Beton aşınmasının iki fazlı malzeme olarak incelenmesi

Abstract

ÖZET Bu çalışmada, iki fazlı kompozit malzeme olarak düşü nülen betonda fazların aşınma özeliklerinin ve hacım oran larının değişiminin betonun aşınma direncine ve ömrüne olan etkileri incelenmiş, betonda aşınma özeliğinin kompozit mal zeme kurallarıyla ifade edilebilmesi amaçlanmıştır. Ayrıca sertleşmiş betonun bazı özelikleriyle aşınma arasındaki ilişkiler araştırılmıştır. Bu amaçla II. ci Bölümde yapılan yayın taramasından, bir birlerine değen katı cisimlerin bağıl hareketleri sırasında malzeme yüzeyinde oluşan sürtünme kuvvetlerinin aşınmaya neden oldukları anlaşılmaktadır. Bu sürtünme kuvvetleri ci simlerin hareketini zorlaştırarak ısı yükselmesi ile enerji kaybına neden olmakta ve sonuçta adezyon, abrazyon, koroz- yon ve yorulma olayları malzeme yüzeyini aşındırmaktadırlar. Sürtünme ve aşınma olaylarının mekanizması özellikle metal lerde detaylı olarak incelenmiş; sürtünme ve aşınma olayla rını açıklamaya çalışan çeşitli teoriler geliştirilmiş ve malzemenin aşınmaya dayanıklığının arttırılması araştırılır ken, sürtünme kuvvetlerinin en düşük değere indirilebilmesi için yağlama gibi önlemler geliştirilmiştir. Beton, tabii taş ve organik ve inorganik esaslı çeşitli zemin kaplama malzemelerinde ise durum biraz farklıdır. Bir yandan kaplama malzemesinin aşınma direnci art tın İmaya çalışılırken, diğer yandan yaya ve araç trafiğinin sağlıklı gelişebilmesi açısından sürtünmenin gerekli olduğu ve kay ganlığın endüşük değere indirilmesi gereği ortaya çıkmakta dır.XI Bu konuda yapılan yayın taramasından beton döşeme ve yollarda adezyon ve abrazyon olaylarının, su yapılarında ise kavitasyon ve erozyon olaylarının aşınmaya neden oldu ğu anlaşılmaktadır. Çeşitli aşınma şartlarına maruz betonun aşınma direncinin arttırılabilmesi için, betonun sert ve sağlam agregalar kullanılarak iyi kalitede üretilmesi ve betona iyi bir bakım uygulanması gerektiği ortaya çıkmakta dır. Ayrıca aşınmaya dayanıklı beton üretimi için çeşitli katkı ve özel yüzey uygulamaları önerilmekte ve servisteki betonların aşınma dirençlerinin arttırılması için bazı acil yöntemler geliştirilmektedir. Son zamanlarda betonun kompozit malzeme olarak ele alınıp, elastik ve ine las tik birçok özeliklerinin çeşitli iç yapı modelleri yardımıyla hesaplanab ilmesi amacıyla çok sayıda araştırma yapılmış olmasına rağmen, yapılan yayın taramasından betonun aşınma özeliği ile ilgili böyle bir çalışmanın bulunmadığı görülmüştür. Daha önceki çalışmalar da betonun aşınma özeliği genellikle çeşitli aşınma şartla rının, simüle edildiği deneysel yöntemlerle araştırılmış ve betonun çeşitli aşınma şartlarına dayanıklı hale getirile bilmesi için önlemler incelenmiştir. Yayın taramasından elde olunan bilgilerin ışığında be tonun aşınma özeliğini kompozit malzeme kurallarıyla araş tırmak ve aşınma ile diğer beton özelikleri arasındaki iliş kileri belirlemek amacıyla deneysel bir çalışma plânlanmış tır. III. cü Bölümde verilen deneysel çalışmalarda kompozit bileşenlerinin niteliklerinin bir seri içinde sabit kaldığı 9 değişik seri beton üretilmiştir. Farklı serilerde, harç ve iri agrega fazlarının aşınma özeliklerini değiştirebil mek için su/çimento oranı ile ince ve iri agrega nitelikle ri değiştirilmiştir. Diğer taraftan harç ve iri agrega faz larının niteliklerinin değişmediği bir seri içinde, iri ag rega fazının hacım oranı 0 dan 0,40 'a kadar arttırılmış ve böylece 35 farklı bileşime sahip harç ve beton üretilmiş* tir. Harç ve iri agrega fazlarının belirginleşmesi için çalışma süreksiz granülometrili karışımlar üzerinde yapıl mıştır. Her karışımdan hazırlanan 10x10x50 cm'lik prizma numuneler üzerinde rezonans frekansı, eğilme, basınç ve Schmidt sertliği deneyleri yapılmış, prizmalardan kesilenXll numuneler üzerinde de BÖHME (DİN 52108) aletiyle aşınma deneyleri yapılmıştır. Deneylerden elde edilen diyagramların incelenmesinden, sertleşmiş betonun aşınma özeliğinin iri agrega hacım ora nıyla değişimi ile aşınmanın sertleşmiş betonun bazı özelik- leriyle değişimi IV. cü Bölümde irdelenmiş ve değerlendiril miştir. Bu değerlendirme V.ci Bölümde özetlenen şu bulguları ortaya çıkarmıştır: îri agrega fazının aşınan kesitteki yüzey oranı betonun aşınma özeliğinin fiziksel