Polished aggregates

Polished aggregates are aggregates that have lost their inherited surface micro-texture and angularity. 

The cause for aggregates to become polished is the polishing action by the traffic. The degree of aggregate polishing is related to the polish and abrasion resistance of the aggregates. 

The only effective treatment is to remove, with the use of a milling machine, the surfacing and to replace it with a surfacing material containing suitable crushed surface aggregate. 

Crushed aggregates derived from sedimentary rocks such as limestone and dolomite are unsuitable for surfacing. The aggregates to be used in surfacing are those mainly produced from a majority of igneous rocks, sandstones from sedimentary rocks and hornfels from metamorphic rocks. 

As surfacing material, the following may be considered asphalt concrete for very thin layers, porous asphalt, SMA and micro-surfacing. Details can be found in Chapter 5. Surface dressing technique may also be considered as an alternative (see Section 15.15). 

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BSE images reveal the different phases because of the atomic number contrast described earlier. Polished sections examined by BSE and characteristic X-rays can provide a wealth of information on length scales from millimetres to less than a micron. Nevertheless, the essential prerequisite is a well-polished sample. This is often challenging, particularly when the cementitious material contains aggregates (mortars and concretes). Aggregates are generally much harder than cement paste, so the paste component (which is the main subject of study) is frequently eroded between well-polished aggregates. 

Polishing. This last process step prepares the product for final formulation or for actual sale. It is designed to remove any aggregated protein, remove residual chromatographic eluent(s), and place the product into a specific solvent. These requirements are admirably served by gel filtration. At this point, the sample volume is small and the product fraction to be applied is fairly clean. The gel and column equipment requirements are now within reason and, the clean samples result in much longer gel life. 

As with any other classical approach in downstream processing of proteins, the process is split into three steps a capture step, followed by a separation step, and finally by a polishing step to remove residual impurities originating from leached compounds of the sorbent and/or fragments and aggregates.72... 

Finally, gel filtration is most often considered an appropriate polishing method when the target antibody is already pure and the only impurities are foreign protein traces or fragments or aggregates that must be eliminated. In this context, separation processes for antibody purification are logical orthogonal combinations of methods. 

Aggregate surface roughness is a measure of the surface texture of the particle and is based upon an arbitrary scale ranging between 1 for very rough and 4 for polished texture. 

There are many similarities between oxide CMP and poly-Si CMP. The main difference between the two processes is that poly-Si CMP slurries contain less abrasives and are, in general, more chemically active. Therefore, the poly-Si CMP process is by nature very sensitive to the polishing temperature. Temperature has a direct effect on removal rate, topography removal, and defect density (pitting and voids). Most poly-Si CMP slurries use colloidal silica that is less likely to form large aggregates than the fumed silica. 

An example of the use of Pourbaix diagrams and electrochemical potential measurements is the polish performance of copper in three slurries based upon NH3 compounds. These slurries consist of 2.5 wt% AI2O3 abrasive with an average aggregate size of 300 nm, DI water, and either NIf,OH, NH4NO3, or NH4CI. The total concentration of ammonia (as NH3, NH4 or NH4X) in each... 

If the aim is to describe precisely the degree of incorporation of one phase into another (for example, the volume percentage or aggregates of zeolite grains in an alumina matrix), it is preferable to work on a polished section. Since a fracture path is not random (it depends on the hardness, distribution and connectivity of the phases), the proportion of phases on a fracture is not always representative of the actual proportions in the material (the dispersed phase can be bypassed). The study of the polished section can be used to obtain a representative description of the distribution of the phases in the cross section. 

Gebalska J(1990) Platdet adherion and aggregation in relation to clinical course of acute myocardial infrrcticxi. M.D. thesis Warsaw, in Polish. 

In the polishing phase the focus is almost entirely on high resolution to achieve final purity. Most contaminants and impurities have already been removed except for trace impurities such as leachables, endotoxins, nucleic acids or viruses, closely related substances such as microheterogeneous structural variants of the product, and reagents or aggregates. To achieve resolution it may be necessary to sacrifice sample load or even recovery (by peak cutting). 

Typically, separations by charge, hydrophobicity or affinity will have already been used so that a high resolution gel filtration is ideal for polishing. The product is purified and transferred into the required buffer in one step and dimers or aggregates can often be removed, as shown in Figure 12. 

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