Rao Arsalan Khushnood

Improvements in self-consolidating cementitious composites by using micro carbonized aggregates

There is growing interest in the use of self-consolidating cementitious systems in construction industry. The present research was conducted to enhance the mechanical performance of cement composites by the utilization of micro-sized inert particles. This paper deals with the synthesis of micro-sized inert carbonized particles from hemp hurds. The synthesized carbonized particles were characterized by field emission scanning electron microscope (FESEM). These particles were further used as additive in self-consolidating cement composites. Total of four different wt% additions (i.e. 0.08, 0.20, 1.00 and 3.00 by wt% of cement) were investigated. The cement composites containing carbonized particles inclusions were characterized by three point bending and compression tests. The results indicate that the carbonized particles additions enhanced the flexural and compressive strengths of the cement composites. It was also observed that the fracture properties and the energy absorption capability of the cement composites were enhanced substantially.

Crack path and fracture surface modifications in cement composites

There is a tremendous increase in the use of high strength and high performance self-consolidating cementitious composites due to their superior workability and mechanical strengths. Cement composites are quasi-brittle in nature and possess extremely low tensile strength as compared to their compressive strength. Due to the low tensile strength capacity, cracks develop in cementitious composites due to the drying shrinkage, plastic settlements and/or stress concentrations (due to 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 macro cracks. The growth of cracks from nanoscale to micro and macro scale is very rapid and may lead to sudden failure of the cement composites. The present paper reports the modifications in the crack growth pattern of the high performance cement composites to achieve enhanced ductility and toughness. The objective was accomplished by the incorporation of the micro sized inert particulates in the cement composite matrix. The results indicate that the incorporation of micro sized inert particles acted as the obstacles in the growth of the cracks thus improving the ductility and the energy absorption capacity of the self-consolidating cementitious composites.

Modified fracture properties of cement composites with nano/micro carbonized bagasse fibers

A novel cost-effective alternative in the form of nano/micro carbonized particles produced from waste bagasse fibers has been explored to modify the mechanical properties and fracture pattern of the resulting cementitious composites. Carbonized bagasse particles were produced at Politecnico di Torino and characterized by Raman spectroscopy and scanning electron microscopy. When added with cement paste up to 1 wt% in six different proportions, the carbonized bagasse particles were found effective in significant enhancement of mechanical strength as well as fracture toughness. From micro-graphical observations it is evident that these heterogenic inclusions either block the propagation of micro cracks which has to deviate from its straight trajectory and has to follow the carbon nano/micro particles contour or distribute it into multiple finer cracks. Crack contouring along the carbonized particle, crack pinning, crack diversions and crack branching are the mechanisms which can explain the increase of toughness in the composite samples.

Experimental Investigation on Use of Wheat Straw Ash and Bentonite in Self-Compacting Cementitious System

In this research, we evaluated the feasibility of wheat straw ash and bentonite (raw and heated at 150°C for 8 hrs) as secondary raw materials in self-compacting paste (SCP). The fresh and hardened properties of SCP formulations including water and superplasticizer demand, flow behavior, compressive and flexural strength development, water absorption, and acid attack resistance were evaluated. Moreover, porosity, microstructural, and mineralogical investigations were also carried out on SCP formulations. Test results showed that the properties of SCP formulations in fresh state depend on the morphology of secondary raw materials. For heated bentonite and wheat straw ash formulations, the 28 days of compressive and flexural strength were higher or almost similar to reference SCP formulation. Among SCP formulations, wheat straw ash formulation was found to be more effective in consuming free lime and showed significant decrease in porosity with time, which in turn improved the resistance of this SCP formulation against water absorption and acid attack. Based on the test results, it can be concluded that the successful utilization of wheat straw ash and bentonite SCP formulations will offer durable and environmental friendly option to construction industry.

Fracture toughness and failure mechanism of high performance concrete incorporating carbon nanotubes

Cement and concrete composites are inherently brittle and exhibit very less tensile/flexural strength capacity as compared to their compressive strength. Use of thoroughly dispersed carbon nanotubes in the concrete matrix is one of the possible solution for enhancing mechanical properties in tension/flexure. In the present research work, small fractions of multiwall carbon nanotube (MWCNTs) i.e. 0.05 and 0.10 wt% of cement have been integrated into the cement concrete to study their effect on the mechanical properties of the resultant concrete mixtures. The enhanced performance of the whole mix lies on a single point that MWCNTs must be thoroughly disperse in the mixture. Hence, special arrangement through usage of high energy sonication along with amended acrylic based polymer (performing as a surfactant) was made to have a uniform dispersion of MWCNTs in the concrete mix. The testing of concrete samples includes i.e., flexure, splitting tensile and compressive strengths after 3, 7, 28 and 56 days of curing. After having comparison with the control mix cured for 28 days, it was observed that the addition of 0.05 wt% MWCNTs increased the splitting tensile strength by 20.58%, flexural strength by 26.29% and compressive strength by 15.60%. Through above results, which verify the increase in concrete mix strength after adding MWCNTs, these MWCNTs may be incorporated in the treatment of Nano/micro cracks completed through process of connecting, branching and pinning. Similarly, as proved in threepoint bending tests, MWCNTs also enhances the breaking strains as well as the fracture energy of the concrete mixes, besides, imparting increase to the strength. The investigations have shown that incorporating lesser amounts of MWCNTs i.e., 0.05 and 0.10 wt% of cement to the concrete mixes after insuring there complete dispersion, unusually improve their properties like mechanical strengths and fracture behavior.

