Geoteknik mühendisliğinde geri dönüştürülmüş beton agregasının dolgu malzemesi olarak kullanımının deneysel incelenmesi
Experimental investigation of the usage of recycled concrete aggregate as a filling material in geotechnical engineering
Yazar: MERVE AKBAŞ
Danışman: PROF. DR. RECEP İYİSAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Zemin Mekaniği ve Geoteknik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Katı atıklar içerisinde yer alan kentsel, endüstriyel ve inşaat yıkım atık miktarı her yıl endişe verici bir oranda artmaktadır. Üretilen inşaat kaynaklı katı atık miktarı dünya ülkelerinde gelişmişlik düzeyine ve nüfusuna bağlı olarak değişmektedir. İnşaat yapım yıkım atıkları Çin Halk Cumhuriyeti'nde her yıl toplam katı atığın %30-40'ını oluştururken, 2016'da İngiltere'de üretilen toplam katı atıkların %61'ini oluşturmuştur. 2015 yılında Amerika Birleşik Devletleri (ABD) Çevre Koruma Ajansı (EPA) ABD'de üretilen inşaat yapım ve yıkım atık miktarının, üretilen belediye katı atıklarının iki katından fazlasına karşılık geldiğini ve bu miktarın yaklaşık 548 milyon ton olduğunu belirtmiştir. Ülkemizde ise başlayan kentsel dönüşüm projeleri ile her yıl yüksek miktarlarda inşaat atığı oluştuğu tahmin edilmektedir. 2012 yılında başlayan, eski ve sismik hasara duyarlı yapıların yıkılmasını içeren kentsel dönüşüm projesine rağmen geri dönüştürülmüş beton agregası üretim miktarı ve çeşitli uygulamalarda kullanım oranı tam olarak belirlenememiştir. Bununla birlikte, artan çevre bilinci ve kurulu tesisler ile geri dönüştürülmüş agregaların kullanım oranının artması beklenmektedir.
Depolanan inşaat yapım ve yıkım atık miktarındaki artış ve doğal kaynakların hızlı bir şekilde azalması nedeniyle, inşaat mühendisliği uygulamalarında atık malzemelerin kullanılması özellikle son on yılda büyük önem kazanmıştır. İnşaat atıklarından elde edilen, çoğunlukla geri dönüştürülmüş beton agregası olarak adlandırılan beton atıklarının yeniden kullanılmasının doğal kaynak kullanımını azaltacağı ve atık depolama sorununa bir çözüm sağlayacağı yaygın olarak kabul edilmektedir. Sürdürülebilirliğe katkı sağlamak amacıyla geri dönüştürülmüş beton agregaları özellikle yüksek miktarda doğal kaynak tüketimine sebep olan yol dolgusu uygulamalarında geniş bir kullanım alanı bulmuştur.
Mühendislik uygulamalarında kullanılacak geri dönüştürülmüş beton agregalarının mühendislik özelliklerini doğru bir şekilde değerlendirmek önemli olup, bu malzemelerin mühendislik özellikleri elde edilen kaynağa bağlı olarak değişmektedir. Ülkemizde oluşan geri dönüştürülmüş beton agregalarının mühendislik özellikleri hakkında literatürde sınırlı sayıda bilgi yer almaktadır. Bununla beraber farklı kaynaklardan temin edilen geri dönüştürülmüş beton agregaları kullanılmadan önce esas alınan şartnamede belirtilen geoteknik özellikleri ile diğer mühendislik özellikleri belirlenmeli ve kullanım uygunluğu tespit edilmelidir. Bu çalışmada İstanbul'daki Kentsel Dönüşüm Projelerinden elde edilen ve Göktürk'te geri dönüşüm tesisinde depolanan beton atıkları kullanılmış, yol alt temel ve temel malzemesi olarak kullanım uygunluğu geoteknik açıdan ele alınmıştır. Bu amaçla tesisten temin edilen geri dönüştürülmüş beton agregası üzerinde laboratuvarda standart zemin mekaniği deneyleri yapılmıştır.
