Environmental Actions and Response - Reinforced Concrete Structures exposed in Road and Marine Environments
Licentiate thesis, 2001
The expenses for repair, maintenance and remediation of existing concrete structures have significantly increased. A large proportion of these expenses are due to problems related to a lacking durability of the structure. Thus, to reduce these expenses it is required to make a design of the service life of the structure in which the service life is explicitly specified.
In this Licentiate thesis it is described how the service life of concrete structures can be determined. The factors that have an influence on the service life of a concrete structure are described. Furthermore it is described how the service life can be mathematically modelled and which techniques that can be used to determine the service life. Three principal factors that influence the service life of a concrete structure have been identified: (1) Material properties, (2) Execution during construction and (3) Environmental actions. Lots of efforts have been made to get a better understanding of how the material properties and to some extent the execution during construction influence the service life. However, very little has been made to clarify, which influence the environmental actions has on the service life.
The deterioration of concrete can be predicted with two principal types of mathematical models namely physical models, which are based on theories of transport in porous materials, and empirical models, which are based on observations from structures. A few examples of different prediction models dealing with chloride penetration, carbonation and corrosion of the reinforcement are shown. It is also briefly described how a prediction of the service life can be made, with a performance based design methodology and probabilistic methods. To achieve reliable results with these prediction models it is required to have knowledge about boundary and initial conditions, e.g. how the environmental conditions vary inside and at the surface of the structure.
Predictions of the service life can be made with two different methodologies: deem-to-satisfy rules, based on rules of thumb, and performance-based design. Usually probabilistic methods are used to predict the service life in a performance based design. However these kinds of predictions require extensive statistical quantifications of the parameters in the mathematical models. These statistical quantifications require good knowledge about which parameters that influence the deterioration of concrete, e.g. concrete composition, environmental actions at the surface of the structure and/or the response from the concrete.
The environmental actions on a concrete structure have a decisive influence on the service life of the structure. The description of the environmental actions can be divided into four different levels depending on the dimensional scale. The objective is to determine the environmental actions at the surface of the concrete structure in terms of temperature, humidity and chloride conditions and the concentration of carbon dioxide. Furthermore the response from the material is of interest, e.g. how moisture and temperature distributions are distributed in a concrete structure. It is described how the environmental actions at the surface of concrete structure can be derived and what the response from the concrete is described.
Finally a study of the response concrete structures exposed in road environments is presented. Seven bridges in and around Göteborg, around 30 years old, have been examined to determine the chloride penetration, moisture conditions and resistance against freeze/thaw actions. The analyses have been made on cores taken from selected positions on the bridges. The results from the study show large variations in chloride penetration between the examined bridges and even on a single part of a bridge, e.g. a column. A large part of these variations are assumed to arise due to variations in environmental actions at the surface of the bridges.