl iş lem parametresi için ultrases hızları ile basınç dayanımları aynı yönde değişirken elastisite modülleri farklı yönde değiş mektedir, ön bekleme süresinin 4 saat olması halinde 1 ve 28 günlük betonlarda hem mutlak, hem de bağıl tokluk değerleri minimum olmaktadır. Isınma hızının 25 °C/saat olmasıyla ise özellikle 28. ci günde maksimum basınç ve eğilme dayanımları elde edilirken, elastisite modülleri 10°C/saat ısınma hızı için maksimum olmaktadır. Silindirik numunelerde sıcaklık ar tışıyla birlikte basınç dayanımları artmaktadır. Bu artış 90 °C içinde görülmektedir. Buna karşılık eğilme dayanımı ve elastisite modülü, 28. ci günde, 50 °C için maksimum olmakta-, dır. îşlem sıcaklığındaki artışla birlikte basınç dayanımla rı artmasına rağmen, süreksizlik sınırındaki `p o is s on `oranla rı azalmaktadır. Araştırma sonuçlarına göre ısıl işlem uygu lamasının hafif beton özeliklerine gösterdiği etkiler normal betonlara göre farklı bulunmuştur. - xıı -

SUMMARY INFLUENCE OF HEAT TREATMENT PARAMETERS ON PROPERTIES OF LIGHWEIGHT CONCRETE It is known that pre fabrication is a proper method in mass building production. For lightweight concrete prefab- rication the reduction of weight is another positive point. Lightweight pumice aggregate is abundantly available in ^ Turkey and is suitable for lightweignt concrete production. Several methods of accelerating the early strength of concrete are used, to increase the effectiveness of the prefabr icat ion. The most widely used method among these is steam curing under atmospheric pressure. Steam curing causes some reduction in strength of normal weight concrete at later ages, whilst increasing the early strength. This phenomenon was investigated by many researchers for normal weight concrete. On the other hand the influence of steam curing on lightweight concrete is somewhat different. The number of investigations on steam curing of lightweight concrete are not so much as those on normal weight concrete and therefore further researche is needed. In literature, mostly use of expanded clay is reported as lightweight aggregate and the unit weight of concrete is kept constant. The thermal conductivity of lightweight concrete at constant unit weight and humidity may vary about 20 % depending on the structure of pores, on the grad ing and the amount of glassy phase in the aggregate. There fore the variation of aggregate type used in concrete may vary its thermal properties, and new investigations may be needed. With this in mind the purpose of the present inves tigation was chosen to make a new study on the properties of lightweight concretes made with pumice lightweight aggregate, which is abundant in Turkey. The unit weights of concretes investigated in this study belong to the classes of lightweight and semi-light weight concretes. The influence of heat treatment parame - ters such as modulus of elasticity and Pöissön's ratio were investigated on the properties of concrete used in struc - tural design at different unit weights. The effects of the preheating period, the rate of heating, and the treatment temperature on elastic and in elastic properties of lightweight concretes, having the same grading but three different unit weights, were studied at both early and late ages. Also by recording the - xiii -temperature variation in the samples the degrees of matu rity of the concretes were calculated and the relations between maturity and the modulus of elasticity and strength of concretes were investigated. The study is presented in 5 chapters. In the first chapter general knowledge about the subject, definitions, related literature, the purpose and the necessity of the study are given. The experimental work is handled in the second chapter. The characteristics of the concrete cons - tituents, the proportions of concrete mixes, the methods of preparation and curing of concretes are given in this chapter. The types of heat treatment cycles and the experi mental techniques used in tests are also given in the second chapter. The test results obtained are listed in the third chapter. Evaluation of test results and their discussion are the subjects of the fourth chapter. The conclusions derived from the study and proposals for further investigations are given in the last chapter. The grading of the aggregate was kept constant and some fractions of the grading were subsituted by normal weight crushed stone in order to obtain concretes with three different unit weights, varying in the range of lightweight and semi-lightweight concretes. The maximum aggregate size was 16 mm. The cement content and effective water/cement ratios were the same for all mixes. Normal Portland cement was used. The following different heat treatment cycles have been applied : the total cycling period and cooling rate were kept constant at the values of 22 hours and 10°C/hr respectively in all cycles, the preheating period, heating, rate, and heat treatment temperature