Autogenous shrinkage properties of high performance cement based products

Yüksek performanslı çimentolu ürünlerin hamur fazının otojen rötre özellikleri

Abstract

Autogenous shrinkage concept has started to gain more importance since the high performance-high strength concretes were began to be utilized for structural purposes. These concretes are generally designed with high cementitious materials contents and low water to cementitious material ratios. It is observed that, due to lack of water in capillary pores in these concretes, autogenous shrinkage increases under the effects of not only the chemical factors but also the physical effects. Autogenous shrinkage has started to draw attention with having the same magnitude of drying shrinkage, and the research dealing with this concept has been intensified. There are numbers of investigations focusing on the effects of various components of cements, chemical and mineral admixtures on this shrinkage. It is well known that, alkalis, which are the minor components of Portland cements, affect the workability, strength, durability, and shrinkage properties of concretes drastically despite their low percentage in cement. These components also affect the hydration process of Portland cement crucially. Therefore, it is inevitable that the soluble alkalis existing in cement will have an effect on the autogenous shrinkage which became more important due to its use in the production of high performance concretes (HPC). Superplasticizers are indispensable ingredients of HPCs. Besides reducing the water to cement ratio, these admixtures also affect the hydration rate and morphology of the hydration products. In this experimental study; effects of cement components (especially soluble alkali content) and superplasticizers on the autogenous shrinkage were investigated. The effects of these two factors on the hydration process were also focused on. Moreover, discussions were done on the definition of autogenous shrinkage. “Physicochemical Autogenous Shrinkage” term was attributed as to define this shrinkage which is mainly caused by physicochemical effects.Cement paste specimens were produced using cements having different chemical compositions. Autogenous shrinkages of these specimens were obtained by means of volumetric measurement technique. Measurements of volumetric changes was taken in water by means of Archimet balance. Hydration degree of these paste specimens were obtained by non-evaporable water technique. Heat of hydration values were also measured by means of semi-adiabadic calorimeter. The rate of hydration reaction was determined. Capillary pore size distributions were also obtained with Mercury Intrusion Porosimeter (MIP) technique. As a result, the effects of cement components (especially soluble alkali content) and superplasticizers on the autogenous shrinkage of HPC’s were determined. Moreover, critical time periods for physcochemical autogenous shrinkage were obtained and the characteristic behavior of this shrinkage were exlained by hydration kinetics. Relations between capillary pore ratio and physcochemical autogenous shrinkage were determined. The main results obtained from this experimental study can be drawn as following:1) Physicochemical autogenous shrinkage has three main phases for the pastes with low water to cement ratios containing superplasticizer. Generally, these stages were not observed for the specimens which do not contain superplasticizer. However, as an exceptional case, some specimens which do not include superplasticizer could show similar steps as the specimens having superplasticizer. 2) Although, lower hydration heats and hydration degrees were obtained for the lower water to cement ratio specimens, higher values of physicochemical autogenous shrinkage were measured. It was determined that the most effective factor in gaining the physicochemical shrinkage was the released heat values which show the rate of hydration. Higher physicochemical autogenous shrinkage values are measured for the cement pastes which have the higher values of released heat. Microporosity test results have shown that the increasing ratio of capillary pores to total pores causes to increase in physicochemical autogenous shrinkage. Soluble al-kali contents were also found to be effective on the physicochemical autogenous shrinkage. Cements having higher soluble alkali contents exhibited higher physicochemical autogenous shrinkage. It was also concluded that besides the soluble alkali contents in cement, C3A, C4AF contents and SO /soluble alkali ratio are effective parameters and they should be taken into consideration.

Özet

Yüksek performanslı ve yüksek dayanımlı betonların üretimiyle, otojen rötre kavramı büyük bir önem kazanmıştır. Çimento miktarı yüksek, su/çimento oranı çok düşük seviyelerde olan bu betonlarda yeterince su bulunmamasından dolayı otojen rötrenin salt kimyasal faktör dışında kendiliğinden kurumanın yol açtığı bir fiziksel etkiyle de arttığı gözlenmiştir. Otojen rötrenin yüksek performanslı betonlarda, kuruma rötresi mertebesine erişmesiyle konu ilgi çekmeye başlamış ve konu üzerindeki araştırmalar yoğunlaşmıştır. Çimentoların çeşitli bileşenlerinin, kimyasal ve mineral katkıların bu rötre üzerindeki etkilerinin araştırıldığı çok sayıda araştırma mevcuttur. Portland çimentolarının minör bileşenleri olan alkalilerin, çok düşük miktarlarına karşın betonların işlenebilme, dayanım, dürabilite ve rötre özeliklerini önemli ölçüde etkilediği bilinmektedir. Bu deneysel çalışmada yüksek performanslı betonların otojen rötreleri üzerinde çimento bileşenlerinin (özellikle çözünen alkali içeriğinin) ve süperakışkanlaştırıcı katkının etkileri araştırılmış, bu iki faktörün hidratasyon süreci üzerindeki etkileri üzerine yoğunlaşılmıştır. Çalışmada değişik kimyasal bileşenlere sahip çimentolar kullanılarak hamur numuneler üretilmiş, bu numunelerin otojen rötreleri belirlenmiştir. Bu çimentoların hidratasyon aşamaları da çeşitli yöntemlerle takip edilmiştir. Sonuç olarak çimento bileşenlerinin (özellikle çözünen alkali içeriğinin) ve süperakışkanlaştırıcı kullanımının otojen rötre üzerindeki etkileri belirlenmiştir. Elde edilen ana sonuç şöyle özetlenebilir; çimentolardaki çözünen alkali miktarları FKOR büyüklüğünü arttırma yönünde etkinlik taşımktadır. Bunun yanında C3A, C4AF, (SO3/çözünen alkali oranı) ve incelik parametreleri de FKOR’yi etkileyen etkenler arasında dikkate alınmalıdır.