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Yapı hasarları ve tamirat metodları

Başlık çevirisi mevcut değil.

  1. Tez No: 46184
  2. Yazar: BARBAROS AKSÖZ
  3. Danışmanlar: PROF.DR. HALİT DEMİR
  4. Tez Türü: Yüksek Lisans
  5. Konular: İnşaat Mühendisliği, Civil Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1995
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 86

Özet

ÖZET a. Betonarme Yapılarda Hasarların Sebepleri Seçilen malzemenin ve yapısal etkilerin bir sonucu olarak beton, gerek plastik halde iken, gerekse sertleştikten sonra çatlaklar oluşturabilir. Sertleştikten sonra yapısal yükler sonucu meydana gelebilecek çatlaklar uygun detaylandırma ile kabul edilebilir limitlere indirgenebilir. Yapısal olmayan çatlaklar üç ana tip olarak incelenebilin Beton döküldükten sonra, büzülme sırasında birkaç saat içinde görülebilecek plastik deformasyon çatlakları, betonda egzotermik reaksiyon sonucu oluşan ısının maksimum seviyeye ulaşmasını izleyen birkaç gün içinde görülebilecek termal çatlaklar ve büzülme çatlakları şeklindedir. Bu çatlaklar betonarme elemanlarda meydana gelebilecek hasarlarda önemli rol oynarlar. Betonda hasara sebep olabilecek diğer bir etken alkali-agrega veya alkali-silika reaksiyonudur. Agregayı saran çimento hamurundaki alkaliler ile agregalar reaksiyon verebilirler. Bunun sonucu olarak meydana gelebilecek hacim artışları erken yaşlarda yüzeyde çatlaklar oluşturabilmektedir. Çevre, betonarme yapıdaki donatıların korozyonuna sebep olabilecek tuzlar, oksijen, nem ve karbondioksidin betona girmesini sağlayacak uygun bir ortam doğurabilir. Çelik korozyona uğrarken çatlaklar ile sonuçlanacak hacimce artışlar meydana getirir. Yüzeyde dökülmeler ve pas lekeleri oluşturur. Bu sorunla bütün betonarme yapılarda karşılaşılabilmektedir. b. Tamirat Malzemeleri Yapıda kullanılabilecek her malzemenin bir servis ömrü vardır ve bu elemanların yapı ömrü boyunca periodik olarak yenilenmesi veya tamir edilmesi gerekmektedir. Çevredeki kimyasal, fiziksel veya mekanik değişiklikler bu elemanların ömürlerini daha da kısaltmaktadır. Bu sebepten ötürü yapılan tamir etmeye veya servis ömürlerini tamamlayabilmeleri için yenilemeye gereksinim duyulmuştur. Sonuç olarak tamirat malzemelerine karşı artan bir gereksinim vardır. c. Koruma ve Tamirat Betonda yapısal hasarlara yol açan çatlaklar, özellikle su yapılarında yapıyı kullanılmayacak duruma getirebilir ve hatta bu yapılar olası bir göçme ile karşılaşabilir. Fakat gerek tasarım aşamasında gerekse yapım aşamasında gereken özen gösterilirse bu gibi etkiler minimize edilmiş olur. Çevreden gelebilecek zararlı etkilere karşı donatıyı paspayı betonu korumaktadır. İdeal olarak paspayı betonunun suya, oksijen, klor iyonların ve neme karşı geçirimliliğinin çok düşük olması gereklidir. Yapıyı tamir etmenin bir yolu yukarıdaki öneriler dikkate alınarak tekrar inşaa etmektir. Fakat ekomomik olarak bölgesel tamiratlar daha uygun olacaktır. Malzeme konusundaki seçenekler ise çimentolu malzemeler, polimerlerle modifiye edilmiş çimentolu malzemeler ve reçine harçlarıdır.