açıklamasın da daha iyi değerlendirme olanağı sağlamakta ve betonda aşınma özeliği kompozi t malzeme kurallarıyla ifade edile bilmektedir. Aşınmaya daha dayanıklı olan fazın miktarı arttıkça, kompozitin aşınmaya dayanıklılığı artmaktadır. Harç fazı niteliği sabit kalırken, aşınmaya dayanıklı iri agrega kullanılmasıyla kompozitin aşınma direnci önemli miktarda artmaktadır. Harç fazı sağlam betonlarda kalker kırmataşı miktarının artması aşınma direncini arttırmamak- tadır. Betonun aşınmaya dayanıklılığı, birim ağırlık, eğil me ve basınç mukavemeti, dinamik elastisite modülü ve Schmidt sertliği ile aynı yönde artmaktadır. Betonun aşın masına etkiyen faktörler, aşınma ömrünü de aynı yönde et kilemektedirler. Böylece diğer bazı beton özeliklerinde olduğu gibi, betonun aşınma özeliğinin de kompozit malzeme kurallarıyla ifade edilebileceği ortaya çıkmıştır.

Xlll SUMMARY Foot traffic and the skidding, scraping or sliding of objects on the floors and stairs of residential and indus trial buildings, and of railway and subway stations result in wear of concrete surfaces. Concrete on roads, highways, bridges and the landing and parking facilities of airports is also subject to wear due to heavy trucking and cars, with and without chains. Concrete on hydraulic structures such as dams, spillways, bridge abutements and tunnels, on the other hand, is subject to wear due to the action of abrasive materials carried by waters flowing at low velocities as well as the cavitation erosion where a high hydraulic gradient is present. So far it has not been possible to develop a single test method to measure the wear characteristics of flooring materials under all conditions, be cause of either the com plexity of the wear mechanisms or the variety of causes of surface wear. Instead, the wearing characteristics of building materials such as stone and concrete are generally determined experimentally using an apparatus under standard wearing conditions and the effects of material properties and environmental conditions on wear are evaluated by comparisons. However, the writer has not encountered in literature, any investigation on wear of concrete, consider ing it as a composite material. For this reason, the aim of this study has been chosen to approach the concrete as a two-phase composite material, hoping to be able to formulate in this way analytical expressions for the amount of wear of concrete in terms of the volume fractions and the wear of each of the two phases, namely the mortar and the aggregate. Also, relations between the wear and some other properties of concrete are investigated.XIV The literature survey presented in Part II is concerned with three topics: Firstly, the nature, laws and types of friction and wear of various materials as well as the theories attempting to explain the friction and wear mecha nisms of metals are reviewed. From the summaries in this section, following important conclusions are drawn: Four main types of wear are distinguished for metals sliding on each other in the absence of a lubricating material between the contacting surfaces. Adhesive wear is the result of cold welding occuring at the asperities of the contacting surf aces, whereas the abrasive wear is the removal of solid material from a surface caused by plowing or gouging out by another body much harder than the abraded surface. Corrosive wear is the result of the combined action of a corrosive environment and mechanical forces acting on a surface. Surface fatigue wear is primarily associated with the rolling motion. The amount of adhesive and abrasive wear is directly proportional to the normal force acting on the contacting surfaces as well as to the sliding distance and inversely proportional to the hardness of the surfaces. The nature of friction and wear of nonmetals may be simu lated to that of metals and the theories explaining the wear mechanisms of metals may be applied to a certain extent to materials other than metals. Secondly, a detailed summary of researches on wear of concrete is presented. Previous investigations on the abrasion of concrete surfaces reveal that resistance of concrete surfaces to abrasion increases with the increase :. in cement content, compressive strength and the