Prefabrike betonarme yapı elemanlarında kalıp teknolojisi

Abstract

Bu çalışmada prefabrike betonarme yapı elemanlarında kullanılan kalıp türleri ülkemizde ve dünyadaki mevcut koşulların ışığı altında açıklanmıştır. İlk olarak yapımda rasyonelleşme ve prefabrikasyon konulan ele alınmış ve özellikle kalıbın yapımda rasyonelleşme konusu içinde önemi vurgulanmıştır. Özellikle konut üretiminin yapı endüstrisi üretkenliği ile bağdaştırılmasının prefabrike yapı elemanı kullanımındaki artış ile sağlanabileceği açıklanmıştır. Bunu söylerken betonarme prefabrike yapı elemanı kullanımındaki artış yalnızca konut üretiminde değil diğer tüm bina türlerinde ve mühendislik yapıları üretiminde sağlandığı takdirde yapı üretiminde en yüksek endüstrileşme seviyesine ulaşılabileceği vurgulanmıştır. Ayrıca yapı elemanlarındaki standardizasyon ve modüler planlama prefabrike eleman kullanımına çok büyük fırsatlar verdiğinin altı çizilmiştir. Bölüm 3' de betonarme prefabrike eleman kalıplarından beklenen genel özellikler açıklanmış ve kalıplar ilk olarak malzemelerine göre sınıflandırılmış daha sonra pozisyonlarına göre de sınıflandırılarak açıklanmıştır. Çalışmanın son iki bölümünde tolerans konusuna değinilmiş, kalıp hataları ve tolerans ilişkisi ayrıca kalıbın beton yüzeyine etkisine değinilerek kalıp içinde kullanılan, ucuz kullanıldıktan sonra atılabilen ve kalıbın ömrünü uzatan kalıp içi astarları konusu açıklanmıştır.

The principles and basic methods of desing and practice of making formwork fo precast products are no different from those for in-situ construction. The differences are ii the details of formwork construction due to the improved accuracy required of size, shap and finish and the additional refinments of the casting processes. While elements of buildings, bridges and other civil engineering constructions have bee: precast, mainly on the actual site of the works or in a precast yard set up for a particula contract, the large scale use of precast items and the application of factory techniques ha not, up to the present time, been widely prefabricated. However the present housing programme can only be met by a considerable increase in th productivity of the building industry, requiring a vast increase in the use of factory mad precast units. This industrilisation of the building process will lead to the increase in use c precast concrete components not only for housing but also for the whole range of buildin and civil engineering. The introduction of Modular Planning will mean standardisation c building components, giving further opportunity for the use of prefabricated elements. The following materials are commenly used for moulds: Plywood, steel, aluminium, plasti< and resins Prefabricated products can be classified into two main groups according to demoldir process: immediate or delayed. In immediate demoulding products are demoulded as soc as the compaction process is complete. Table 4.1 indicates the range of products included this category, and the manufacturing processes involved. Vlll Generally the compaction process involves various applications of compacting force by means of spinning, vibrating, pressing, tamping or combination of any two of these processes. Demoulding involves pushing the unit out so that it is really an extrusion of the unit from the mould which usually takes place as soon as compaction is completed. In some cases green strentgh may not be high enough for immediate demoulding and part of the mould might remain on the unit for a few hours. In delayed demoulding products are cast in the conventional way in the moulds. Although there may be very high degree of mechanization in the casting process and placing the concrete itself, the important distinction is that demoulding does not take place until the unit has been allowed to develop a high degree of strenght. Delayed methods can be classified into three main groups (a) horizontal (b) vertical (c) special. Production techniques can be devided into three main groups; (a) production of separate items (b) mass production (c) process production. In seperate item productions units are usually not moved about during manufacture; they mature where they are moulded and are completed stage by stage. Mass production technique involves a grater degree ol mechanization than seperate item production. Process production is the most advanced method of manufacture ; successive production stages form a uniform and progressive line which is to great extent automated. Mould desing is one of the most important consadirations for efficient precast concrete production. The designer must have a detailed knowledge of methods of manufacture Vibration, re-use, self location, mould joints, mould movements, mould durability an important factors which should be considered. Obviously the cost of a mould will affect th< cost of the product formed threin. Re-use factor must be considered as well as first cost First cost of a steel mould, for instance is high, but if large number of uses is required, it high re-use potential will compansate for this. The traditional materials for moulds are timber and steel, the use of timber usually being limited to moulds which will have a relatively small number of uses, while steel is genarally used for repetition processes. Inaccuracy in precast concrete units, and their use in constructing buildings can result from distinct sets of causes. (a) inherit inaccuracies (deriving from the actual manufacturing process: ie mould inaccuracy and concrete shrinkage), (b) induced inaccuracies (deriving from factors associated with subsequent use of the component: ie inaccuracies in setting out the building grid, and in physical assembly of components in relation to the grid) The often appalling apperance of exposed concrete surfaces on the structures in our country can be ascribed to two primary factors: atmospheric pollution and the placing of low quality concrete in unsatisfactory formwork. In order to achieve a plasing cocrete surface cooperation between the engineer, architect, formwork designer and the contractor is essential, and particular consadiration should be given to the location of the work and the degree of pollution of atmosphere. Rough textured surfaces weather better than smooth, and in sooty areas the effect of light and shade is retaind, whereas the colours and texture of finer finishes is soon lost. In order to obtain the desired finish, it is essential that trial panels are constructed, and the formwork should be designed to repeat the finish of the trial panel. The formwork must be watertigth and rigid, and any releasing agents, such as mould oil or other protective coatings with which the formwork is treated, must not attack the concrete. Erection of formwork is usually carried out in exposed conditions where it is often quite impracticable to avoid contamination and damage. The consequence is that repeatec erection and dismantling leads to deterioration of the form face that eventually ends it; useful life long before its theoretical re-use potential is exhausted. Thus in theory a cheap disposable formwork liner should be a viable economic proposition, and in fact absorbent linings already promise to equal if not excel the cheapness of untreated plywood. Formwork costs should be considerably reduced by the low quality materials needed to back up the linings, and the improved re-use potential of such backing when it is neither exposed directly to concrete nor put out of action by minor damage from handling indicates that more widespread use of the technique is likely in the future. For precasting, use of an expendable lining inside the mould has several advantages such as simplifying mould cleaning, free of blowholes, extending the re-use potential, reduced mould costs, using non- standart units for fair faced finishes, avoding drying discoloration.