T RANSPORT IN CRACKS

As shown by the cyclic degradation process [MEH-94] also transport through cracks needs to be investigated.

Recently, the previously mentioned migration test (§2.2.2.2) was also applied on cracked concrete samples [DJE-06]. Different concretes with traversing cracks, with crack widths ranging from 30 to 400 µm, are tested. The results of the steady state migration tests showed an increase of the diffusion coefficient with increasing crack width, with a limit for crack widths of 80 µm or more. Nevertheless, it can be questioned whether a global diffusion coefficient for the transport through cracks is a proper approach. Moreover it is questioned, if forced migration tests can be considered as adequate tests to investigate salt ion transport though cracks.

In the past the effect of cracks on the generation and development of corrosion in reinforced concrete has been analysed. Some state that not the crack width, but the total area covered by cracks next to the surface of the steel bar plays an important role for corrosion to occur [MET-82]. Others show that cracks in concrete may cause localized chloride ingress and the initiation of rebar corrosion [SAG-98], because these

cracks may open a direct path to the rebar. Further it is reported [FRA-99] that crack widths of less than 0.5 mm affect the development of corrosion, but that this width has not a significant influence at later stages in the corrosion process. Other factors such as environment, quality and thickness of the cover seem more important according to their results. Montes [MON-04] showed, using linear polarization resistance measurements on high performance concrete subjected to a simulated marine environment, that crack conditions (0.25 and 0.5 mm) strongly affect the corrosion process.

Win [WIN-04] investigated the penetration profile of chloride in cracked reinforced concrete and the effect on corrosion. He emphasized the fast transport of chloride ions by water or moisture movement into dry material. In order to investigate this phenomena he performed uptake experiments. Single-cracks and multi-cracks were introduced by using respectively a three-point and a four-point bending test. The initiated crack widths were ranging from 0.1 to 0.5 mm. The liquid uptake fastly reached the top of the sample in all tested samples, except for mixes with low w/c of 0.25. Besides to the capillary suction within the crack, a secondary flow perpendicular to the crack took place (Fig.

2.13). Additionally, a flow along the steel bar was present, because this zone was more porous due to bleeding. The penetration depth around the crack was similar to the penetration depth from the exposed surface, only a slight increase was noticed. This is possible due to the microcracks along the crack surface resulting in a local increased permeability. Although it could be concluded that the quality of concrete has a high influence on the penetration process, also increase in w/c, higher NaCl solutions concentrations and wider cracks led to deeper penetration. Concerning the crack widths however, they all showed more or less the same result, except the crack with a width of 0.5 mm showed a relatively higher penetration. Finally, it was observed that despite the high concentrations of chlorides, no corrosion was observed. Probably this might be due to an insufficient oxygen supply.

These different results emphasize the importance of further investigation of transport in cracks.

Fig. 2.13 Electron probe microanalysis (EPMA) images of Cl- ion concentration for w/c ratio series (a) 0.25; (b) 0.45 and (c) 0.65, after 1 month of exposure. The exposure surface is at the top of the images and the dotted line represents the crack.[WIN-04]

As can be concluded from the previous paragraph, a lot of findings concerning the influence of cracks in concrete are contradictory. The reasons for these discrepancies vary: different mixes, curing conditions and ages, varying tests and test conditions, but also different determination methods and measurement techniques of cracks. Ringot [RIN-01] pointed out that difficulties appear when studying the geometry of microcracks and cracks in cement-based materials. Two methods seem to be suitable: scanning electronic microscopy (SEM) coupled with the replica technique and optical microscopy.

However, these images provide local information. Moreover, it is difficult to visualize crack evolution. The ultimate goal of cracks analysis following Ringot is to relate crack parameters with the transport properties of cracked concrete. The crack density often appears in transport models, but also crack width, roughness, connectivity and tortuosity are required.

The self-healing property of cement-based materials is also a well-known influencing parameter. Various types of crack healing have been reported: physicochemical processes [HEA-94] or mechanical phenomena such as blocking of cracks. Self-healing leads to crack closing. Consequently, it improves mechanical, durability and permeability properties [EDV-99]. Cracked concrete specimens were put in water for three months in order to induce self-healing. Jacobson [JAC-96] found a significant reduction on the rate of chloride migration (28% – 35%) compared to migration in newly cracked specimens.

Ismail [ISM-04] studied the effect of crack width on the local diffusion of chloride in inert materials. To eliminate crack self-healing, tests were carried out in an inert brick material. The tested cracks had a width ranging from 21 to 128 µm. To generate these cracks with constant width across the thickness of the sample, a mechanical expansive core and a confining ring were used (Fig. 2.14). After saturation with demineralized water the cracked samples were exposed to a chloride solution for 10 h in a chloride

penetration cell. Afterwards the chloride concentration profiles were measured in two directions: normal to the surface and to the crack wall (Fig. 2.15, Fig. 2.16). The powder samples at different depths were collected using a Rapid Chloride Test (RCT) sampling device.

Fig. 2.14 Brick disc and the expansive core [ISM-04]

Fig. 2.15 Reference, surface, and perpendicular-to-crack penetration profile in the samples after 10h in chloride solution [ISM-

04]

Fig. 2.16 Location of surface and perpendicular-to-crack concentration profiles The results for crack widths less than 53 µm show a chloride concentration drop near the surface (Fig. 2.15). It is explained that by mechanical interaction between the fracture surfaces, chloride diffusion through the cracked material is limited. This phenomenon also controls the diffusion process perpendicular to the crack wall. For crack widths of 60 µm and more, the perpendicular-to-crack profiles are very similar to the surface profiles. In these cases it is assumed that the rate of chloride penetration perpendicular to crack walls is not restricted by the diffusion of chloride along the crack path.

Dormieux [DOR-01] used a homogenization technique for the prediction of the transport in cracked porous materials. Starting with the mathematical description of phenomena at

microscopic level, an upscaling of the phenomena to the macroscopic level is performed.

This strategy makes it possible to define transport parameters such as permeability starting from the geometry of the microstructure. The disadvantage of this technique is the need for detailed information at microscopic level.

A lot of other publications exits on transport simulations in cracks and the effect on the