Betondaki boyut etkisinin bileşimle ilgisi

Abstract

ÖZET Bu çalışmanın amacı betonun basınç dayanımı ve ultrases hızı üzerindeki boyut etkilerinin ve bu etkilerin beton bileşimiyle ilgilirinin araştırılmasıdır. Deneysel çalışmalar için 6 adet karışım granülometrisi seçilmiştir. Çimento dozajı üretilen tüm betonlar için 400 kg/m alınmıştır. Her granülometri için birbirinden farklı 5 değişik beton üretilmiştir ve üretilen ilk betonun su miktarı saptanırken vebe derecesinin 7-10 sn arasında olması sağlanmıştır. Aynı granülometrideki diğer 4 betonun su/çimento oranları, ilk üretilen betonun oranına her defasında 0.05 eklenerek bulunmuştur. Her karışımdan 4 tane 20x20x20 cm'lik ve 5'er tane 15x15x15 cm ile 7x7x7 cm'lik küp numuneler hazırlanmış tır. Numuneler üzerinde 28. gün sonunda önce ultrases değerleri saptanmış, daha sonra basınç dayanımları ölçülerek, deneysel çalışmalar tamamlanmıştır. Elde edilen değerlere göre ortaya çıkanlar şöyle özetlenebilir. * Numune boyutunun azalması, basınç dayanımı arttır mıştır. * Numune boyutunun azalması ultrases hızımda yükseltmiştir. Ancak bu yükselme, heterojenliğin artışından değil `Numune boyutu/Dalga boyu` oranının değişmesinden dolayıdır. * Agrega karşınımın incelik modülü azaldıkça basınç dayanımının ve ultrasses hızının azaldığı gözlenmiştir. Ancak bu azalmanın kısmen veya tamamen hava boşluğundaki artıştan doğduğu düşünülebilir. * Aynı ultrases hızına sahip numunelerden, daha büyük boyutlu olanının daha yüksek basınç dayanımına sahip olduğu yönünde bir eğilim gözlenmiştir. xiii

