CB
C.R. Braam
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1
When designing a concrete structure, a structural engineer often assumes that it is initially free of stresses and cracks. Serviceability limit state analyses are therefore often carried out only taking into account the structure’s self weight and the other imposed loads. As a result, initial tensile stresses and (internal) cracking that can arise from, e.g., partially restrained shrinkage is not accounted for. This might imply that service life predictions can be too optimistic since micro-cracks can promote the penetration of chlorides, with the result of an accelerated corrosion of the reinforcement.
The impact of restraint can be at both the structural level (plane sections must remain plane) and the material level (concrete is a non-homogenous material from components having different shrinkage behaviour and stiffness).
In this contribution the results of the numerical analysis of eigenstresses and micro-cracking due to the shrinkage of the cement paste in concrete are highlighted.
The two-dimensional Delft beam lattice model (BLM) is used to simulate concrete on meso-level as a three-phase material. Stress relaxation in the cement paste is calculated with the activation energy method.
For the comparison of the calculated micro-cracking the results of laboratory tests on concrete are used by which the chloride penetration is measured under different restraint conditions with different levels of micro-cracking.
The results of the numerical analyses with BLM specimens showed that micro-cracking can increase the diffusion coefficient of (uncracked) concrete with a factor 1.2–1.6, depending on the magnitude of stress relaxation.
The residual tensile eigenstresses in concrete reduces due to micro-cracking and stress relaxation till values in the order of 0.5–1.0 MPa which are not taken into account in global calculations and can speed up (unexpected) cracking in the concrete.
...
When designing a concrete structure, a structural engineer often assumes that it is initially free of stresses and cracks. Serviceability limit state analyses are therefore often carried out only taking into account the structure’s self weight and the other imposed loads. As a result, initial tensile stresses and (internal) cracking that can arise from, e.g., partially restrained shrinkage is not accounted for. This might imply that service life predictions can be too optimistic since micro-cracks can promote the penetration of chlorides, with the result of an accelerated corrosion of the reinforcement.
The impact of restraint can be at both the structural level (plane sections must remain plane) and the material level (concrete is a non-homogenous material from components having different shrinkage behaviour and stiffness).
In this contribution the results of the numerical analysis of eigenstresses and micro-cracking due to the shrinkage of the cement paste in concrete are highlighted.
The two-dimensional Delft beam lattice model (BLM) is used to simulate concrete on meso-level as a three-phase material. Stress relaxation in the cement paste is calculated with the activation energy method.
For the comparison of the calculated micro-cracking the results of laboratory tests on concrete are used by which the chloride penetration is measured under different restraint conditions with different levels of micro-cracking.
The results of the numerical analyses with BLM specimens showed that micro-cracking can increase the diffusion coefficient of (uncracked) concrete with a factor 1.2–1.6, depending on the magnitude of stress relaxation.
The residual tensile eigenstresses in concrete reduces due to micro-cracking and stress relaxation till values in the order of 0.5–1.0 MPa which are not taken into account in global calculations and can speed up (unexpected) cracking in the concrete.
Conference paper
(2017)
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Tomas Weeda, Jarit De Gijt, Klaas Jan Bakker, Rene Braam, Eric Broos, Bas Jonkman
As a result of the persistent increase of container vessel dimensions, future problems regarding navigation draught in the presence of tunnels are becoming more likely to occur. Hence, possible solutions to this problem have been investigated for the Beneluxtunnel. Several design options have been elaborated to determine the technical and economic feasibility of a possible lowering of the tunnel. Important subjects involved are the consequences of increasing the slopes, the cross-sectional concrete capacity, the use of joint rotations, the construction methods and the costs. This initial exploration of the subject shows lowering the Beneluxtunnel seems to be possible and is expected to be economically attractive. However, further research into certain boundary conditions and risky aspects of construction is required to ascertain this statement.
...
As a result of the persistent increase of container vessel dimensions, future problems regarding navigation draught in the presence of tunnels are becoming more likely to occur. Hence, possible solutions to this problem have been investigated for the Beneluxtunnel. Several design options have been elaborated to determine the technical and economic feasibility of a possible lowering of the tunnel. Important subjects involved are the consequences of increasing the slopes, the cross-sectional concrete capacity, the use of joint rotations, the construction methods and the costs. This initial exploration of the subject shows lowering the Beneluxtunnel seems to be possible and is expected to be economically attractive. However, further research into certain boundary conditions and risky aspects of construction is required to ascertain this statement.
Brand en de Eurocode 2
Toelichting aan de hand van een rekenvoorbeeld
Met de introductie van de Eurocodes verandert ook de manier waarop brandwerendheid van betonconstructies moet worden bepaald. Eurocode 2, deel 1-2, vervangt dan NEN 6071. In dit artikel worden enkele van de mogelijkheden getoond die Eurocode 2 biedt om aan te tonen dat de constructie zijn draagvermogen behoudt gedurende de blootstelling aan brand tijdens de vereiste tijdspanne. De mogelijkheden worden geïllustreerd aan de hand van het toetsen van de brandwerendheid van kolommen, waarbij een rekenvoorbeeld is toegevoegd. In een volgend artikel komt de brandwerendheid van liggers ter sprake.
...
Met de introductie van de Eurocodes verandert ook de manier waarop brandwerendheid van betonconstructies moet worden bepaald. Eurocode 2, deel 1-2, vervangt dan NEN 6071. In dit artikel worden enkele van de mogelijkheden getoond die Eurocode 2 biedt om aan te tonen dat de constructie zijn draagvermogen behoudt gedurende de blootstelling aan brand tijdens de vereiste tijdspanne. De mogelijkheden worden geïllustreerd aan de hand van het toetsen van de brandwerendheid van kolommen, waarbij een rekenvoorbeeld is toegevoegd. In een volgend artikel komt de brandwerendheid van liggers ter sprake.