were varied. Three different preheating periods used were 1, 4 and 7 hours. Heating rates were chosen as 10, 25 and 40 °C/hr,.and heat treatment temperatures of 50°, 70° and 90 °C were applied. In cycles with variable preheating period the heating rate of 10°C/hr and the heat treatment temperature of 70 °C were kept constant. On the other hand in cycles with variable heating rates the preheating period of 4 hours and the heat treatment temperature of 70 °C were chosen constant. Finally, in cycles with variable treat - ment temperatures the preheating period and the heating rate were kept constant at the values of 4 hours and 10°C/hr respectively. In order to find out the influence of maturity on compressive strength and modulus of elasticity, two addi tional types of cycling with the constant preheating period of 1 hour and the heating rate of 25 °C/hr at two different treatment temperatures of 60° and 70°C were used also. Including this last two cycles the total number of cycles amounted to 9. During the heating cycles variation of the temperature was recorded in the center and on the surface of the samples. - xiv -Control mixes cured at normal conditions., were tested, in order to demonstrate the relative influence of the heat treatment cycles on the properties of lightweight concretes, with respect to normal curing. On the hardened concrete specimens the same testing program has been repeated at the ages of land 2 8 days. The ultrasonic velocity was measured on prismatic specimens of 7x7x28 cm. dimensions. After which simple beam flexure test has been performed on the same prismatic specimens, and the modified compressive strength tests were conducted on their broken pieces. Uniaxial compressive strength tests were performed on the standard cylinder specimens of 15x30 cm. at a constant strain rate with a closed looped servo- controlled compression machine. The stress-strain curves of the specimens were recorded by an X-Y recorder. Longitudinal and lateral strains were measured by two dif ferent frames mounted on the specimen. Poisson's ratios were calculated from the measured stains and discontinuity limits and loosening points were obtained from the stress- Poisson's ratio curves. The stress-strain curves were as sumed to be linear up te one third of the maximum stress (compressive strength) and the modulus of elasticity was calculated from this portion of the curve by using the least squares method. The toughness values of the concretes were obtained from the area under the stress-strain curves up to the maximum stress. The variation of concrete properties mentioned above with the change in heat treatment parameters were shown on graphs and the evaluations and discussions were based on these graphs. Important results obtained in the study can be grouped as follows : 1) Results related to compressive strength o The highest strength values were obtained when the preheating period was 7 hours. Although maturity decreased, when the preheating period is increased, the reason for the higher compressive strength can be explained due to reduc tion of physical imperfections in the longer preheated concretes. The worst results were obtained with a preheat ing period of 4 hours. o Although the influence of the heating rate was not important on prismatic specimens, the same parameter was effective on cylindirical specimens. The highest 28 day strength values were obtained with a heating rate of 25 °C/hr. At this heating rate, the heating was probably more uniform through the specimen and the adverse effect of differential heating between the interior and the sur face of the specimen was reduced. - xv -o Whilest the increase of heat treatment temperature had no effect on relative strength of prismatic specimens, the relative strength of cylindrical specimens increased with increasing heat treatment temperature. Even at the heat treatment temperature of 90 °C an increase in relative strength of cylindrical specimens was observed. o Prismatic specimen with their greater surface area per unit volume than the cylindirical ones absorbed more heat and hence hydrated more quickly. This caused the higher increase in relative strength of prismatic specimens. o The effectiveness of heat treatment on the relative strengths increased, with decreasing unit weight of concrete. o The results given above show that the effects of heat treatment on lightweight concrete are different from the those on normal weight concretes. 