SUMMARY SIZE EFFECTS ON THE COMPRESSIVE STRENGTH AND ULTRA SOUND VELOCITY OF CONCRETE The most important qualification in concrete is compressive strength. In various researches carried out on concrete it is seen that some qualification of concrete directly changes parellel to compressive strength. Because of this relations compressive strength of concrete is used as a major of concrete's quality. However the relations between the other qualifications and the compressive strength is not absolute. Control over the quality of concrete is needed in all countries, especialy in our contry. So as in other countries standart the compressive strength of concrete at 7 and 28 days must be greater than the characteristic strength of a given concrete class. The main factors that effect the compressive strength of concrete are; the ratio of water/cement, the quality of aggregate, the cure of concrete, the kind of cement and dosage of cement. Since there is a relation between the compressive strength and size and glometry of the specimen i.e., the height/lateral dimension, there has been a standartisation of size and form in specimens. Order to have a right idea about compressive strenth. As a result of this compressive strength is calculated in experiment that are made on specimens formed as cube and cylinder. The size of the cubes are 20 cm length in Germany Standart and Germany, 15 cm length in England. The diameter and height of the cylinders are 6 inch and 12 inch, respectively in U.S.A., 16 cm and 32 cm in France, and 15 and 30 cm in Turkey. Although 20 cm cubic specimans had been proposed in old addition of Turkish Standarts, 15 cm cubic mould are widely used in the application. Variation of compressive strength with the to form of the specimen is related to fractional forces between thr surface of the specimen and plate of the compressive xivtesting machine. Because of this forces the lateral expansion of the specimen is prevented and dispersion is delayed. Relative to slenderness of specimen (height/diameter) the friction force becomes lessor more. When the slenderness decreases, the frictional forces become active through the whole height of the specimen and prevent lateral expansion, thus compressive strength increases. If slenderness increases, lateral expansion formes freely and the compressive strength becomes less. As in the cylinder specimens the compressive strength goes down by the rise in sizes in cube and prism formed specimens. The values of slenderness effect on the strength of concrete in American and British Standarts are as follows; L'Hermite who reseached the size effect on concrete compressive strength on cylinders that have 16 cm diameter, determined below values. Size effect was investigated on cube specimen by Kuczynski and the values of size effect given below XVSize effect is closely related to the maximum aggregate size. Some standarts bring restrici tions about the specimen size and the maximum aggregate diameter. The size effect in concrete compressive strength is changeable with concretes geometric heterogenity based on aggregate size. Because of this ratio of specimen diameter, which is a characteristic of concrete geometric heterogenity, to avarage aggregate diameter is used in concrete compressive strength. The size effect also changes with the experimental conditions. Such as wall effect, inner tensions formed after tempature changes, size of specimen/max. aggregate diameter, moisure inside and surface of specimens, cure conditions and the rate of loading. Size effect might be doserved differently according to elastic modulus of concrete and compressive strength. If the strength of material is high, the size and form factors have slight effecion experimental result. In the oppositte situation great changes will happen in strength values by the effect of these factors. Size effect of light weight aggregate concrete in compressive strength is less influenced by grading curve of aggregate. That means, size effect on light aggregate concrete has small values compared to normal concrete. There is a close relation between compressive strength and ultrasound velocity in concrete. But we have to know the ultrasound and it's properties. The ultrasound waves have of requency over 16,000 Hz. Sound waves of experiments in order to control the quality of concrete in buildings has a application in handestructive testing technology today. By measuring sound velocity we can have an idea about elastic constant of concrete as well as the strength of it. There are some factors that effect the sound velocity in concrete, for example grading of aggregate, normal changes in grading might change the velocity results belove 12%, but the changes might be more in thick or thin grain size. The overdose of cement also effects sound velocity. In high dosed concrete compressive strength increases but sound velocity dosen't change. The purpose of this investigation is to search the size effects on concrete compressive strength and ultra XVIsound velocity and to find relations between this effects and concrete. For experimental works, six grading curves were chosen. The dosage of cement was taken constant 400 kg/m for all concretes produced. For each aggregate mixture five different kinds of concrete were produced. While calculating the water quantity for the first concrete, attention has been taken that vebe time is between 7 or 10 second. For the other four concrete mixes water/cement ratios were found by increasing the water/cement ratio, first concrete mix in 5% increments. From each mixture four 20x20x20 cm, five 15x15x15 and five 7x7x7 cm cubic specimens were cast. After keeping the specimens in the formwork for one day, they were demoulded and kept under wet cloth for one week and than remained in room tempature until the age of 28 days. After 28 th. day, first ultra sound velocity was measured and than the compressive strength of specimens were obtained so experimental works have ended. The conclusions drived in this study can be given briefly as follows; * Decrease in specimen sizes, increase in compresive strength. ** Smaller specimen sizes, higher the sound velocity but this is not because of the increase of hetorogenity it'is because of the change in the ratio of specimen size to wawe length. * In concretes of which grading curve is between A-B curves or close to B curve, the increase in maximum aggregate diameter decreases the compressive strength. Besides this concrete of which grading curve is between B-C curves, in when maximum aggregate diameter, increases the compressive strength decreases. * In cubes with 20 cm size, maximum aggregate diameter dosen't have an effect on sound velocity. Moreover as the specimen sizes decrease and maximum aggregate diameter climbs, sound velocity goes up. * The increase in ratio of `specimen size/maximum aggregate diameter` causes the compressive strength and the sound velocity drog down. xvn* When the modulus of fineness of aggregate mixture goes down, it is observed that the compressive strength and ultra sound velocity also drop. But it might be estimated that this decrease might be partly or completly because of the increase in air vuids in concrete. * It has been seen that, larger the size of specimen, which has the same sound velocity with the others, greater the compressive strength than the onesin small size. * It has been seen that grading curve dosen't have a specific effect on the relation between compressive strength and sound velocity. xvi i i