Farklı Zamanlarda Dökülen Beton Tabakaları Arasındaki Yüzey Koşullarının Kayma Dayanımına Etkisinin Deneysel Olarak İncelenmesi

Abstract

Yapı mühendisliğinin saha uygulamalarında sıkça karşılaşılan, farklı zamanlarda dökülen betonların birbirleri arasındaki kayma dayanımını etkileyen birçok faktör vardır. Bu faktörlerin başında eski beton yüzeyinin pürüzlülüğü, nem durumu, eski ve yeni betonun dayanımı ve kalitesi gelmektedir. İki beton tabakası arasındaki yapışma dayanımının belirlenmesine yönelik farklı yöntemler kullanılmaktadır. Bu çalışma kapsamında, hazırlanan 200 mm kenar ayrıtlı farklı yüzey koşullarına sahip küp numuneler üzerinde, sabit düşey yük altında, taşınan yatay yükün kayma deneyi ile belirlenmesi amaçlanmıştır. Bu doğrultuda, yüzey pürüzlülüğü, yüzeyin nem durumu, eski ve yeni beton arasındaki döküm zamanı, çimento şerbetinin ve epoksi esaslı aderans artırıcının yapışma dayanımına etkisi deneysel parametre olarak belirlenerek, her parametre için bir yapışma dayanımı ve sürtünme katsayısı elde edilmeye çalışılmıştır. İstanbul Teknik Üniversitesi Yapı ve Deprem Mühendisliği Laboratuvarında 108 adet numune üzerinde gerçekleştirilen deneyler sonucunda, kayma dayanımını artıran en önemli faktörün yüzey pürüzlülüğü olduğu görülmektedir. Bununla birlikte genel davranış açısından pürüzlü numunelerde çimento şerbeti ve aderans artırıcı kullanımının kuru ve ıslak yüzeylere göre daha etkin olduğu görülmüştür. Deneysel elde edilen sonuçlar, ilgili tasarım yönetmeliklerinde yer alan hesap esaslarına göre kıyaslandığında, Türk Standardı TS500, American Concrete Institute ACI318-11 ve Eurocode 2 `nin deneysel sonuçlara göre hem düz yüzeyli hem de pürüzlü yüzeyli durumlar için oldukça güvenli değerler verdiği görülmüştür. Bununla birlikte, FIB Model Code 2010`da pürüzlü yüzeyli durum için verilen yaklaşım ile elde edilen kayma dayanımı deneyde elde edilen kayma dayanımının üzerinde bir değer vermektedir.

In structural engineering, adding a concrete overlay to an existing concrete is a common encountered case that has to be considered. Concrete-to-concrete interfaces are exist when two concretes are cast at different times. In practice the following situations are generally encountered; existing structures strengthening and repair of concrete structures, adding concrete to the precast elements at site, adding new concrete layer to hardened concrete which cause cold joints. In these applications, the bond strength at the interface between the old and new concrete shows a weak connection. In this context, concrete-to-concrete layer behaviors have to be researched. The bond strength of the concrete-to-concrete interface mainly depends on the preparation on the surface, using bonding agents, moisture content of the concrete, curing condition, cleanness and roughness of interface. In this research, an experimental study was performed to analyze the shear strength of the concrete-to-concrete layers. This experimental study was performed to evaluate the bond strength between the two concrete layers. Two different times were considered for the time gap between casting the first layer and the added concrete layer. Three and fourteen days were chosen for casting the second layer of the concrete. After first layer casted to the formwork, three and fourteen days later, other parameters were applied and the new concrete was added. Epoxy based bonding agent, cement mortar bonding agent, humidity of the layer, dry surface and roughness of the concrete layer were decided for the testing parameters. Three different axial load level were applied to the specimens with the aim of understand the changes on the bond strength. For the experimental study, 200 x 200 x 200 mm cube formworks were prepared. First 100 mm height concrete were casted, then the second concrete layer were casted. Before casting the second concrete layer, the surface parameters were applied to the first layer. The specimens were cured indoor and the laboratory conditions. All the tests were performed in Istanbul Technical University Structural and Earthquake Laboratory. The experimental research include 108 specimens tested in shear test method. There are several test methods to evaluate the bond strength between the old and new concrete. First of all, tests that measure the bond strength under tension stress are one part of this methods. Pull off, direct tension and splitting prism tests are the examples of this part. Tests measures the bond strength under shear stresses are the second group. Direct shear tests is an example of this group tests. In this experimental study, direct shear tests was used for evaluate the shear strength between the layers. The third category measures the bond strength under the combination of shear and compression. Slant shear test method is an example for this group. Pull off and slant shear tests are the most used methods in the literature. The test results about the bond strength between the new and old concrete layers are compared with relevant codes. In this research, for the comparison TS500, FIB Model Code 2010, Eurocode 2, ACI Structural Concrete Building Code (ACI 318), AASHTO LRFD Bridge Design Specifications codes were used. The shear strength results according to codes are compared with the experimental test results of 108 specimens. With the aim of compare the results clearly, the code results and the test results are given in the same chart. Therefore, the experimental test results are shown easily if it is conservative or not according to the codes. From an analysis of the experimental results it was possible to conclude that: The shear strength between the concrete-to-concrete interface increase with increase the roughness of the concrete layer. The shear strength between the roughness surfaces are shown better performance than the smooth surface for the all surface conditions (using bonding agent on the concrete surface and moisture of the surface). Smooth surface moisture and dry specimens were compared according to cast at three and fourteen days concrete. Casting at three days moisture and dry concrete specimens were performed better shear strength than fourteen days specimens. According to the experimental test results, significant difference on the shear strength was not observed dry and humidity surface specimens. The cement based bonding agent concrete-to-concrete surface specimens especially roughness surface specimens showed better shear strength performance than the specimens that dry and humidity surface specimens, beside this increasing the smooth concrete surface specimens performance has been limited. Experimental test results showed that, using epoxy based bonding agent increased the shear strength on the roughness surface specimens, besides this the influence of the epoxy based bonding agent was insufficient on the smooth concrete-to-concrete layers. In chapter one, for evaluate the bond strength between the concrete-to-concrete layers various test methods are explained. In chapter two, the previous published experimental works were researched. The using tests methods were examined in that part. Furthermore, the experimental test parameters that influence the bond strength were analyzed. Using concrete-to-concrete surface testing parameters and techniques of applying that parameters are explained in that part. In chapter three, chosen test method, experimental test set up, material properties are explained. The experimental concrete-to-concrete surface layer parameters and concrete casting age parameters are specified clearly by a table in this chapter. In additionally, production steps are presented by the help of photos. In chapter four, all relevant conclusions for each specimen are presented by tables and photos. Moreover, the bond strength of 108 specimens has been reported in that part. In chapter five, all the 108 specimens test results are compared with the available codes. The relevant parts of these codes are detailed in that part. All the results according to these codes are clearly given with a chart. In chapter six, based on the results obtained, the conclusion of the experimental study is given.