The following is a general guide for adjusting the trial mix, but the 

 suggestions outlined may not necessarily apply in all cases. 



(a) Voids Low, Stability Low . Voids may be increased in a 

 number of ways. As a general approach to obtaining higher voids in the 

 mineral aggregate (and therefore providing sufficient void space for an 

 adequate amount of asphalt and air voids) the aggregate grading should be 

 adjusted by adding more coarse or more fine aggregate. 



If the asphalt content is higher than normal and the excess is not 

 required to replace that absorbed by the aggregate, the asphalt content may 

 be lowered to increase the voids. It must be remembered, however, that 

 lowering the asphalt content increases the void content and reduces the 

 film thickness, which decreases the durability of the pavement. Too great 

 a reduction in film thickness also may lead to brittleness, accelerated 

 oxidation, and increased permeability. If the above adjustments do not 

 produce a stable mix, the aggregate may have to be changed. It usually is 

 possible to improve the stability and increase the aggregate void content 

 of the mix by increasing the amount of crushed materials. With some 

 aggregates, however, the freshly fractured faces are as smooth as the 

 waterworn faces and an appreciable increase in stability is not possible. 

 This is generally true of quartz or similar rock types. 



(b) Voids Low, Stability Satisfactory . Low void content may 

 result in instability or flushing after the mix has been exposed to design 

 loads for a period of time because of reorientation of particles and addi- 

 tional compaction. It also may result in insufficient void space for the 

 amount of asphalt required for high durability, even though stability is 

 satisfactory. Degradation of the aggregate under the action of use may also 

 lead to instability and flushing if the void content of the mix is not suf- 

 ficient. For these reasons, mixes low in voids should be adjusted by one of 

 the methods given above, even though the stability appears satisfactory. 



(c) Voids Satisfactory, Stability Low . Low stability when 

 voids and aggregate grading are satisfactory may indicate some deficiencies 

 in the aggregate. Consideration should be given to improving the quality 

 as discussed above. 



-—(d) Voids High, Stability Satisfactory . High voids are fre- 

 quently, although not always, associated with high permeability. High 

 permeability by permitting circulation of air and water through the asphalt 

 cement may lead to premature hardening of the asphalt. Even though stabili- 

 ties are satisfactory, adjustments should be made to reduce the voids. This 

 usually may be accomplished by increasing the mineral dust content of the 

 mix. In some cases, however, it may be necessary to select or combine 

 aggregates to more closely approximate the gradation of a maximum density 

 grading curve . 



(e) Voids High, Stability Low . Two steps may be necessary 

 when the voids are high and the stability is low. First the voids are 

 adjusted by the methods discussed above. If this adjustment does not also 

 improve the stability, the second step should be an improvement of aggregate 

 quality as discussed above. 



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