decrease in water-cement ratio. Concrete becomes more resistant to abrasion as the fine and coarse aggregate used are harder, better graded and more resistant to abrasion. Curing and surface finish are among the important factors which affect the abrasion of concrete surfaces. In some cases, special surface hardeners, liquid or dry-shake, are found to increase the resistance of concrete surfaces to abrasion. Thirdly, literature on composite materials are reviewed briefly. Even though concretes are multiphase materials,XV composite two-phase models for their mechanical behaviour have found general acceptance. Considering that concrete is an aggregate-cement paste composite, elastic constants, shrinkage and creep of concrete have been evaluated using relations derived from various two-phase models. Part III presents the description of the tests and the test methods. In the experimental part of this study 9 different series of concrete mixes have been used. The qualities of the phases of the composite material have been changed from one series to the other. In each series, however, the volume fraction of the coarse aggregate phase was changed from 0 to 0.40 only. Thus, concretes with 35 different mix proportions were produced. In order to change the quality of the mortar phase, either the type of the fine aggregate or the water-cement ratio was varied. In order to change the nature of the aggregate phase, the type of the coarse aggregate was varied. Prismatic specimens of 10x10x50 cm in size were fabricated and tested nondestructively for surface hardness by rebound hammer test, and dynamic modulus of elasticity by the resonance test respectively. Then, the modulus of rupture and the compressive strength of concretes were determined by flexure tests and equivalent cube tests respectively. Finally, the abrasion tests were done on samples cut from the pieces of the prisms which had not been tested for compressive strength. The abrasion tests were made using the German standard (DIN 52108) method of test apparatus called BÖHME abrasion table, with the modification that the surface areas of the samples subjected to abrasion were 100 sq.cm., but not 50 sq. cm. as specified in the German test method. Therefore the vertical load applied to the sample on the abrasion table was increased from 30 kg. to 60 kg., and the amount of the abrasive material, renewed after every 22 turns of the abrasion table, was increased from 20 gr. to 40 gr. in order to account for the change in the surface area. The sample was rotated 90° at the end of every 22 turns of the table and the decrease in the height of the sample was measured at the end of every 88 turns. The loss in the specimen after a total of 440 turns of the abrasion table was taken as the depth of abrasion of the sample.XVI In Part IV discussions and evaluations of the test results are presented. In the discussion of the abrasion test results, the variation of the depth of abrasion with the surface ratio (Ap) of the coarse aggregate was considered. However, the volume ratio and the surface ratio of the coarse aggregate have not been found to differ very much. Parallel and series phase distribution models as well as the relation of two-phase suspensions were considered for the variation of the depth of abrasion with the surface ratio of the coarse aggregate. It has been shown that an expression of the form of lower bound relations, offered for the modulus of elasticity of concrete, represented the variation of abra sion depth with coarse aggregate content best. The abrasion resistance of the concrete has been defined as the reciprocal of the depth of abrasion. When the difference between the abrasion resistances of the two phases was large, the depth of abrasion of the composite has been found more sensitive to the changes in the coarse aggregate volume ratio. For concretes, containing gravel or granite coarse aggregate, with a water-cement ratio of 0.50, the abrasion resistance of the coarse aggregate was much higher than that of the mortar. The addition of these kinds of hard aggregates in to the mortar increased the abrasion resistance of the concrete. With the increase in the volume fraction of the hard coarse aggregate in the mix, the