Macrocells

The first term represents the contribution to the specific surface of microcells (R < 1000 A), the second one that of macrocells (R > 1000 A). 

Fig. 2). Cells with a radius of 100 A or less make up about 50% of all cells the entire population of microcells (the size of which is equal or less than 1000 A) is by at least three orders of magnitude higher than that of macrocells. Due to the very small size of microcells and their large number, the specific surface S of foams for the light specimen (y = 40 kg/m ) is as high as 100 m /g and for the heavy specimen (y = 500 kg/m ) S = 30 m /g.

In all photographs obtained by a scanning electron microscope Lowe and coworkers have observed interstices between macrocells, the cross section of which is 1.5-2 fim. A large number of interstices of approximately the same size have also been ob rved in the structure of phenolic foams Lowe et al. deduced the... 

Let us now examine the formation of aich microstructures. Lowe and co-workers ) consider that the formation of microcells is caused by a rupture of macrocell walls by formaldehyde vapors. Indeed, oligomer polycondensation of 2-methylolphenol in the presence of acidic catalyst may partially proceed via 2-hydroxydibenzyl ethers ... 

This idea has been confirmed by Lowe et aL l describing the dependence of water absorption of phenolic foams (7 = 35 kg/m ) on the fraction of closed cells this dependence recalculated for the fraction of open cells is shown in Fig. 9. It is noteworthy that an increase of d< from 10 to 98% corresponds to an increase of the maximum water absorption only from 6 to 8 g/lOOmL On the basis of these data, Lowe et al. concluded that surface chemistry is more important for foamed polymers than the closed cell content They explain the obtained data by the presence of microcells and interstices between macrocells (see Chap. 5.2). 

By varying the volume fraction of polymer, i.e. the apparent denaty, we may, in principle, change and even predetermine the type of macrocell packing and the content of microcells (see Fig. 2), that is to affect moisture and water absorption through macro- and micromorphological parameters of the foamed polymer. 

Additional evidence of the different states of sorbed water in micro- and macrocells is provided by the calculation of water viscosity (n) according to the model of relaxation of molecular diffusional motion ... 

It has been found that the viscosity of water in microcells (micropores) is by an order of magnitude higher than that of common free water and is about 10 cps in the temperature range from +10 to -6 °C (the effect of oxygen dissolved in free water which leads to a two-fold reduction of was taken into account). In macrocells at 0—10 °C, the water viscosity is 2 cps and at temperatures below -4 °C,about 4cps ... 

Fig. S. Relationship between the volumetric weight of polystyrene foam, y, and mean diameter d of macrocells (1) and (2) curves plotted according Eq. (20) for grades PSB and PSB-S respectively...

From the data (Fig. 20) of the author it follows that microcells are the most frequently occurring form of gas voids in foamed plastics bas on reactive oligomers. Another characteristic feature of microcells is that their absolute size is practically independent of the volumetric weight of the foam when the latter is varied from 40 to 500 kg/m, the mean size decreases only by a factor of 2 (from 1 to 0.5 microns). A third feature of microcells is their shape the bulk of microcells are spherical whereas most macrocells are oblong (see Chapter 7.3). 

The internal surface area of plastic foams may be measured not only by direct physico chemical methods but also indirectly by morphological parameters of the structure. Thus, if we assume that the solid phase of the plastic foam contains no microcells and that the macrocells are spherical, the value S. will be given by... 

These results suggest than there is no straightforward relation between V ff and y, Vjff fluctuating about a mean value of 1.26 cm /g. This means that (just as for microcells) the macrocell distribution pattern is independent of the volumetric weight of the foam and, judging from the asymmetrical form of the bar charts, is logaithmically-normal. 

In colloid chemistry all kinds of foams, both liquid and solid, are classified as lower coarse-disperse systems, i.e. such in which the minimum pore diameter is not less than 1 mm. The grouping of plastic foams into coarse disperse systems has until recently, been justified, since the minimum size of the cells that could be observed did not exceed this value. However, microcells have been discovered in polymer foam structures, this requires a revision of the old concepts regarding the dispersity. The real minimum size of microcells is a few hundredth of a micron and their number is well above that of macrocells. These results support the conclusion that plastic foams belong to the group of finely dispersed or colloid systems... 

