Almost all documents on SMA reviewed for the purposes of this book contain the compaction factor as a specified requirement. As in many other cases, the differences between European and U. S. specifications are clear.
That factor has been defined differently in different countries. The differences are grounded in the different reference density used related to the bulk density of the layer achieved on the construction site. That is the source of the sharp differences in numerical values: from 94% (the United States.) to 98% (Norway). The description of two basic definitions of the compaction factor follows later on.
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Mix design After construction End of service life |
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FIGURE 10.13 Change in the content of air voids in an SMA layer. (From Voskuilen, J. L.M., Proceedings of the 6th International Conference Durable and Safe Road Pavements, Kielce [Poland], 2000. With permission.) |
10.6.2.1 Compaction Factor as a Quotient of Bulk Densities
In European practice, compaction factors based on a quotient of bulk densities have been commonly used. That quantity is calculated in percent according to the formula
c = Ps — -100%
Psl
where
c = compaction factor, %
ps = the bulk density of a specimen cut out of a pavement, g/cm3
psl = the bulk density of a specimen prepared of the same constituents and com-
pacted in laboratory conditions (so-called reference specimen), g/cm3
The most common requirement for the SMA compaction factor calculated according to this equation is c is at least 97%, though the requirement of c being no less than 98% can also been found.
A sine qua non for the correct calculation is taking into account the following assumptions:
• The SMA mixture used for preparing reference specimens compacted in a laboratory should come from the mixture being placed (e. g., from a truck just leaving the asphalt plant).
• Compaction conditions for reference specimens, specifically the temperature and compaction, should be adequate to meet the real conditions prevailing on the construction site.
• The control specimens cut out of the pavement should not come from areas close to the edges or at the beginning or end of a working lot.
The German guidelines for compacting asphalt surfacing (M VA 2005) have included additional procedures and directions to facilitate control of the compaction process for difficult-to-compact mixtures. The compaction factor of an asphalt layer, denoted as k, is calculated according to the equation:
к =—P——— 100%
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P |
lab/
s (E=50)
where
p/ = the bulk density of a specimen cut out of a placed layer pslab(E=50) = the bulk density of a specimen prepared in a laboratory with the use of 50 strokes of a Marshall hammer on each side of a specimen
The bulk density of laboratory specimens is determined on material prepared with 2 x 50 strokes of a Marshall hammer. The compaction temperature should be appropriate for the applied binder. Due to various interpretation problems and mistakes in the calculation of the index k, the new German guidelines (M VA 2005) have introduced an additional parameter—the compaction index K referring to the bulk density achieved on the construction site relative to the maximum density
K = ^ • 100%
P”
where
K = compaction index, %
psb = the bulk density of a specimen cut out of an executed layer
p" = the maximum analytical density of a given mixture
Having substituted the maximum density, the values of the compaction index will be higher than 94%. The method of using the quotient of two bulk densities for determining the SMA compaction factor has been called out by the European standard EN 13108-20. Its clause C.3 stipulates that the reference density for indicating the compaction factor is the bulk density. The standard also provides that detailed conditions for preparing specimens and determining that density using EN 12697-5 and EN 12697-6 shall be declared.
