Yarı hafif betonun inelastik davranışının incelenmesi

Abstract

Bu çalışmada normal betondaki bazı agrega gruplarını ponzatası agregası ile ikame ederek üretilen hafif ve yarı hafif betonların basınç altındaki inelastik davranışı üzerine hafif agrega boyutlarını ve "hafif agrega mutlak hacmi "V "toplam agrega mutlak hacmi" oranının etkileri araştırılmıştır. Çalışmada su/çimento oranı, çimento dozajı ve agrega granülometrisi sabit tutulmuştur. 5 değişik hafif agrega boyutu ve 3 değişik "hafif agrega mutlak hacmi "/"toplam agrega mutlak hacmi" oranı kullanılmıştır. Yani 15 adet karışım üretilmiştir. Her bir karışımdan 15/30 cm boyutlu 3 adet silindir dökülmüştür. Su/çimento oranını düşük tutmak amacıyla süper akışkanlaştırıcı katkı maddesi kullanılmıştır. Su/çimento oranı bütün karışımlarda 0.39 dir. Mekanik deneyler 28 inci günde yapılmıştı r. Çalışmada başlıca şu sonuçlar elde edilmiştir. 1- Ortalama hafif agrega boyutlu (dor) nm artışıyla kritik gerilmeler ve basınç dayanımı bir miktar azalmaktadır. 2- dor nın artışıyla şekil değiştirme yeteneği bir miktar azalmaktadır. 3- dor'nın artışıyla tokluklar bir miktar azalmaktadır. 4- Kritik gerilmnelerin basınç dayanımına oranı, kritik gerilmelerdeki şekil değiştirmelerin basınç dayanımındaki şekil değiştirmeleri oranı ve basınç dayanımındaki ve (3.ec) deki bağıl tokluklar dor dan etkilenmemiştir. 5- dor nın 4 mm den 20 mm ye yükselmesiyle elastik modülü % 38 azalmış

