Published: Jan 1957
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The design of bituminous paving mixtures for heavy-duty service under today's traffic requires great emphasis on stability or resistance to lateral displacement under load. Maximum stability is obtained by adequate provision for all of the following factors: 1. Angular aggregates with rough or granular surface texture. 2. Properly graded aggregate with maximum size particles as large as is consistent with good finish and acceptable pavement surface texture. 3. Bitumen of as heavy a consistency as can be used with proper regard for limits imposed by cold weather brittleness. 4. Carefully controled bitumen content to provide not less than 2 or 3 per cent of unfilled voids in the fully compacted pavement. 5. Complete compaction during construction. If it can be done without discounting the importance of the other four, I should like to put a great deal of emphasis on the importance of the fifth factor, compaction. This does not mean that specific gravity as a subject is being abandoned. It is simply recognizing the fact that unless we concern ourselves ultimately with compaction and the closely related matter of voids in the compacted aggregate we might as well forget about specific gravity. Specific gravity is the important tool with which we determine the volume of the ingredients of the bituminous mixture and the size of the reservoir that must contain all the bitumen we put into the mixture. Basically, this reservoir is the total volume of the interstitial or inter-particle voids when the aggregate is compacted to refusal. Account must also be taken of the fact that if the aggregate particles are absorptive, the size of the reservoir is increased by the amount of asphalt the pores in the aggregate particles are capable of absorbing. After we have determined the specific gravities of the ingredients and made laboratory compaction tests to determine the refusal point of the combined aggregate we can proceed to the final step in design, determination of the bitumen content so that the capacity of the reservoir will not be exceeded by using too much bitumen. If the specific gravity we use in computing the capacity of the reservoir is wrong or we do not in some way take into account the absorptive properties of the aggregate the design bitumen content will be wrong and very probably the pavement will not perform satisfactorily.
Carpenter, C. A.
Assistant Chief, Bureau of Public Roads, Washington, D. C.,