Factores psicológicos que influyen en la decisión de compra

It is clear from the theories that were described in Chapter 2 that, as regards frost resistance, the most important parameter of the air-void system is the spacing of the air voids. (The method recommended by ASTM Standard C 457 (which is based on the work of Powers and others) to determine the various characteristics of the air-void system (mainly the total air content, the specific surface and the spacing of air voids) is explained in the next chapter.) The most obvious question that can be asked concerning frost resistance is ‘what value of the spacing factor is necessary to protect concrete against frost damage?’

Powers in his 1949 paper mentioned the value of 250 µm, and later, in 1954, published results from rapid freezing and thawing tests carried out in his laboratory that indicated that this value was applicable to a relatively wide range of concretes. Many engineers still consider this value to be correct and use it as a reference point.

The most often mentioned value, 200 µm (or 0.008 in!), comes from a paper by Backstrom

et al. published in 1958. This paper states (again on the basis of rapid freezing and thawing

tests) that the required value varies between 100 and 200 µm (0.004 to 0.008 in), depending on the type of concrete. It is, however, the value of 200 µm that was adopted afterwards by many regulating bodies and agencies and that is now the most widely recognized value.

In most countries where natural freezing and thawing cycles occur, the rate of cooling or freezing rarely exceeds 2 or 3 °C/h (Pigeon and Lachance, 1981; Cordon, 1966). Why then was the value of 200 µm obtained from rapid freezing and thawing cycle tests (at more than 8 °C/h) so rapidly accepted, since Powers’ work had clearly shown the importance of the freezing rate? The answer is most probably because that value is typical of correctly air-entrained concretes and it was then evident from all the data being published (field as well as laboratory) that air entrainment was a necessary protection against frost action.

The reasons behind this apparent contradiction between field experience (showing that a value of 200 µm is necessary to give concrete an adequate protection against ‘slow’ natural freezing and thawing cycles) and laboratory testing (showing that a value of 200 or 250 µm is necessary to protect concrete against rapid cycles) were studied recently by Pigeon and co-workers using what they termed the critical spacing factor. This concept is based on the assumption that for any given concrete subjected to any given freezing and thawing cycle test, there exists a critical value of the spacing factor beyond which concrete deteriorates rapidly when subjected to the test. Laboratory data has shown this assumption to be correct in a very large number of cases (Pigeon, 1989).

The critical spacing factor concept can be used to evaluate the relative frost durabilities of various concretes and also to compare various types of freezing and thawing cycle tests. For instance, if, for the same test, the critical spacing factor is lower for one type of concrete than for another, it means that this concrete has a lower frost durability since it requires more protection (closer air voids) for good durability. In the same way, if, for the same concrete, the critical spacing factor is lower for one type of test than for another, it means that this test is more severe since more protection is required for good durability.

The determination of a single value of the critical spacing factor represents a large amount of work because it requires the fabrication and testing of many concrete mixtures with the same basic characteristics but different air-void systems. Figure 3.9 shows an example of the results of the determination of a critical spacing factor (Pigeon et al., 1986). In this particular case, 17 similar concrete mixtures were fabricated with the same ordinary Portland cement, the same fine aggregate, the same limestone coarse aggregate, the same

E E

I

Length change greater than 6000 jam/m | for C values of 731, 656, and 1172 ^im

Lent * 500 ^imi

200 400 _ 600

Air-void spacing factor, L (|im) 800

Figure 3.9 Freezing and thawing test results showing the existence of a critical value of the air-void spacing factor for an ordinary Portalnd cement concrete with a water/cement ratio of 0.5 (the tests were performed according to ASTM C 666, procedure A, on 17 similar concretes with various air-void spacing factors; after Pigeon et al., 1986).

water/cement ratio and various dosages of an air-entraining agent. These concretes were all cured 14 days in water at 23 °C and then subjected to freezing and thawing cycles in water at an approximate freezing rate of 8 °C/h. The results of the freezing and thawing tests (i.e. the length change after 300 cycles) are given in the figure as a function of the air-void spacing factor (determined on samples of each of the hardened concretes). For all values of the spacing factor lower than about 500 µm, the length change after 300 cycles is extremely small and indicates that no significant cracking has occurred in the specimens. For higher values, length change is very significant and increases extremely rapidly with the spacing factor. The critical value of the spacing factor is thus approximately 500 µm (the precision of the determination of course increases with the number of concretes tested) for this particular concrete subjected to this freezing and thawing test.

