Betonun Mod I durumundaki göçmesine agrega konsantrasyonunun etkisi /
Yazar: A. FERHAT SARISU
Danışman: DOÇ. DR. MEHMET ALİ TAŞDEMİR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1996
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Agrega = Aggregate ; Çimento = Cement

Özet:
Bu çalışmada yüksek mukavemetli bir betonda agrega hacim konsantrasyonunun betonun açılma modu (Mod I) durumundaki göçmesine etkisi araştırıldı. Üretilen betonlarda en büyük agrega boyutu, granülometri ve su/çimento oranı sabit tutularak agrega hacim konsantrasyonu değiştirildi. Yüksek mukavemetli beton gibi çekme halinde yarı gevrek davranış sergileyen bir malzemede Mod I halinde gerilme şiddet çarpanını saptayabilmek için Brezilya yöntemi olarak bilinen aynı bir disk numune geometrisinin kullanıldığı deney tekniği seçildi. Mod I halinde çatlak oluşumu ve yayılmasını inceleyebilmek için merkezinde çentik bulunan disk yarma numuneleri kullanıldı. Bu numunelerin merkezlerindeki çentikler üretim sırasında çelik levhalar yardımıyla oluşturuldu ve levhalar priz sona ermeden önce çekilip çıkarıldı ve böylece çelik levhaların yerinde çentik oluşturuldu. Ayrıca üretilen silindir numuneler üzerinde basınç deneyleri de yapıldı. Basınç halinde hem elastisite modülleri hem de basınç mukavemetleri bulundu. Agrega konsantrasyonu arttıkça Mod I halindeki gerilme şiddet çarpanı artmaktadır. Elde edilen sonuçlar aynı mukavemetlerdeki betonlar için literatürde verilen sonuçlara yakın bulundu. Disk numune geometrisinin şimdiye dek önerilen ve üzerinde çok çalışılan çentikli kiriş numuneye bir alternatif olabileceği düşünülmektedir. Disk numunede K| 'in saptanması başarıldığı karot numuneler için de bu geometrinin ayrı bir üstünlüğü söz konusu olabilir. Disk numune geometrisi için Boyut etkisi Modeli veya İki Parametreli Modelin kullanılması halinde belirli bir betonda boyuttan bağımsız K] kritik değeri (K|C) bulunabilir. Böylece yükün kritik değeri de farklı boyutlar için hesaplanabilir. Agrega hacim konsantrasyonu arttıkça basınç mukavemeti başlangıçta hızla azalmakta daha sonra sabit kalma eğilimi sergilemektedir. Üretilen betonların basınç altında bulunan elastisite modüllerinin agrega konsantrasyonunda^ artma ile belirgin biçimde arttığı görülmektedir. Öte yanda betonların elastisite modülleri iki fazlı kompozit malzeme modeli yardımıyla hesaplandı ve elde edilen sonuçların deneysel değerlere çok yakın olduğu görüldü. Bütün çentikli disk numunelerde agrega hacim konsantrastonu arttıkça yarılma çekme mukavemetleri artmaktadır. Bu artışlar kısa çentikli disk numunelerde daha belirgindir. Çatlak önce çentik uçlarında oluşmakta, düzleminde yayılarak ilerlemekte ve yük uygulanan noktalarda çatlaklar çatallanmakta ve kama oluşumu gözlenmektedir. Hamur veya düşük konsantrasyonlu betonlar lineer elastik teoriye daha uygun sonuç vermekte, agrega konsantrasyonu arttıkça beton daha sünek davranış göstermekte ve kırılma süreci bölgesi özellikle kısa çentikli halde teoriden belirgin biçimde farklı çıkmaktadır.