Özet (Çeviri)

SUMMARY DETERIORATIONS OF STRUCTURES AND REPAIR METHODS a. Historical Development Of Concrete The development of concrete as a construction material dates back several tousand years to the days of ancient Egiptians, the Greeks and the Romans. These early concrete compositions were based on lime, although the Romans are known for their development of puzzolanic and lightweight concrete based on pumice. The domed roof of the Pantheon is testimony to the durability of this material. Apart from a brief revival during the Norman period when structures such as Reading Abbey and the foundations to Salisbury Cathedral were constructed from concrete, there was little further development until the eighteenth centtury. In 1756 Smeaton Constructed the Eddystone lighthouse which stands to this day on Plymouth Hoe. In 1824 Aspdin took out his patent for 'a superior cement resembling portland stone' which was used for concrete in the construction of the Thames Tunnel. The Axmouth Bridge in Devon, a three-span all-concrete arc bridge, was built in 1887 and still carries traffic to this day. The credit for the introduction af steel as reinforcement is variously attributed to Lambot in 1855 for ferrocement boats, to Monier in 1867 and to Hennebique in 1897 who built the first reinforced concrete frame building in Britain at Weaver's Mill, Swansea. It is interesting to note that one of Lambot's boats was still afloat 100 years later and reportedly in very good condition despite in the seawater environment. However, the same cannot be said for Hennebique's first reinforced concrete bridge in Scotland which was reported in 1932 to be suffering from 'severe spalling as a result of the harsh environment'. Neverteless, gunite repairs have extended its life to the present day. Notable steps forward in this century have been the introduction of prestressed concrete by Freyssinnet in the 1940's, the extensive use of reinforceed concrete during World War II including the famous Mulberry Harbours, the rapid post-war concrete-building expansion promoted by shortages of steel, the motorway-building boom of the 1960's involving concrete to very large offshore structures. Although the vast majority of concrete structures have performed satisfactorily for many years such progress has not been made wihout its problems. Structurally these range from the wind-induced failure of cooling towers at Ferrybridge Power Station to the progressive collapse due to a gas explosion of the Ronan Point tower VIIblock in East London in 1960's However, attention today is primarily focused on environmental attack which is substantially reducing the lives of many concrete structures arround the world, in many cases due to corrosion of reinforcement steel. [1] b. Concrete The word 'concrete' comes from Latin 'concretus' meaning compounded. Today concrete is understood to consist of a graded range of stone aggregate particles bound together by a hardened cement paste. Concrete is required to be strong, free from excessive volume changes and resistant to penetration by water. It may also need to resist chemical attack or possess a low thermal conductivity. Concrete's strenght is derived from the hydration of the cement by water. The cement consistuents progresiveiy crystalize to form a gel or paste which surrounds the aggregate particles and binds them together to produce a conglomerate. Generally, the strenght and permeability of the concrete and governed by its water-cement ratio. For high strenght and low permeability the water-cement ratio should be low. Conversely, for ease of placing and compaction for easiest way of increasing workability is to increase the free water content, although nowayds chemical plasticizers are avaible to assist. In normal concrete the aggregate type has little effect on strenght but it does affect workability. Normal procedures for the design of mix proportions involve selecting a water-cement ratio to provide the required strenght and then determining water and additive content from considerations of workability and aggregate type. In high strenght concrete the aggregate- cement ratio and aggregate type must also be taken into account. For all structural concrete it is also usual to specify a minimum cement content to ensure a level of alkalinity which inhibits any tendency for corrosion to embedded steel. Conversely, very high cement contents are inadvisable since early thermal contraction leading to cracking may result from the rise in temperature induced by cement hydration. Concrete is a material which, although relatively strong in compression, is weak in tension and for structural members subject to tensile stress may be reinforced by steel bars. The effectiveness of reinforced concrete as a structural material depends on the following: a. The interfacial bonding between steel and concrete which allows it to act as a composite material; b. The passivating effect of the concrete environment to inhibit steel corrosion; c. The similar coeficients of thermal movement of concrete and steel. Alternatively, the concrete may be precompressed by applying load through tendons of high tensile steel anchored to the concrete. Under load the effect is to unload the compression and avoid significant tensile stresses. vinc. Mechanism and Causes of Failure It is accepted that concrete cracks in both the plastic and hardened state as a result of constituent materials and structural loads. The cracks caused by structural loads in hardened state can be limited to acceptable levels by appropriate detailing. The non-structural cracks may be three main types; plastic cracks which appear in first few hours due to shrinkage of restrained settlement, early thermal contraction cracking which develops over the first few days following to exothermic peak and longer term drying shrinkage. These cracks in concrete may be significant in terms of rate subsequent reinforcement deterioration. Problems