Moncef L. Nehdi

Performance of rice husk ash produced using a new technology as a mineral admixture in concrete

This article investigates the use of a new technique for the controlled combustion of Egyptian rice husk to mitigate the environmental concerns associated with its uncontrolled burning and provide a supplementary cementing material for the local construction industry. The reactor used provides efficient combustion of rice husk in a short residency time via the suspension of processed particles by jets of a process air stream that is forced though stationary angled blades at high velocity. Investigations on the rice husk ash (RHA) thus produced included oxide analysis, X-ray diffraction, carbon content, grindability, water demand, pozzolanic activity index, surface area, and particle size distribution measurements. In addition, concrete mixtures incorporating various proportions of silica fume (SF) and Egyptian RHA (EG-RHA) produced at different combustion temperatures were made and compared. The workability, superplasticizer and air-entraining admixture requirements, and compressive strength at various ages of these concrete mixtures were evaluated, and their resistance to rapid chloride penetrability and deicing salt surface scaling were examined. Test results indicate that contrary to RHA produced using existing technology, the superplasticizer and air-entraining agent requirements did not increase drastically when the RHA developed in this study was used. Compressive strengths achieved by concrete mixtures incorporating the new RHA exceeded those of concretes containing similar proportions of SF. The resistance to surface scaling of RHA concrete was better than that of concrete containing similar proportions of SF. While the chloride penetrability was substantially decreased by RHA, it remained slightly higher than that achieved by SF concrete.

Experimental Investigation on the Seismic Behavior of Beam-Column Joints Reinforced with Superelastic Shape Memory Alloys

Superelastic Shape Memory Alloys (SE SMAs) are unique alloys that have the ability to undergo large deformations and return to their undeformed shape by removal of stresses. This study aims at assessing the seismic behavior of beam-column joints reinforced with SE SMAs. Two large-scale beam-column joints were tested under reversed cyclic loading. While the first joint was reinforced with regular steel rebars, SE SMA rebars were used in the second one. Both joints were selected from a Reinforced Concrete (RC) building located in the high seismic region of western Canada and designed and detailed according to current Canadian standards. The behavior of the two specimens under reversed cyclic loading, including their drifts, rotations, and ability to dissipate energy, were compared. The results showed that the SMA-reinforced beam-column joint specimen was able to recover most of its post-yield deformation. Thus, it would require a minimum amount of repair even after a strong earthquake.

PREDICTING PERFORMANCE OF SELF-COMPACTING CONCRETE MIXTURES USING ARTIFICIAL NEURAL NETWORKS

This paper shows that artificial neural networks (ANN) can be used to effectively predict the performance of self-compacting concrete (SCC) mixtures. Inspired by the internal operation of the human brain, the ANN method has learning, self-organizing, and auto-improving capabilities. Thus, it can capture complex interactions among input/output variables in a system without any prior knowledge of the nature of these interactions, and without having to explicitly assume a model form. Indeed, such a model form is generated by the data points themselves. The database assembled, the architecture of the network selected, and the training process of the ANN model used are described. Initial tests show that the ANN method can accurately predict the slump flow, filling capacity, segregation, and compressive strength test results of SCC mixtures. A model for the acceptance or rejection of SCC mixtures based on knowledge of their mixture proportions is proposed and may be used after sufficient development of a more comprehensive database on an industrial scale for the proportioning of SCC with tailor-made properties.

Fiber Synergy in Fiber-Reinforced Self-Consolidating Concrete

Self-consolidating concrete (SCC) was developed to respond to the need for concrete that can improve durability while eliminating the need for compaction and vibration work. SCC can compact itself into complicated formwork and congested structural elements under its own weight without the need for mechanical vibration. Thus, SCC decreases construction time, labor, and equipment needed on construction sites; makes the construction of heavily congested structural elements and hard-to-reach areas easier; reduces noise and injuries related to vibration work of concrete; and helps achieve higher-quality finish surfaces. Fiber reinforcement can extend the technical benefits of SCC by also providing crack bridging ability, higher toughness, and long-term durability. This article reports on a study of the potential synergistic effects in SCC incorporating different steel and synthetic polymer macro- and microfibers in various hybrid (single, binary, and ternary) combinations. The study included a total of 31 SCC mixtures, with combinations of fibers in varying proportions from 0.25 to 1.0%. The fiber length varied from 30 to 50 mm for the macrofibers, while microfibers were approximately 12 mm long. Each mixture was evaluated using the slump flow and L-Box flow tests, compressive strength tests, flexural toughness, first-crack strength in bending, and post-first-crack behavior. Results of this study show that fibers can have rheological and mechanical synergistic effects. The authors conclude that optimized fiber combinations can better increase toughness and flexural strength while maintaining adequate flow properties for fiber-reinforced SCC.

