Emmanuel Annerel

Full-Probabilistic Analysis of Concrete Beams During Fire

Asimplified full-probabilistic calculation tool is developed which is capable of calculating the bending moment capacity of simply supported beams exposed to fire. It is found that the uncertainty with respect to the reduction factors for the material properties at elevated temperatures and the uncertainty with respect to the concrete cover have a negative effect on the safety level corresponding to the design value of the bending moment capacity calculated according to the Eurocode. Furthermore, the model allows an objective comparison of different design alternatives with respect to the safety level.

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 tools for this assessment. Firstly, the effect of both methods is studied 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.

Assessment of the Safety Level of Concrete Slabs during Fire

During the last decades research has provided many new insights in the structural behavior of concrete structures subjected to fire. However, most studies and also the semi-probabilistic design methods in the Eurocode EN 1992-1-2 focus on the maximum time of fire resistance, without mentioning the impact of fire on the safety level of the structure. In order to assess the effect of fire on the safety level of concrete slabs, a full-probabilistic model is developed, using Monte Carlo simulations for modeling the uncertainty regardind the bending moment capacity. Results indicate that the calculated safety level is highly sensitive to the uncertainty of temperature effects on material properties. Furthermore, it is shown that the safety level at the fire resistance time is not the same for all the design approaches mentioned in EN 1992-1-2. Also, a generalized target value for the safety index beta during fire is proposed, which allows for an objective comparison of the fire resistance time for different design alternatives.

Basic Approach for the Diagnosis of Concrete after Fire Exposure

Concrete structures have a good fire resistance. After the fire, depending on the amount of damage, they may be repaired and reused. However, knowledge is needed to do this in a systematic and scientific way. This paper describes the parameters influencing the residual compressive strength of heated concrete. Since this strength decreases with temperature, the temperatures inside the concrete need to be known to assess the remaining load bearing capacity of concrete members. Two assessment techniques are discussed in this paper. Firstly, the colour alterations of the concrete surface and of the aggregates are measured, from which colour paths are derived. The shape of these paths allows to distinguish different temperature zones. Secondly, due to thermal decomposition of the cement matrix and thermal cracking at the interfacial transition zone, an increase of the porosity is found. This internal damage can be measured by the weight increase found after immersing concrete cores under water.

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.

Robustness of a Typical Beam-Column Concrete Structure Exposed to Fire

Concrete structures behave mostly well during the fire, however, in some cases collapse is found before the calculated fire resistance is obtained. This premature collapse during fire can be explained when studying the global structure response, as stresses are induced by restraint actions of the thermal deformations. This paper studies the behaviour of a structure consisting of beam-column connections exposed to the ISO 834 fire by means of the finite element package Diana. It is found that the effects of thermal restraint can be modeled. Internal thermal restraint may cause vertical cracks in the concrete element which do not extend towards the bottom surface of the heated beam. External thermal restraint is found as the gradual development of plastic hinges and the increase of shear forces which first appear well before the fire resistance according to the simplified methods of EN 1992-1-2 is attained. Due to the thermal deformations, cracks may also occur in building elements not directly exposed to the fire.

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.