(within the limits of the project specifications] and asphalt that yields a 

 mix having: 



(a) Sufficient asphalt to ensure durability; 



(b) sufficient mix stability to satisfy the demands of 

 designed use without distortion or displacement; 



(c) sufficient voids in the total compacted mix to allow 

 for a slight amount of additional compaction under loading without 

 flushing, bleeding, and loss of stability, yet low enough to keep 

 out harmful air and moisture; and 



(d) sufficient workability to permit efficient placement 

 of the mix without segregation. 



(.2) Evaluation and Adjustment of Mix Designs . Often, in the 

 process of developing a specific mix design, it is necessary to make 

 several trial mixes to find one that meets the criteria of the design 

 method used. Each trial mix design, therefore, serves as a guide for 

 evaluating and adjusting the trials that follow. For preliminary or 

 exploratory mix designs it is advisable to start with an aggregate of a 

 gradation that approaches the median of the specification limits. Initial 

 trial mixes for establishing the job-mix formula, however, must have an 

 aggregate gradation within the specification limits that the central mixing 

 plant is producing or is capable of producing. 



Where the initial trial mixes fail to meet the design criteria it will 

 be necessary to modify or, in some cases, redesign the mix. Adjustments in 

 the grading of the original aggregate blend will be required to correct the 

 deficiency. 



For many engineering materials, the strength of the material frequently 

 is thought of as denoting quality; however, this is not necessarily the 

 case for hot-mixed asphalt paving. Extremely high stability is often 

 obtained at the expense of lowered durability, and vice versa. Therefore, 

 in evaluating and adjusting mix designs always keep in mind that the 

 aggregate gradation and asphalt content in the final mix design must strike 

 a favorable balance between the stability and durability requirements for 

 the use intended. Moreover, the mix must be produced as a practical and 

 economical construction operation. 



Grading curves are helpful in making necessary adjustments in mix 

 designs. For example, curves determined from the Fuller equation, a 

 version of the maximum density equation using the power 0.5, represent 

 maximum density and minimum voids in mineral aggregate (VMA) conditions. 



The Fuller equation is: 



p = 100(d/D)* 5 



where p is the total percentage passing given sieve, d the size of sieve 

 opening, and D the largest size (sieve opening) in gradation. Mixtures 

 described by such curves tend to be workable and readily compacted. 



170 