number of hard coarse grains on the surface subjected to abrasion has been tought to increase and hinder the abrasion of the mortar. As the quality of the mortar was improved by reducing the water-cement ratio from 0.50 to 0.35, the effect of the increase in the volume ratio of granite or gravel aggregates became less pronounced in increasing the abrasion resistance of the concrete. On the other hand, for concretes with a water-cement ratio of 0.50 but containing crushed limestone aggregate, the abrasion resistances of the two phases were almost equal and the changes in the volume ratio of limestone coarse aggregate did not influence the depth of abrasion of the concrete. Increasing the water-cement ratio to 0.65 orXV11 using an inferior sand in the mix resulted in a decrease in the abrasion resistance of the mortar. In these cases, an increase in the volume ra>tio of the limestone coarse aggregate decreased the abrasion loss of the concrete surface. Furthermore, as the water-cement ratio was reduced from 0.50 to 0.45 and 0.35 successively, the abrasion resistance of the mortar phase was improved, and hence the abrasion loss of the concrete increased with an increase in the volume ratio of the softer limestone coarse aggregate. The type and quality of the fine and coarse aggregates used in the mix affected the abrasion resistance of the concrete. Good graded sand of granite origin improved the abrasion resistance. Also, the use of gravel coarse aggregate, composed mainly of quartz and quart zites, increased the abrasion resistance of the concrete much more, than the crushed granite or limestone. Water-cement ratio, on the other hand, had also a great effect on the abrasion resistance of the concrete. The lower the water-cement ratio of the mix, the higher the abrasion resistance. In this investigation, it has been found that there is a critical value of water-cement ratio of 0.475 below which the use of a higher volume ratio of limestone coarse aggregate in concretes containing a good graded hard sand did not improve the abrasion resistance of concrete. It has been established in this study that abrasion resistance may also be related to various other properties of concrete. The abrasion depth decreased with the increase in unit weight, dynamic modulus of elasticity, rebound number, modulus of rupture and compressive strength of concrete. A linear relation between the rebound number and abrasion loss gave the highest correlation coefficient. The abrasion depth was found to be inversely proportional to some power of each of the properties cited above. The correlations between abrasion and each of these properties were also found to vary with the type of the coarse aggregate used. The correlations between the abrasion resistance and some other property of the concrete seemed to be high for concretes containing poor quality mortar or coarse aggregate. Improving the quality of the components of the composite lowered thi^s correlation.XVI 11 Factors which affect the abrasion resistance of concrete also seemed to affect the abrasion life of the concrete in the same way. Average abrasion life may be calculated by dividing the allowable depth of abrasion, chosen for specified conditions of wear, by the rate of abrasion of concrete surfaces tested at an abrasion apparatus simulating the same specified conditions of wear. On the other hand, it is also necessary to determine the abrasion life of a concrete surface with a certain confidency by evaluating statistically the results of a large number of abrasion tests. Part IV contains conclusions for some practical evaluation of the test results. A few mix proportions for abrasion resistant concretes are also given. Part V presents `a summary of results and some proposals for future study. The most important finding in this investigation is the demonstration that abrasion of concrete may be evaluated by approaching the concrete as a two-phase composite material. The abrasion of the concrete is deter mined by the abrasions of the phases. The abrasion resistance of the concrete becomes higher, as the volume fraction of the phase more resistant to abrasion is increased. The Appendix includes tables of results and evaluation diagrams.