Issue34

S. Ahmad et alii, Frattura ed Integrità Strutturale, 34 (2015) 524-533; DOI: 10.3221/IGF-ESIS.34.58 525 I NTRODUCTION ement based composites i.e. paste, mortar and concrete are the most utilized materials in the construction industry all over the world and their quantities produced and utilized are rapidly increasing due the fast pace of development in the modern world [1,2]. Besides many advantages, there are few limitations in the utilization of the cementitious composites and their quasi-brittle nature is one of them. The cement composites possess extremely low tensile strength as compared to their compressive strength. Due to their low tensile strength, cracks develop in cementitious materials due to the drying, shrinkage, plastic settlements, stress concentrations (due to the external restrains and/or applied stresses) etc. These cracks developed at the nanoscale may grow rapidly due to the applied stresses and join together to form micro and subsequently macro cracks in the composite. The growth of cracks in the cementitious composites from nanoscale to micro and macro scale is very rapid and may lead to sudden failure. Generally the paths taken by the cracks for their growth in a composite structure determines whether the failure will be brittle and catastrophic or elastic and safe [3]. In the traditional cement composites, there is absence of crack trapping mechanisms such as fibers and hard inclusions at nano/micro scale and stress relieving mechanisms such as dislocations or crazing in metals and polymers. As a result, the stresses required to propagate the cracks are much lesser as compared to the stresses required for their initiation [4]. The large variations in the stresses required for crack initiation and growth in cement composites leads to unstable and rapid growth of cracks after their nucleation [5]. As it is a well-known fact that brittleness of cement composite is directly related to their strength so the issue of brittleness and crack susceptibility is more pronounced in the high strength cement composite, which have been developed to meet the modern world requirements [6]. From the viewpoint of high performance in terms of eco-efficiency, durability and sustainability, it is highly desirable to enhance the ductility and energy absorption capability of the cementitious composites. Literature indicates that researchers have investigated several types of materials/fibers for enhancing the ductility of the cement composites such as hemp, sisal, jute, cellulose whiskers, steel, polyvinyl alcohol (PVA), polypropylene (PP), carbon nanotubes (CNTs), carbon nano fibers (CNFs) and many others [7–17]. Recent studies also show the utilization of nano materials such as graphene, nano crystalline cellulose, calcium carbonate whiskers and nano SiO 2 particles for improving the ductility and strength of cement composites [18–21]. The fibers in the cement matrix behave as crack arresters and restrain their growth. The crack arresting mechanisms of fibers in cement composites impart toughness and enhance their energy absorption capacity. In the crack arresting mechanism of fibers, two crucial parameters limiting their performance are their dispersion in the host matrix and the interfacial bonding between the fiber and the composites. The uniform fibers dispersion in the cement matrix is usually a difficult task and requires special procedures. This factor greatly limits the fibers utilization in the cement composites [22–24]. In the present research, the objective of crack growth pattern modification in the high performance cement composites to achieve enhanced ductility and toughness has been accomplished by the incorporation of the micro sized inert particulates in the cement composite matrix. The inert micro sized inclusions increase the heterogeneity in the matrix as they help to reduce the void in the matrix, increasing the strength, but at difference of silica fume particle do not interact with the cement matrix. This results in complex crack tip stresses around the small particulates [25,26]. The presence of complex stress field at the growing crack tip restricts its usual in plane movement and the result is a crack deflection and sometimes crack contouring around the particulates. The random and multi-plane movement of the crack path in the cement composite matrix results in higher fracture surface area involving more and more material in the cracking process as compared to the cracking in a single plane. For that purpose, a novel inert material was synthesized by the carbonation of coconut shells and was subsequently used in the preparation of cement composites. M ATERIALS AND METHODS Materials n this experimental study the cement composites were prepared with Type-I, Ordinary Portland Cement (OPC) [27]. The chemical composition and the physical properties of OPC are reported in Table 1. High range water reducing agent (HRWRA) “Mapei Dynamon SP-1” which is based on the second generation of modified acrylic polymers was used to ensure sufficient workability of the cement composites at the given w/c ratio. Physical properties of the HRWRA are reported in Table 2 [28]. C I