H.S. Wong, R.W. Zimmerman, N.R. Buenfeld

All major deterioration processes affecting concrete are rate-controlled by penetration of water and aggressive agents via the inevitably porous microstructure. This project aims to enhance the understanding of how the microstructure of concrete controls penetration of aggressive agents and to establish models to predict transport properties from the microstructure. This would allow more reliable assessment of deterioration rate and remaining life, and facilitate the development of more durable structures.

The interface between aggregates and cement-paste, the ‘interfacial-transition zone’ (ITZ), is of particular importance since it contains higher porosity and lower cement content compared to other regions (Figure 1). Microcracks often initiate and propagate preferentially in the ITZ. It is generally thought that penetration of deleterious agents occurs mainly through the ITZ and this idea forms the basis of many transport models for concrete.

However, a review of the ITZ by us highlighted inconsistencies in previous studies. We subsequently carried out an extensive experimental study to determine the relative importance of ITZ and microcracking on three different transport mechanisms [1]. Over 200 samples of pastes, mortars and concretes were tested. Variables include w/c ratio, cement type, aggregate content, curing age and conditioning temperature.

Our tests found that transport properties of mortars decreased with increasing ITZ fraction for all cases (Figure 2). No critical threshold sand content linked to an ITZ percolation effect was found, even in samples that were deliberately damaged by drying at 105oC. Severe oven-drying induces microcracks with widths of 0.5-10μm that are interconnected and randomly orientated. The microcracks have a significant influence on permeability, which increased by up to a factor of 30, when comparing the same sample dried at 50°C and 105°C.

Diffusivity and sorptivity increased by only a factor of 2. Despite a lower ITZ fraction, concretes have about the same diffusivity and sorptivity, but significantly higher permeability than mortars with the same aggregate fraction (Figure 2). Results from image analysis (Figure 3) suggest that the higher permeability of concrete is attributable to more microcracking that forms in concrete compared to mortar.

Experimental results are often influenced by many varying parameters that are difficult to isolate, masking important trends. Thus, numerical modelling was carried out to complement our experimental work and to support our understanding of the underlying mechanisms. A three-phase composite model was developed to estimate the steady-state chloride diffusivity of mortars and concretes [2].

The model was used to examine the influence of several interacting parameters and to identify those having the most significant effect on diffusivity. A nonlinear finite-element model was used to simulate shrinkage-induced micro-cracking and to investigate its effect on permeability of concrete and mortars [3].

The findings from both studies were in agreement with experimental observations and highlighted the significance of micro-cracks to transport. For example, the simulations of shrinkage-induced microcracking (Figure 4) showed that the higher permeability in concrete is due to wider cracks that form as a result of the larger aggregate size in concrete, in agreement with experimental findings.

The study concludes that the net influence of the ITZ is small, even for samples with a large fraction of overlapping ITZs. The effect of total porosity and presence of micro-cracks, far outweigh any effects of the overlapping ITZs on transport. Thus, mass transport is governed by the entire pore structure within the cement paste and not just within the ITZ.

Acknowledgements:This project was supported by the EPSRC (EP/F002955/1)

References

• H.S. Wong, M. Zobel, N.R. Buenfeld, R.W. Zimmerman (2009), Influence of the interfacial transition zone and microcracking on the diffusivity, permeability and sorptivity of cement-based materials after drying, Mag. Concr. Res., 61, 571-589.
• J.J. Zheng, H.S. Wong, N.R. Buenfeld (2009), Assessing the influence of ITZ on the steady-state chloride diffusivity of concrete using a numerical model, Cem. Concr. Res., 39, 805-813.
• P. Grassl, H.S. Wong, N.R. Buenfeld (2010), Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar, Cem. Concr. Res., 40, 85-93.