Sentetik lif,çelik lif,çelik hasır,burgu çelik içeren betonların mekanik özelliklerine yüksek sıcaklığın etkisi
The effect of high temperature on mechanical properties of concrete containing synthetic fiber, steel fiber, steel wire mesh, auger steel
Yazar: OSMAN ALPTEKİN ÖZDEMİR
Danışman: DOÇ. DR. HASAN YILDIRIM
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Betona eklenen katkı malzemeleri, servis ömrü süresince betonun eğilme, çekme dayanımı, kayma dayanımı, yorulma dayanımı, deformasyon kapasitesi, aşınma direnci,  çatlama sonrası yük taşıma dayanımı, enerji yutma kapasitesi, parçalanma direnci (darbeli yükler), vb. gibi betonun zayıf olan mekanik özelliklerini iyileştirmek amacıyla kullanılmaktadır. Kullanılan betona eklenen katkı malzemelerin bahsedilen mekanik özelliklere olan etkilerinin, yapının kullanım amacı ya da yangın sebebi ile yüksek sıcaklığa maruz kalması halinde yine bu mekanik davranışlara olan etkilerinin nasıl olduğu da dikkate alınması gereken bir durumdur. Bu nedenle inşaat teknolojisinde uzun yıllardır, yüksek sıcaklık etkisinde betonarme yapının performansını geliştirme amacıyla betona eklenerek istenilen performansı sağlayabilecek katkı malzemeleri üzerine çalışmalar yapılmaktadır.
Bu araştırma konusu kapsamında, betonda kullanılan katkı malzemelerinin betonun yüksek sıcaklık altında mekanik davranışlarına olan olumlu veya olumsuz etkilerinin gözlemlenmesi ve bu malzemeleri içeren betonların mekanik davranışlarının birbirleri ile kıyaslanması amaçlanmıştır.
Yapılan deneylerde, deney betonuna; sentetik lif, çelik lif, çelik hasır, burgu çelik ilave edilmiş ve tüm bu katkı malzemelerine ek olarak katkı malzemesi içermeyen numuneler hazırlanmıştır. Hazırlanan numuneler eğilmede çekme dayanımını tayin etmek amacı ile 23x48x10cm boyutlarında olup 28 günlük basınç değerlerini kontrol amaçlı 15x15x15cm ölçülerinde küp numuneler hazırlanmıştır.
Numuneler fırın sıcaklığının deney sıcaklığına ulaşması için geçen kontrollü yükselme zamanı dâhil kararlı durumdaki ısıtılma süresi olarak 6 saat süre ile 100 °C ve 200 °C sıcaklıklarına maruz bırakılmıştır. Tüm katkı malzmeleri içeren betonların kendi içlerinde yüksek sıcaklık etkisine karşı davranışlarının tayini adına her bir numune grubu için 2 adet 100 °C'lik, 2 adet 200 °C'lik fırına koymak üzere ve 2 adet oda sıcaklığında 25 °C'de tutulmak üzere numuneler üretilmiştir. 6 saat süre ile belirlenen sıcaklık değerlerinde tutulan numuneler etüv içerisinde oda sıcaklığına erişinceye kadar bekletildikten sonra gerekli hazırlıklar yapılarak deneye tabi tutulmuştur.
Tez çalışması ile elde edilen deneysel veriler sonucunda, eğilme altında maksimum taşınabilecek yük konusunda betona çelik hasır ve burgu çelik ilave etmenin olumlu sonuçlar doğurduğu, sentetik ve çelik lif ilavesinin bu konuda beton özelliklerine olumlu etkisinin olmadığı, eğilme sırasında enerji yutabilme kapasitasi olarak tüm katkı malzemelerinin faydasının olduğu görülmüş, ancak  sıcaklık artması durumunda sentetik lif içeren numunelerin tokluk değerinde azalma saptanmıştır.

Abstract:
While the service life of the structure concrete additives, bending, tensile strength, fatigue strength, deformation capacity, shear strength, post-cracking load bearing strength, abrasion resistance, energy absorption capacity, fragmentation resistance (impact loads), etc. It is used to improve the weak mechanical properties of concrete. Since concrete is a poor material in terms of thermal conductivity, it is likely that cracks will occur in the concrete as the temperature difference between the inside and outside of the concrete will be high during sudden heat increases. The behavior of concrete material against high temperature depends on the thermal properties of the concrete such as specific heat and thermal conductivity, and temperature application conditions.The effects of the additive materials added to the used concrete to the mechanical properties should be taken into consideration as well as the effects of the structure on the mechanical behavior if the structure is exposed to high temperature due to its intended use or fire.
