Betonun ultrasonik tomografi yöntemi ile incelenmesinde frekansın etkisi
The effect of frequency on the analysis of the concrete by ultrasonic tomography method
Yazar: MUSTAFA ALTUĞ PEKER
Danışman: DOÇ. DR. BEKİR YILMAZ PEKMEZCİ
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:
İnşaat mühendisliği uygulamalarında en fazla kullanılan malzemelerden biri olan beton; günlük hayattaki birçok önemli yapıda bulunmaktadır. Bu önemli yapıların kullanım ömrünün ve güvenliğinin doğru şekilde belirlenmesi gerekmektedir.
Yapıların mevcut durumunu ve yapının kalitesini belirlemek için tahribatlı ve tahribatsız muayene yöntemleri kullanılmaktadır. Tahribatsız muayene yöntemleri, tahribatlı yöntemlere göre maliyet ve zaman açısından daha kullanışlıdır. Bu tez kapsamındaki deneylerde kullanılan Ultrasonik Tomografi yöntemi yapıdaki nesneleri, arayüzleri ve anomalileri tespit etmek için ultrasonik dalgalar kullanan modern bir tahribatsız yöntemdir.
Deneylerde kullanılmak üzere üç adet 50x50x22 cm boyutlarında beton numune üretilmiştir ve bu numunelerin içerisine farklı malzemeler yerleştirilmiştir. Her bir numune üzerinde dört farklı frekans değerinde deneyler yapılmıştır.
Yapılan deneyler sonucunda numunelerin kalınlık değeri üç numune için de 25, 50 ve 100 kHz frekanslarında %91 doğrulukla tespit edilmiştir. 200 kHz değerinde ise numunelerin kalınlığı hakkında net bir yorum yapmak mümkün olmamıştır. Numune-1 içerisindeki metal levha, 5,5 cm ve 6,5 cm çapındaki toplar ise sadece 25 ve 50 kHz değerlerinde görülebilmiştir. Sırasıyla %89, %85 ve %79 ortalama doğruluk oranları elde edilmiştir. Numune-2'nin içerisine yerleştirilen 24 mm, 16 mm ve 8 mm çaplı beton çelikleri de 25 ve 50 kHz değerlerinde %89, %88 ve %94 ortalama doğruluk oranlarıyla görüntülenmiştir. Numune-3 içerisindeki metal levha hiçbir frekans değerinde görülemezken, çelik lifler ve cam bilyeler ise 25 ve 50 kHz değerlerinde sırasıyla %79 ve %87 ortalama doğruluk oranlarıyla belirlenmiştir.
Deney sonuçlarına göre; beton malzemelerde yapılacak Ultrasonik Tomografi deneylerinde düşük frekanslar kullanılmalıdır. 25 kHz ve 50 kHz frekansları malzeme boyutlarının belirlenmesini yüksek doğrulukla yapabilmekteyken 100 kHz frekans değeri sadece yapı boyutlarının belirlenmesi için yeterli kalmıştır. Yabancı madde boyutlarının belirlenmesinde 25 kHz daha yüksek doğruluk vermişken, beton çeliği boyutları 50 kHz eğerinde daha yüksek doğruluk oranlarıyla belirlenmiştir.
Bu çalışma kapsamında yapılan deneyler sonucunda Ultrasonik Tomografi yönteminin beton malzemelerde yüksek doğrulukla sonuç vereceği sonucuna ulaşılmıştır. Bu yöntem bütün beton ve betonarme yapılardaki kusur ve anomali tespitinde yapıya hasar vermeden kullanılabilir ve sorunların tespit edilmesiyle daha sonra çıkabilecek sorunlara önceden önlem alınması sağlanabilir.

Abstract:
Concrete is one of the most widely used material for civil engineering applications. There are many important concrete structures used in the daily lives of human beings such as buildings, roads, tunnels, bridges, harbors, and dams. Due to the widespread use of concrete structures in every branch of life, it has great importance for humanity to accurately determine the performance, service life and safety of these structures.
Turkey is located in an active earthquake-prone region and the structures in this country are continuously faced with natural disasters. In consequence of these disasters, civil engineering structures need repairment, maintenance and strengthening on a regular basis. Therefore, it is very important to determine the quality and performance of these structures and the condition of the damages and defects in order to perform the maintenance, repairment, and strengthening in the correct way.
Destructive and non-destructive testing methods are used to determine the performance and quality of the concrete structures. Due to the nonhomogeneous characteristics of these materials, it is difficult to observe the differences and discontinuities in properties with traditional destructive testing methods. Destructive testing methods damage the structure or material and these testing methods take a long time since the specimens taken from the structures are sent to the laboratories for the test results.
