Çimento hamuru konstrasyonunun betonun dona dayanıklılığına etkisi

Abstract

Bu çalışmada süperakışkanlaştırıcı katkılı betonlarda çimento hamuru hacmini normal ve yüzeysel dona dayanıklılığa etkisi incelenmiştir. Bunun için çimento dozajının 200 kg/m3'den 475 kg/m3'e kadar değiştiği 12 adet karışım üretilmiştir. Yüzeysel donma deneyleri beton yüzey üzerinde dondurulmuş suyun NaCl çözeltisi ile çözülmesi ile yapılmıştır. Su/çimento oranı 0.55 alınırken, süperakışkanlaştırıcı katkı maddesi karma suyunun %3'ü kadar alınmıştır. Böylelikle düşük dozajlı betonların da yeteri kadar sıkıştırılabilmesi amaçlanmıştır. Bütün karışımlarda agrega granülometrisi aynı olup, A16-B16 sınır eğrileri arasındadır. Her karışımdan 8 adet 7x7x28 cm numune (4'ü şahit, 4'ü yüzeysel don deneyinde) ve 6 adet 15x15x15 cm numune (3'ü şahit, 3'ü normal don deneyinde) üretilmiştir. Don deneyleri başlayana kadar numuneler 20o ± 2oC sıcaklıktaki kirece doygun suda (28 gün) saklanmıştır. Yüzeysel ve normal don deneyleri 20 tekrar olacak şekilde ve her 5 tekrarda bir ultrases hızı şahitleri ile birlikte ölçülerek yapılmıştır. Don tekrarlan bittikten sonra yüzeysel don deneyine girenlerde eğilme dayanımları, normal don deneyine girenlerde basınç dayanımları (şahitleri ile birlikte) ölçülmüştür. Elde edilen değerlere göre ortaya çıkan şöyle özetlenebilir: * 250 kg/m3 dozajın altında beton mukavemeti ve durabilitesi düşük çıkmıştır. * Dozajın 250kg/m3 'den 475 kg/m3 'e yükselmesi ile basınç dayanımında küçük artış olurken eğilme dayanımında azalma olmuştur. Ama normal donmaya dayanıklılığında küçük bir azalma olurken yüzeysel donmaya dayanıklılığında belirli bir yükselmeye neden olmuştur. * Ultrases hızları C=275 kg/m3 civarında maksimumdan geçmiştir. Bu bölgede çimento ve agrega dağılımı iyi olduğundan maksimum sonuç çıkmıştır.

