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Selecting the Best Pavement

qualities, values, quality, value, conditions and district

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SELECTING THE BEST PAVEMENT. The problem of se lecting the best pavement for any particular case is a local one, not only for each city but also for each of the various parts into which the city is imperceptibly divided, and it involves so many elements that the nicest balancing of the relative values for each kind of pavement is required to arrive at a correct conclusion.

In some localities, the proximity of one or more paving materials determines the character of the pavement, while in other cases it may require a careful investigation to select the most suitable material. Local conditions should always be considered, and hence it is not possible to lay down any fixed rule as to what material makes the best pavement; but a careful study of the requirements of the ideal pavement and of the qualities of the different kinds of pavements will promote an intelligent selection in any particular case. The decision must always be largely a matter of judgment; but the engineer should reach his conclusion by a series of carefully considered steps, and not by a single hap-hazard leap. He should weigh all the evidence and not base a decision upon a single item, es is too often the case; nor should he adopt the practice of some other locality without a careful consideration of the local re sources and of the needs of the place in which the pavement is to be laid, as is frequently done.

Relative Merits.

It is proposed to compare different kinds of pavements by assigning percentages to the different qualities of an ideal pavement, and then with this as a guide to assign numerical values to the various qualities of the several kinds of pavements.

The various qualities of a perfect pavement have been dis cussed in § 877 to § 896, and these qualities have been grouped in Table 61, page 583, under the three heads: (1) economic qualities, (2) sanitary qualities, and (3) acceptability. Oppo site each of these qualities in the first column of Table 61 is placed a number which is believed to represent the average rela tive importance of that particular quality on a scale of 100.

The assignment of these numbers is wholly a matter of judgment, and different individuals will differ greatly as to the relative values to be given to each quality; but the table is only to show a method whereby the good and the bad qualities of one kind of pavement may be balanced against those of another kind, and a conclusion may be reached step by step, -which represents the algebraic sum of the judgment on each item.

Different values should be assigned to the same quality according to the attendant conditions. If the street is in a manufacturing district and subject to heavy traffic, ease of trac tion should be assigned a comparatively high value, and noise a very low value. For an office district, quietness is the con trolling factor, and should therefore have a relatively high value. Similarly, for a residence district with its light driving, he althfulness and freedom from dirt and dust may be the most important element ; for a residence district where the property owners can not afford an expensive pavement, the first cost may determine the kind of pavement; and on a steep grade slipperiness may out-weigh all other conditions in determining the kind of pavement to be employed. The application of these principles is likely to be complicated by the personal interests of the residents or property holders, since opinions are likely to differ according to whether the point of view is that of a tenant, a resident property-holder or a non-resident prop erty-holder.

Each quality of a pavement will now be considered, and the degree of perfection of this quality possessed by each kind of pavement will be indicated by a numerical value.

Importance of First Cost.

Since the cost of a pave ment varies greatly with local conditions, it is not possible to state a general value for the cost of each kind; but for the sake of illustration, the values in the exhibit below will be assumed, which values are believed to be roughly approximate averages for the best of each kind of pavement.

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