Damage assessment of concrete structures exposed to fire

During a fire, concrete structures behave in most cases very well. It could therefore be of economic interest to repair the damaged structures, as costs for demolition and rebuilding can be avoided and the building can be reused faster. To assess the remaining loadbearing capacity in a scientific based way, information is necessary about the temperature distribution inside the concrete element and the residual material properties of both concrete and steel. But, at least of equal importance is a fundamental insight in how a concrete structure could behave during a heating cycle, as indirect actions due to thermal restraints can occur and cause significant cracking. These effects should be noticed during a visual inspection of the structure, however, cracks introduced by internal thermal restraints are not visible from the concrete surface. In this paper, fundamental knowledge is given about the effect of heating and cooling on the compressive strength of concrete. Diagnosis tools are discussed to obtain the temperature distribution, especially in the neighborhood of the reinforcement. Those techniques are based on the physico-chemical transformations of the cement matrix and the aggregates that occur during heating. To determine the effect of thermal restraints on the structural behavior, a methodology based on finite element methods is illustrated

Combined effects on residual strength of a high performance concrete exposed to fire

Concrete structures exposed to fire suffer from damage, but can remain a certain degree of residual strength. International research has shown that the compressive strength of concrete decreases not only with temperature, but also by the way of cooling and the storage conditions after fire. Fast cooling introduces a thermal shock which, based on experiments by the authors, could result in a 30% additional strength loss with respect to the loss during heating. When storing the concrete after the fire in air or under water, additional strength losses of about 20-30 % are found within 14 days after the fire. In this paper it is investigated for a high performance concrete what the combined effect is of heating, cooling and storage. One of the conclusions – but with respect to the specific test conditions (e.g. slow heating, 550°C max, pre-dried samples) – is that superposing both expected strength losses of about 30% in case a fast cooling is followed by a period of post-cooling storage results in too conservative strength estimations. It is deemed that the cracks resulting from fast cooling, will act as expansion chambers for the newly produced portlandite, thus strongly reducing additional stresses, which results in expected lesser damage

Assessment techniques for the evaluation of concrete structures after fire

As concrete structures exposed to fire behave in most cases very well, it could be of economic interest to repair the fire damaged structure. For this purpose a damage assessment based on scientific research is required as first step. In this paper, the Schmidt Rebound Hammer and colorimetry are addressed as toos for this assessment. Firstly, the effect of booth methods is studies on heated siliceous concrete specimens under laboratory conditions. Secondly, the practical applicability of both methods is examined by evaluating the fire damage of a concrete girder exposed to a real fire. Both techniques show to be very useful in evaluating the fire damage of the girder

Approaches for the assessment of the residual strength of concrete exposed to fire

Generally, concrete structures have a high fire resistance. After fire, it is of economical interest to reuse the structure after appropriate repair based on a reliable assessment of the residual strength. This paper deals with some fundamental aspects of a scientific and systematic methodology to assess the damage and to estimate the residual concrete strength on the basis of the change in colour and the crack development. This method seems to be promising, but the number of cracks and the change in colour are influenced by the test set up. Furthermore, these relationships change when the concrete ages after heating. Other methods such as water immersion, the Rebound Index and microscopy also provide an adequate basis for the assessment of the temperature in heated concrete

NDT methods for the assessment of concrete structures after fire exposure

This paper studies the application of the Schmidt Rebound Hammer and colorimetry as tool to assess the fire damage of concrete structures. Firstly, experimental data is acquired under laboratory conditions on small specimens. Secondly, this information is used to evaluate the damage of a case study consisting of a girder exposed to a real fire. Both techniques show to be very useful in evaluating the fire damage and can provide the necessary information for a calculation of the residual load bearing capacity

Probabilistic analysis of concrete beams during fire

In this paper a simple computational tool is presented, which provides insight in the time and temperature dependent reliability of concrete beams during fire. The uncertainty of basic variables is taken into account through Monte Carlo simulations, resulting in a quantification of the uncertainty regarding the bending of these full-probabilistic simulations are compared with the semi-probabilistic calculation methods as specified in EN 1992-1-2

Post-cooling stress-strain model of traditional and high-strength concrete

As concrete structures suffer from severe fire damage, but may retain a certain remaining loadbearing capacity, it is important to have material properties for assessment by calculation after fire. This paper proposes a full stress-strain model for post-cooling conditions of a traditional calcareous concrete (TCC) and a high strength siliceous concrete (HSSC)