Effect of mixing proportions on the compressive strength and permeability of concretes containing mineral admixtures

Betonda dayanıklılığın (dürabilite) en az dayanım kadar önemli olduğu son yıllarda daha iyi anlaşılmaya başlamıştır. Dayanıklı beton üretmek için düşük su/çimento oranının yanında puzolan (mineral katkı) kullanımının da gerekli olduğu bilinmektedir. Bu deneysel çalışmada farklı oranlarda uçucu kül veya silis dumanı mineral katkıları içeren iki seri beton üretilmiştir. ‘Merkezi Kompozit Tasarım’ adındaki deneysel tasarım ve analiz yapan istatistiksel yöntem kullanılarak her seride karışım parametrelerinden toplam bağlayıcı miktarı, su/bağlayıcı oranı ve puzolan/bağlayıcı oranı bağımsız değişken olarak seçilmiştir. Değişken sınırları puzolan özelliklerine ve betonların belirli bir kıvamda bulunması koşuluna göre belirlenmiştir. Her seride, kullanılan istatistiksel programın belirlediği 20 adet karışım hazırlanmıştır. Betonların basınç dayanımı ve klorür geçirimliliğini ölçmek üzere alınan numunelerin yarısı laboratuvar ortamında havada diğer yarısı da kirece doygun su içerisinde 90 gün boyunca saklanmıştır. Basınç dayanımının yanında ASTM C 1202 standardına uygun olarak hızlı klorür geçirimliliği deneyleri gerçekleştirilmiştir. Tüm deney sonuçları varyans analizi ile değerlendirilmiş ve her iki kür koşulu için ölçülen beton özelliklerinin karışım parametreleri cinsinde ayrı ayrı modelleri belirlenmiştir. Betonun seçilen bileşim parametrelerinin (toplam bağlayıcı miktarı, su/bağlayıcı oranı ve puzolan/bağlayıcı oranı) basınç dayanımı ve klorür geçirimlilği üzerine etkisi elde edilen istatistiksel modellere dayanılarak değerlendirilmiştir. Hızlı klorür geçirimliliği deneyinin havada saklanan betonların karşılaştırılmasında kullanılabileceği ancak su kürü uygulamasında mineral katkılı betonlarla katkısız betonların karşılaştırılmasında hatalı sonuçlara neden olabileceği belirlenmiştir.  Anahtar Kelimeler: Uçucu kül, silis dumanı, kür koşulları, istatistiksel analiz, merkezi kompozit tasarım.Concrete is a material which contains different amounts of pores in various sizes. On this account, penetration of liquid and/or gas into the concrete occurs by diffusion, absorption under pressure or capillary sorption through the pores. In no matter what way the transfer of liquid and gas occurs, the phenomenon is called as permeability. Permeability depends on amount, distribution, size and connectivity of pores within concrete for as much as rate of penetration of aggressive substances into concrete mainly related to the continuity of the pores and their shape and distance. 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 problem occurred during the service life. However, in recent years it is well known that in order to obtain a durable concrete, which shows the expected performance throughout its service life, selection of suitable materials in accordance with environmental conditions, the best proportion of concrete mixture and sufficient curing after casting are the most important steps to pay attention. Ingredients of concrete should be selected according to their participation in durability. Pozzolanic materials, most of which are by products of industries, are widely used in concrete production, especially to increase the durability properties. Pozzolanic materials react with Ca(OH)2, generated during the hydration of C3S and C2S components of cement, to form calcium silicate hydrate (C-S-H), hence reduce a weak part of concrete against durability. Furthermore, pozzolans improve the impermeability of concrete which is the main parameteraffecting the durability of concrete. In this experimental research, two series of concrete incorporated with fly ash and silica fume were produced. The statistical program of 'Central Composite Design' was used to design experiments and to analyze the testing results. In each series, total binder content, water/binder ratio and pozzolan/binder ratio were chosen as independent variables and the ranges of the variables were adjusted according to the properties of pozzolanic material and to obtain certain workability for all concretes. The range of the factors were selected between 300-400 kg/m3 for binder content, 0.48-0.58 (when silica fume was used) or 0.46-0.60 (when fly ash was used) for water/binder ratio and 0.03-0.12 for silica fume/binder ratio or 0.10-0.40 for fly ash/binder ratio. For each series of concretes, the total number of batches was 20. Lime saturated water curing was applied over the half of the specimens, cast to measure compressive strength and permeability, while the remaining ones were stored in air at laboratory conditions for 90 days. In addition to the compressive strength test, rapid chloride permeability test was conducted in accordance with ASTM C 1202. Test results were analyzed and evaluated by using ANOVA (Analysis of Variance). Statistical models are derived for each measured concrete property in two curing conditions by means of three independent components of concrete. Influence of selected mixture parameters on compressive strength and chloride permeability was evaluated by using the test results. It has been found that total binder content of fly ash added concretes was not influential on the compressive strength and chloride penetration. On the other hand, increasing binder content reduced the overall performance of silica fume added concretes because of the higher porosity of cement paste than that of aggregate. It was also seen that the optimum supplementary cementing material contents determined to obtain the minimum chloride permeability are shifted to higher values for the fly ash added concretes under the proper curing conditions rather than storing in air. However, the optimum percentages of the silica fume added concretes cured under both conditions are not changed. Besides, it is found that RCPT method can be employed to make comparisons for all of the air stored concretes produced with or without supplementary cementing material (SCM). However, for water cured SCM added specimens this method can be used for only internal comparisons, it is not appropriate to compare the chloride permeability of OPC concretes with those of pozzolan added concretes. Keywords: Fly ash, silica fume, curing conditions, statistical analysis, central composite design