The Justification of a Program to Beautify Waterworks Property.... 376 By Arthur H. Blanchard, Professor of Highway Engineering By Chester E. Rightor, Chief Accountant the Detroit Bureau of Government Research, Detroit, Michigan. By George C. Warren, Chairman of Executive Committee, Warren Brothers Company, Boston, Mass. Chapters XI to XX, Specifications Committees DEVELOPMENT IN THE THEORY AND PRACTICE OF THE DESIGN AND CONSTRUCTION OF BITUMINOUS PAVEMENTS By Hugh W. Skidmore, Chicago Paving Laboratory, Inc., 536 Lake Shore Drive, Chicago, Ill. The new method of designing mixtures for asphalt pavements that has rapidly developed during the past five years has as quickly passed the theory period as it has come into usage. It is based primarily upon one fundamental, that is, denseness. Density does not necessarily mean very much unless it is defined. A mixture may be dense and yet very unstable. Two other very important factors enter into the problem of designing a mixture capable of withstanding the distorting effect of traffic; these are the voidage of the mineral aggregate and the plasticity of the final mixture. Plasticity is a direct function of voidage in relation to bitumen content. The higher the percentage of voids in the aggregate, the greater the plasticity of the mixture provided sufficient bitumen is added to approximately fill those voids. It will readily be seen then that plasticity is likewise a direct function of the thickness of the film of bitumen coating the particles of aggregate. If, then, the problem of producing greater stability of mixture resided alone in the reduction of film thickness, stability could be controlled at will in any mixture by simply reducing or increasing the amount of asphalt. That this would be dangerous practice, however, is immediately apparent; because, unless reduction in cementing medium is accompanied by a reduction of voids in the aggregate the mixture is immediately weakened in structure, and likewise because of increased porosity is weakened in its natural resistance to the attack of moisHence, if structural soundness is to be preserved, the voids must be reduced before the bitumen is reduced. Strange as it may seem, the more both are reduced the stronger the mixture becomes, providing two things are also done. First, the bitumen must be maintained at such a percentage as will almost fill the voids (or will fill them at, say, 350° F.) and the mixture must be capable of compression to within, say, 98% of maximum density. The first consideration in the reduction of voids is the grading of the mineral aggregate, and the second is the filler to be added. Aggregates of high voidage will naturally require more filler to reach minimum voids than will be necessary with an aggregate of low voidage-likewise, lower ultimate voids in the "filler-aggregate" combination will result in the case of the low voidage initial aggregate. Reverting for a moment to the old standards of mixture design. Richardson's ideal heavy traffic sand grading for sheet asphalt was the well known: Passing 80-mesh... Passing 40-mesh Passing 10-mesh.. The voids in this combination vary only slightly according to the aggregate, say, between 30% and 32%. ture for heavy traffic was: Bitumen Passing 200-mesh. Passing 80-mesh. Passing 40-mesh. 34% 43% 23% 100% His ideal mix 10.5% .13.0% 26.0% 33.0% .17.5% Passing 10-mesh. The addition of this amount of filler reduced the voids from 30.7% in the raw sand to, say 25% in the combined aggregate (the exact voidage would depend upon the filler used, but 25% is approximately correct for minimum voids possible with ordinary filler). At 77° F. this mixture will require 11.6% bitumen to fill the voids. Under the requirements of modern practice such a mixture would be classified as quite plastic. |