Agrega-Çimento hamuru arayüzeyi mikroyapısının yüksek mukavemetli betonların kırılma parametrelerine etkisi

Abstract

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.

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 important role 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 XI addition 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.