Agrega-çimento hamuru arayüzeyi mikroyapısının yüksek mukavemetli betonların kırılma parametrelerine etkisi
Effect of aggregate-cement paste interface microstructure on the fracture parameters of high strength concretes
- Tez No: 46495
- Danışmanlar: PROF.DR. SAİM AKYÜZ
- Tez Türü: Doktora
- Konular: İnşaat Mühendisliği, Civil Engineering
- Anahtar Kelimeler: Agrega, Yüksek dayanımlı beton, Çimento, Aggregate, High strength concrete, Cement
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bu çalışmada agrega-çimento hamuru arayüzeyinde oluşturulan mikroyapısal değişikliklerin betonun kısa süreli mekanik davranışına etkileri araştırılmıştır. Merkezinde silindirik model agrega içeren harç disk numunelerde yapılan modelleme çalışmalarında malzeme; agrega, çimento harcı ve agrega-harç arayüzeyinden oluşan üç fazlı bir kompozit varsayılmış, bu model yardımıyla arayüzeyde, harçta ve agrega yüzeyinde gerilme dağılımları ile arayüzey bölgesinin kalınlığı hesaplanmıştır. Silis dumanı içeren ve içermeyen karışımlarda agrega-çimento hamuru mikroyapısal incelemeleri elektron mikroskop yardımıyla doğrudan gerçek betonda yapılmıştır. Eğilme halinde çentik içeren beton kiriş numunelerinin kırılma enerjisi ve karakteristik boy gibi kırılma parametrelerine mikroyapısal etkiler kantitatif olarak araştırılmıştır. Ayrıca basınç halinde tepe noktası öncesinde yükleme ve boşaltma yapılarak betonların gevreklik indisleri bulunmuştur. Silis dumanı içermeyen betonlarda hem kırılma enerjisi hem de karakteristik boy en büyük agrega boyutu arttıkça artmış, bu betonlarda yük-deplasman eğrisinin inen kolu agrega boyutu arttıkça uzamıştır. Buna karşın silis dumanı içeren betonlarda ise inen kol aniden düşmüş ve daha kısa bir kuyruk elde edilmiş, bu betonlarda agrega boyutunun bir önemi kalmamış ve aynı bir kırılma enerjisi elde edilmiştir. Silis dumanı içermeyen betonlarda en büyük agrega boyutunun küçülmesiyle basınç mukavemetine kadar kırılma enerjisi artmış, buna karşılık silis dumanı içeren betonlarda ise sözkonusu pik noktasına kadar kırılma enerjisine agrega boyutunun etkisi olmamıştır. Bütün betonlarda silindir basınç mukavemetinin 60 N/mm2 değerinden itibaren gevreklik indisinde hızlı bir artış olmuştur. Silis dumanı içermeyen betonlarda agrega-çimento hamuru temas yüzeyinde büyük boyutta kalsiyum hidroksit (CH), monosülfat (AFm) ve/veya etrenjit (AFt) gibi hidrate ürünlere bol miktarda rastlanmış ve söz konusu bölgenin daha heterojen yapıda olduğu görülmüştür. Hem agrega-çimento hamuru temas yüzeyinde hem de matris içindeki çimento hamurunda boş hacimlerin CH ve AFm kristalleriyle dolu olduğu görülmüş arayüzeydeki Ca/Si oranı matristekine göre daha yüksek bulunmuştur. Silis dumanı içeren betonlarda agrega-çimento hamuru temas yüzeyini silis dumanı değiştirmiş, arayüzey daha homojen ve yoğun olmuştur. Temas yüzeyinde ve matriste Ca/Si oranı düşük bulunmuş, boşlukların CH ve AFm ile dolu olmayıp boş oldukları amorf kalsiyum silikat hidrateye dönüşümün belirgin olduğu görülmüş, böylece daha homojen olan malzemede eğilme ve basınç deneylerinde çatlaklar genelde agreganın içinden geçmiş ve en büyük agrega boyutunun etkisi ortadan kalkmıştır. ıx
Özet (Çeviri)
In microstructural terms, normal concrete is an extremely complex system of solid phases, pores and water, with a high degree of heterogeneity. This heterogeneity can be considered on several levels. For material modelling purposes, Witmann introduced the idea of three levels, such as micro-level, meso-level and macro-level. This hierarchic system may be explained as follows: i) Micro-level: Characteristic features of this level are structures of hardened cement paste and xerogel. In this level, materials science type of models are used. Hardened cement paste is considered as a multi-phase material composed of unhydrated cement particles embedded in a continuous matrix of cement gel, which, in turn, is interpenetrated by capillary pores and cavities. ii) Meso level: The important factors are pores, cracks, inclusions and interfaces. Materials engineering models or mechanical and numerical models can be used for material modelling. At this level, concrete may be defined as two-phase material, where aggregates are embedded in a homogeneous matrix of cement paste (or mortar). Typical phenomena to be studied at this level are crack-formation and fracture mechanisms. iii) Macro level: At this level, concrete is modelled as a homogeneous material. However, macroscopic fracture mechanics parameters may be included. To establish a realistic failure model at the macro-level, an insight into the fracture mechanisms at the meso-level is required. At the meso-level the heterogeneity results in a non-uniform internal strain distribution within the concrete composite. Since the interface between the aggregates and cement paste is the weakest link, the mechanical behaviour of concrete is largely affected by the properties of interfacial zone. Especially, the fracture of concrete is very sensitive to the properties of this zone. The interface failure may be considered at meso-level. The development of bond cracks at the aggregate-matrix interfaces plays an importantrole in the inelastic behaviour of concrete. A considerable portion of the total strain is concentrated at interfaces and the final failure occurs in mortar, bridging bond cracks. Recent research efforts show that there are two principal aspects of interfaces in cement and concrete: 1°) the microstructural features of the interfacial regions, including their effets on concrete properties; and 2°) models of the effects of interfaces on the properties of concrete through the application of continuum mechanics and fracture mechanics. In recent years, three developments have permitted modern concrete to approach its potential as a construction materials, namely the introduction of air- entraining admixtures to improve freeze-thaw resistance, the use of superplasticizers to enable easier placing and improvements in physical properties and the addition of silica fume to enhance overall durability and strength. As it is known silica fume is a by-product material of Si-metal and Fe-Si-alloys industry. It consists of 85-95% amorphous Si02 with an average particle diameter of about 0. 