Flow duration curve regionalization with enhanced selection of donor basins

A non-parametric regionalization procedure for the assessment of flow duration curve (FDC) at ungauged basins is presented. This modeling approach is fundamentally based on the quantification of dissimilarity between FDCs, thus allowing the grouping of most similar basins. An analogous grouping procedure, performed in the space of selected basin characteristics, allows the estimation of FDCs also at ungauged sites; however, for a fixed set of basin characteristics, some ungauged basins cannot be properly represented due to the scarcity of close (similar) donor basins. For these cases, the proposed method allows for the selection of an alternative set of basin characteristics as a support for similarity grouping. The results of the study show that the statistical error can be significantly reduced by following the proposed methodology. About 10% of all the basins involved in the analysis can benefit from the model swapping procedure, thus improving the final predicted curve.

Bioimmobilized Limestone Powder for Autonomous Healing of Cementitious Systems: A Feasibility Study

For preserving concrete structures and hindering ingress of chemicals through cracks and fissures, repair is inevitable. Microbial calcite precipitation is an intrinsic approach for crack rectification and emulating way of sustainability for reducing anthropogenic greenhouse gases (GHGs) along with conserving the natural resources. In this study, Bacillus subtilis strain is applied for intrinsic repair of concrete’s cracks because of its high pH endurance and capability of sporulation. For prolonged survival of microorganisms, immobilization technique was employed. B. subtilis was immobilized through limestone powder (LSP) before adding into cement matrix. Self-healing proficiency of B. subtilis was deliberated in terms of mechanical strength regain after cracking at 3, 7, 14, and 28 days. To examine the microstructure and characterization of healing precipitate, micrographical (field emission scanning electron microscopy), chemical (energy dispersive X-ray), and thermal (thermogravimetric analysis) analyses were performed after the healing period of 28 days. The results revealed evident signs of calcite precipitation in nano-/microcracks subsequent to microbial activity. Furthermore, immobilized LSP improved the compressive strength of the analyzed formulations.

High Performance Self-Compacting Cementitious Materials Using Nano/Micro Carbonaceous Inerts

Cementitious materials are commonly and extensively used worldwide by construction industry for various types of infrastructures. Despite of their exceptional strength in compression they still possess limited tensile strength and tensile strain capacity. Different types of fibers have been investigated since last fifty decades to reinforce the cementitious matrix against tensile failures and to impart ductility. The size of the reinforcing fillers has diminished from macro to micro and now even to the nano scale with the recent advancements in nanotechnology. Due to exceptional intrinsic properties and large aspect ratio, carbon nanotubes have been successfully investigated as a reinforcing filler to modify the mechanical strength, fracture toughness, electrical and electromagnetic wave absorbing properties of cementitious composites. However the problems associated with its effective dispersion and bonding with the host material limit its widespread applications on large scale. To overcome the aforementioned issues concerning the dispersion and bonding of nano reinforcing materials with the host matrix, graphene nano sheets were explored for the first time as a reinforcing agent for high performance cementitious matrices. Graphene sheets are free form entanglement problems and therefore need comparatively lesser energy for proper dispersion. Due to very high specific surface area and large aspect ratio in comparison with carbon nanotubes they are much capable to develop strong interfacial bond with the host medium. In the commercialization of these nano carbon particles filled cementitious composites, another major concern would be the related expenses. Therefore in parallel, research work was also done to explore the cost effective alternatives for the production of carbon nano particles to be used for modification or improvement in the properties of cement matrices. In recent wok by Prof. Ferros research team it has been explored that carbon nano particles produced from coconut shells can be effectively used to improve the mechanical strength and fracture toughness of cementitious composites with limited dispersion issues (G. Ferro et al. 2014, 2015). To continue with the productive research pertaining the cost effective production of carbon nano particles for high performance cementitious composites, bio-waste in the form of bagasse fibers, hazelnut shell and peanut shell was investigated. These particular types of agricultural wastes were selected keeping in view their economic availability as well as the excellent conversion efficiency via pyrolysis. The present work encompasses complete characterization of the investigated materials, detailed study on their dispersion ability in water and the cement matrix, entire mechanical characterization of reinforced cementitious composites at varying proportions as well as their electromagnetic wave absorption properties in 2-10 GHz frequency range. It was determined that graphene nano-platelets can be uniformly dispersed in water as well as in the cementitious matrix without any addition of separate dispersant or surfactant or stabilizing agent. It was found that even at a very low content of addition remarkable improvements in the mechanical strength and fracture toughness were attained. The optimum content of addition for the grade 4 graphene nano-platelets was found as 0.08 wt% providing with a significant increase of 89% and 29% in compressive and flexure strength along with 115% improved fracture toughness. Similarly the carbonized particles produced for bio-waste were found quite effective in modifying the mechanical performance of cementitious composites. Maximum enhancement by 139% and 88% in flexural and compressive strength were achieved on 0.2 wt % addition of nano/micro carbonized particles produced from peanut shell with an increase of 69% in the fracture toughness as well. Microstructural investigations evidenced the proper homogeneous dispersion of GNPs and NMCPs throughout cementitious matrix along with their efficient filling action to refine the pore-structure of the cementitious composite. The phenomena of crack bridging, crack deflections, crack contouring and crack branching were observed via scanning electron microscopy revealing the mechanism behind the remarkable improvements of mechanical properties achieved in the present research. A novel cost effective material in the form of cement composites containing carbonized agricultural residue (comprising CPS and CHS) was proposed for shielding against electromagnetic waves. The investigated material was found much efficient for electromagnetic interference shielding applications, providing the advantage of better dispersion, simple manufacture at a much lower cost (cost saving ˃ 85%) compared to the corresponding carbon nanotubes based cement composite material