Hafif Agrega İçeren Yalın Ve Pva Lif İle Güçlendirilmiş Yapısal Betonların Mekanik Özelliklerinin İncelenmesi

Abstract

Yapısal uygulamalar çerçevesinden bakıldığında, hafif agregalı betonların son derece hafif olma, ısı ve ses yalıtımı sağlama gibi üstünlükleri mevcuttur. Özellikle geleneksel normal betonlarla kıyas yapılabilecek kadar yeterli dayanım performansı sağlaması durumunda bu betonların yapısal uygulamalardaki potansiyeli kritik bir öneme sahip olmaktadır. Hafif agregalı betonların hafiflik özelliği mühendislere tasarımlarda esneklik sağlamaktadır. Yapılardaki zati ağırlığın azaltılmasıyla ince kesitli ve boyutu azaltılmış yapı elemanları, daha az temel maliyeti, sismik etkilere karşı artırılmış performans ve ısı ve akustik yalıtım performansı gibi önemli üstünlükler elde edilmektedir. Ancak bununla birlikte, betonların mekanik ve fiziksel özelliklerinin içerdikleri agregaların özellikleriyle doğrudan alakalı olması sebebiyle hafif agregalı betonlar yeterli dayanım seviyesine ulaşsalar dahi kimi zaman elastik özellikleri bakımından daha olumsuz durumda olabilmektedirler. Beton, agrega, sertleşmiş çimento hamuru ve agrega – çimento arayüzü olmak üzere üç fazlı bir malzeme olarak tanımlanabilir. Bir beton karışımında agregalar toplam hacmin %60 ila %80’ini oluştururlar. Bu sebepten ötürü, agrega özellikleri betonun mekanik davranışını önemli ölçüde etkilemektedir. Hafif agregalı betonlarda en zayıf unsur sertleşmiş çimento hamuru ve agrega – çimento hamuru arayüzünden ziyade hafif agregalardır. Yapılan son çalışmalar, betonun dayanımını betondaki en zayıf unsurun belirlediğini göstermiştir. Bu çerçeveden bakıldığında, elastik ve mekanik davranışların yanında, hafif beton üretmek amacıyla kullanılan hafif agregalar betonun kırılma davranışını da önemli ölçüde değiştirmektedir. Betonun elastik özelliklerini tahmin etmek için hesaba dayalı cebirsel tahmin modelleri, bilimsel beton araştırmaları dahilinde çoğu zaman kullanıma elverişli olmaktadır. Hesaplanan elastik modüllerinin ana dayanaklarını agregaların ve çimento matrisinin elastik özellikleri ile bunların hacimsel oranları oluşturmaktadır. Yapılan bu çalışmada, yalın ve PVA lif içeren hafif agregalı betonların basınç dayanımları, elastik modül değerleri, Poisson oranları ve kırılma toklukları deneylerle incelenmiş ve geleneksel normal betonlardaki benzer parametrelerle kıyaslanmıştır. Bu amaçla, iki farklı genleştirilmiş kil agregası kullanılarak fiziksel ve mekanik özellikleri farklı iki ayrı hafif agregalı beton üretimi yapılmıştır. Bu betonların kuru birim ağırlıkları nihai olarak 1600 – 2000 kg/m3 arasında değişkenlik göstermiştir. Karışımlarda matris dayanımlarının etkisini gözlemlemek amacıyla tüm karışımlarda agrega hacmi yaklaşık %62 olarak sabit tutulmuştur. Sonuç olarak, betonların basınç dayanımı değerleri, kuru birim ağırlıklarına ve karışım özelliklerine göre değişkenlik göstermiş ve 20 – 80 MPa arasında elde edilmiştir. Normal beton karışımlarına kıyasla, hafif agregalı betonların elastik ve süneklik özellikleri büyük farklılıklar göstermiştir. Aynı basınç dayanımı mertebesinde hafif agregalı betonların elastik modüllerinde önemli düşüşler görülmüştür. Cebirsel hesaplamalar ve deneysel veriler ışığında betonların elastik modüllerinin tahmini için yeni tahmin modeli geliştirilmiştir. Bunun yanında, betonların kırılma enerjilerinin karışımlarda kullanılan agrega tipine ve çimento matrisine bağlı olduğu anlaşılmıştır. Hafif agregalı betonların normal betonlara kıyasla daha gevrek yapıda olduğu görülmüştür. Tüm betonların Poisson oranları ise birbirlerine çok yakın değerlerde kalmıştır.