Table 3.1 presents the results of the determination of the critical spacing factor for various types of concretes tested in similar conditions (the value of 500 µm from Figure 3.9 is given in this table). The influence of the various parameters involved (such as the type of cement and the value of the water/cement ratio) will only be discussed in Chapter 5, but the results in the table can serve to illustrate the use of the critical spacing factor concept. For instance, the critical spacing factors for the normal Portland cement concretes made with and without a superplasticizer (for the same water/cement ratio of 0.5) are approximately equal, which shows that superplasticizers per se have little effect on frost resistance. The results indicate, however, that at the same water/cement ratio of 0.5, the use of silica fume tends to decrease frost resistance since the critical spacing factor for the silica fume concretes is significantly lower than that for the normal Portland cement concretes. Table 3.1 Critical spacing factors (µm) obtained for various types of concretes subjected to 300 freezing and thawing cycles in water and in air (after Pigeon, 1989).

Water/binder ratio Freezing and thawing in water Freezing and thawing in air Type 10 Type 10+SF Type 30+SF Type 10+SF

0.50 500 250 – 400

0.50 (SP) 500 200 – 400

0.30 (SP) 400 300 >800 450

0.25 (SP) 750 – >800 –

Type 10: normal Portland cement.

Type 30: high early strength Portland cement. SF: silica fume.

SP: superplasticizer.

Pleau (1986) has reviewed in depth the technical literature on this question of the relationship between frost durability and spacing factor. In most cases where the values of the spacing factor were available and the characteristics of the concretes as well as of the freezing and thawing cycles clearly described, the results were found to be consistent with those in Table 3.1.

Pigeon et al. (1985) have used the critical spacing factor concept to analyse the influence of the freezing rate (Figure 3.10). Their results have confirmed what Powers and Litvan had stated earlier on the basis of theoretical considerations. They also tend to show that

concretes with values of the air-void spacing factor much higher than 200 or 250 µm can satisfactorily resist 300 cycles of freezing and thawing when the value of the freezing rate is close to the highest values observed under natural conditions (about 2 °C/h). This particular aspect of the problem of frost resistance will be specifically addressed in the chapter on exposure conditions. It should be stated right away, however, that 200 µm is nevertheless a good and safe design value for concretes exposed to severe weathering in northern climates, probably in good part because values of that order of magnitude are generally required for good resistance to scaling due to freezing in the presence of de-icer salts. It is also possible, though this has never been clearly demonstrated, that, as mentioned in Chapter 2, freezing cycles with long freezing periods are more harmful to concrete and that lower values of the spacing factor are thus required for satisfactory protection against such cycles. It is important to note that the values in Table 3.1 represent the spacing factors required to resist a particular test only and that other results would be obtained with other tests. These values should therefore never be considered or used as safe design values.

1000 £ 800 •c_o l-l I 600 CO »*-_O) C 9 ₄₀₀ CO O 200 • _{Test results} (water-cement ratio: 0.5) ■ Powers' data i • _{Test results} (water-cement ratio: 0.5) ■ Powers' data i • " ^ ^ - ■ 4 6 8 Rate of freezing (°C/h) 10 12

Figure 3.10 Variation of the critical air-void spacing factor value with the rate of freezing (during freezing and thawing cycle tests) (after Pigeon et al., 1985).

Apart from the known fact that correct air entrainment is needed in most cases for good de-icer salt scaling resistance (Verbeck and Klieger, 1957; Sommer, 1979), little information is available concerning the exact relationship between the spacing factor and the de-icer salt scaling resistance. The data that exist tend to show that there is no critical spacing factor for scaling resistance, and that surface deterioration increases gradually with an increase in the spacing factor (Bordeleau et al., 1992). The available information also indicates that, although 200 µm should normally be considered as a safe design value, the optimum value of the spacing factor for scaling resistance varies with the type of concrete. This value can be higher than 200 µm, for instance, for certain high performance concretes (Gagné et al., 1991).

The field data in Figure 3.11 illustrate the influence of the spacing factor on the scaling resistance. In this figure the surface deterioration of a number of sidewalks only a few years old is plotted against the air-void spacing factor measured in cores taken from the sidewalks. For low values of the air-void spacing factor, the performance can be good or bad (because factors other than air-void characteristics can of course affect the scaling resistance), but at high values, the performance seems always to be bad and there is certainly no clearly defined critical value.

2 © CO 2 1 ₃

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♦-*♦*—♦—•

Air-void spacing factor (\im) 1000

Figure 3.11 Relationship between the intensity of surface scaling and the value of the air-void spacing factor for a number of sidewalks from an urban area in the province of Québec.