770 



SCIENCE 



[N. S. Vol. XLI. No. 1064 



culties exist even to-day in regard to fixing the 

 desirable qualities of many paving materials is a 

 certain indication that this subject has not re- 

 ceived the close study and systematic observation 

 that its importance merits; furthermore, while 

 there has been too great a tendency in some lines 

 to charge all failures to the materials used, or 

 some one of them, rather than to the methods of 

 construction, it is also certain that a considerable 

 proportion of failures in paving work can still be 

 traced to the use of materials, which, if not actu- 

 ally of poor quality, were unsuitable for the con- 

 ditions at hand. 



The desirable scope of plant inspection must 

 first be established before the actual duties and 

 details can be determined. The work may be con- 

 fined to the general inspection and sampling of 

 materials and mixtures, requiring nothing more than 

 that the contractor shall keep within the more or 

 less broad limits of the specifications, but allow- 

 ing hhn discretion and variations within these lim- 

 its. Going a step beyond this idea, the inspection 

 may be carried on as actual plant control, in 

 which the highway organization assumes the right 

 to specify narrower limits for a given piece of 

 work as to amount of bitumen, hardness of the 

 asphaltic cement, temperature of mixtures, and 

 even to some extent the exact details of the 

 method by which these mixtures are to be ob- 

 tained. The latter plan, that is, plant control, is 

 the logical one to follow on standard, if not 

 patented pavements as well, from the standpoint 

 that the organization which formulates the specifi- 

 cations should also be most capable of regulating 

 their application. 

 Specifications Covering the tolling of Boad Crusts 



of Various Types: Major W. W. Ceosbt. 



The assumption is made that the contract and 

 specifications are to be in the more usual form 

 under which the contractor is "to furnish all the 

 labor and materials and do all the work." 



Before proceeding to details, it seems necessary, 

 for the sake of clearness, to state certain general 

 principles in regard to specifications. 



In the first place, whUe it may be necessary 

 sometimes to restrict in details the methods to be 

 followed, generally it will be found more satisfac- 

 tory to specify the results to be obtained rather 

 than one exact method for reaching the result. 

 Elasticity for meeting variations in conditions en- 

 countered will then not be wanting. This is espe- 

 cially true as regards rolling. 



Secondly: Where necessary the methods of pro- 

 ducing the result may be limited by specific de- 



scription but this should be done only when un- 

 avoidable for the insurance of proper results and 

 for preventing the production of a result which 

 will be offered for acceptance as "just as good." 



Thirdly: For economic reasons as much elastic- 

 ity in the provisions for limits, in the descriptions 

 of the machinery or tools allowed for use, should 

 be given as is practicable. 



Fourthly: The specification of the result to be 

 secured should be absolutely definite, clear, and as 

 brief as may be consistent. The specification 

 should so describe the product that no more room 

 for argument as to the fulfillment of the specifica- 

 tion will exist than will be occupied by a few 

 questions whose answers can and must be deter- 

 mined by scientific methods, such as physical or 

 chemical analyses and arithmetical calculations or 

 measurements. 



The author cites the following specification cov- 

 ering the rolling of the second course of a ma- 

 cadam road as embodying the fundamental prin- 

 ciples cited. 



Second Course 



"After the metal for the second course shall 

 have been spread to the proper thickness and 

 cross-sections, it shall be rolled as hereinbefore 

 provided under the head of 'First Course,' ex- 

 cept that water, in connection with the rolling, 

 shall be used as follows: When the rolling shall 

 have been carried on to the point where the metal 

 of the second course will not push or 'weave' 

 ahead of the roller and any depressions or un- 

 evennesses have been properly remedied, as pro- 

 vided, the rolling shall be interrupted and a thin 

 layer of sand, screenings or other approved bind- 

 ing material, shall be evenly spread over the sur- 

 face of the second course metal with as little dis- 

 turbance of the latter as possible. The quantity 

 of fine material so applied shall be just sufficient 

 to cover the metal and care shall be exercised to 

 avoid its use in excess. Water shall then be 

 sprinkled on the roadway surface and the rolling 

 at the same time resumed, the quantity of water 

 used being such as will prevent the fine material 

 from sticking to the wheels of the roller. The 

 combined watering and rolling shall be continued 

 until the voids of the metal shall become so filled 

 with the fijier particles as to result in a wave of 

 water being pushed along the roadway surface 

 ahead of the roller wheel. The watering and roll- 

 ing shall then be discontinued until the macadam 

 shall have dried out. If then the metal shall be- 

 gin to loosen and to appear on the roadway sur- 

 face, or if the voids in the metal shall appear to 



