The application of Ag/AgCl electrodes as chloride sensors in cementitious materials

Determination of the chloride content in a reinforced concrete structure is important for evaluation of the risk of chloride-induced corrosion of reinforcement. The traditional techniques for chloride determination in concrete are laborious, time-consuming and cannot be used for continuous monitoring of the chloride content. The investigation on the use of Ag/AgCl electrodes as chloride sensors in cement-based materials dates back to 1990s. Interpretation of the sensor’s response in cementitious materials requires the knowledge of chloride sensor’s characteristics and the interaction between the sensor and the surrounding medium. Hence, the stability of the chloride sensor’s response in cementitious materials depends on the properties of Ag/AgCl interface, AgCl/cement paste interface and the pore solution composition of cementitious materials. The influence of these factors on the stability of the sensor’s response was studied in this thesis. In Chapter 1 the background and motivation for the thesis were presented. In Chapter 2 the advantages and drawbacks of available test methods for determination of the chloride content in cementitious materials were explained.Materials and Environmen

Resource optimization techniques in scheduling:applications to production and maintenance systems

Abstract Optimizing the use of resources plays an important role in today’s modern manufacturing and service organizations. Scheduling, involving setup times and costs, leads to better allocation of resources over time to perform a collection of required tasks. This compilation dissertation examines how the learning effect of workers and a combination of setup activities can be used to optimize resource utilization in manufacturing systems and maintenance services. The learning effect is a technique that can model improvement in worker’s ability as a result of repeating similar tasks. By considering the learning effect, setup times will be reduced, and a schedule can be determined to place jobs that share similar tools and fixtures next to each other. The purpose is to schedule a set of jobs in a hybrid flow shop environment while minimizing two criteria that represent the manufacturers’ and consumers’ concerns: namely maximum completion time (makespan) and total tardiness. Combining setup activities can also reduce setup times and costs. In the maintenance of systems consisting of multiple components, costs can be saved when several components are jointly maintained. By using this technique, a schedule can be determined to minimize the total cost of maintenance and renewal projects for various components and their relevant setup activities. Mathematical programming models that incorporate these aspects of the problem are developed in this research and the performance of the proposed models are tested on a set of problem instances. The results of this work show that the proposed techniques perform well in reducing setup times and costs and eliminate the need for setups through scheduling. This work proposes several exact, heuristic, and meta-heuristic methods to solve the developed models and compare their efficiency. This study contributes to the theoretical discussion of multi-criteria production and maintenance scheduling. For practitioners, this dissertation work provides optimization techniques and tools through scheduling that can help keep costs down and allow companies to operate according to time and budget constraints.Tiivistelmä Resurssien käytön optimoinnilla on tärkeä rooli nykypäivän tuotanto- ja palveluympäristöissä. Joukko tehtäviä voidaan toteuttaa resurssitehokkaammin niille varatussa ajassa huomioimalla aikataulutuksessa asetusajat ja –kustannukset. Tämä kokoomaväitöskirja tarkastelee, kuinka työntekijöiden oppimisefektin mallinnus ja asetustoimien yhdistäminen tukevat resurssien optimointia tuotantojärjestelmissä ja kunnossapitopalveluissa. Oppimisefekti on tekniikka, jolla voidaan mallintaa työntekijän osaamisen kehittymistä samankaltaisia työtehtäviä toistettaessa. Huomioimalla oppimisefekti asetusaikoja voidaan pienentää, ja töille luoda aikataulu jossa samankaltaiset työkalut ja laitteet ovat lähellä toisiaan. Osana väitöskirjaa esitetään työerän aikataulutus tietyssä yksittäistuotantoympäristössä minimoiden kahta kriteeriä: valmistajan tavoite kokonaisläpimenoaika ja asiakkaan tavoite yksittäisten töiden aikataulussa valmistuminen. Toinen väitöskirjassa esitetty tekniikka asetusaikojen ja –kustannusten pienentämiseen on asetustöiden yhdistely. Useista komponenteista koostuvassa systeemissä kustannussäästöjä voidaan saavuttaa huoltamalla useita komponentteja yhtä aikaa. Tämän yhdistelyn avulla voidaan luoda aikataulu, joka minimoi useiden komponenttien ylläpidon, uusimisen, ja asetuskustannusten kokonaiskustannuksen. Työssä mallinnetaan näitä tekniikoita matemaattisen ohjelmoinnin keinoin, ja luotuja malleja testataan joukolla esimerkkiongelmia. Väitöskirjan tulokset osoittavat, että ehdotetuilla tekniikoilla voidaan vähentää asetusaikoja ja –kustannuksia, tai poistaa asetustöistä aiheutuvia kustannuksia kokonaan. Siinä esitetään useita eksakteja, heuristisia ja metaheuristisia menetelmiä kehitettyjen mallien ratkaisuun ja niiden suorituskyvyn vertailuun. Työn tulokset edistävät tieteellistä keskustelua monikriteeriskeduloinnin alalla, erityisesti liittyen tuotanto- ja kunnossapitosysteemeihin. Käytännön toimijoille väitöskirja tarjoaa optimointitekniikoita- ja työkaluja aikataulutukseen ja sen kautta taloudellisissa ja ajallisissa rajoitteissa toiminnan mahdollistamiseen

