Fracture Mechanics of Concrete Structures, de Borst et al (eds)© 2001 Swets & Zeitlinger, Lisse, ISBN 90 2651 825 0 Fracture process zone in high strength concrete G.Appa Rao & B.K.Raghu Prasad Department of Civil Engineering, Indian Institute of Science, Bangalore-560 012, India ABSTRACT: An experimental investigation on the influence of maximum size of coarse aggregate, cement and coarse aggregate contents on the size of fracture process zone (FPZ) in high-strength concrete (HSC) is reported. Wide ranges of concrete mixes were designed to vary the compressive strength of concrete between 55 MPa and 80 MPa. Three different maximum sizes of coarse aggregates namely lOmm, 16mm, 20mm and combination of the above three were adopted. The size of FPZ was obtained qualitatively using ultrasonic pulse velocity technique. Single edge notched compact tension specimens (SENCTS) of size 500 mm x 500 mm x 80 mm with 250 mm edge notch subjected to uniform far field stress were used. It has been observed that the size of FPZ was influenced by the size of coarse aggregate. The size of FPZ increases as the maxi- mum size of coarse aggregate increases up to 16 mm size and then tend to decreases with aggregate size. The size of FPZ was in the order of the magnitude of the size of coarse aggregate. It seems that the FPZ in HSC is localized indicating possible application of LEFM to HSC. Further, the FPZ decreases as the cement content increases, and relatively higher values of FPZ have been observed at higher coarse aggregate contents. 1. INTRODUCTION Accurate experimental investigation of the fracture parameters of concrete is very important. Generally the fracture parameters are determined in the re- search laboratories on relatively small size speci- mens, which are significantly dependent on the specimen size. The structure of concrete is viewed as a multi-level hierarchy system (Zaitsev and Witt- mann (1984)). The presence of inhomogeneities in concrete results in stress concentration at different interfaces of the system at higher stress levels. How- ever, the information on the characteristics of frac- ture process zone (FPZ) and its extension in HSC are very limited. Research efforts have been reported to estimate the extent of FPZ using different tech- niques, which are classified under (i) Indirect or Non-destructive Techniques and (ii) Direct or De- structive Techniques. The non-destructive tech- niques are ultra sonic pulse velocity technique, acoustic emission technique, photo elastic coating techniques, laser speckle interferometry, Moire inter- ferometry, holographic interferometry, compliance techniques, and infrared vibrothermography. These non-destructive techniques enable data collection during specimen loading. The destructive techniques 327 are based on the direct observation of material frac- ture using, optical microscopy, scanning electron microscopy, X- ray diffraction techniques, or high- speed photography. These techniques are limited to surface observations only. 2. FRACTURE PROCESS ZONE Before the peak load, tip of a crack in concrete is characterized by cluster of microcracks, which is often known as fracture process zone (FPZ). Signifi- cant amount of fracture energy is absorbed in this zone due to large extent in conventional concrete. However, high strength concrete is more brittle and tend to behave more like a composite material due to strong interfacial bond between cement paste and aggregate, which lead to show localized FPZ in HSC. Hu and Wittmann (1992) introduced local fracture energy, which decreases in HSC due to very small FPZ. Two basic mechanisms take place in the softening regime of concrete (van Mier et al. (1990)). The post peak softening response of con- crete and its ductility are primarily related to maxi- mum size of coarse aggregate, grading of aggregates, and confinement stress on the test specimen (Rado-