The appearance of significant differences in density among individual fractions of the aggregate mix compels us to discuss volume relations in the aggregate mix and necessary adjustments to the binder content
Substantial differences in the densities of aggregates combined in an SMA can cause numerous problems. This situation happens rather seldom; however, it is possible to find very light material combined with very heavy aggregate (e. g., densities of approximately 2.400 and 3.100 Mg/m3, respectively). When designing a particular mix, results of the sieve analysis of constituent aggregates, as well as the overall gradation curve of a mix, indicate the gradation sieve distribution in mass units. In fact, mass and volume distributions do not correspond with each other if there are substantial differences in the densities of aggregates. Hence, guidelines have been created to regulate the allowable difference between densities. For example, a difference of approximately 0.2 Mg/m3 is allowed according to AASHTO M 325-08; if it is higher, the sieve distribution should be converted into volume units. Conducting the sieve analysis and determining the results in volume units have not always been practiced outside the United States
Besides problems with the aggregate mix, the use of aggregates with different densities brings about the necessity of correcting the binder content. For that purpose, correction coefficients have been used all over the world. Approximate, or “framework,” binder contents in SMAs have been detailed in various reference documents (e. g., standards, guidelines, recommendations) from many countries. The given minimum quantities of binder have been established based on a reference density of an aggregate mix.
For example, in the NAPA SMA guidelines QIS-122 the minimum content of binder in SMA amounts to 6.0% (m/m), but that is the value for a reference aggregate density equal to 2.750 Mg/m3. If the aggregate mixture density is different from the reference one, an adjustment should be made according to the following principles:
• 0.1% of the binder for each 0.05 Mg/m3 of difference between the density of an aggregate mix and the reference density (2.75) could be added or subtracted.
• For a density smaller than 2.75 Mg/m3, the correction bears the plus sign (+); for a density larger than 2.75 Mg/m3, it bears the minus sign (-).
Since 2006 the rules for correcting binder contents have been standardized in European member states of the CEN. The correction coefficient a for aggregate mixes with densities different than 2.650 Mg/m3 has been adopted in the European standard EN 13108-5 on SMA. The minimum content of binder stipulated by the requirements of this standard should be adjusted depending on a calculated as follows:
2.650 a =
Pa
where:
• a = The coefficient adjusting the binder content
• pa = The density of the aggregate mix determined according to EN 1097-6
For a thorough description of the requirements of the EN 13108-5 standard, see Chapter 14.
