TRANSPORTATION AND PLACING TECHNIQUES

The characteristics of the air-void system of concrete can be altered during transportation in ready-mix trucks (in which concrete is continually agitated). It is quite common to observe a decrease in air content from the time when concrete is mixed at the plant to the time when it is placed in the formwork at the job site. In properly air-entrained concretes

however, the decrease in air content is mainly due to the loss of large air bubbles, and it seems that the value of the spacing factor is little affected because it is mainly related to the presence of the smaller air voids (Pigeon et al., 1987; Gjorv et al., 1978; Pleau et al., 1990). The characteristics of the air-void system of concrete can also be affected by pumping or conveying over long distances at the job site, although very little data is available on this subject. When pumping or conveying is required, it is highly recommended to carry out field tests to verify that the spacing factor of the air voids will not be adversely affected.

Placing operations can influence the frost durability of concrete in two different ways. First, the use of vibration for placing concrete in the formwork may cause air losses. Fortunately, most of the time, the spacing factor of correctly air-entrained concrete is little affected by vibration because, as previously mentioned, the air loss is primarily due to the loss of a relatively few large air bubbles. Nevertheless, it has been observed that the use of vibrators operating at a high frequency (which is a very uncommon practice) can be detrimental to the stability of the spacing factor (Stark, 1986).

The second problem associated with the placing of concrete is related to the resistance to surface scaling. This resistance is strongly dependent on the quality of the surface layer of concrete (i.e. the first few millimetres) and all phenomena which can be detrimental to the overall quality of this zone, such as excessive bleeding or plastic shrinkage, can be detrimental to the scaling resistance, and must therefore be avoided as much as possible. Finishing operations are also particularly important. Care should be taken to avoid overworking the surface, since overworking can remove some of the entrained air voids (Cordon, 1966) and also attract water from the body of the concrete to the surface layer (which is then weakened). It was found that the use of wooden trowels is preferable to steel trowels since it reduces the risks related to overworking.

9.6 CURING

Good curing (to ensure proper cement hydration) is important to obtain frost-resistant concrete, and particularly de-icer salt scaling-resistant concrete, since curing mostly affects that part of the concrete which is close to the surface. The influence of different curing procedures on the resistance of concrete to surface scaling was analysed by Beaupré (1987). The results obtained are summarized in Figure 9.3 which shows the mass of the scaledoff particles versus the number of daily freezing and thawing cycles in the presence of de-icer salts (according to ASTM Standard C 672). Each curve on the figure represents the mean value obtained from six concretes (with or without silica fume as partial Portland cement replacement). Five different curing procedures were tested: moist curing for 2 and 14 days, respectively, 7-day curing with two different curing compounds, and accelerated heat curing (6 h after mixing, the specimens were stored at 65°C for approximately 18 h).

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Figure 9.3 Average mass of scaled-off particles (for three concretes) versus number of freezing and thawing cycles in the presence of a de-icer salt solution for four different curing methods (after Beaupré, 1987).

The data in Figure 9.3 indicate that increasing the moist curing period from 2 to 14 days significantly reduces the extent of surface scaling. It also indicates that the use of curing compounds is beneficial since a 7-day curing period with each of the two different curing compounds gave better results than a 14-day moist curing period. This positive influence of curing compounds (also observed by other investigators (Marchand et al., 1991; Bilodeau

et al., 1991)) cannot be attributed to a lower chloride ion penetration during the tests

because microprobe analyses have shown that the concentration of chloride ions was about the same in the specimens cured for 14 days and those cured with a curing compound. It is also not just related to better cement hydration, since a loss of mass (due to evaporation) was recorded during the curing period with the curing compounds. The exact role of curing compounds is not perfectly understood yet, but one of their main advantages could simply be to slow down the drying process and thus reduce the surface damage due to drying. As explained in Chapter 3, the moisture history of concrete is very important as regards its frost behaviour and any product that can influence the way in which concrete dries (and reabsorbs water) can exert an influence on its frost behaviour.

It has been clearly established, again as mentioned in Chapter 3, that, although the mechanisms involved are not fully understood, drying can significantly increase the amount of freezable water, probably because the drying process leads to an ‘opening’ of the pore structure leading to a coarser pore-size distribution and a better interconnection between these pores, both of which promote ice formation (Bager and Sellevold, 1986). Research is needed to investigate the phenomena related to the influence of drying on the microstructure of cement paste and concrete. Nevertheless, it is commonly agreed that a sufficiently long curing period (using an adequate technique such as, for example, the use of a curing compound) before concrete dries is a very good practice, especially in the summer period when drying can be particularly severe. High drying temperatures have definitely been shown to be detrimental to the scaling resistance of many concretes (Sorensen, 1983), and preliminary observations from a study carried out recently in the province of Québec (Canada) tend to suggest that, contrary to common belief, sidewalks cast during the summer could be more susceptible to de-icer salt scaling than sidewalks cast in the fall.

The curing temperature is another important parameter, and recent experimental data tend to show that a high curing temperature is detrimental to the surface scaling resistance of normal concrete. As can be seen in Figure 9.3, the deterioration due to surface scaling is much higher for the specimens cured at 65 °C than for all the others (after only five cycles, the mass of scaled-off particles is already larger than the commonly accepted limit of 1 kg/m2_{). This could explain the poor service record of certain pre-fabricated}

steam-cured concrete elements exposed to de-icer salts.