1 um. Concretes with strength exceeding 80 MPa are now commonly being used in the construction of high-rise buildings and off-shore structures. The high-strength concrete (HSC) is characterized by a much lower total porosity and by an internal structure which is much more uniform in the bulk paste matrix itself, as well as at the aggregate-paste interface than the normal strength concrete. High strength silica fume concretes are characteristically brittle and may shown catastrophic structural failure under certain conditions. It is now possible to produce HSC with a strength up to 115 MPa in routine concrete production without special competence or materials. The availability of HSC in some countries has let to increased interest in the use of HSC for structural purposes and this caused intensive efforts to establish design criteria in the national code for HSC, up to a strength class of C 115. There is a growing interest in the study of aggregate-matrix interfaces to improve the strength of concrete. Previous studies on the fracture and micro- structure of this region in normal strength concrete has given some useful information, however, more research and quantititative measurements are needed for the better understanding of the fracture process in HSCs with and without silica fume. In particular, a more realistic approach is required to investigate behaviour of concrete instead of mortar containing model aggregate. The main objective of this work was to investigate the influence of silica fume replacement of cement and aggregate size on the strain localization, softening response and brittleness of HSCs by determining fracture parameters such as fracture energy GF (according to the reecommendation of the RILEM 50-FCM Technical Committee) and characteristic lengths ldl. The fracture energy tests are based on the measurement of the energy absorbed until the beam is broken into halves. The influence of introducing silica fume in concrete and aggregate size on the brittleness index of HSCs was also investigated under uniaxial compression. In XIaddition by three point bending test, the effects of aggregate size and silica fume on the shape of the descending brach in the curves of load-CMOD (Crack Mounth Opening Displacement) and also load-displacement at mid-span were investigated and the results were also examined by the light of the microstructural studies of the aggregate- mortar interfaces and fracture surfaces. The effect of silica fume and aggregate size on the brittleness index of HSCs were also supported by using microscopic studies at the aggregate-matrix interface. In microstructural studies Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Analizer (EDX) were used. Concrete was considered as a three phase composite material consisting of hardened cement paste, aggregate and the interfacial zone between cement paste and aggregate. The stress distributions in these zones were obtained as functions of B1[E2 and Ej/Ej ratios where Ex, E2 and E3 are the moduli of elasticity of matrix, interfacial zone and aggregate respectively. The calculations were based on the theory of elasticity and the assumption of isotropy and small deformation. The stress distributions were found in cases of E!<E3 and E^Ej. For bending test, four different HSC batches with constant water/binder ratio (water/cement or water/cement + silica fume) were prepared by using the same portland cement and natural sand. Two different size of crushed limestone coarse aggregate (5-10mm or 10-20mm) were used in the study. For each maximum aggregate size, two different concretes, one with and one without silica fume, were cast. For uniaxial compression tests, four different HSC mixes with constant water/binder ratio were used. The sizes of limestone coarse aggregates were 8 to 16 mm and 16 to 32 mm In each series, the silica fume (in slurry form and approximately 96 percent SiO, content) was 10 percent by weight of cement. The water/binder ratio was kept at 0.36 in preparing bending test specimens, and this ratio was kept at 0.45 in compression test specimens. A sodium naphthalene sulphonate type superplasticizer was used for all eight mixes. All concretes had the same nominali slump (60 mm to 80 mm). This thesis consists of six parts: In the first part, an introduction is made to the subject matter of the investigation, main objectives and scope of the presented work are given. Chapter 2 gives a survey of the present knowledge on the microstructure of aggregate-paste interface. Microstructural effects on the properties of concrete are summarized. In the third part, concrete is considered as a composite material. A summary of the works dealing with the effect of the phases on the concrete composite are given. A three phase composite model is proposed to calculate stress distributions at the interface or at the matrix. The model test results are discussed and evaluated xnin this chapter. The fourth part is devoted to the experimental determination of fracuture parameters in concrete. The effects of introducing silica fume and aggregate size on the strain localization, softening response and brittleness of HCSc were investigated by measuring the fracture energy GF and the characteristic length. The definitions of these parameters are made. The main lines of the fracture tests carried out on the notched beams are explained. In the fift part, the experimental results related to the compression tests