Non-destructive testing methods are more practical in terms of cost and duration of testing compared to destructive testing methods. This methods also come to the forefront with its on-site inspection without damaging the structural member. By means of the non-destructive testing methods, the entire structure can be examined faster and economical without the requirement of core samplings. Non-destructive tests can be applied to the same elements and structures several times and at different times and give immediate results. Standardization of non-destructive methods has begun in many countries.
Among the numerous non-destructive testing methods, acoustic non-destructive methods become prominent as an innovative and useful technique for structures in the civil engineering applications. During recent years, the use of non-destructive acoustic methods in civil engineering researches and developments has been increasing.
Ultrasonic tomography method can be classified as one of these acoustic non-destructive techniques. To explain briefly; in this method, the flight time of the elastic shear waves reflected by the discontinuities in the material is measured and recorded by an ultrasonic tomography device. This device generates, sends and receives the elastic shear waves by the transducer arrays that has both source and receiver functions. A visual image of the discontinuities where the sound waves are reflected is obtained from the measured flight times. A mathematical algorithm named Synthetic Aperture Focusing Technique (SAFT) is used to reconstruct the images.
In the scope of this thesis, three pieces of 50x50x22 cm concrete specimens were produced for ultrasonic tomography experiments. Four different types of aggregates were used in the specimens and the maximum size of these aggregates was 22 mm. CEM 1 PC 42.5 cement was used in the production of the concrete specimens. The cement dosage in the concrete mixture is 387 kg / m3 and the water / cement ratio used in the mixture is 0.51. The amount of super plasticizer is taken as 1% by weight of cement.
The concrete specimens produced within the scope of the study were named as Specimen-1, Specimen-2, and Specimen-3 according to the order of the experiments. Different materials such as plastic balls and reinforcement bars are placed in these three concrete specimens. Specimen-1 has two different diameters of plastic balls and a metal plate. Specimen-2 has three different diameters and lengths of concrete reinforcing steels and Sample-3 has a metal plate, a bag of steel fibers and glass balls.
Ultrasonic tomography experiments were performed on each specimen with the frequencies of 25 kHz, 50 kHz, 100 kHz, and 200 kHz. In the experimental studies, A1220 Monolith was used as the ultrasonic tomography test device. The specimens were marked with 6x3 cm intervals in order to perform the test properly. After the measurements were made at each point of the specimens, the obtained data was transferred to the computer.
As a result of the ultrasonic tomography experiments, the thickness of the specimens was determined with 91% accuracy rate at the frequency levels of 25 kHz, 50 kHz and 100 kHz for the three specimens, but it was not possible to make a clear comment about the thickness of the specimens at the frequency level of 200 kHz. At th3 frequency level of 200 kHz, the thickness value could only be determined with an accuracy of 73%. The metal plate in the Specimen-1, the plastic ball with the diameter of 5,5 cm and the plastic ball with the diameter of 6,5 cm were only observable at 25 kHz and 50 kHz and the average accuracy rates were 89%, 85%, and 79%, respectively.
Concrete reinforcing steels of 24 mm, 16 mm and 8 mm diameters placed inside the Specimen-2 were also visualized with the average accuracy rates of 89%, 88% and 94% at 25 kHz and 50 kHz frequency levels. While the metal plate in the Specimen-3 could not be visible at any frequency level; the steel fibers and the glass balls were determined with the average accuracy rates of 79% and 87% at 25 kHz and 50 kHz frequency levels.
According to the obtained experiment results; lower frequencies should be used in Ultrasonic Tomography method to be performed in concrete materials or structures. The frequency levels of 25 kHz, 50 kHz, and 100 kHz can be used to determine the material dimensions with high accuracy rates, while the 200 kHz frequency level is not satisfactory to determine the structural dimensions of concrete materials. Only the thickness is determined at this frequency level of 200 kHz and the accuracy is low.
It can be said that the frequency of 100 kHz is only useful in determining the material dimensions. The concrete reinforcing steel bars dimensions were determined with higher accuracy rates of 50 kHz frequency level, while 25 kHz frequency gave higher accuracy rate in determining the dimensions of the non-structural objects in the concrete specimens.
The sample images obtained in the experiment are only in some colors. These colors range from black to red according to their reflectivity. The colors used are black, blue, green, yellow and red. These colors do not provide information about the material and only indicate discontinuity.
As a conclusion of the experiments conducted within the scope of this study, it was concluded that the Ultrasonic Tomography method will give the high accuracy results in the concrete materials. This method can be used to detect defects and anomalies in all concrete and reinforced concrete structures without damaging the structure and can provide the determination of structural performance and, especially quality.
The thickness of all concrete and reinforced concrete structures can be determined with this method. The void and other defects in the concrete can be measured with certainty. Ultrasonic tomography is a preliminary study. Investigations into this method should continue and be developed in order to obtain accurate non-thickness properties.