İnce malzeme oranı ve cinsinin 10 mm iri agregalı kendiliğinden yerleşen betonların durabilite özelliklerine etkisi
Effect of types and amount of fine materials on the durability properties of self-compacting concrete with 10 mm coarse aggregate size
Yazar: SİDAR NİHAT BIÇAKÇI
Danışman: PROF. MUSTAFA HULUSİ ÖZKUL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Mühendislik Bilimleri = Engineering Sciences ; İnşaat Mühendisliği = Civil Engineering

Özet:
En genel anlamda beton, su, çimento, agrega ve belirli katkı malzemelerinden oluşan kompozit bir malzemedir. Kendiliğinden Yerleşen Beton ise yüksek akıcı kıvama ve yüksek vizkoziteye sahip, hiçbir yerleştirme işlemi gerektirmeksizin kendi ağırlığı ile yerleşebilen özel bir beton çeşididir. Kendiliğinden Yerleşen Beton ilk olarak 1986 yılında Okamura tarafından bulunmuştur. Su altı beton uygulamalarında kullanılmaya başlanmıştır. Daha sonrasında ise kullanım alanı gittikçe artmıştır. KYB'ler normal betondan farklı olarak daha yüksek akıcı kıvama sahiptir. Bu akıcı kıvamı sağlamak için genel olarak polikarboksilat kökenli yeni nesil süper akışkanlaştırıcı katkı kullanır. Betonda akıcılık arttıkça ayrışma ihtimali artar. Bu nedenle vizkozitesi yüksek beton üretmek gerekir. KYB'lerde yüksek vizkozite elde etmek için ince malzeme miktarı yüksek tutulur veya vizkozite düzenleyici katkı kullanılabilir. İnce malzeme miktarı sadece çimento ile sağlandığında yüksek hidratasyon ısısından dolayı belirli problemler ortaya çıkabilir. (kuruma ve termal rötre gibi) Bu nedenle KYB'lerde genellikle çimentoya ek olarak mineral katkılar kullanılır. 
Beton değişik boyutlarda ve miktarda boşluk içeren bir malzemedir. Bu boşluklardan betonun içine sıvı veya gaz girişleri olur; buna betonun geçirimliliği denir. Betonun boşluklarından sıvı veya gaz girişi difüzyon, basınç altında su emme veya kılcallık yoluyla gerçekleşebilir. Betonun geçirimliliği boşlukların miktarına, çapına, dağılımına ve birbirleriyle ilişkili olmasına bağlıdır. 
Betonun geçirimliliği betonun dayanıklılığında en önemli etkendir. Uzun yıllar boyunca betonun mekanik dayanımı uygun bir beton üretimi için yeterli bir koşul sayılmıştır ve ufak onarımlarla çıkabilecek sorunların üstesinden kolayca gelinebileceği düşünülmüştür. Ancak günümüzde ̏betonun kendisinden beklenen performansı hizmet süresi boyunca göstermesi ̋ olarak tanımlanan dayanıklılık kavramı, yapının bulunduğu ortam şartlarına göre malzeme seçimi ile en uygun bileşimin belirlenmesi, buna ek olarak asgari bakımın ihmal edilmemesi gerekliliğini ortaya koymuştur. 
Bu çalışmada kalker tozu ve uçucu kül olmak üzere iki farklı mineral katkı içeren KYB'ler üretilmiştir. Üretilen betonlarda çimento ve su miktarları sabit tutulup, mineral katkı hacmi m3'te 0-36-72-108 dm3 şeklinde arttırılmıştır. (Her iki mineral katkı türü için) KYB'lerde maksimum agrega çapı ise 10 mm'dir. Bu karışımlara ek olarak aynı su ve çimento dozajına sahip, bağlayıcı içermeyen maksimum agrega boyutu 22 mm olan bir normal beton da üretilmiştir.
KYB'lerde taze beton deneyi olarak yayılma deneyi ve birim ağırlık deneyi yapılmıştır. Normal betonda ise çökme deneyi ve birim ağırlık deneyi yapılmıştır. KYB'lerde yayılma çapının minimum 650 mm olması istenmiş ve üretim esnasında bu şart dikkate alınarak üretimler gerçekleştirilmiştir. Normal beton ise çökme deneyi sonucunda S3 sınıfında çıkmıştır.
Sertleşmiş beton üzerinde yapılan deneyler şunlardır; Basınç deneyi, kılcal su emme deneyi, hızlı klor geçirimliliği deneyi, vakumlu su emme deneyi, karbonatlaşma deneyi, yüzeyde elektriksel özdirenç deneyi, donma-çözünme deneyi, magnezyum sülfat deneyi ve basınçlı geçirimlilik deneyi.
Deney sonuçları incelendiğindeyse; genel olarak hem kalker tozu hem uçucu küllü KYB'lerde bağlayıcı miktarının artmasının durabiliteyi olumlu anlamda iyileştirdiği söylenebilir. Ayrıca karbonatlaşma deneyi hariç, uçucu küllü karışımlar kalker tozuna göre daha iyi sonuç vermiştir. Hızlı klor geçirimliliği deneyi ve yüzeyde elektriksel özdirenç deneyinde kalker tozu miktarının artması iletkenliği arttırmıştır. KYB'ler normal betona göre durabilite açısından daha olumlu sonuç vermiştir.

