Betonda sıvı hareketinin kompozit malzeme kuralları ile incelenmesi

Abstract

Bu Çalışmada harç ve iri agrega fazlarından oluş muş iki fazlı kompozit malzeme olarak düşünülen betonda, iri agrega fazında hacım oranının ve dane boyutunun, harç fazında da su-çimento oranının değişmesinin betonun basınçlı su geçirimliliği, kılcal su emme ve hacımsal su emme özelikleri üzerindeki etkileri incelenmiş ve beton boşlukları içinde su hareketi ile ilgili bu özeliklerin, kompozit malzeme kuralları ile değerlendirilebilmesi a- maçlanmıştır. Bunlar yanında aynı faktörlerin sertleşmiş betonun mekanik özeliklerine etkileri de elde edilmiş ve mekanik özelikler ile basınçlı su geçirimliliği ve kılcal su emme arasındaki korrelasyon incelenmiştir. Çalışmanın ikinci bölümünü oluşturan yayın tara masında, betondaki geçirimlilik olayının teorisi, bu olaya etkiyen faktörler ve betonu kompozit malzeme olarak ele alıp mekanik özelikleri ile bileşimi arasındaki iliş kileri belirten matematik modeller yayınlara dayanılarak özetlenmiştir. Son yıllarda çok sayıda araştırmada, betonun kompozit malzeme olarak ele alınmasına ve bileşim-özelik i- lişkilerine çeşitli iç yapı modelleri yardımı ile yaklaşım yapılmasına rağmen, incelenen yayınlarda beton için deki sıvı hareketi özeliğine bu yönden yaklaşan bir çalışmaya rastlanmamıştır. Yayın taramasından elde edilen bu sonucun ışığın da, beton içindeki sıvı akımı özeliklerinin kompozit malzeme kuralları ile araştırılmasına karar verilmiş ve Bö lüm. 3 de açıklanan basınçlı su geçirimliliği, kılcal su emme ve toplam su emme deneysel araştırmaları planlanmış tır. Deneylerden elde edilen sonuçların değerlendirilmesi ve irdelenmesi Bölüm. 4 de yapılmış ve sonucunda be tondaki sıvı akımı özeliklerinin de diğer bazı beton özelikleri gibi, kompozit. malzeme kuralları ile iyi bir şekilde açıklanabileceği saptanmıştır. Bölüm. 5 de ise sonuçların kısa bir özeti verilmiştir.

