H.S. Wong, A.M. Pappas, R.W. Zimmerman, N.R. Buenfeld

Concrete contains air voids that are inadvertently entrapped or deliberately entrained to enhance its resistance to frost damage and salt scaling. Whilst an extensive body of work exists on determining the requirements for frost protection, very little research has been carried out to understand the effects of entrained air voids on other aspects of hardened concrete such as mass transport properties and resistance to other deteriorations.

Air voids are penetrable, but because they appear isolated in the microstructure and do not form a continuous flow channel, they are often assumed to make little or no contribution to the transport properties of concrete. Thus, air voids are treated as inert inclusions similar to aggregate particles. However, concrete contains 70% aggregate and all the air voids reside in the cement paste. Thus, a small amount of air entrainment drastically changes the microstructure, in particular the pore structure of concrete. This in turn may have a significant effect on the properties of the hardened concrete.

The aim of this study is to carry out a systematic investigation into the influence of entrained air voids on the microstructure and bulk transport properties of concrete in saturated and non-saturated conditions. Concretes with a range of air contents (0.5-11.5% vol.), w/c ratios (0.35, 0.50), curing ages (7, 365 days), curing conditions (sealed, fog) and conditioning regimes (52% r.h., 75% r.h. and 50˚C oven drying) were tested.

Several transport mechanisms were examined because the influence of entrained air on each may not be the same. The spatial distribution of capillary porosity and cement particles were measured using backscattered electron microscopy and image analysis.

Fig. 1 a) Backscattered electron micrograph of concrete with 8.2% vol. air, b) Image analysis shows the microstructure near the air void interface contains higher capillary porosity and lower cement content compared to the bulk paste farther away.
Fig. 1 a) Backscattered electron micrograph of concrete with 8.2% vol. air, b) Image analysis shows the microstructure near the air void interface contains higher capillary porosity and lower cement content compared to the bulk paste farther away.

It was found that air voids increases the heterogeneity of the microstructure by disturbing the packing of cement grains and the distribution of capillary porosity. Results show that the microstructure of the air void-paste interface is similar to that of the aggregate-paste ‘interfacial transition zone’, in that it contains significantly lower cement content, higher porosity, and higher initial w/c ratio compared to bulk paste farther away from the interface (Figure 1).

The porosity near the air void boundary is about 2-3 times that of the bulk paste, and the width of the affected interface is around 30 μm. However, no significant precipitation of calcium hydroxide was observed at the air void-paste interface. Transport measurements show that gaseous diffusivity and permeability are increased by a factor of 2-3 at the highest air contents, regardless of the w/c ratio, curing age and conditioning regime.

This effect is similar to increasing w/c ratio from 0.35 to 0.50 when samples are conditioned at 52% r.h or 50˚C. The effect on sorptivity is less consistent, while the effect on electrical conductivity is influenced by the moisture condition of the air voids. Under non-saturated conditions, empty air voids act as insulators and the bulk electrical conductivity is decreased. However, saturated air voids behave as conductors and increase electrical conductivity.

Fig. 2 Effect of air content on oxygen diffusivity (a), oxygen permeability (b), water sorptivity (c) and electrical conductivity (d). The data are normalised to that of the control. For electrical conductivity, measurements were made after sorptivity testing (I), conditioning at 75% rh (II) or vacuum saturation (III).
Fig. 2 Effect of air content on oxygen diffusivity (a), oxygen permeability (b), water sorptivity (c) and electrical conductivity (d). The data are normalised to that of the control. For electrical conductivity, measurements were made after sorptivity testing (I), conditioning at 75% rh (II) or vacuum saturation (III).

A model based on Maxwell’s equation was proposed to predict the effect of air voids on transport properties. It was estimated that every 1% increase in air content increases the transport coefficient by about 10% or decreases it by 4%, depending on whether the air voids act as conductors or insulators. Good agreement was observed between the estimated and measured transport coefficients, which span up to two orders of magnitude in the case of gas diffusion and gas permeation.

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

Reference

H.S. Wong, A.M. Pappas, R.W. Zimmerman, N.R. Buenfeld (2011), Effect of entrained air voids on the microstructure and mass transport properties of concrete, Cem. Concr. Res., 41, 1067-1077