Abstract:
In most general sense, concrete is a composite material, which contains water, cement, aggregate and mineral and chemical admixtures. Self-Compacting Concrete (SCC) with high flowing property is a special concrete type due to its compacting ability under its own weight without vibration. SCC was found by Okamura in 1986 and was used in underwater concrete applications. Then the area of using SCC has increased. SCC has a more fluent consistency than normal concrete. In SCC production, new super plasticizer based on polycarboxylate is used to high fluent consistency. As the flowing increases in concrete, the possibility of segregation may also increase. Therefore, it is necessary to produce concrete without reducing viscosity to low levels. For preventing the segregation, the amount of fine material should be high or a viscosity modifying admixture should be added. When only using of cement as a fine material, some obvious problems may occur due to high heat of hydration. That's why mineral additive is used addition to cement in SCC. 

Concrete is a material which contains different amounts of pores in various sizes. Penetration of liquid and/or gas into the concrete occurs because of these pores. This phenomenon is called as permeability of concrete. Penetration of liquid and/or gas into the pores is occured by diffusion, absorbtion under pressure or capillary absorption. Permeability of concrete depends on amount, distribution, size and connectivity of pores within the concrete.

Permeability is the most important factor in durabilty of concrete. It has been widely accepted for many years that mechanical strength of concrete is adequate enough for a structure and little maintenance would be sufficient to overcome any problems during the service life. However, it is well known that in order to obtain a durable concrete showing the excepted performance throughout its service life, selection of suitable materials according to environmental conditions, the best proportion of concrete mixture and sufficient curing are the most important steps which should be considered. 

In this experimental study, SCCs were prepared with limestone fine material content between 350-650 kg/m3 (for fly ash added SCCs 350-596 kg/m3) changing with 100 kg (for fly ash 82 kg to keep the volume constant) increments. In all production water/cement ratio is constant (water=178 kg, cement=350 kg) but volume of fly ash and/or limestone powder was increased at a constant rate (0-36-72-108 dm3). SCCs have 10 mm maximum aggregate size. Also a normal concrete consisting only cement as powder were prepared with 22 mm maximum aggregate sizes.

As fresh concrete test, unit weight was performed on all concretes produced. To measure fluency, slump flow test was performed on SCCs and slump test was performed on normal concrete. 
While producing SCC, result of slump flow test taken into account as  minimum 650 mm. Normal concrete were produced in S3 class as a result of slump test.     




The experiments carried on hardened concretes are as follows; compressive strength test, rapid chloride permeability test, capillary water absorbtion test, permeability under pressure test, water absorbtion under vacuum test, carbonation test, electrical resistivity test on surface, freeze-thaw test and magnesium sulfate resistance test.