Why some carbonate fillers cause rapid increases of viscosity in dispersed cement-based materials

Abstract Fine carbonate fillers complement the deficiency in fine particles of the cements particle size distribution, which can enhance both the flowability and stability of fresh concrete. They also fill in between the relatively coarser cement grains, reducing the room available for water and consequently the water demand. In conventional concrete mixtures, slight reductions in the setting time have been often reported when carbonate fillers were used, without significant effects on the workability. However, as the use of high-performance concrete (HPC) continues to rise, carbonate fillers are being added in low water/cement (w/c) ratio superplasticized-mixtures. Increasingly finer fillers are being specified because they enhance the packing density of the particulate system. In such conditions, rapid losses of workability have been reported. Yet, this did not seem to occur with all carbonate fillers. This paper investigates the potential causes of this phenomenon using a model system. Chemical oxide analysis was conducted on various carbonate fillers, which were then ground to sub-micron particle sizes in the presence of polyacrylate dispersants using an agitated ball mill-grinding machine. A rotational co-axial cylinders viscometer was used to study the rheology of the slurries thus obtained. Laser diffraction was employed to establish the particle size distribution of the input and output slurries. It was discovered that the magnesium (MgO) content of the carbonate filler can hinder the effect of the dispersant, which may combine with the acceleration of C 3 S hydration due to calcite and possibly the formation of carboaluminates in Portland cement-based mixtures to cause rapid losses of workability.

Shear Behavior of Fiber-Reinforced Self-Consolidating Concrete Slender Beams

A detailed investigation on the shear behavior of fiber-reinforced self-consolidating concrete (FR-SCC) beams was carried out. FR-SCC mixtures were designed to study the influence of: a) fiber type; b ) fiber anchorage; c) fiber aspect ratio; and d) fiber content on the shear performance of reinforced concrete slender beams without stirrups, and to determine the suitability of using fibers to satisfy minimum shear reinforcement requirements. SCC was found to be particularly suited for fiber addition owing to its mixture design and fundamental rheological characteristics. It was observed that the short discrete fibers could significantly improve the shear behavior of reinforced SCC slender beams and beams incorporating 1% steel fiber addition could achieve a 128% increase in shear capacity over that of the reference beam without fibers. Furthermore, the FR-SCC beams performed better under shear loading than conventional fiber-reinforced concrete (FRC) beams. The experimental results obtained on 13 FR-SCC slender beams indicate the possibility of using fibers as minimum shear reinforcement. Finally, a design equation capable of estimating the shear capacity of both normal and FRC beams is proposed and its accuracy is established using experimental data from the present study along with other data in the literature.

Recycling waste latex paint in concrete

Abstract Currently, in Ontario, Canada, around 21.7% of the total hazardous waste (HZW) collected by municipalities is waste paint. Waste latex paint (WLP) alone constitutes 12% of the total HZW. It is estimated that only 10–30% of this waste is presently being collected but this proportion is growing with public education efforts. In addition, due to increasingly more stringent environmental regulations on volatile organic compounds (VOCs), more latex-based paints will be produced compared to solvent- and oil-based alkyds. This will result in more WLP being generated annually in Ontario and across North America. The disposal cost of such waste currently varies between Can

Effect of Mixture Design Parameters on Segregation of Self-Consolidating Concrete

0.90 and Can

ARTIFICIAL INTELLIGENCE MODEL FOR FLOWABLE CONCRETE MIXTURES USED IN UNDERWATER CONSTRUCTION AND REPAIR

1.40 per litre. This study was conducted in collaboration with the City of London, Ontario and the Ontario Paints and Coatings Association and aims at investigating the benefits of recycling WLP in concrete with a special focus on concrete sidewalks. WLP was used in concrete mixtures both as a partial replacement for virgin latex and for mixing water. This paper demonstrates that concrete mixtures incorporating WLP can have improved workability, higher flexural strength, lower chloride ion penetrability, better resistance to deicing salt surface scaling and can be more economic because they require less water-reducing and air-entraining admixtures. The results also indicate that the annual urban concrete sidewalk construction could use the yearly production of WLP while producing sidewalks with enhanced properties and durability.

Recycling Waste Latex Paint in Concrete with Added Value

This paper presents research that evaluates the segregation potential (defined as the tendency of coarse aggregate particles to separate from the mortar mix during the transportation, placement, and setting of fresh concrete) of a wide range of self-consolidating concrete (SCC) mixtures. The evaluation involved 123 SCC mixtures, comprising a large scope of mixture designs. Mixture properties (e.g. slump flow, slump 50, V-funnel flow, L-Box flow), along with compressive strength at various ages, were also examined. The resulting data were used to train an artificial neural network (ANN) to accurately predict segregation resistance and to conduct sensitivity analysis.