are discussed and evaluated. The definition of the brittleness index is made. The relation between brittleness index and compressive strength of concrete is discussed. Conclusions and suggestions for further study are given in the sixth chapter. On the basis of microstructural studies at the aggregate-paste interface, fracture tests in bending and test results in compression the following conclisions can be drown: 1°) The results related to model experiments i) There is no significant difference between moduli of elasticity of mortars with and without silica fume for the same mixture. ii) In mortar without silica fume, the interface between aggregate and matrix is more heterogeneous and porous, however, by introducing silica fumes in mortars, the interfacial zone becomes stronger and more homogeneous and the fracture occurs in more brittle manner and exhibit shorter crack pattern around model aggregate. iii) Stress distributions at the interface and in the matrix can be calculated by means of a three phase composite material model. The thickness of the interfacial zone can be also obtained using the proposed model. It is concluded that the thickness of the interface is decreased in HSCs. 2°) The results related to the microstructural studies at the aggregate-cement interface i) In concrete without silica fume, one observes a profusion of calcium hydroxide (CH) at the aggregate-paste interface. Not only are the CH crystals massive, often up to 20 jam in size, they are mostly tabular and oriented. This is a characteristic feature of normal concrete, or concrete without any mineral admixture. Apart from CH, some platy mono-sulphate (AFm) crystals were also identified in this region. The calcium silicate hydrate (C-S-H) is also much less dense, the very high ratio of Ca/Si in the C-S-H indicates that it is mostly CH. The paste is also porous and is characterized by the presence of fibrous or Type I C-S-H. Platy AFm and tabular CH crystals are also identifiable in the paste. The air voids are full of xiuplaty CH crystals. ii) In concretes with silica fume, the interfacial zone is composed of dense C-S-H. The air voids and other vacant spaces in this region are empty, and show no deposition of CH, mono-sulphate or ettringite crystals. The dense paste identical to that of the interfacial zone was identified. As in transition zone, the air voids in the paste are empty. From the general absence of CH and compactness of the bulk paste, it can be presupposed that this concrete will yield high mechanical strength. 3°) The experimental results related to the fracture energy tests using three point bending specimens i) In the concretes with silica fume, the cracks usually travel through the aggregates, as a result fracture energy does not vary with varying the maximum aggregate size. ii) In the concretes without silica fume, with 20 mm. maximum size of aggregate the descending branch of the load-displacement curve decreases more slowly and a longer tail is observed due to heterogeneity. However, in the concretes with silica fume, a steeper gradient of the softening branch with a shorter tail is obtained. iii) In the concretes without silica fume, the fracture energy (GF) and the characteristic length (1^,) depend strongly on the maximum size of aggregate. Both parameters increase as the aggregate size increases on the contrary, in the concretes with silica fume, the fracture energy and especially the characteristic length decrease dramatically with increasing aggregate size. iv) In the concretes with microsilica, the cracks usually travel through the aggregate and fracture tends to be brittle in nature. However in the concretes without microsilica, the cracks usually develop around the coarse aggregate resulting in an inter-coarse aggregate type of fracture. This different crack pattern can be attributed to the zone becoming of interfacial zone stronger and more homogeneous, as a result of microsilica replacement, and hence the material exhibits a more brittle type of behaviour. 4°) The results related to uniaxial compression tests In the light of experimental measurements, examinations of the fracture surfaces and the microstructural studies of the aggregate matrix interfaces, the following conclusions can be drawn: i) In concrete with silica fume, as the compressive strength of concrete increase the brittlenes index also increases. The concretes become stronger and more homogeneous, as a result the fracture occurs in a more brittle manner. ii) In concrete without silica fume, up to the peak point, the fracture energy of concrete containing large size aggregates is less than that of small size xivaggregates. However, hysteresis loop does not change with varying the maximum aggregate size. iii) Within the limits of this work, the brittleness index does not vary for the concretes with the compressive strength between 47 N/mm2 to 60 N/mm2 compressive strength. That is why the value of 60 N/mm2 can be taken as a threshold strength for the typical brittleness of HSCs. iv) Based on the fracture tests and microscopic studies at the aggregate matrix interface, it is concluded that in concretes which contain silica fume, the cracks usually travel through the aggregates the interfacial zone for these concretes becomes stronger and more homogeneous, and the fracture occurs in trans-granular type. However, in concretes without silica fume, the cracks usually develop around the coarse aggregate resulting in an inter-granular type of fracture. Finally, recommendations for further study may be summarized as follows: Experimental work should be expanded to study the size effects of test specimens, the effects of the surface texture and mineralogy of the aggregates, curing conditions, age effects, loading rate and loding conditions. xv
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