From the point of view of structural applications, lightweight concretes (LWC) have the advantages of being light and having improved thermal and sound insulation properties. Especially when it is designed with adequate strengths, which is comparable to conventional concretes (CC), its use for structural applications gains critical importance. The property of “lightness” of LWC’s can offer engineers flexibility on design. By reducing the dead load of a structure, advantages such as thinner sections, structural elements in smaller sizes, lower foundation costs, and improved seismic response of structures can be obtained. However, even the adequate strength levels can be reached, hence the overall mechanical and physical properties of concretes are closely related to aggregate properties, elastic properties and overall mechanical behavior of LWC’s, in which the lightweight aggregates are utilized, are greatly influenced. Concrete can be defined as a three-phase material consisting of aggregate, hardened cement paste and the interfacial zone (ITZ) between the aggregates and the cement paste. It can be said that the aggregates occupy a 60% to 80% of the volume of a concrete mixture. By virtue of this fact, aggregate properties have significant effects on the mechanical behavior of concretes. Among LWC’s constituents, lightweight aggregates are the weakest component in the mixture rather than the hardened cement paste and ITZ. Recent researches on aggregate properties and their influences on high-strength lightweight concretes have shown that the weakest component of the concrete determines the strength of the concrete. In light of this fact, it should be noted that when lightweight aggregates are used with the aim of producing structural lightweight concretes, besides the overall elastic properties and strength, fracture behavior of the concrete is also greatly affected. Most of times, using algebraic models for the prediction of the elastic properties of concrete is favorable within the limits of concrete researching. The elastic properties of the aggregates and the matrix, and respective volume fractions are the main bases of the calculated elastic modulus. In this experimental work, properties such as compressive strength, elastic modulus (MOE), Poisson’s ratio and flexural and fracture behavior of the concretes consisting of a total of 18 particular designs were investigated. The compressive strength and elastic modulus tests were conducted at the age of 28 and 120 days. The 3 point bending test for investigating the flexural and fracture behavior of the concrete were conducted only at the age of 120 days. 28 and 120-day old age samples were cured in lime saturated water at 21 °C until the test ages. In the design work of conventional concrete mixtures, natural sand (0-1 mm), limestone fines (0-5 mm) and crushed limestone (4-16 mm) were used as fine and coarse aggregates. In the design of lightweight concrete mixtures, on the other hand, some portions of the natural coarse aggregate, corresponding to 4-8 mm size distribution, have been replaced with expanded clay (EC) aggregates. For all mixtures, the maximum aggregate size was kept constant at 16 mm. Depending on the targeted unit weight of lightweight concretes, two different types of EC aggregate were used having different strengths, water absorption capacities and density values, but having approximately the same size distribution. Lightweight concrete mixtures produced plain and with PVA fibers in the ratio of %0,5 were casted and the results were compared with the conventional concrete mixtures, again produced plain and with PVA fibers. For this purpose, 10 cylinder specimens of 100x200 mm and 5 beam specimens of 100x100x500 mm were moulded from each of the 18 designs. Dry unit weights of the lightweight concretes were targeted as approximately 1700 and 2000 kg/m3. At this point, HB17 notation was decided for the 1700 kg/m3 concrete and HB20 was decided for the 2000 kg/m3 concrete. NB23 notation was used for the CC mixtures of which dry unit weight was around 2300 kg/m3. For the concretes including PVA fibers, L letter was put at the end of the each notations, such as HB17L. In order to express the effect of matrix strength on determined properties, on the other hand, concrete mixtures having different matrix strengths were designed but the total aggregate volume kept constant at approximately 62%. For this purpose, water to cementitious material ratio and the amount of cementitious materials were changed. In order to eliminate the effect of the fly ash concentration in the total amount of cementitious material, the ratio between the mass of cement and fly ash was also kept constant for all mixtures. Water to cementitious material ratios for conventional and lightweight concrete mixtures were selected as 0,34, 0,42, and 0,50. An ordinary Portland cement (CEM