Betonun dona dayanıklığının çimento hamurunun boşluk yapısına duyarlılığı

Abstract

VI ÖZET Bu çalışmada betonun dona dayanıklılığının çimento hamurunun boşluk yapışma duyarlılığı incelenmiştir. Bunun için A16 - B16, B16, B16 - C16 granülometri referans eğrilerine uyan 3 ayrı grup için ve bu grup içinde çimeno/su oram farklı 5 ayrı karışım üretilmiştir. Her karışımda 8 adet 7x7x28 cm.'lik numune (4'ü şahit, 4'ü yüzeysel don deneyinde) ve 6 adet 15x15x15 cm.'lik numune (3ü şahit, 32ü normal don deneyinde) üretilmiştir. Don deneyleri başlayana kadar numuneler 20 °C ± 2 ° C sıcaklıktaki kirtece doygun suda saklanmıştır. Yüzeysel ve normal don deneyleri 20 tekrar olacak şekilde ve her 5 tekrarda bir ultrases hız şahitleri ile birlikte ölçülerek yapılmıştır. Yüzeysel don deneyleri beton yüzey üzerindeki dondurulmuş suyun NaCl çözeltisi ile çözülmesi ile yapılmıştır. Don deneyleri bittikten sonra yüzeysel don deneyine girenlerde eğilme dayanımları normal don deneyine girenlerde basınç dayanımları (şahitleriyle birlikte ) ölçülmüştür. Elde edilen değerlere göre ortaya çıkan şöyle özetlenebilir. Ultrases hızı ölçümleri yüzeysel ve hacimsel don hasarlarım belirlemede yetersiz kalmıştır. Hacimsel don deneyi gören numunelerdeki don hasan betondaki hava boşluğundan etkilenmemiş çimento hamurundaki kılcal boşluklardan etkilenmiştir. Don hasan ile çimento hamuru yapısı arasındaki ilişkiyi gösteren bağıntılar Yüzeysel don deneyine giren numunelerde Ff / (Ff)şahit = 0,013393. (C/ a) + 0,972024 Korelasyon Katsayısı= 0,595164 Hacimsel don deneyi gören numunelerde Fc / (Fc)şahit = 0,046793. (C / w) + 0,961244 Korelasyon Katsayısı^ 0,297175 Hacimsel don deneyi hasarında hava boşluğunun etkisiz olduğu bulunduğundan bağıntı (C / w) değişkenine göre hesaplanmıştır.

vıı SUMMARY In addition to the development in the concrete technology 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 concrete service ages importance becomes more imported point. The service life of the concrete can be defined as the ability of the concrete to continue its properties like its first shape under the effects of the surrounding conditions and the ability of serving for along time without missing any of them. It a pears as a result of the technical and the economical desingofthe concrete In order to reach this aim, the quality control and tests of concrete before and after production have & big importance. Before production concrete the effects which the concrete will he exposed to and 4he role of the concrete in the structure should be taken into account. The materials should be chosen according to purposes of the concrete usage and the probable effects which may come from the soil should be avoided The produced concrete with the selected gradients should be guaranteed to be compacted segregation with the suitable means. After pouring, the concrete should be cured until it reaches enough durability. As it was mentioned before, the concrete which was not made quality control it will always be under the threat of failure under chemical and physical effects There are three main 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 usage of concrete, other properties are also wanted. Like casting under water, and durability againts abrasion and freezing, Durability againts freezing and thawing, is very important especially to the areas in which freezing and thawing is repeated many times. The water which is found inside the porous material increases its volume when it is exposed to freezing. When the stresses which results from the change in volume exceeds the tensile strenght of concrete fracture appears followed by rupture of the material. Chemical air entraining admixtures are added to concrete to prevent the progress rate ofvm water and decrease the hydraulic pressure and as a result of this the durability of concrete againts freezing and thawing is increased. The observation that the durabilir)' 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 againts the combined influence of freezing and deicing salts are generally iower man its resistance to frost alone. The causes of this puzzling phenomenon are fully yet known Combination of Freezing and Deicing Salts The use of deicing salts in practice has negative as well as positive conquences with respect to the durability of concrete. The effects 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 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 retailed. 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 to freezing and deicing salts depends mainly on the durability of its outermost zones. The materail 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 theIX order of magnitude of the tensile strength of concrete under unfavorable circumstances (thickness of ice 0,5 mm and more, high sah concentration) The places which are much exposed to freezing are roads, airports, canals and the surfaces which are exposed to weather conditions. At the roads, part of the accidents happen, are 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 quality concrete made by freezing, the unsuitable conditions are taken into account the creation of ice must be stopped. In order to prevent the formation of ice in airports and roads, an abrasion material, or a combination of abrasion and chemical materials, or only chemical materials are used In order to reduce or remove ice and compacted snow, chemical material? like rock salts and sodium clorid (NaCl) are used. In addition to these materials cruched stones, sand and colliery are used in the icy roads, especially rock salts are more suitable for use It is proposed that rock salts and sodium klorid have no effects on concrete But their presence with ice may make damages at the surfaces for repeated applications for removing the ice The degree of the damage depends on the concrete quality, the concentration of the salt, and the repeated drying and wetting. Another reason for damage is the crystallisation of salts m pores during drying resulting in increasing the volume Three different groups of concrete, each have five different E/C ratios were prepared. The specimens which were not exposed to freezing and thawing were kept under wet sack, The samples at the age of 28 days had been frozen and thawed for 20 times. In addition to the classical experiments of freezing and thawing of whole specimen, the experiments on frozen thawing by making use of NaCl (rock salt) had also been done. In these tests one salt concentration of 0,200 gr/cm2 had been used After having done the freezing and the thawing for 5 times, the resonance frequencies (or ultrasound velocity) of the prismatic specimens, as well as cubic specimens were measured and the reductions in the dynamic modulus of elasticity 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 thawing for 20 times, the mechanical strenghts of the concretes were obtained, by simply exposing the specimens to bending and compression tests 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. In the boundary of this study we find mainly the following results:1- Ultrasound velocity measurements are unsufficient for determining superfical and volumetric freezing damages. 2- Freezing damage of samples which are used in volumetric freezing experiments, is not affected by the large pores in concrete. But it is affected by capillary pores in the cement paste 3- Results which indicate relation between freezing damage and structure of cement paste, as follow Samples which are used in superfical freeze experiment: Ff / (Ff)şahit = 0.013393 x (C/a) + 0.972024 correlation coefficent = 0.595 164 Fc / (Fc)şahit = 0.046793 x (C/w) + 0.96İ244 correlation coefficent = 0.297175