Tesisten temin edilen geri dönüştürülmüş beton agregası üzerinde ilk olarak ASTM D-422'ye göre elek ile hidrometre analizleri yapılmış ve Birleştirilmiş Zemin Sınıflandırma Sistemine göre malzeme Siltli Kum (SM) olarak belirlenmiştir. Ülkemizde yol dolguları alt temel ve temel tabakasında kullanılacak malzemelerinin sağlaması gereken geoteknik özellikler Karayolu Teknik Şartnamesinde (KTŞ, 2013) belirtilmiş olup, tesisten temin edilen geri dönüştürülmüş beton agregasının dane çapı dağılımının KTŞ'de alt temel ve temel malzemeleri için belirtilen dane çapı dağılım sınırlarının dışında kaldığı tespit edilmiştir. KTŞ'de alt temel ve temel tabakası dane çapı dağılımı için belirtilen üst ve alt limitler göz önüne alınarak farklı dane çapı dağılımına sahip alt temel ve temel numuneleri hazırlanmıştır. Hazırlanan alt temel ve temel numuneleri üzerinde yapılan deneyler dane çapı dağılımı şartname limitleri dışında kalmasına rağmen tesis numunesi üzerinde de yapılmıştır. Numunelerin kompaksiyon özelliklerini belirlemek için Standart ve Modifiye Proctor, Islak ve Kuru California Taşıma Oranı (CBR) değerleri ile geçirimlilik özelliklerini belirlemek için Kuru ve Yaş CBR ile Sabit Seviyeli Permeabilite ve esneklik özelliklerini belirlemek için Esneklik Modülü deneyleri yapılmıştır.
Geri dönüştürülmüş beton agregalarının kullanım uygunluğu belirlenirken bölgedeki iklim koşulları da büyük önem taşımaktadır. Geoteknik mühendisliğinin inceleme konuları arasında ayrı olarak ele alınan donma çözülme olayı ülkemizin iklim koşulları göz önünde bulundurulduğunda yol dolgularında kullanılacak geri dönüştürülmüş beton agregaları için de ele alması gereken bir problem haline gelmiştir. Donma çözülme olaylarının sık görüldüğü yerlerde geri dönüştürülmüş beton agregaların mukavemet özellikleri değişmektedir. Donma çözülme çevrimleri sonrası mukavemet özelliklerindeki değişimi belirlemek amacıyla hazırlanan alt temel ve temel numuneleri farklı sayılarda donma çözülme çevrimlerine maruz bırakılmış ve donma çözülme sonrası mukavemet özelliklerindeki değişim CBR değerlerindeki değişim ele alınarak belirlenmiştir.
Geoteknik deneylerde bulunan değerler Karayolu Teknik Şartnamesinde (KTŞ, 2013) alt temel ve temel malzemesi için istenen sınır değerler ile karşılaştırılmış ve geoteknik açıdan şartname limitlerini sağladığı belirlenmiştir. Donma çözülme sonrası elde edilen CBR değerlerinin ise şartnamede belirtilen sınır değerleri sağlamamasından dolayı don olaylarının görüldüğü bölgelerde bu malzemelerin doğal agrega ile karıştırılarak, katkı maddesi eklenerek veya geosentetik kullanarak mukavemet özelliklerinin iyileştirilip kullanılması önerilmektedir.

Abstract:
The amount of annually produced municipal, industrial and construction solid waste has been increasing at an alarming rate. In particular, large amount construction and demolition (C&D) waste deposited in landfills worldwide. It is a well-known fact that development level and movement to urban areas are linked to the generation of construction and demolition waste. C&D waste accounts almost for 30-40% of the total solid waste in the People's Republic of China every year, C&D waste accounted for 61% of total generated solid wastes in the United Kingdom (UK) in 2016. The Environmental Protection Agency (EPA) stated that the amount of C&D waste produced in the United States (US) was 548 million tons, corresponding to more than twice the amount of municipal solid waste generated in US which emphasizes the need to find alternative ways of utilizing them. Due to dramatic increase in the stockpiled amount of construction and demolition waste and rapid diminishment of natural resources, using waste materials in civil engineering applications has come into importance particularly in the last decade. Highway constructions may be deemed as a suitable application since large volumes of materials might be utilized. The satisfactory mechanical properties of recycled concrete aggregate (RCA) enables using it as a pavement base or subbase material. Several countries have been using recycled C&D materials as a high-value construction material in civil engineering applications It is widely accepted that the reuse of concrete wastes, mostly referred as Recycled Concrete Aggregate (RCA), obtained from C&D waste will reduce the use of natural resources and will provide a solution to the problem of waste storage.
Turkey is a country where the urban transformation project was initiated in 2012, however, concrete aggregates derived from construction waste in Turkey was not used up to now as a subbase or base materials in the pavement. Despite the urban transformation project that started in 2012 and includes demolishing of the old and seismic damage-susceptible structures, neither the generation amount of RCA nor utilization rate in various applications has been fully determined. However, the utilization ratio is expected to increase with increasing environmental awareness and installed facilities, and in this case, it is important to correctly assess the characteristics of the RCA materials to be used. In addition, there is very little information about the geotechnical properties of these materials. The results of the research studies carried out in recent decades made it possible to utilize RCA as subbase and base materials of the pavement. However, the results of the previous research studies have shown that the both engineering properties of RCA are strictly dependent on the origin. Therefore, in this thesis utilization of RCA from Istanbul, as subbase or base layer in the highways was evaluated through a series of geotechnical laboratory tests. In this study, RCA obtained from C&D wastes in Istanbul were utilized. The RCA material was collected from a plant in Istanbul Gokturk region and sorted with respect to its particle size.
In order to determine the physical properties of the RCA, grain size distributions were determined and then Proctor, California Bearing Ratio (CBR), Constant Head and Resilient Modulus (RMT) Tests were performed respectively. In addition to, when determining the suitability of use of recycled concrete aggregates, the climatic conditions in the region have great importance. Strength properties of recycled concrete aggregates where frost events are common should be reevaluated by taking these conditions. It should be noted that recycled concrete aggregates designated as usable for the subbase and base layer can lose their suitability after freeze-thaw. Therefore, strength losses were determined by using CBR test on prepared subbase and base samples that completed the freeze-thaw cycle.
Sieve and hydrometer analyses were performed on the as-received material according to ASTM D-422. The material was classified as Silty Sand (SM) using the Unified Soil Classification System (USCS). Once the particle size distribution characteristics of the as-received material have been determined, the suitability of utilization in as- received condition was investigated. However, when Turkey's General Directorate of Highways (KGM) Highway Technical Specification (KTS), which show upper and lower limits for grain diameter distribution of subbase and base material are investigated, it is seen that as-received RCA grain size distribution is out of these limits. Thus, two different grain size distributions were prepared for subbase and base materials considering Highway Technical Specification (KTS) upper and lower limits.
Standard and modified compaction tests were performed in accordance with ASTMD- 698 and ASTM D-1557, respectively. The results of the standard compaction test on the as-received material yielded an optimum moisture content (wopt) of 13% and a maximum dry unit weight (γdry,max) of 19.3 kN/m3, whereas the optimum moisture content and the maximum dry unit weight obtained by applying modified compaction energy to the same material was 9% and 19.7 kN/m3, Since the material remaining on the 19.0 mm sieve was less than 10% in both the subbase and base layer grain size distributions, Method C was used for Standard and Modified Proctor Test. The samples were compacted as 3 layers in a 152.4 mm diameter mold with 24.5 N rammer and, the rammer was dropped 56 times from 305 mm for Standard Proctor Test. For the Modified Proctor Test, the material was compacted as 5 layers using a 152.4 mm diameter mold and the rammer was dropped 56 times. respectively. The standard compaction test results for subbase materials show that maximum dry unit weight is 19.40 kN/m3 and optimum water content is 11%. Due to increasing energy in the modified compaction Test, the maximum dry unit weight was found to be 19.80 kN/m3 and the optimum water content was obtained as 8%. Similarly, for base composition, the maximum dry unit weight and optimum moisture content under standard compaction effort was 20.10 kN/m3 and 10%, respectively, whereas when modified compaction test was performed, the optimum moisture content decreased to 7% and maximum dry unit volume weight increased to 20.50 kN/m3. The higher dry unit weight and lower optimum moisture content of base material under both standard and modified compaction energies compared with subbase material can be attributed to coarser particle size distribution of the base material.
CBR test is used to determine the materials CBR value and to evaluate their suitability as subbase and base course material. Standard or modified compressed samples are used to determine the CBR value in the laboratory and, it is aimed to evaluate the strength of the material having a particle size of smaller than 3/4 inches (19 mm). The subbase and base samples were compacted in a CBR mold (7-inch high and 6-inch diameter) at maximum dry unit weight obtained from the modified compaction test.
The test was performed in both dry and wet conditions to evaluate the strength properties under drought and heavy precipitation. In dry condition, the sample was subjected to CBR test immediately after compaction whereas for wet condition, the sample was kept soaked for 3 days and the vertical deformation was measured constantly via a dial gauge. The sample was subjected to loading once the vertical displacement was stabilized. In order to perform the test, the tests were performed at a strain rate of 1.27 mm/min using a loading frame with a maximum loading capacity of 45 kN. The dry CBR value at the optimum water content of the as-received material was 76% and the wet CBR value was 79%. The dry and wet CBR values of the base layer were found as 133% and 121%, respectively. On the other hand; the dry and wet CBR values of the subbase layer obtained %109 and %106. In all cases, the CBR value is significantly higher than 50, a generally accepted global limit for base materials. The higher CBR values for wet condition can be attributed to the potential reaction of anhydrate cement in the concrete aggregate when exposed to water. It is thought that the higher CBR value of the base layer both in wet and dry conditions is caused by a better packing of the sample and minimization of voids. With increasing percentage of coarse material, the amount of load to be applied for 5 mm and 2.5 mm penetration increased significantly. The required CBR value for the subbase layers for the KTS in Turkey is minimum 30 for Type A and the CBR value (wet CBR = 121%) of the sample prepared considering Type A boundary grain size distribution is significantly higher. This fact once again emphasizes the need to perform tests when using RCA since the wide range of index and strength properties exhibited by the materials heavily depend on the origin. The hydraulic conductivity is one of the most complicated and important properties of soils, since it is heavily affected by other factors, however, it needs to be determined in particular for materials to be used in road pavements, bases or subbases. The constant-head test method is used to determine the hydraulic conductivity for the laminar flow of water through granular soils in embankments or base courses under pavements. In order to determine the hydraulic conductivity, RCA samples for subbase and base were prepared in accordance with ASTM D-2434. The test was triplicated for each sample and was repeated on different hydraulic gradients. The results indicate that the hydraulic conductivities of all materials are within the same order of magnitude. The average hydraulic conductivity of as-received material was found to be 8.86x10-5 cm/s. The prepared subbase and base samples have a slightly lower permeability than the as-received sample. This result can be explained by the fact that the prepared sample has a better gradation and a packing, hence a lower void ratio.