A system dynamics analysis of value creation in project context

Abstract The focus of this paper is on analyzing the value creation dynamics in the project implementation phase. By value creation, we mean the activities, processes, and strategies that the project team uses to increase benefits and/or reduce costs in the project. By synthesizing the literature on project management and system dynamics, we developed a simulation model with various structures underlying project dynamics. We considered four structures that influence project realized value: project team features, project characteristics, project controls and value creation processes, and project remedial actions due to ripple effects. The resulting model can systematically examine the interplay of value creation processes: work progression, rework, redesign and innovation, and rescheduling. We used the model to explain how the project team’s capability, motivation, and speed of making the best- for-project decisions ensure that the value creation goals are met. We simulate various scenarios that show the significance of the processes and their influencing structures on the realized value. The results present how endogenous and exogenous drivers of system behavior unfold over time and provide a richer understanding of the effect of various model structures such as project complexity and uncertainty on value creation

Polarization Behaviour of Silver in Model Solutions

When studying chloride-induced corrosion in reinforced concrete structures, essential information of interest is the concentration of chloride ions in the system. The absence of a reliable method for monitoring the free chloride ions justifies the attempts towards establishing a feasible practice in the application of the already known Ag/AgCl electrode, as a chloride sensor. To identify the governing mechanism and cognition of causes for instability of the chloride sensors in highly alkaline medium (as concrete), it is necessary to study the polarization behaviour of silver in different aqueous solutions resembling the concrete environment. Following expectations and well-known fundamental background, the results from this work confirm that in the presence of chloride ions, silver chloride is the predominant reaction product, forming on the silver surface. Whereas, in the absence of chloride ions and/or presence of interfering ions, such as hydroxide ions, the oxidation process of AgCl formation is significantly dependent on the chloride concentration in the medium. Therefore, the formation of a stable AgCl layer on a Ag substrate (as would be required for sensors application for example) is a function of the presence and amount of interfering ions, together with the chloride concentration in the medium.Materials and Environmen

Determination of chloride content in cement-based materials: Comparison of results derived by conventional methods and chloride sensor readings

In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides

Potentiometric response of chloride sensors in cementitious materials of varying chemical composition and water-to-cement ratio

Free chloride ions are the main cause of steel corrosion in reinforced concrete structures. Ag/AgCl electrodes, as chloride sensors, are predominantly sensitive to chloride ions and respond electrochemically to the chloride ion activity in the environment. This is reflected by changes of the open circuit potential (OCP) of the sensors. To investigate the effect of the cement-based matrix on OCP readings, this paper presents the electrochemical responses of chloride sensors, embedded in cement paste specimens of hydrated pure cement phases of different water-to-binder ratios. The specimens were immersed in alkaline solutions with various chloride concentrations. The results show dependency of the OCP of the sensor on the surrounding hydrated cement-based matrix. This effect was more pronounced when the specimens were immersed in solutions of lower chloride concentration. In contrast, these effects were negligible at high (> 500 mM) chloride concentrationsMaterials and Environmen