As the physical and chemical properties of cement paste under the influence of temperature have changed significantly, it is the most sensitive material against temperature among concrete materials. Since water is a phase changing material at temperatures like 100-150 °C, it has an important place in the behavior of cement paste under high temperature effect. Chemical decomposition and water loss are predominant at temperatures of 100 °C and above, while thermal changes at temperatures of 600 °C and above become important. Water loss and chemical decomposition in cement paste cause changes in the microstructure of cement paste. The porosity of cement paste, homogeneity of pore distribution, cohesive forces in its internal structure (Van der Waals) and chemical bonds are altered.
During the process of exposure to temperature, the size of the cracks in the concrete increases in the cooling process. The reason for this significant volume increase in concrete is the decomposition of Calcium Hydroxide in concrete. Fly ash, pumice, silica fume, etc. used as cement additives. It has been found that the materials help to reduce Calcium Hydroxide due to the pozzolanic reaction in the cement paste and thus prevent some growth of cracks in cooling. However, when the temperature exceeds 500 °C, Calcium Hydroxide starts to decompose due to the breakage of the bonds in its structure, so when the temperature exceeds this value, the physical and chemical properties of the concrete deteriorate. This necessitates the addition of additives in order to maintain the function of the structure under the influence of high temperature.

Within the scope of this research, it is aimed to observe the positive or negative effects of the additives used in concrete on the mechanical behavior of concrete under high temperature and to compare the mechanical behavior of the concrete containing these materials with each other.
In the experiment, the test concrete; Synthetic fiber, steel fiber, steel mesh, auger steel were added and samples without additives were added in addition to all these additives. The samples were prepared as 23x48x10 cm in order to determine the tensile strength of the bending specimens and 15x15x15 cm cube samples were prepared for the purpose of controlling the pressure values of 28 days.
The samples were exposed to the temperature of 100 °C and 200 °C for 6 hours as a continuous heating period including the controlled rise time for the furnace temperature to reach the test temperature. In order to determine the behaviors of the concretes containing all the additive materials against high temperature effects, samples for each sample group to be placed in 2 100 °C, 2 200 °C furnaces and 2 at room temperature for 25 °C. it is produced. The samples kept at the temperature values determined for 6 hours were kept in the oven until they reached room temperature and then they were subjected to the necessary preparations.
The effects of synthetic fiber, steel fiber, steel mesh and auger steel concrete admixtures on the properties of concrete in fresh concrete form and mechanical properties under the influence of temperature were investigated according to the results of the researches and experiments obtained in this study;
1. The use of synthetic and steel fibers had a negative effect on machinability by reducing the flowability of fresh concrete. There is no effect on machinability for steel mesh and auger steel materials.
2. According to the results of the ultrasound test, low temperature of ultrasonic sound was observed as temperature increased in samples containing all additives. A similar rate of decrease was observed in synthetic and steel fiber samples, whereas the rate of decrease in ultrasonic sound velocity was higher depending on the temperature at which the samples containing steel mesh and auger steel were held.
3. According to the results of the bending test, 25 oC temperature applied sample; The same bending strength values were observed in the samples containing blank, synthetic fiber and steel fiber, whereas the bending strength values of the samples containing steel mesh and auger steel were higher.
4. Bending strength; Between 25 oC and 100 oC, there was no decrease in the sample and steel mesh samples, but the decrease in the synthetic and steel fiber samples. Since the bending value increase at 100 oC was observed for the auger steel containing samples, it was commented that there was no change in strength value between the samples held at 25 oC and 100 oC, considering that the samples held at 25 oC could be reduced due to different reasons.
5. 200 oC temperature applied samples; 21,87 % of the witness sample, 16,82 % of the synthetic fiber sample, 17,57 % of the steel fiber samples were observed to decrease in strength, bending strength of the samples containing steel mesh and auger steel did not decrease.
6. Bending energy absorption capacity comparisons according to the sample under 100 oC temperature; 4 times for synthetic fiber samples, 5 times for steel fiber samples, 11 times for auger steel samples and 15 times more for steel mesh samples.
7. For samples with 100 oC temperature application; while there was no decrease in toughness in the samples, synthetic fiber, steel mesh and samples, toughness increase was observed in steel fiber and auger steel samples.
8. While the toughness of the samples and synthetic fiber samples decreased at     200 oC, the toughness of steel fiber and twisted steel samples increased. On the other hand, it was found that the values of toughness were preserved in the samples containing steel mesh.
9. In the future, by increasing the number of samples studied and working with higher temperature values, the results can be obtained under more precise and different conditions.