The own weight of structural concrete represents a very large proportion of the total load in structures and therefore it is advantatageous to reduce the density of normal concrete, to reduce the loads and the thermal conductivity, if lightweigt concrete is used in concrete construction, a decrease of about 25 % of the weight in the structural system is obtained which causes a considerable decrease in gravitional and seismic forces, Hence, it is expected that the use of structural light wieght concrete is a good solution for the construction of building specially in eartquake regions. Further more, lightweight concrete in construction can show considerable savinys on transport, creenage, f ormwork, concrete placement and plant maintenance. The reduction in self -weight is also of considerable importance in multy-storey and long- span structures and it makesi t possible to have faster building rates and lower haulage and handling costs. Structural lightweight concretes are more fire resistant than normal weight concretes because of their lower thermal conductivity lower cofficient of thermal expansion and the inherent fire stability of an agregate already to over 1100 C.This has obviows important effects on thermal insulation and heating costs. It is well known that air is a good heat insulator ahd that the insulation property of a material increases as its density decreases i.e. as its porosity increases.' The continued and expanding extraction of normal, natural aggregates can cause severe environmental problems, sometimes leading to irreversible deterioration of the country side. On the other hand artificial aggregates can often be manufactured from indurstrial waste materials such as pulverised fuel -ash collieng shake slag and slate waste, in industrial area the use of such materials for the production of aggregates could relieve the economy of troublesome pollutants, a social benefit of increasing im portance. The complete behaviour of materiasls cannot be studied throughly in the laboratory because of difficulties in accurate simulation the complex stress regimes and environmental couditions commonly found in the structures. XI Therefore full-scale experiments are reguired in order to check, supplement or modify the laboratory data and this is clearly a long-term process. Nevertheless, laboratory studies are of value because they provide a basic understanding of materials in terms of their fundamental proterties. Further more, laboratory studies allow comparative investigations of diffirent materials to be under taken under controlled conditions so that the influence of various factors can be exmained relativiy at low cost. The following comments could be made from the reviev of literature at the time of drawing up the ressearch programme. / a) Information on the tensile propertiens of light weight aggregate concretes, parcicularly based on uniaxial tests, was limited in extend. b) An investigation was needed the strenght and deformation properties of high strength lightwe ight concretes. These properties are reguired mainly to establish criteria which would not cause unacceptable structural deformation or cracking during the design life of structure. c) Doubt existed with respect to the quality, or otherwise of the mudulus of elasticity in tension > and in compression It was therefore decided to study the enlastic pro perties of selected lightweight aggregate concretes. The present work has been undertaken for this purpose. It con sists of four parts. In the firs part, an introduction and related definitions, a classification of lightweight concrete specially their inelastic behavior and a general evaluation of the present work. The second part is devoted to the experimental studies. The materials used the prin ciples assumed, the mix compositions, the methods of mixing and curing, the types of loading, the equipment and the met hods employed in the testing and measurements made are described. In the third part, the experimental results are presented. The experimental results are discussed and evaluated. XII In the forth part the councl usions are summarized. In this work, the maxsimurn particle size and grading of the aggregate, the cement content and the cementwater ratio were kept constat. In addition to sand four different fractions of the aggregate grading were substituted with lightweight aggregate and concretes were produced using constant effec tive water/cement ratio W/C=0. 39 in order to obtain 21 N/mm cylinder compressive strength using plastisizer five light weight aggregate rations were used the ratios were 0. 0.3, 0.35, 0.40, 0.45 five different mixes were prepared and 3 standart 6x12 "cylinders were cast from each mix. They were cured for 2 weeks in water and then in a humed room. At the age of 28 days they were tested under uniaxial compressive loadings at constant strainrates, using a ser vo-controlled close-loop testing machine, using an x- y recordet. At the next stage the relation between the inelastic behavior and the volume of max light weight particles was studied. These ratios of the volume of the light weight aggregate to the total volume of the aggregates, 0.35, 0.40 0.45 were used to produce the next semi lightweith concrete mixes a) semi lightweight concrete containing lightweight aggregates between 2-8 mm b) semi lightweight concrete containing lightweight aggregates between 4-16 mm c) semi lightweight concrete containing lightweight aggregates between 8-16 mm d) semi lightweight concrete containing lightweight aggregates between 16- 25 mm e) semi lightweight concrete countaining lightweight aggregates between 2-25 mm the same procedure which was used in the first stage was also used in the second stage, also the same curing and testing conditions were used. Poisson's ratios were also obtained form measurements of lateral strains. In the specimen, while it was subjec ted to low cyclic loading, the slope has been accepted to XIII be a measure of the rate of damage of the specimen, from the results of the experiment, following proporties were evaluated. The conclusions reached in this work can be sumrne- rized in the following groups. - The increase in the average light weight aggregates size causes a certain decrease in the critical tresses and in the compressive strength. With the increase of the average size from 4 mm to 20. 5 mm adecrease of about %41 in on % 39 in ox, and % 41 inFc had been abserved. On the other hand no certain effect had been observed from h/a ratio on the critical stresses and compressive strength. - The ability of displacement had been decreased by the increase of the average size of the lightweight aggregate. The increase of the average size of lightweight aggregate from 4 mm to 20.5 mm causes a decrease in sr> about % 33, in £:l °i 33 in sc about % 32, in <£gi> about % 23, in £gl about % 26, in £?c about % 36, in about % 44, in CAV/V)l about % 42 and in (AV/V) about % 17, on the other hand no effect had been found from the ability of displacement. - An inrease in the average size of the lightweight aggre gate fraction decreases the toughness. The increase in the average size at the light weight aggregate fraction from 4 mm to 20.5 mm causes adecrease in the tougheness of the compressive strength of about % 62 and adecarease in the 3^c s toughness of about % 65. The ratio of h/a have no certain effect on the toughness at the compressive strength. But at the toughness of (3.£c) the ratio H/a =0.40 is less then the others. - The ratio of the critical stresses to the compressive strength the ratio of the displacements in the critical stresses to the displacements in the compressive strength and the relative toughness in the (3sc) can give an idea about the length of the fracture period which comes out by the development of the microcracks. The mentioned characteristics hadn't show any important change with the average size of the lightweight aggregate faction and with h/a ratio but the increase of the average size of the light weight aggregate from 4 mm to 20.5 mm causes adecrease in the relative toughness of the compressive strength of about "A 12 and in the relative toughness of (3£c) of about % 14 - The elasticity modulus had decreased about % 38 by the increase of the average size of the light weight aggregate XIV from 4 mm to 20.5 mm The poissons ratio icrensed about%10 the h/a ratio hadn't show an important change for the mentioned two charactaristics. The results obtained in this investigation showed that it is necessary to have a small average size of light weight aggregate in order to obtain alow rate of damage under loading. A comparison can be made between this research and other researchs which heve been done before and this is a summary of this comparison. Def ormational stability is usually desirable for all structral concretes, particularly for conretes which may be fairly highly stressed in service. Insufficent def ormational stability will probably result in cracking def ormational stability which probably result in failure of the structure or of the member or in lack of durablity. Creap, shrinkage, modulus of elasticity thermal movement and band to reinforcement are the most important properties in this subj ect. ) Creep properties of concrete may be either benificial or detrimental, depending on the structural conditions. Concentrations of stress, either compressive or tensile, may be raduced by stress transfer trough creap, but creap may lead to exessive long-term deflection or prestress loss The band of scatter of creap properties of all light weight aggregate concrete is wide for concrete having alow 28-day compressive strength but it sharply decreasses as compressive stregth increases. The band for sand-lightweight concrete is narrower than taht for the all -lightweight concrete for all 28-day compressive strengths. / The creep may be significantly reduced by low pressure steam curing and very greatly reduced by hight pressure steam curing. The reduction for low pressure steam cured concrete may be from 25-40 % of the creap of similar concretes subjected to moist curing. The corresponding values for high pressure steam cured concret may be from 60-80 %. Hight pressure steamed cured concrete has the lowest creep values and the lowes prestree loss due to creep and shrinkaqe. In general lightweight concrtes exhibit higher shrinkage than normal concrete. The order of magnitude for the former propably varies from about equal shrinkage to double that of the later.