Harçta alkali-silika reaktifliğini bastırmada silis dumanı ve metakaolinin etkinliklerinin karşılaştırmalı değerlendirilmesi

Öz

Arka Plan—Ne yazık ki, beton ve betonarme yapılar hizmet verdikleri ortamlarda çok sayıda düşmana sahiptirler. Bu nedenle, böylesi ortamlara maruz bu yapıların hizmet ömrünü uzatmak amacıyla gerekli önleyici tedbirlerin alınması hayati önemdedir. Örneğin, bu tür yapılar sıklıkla sülfat veya klorür içeren ortamlara maruz kalabildiği gibi donma-çözülme çevrimlerine ve/veya ıslanma-kuruma çevrimlerine maruz kalabilir. Yapısı dolayısıyla beton türü çevresel etkilere karşı potansiyel olarak hasar görme olasılığı yüksektir. Neticede mukavemet ve yapısal bütünlük kaybına uğrayan beton tahmin edilenden daha kısa sürede ömrünü kaybedebilmektedir. Betonun zamanla mukavemetini ve yapısal bütünlüğünü kaybetmesine neden olan süreçlerden biri de alkali-silika reaksiyonudur (ASR). Alkali-silika reaksiyonu, agregadaki reaktif silis ile betonun gözenek suyundaki alkaliler arasında meydana gelen kimyasal bir etkileşimdir. Bu etkileşim sonucu oluşan alkali-silika jeli ortamda bulunan suyu emerek şişer ve betonun zamanla genleşmesine ve sonunda çatlamasına neden olur. ASR için öncelikli koşul, nemli bir ortamın varlığıdır. Ne yazık ki, beton ve betonarme, hizmet verdikleri ortamlarda çoğunlukla suya maruzdur. Bu nedenle, beton ve betonarme yapılar bu açıdan risk altındadır. ASR çok yavaş bir süreç olup yalnızca ASR etkisi nedeniyle betonun hasar görmesiyle işlevselliğini ve yük taşıma kapasitesini kaybetmesi yıllar alabilir. Bununla birlikte, ASR sık karşılaşılan bir hasar süreci olduğundan, önlenmesi son yıllarda önem kazanmıştır. Bu amaçla günümüze değin çeşitli önleyici yöntemler denenmiş olup bunlardan bir tanesi puzolanik malzeme kullanımıdır.

Amaç—Çalışmada harç ölçeğinde alkali reaktif bir agreganın kullanıldığı bir incelemede alkali-silika reaktifliğini bastırmada silis dumanı ve metakaolinin etkinliğinin karşılaştırmalı değerlendirmesi amaçlanmıştır.

Yöntem—Bunun için silis dumanı ve metakaolin, ağırlıkça %5, %10, %15 ve %20 oranlarında çimento ile ayrı ayrı ikame edilerek karışımlar üretilmiş ve elde edilen sonuçlar karşılaştırmalı olarak değerlendirilmiştir. Karşılaştırma, silis dumanı ve metakaolin içermeyen referans karışımı dikkate alınarak gerçekleştirilmiştir. Alkali-silika ölçümleri ASTM C1260 standardı uyarınca gerçekleştirilmiştir.

Bulgular—Bulgular—Genelde test sonuçları, silis dumanı ve metakaolinin ASR kaynaklı genleşmeyi önemli ölçüde azalttığını; hatta önlediğini doğrulamıştır. Elde edilen sonuçlar, %5 ikame oranının her iki mineral katkı maddesi için de geçerli olmak üzere alkali-silika kaynaklı genleşmeyi bastırmak için yeterli olmadığı, buna karşın %10 ikame oranının ise alkali-silika kaynaklı genleşmeyi ilgili standart ASTM C1260'ta zararsız sınır olarak belirtilen %0,10 limitinin altına indirdiğini göstermiştir. Ayrıca elde edilen sonuçlardan ikame oranı arttıkça genleşmedeki azalmanın daha da belirginleştiği açıkça görülmektedir.