I-42,5R) with a density of 3,14 was used in the mixtures. For both normal and lightweight concrete mixtures, an F type fly ash with a density of 2,54 was also used as a supplementary material. For the desired fresh properties of the mixtures, a naphthalene-based high range water reducer (HRWR) was used for both types of concretes if needed. Based on the water absorption amounts determined after 30 min. of water exposure, EC aggregates were absorbed with precalculated amount of water before the concrete mixing process has started. By this procedure, especially for the lightweight concrete mixtures, the mix water was prevented from being absorbed by the EC aggregates. Results showed that the compressive strength of LWC’s varied between 20 MPa to 80 MPa, depending on the unit weight and mixture design properties. The compressive strength of the concretes increased as the unit weight of the concretes increased as a general trend. It should be noted here that, the coarse aggregate characteristics for HB17/L, HB20/L and NB23/L mixtures differed from each other significantly. Mechanical performance of HB20 concrete was higher than the HB17 concrete as HB20 mixture was casted with a different type of EC aggregate having different physical properties. It can be said that the second type EC aggregate, the one used in the HB20 design, had adequate performance compared to the limestone aggregates by means of strength as the compressive strength values of HB20 and NB23 concretes were found almost the same. EC aggregates having a high water absorption capacity was thought to have provided a better internal curing with the related property of its in the concrete so that HB20 concretes showed improved strength capability. It was understood that with the passing of time, as a result of a better hydration and as the matrix strength of the concretes increased, the aggregate characteristics started playing an important role in determining the maximum concrete strength that can be reached. PVA fiber reinforced concretes were showed lower compressive strength values when compared to the plain concretes. It can be indicated that the workability issues of the PVA reinforced concretes might have had the compressive strength values decreased. When the compressive strength results of NB23L concrete at the age of 28 and 120 days were taken into consideration, on the contrary of HB20 and HB20L concretes, as the times passes by 28 days to 120 days, the inclination of compressive strength development was found to have decreased. When the curing age was up to 120 days, it was clearly seen that the compressive strength of all kind of concretes was increased. Results also showed that, at the same compressive strength range, LWC mixtures exhibited remarkable reduction in modulus of elasticity. It can be stated that as the coarse aggregates used in the mixtures were stiffer, in this case the limestone aggregates were stiffer than the expanded clay aggregates, elastic properties of the concretes were found to have improved. Within the limits of this study, the MOE values determined from LWC and CC mixtures were also compared to some of the common prediction models such as ACI318, ACI363, CEB-FIB and TS500. Taking the unit weight or the aggregate type used in the mixture design into consideration was decisive on predicting the MOE values of the concretes within the acceptable limits. In this experimental work, a new model for predicting the MOE of CC and LWC concrete mixtures, considering the unit weight and aggregate type, was suggested. From the point of view of Poisson’s ratios of concretes, negligible differences were observed between the values of LWC and CC mixtures. The Poisson’s ratios determined varied between 0.19 and 0.22. The average Poisson’s ratios determined for LWC and CC mixture were found same, i.e. 0.21. Three point bending tests conducted to 120 days old samples provided valuable results on the flexural and fracture behaviors of the concretes. As a general result, it was observed that the flexural performances of the concretes were satisfactory as the overall mechanical and elastic properties of the aggregates were improved. Independently of the water to cementitious material ratios of the concretes, plain concrete samples had higher flexural strength values than the PVA reinforced concrete samples. Furthermore, the fracture energy of the concretes was found to have significantly affected by the aggregate type used and matrix mixture properties. It was observed that LWC mixtures showed more brittle behavior compared to CC mixtures. The use of PVA fibers on all kind of concretes was found to have had an improving effect on the fracture toughness mechanism of the concretes. The post-peak behavior of concrete samples under the flexural stress showed that the PVA fibers had a substantial effect on the fracture energy of its massive enhancement.