Concrete, which Is the most widely used material in civil engineering, may come in contact with water, which may be in liquid or vapor from. Water can migrate through the pores of concrete due to hydraulic pressure, capillary absorption or vapor pressure difference. Particularly in concrete dams, water tanks and irrigation channels permeability must have a low value in order to keep the water in and also to resist chemical attack. In addition to this, ground water may affect concrete road and foundations. Water also may rise in the building walls, causing moisture in the living spaces and also effloration in the walls. All these phenomena show that high permeability is not desirable. In practice, concrete which is exposed to water under pressure, generally must be practically inpermeable. Concrete which is not under the effect of water under pressure, must show a low capillarity. Providing low permeability and low capillarity in concrete will prevent these harmfull effects. Water permeability under pressure and related phenomena like vapor diffusion and capillary absorption have drawn the interests of many researchers. Studies have been made, related to the conduction of water and or vapor through porous materials. To explain the character of these phenomena, different theories have been suggested. Also several investigations have been made for obtaining impermeable concretes and it has been determined that properties and mix proportios of aggregates, cement and water have great effect on permeability and that the largest effect comes from the water-cement ratio and the consistency of the concrete. In spite of the fact, that in most of the recent works on concrete, properties like elastic and inelastic properties have been studied with the help of internal structure models. / the survey presented in the second part of this thesis indicated, that concrete's permeability has not been studied yet as a composite material. xiii Therefore the first aim of this work, was to consider concrete as a composite material, made up of a mortar phase and a coarse aggregate phase, and to study experimentally the effect of these two-phases and their volume fractions on the permeability of this composite. The second aim of our study was to establish relationships between the components of this composite and its permeability. In addition to these the possibility of correlation between permeability and some other characteristics! of concrete have also been investigated. The second part of the study presents a wide summary of literature. This literature survey indicated that the mathematical treatment of water flow through concrete, in all experimental studies up to present, has been based on D'arcys law, and that it has been verified, that permeability of concrete generaly obeys this law. In this literature survey, the degrees of the effects of factors, influencing permeability have also been examined. The effects of factors related to the composition of concrete like the properties of cement, water, aggregates, integral and surface water proof ers on permeability of concrete have been examined. No information on studies handling concrete as a composite material and examining its permeability from this point of view has been encountered. As a result of the literature survey it has been established that the most important effect on the permeability of concrete is its water-cement ratio. Increasing the water-cement ratio increases the porosity of the concrete. This couses increase in water permeability under pressure and in capillary absorption, Also very important effects come from curing conditions. For example, it is known that permeability of concrete which is kept dry is much higher than, that of a concrete which is kept wet. The duration of curing also affectes permeability. Increasing the age of the concrete decreases permeability. The types of cement, aggregates and integral water proofing materials also affect permeability. For example, the use of crushed stone instead of gravel increases permeability. There were improving effects xiv due to integral water proofing materials on permeability. It has been shown experimentally that increasing the amount of the cement and it's fineness, decreases permeability. The last paragraph of the second Part presents a review of investigations on some of the characteristics of concrete, with composite materials theory, and internal structure models. Here it has been pointed out that composite materials theory is in harmony with experimentally determined properties of concrete. Under the light of information obtained from this preparatory survey, the objective of the present study has been chosen to investigate the permeability of concrete applying composite materials theory, to determine its composition-property relationships, and to search formulations for the permeability and other characteristics of concrete. An experimental program which will be explanied in the following has been planned: In the experimental program, which has been presented in the third Part of the present study, concrete has been examined as a composite material, made up of two-phases; a mortar phase and a coarse aggregate phase. In the mortar phase 0.50 volume fraction of 0-7 mm particle size natural sand has been used. The particle size of the coarse aggregate phase added into this mortar phase was 7-15 mm. The volume proportions of the coarse aggregate phase in the composite material have been increased from % 0, to % 15, % 20, % 25, and % 30 and five different concrete mixtures have been prepared. In the mortar phase, the water-cement ratio has been varied from 0.80 to 0.65, 0.50 and 0.35 and several concrete series each with five different mixtures have been obtained. For the water-cement ratio value of 0.65 an additional series of concrete has been produced in which the particle size of the coarse aggregate has been increased to 15-30 mm. This additional series represented a gap graded aggregate and also a larger maximum particle size. Thus, a total of 21 different concretes of different compositions have been prepared. For each mix six specimens in from of truncated cones of (15/20) cm upper and lower diameters and 15 cm hight have been prepared for the.permeability under pressure, capillary water absorption and total water absorption tests. Also two 20.20-20 cm cubic specimens have been prepared from the same compositions for the compressive strength and Schmidt hardnees tests and two cylindrical specimens of (15/3 0) cm size for the cylinder I strengh and modulus of elasticity tests. On the xv fresh concrete slump and the unit weight have been determined. The unit weight has been determined also on the 28 day hardened concrete. The samples have been cured for 21 days in water and then in the laboratory air until they were tested at the age of 28 days. The results obtained in the experiments have been presented in Tables in the appendix and in the form of diagrams in the text. Examination of the diagrams showed that properties like permeability of hardened concrete under water pressure, capillary water absorption and total absorption depend on the properties and volume fractions of the coarse aggregate and the mortar phases and on the maximum particle dimension of the coarse aggregate. Variation of the permeability of the hardened concrete with these characteristics has been studied and evaluated in the fourth part of the study. Also a graphical mix optimizition with current 1988 unit prices has been presented in which restrictions for unit weight of the concrete (A), water permeability (Kg), modulus of elasticity (Ed), cube compressive strength (Km. (1- Vs)n for the " above critical " values of mortar volume ratio. Here Vs^ 0.625 xvx 2- The effect of the maximum particle size of the coarse aggrega phase: It has been- shown that increasing the maximum particle dimension of the coarse aggregate made a negative effect on the permeability of concrete. Increasing the coarse aggregate dimension made also negative effects on compressive strength and modulus of elasticity of the concrete. But it showed no important effect on unit weight of the fresh or hardened concrete. 3- The effect of the water-cement ratio of the mortar phase: It has been determined that increasing the water-cement ratio of the mortar phase increased the permeability and that decreasing the water-^cement ratio, decreased it. But this decrease had a certain lower limit. Below this certain lower limit of the water-cement ratio, it was not any more possible to compact the concrete properly, and permeability therefore started to increase again. For the conditions in the present study, this lower limit has been determined as 0.45. 4- Besides permeability also other mechanical characteristics of concrete. have been determined, and the degree of their dependence on composition and their correlation with permeability have been investigated: a) These other properties also varied with increasing volume fraction of the coarse aggregate phase. For instance the increase of the unit weight of concrete with increasing volume fraction of the coarse aggregate phase was obvious, but it also showed a turning point and started to decrease af ter a- certain limit value of the j aggregate' volume fraction. The increase of the maximum particle size of the coarse aggregate showed no difference on unit weight. The static modulus of elasticity and the Schmidt hardness increased- first with increasing coarse aggregate volume fraction and then after the critical volume fraction, they started to decrease again. Cylinder and cube compressive strengths of the concrete also showed similar patterns. They first decreased with increasing coarse aggregate volume fraction when the water-cement ratio of the mortar was 0,80, But for water-cement ratios of 0.65 and 0.50.the cylinder and cube strengths increased first and after the critical volume fraction they began to decrease again. b) With increasing water-cement ratio an obvious decrease in the unit weights of the fresh and hardened concrete has been observed. Increase of the unit weight with decreasing water-cement ratio had again a specific lower limit which was obtained at a water-cement ratio of 0.45. The reason of this turning xvi x point lyed in the change of the consistency of the concrete to become unworkable ' and incompactable. Below this limiting water-cement ratio the mortar phase had a great deal of pores left. Increase in the values of the modulus of elasticity and Schmidt hardness also have been observed with decreasing water-cement ratio. c) The degree of correlation established between the properties of water permeability under pressure, capillarity and total water absorption with some other characteristics of concrete were as follows: With decreasing porosity of the concrete the permeability, capillarity and total water absorption decreased and showed good correlation. (Equation 4.7) No definit relation could be established between mechanical properties of concrete like its modulus of elasticity and compressive strength with its permeability. Plots of the results between these pairs of properties showed wide scatter. (Figs: 4.12, 4.13 4.14, 4.15, 4.16, 4.17, 4.18, 4,19) Finally as a results of the study, it has been established, that like its mechanical characteristics, also the permeability of concrete could be explained succesfully employing composite materials theory, relating them to the internal structure. The literature reference list and Tables indicating experimental and calculated results were presented in an appendix