Estimate of service life of concrete structures in marine environment

Abstract

Deterioration and distress of concrete structures in service is a result of a variety of physicochemics processes. These processes include attack by acids or alkalis, cycles of wetting and drying, freezing and thawing, alkalis-aggregate reaction, etc. However, the most serious deterioration process is caused by chloride diffusion mechanism which then leads to corrosion of reinforcing steel. Moreover, up to date, the accurate service-life prediction of reinforced concrete structures is getting more and more attention. Needless to say, the most advanced models on durability of concrete are found in the field of chloride-induced corrosion. However, most of them are built and developed based on the marine environment of Europe, America and a few countries from Asia, excluding Thailand and Vietnam. For this reason, the requirement of an accurate and suitable model for the local marine environment conditions of Thailand and Vietnam is very imperative. Because Thailand and Vietnam not only have the same environment conditions, but also have a long coast which mostly affect the durability of concrete structures. The aim of this current paper is computing a service life model, which is the most suitable for the local environment of Thailand, and Vietnam based on the available models of previous researches. In order to fulfill objective and satisfy the time restraint, a rapid or short term experiment named Nord test NT build 492: “Chloride migration coefficient from non-steady state migration experiments” with the input parameters including water binder ratio, partial replacement of Portland cement by fly ash and rice husk ash will be conducted. After that, based on these results obtained from the experiment, the chloride diffusion coefficients are got. Moreover, using the data, the expressions for prediction the value of chloride diffusion coefficient are proposed in term of water binder ratio, percent of fly ash and rice husk ash. These expressions are the key for computing the service life model of a certain concrete structure. Furthermore, the influences of pozzolana (fly ash and rice husk ash for the current case) on chloride resistance are investigated as well. Results of the experiment reveal the significance of pozzolanic effect of both fly ash and rice husk ash on decrease of the chloride diffusion coefficient, in compared with the ordinary Portland cement case. However, rice husk ash has more influence on chloride resistance than fly ash at the early age 28 days. In addition, ternary blend incorporating Portland cement with both fly ash and rice husk ash also shows the remarkable impact. However, in ternary blend at the early age 28 days (the current work), the decrease of chloride diffusion coefficient is only due to the effect of rice husk ash and fly ash on the other hand has no significant influence on chloride diffusion coefficient. Finally, two main approaches used in practice for civil engineers to compute service life of concrete structures in marine environment are proposed. In addition, in order to illustrate these both approaches, case studies of certain concrete structures in Thailand and Vietnam are also investigated.