have arisen with the use of high alumina cement for the construction of precast concrete products in the 1950's and 1960's. The material was first developed to resist sulphate attack, it's resistance being derived from the absence of calcium hydroxide in its hydrated form. It also provides a very high rate of strenght development which is of economic benefit in precast industry. Howewer, a number of dramatic failures in early 1970's, notably in schools and swimming pools, led to a complate reappraisal of it's use for structural concrete. It has been found that hydrates are unstable, particularly at higher temperatures and in presence of water. Sulphate-resisting portland cements are now available for use with concrete in contact with sulphate-bearing soils. Sulphate-resisting properties are also exhibited by concrete containing blasfurnace slag as partial cement replacements. Another form of attack from within which is now seen on a significant scale is the phenomenon of alkali-aggregate or alkali-silica reaction. Under most conditions certain aggregates appear susceptible to attack by the alkalies present within the surrounding cement paste. The aggregates gel and expand causing a distruptive effect on the concrete evidenced in the early stages by cracking at the surface. d. Environmental Attack Environmental processes may cause salts, oxygen, moisture or carbon dioxide to penetrate the concrete cover and eventually lead to corrosion of embedded steel reinforcement. As steel corrodes it expands in volume causing cracking, rust staining and spalling of concrete cover. The problem is widespread in all forms of concrete structures. The two most significant penetration processes are the following. i. Carbondioxide is caused by atmospheric carbon dioxide dissolving in concrete pore water and creating an acidic solution. This starts to neutralize the natural alkalinity of the concrete, and when pH value around the steel bars falls below about 1 1.5 corrosion can begin.ii. Chloride salts may be presented in concrete either because calcium chloride was added at the time of construction as an accelerating admixture or because of ingress of de-icing salts, particularly in highway structures, ingress from seawater or spray in the case of marine structures or impurities in the aggregates and/or mixing water. Once chloride ions reach the reinforcing bar localized breakdown of passivating environment can occur, forming sites of corrosion attack. The simple process of freeze-thaw effects of concrete must be mentioned. Porous concrete is particularly liable to demage from frost attack. Use of de- icing salts containing cholorides greatly increases the chance of frost damage. [1] e. Repair Materials Each material or component in a structure has an expected service life and requires periodic repair or replacement during lifetime of the structure. The finite life of material is a consequance of the gradual chemical, physical or mechanical changes that degrade them and reduce their ability to perform as required. Consequently, there is a need for repairing or upgrading structure to designed level of serviceability. Recent years have seen an increased emphasis on the repair and refurbishment of all types of structures in preference to demolition and rebulding. As a result, there is an increasing demand for repair materials. Every repair job has unique conditions and special requirements. Once this criteria are known, it will often be found that more than one material can be used with equally good results. Final selection of the material or the combination of materials must than take into account the ease of application, cost, available labor skills and equipment. Data on the service life of the repair materials are essential for effective selection, use, and maintenance of these materials. Service life data are also needed to assess the performance as a function of cost and thereby permit the selection of the most economically attractive option. [2] f. Prevention and Repair Cracking resulting form intrinsic effects within concrete is more likely to result in unserviceability, particularly in the case of water-retaining structures, than actual collapse. However, certain fundamental precautions both in the desing process and during construction can minimize such effects. In the case of environmental attack it is the concrete cover to the reinforcement which traditionally in prime defence. Ideally this cover concrete should have low permeability to water, oxygen, chloride ions and water vapour. Factors which may be beneficial in reducing penetration are the following:i. incresed depth of concrete cover; ii. Low water / cement ratio to minimize capilaries formed as the fresh concrete 'bleeds'; iii. Efficient curing of adequate duration to minimize the formation of capillaries; iv. Coatings and barrier treatments applied to the surface; One repair method is complete reconstruction with members constructed from fresh concrete designed, placed and cured with the above factors in mind. However, it is often more economical to consider local patching of the damaged area. The patching process may be multi-staged and the choice of material for reinstating the cover is often a difficult one. The options are cementitious, polimer-modified cementitious or resin mortars generally selected in that order as the thickness of the patch decreases. Of prime importance to the long-term stress characteristics of the repair is the bond to the concrete substrate. There is also a need to consider the level of mismatch between the properties of the repair material and those of the original concrete with regard to resisting structural loading, thermal and creep effects and assessing the level of composite action between the two materials. An alternative method, particularly for protecting chloride-contaminated structures, is to use cathodic protection. XI

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