Today the failures in the concrete creates serious technical and economical problems. These problems begins from the structure becoming not valid for use to the collapsing of it reasoning to the loose of lives and even threats the economy of the country. In our country according to the durability of concrete and the problems which appeared in this area there is not statistics referring to it but the known example are too much. In addition to the development in the technical properties of concrete the fields of use also developed according to these developments the durability of concrete, permeability, elasticity and many other properties were also developed. The fields were the concrete used and chemical and physical conditions which it faces, if they were taken into account, as a result to the durability of concrete, the concretes service ages importance becomes more imported point. The service age of the concrete can be defined as: under the effects of the surrounding conditions the ability of the concrete to countinve its properties like its first shape and the ability of serving for along time without missing any of them. This case appears as a result of the technical and the economical design of the concrete. In order to reach this aim: the quality control and tepts of concrete before and after production have a big importance. Before production concrete the effects which will the concrete will be exposed to and taking into account the role of the concrete in the structure. The materials should be chosen according to purposes of the concretes uses and the probable effects which may come from the oil can be avoided by mending the soil. The produced concrete with the selected contents should be guaranteed to cast after being maximum compressed without segregation with the suitable means. After being pouring (casting) the concrete should be cured until it reaches enough durability. As it was mentioned before the concrete which wasn't quality controlled always it will be under the threat of fail under chemical and physical effects. There are three wanted properties of concrete: - The workability of fresh concrete - Enough mechanical strength for the hardened concrete - Durability to the outside conditions In addition to these general properties, according to the use of concrete other properties are wanted. Like casting under water, and durability against abrasion and freezing. Durability against freezing and thawing, is very important especially to the areas in which freezing and thawing is repeated very much. The water which is found inside the porous material increases its volume when it is exposed to temperature under 0°C because it freezes. When the stresses which results from the change in volume passes the tensile strength of concrete fractures appears followed by rupture of the material is the amount of water inside it. Chemical air entraring additives are added to concrete to stop the progress velocity of water and decrease the hydraulic pressure and as a result of this the durability of concrete against thawing is increased. The observation that the durability of concrete often is lower under the combined influence of frost and deicing salts than under frost influence alone is discussed with regard to several physical aspects and mechanisms, for example, hydrodynamic effect, capillary effect, super cooling, lowered melting point of water in smaller pores, layer by layer freezing. It is concluded that some of the most detrimental factors with regard to the durability of concrete are super cooling of water and aqueous solutions and a higher degree of saturation of the concrete in the presence of salts. The differing effects of dry application of deicing salts on snow and ice covered concrete (temperature shock) compared to the preventive salt application on humid concrete (prevention of ice formation but with some negative aspects) are discussed. For many years it has been observed that the resistance of concrete against the combined influence of freezing and deicing salts seems to be generally lower than its resistance to frost alone. The causes of this puzzling phenomenon are yet fully known Combination of Freezing and Deicing Salts The use of deicing salts in practice has negative as well as positive consequences with respect to the durability of concrete. The effects XI depend on the aggregate state of the salt (dry, humidor in solution) and on the method of application on ice and snow, or preventively applied on humid or wet concrete). Negative Consequences=> Degree of saturation, super cooling due to preventive salt application, layer by layer freezing, temperature shock, displacement of 0°C limit due to temperature shock, crystallisation pressure. Positive Consequences=> The higher the concentration of soluble salts in water, the lower will be the crystallisation of deicing salts positively effects the durability of concrete since the salt delays the ice formation. Even if some ice is formed, the salt concentration in the residual solution will increase and in this way any further ice growth will be retarted. In addition, the capillary effect occurs only at lower temperatures and then to a lesser extent than by freezing without salts. The resistance of concrete of freezing and deicing salts depends mainly on the durability of its outermost zones. The material properties of these outermost zones appear to be rather in homogeneous. Thus, variability of material properties is inevitable in these zones. In addition, external influences may cause other in homogenates in the concrete, for example, gradients of water saturation, salt concentration or temperature. Thermal gradients, especially those created by the temperature shock during the process of ice melting by means of deicing salts, can cause the development of internal stresses in the concrete. An estimation of the internal tensile stresses is determined on the basis of temperature shock experiments; they may reach the order of magnitude of the tensile strength of concrete under unfavourable circumstances (thickness of ice 0,5 mm and more high salt concentration). The places which are much exposed to freezing are roads, airports, canal sand the surfaces which are exposed to weather conditions. At the roads part of the accidents happen as a result of freezing. In airports the runways must be clean and suitable for planes to land. In addition to the looses in the concrete made by freezing if the unsuitable conditions are taken into account the creation of ice must be stopped. In order to prevent the ice to be produced in airports and roads, abrasion material, abrasion and chemical materials or only chemical materials are used. In order to reduce or remove ice and compacted snow chemical materials like rock salts and sodium kîorür (NaCl) are used. In addition to these materials crushed stones, sand and colliery are used in the icy roads, using rock salts in more suitable for use. Xll It is proposed that rock salts and sodium have no effects on concrete. But these presence with ice may make damages at the surfaces appear at using the salts many times for removing the ice. The degree of the damage depends on the concretes quality the concentration of the salt and drying and wetting are coming after each another many times. Another reason for damage is the crystallisation of salts in pores during drying resulting in increasing the volume. The purpose of this investigation is to determine the in flounce of salt on hinge range water reducer concrete. Example: concrete highways and bridges, bridge railing, retaining walls, Twelve types of fresh concrete having different 6 (cement) ratios which 2000, 235, 250 and 475 were produced. These concretes were produced by using hinge range water reducer admixtures. The specimens which were not exposed to freezing and thawing were kept under wet sack. The samples which were at ages of 28 days had been frozen and thawed for 20 times. In addition to the classical experiments such as freezing and thawing of whole specimen, the experiments on freezing and thawing of the specimen by making use of NaCl (rock salt) had also been done. In order to do that one type of concentration being 0,200 gr/cm2 had been used. D* After having done the freezing and the thawing for 5 times, the resonance frequencies (or ultrasound speeds) of the prismatic specimens, cube specimens were measured and the reductions in the dynamic elasticity modulus of the specimens were compared with reductions in the dynamic modulus of elasticity of the specimens which had not been exposed to freezing and thawing. In addition to those having repeated the freezing and the thawing for 20 times, by simply exposing the specimens to bending and compression tests the mechanical strengths of the concretes were obtained. The theoretical ice formation and the theory of destruction of freezing had been investigated and the results of the experiments were explained basing on the theories. The conclusions obtained in this study can be in breaf as follows: 1 The concrate streight and durability have been found lower then the dosage under 250 kg/m-\ The doughes of concrate under the dosage is in sufficient to fill in cavity between aggregates. Xlll 2 if dosage of the cement increase from 250 kg/m3 to 475 kg/m3, strength of compression will be increase a iittle, but bending strength will be decrease. 3 Ultra-sound speeds having maximum at 275 kg/m3. Because cement and aggregates are better scattering than the other area. The relation between cement and strength of compression are as follows: fe= 0.00067C + 39.616366 The relation between cement and bending strength are as follows: