Erdoğan Özbay

Prediction and multi-objective optimization of high-strength concrete parameters via soft computing approaches

The optimization of composite materials such as concrete deals with the problem of selecting the values of several variables which determine composition, compressive stress, workability and cost etc. This study presents multi-objective optimization (MOO) of high-strength concretes (HSCs). One of the main problems in the optimization of HSCs is to obtain mathematical equations that represents concrete characteristic in terms of its constitutions. In order to solve this problem, a two step approach is used in this study. In the first step, the prediction of HSCs parameters is performed by using regression analysis, neural networks and Gen Expression Programming (GEP). The output of the first step is the equations that can be used to predict HSCs properties (i.e. compressive stress, cost and workability). In order to derive these equations the data set which contains many different mix proportions of HSCs is gathered from the literature. In the second step, a MOO model is developed by making use of the equations developed in the first step. The resulting MOO model is solved by using a Genetic Algorithm (GA). GA employs weighted and hierarchical method in order to handle multiple objectives. The performances of the prediction and optimization methods are also compared in the paper.

Permeation Properties of Self-Consolidating Concretes with Mineral Admixtures

This paper addresses the permeation properties of self-consolidating concretes (SCCs) with different types and amounts of mineral admixtures. Portland cement, metakaolin, fly ash, and ground-granulated blast-furnace slag were used in binary, ternary, and quaternary cementitious blends to improve the durability characteristics of SCCs. For this, a total of 22 SCCs were designed that have a constant water-binder ratio (w/b) of 0.32 and a cementitious materials content of 926.75 lb/yd³ (550 kg/m³). In addition to compressive strength and ultrasonic pulse velocity, the permeation resistance of SCCs was determined by means of chloride ion permeability, water permeability, and sorptivity tests. The test results indicated that the permeation properties of SCCs appeared to be very dependent on the type and amount of the mineral admixture used; the SCC mixtures containing metakaolin were found to have considerably higher permeability resistance than the control mixture.

Repeatability and Pervasiveness of Self-Healing in Engineered Cementitious Composites

This paper investigates the intrinsic self-healing ability of engineered cementitious composites (ECCs) coupled with multiple microcrack formation under mechanical loading based on two robustness criteria: repeatability and pervasiveness. To this end, two different composites containing Class F fly ash and slag were investigated. To generate microcracks, specimens were repeatedly preloaded up to 70% of their deformation capacities under mechanical loading at the end of each specified cyclic wet/dry conditioning period. Resonant frequency (RF) and rapid chloride permeability tests (RCPT) were used to assess the extent of damage and self-healing, and final results were supported by microscope observations. RF measurements were recorded from two different parts of each specimen (the top and middle portions) to monitor whether self-healing takes place in certain regions or whether it is pervasive over the entire specimen. Results of the experimental study show that depending on the type of mineral admixture used and the duration of initial curing before deterioration, ECC specimens can recover up to 85% of their initial RF measurements, even after six repetitive preloading applications. The recovery rates observed in the middle portion are similar to those in the top portion for both ECC mixtures (to a slightly lesser extent), which implies that self-healing is quite pervasive. Furthermore, after repeated application of severe preloading, RCPT results for both mixtures satisfy low or moderate chloride ion penetrability levels in accordance with ASTM C1202. Due to the enhanced self-healing capability of specimens, maximum crack width observed over the specimen surfaces was restricted to 190 µm (0.008 in.), even after nine preloadings. These findings suggest that under certain conditions, the ECC materials produced in this study may significantly enhance the functionality of structures by reducing the need for repair and/or maintenance.

Contractor selection with Multi Criteria Decision Support tools

The selection of the most suitable contractor is crucial for the success of construction projects. An important task in contractor selection is to establish a set of decision criteria that can measure and judge the capabilities of contractors. Multiple Criteria Decision Support (MCDS) methods can be very useful to solve this problem. Several MCDS tools are used in the literature for this problem. In order to promote the usage of these methods a set of them are applied to a real problem. It is also aimed to present the relative performances, advantages and disadvantages of these methods.

Properties of Mortars with Natural Pozzolana and Limestone-Based Blended Cements

This paper presents the results of an experimental investigation on the consistency, compressive strength, water sorptivity, chloride ion permeability, electrical resistivity, and sulfate resistance of mortars made with plain and blended cements. Plain (CEM I 42.5 R) and blended cements, including portland pozzolana cements (CEM II A-P 42.5 R and CEM II B-P 32.5 R) and portland limestone cements (CEM II A-LL 42.5 and CEM II B-LL 32.5 R), were used in this study. Mortars with three different water-cement ratios (w/c) of 0.420, 0.485, and 0.550 were produced by using the plain and blended cements. In all the mixtures, the cement:sand ratio was kept constant at 1:2.75 by weight. The compressive strengths of the mortar specimens were tested at 1, 3, 7, 28, 90, and 180 days. Moreover, the water sorptivity, chloride ion permeability, and electrical resistivity of the mortar specimens were measured at 7, 28, 90, and 180 days. The sulfate resistance of the mortars was evaluated by the length change of the mortar specimens up to 30 weeks of exposure. The test results revealed that the use of blended cements decreased the water sorptivity and chloride ion permeability while increasing the electrical resistivity and sulfate resistance of the mortars at later ages compared to the normal portland cements