When the fresh concrete test results are examinated; according to flow test, it is seen that limestone powder needs more superplasticizer than fly ash the amount of mixture taken into account. Similar results have been obtained in studies on this object. It is because the shape of the fly ash particles is round but that of limestone is angular, as seen under microscope, so it is more easy for the SCC with fly ash to consolidate and compact than that with limestone powder. As a result of unit weight test, mixtures containing limestone powder gave the highest result due to the high spesific gravity of limestone powder.

According to the results of compressive strength test, capillary water absorbtion, permeability under pressure  and water absorbtion under vacuum tests; the increase in the amount of fine material (cement+limestone powder or fly ash) in both types of powders positively affected the impermeability performance of SCCs. This result can be due to the filling effect of both powders; furthermore fly ash shows pozzolanic, which also reduces the porosity of a concrete. 


Results of freeze-thaw test shows that; as the amount of fly ash increases, due to filler and pozzolanic effect, SCCs becomes more resistant to the freeze-thaw cycles. On the contrary, for the limestone powder added mixtures the resistance of the SCCs against freezing thawing effects reduced when the amount of the powder is increased. When similar studies are examinated, it is seen that such problems are encountered. For this situation, it can be shown that the limestone powder is collected around the aggregate interface, unlike the fly ash which showed good dispersion and pozzolanic effect.

Carbonation test results showed that using of limestone powder has reduced CO2 transmission because of filler effect. Another result is that increasing the amount of fly ash in the mixtures caused an increase in carbonation penetration depth. İt is known that pozzolans such as fly ash containing active silica bind Ca(OH)2 crystals. Ca(OH)2 crystal turn Calcium Silicate Hydrate which insoluble in water and more difficult to carbonate. The negative aspect of this application, which makes concrete more resistant to chemical attack due to decreasing the permeability of concrete, is thereduction in Ca(OH)2, which affects the passivation of reinforcement in concrete negatively. So when the fly ash is used too much in concrete, it can bind all of the Ca(OH)2 crystals and cause negative results in terms of carbonation. Therefore, the amount of use is very important. The use of low amounts of fly ash has generally led to positive results in examined studies. However, in this study, it can be said that the ratio of fly ash is high and results are negative.

When rapid chloride permeability and electrical resistivity test on surface results are examinated, in SCCs containing fly ash, it is observed that chloride permeability decreases with increasing amount of fly ash and electrical resistivity increases with high rate. As in similar studies, it can be concluded that pozzolanic materials, such as fly ash, reduce chloride permeability highly due to filler and pozzolanic effects. Another important result is the increase of choloride permeability and decrease of electrical resistivity as the amount of limestone powder increases. Limestone powder has no pozzolanic effect unlike fly ash which has hydration property (pozzolanic effect). It has been seen that using limestone powder increases the ion density which can be seen with electrical resistivity test. This leads to an increase in chloride permeability. In similar studies it was observed that limestone powder did not give a positive result for chloride permeability.

In the magnesium sulfate test, ultrasound velocity and weight loss were measured. According to the results of these measurements, with the increase the amount of limestone powder, the durability of the concrete againist the MgSO4 attack increased and  limestone powder showed slightly better performance than the fly ash. It has been determined that MgSO4 salt resistant increases with increasing with fly ash content in the SCCs. MgSO4 considered to be the most dangerous salt since it cause sulfate attack by SO4 and acid attack by Mg on concrete. When evaluated in terms of sulphate attack, calcium hydroxide (Ca(OH)2) components decrease due to pozzolanic effect with the use of fly ash in concrete. Hence, fly ash may have positive effects but increased calcium silicate hydrate components can cause negative concequences due to Mg attack. As is known, Mg ions replace calcium in C-S-H and weaken the concrete and cause damage.

In all experiments, SCCs was showed better durability properties than normal concrete produced as reference which have same water/cement ratio. This can be explained by the fact that SCCs have less pores due to the compaction of SCCs under own weight without any vibration.

In general, in terms of durability SCCs give positive results. In addition, the amount and type of fines used in SCCs significantly affect the durabilitiy properties of SCCs.