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10978: Measuring, Modelling and Monitoring Chloride ingress and Corrosion initiation in Cracked Concrete (M3C4)

Chloride ingress and subsequent reinforcement corrosion determine for a large part the service life of concrete structures. Various factors are highly uncertain, however. Previous research has addressed chloride ingress and various models and test procedures are available for concrete which is not loaded and not cracked. In practice, all concrete structures are loaded and cracked to a certain extent. Chloride transport through cracks is fast, potentially causing early corrosion of reinforcement. Corrosion at cracks in practice is less severe than expected, but the limits are not understood. Apparently counteracting factors exist that mitigate corrosion in cracks, possibly due to (self)healing and blocking by corrosion products. Presently there is no non-destructive measurement technique available for studying chloride transport in concrete. NMR promises to provide such a technique. Moreover, it can be used to trace transport of moisture and changes in pore structure in and around cracks, thus visualising the presumed blocking of cracks. High uncertainty also exists for the critical chloride content, which is the content above which corrosion of steel starts. The wide variation in literature values is unfortunate, as the critical content is the resistance term in the service life equation (see e.g. fib New Model Code). This variation is due to strong variation in materials and external conditions. The main influencing factors are only qualitatively known. Experimental work on micro and meso level is needed to elucidate the mechanism and to quantify the main influences.

Main goals:

The main goals of the project are:

- To measure and model the chloride ingress in cracked concrete in relation to un-cracked concrete, taking into account self healing, crystallisation and crack blocking effects.

- To develop a model for corrosion initiation of steel due to chloride ions in cracked and un-cracked concrete.

- To develop a non-destructive measuring technique (based on NMR) for detecting chlorides in cracked an un-cracked concrete.

The proposed multi-disciplinary approach to these issues consists of the following tasks:

1. Experimental research on the mechanism of chloride ingress in cracks. Specimens will be prepared, cracked and an accelerated test for ingress will be developed especially suited for cracked concrete and/or concrete under loading. Influencing processes will be studied, like self healing, crystallisation, chloride binding and crack blocking in addition to factors like blended binder (slag, fly ash) and different execution variables with regard to curing and compaction. These items will be studied on a micro scale using techniques like ESEM and NMR. This task will result in a model for chloride transport in cracks taking into account healing and blocking effects.

2. Development of Nuclear Magnetic Resonance (NMR) to measure and visualize the transport of chlorides in (un)cracked concrete. Measuring the chloride distribution in time and place non-destructively is essential for this project. NMR is the only technique with the possibility to selectively and non-destructively measure Cl in a porous material. As Cl is a quadrupolar nucleus, its interactions with the matrix will have to be determined to obtain quantitative measurements. NMR will be used for further developing the model to simulate chloride ingress in cracked concrete. This technique can be used to validate sensors that are developed in other IS2C projects to measure chlorides in concrete.

3. Experimental work on and modelling of the critical chloride content in (un)cracked concrete. The main factors are cement chemistry, air voids, cracks, steel imperfections, moisture variation, compaction defects, curing. These factors must be studied on a microscale in the steel/concrete interface using ESEM, LIBS and micro-electrochemical techniques. From understanding microscale effects, a test method will be developed for determining the critical content in concrete. It should have a relatively short duration, while involving realistic chloride transport up to corrosion initiation determined by non-destructive (electrochemical) detection. The results will be processed with emphasis on the low end of the statistical distribution, as service life design aims at low probabilities of failure. The final result should be a probabilistic model for corrosion initiation as a function of concrete and environmental variables, providing limit values for design of new structures and redesign of existing structures for (remaining) service life.

Main deliverables:

The main deliverables of the project are:

- Experimental data of chloride ingress in (un)-cracked concrete, which can be used to determine input parameters for chloride ingress models.

- A model for chloride ingress taking into account the influence of cracks in structures which can be used for service life predictions

- A model for corrosion initiation for (un)-cracked concrete as a function of concrete variables.

- A non-destructive measuring technique to determine chlorides in concrete.

- A technique to validate and calibrate chloride monitoring sensors.

 

For more information related to the project, please contact:

Branko Šavija
b.savija@tudelft.nl
TU Delft – CiTG
Materials & Environment
Stevinweg 1 / Postbus 5048,
2628 CN Delft / 2600 GA Delft
T: +31 (15) 27 88986
F: +31 (15) 27 86383
Room: 6.05

Jose Pacheco
j.pachecofarias@tudelft.nl
TU Delft – CiTG
Materials & Environment
Stevinweg 1 / Postbus 5048,
2628 CN Delft / 2600 GA Delft
T: +31 (15) 27 88990
F: +31 (15) 27 86383
Room: 6.05

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