2) Results related to flexural strength o In general, the way of effecting of heat treatment the flexural strength of lightweight concrete is different from that of compressive strength. o The 1 day relative bending strength of concrete with a unit weight of 1.46 was greater than that of the control concrete and had a value of 10 3-10 4 % of the control concrete. On the other hand the 28 day bending strength of concrete preheated 1 hour was less than the 1 day bending strength of the same concrete. This contradiction may be due to shrinkage stresses occurring in the concrete because of differential drying of the interior and exterior parts of the specimens. o For concretes with unit weights of 1.46 and 1.9 3 a preheating period of 4 hours was an optimum period which caused maximum 28 day relative flexural strength. o The 1 day relative bending strength for all unit weights decreased when heat treatment temperature increas ed. The 28 day bending strengths became maximum at a heat ing rate of 25°C/hr. At this heating rate the relative strengths were greater than 100 %. o The largest 28 day relative bending strengths were obtained with a heat treatment temperature of 50 °C. o The 1 day relative bending strengths increased when the unit weights of concretes decreased. The largest 28 day relative flexural strength was obtained for concretes with a unit weight of 1.69, kg/dm3. o It has been found that the results on flexural strength obtained in this study are different from results of other investigators for both normal and lightweight - xvi -concretes, which may be due to differences in curing condi tions. 3) Results related to ultrasonic velocity o The preheating period affects ultrasonic velocity in the same way as compressive strength. o Generally, ultrasonic velocity increases with in creasing unit weight. This behaviour was observed for both the 1 and 28 day ages. o The highest heat treatment temperatures, that is at 90°C. This showed that higher heat treatment tempera - tures were more effective on lightweight concretes as compared to normal weight concretes, for which the optimum heat treatment temperature is reportedly around 75 C. o Generally, it was not possible to find a clear correlation between the variation of ultrasonic velocity and the variation of the compressive strength due to heat treatment of lightweight concrete in this study. 4) Results related to elastic constants o Moduli of elasticity were maximum for the concretes with unit weights of 1.46 and 1.93 which were preheated for 4 hours. b In general, the largest relative moduli of elasticity were obtained with a heating rate of 10°C/hr for concretes of all three unit weights of 1.46, 1.69 and 1.93, and at both ages of 1 day and 28 days. o When the heat treatment temperature increased, the 1 day modulus of elasticity increased but for heat treat ment temperatures exceeding 50 °C a decrease was observed for 28 day results. Only for the concretes with unit weights of 1.46 and for the ages of 1 and 28 day, a peak was observed at the heat treatment temperature of 70 °C. o As a general rule, it can be said that, at the age of 28 days, every heat treatment parameter affected both the ultrasonic velocity and the compressive strength in the same way, but it affected the modulus of elasticity in different ways. o Although the compressive strengths increased with increasing heat treatment temperature, the Boisson's ratio at discontinuity limit decreased. This result contradicted the results reported in literature. 5) Results related to inelastic properties and the stress-strain curves of concrete o Concretes cured for a preheating period of 4 hours -r xvii -showed the smallest values of strains at maximum stress and the smallest absolute and relative toughnesses at 1 and 28 days of age. o When heat treatment temperatures increased, the strains at maximum stress and the absolute toughness dec reased at 1 day age but the same properties increased at the 28 days age. The relative toughness value at both ages, was less than 100 %. o Generally, heat treatment parameters affected both the absolute and relative toughnesses in the same way. 6) Results related to degree of maturity o The maturity degree varied linearly with both the absolute and the relative compressive strengths at the same heat treatment temperatures. Similar linear relations were obtained also for both the absolute and relative moduli of elasticity. o The coefficients of correlation for the linear re lations between maturity and compressive strength increas ed with decreasing unit weight of concrete. Whereas for the coefficients of correlation between maturity and modulus of elasticity the reverse was true. This was an interesting observation worth to be checked and explained. 7) General results o The results presented in this investigation on : inelastic properties of lightweight concretes and on their material constants such as modulus of elasticity and Poisson's ratio show for the first time in which way these properties may vary due to the effect of heat treatment. Also the results on the effect of heat treatment upon the toughness of lightweight concrete may be another contribu tion towards the behavior in earthquake resistance. o This study showed that it was possible to obtain better and more economical results in prefabricated light weight concrete by choosing proper heat treatment para meters. - xvin -