PRACTICAL CONSIDERATIONS

The purpose of air entrainment is to protect concrete from frost action by providing it with a closely spaced system of air voids. The commonly used limit of 200 µm for the spacing factor can be considered as a safe design value although, depending on the exposure conditions, some field and laboratory concretes can be frost resistant when the air-void spacing factor is significantly higher than 200 µm, as has already been described.

Considering the large number of parameters that affect air-void production and stability, and considering also the compatibility of the various admixtures between themselves as well as with the cement, it must be emphasized again that testing is the only way to verify

whether the use of a particular air-entraining admixture in a given mix will yield a system of closely spaced air voids (with a spacing factor of approximately 200 µm), which will stay stable until the concrete has set.

Field tests are much more preferable than laboratory tests. Laboratory testing involves relatively small amounts of concrete where heat of hydration effects, for instance, can be very different from those in large volumes of concrete. Many laboratory mixers are more energetic than field or plant mixers, and tend to overestimate the stability problems since they tend to increase the production of smaller voids at the beginning when the concrete is more fluid. Some of these voids gradually disappear as the concrete which is continually agitated stiffens, and the spacing factor increases. Figure 6.16 illustrates the variation of the spacing factor with the time of casting for a typical laboratory mix with an insufficient dosage of the air-entraining agent.

When field tests are carried out, samples for the determination of the air-void characteristics of the concrete should be taken after a sufficiently long period (one hour or more) in order that stability problems, if any, will be observed. If the air-void system after such a period is correct, it can safely be assumed that it was also correct immediately after initial mixing, although tests to verify this should also be carried out. These tests will also serve to verify if the initial mixing period is sufficiently long. If a superplasticizer is to be added to the mix on site to increase the slump, samples should be taken immediately before and after the concrete has been remixed with the superplasticizer, in order to determine the effect of the superplasticizer on the air-void stability.

400 I 300 O

1

Time of casting (min)

Figure 6.16 Variation of the air-void spacing factor with the time of casting for a typical laboratory air-entrained concrete with an insufficient dosage of the air-entraining admixture (after Pigeon et al., 1990).

Once the dosage of the air-entraining agent required to obtain a correct spacing factor in a mix is determined, it should not be changed significantly because it is almost certain that reducing the dosage will result in a higher spacing factor even if the air content stays

constant. Figure 6.17 shows the relationship between the dosage and the spacing factor for a large number of superplasticized and non-superplasticized concretes containing various air-entraining agents and cements. Even if, for a given dosage, there is a wide range of spacing factor values, the trend is clear: the higher the dosage, the lower the spacing factor. Air-entraining agents, it will be recalled, concentrate at the surface of the air voids and help to stabilize them. The higher the amount of admixture available, the larger the amount of surface area that can be stabilized. Large surface areas mean smaller voids and smaller voids correspond to lower spacing factors.

The spacing factor for a given air content, due to the importance of the specific surface, can be quite variable (Figure 6.6). It has been shown that air content variations, particularly when superplasticizers are used, are not necessarily a very good indication of the variation of the spacing factor, unless these air content variations are large (Saucier et al., 1991). The use of total air content variations to predict spacing factor variations should therefore be accompanied as much as possible by measurements of the spacing factor in hardened concrete samples.

It is possible that, in certain mixes, the air content needed to obtain a correct spacing factor will be unacceptably high. This is more likely to occur in mixes with water/cement ratios higher than 0.45 which, in North America, is the upper limit for concrete exposed to freezing and thawing cycles in the presence of de-icer salts. This phenomenon can be considered as a compatibility problem, and other types of admixtures should be tried. Fortunately, the number of commercial products available is large, and it should be possible in most cases to find an admixture (or a combination of admixtures: air-entraining agent and water reducer) that will be compatible with the cement and aggregates used.

800 | 600 o

i

8?e

a>°o

Dosage of air-entraining admixture 1.0

Figure 6.17 Relationship between the dosage of air-entraining admixture and the air-void spacing factor for superplasticized and non-superplasticized concretes containing various cements and air-entraining admixtures (after Saucier et al., 1991).

It is also possible, and this is a more delicate problem, that in certain mixes there will be unpredictable increases of the air content even if the dosage of the air-entraining admixture is kept constant. These increases can be due to temperature variations for instance, or to changes in the grading of the sand, or even to variations in the quality of the mixing water. Reducing the content of air-entraining agent to solve this problem is not desirable considering that any decrease in the dosage of the air-entraining agent can cause an increase in the spacing factor. There is no simple answer to this problem, and research is still needed to understand the factors that influence air content (i.e. the factors that influence the production of large entrapped air bubbles as opposed to small entrained air bubbles). In a brochure published by the Association Technique de l’Industrie des Liants Hydrauliques (the French equivalent of the Portland Cement Association in North America), a solution to this problem is suggested. Since a decrease in the water/cement ratio tends to decrease the average size of the air voids, but has little influence on the spacing factor for a given dosage of the air-entraining agent (Backstrom et al., 1958b), it is simply suggested to increase the cement content of the mix. This should reduce the production of unwanted large bubbles and also increase strength (which is necessary to compensate for the reduction due to the increase in air content).

Recently, Saucier et al. (1991) have suggested the use of a ‘performance index’ to characterize the air-void system in concrete. This index incorporates the spacing factor and the air content in a single parameter. It is based on the assumption that spacing factors lower than approximately 200 µm do not significantly increase the frost resistance of concrete, and that analysis of field concretes often shows little frost damage in concretes with a spacing factor lower than about 300 µm (Fournier et al., 1987). It also takes into consideration the fact that air content per se does not influence the resistance to freezing and thawing cycles, but affects significantly the strength, which is an important concern for producers.

Isocurves that can be used to calculate the performance index are shown in Figure 6.18. Saucier et al. (1991) consider 80% as the lower limit for an acceptable air-void system. It corresponds to a spacing factor of 200 µm at an air content of 8% (a good air-entraining agent should yield such a spacing factor with no more than 8% air) and to a spacing factor of 300 µm at an air content of 2%. The value of 300 μm is the upper limit for frost resistance; it was verified in more than 1000 field and laboratory mixes that this value cannot be practically obtained with an air content lower than 2%. It can be seen in Figure 6.18 that a spacing factor of 200 µm at an air content of 5% (which Saucier et al. consider a very good ‘performance’) corresponds to a performance index of 100%. Values higher than 100% are of course always possible.

100 200 300 400 Spacing factor (^m)

Figure 6.18 Isocurves used in the computation of the performance index (after Saucier

et al., 1991).

The performance index could be very useful for concrete producers faced with difficult decisions when, for instance, unforeseeable variations in the air content occur.