Potentiometric response of Ag/AgCl sensor in Portland and slag cement pastes

Determination of the free chloride content in a concrete structure is important for evaluation of the risk of corrosion of reinforcing steel. The Ag/AgCl sensor is sensitive to chloride ions and exhibits an open circuit potential (OCP) that depends on the chloride ions activity in the environment. The interference of hydroxide and sulfide ions in the pore solution of cementitious materials with the sensor affects the stability of the sensor’s OCP. In this paper, the sensor’s OCP (i.e. sensor reading) in Portland and slag cement pastes is monitored over 300 days of immersion in solutions with different chloride concentration. The results show the applicability of the chloride sensor for continuous and non-destructive determination of the free chloride content in Portland cement paste. The significantly different OCP of the sensor in slag cement paste was attributed to the interference of sulfide ions and formation of Ag2S on the sensor’s surface. Hence, the content of free chloride ions in slag cement paste could not be determined from the sensor’s OCP. The acid-soluble chloride and water-soluble chloride in the cement pastes were also determined after 300 days of immersion of specimens in the solutions. In case of Portland cement paste, the measured acid and water-soluble chlorides were compared to the sensor reading. The acid and water-soluble chlorides were higher than sensor reading. The release of physically and chemically bound chlorides into the acid and water solvents were the main cause for this trend. The results show the importance of cement type and chloride binding ability of hydration products for the sensor’s response and the acid and water-soluble chloride contents. Materials and Environmen

The importance of chloride sensors stability in monitoring ageing phenomena in concrete structures: Ag/AgCl electrodes performance in simulated pore-water environment

One of the reported problems associated with the performance of Ag/AgCl electrodes in alkaline environment, as concrete pore water, has been their poor stability. Open circuit potentials are typically observed to be stable for a time period of a few minutes to some days depending on the thickness and microstructure of the sparingly soluble AgCl and the subsequently developed mixed potentials at the electrode/solution interface. In this paper the open circuit potential of the chloride sensors were monitored over time in simulated pore solution with and without chloride and their electrochemical response recorded via electrochemical impedance spectroscopy (EIS). The alterations in response of the sensors can be denoted to transformation of the AgCl layer to Ag2O in chloride-free high pH environment as in concrete pore water. Recovery of the AgCl layer takes place in chloridecontaining medium, where establishing of sensor stability is determined by the chloride concentration

Determination of chloride content in cement-based materials: Comparison of results derived by conventional methods and chloride sensor readings

In this paper the potentiometric response of a Ag/AgCl electrode as a chloride sensor in cementitious materials of different mix design was studied. The chloride sensor’s response was discussed with respect to the presence of hydration products around the sensor. The free chloride content inferred from the sensor’s response was compared to the one obtained from destructive water and acid soluble chlorides. The measured free chloride content, obtained via sensor’s reading, was lower than the obtained water and acid soluble chlorides. The results indicated the influence of the cementitious mix design on the correlation between the free chloride content obtained via sensor’s reading, water and acid soluble chlorides.Materials and Environmen

The effect of worker learning on scheduling jobs in a hybrid flow shop:a bi-objective approach

Abstract This paper studies learning effect as a resource utilization technique that can model improvement in worker’s ability as a result of repeating similar tasks. By considering learning of workers while performing setup times, a schedule can be determined to place jobs that share similar tools and fixtures next to each other. The purpose of this paper is to schedule a set of jobs in a hybrid flow shop (HFS) environment with learning effect while minimizing two objectives that are in conflict: namely maximum completion time (makespan) and total tardiness. Minimizing makespan is desirable from an internal efficiency viewpoint, but may result in individual jobs being scheduled past their due date, causing customer dissatisfaction and penalty costs. A bi-objective mixed integer programming model is developed, and the complexity of the developed bi-objective model is compared against the bi-criteria one through numerical examples. The effect of worker learning on the structure of assigned jobs to machines and their sequences is analyzed. Two solution methods based on the hybrid water flow like algorithm and non-dominated sorting and ranking concepts are proposed to solve the problem. The quality of the approximated sets of Pareto solutions is evaluated using several performance criteria. The results show that the proposed algorithms with learning effect perform well in reducing setup times and eliminate the need for setups itself through proper scheduling