Sonuç—Sonuç itibarıyla, deneysel çalışmanın yürütüldüğü koşullar ve kullanılan silis dumanı ve metakaolinin karakteristikleri özelinde metakaolinin ASR kaynaklı genleşmeyi bastırmada silis dumanından biraz daha etkili olduğu görülmektedir.

Anahtar Kelimeler

• Alkali-silika reaksiyonu
• Genleşme
• Silis dumanı
• Metakaolin
• Permeabilite
• Durabilite

Etik Beyan

Bu makale için etik komite onayı gerekmemektedir.

Teşekkür

Yazarlar, Karadeniz Teknik Üniversitesi Yapı ve Malzeme Laboratuvarı personeline deneysel çalışmaların yürütülmesinde gösterdikleri destek için ve fiziksel ortam ile deneysel ekipman sağlayan kuruma teşekkürlerini sunarlar.

Comparative assessment of the effectiveness of silica fume and metakaolin in suppressing alkali-silica reactivity in mortar

Öz

Context—Unfortunately, concrete and reinforced concrete structures face numerous adversaries in their environments. Therefore, taking necessary preventive measures to extend the service life of these structures exposed to such environments is vital. For example, such structures are frequently exposed to environments containing sulfates or chlorides, as well as freeze-thaw cycles and/or wetting-drying cycles. Due to its structure, concrete is potentially susceptible to damage from environmental influences. As a result, concrete, which suffers a loss of strength and structural integrity, can lose its lifespan sooner than expected. One of the processes that causes concrete to lose its strength and structural integrity over time is alkali-silica reaction (ASR). ASR is a chemical interaction that occurs between reactive silica in the aggregate and alkalis in the pore water of the concrete. The alkali-silica gel formed because of this interaction absorbs the water in the environment, swells, and causes the concrete to expand over time and eventually crack. The primary condition for ASR is the presence of a humid environment. Unfortunately, concrete and reinforced concrete are often exposed to water in their environments. Therefore, concrete and reinforced concrete structures are at risk in this respect. ASR is a very slow process, and it can take years for concrete to lose its functionality and load-bearing capacity due to ASR alone. However, since ASR is a frequently encountered damage process, its prevention has gained importance in recent years. For this purpose, various preventive methods have been tried to date, one of which is the use of pozzolanic materials.

Objective—This study aims to comparatively evaluate the effectiveness of silica fume and metakaolin in suppressing alkali-silica reactivity in a mortar-scale study using an alkali-reactive aggregate.

Method—For this purpose, mixtures were produced by substituting silica fume and metakaolin separately with cement at 5%, 10%, 15%, and 20% by weight, and the results obtained were evaluated comparatively. The comparison was carried out considering a reference mixture without silica fume and metakaolin. Alkali-silica measurements were performed in accordance with ASTM C1260 standard.

Results—In general, the test results confirmed that silica fume and metakaolin significantly reduce, or even prevent, ASR-induced expansion. The results showed that a 5% substitution ratio was insufficient to suppress alkali-silica-induced expansion for both mineral additives, whereas a 10% substitution ratio reduced alkali-silica-induced expansion below the 0.10% limit specified as the innocuous limit in the relevant standard ASTM C1260. Furthermore, the results clearly show that the reduction in expansion becomes more pronounced as the substitution ratio increases.

Conclusion—In conclusion, considering the conditions under which the experimental study was conducted and the characteristics of the silica fume and metakaolin used, metakaolin appears to be slightly more effective than silica fume in suppressing ASR-induced expansion.

Anahtar Kelimeler

• Alkali-silica reaction
• Expansion
• Silica fume
• Metakaolin
• Permeability
• Durabillity

Etik Beyan

Ethics committee approval is not required for this article.

Teşekkür

The authors express their gratitude to the staff of the Structural and Materials Laboratory at Karadeniz Technical University for their support in conducting the experimental studies and to the institution that provided the physical environment and experimental equipment.

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