Resilient Modulus test (RMT) is used to determine of resilient modulus (MR) under conditions representing the stress states of base/subbase materials beneath flexible pavements subjected to moving wheel loads. According to AASHTO T-307 as-received, base and subbase samples were subjected to resilient modulus test. Prior to loading, the type of material is determined based on the grain size distribution. Since the passing material through the sieves No.10 (2.00 mm) and No.200 (0.075 mm) is less than 70% and 20% respectively, both subbase and base RCA were classified as Type-1. Thus, all materials were compacted in six layers at their optimum moisture contents and maximum dry unit weights based on modified compaction test results. The test was conducted via external linear variable displacement transducers (LVDTs) with a measurement capacity of 50.8 mm, a Geocomp LoadTrac-II loading frame and associated with hydraulic power unit system. The conditioning stage was performed on the specimens under the same confining and axial stress of 103 kPa for 500 repetitions.
Once proceeded to the actual loading stage, the confining stress was varied between 20.7 and 138 kPa during loading stages with 100 repetitions applied at each stage. Resilient modulus 6.0 software was used to keep track of the loading. The last five repetitions of each stage were used to calculate the resilient modulus of each corresponding stage. The value of resilient modulus is a measure of the elastic modulus of base and subbase materials having certain nonlinear characteristics. For calculation of the resilient modulus, Bulk Stress Model and Mechanistic-Empirical Pavement Design Guide (MEPDG) were used. The resilient modulus test (RMT) results showed that there is a linear relationship between the bulk stress and the MR as reported in previous studies. When the MEPD Model Parameters are analyzed, it is seen that there is not much difference between the regression coefficients (k1, k2, k3) of all samples, but the correlation coefficient is close to 1 for all square values. Using regression coefficients for the as-received sample, under the 34.5 kPa (5 psi) confining stress and 103.4 kPa deviator stress, resilient modulus was calculated as 143.8 MPa according to the MEPD Model, and for the base and subbase samples this value was found to be 154.3 MPa and 128.7 MPa respectively.
In the last part of the laboratory study, the subbase and base samples, whose geotechnical properties were determined by taking into consideration the KTS, were subjected to freeze-thaw cycles. The freeze-thaw cycle numbers were chosen 1-3-5- 10 and 20. When preparing samples for freeze-thaw cycles, special plastic mold (same dimensions with CBR mold) and vibratory compression device was used. In order to observe the changes in CBR values after freeze and thaw cycles, subbase and base samples were prepared at the maximum dry unit volume weight obtained in the Modified Proctor experiment. The samples were compressed in 5 layers with an energy equivalent to the modified proctor compression energy during the vibration time determined before the experiment. The samples were compacted in molds made in CBR mold sizes in order to minimize the shaking of the samples during carriage during freeze-thaw cycles. To make comparison, CBR experiments were performed on subbase and base samples which was not subjected to freeze-thaw cycle and prepared by vibration device in plastic mold. CBR values before and after freeze thaw cycles were determined and the suitability of use was re-evaluated considering the effect of freeze thaw on CBR value. CBR values after 1-3-5-20-20 freeze and thaw cycles were determined to be below the limit values specified for the aggregates to be used in the subbase and foundation layer of the Highways Technical Specification. After the freezing and thawing cycles, the change in CBR values was determined and the suitability of use in the regions where the climatic conditions were variable and frost events were discussed separately in terms of strength properties. In order to meet the limits of the CBR values falling after the freezing and thawing cycles, it is recommended to mix the natural aggregate or using geosynthetics.
In conclusion, the use of recycled concrete aggregate obtained from the plant in the prepared subbase and base layer is suitable in terms of geotechnics. However, other engineering features should be determined before being used by the relevant civil engineering applications and their conformity with the values specified in the Technical Specifications of the Highways should be indicate.