Void spacing factor

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

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... 

Air content of field mixes varies very little for mixing periods between 10 and 25 min and retempering with water to increase the slump from 50 to 100 mm has no significance on the air-void spacing factor, although it results in a small increase in the air content. 

Fig. 6.78 a Effect of air-void spacing factor on the durability of concrete. (According to [287]). b Relationship between the air-void spacing factor and w/c ratio of concrete the curve fixes the critical air-voids spacing factor. (After [333])... 

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

In the pharmaceutical industry, surface area is becoming more important in the characterization of materials during development, formulation, and manufacturing. The surface area of a solid material provides information about the void spaces on the surfaces of individual particles or aggregates of particles [5], This becomes important because factors such as chemical activity, adsorption, dissolution, and bioavailability of the drug may depend on the surface on the solid [3,5]. Handling properties of materials, such as flowability of a powder, can also be related to particle size and surface area [4],... 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

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]. 

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. 

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