A further example, which confirms the necessity of evaluating the resistivity of the medium very carefully, concerns the corrosion of rebars in reinforced concrete. In this caae the intensity of the current flowing between the anodic and cathodic zones of a macrocell depends on the resistivity of the concrete and the extent of the region involved. To determine the concrete resistivity various methods have been developed, which can be applied in the laboratory [14] as well as in the field [15]. It should be noted, however, that in the latter case most researchers have pursued the approach suggested by Wenner [16] for the evaluation of the resistivity of soils. The contribution of the ohmic drop to the electrode overvoltage cannot be neglected when the values of the corrosion rate of the rebars are appreciable, even if the current intensity is small within a given polarization potential interval, because under such conditions the interpretation of experimental results could be completely distorted. 

Electrical current flow by ion migration in concrete is important for electrochemical rehabilitation techniques such as chloride removal (Chapter 20), but also for (macrocell) corrosion processes (Chapter 8). 

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

The most frequent type of macrocell in reinforced-concrete structures exposed to the atmosphere is that established between more superficial rebars that have been depassivated by carbonation or chloride penetration, and internal passive rebars. Another example may be walls where chloride penetrates from one side and oxygen penetrates from the other side, which may occur in hoUow structures Hke tunnels and offshore platform legs or with ground retaining walls. 

Protection effect. MacroceU currents can have beneficial effects on rebars that are polarized cathodically. This is indirectly evident for patch repair of chloride-contaminated structures when only the concrete in the corroding areas is replaced with alkaline and chloride-free mortar, but surrounding concrete containing chlorides is not removed. Before the repair, the corroding rebars behave as an anode with respect to those in the surrounding areas, which are polarized cathodically and thus are protected by the macrocell. After the repair, formerly anodic zones no longer provide protection, and corrosion can initiate in the areas surrounding repaired zones (these have been called incipient anodes) [3]. Consequences for repair are discussed in Chapter 18. 

Other macrocell effects. A special case of macrocell effects has been observed on structures contaminated by chlorides where an activated titanium mesh anode was installed in order to apply cathodic protection when the cathodic protection system is installed but is not in operation, locahzed corrosion on steel can be slightly enhanced by the presence of the distributed anode [4]. 

The action of macrocells in structures buried in the soil or immersed in water is different from that of structures exposed to the atmosphere two circumstances promote macrocell effects while another reduces them. First, concrete is wetter than in aerated structures and its resistivity is lower, particularly in structures immersed in seawater. This reduces the ohmic drop in the concrete and increases the size of the effective cathodic area in relation to the anodic one. Secondly, the soil or the seawater around the concrete is an electrolyte of low resistivity, and the macro-cell current can also flow outside the concrete. This further reduces the ohmic resistance between the anodic area and passive reinforcement. Thirdly, there is, however, a mitigating aspect. Oxygen can only diffuse with great difficulty through wet concrete and thus it hardly reaches the surface of the embedded steel. Depletion of oxygen at the surface of the rebar that is observed in this case makes initiation of corrosion very difficult, and, even when corrosion initiates, the driving voltage for the macrocell is very low. 

Nevertheless, there are specific situations in which macrocells may form and promote localized attack. 

Differential aeration in buried structures. A clear example of macrocell action was documented in diaphragm walls in Berlin, illustrated in Figure 8.1 [6]. In this case, anodic areas had formed at the lower, non-aerated parts of the reinforcement at the ground side, while steel on the free side and higher up acted as cathode. Large amounts of corrosion products were found inside the concrete at various distances from the anodes and in the soil, suggesting that relatively soluble iron(II) oxides had formed that were able to move away from the anodes. Chlorides originated... 

Figure 8.1 Diagrammatic representation of the macrocell formed on a diaphragm wall [6]...

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Where oxygen access is low, it can be seen that the macrocell current tends to diminish in time because of oxygen depletion at the surface of the passive steel. The potential of passive steel consequently decreases in time until it reaches a value similar to that of corroding bars. 

Rebars not entirely embedded in concrete. Macrocell corrosion can occur when there are macroscopic defects in the concrete (cracks with large width, honeycombs, delaminations, etc.) or when there are metallic parts connected to the rebars that are only partially embedded in the concrete. This case is important for structures immersed in seawater or in aggressive soil. Besides being subjected to direct attack, those parts in direct contact with water or soil may also undergo more severe attack caused by the galvanic coupling with steel embedded in concrete. 

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