Abstract:
Since concrete is widely used material in structural engineering, engineers need sufficient information about crack initiation and propagation in the material. The information is especially needed for finite element studies of concrete behaviour. Although intensive studies on fracture of concrete have been conducted during the past two decades, many fundamental aspects on fracture process in concrete still remain unclear because of heterogeneity of concrete materials. Further progress in this field may primarily depend on broad cooperation and development of new experimental techniques. Concretes of compressive strength exceeding 80 N/mm 2 are now commonly been used in the construction of some important structures such as long span bridges, dams, nuclear reactors and off shore structures. In some countries it is now possible to produce concrete with a compressive strength of 115 N/mm 2. Typical high strength concrete has a matrix that is very strong and stiff, is compact, and possesses well-bounded aggregate-mortar interfaces. Due to its composition, several of the toughnening mechanisms found in normal concrete are absent during the fracture process. Microcracking at interfaces, flaws, and voids are infrequent, and cracks propagate through the coarse aggregates instead of being deflected by them. Several researchers observed these differences and concluded that as its compressive strength increases, concrete behaves more like a homogeneous material. The decrase in toughening leads to an increase in brittleness. Designers have been forced to confine high strength concrete with steel in order to prevent catastrophic failure, especially under seismic loading. J^bviously, a less brittle material would make design more economic and safer. There is a growing interest in the field of fracture mechanics of concrete. Draft recommendations have been proposed : to determine fracture energy using a notched beam test, determination of fracture parameters (critical value of Mode I stress intensity factor and crack tip opening displacement) of plain concrete using three-point bend tests and size-effect method for determining fracture energy and process zone size of concrete. Studies to determine the influence of test set-up and geometry of specimens on various materials parameters are being conducted by various technical committies. XIHowever, there is no acceptable testing technique to determine fracture parameters of concrete. Recent advances show that a single fracture parameter, such as the critical stress intensity factor can not be simply applied to quasi-brittle materials such as concrete. This inapplicability of conventional fracture mechanics is due to determine fracture process zone and precritical crack growth. In order to determine the size independent fracture parameters, nonlinear type of fracture mechanics models based on observations of fracture processes using well developed experimental techniques will be used in the near future. Generally, the critical value of the parameter during fracture can be determined with laboratory specimens. Hence, the value obtained as a material property can be used in a wide range of engineering structures of different geometric shapes and sizes. Accuracy in the determination of fracture parameters is needed for readable predictions of crack growth and crack propagation in structures. Due to the difficulty in applying direct tension to concrete, the so-called Brazilian test or split-cylinder test is widely used for determining the tensile strength of cement-based materials. In this test, a cylinder is loaded in compression diametrically between two platens. The split-cylinder test method to determine tensile strength is attributed to Camerio and was adopted by the national standard method of Brazil, thus the method was named as "Brazilian Test". The loading produces a nearly uniform maximum principal tensile stress along approximately seventy five percent of the diameter which causes the cylinder to fail by splitting. The tensile strength at failure is calculated using the theory of elasticity and ignoring the effect of multiaxial state of stress near loading platens. The tensile stress value at failure has proven to be useful measure of tensile strength, although the state of stress is not uniaxial due to significant compressive stress in the transverse direction and there is redistribution of stress due to nonlinearity of materials. Recent experimental test results have indicated that the flexural strength of concrete usually decreases with increasing size of specimens and then remains constant. The size effect on concrete strength may be primarily explained by the fracture process zone. When a concrete structure is loaded, the strain energy produced by applied load is converted to the energy absorbed in the fracture process zone. For large sized structures, the latter is negligible compared to the former, whereas for small sized structures, the latter can be comparable compared to the former. However, the concrete strength approaches a constant when sizes of concrete structures become very large. Since concrete is weak in tension, its tensile strength is frequently neglected in the conventional design of reinforced concrete structures. In recent years, research on concrete have primarily concentrated on increasing its compressive strength, however, more information needed for many aspects of mechanical properties such as fracture behaviour and tensile properties. MlThe information about tensile properties of concrete is required for calculating the response of a structure and especially for determining the risk of cracking. The wide use of modern computer-aided analysis in design and use of concrete for special structures mentioned have let to growing interests in cracking behaviour of concrete. It is expected that in the next century more sophisticated approaches will be used in design and as a result, more research will be required on concrete for a better understanding of its material behaviour. The main objective of this work was to investigate the influence of aggregate volume concentration on the fracture parameters of concrete in Mode I loading condition. After a brief introduction to the notched cylinder and the experimental procedure employed, the observed modes of failure of the cylinder are discussed. This is followed by a quantitative description of fracture and notch effect, and a critical examination of alternate ways of computing Ki stress intensity factor. The aggregate volume concentrations have been changed in the range of 0%-60%. The aggregate grading of concrete, water/cement ratio and the maximum particle size of aggregate