Spacing factor

Figure 28.7 Space factor for rectangular conductors (Courtesy The Copper Development Association) Table 28.3 Derating factors on account of higher ambient temperature or restricted temperature rise...

Isc = r.m.s. value of fault current in Amperes = 45 000 A K = space factor for rectangular conductors, determined from the curves of Figure 28.7, corresponding to... 

Three types of flare systems are commonly used the elevated flare, the ground flare, and the burning-pit flare. Although the three basic designs differ considerably in required capital and operating costs, selection is based primarily on pollution/public relations considerations such as smoke, luminosity, air pollution, noise and spacing factors. Table 1 summarizes the advantages and... 

A popular method of smoothing analytical measurement data is the least squares technique presented by Savitzky and Golay (1964) [see also Steinier, Termonia, and Deltour (1972)]. The technique is useful for data consisting of a single response as a function of a single factor with equally spaced factor levels. This type of data is... 

Establishing a design space (factor s responses spaces)... 

Water-cement ratio (by wt) Nature of mix Air content (%) Paste content (%) Voids in concrete (%) Specific surface area (mm) Void spacing factor (mm)... 

In air-entrained superplasticized concrete, the commonly accepted minimum value of the spacing factor of the air void system (0.2 mm) to provide adequate freeze-thaw protection is usually exceeded [68-71]. 

Despite the fact that the minimum spacing factor is exceeded, the concrete freeze-thaw resistance of the concrete does not appear to be adversely affected [71]. Table 2.7 clearly shows how the presence of an SNF superplasticizer increases the spacing factor of the air void system at each of the three water-cement levels evaluated [71]. 

Table 3.4 summarizes similar data, but also includes the sodium dodecylbenzene sulfonate type and the resultant effect on the specific surface area and computed spacing factor of the bubbles [14],... 

An increase in the water-cement ratio of cement pastes leads to greater air entrainment and a decrease in the specific surface area of bubbles. However, the spacing factor is relatively unchanged, as shown in Table 3.5 [14]. 

Table 3.5 The effect of water-cement ratio of cement pastes on the air content, specific surface area and computed spacing factor...

Table 3.23 The effect of spacing factor on freeze-thaw durability...

Air-entraining admixture % air by volume Average spacing factor (mm) Durability to freeze-thaw cycling ... 

Other parameters of the air-void system in the hardened concrete should also be determined to confirm that the air-void spacing factor is within the hmits specified in the standards. 

The use of superplasticizers in air-entrained concrete has caused much debate. Two main problems are associated with superplasticized air-entrained concrete (1) a decrease in air content by 1-3% when slump is increased from 75 mm to 220 mm after the addition of the superpiasticizer to create flowing concrete, and (2) a change in the air void system to less desirable values. However, most investigators [10-11, 12] have shown that, although the air-void spacing factor required for adequate frost resistance is altered, the change did not necessarily affect the freeze-thaw durability of... 

Water-soluble alkalis in Portland cement or fly ash can have a harmful effect on the characteristics of the air-void stmcture (AVS), particularly the stability of the spacing factor. Because of the more restrictive environmental... 

From the ready-mix producer s point of view, considerations about instability of the air-void system must be linked to a practical measurement of the air content. This raises the question as to how unacceptable spacing factor values can be detected from air content measurement. Since each combination of cement and AEA will produce its own values and the addition of SP will modify the relationship between the air content and air-void spacing, the only way to assess the stability of the AVS in superplasticized concrete is to carry out preliminary test for each cement-AEA-SP combination. It is probable that with several tests of a specific combination of cement-AEA-SP, a relationship between the air content and spacing factor could be estimated with a reasonable degree of precision. But any change of AEA type or brand of cement will make necessary a new evaluation of this relationship. 

Using 2-5% of the original dosage of AEA is sufficient to increase the air content to the original level and provide a suitable spacing factor [136],... 

Adding water to increase the slump by an average of 25 mm had little influence on the spacing factor. 

Air-content variations do not necessarily correspond to spacing-factor variations and air-content variations of the order of 1-2% should not be used to predict spacing-factor variations. 

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