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The fresh and hardened properties of self-compacting concrete (SCC) depend on number of factors such as paste composition, paste content, aggregate content, aggregate gradation etc. In the present investigation, the influence of the packing density of aggregates on the properties of SCC was evaluated. Experiments were conducted to measure the packing density for different combinations of aggregates precisely. A ternary packing diagram (TPD) was developed based on the packing density of measured and interpolated data. Considering the limitations in generalising the TPD and the difficulty involved in adopting mathematical models for aggregates, an attempt was made to establish a simple method for the selection of the combination of aggregates resulting in maximum packing density from the particle size distribution of aggregates (represented by the Coefficient of uniformity—C u). Further, studies were extended to investigate the effect of aggregate packing density on fresh and hardened SCC properties. The results indicate that for a constant paste volume and paste composition, with increase in packing density of aggregates, the fresh properties and the compressive strength of SCC were improved positively. An attempt was also made to identify the influence of 10 different proportions of aggregates having the same packing density on the properties of SCC. The results indicate that at the same aggregate packing density, the fresh concrete properties were influenced significantly by the choice of the aggregate combination, while there was little or no influence on the hardened properties. Furthermore, the experimental data obtained was used for supplementary validation of the existing model (compressible packing model) for predicting the packing density and the fresh behaviour of SCC.

In this work, the flowshop scheduling problem is considered with the objective of minimising the completion-time variance (CTV) of jobs, and an Ant Colony Optimisation (ACO) algorithm is presented. Two implementations of the Modified Ant Colony Optimisation algorithm (MACO-I and MACO-II) are proposed to solve the permutation flowshop scheduling problem. The proposed ant-colony-algorithm implementations have been tested on 90 benchmark flowshop scheduling problems. The solutions yielded by the proposed MACO implementations are compared with various algorithms and with the best CTV of jobs reported in the literature. The proposed MACO implementations are found to perform very well in minimising the chosen performance measure. � 2012 Copyright Taylor and Francis Group, LLC.

Designs for improved railroad track structure are needed to ensure safety, reliability and profitability as the railroads strive to compete with other means of transport. The large number of precast sleepers produced in the country calls for an optimal design inthe interest of economy. Use of optimisation methods can provide a scientific approach toarrive at a leastcost design. In this paper, optimal design of broad gauge prestressed concrete sleepers for tangent tracks is presented considering the current design standards by Research Designs and Standards Organisation (RDSO). Formulations for optimal design for the conventional 22.9 t and proposed 25 t axle loads are presented along with identification of design variables, objective function and constraints. Optimisation was carriedout in two phases: first, by keeping the geometry constant (for a constant mould size, as presently used) and secondly, also by varying the sleeper cross-section geometry. Among the non-traditional optimisation techniques, genetic algorithm for a constrained optimisation problem using adaptive penalty parameter and adaptive genetic algorithm was used. A MATLAB program has been developed based on the formulations. The optimal solutions are summarised here.

The simultaneous heat exchanger network (HEN) synthesis problem is generally formulated as a mixed integer non-linear programming (MINLP) problem by using superstructures, which embed several possible stream matches and various alternative HEN designs. The superstructures proposed by Yee et al. [1] and by Floudas et al. [2] are used for this purpose. In the present work, the graph theoretic principles used by Shivakumar and Narasimhan [3] to develop an efficient nonlinear program (NLP) formulation for Yee superstructure has been extended to the Floudas superstructure. A comparison of the NLP formulations based on the two superstructures, in terms of efficiency, scalability, convergence and quality of solution has been carried out using several test problems available in the literature. It has been shown that the Floudas superstructure is superior in terms of these performance metrics, and appears to be the best suited for optimisation based HEN synthesis. � 2010 Indian Institute of Chemical Engineers.

Viscosity modifying admixtures (VMAs) are high molecular weight, water soluble organic polymers that are used to stabilise the rheological properties and consistency of self-compacting concrete (SCC). Most studies on VMA focus on the influence of different types and dosages of VMA on the rheological properties and the compatibility issues between superplasticiser and VMA. To obtain the desired rheological properties and to reduce the cost, optimisation of the VMA dosage is essential. In the present study, a simple method was developed for optimising the VMA dosage for the cementitious pastes of different water to powder ratio. The principle of this method was based on Stokes' law. Rheological properties of SCC pastes with VMA were studied using a viscometer. The studies were also extended to concrete for validation of the optimised dosage obtained from the cementitious paste tests. The results obtained clearly indicate that the optimised dosage obtained by using the empirical method yields SCC without segregation. The lower dosages are not sufficient enough to control the segregation. On the other hand, dosages higher than optimal resulted in reduced slump flow. � 2009 RILEM.

Precast concrete pipe racks are an economically advantageous alternative to steel pipe racks in industrial projects like refineries and petrochemical plants. One major reason for this is the elimination of fire proofing needs for the precast element. Optimal design of the concrete elements would further lower the cost. This paper deals with the optimal design of precast concrete pipe racks, considering design variables such as member sizes, grades of concrete and steel, and reinforcement area. Genetic algorithms (GA) have been used for optimisation and a program in VC++ has been developed to arrive at the optimal solution, which corresponds to overall cost, satisfying design criteria for strength and limiting deflection. Proposals for most favourable jointing mechanisms in terms of cost and constructability have also been given.

The fresh concrete properties of self-compacting concrete (SCC) greatly depend on the paste composition and paste content. As far as high performance concrete is concerned, apart from cement and water, mineral and chemical admixtures play a vital role especially in the paste formulation of SCC. Most of the studies focus only on the influence of the different dosages of mineral and chemical admixtures on the performance of SCC. Moreover, the production cost of SCC is high due to the usage of these admixtures, especially the chemical admixtures for enhanced performance. Hence there is a need for optimised use of the mineral and chemical admixtures. In the present investigation, a new systematic approach based on the concepts of particle packing and rheology was adopted for optimising the powder combinations (cement and fly ash) and superplasticiser (Polycarboxylate based ether) dosage respectively. An empirical test was conducted to optimise the cement - fly ash combination, while the superplasticiser dosage was optimised for 5 different w/p ratios using mini slump tests on cement paste. An attempt was made to understand the influence of optimised dosage of superplasticiser for different w/p ratio on the rheological properties of the cementitious paste. The optimised dosage of mineral and chemical admixtures of the paste was validated in concrete. The results indicate that the slump flow of concrete for the optimised dosage of superplasticiser for different w/p ratio ranges from 550 mm to 650 mm with T500 time ranging from 1 to 6 seconds.