were 0.32 and 8 mm, respectively. The compressive strength and modulus of elasticity were determined according to standard procedures using 100 mm diameter and 200 mm long cylinders. Notched disk specimens (the height =60 mm and the diameter =150 mm, and the height =50 mm and the diameter =100 mm) were cast in steel molds to simplify notched. A strip of thin sheet metal was left in the diameter of the fresh concrete to form the notch. The steel sheet was removed before setting for easy removal. The ratio of a/R was chosen as 0.1, 0.3 and 0.5 where 2a» length of notch and 2R= diameter of the disk specimen. Test results obtained are evaluated in terms of following properties: fresh concrete properties, elastic and fracture properties. The static moduli of elasticity were calculated from the ascending part of stress-strain curves in compression for stresses below approximately 30 percent of the ultimate strength using standard cylinder specimens. The objective of this thesis has been to investigate the effect of aggregate volume concentration on the fracture parameters of concrete using the notched cylindrical specimens and the hypothetical material property of fracture toughness as defined in the context of the Linear Elastic Fracture Mechanics. From theoretical and experimental analysis of models of the notched cylinder, and from experiments on concrete disks of various axial and initial notch lengths, several conclusions may be drawn This thesis consists of six parts: In the first part, an introduction is given containing the objective of the investigation and relevant general information. In this part the aim and scope of the present work are also given. The second part is devoted to a survey of the present knowledge on the mechanical behaviour of high strength concretes. MilThe third part gives the experimental studies. The material used, the principles assumed, the mix compositions, the methods of mixing and curing, the types of loading and the methods employed in testing and measurements made are described. The fourth and fifth parts are devoted the experimental results. In the sixth part the experimental results are disused and evaluated in the light of experimental findings. In the seventh part the conclusions are summarized and suggestions for further studies are given. The results obtained in the experimental work can be outlined as follows: 1) Aggregate concentration has an important effect on the modulus of elasticity, such that, when it increases the modulus of elasticity of concrete increases significantly. 2) As the aggregate volume in the mixes increases, the splitting-tensile strength also increases. This increase is significantly high in short notched specimens. 3) The compressive strengths of concrete decreases, when the aggregate concentration increases. The strength first decreases dramitically, then a plateau is obtained with the increase in aggregate volume concentration. It can be concluded that the stress concentrations increase at the weak matrix- aggregate interfaces as the volume of aggregate in the mix increases. This increase in stress concentration may be responsible of the drop of the compressive strength. 4) As the aggregate volume concentration increases, the Mode I stress intensity factor calculated based on LEFM increases significantly. Experimental test results show that a single fracture parameter is not enough to describe fracture of concrete. It is obvious that more than one parameter, or at least two parameter is required. 5) A central crack initiates at the tip of a notch propagates towards the loading points, and is followed by wedge formation. It has been observed that the wedge formation in the disk test configuration constitues a secondary failure mode and occurs after a single crack has propagated through the diameter of the specimen. 6) For the modelling purposes, the modulus of elasticity of concretes can be estimated from the modulus of aggregate and cement paste phases by using composite models. The predicted values showed reasonable approximations to the experimental results. XIV7) It can be concluded that fracture process zone becomes more important in specimens with shorter initial notches, thus making them more ductile behaviour with the increase in aggregate volume concentration. Future resarch, both theoretical and experimental, should establish criteria for crack initiation and propagation at every instance in an experiment. At this point, useful directions seem to be related to the use of crack tip opening displacement and rotation between two cracked parts, measured using CMOD (Crack Mouth Opening Displacement) and CTOD (Crack Tip Opening Displacement), for determining the crack length and the onset of crack propagation. The fracture toughness may then be calculated from this length and the corresponding load, and evaluated with respect to order specimen and material parameters. Because of abserved ductility and different crack propagation rates, the dynamic aspects of crack propagation, continuous stress redistribution ahead of a crack tip and the size effect of specimen on the fracture parameters of concrete should also be investigated. The notched cylinder is a good fracture mechanics specimen with many advantages, and future investigations of cracking in concrete may be carried out on this specimen with a strong potential for standardization The limitations of the present work also suggest new directions for further studies on combined mode fracture of quasi-brittle materials like concrete. In testing such a disk shaped specimen with inclined crack subjected to diametral compression test, there are the following advantages: I) By changing the inclination of the notch with respect to the loading direction, it will be possible to change the fracture mode from Mode I (tensile) to Mode II (shear) and to obtain combined loading condition with a wide range of K|/K!C versus Kn/K|C, where K» is the mode stress intensity factor, and K( is the Mode I stress intensity factor. The value of K)c (critical value of stress intensity factor in Mode I) is a material parameter and may depend on the rate of loading. These efforts will help in better understanding of crack growth in concrete under combined Mode I and Mode II loadings. ii) It is not diffucult to obtain higher Kn/K, ratios using this type of loading configuration. One can also obtain both positive and negative values of Kıı/Kı ratio. The same specimen can be used to obtaine test results in pure Mode I and combined Mode cases. iii) To obtain loading rate effects on both Mode I and mixed mode fracture of concrete, crack propagation studies should also be used at slow and high rates. XV