Properly hydrated

Addition of lime to Superpave shows benefits

April 28, 2015

In the mid-1990s the Louisiana Department of Transportation and Development (LADOTD) revised its hot-mix asphalt (HMA) mixture specifications.
 
Some of those major changes included requiring larger nominal maximum aggregate size in its aggregate structure and significantly reducing the amount of fine aggregates used in HMA mixtures. Later, in 2003, LADOTD implemented the Superpave mixture design method. Mixtures under the Superpave system are encouraged to use coarse gradations that pass below the maximum density line, which eventually led to a reduction in the amount of fine aggregate materials from the aggregate structure. This lower fine aggregate content in combination with relatively high in-place air voids and high voids in mineral aggregate resulted in Superpave mixtures with high permeability, less resistance to age hardening and poor rut resistance. The addition of quality filler materials can be a possible solution to rutting, fatigue cracking and moisture damage in asphalt concrete mixtures.
 
This study evaluated the influence of hydrated lime on the fundamental engineering properties of HMA mixtures produced with the LADOTD Superpave specifications. In addition, the effects of the method (i.e., slurry or dry) in which hydrated lime was introduced to the HMA mixture also were evaluated.
 
With and without lime
The primary objective of this study was to compare the fundamental engineering properties of HMA mixtures containing hydrated lime with the conventional mixtures designed to meet the Louisiana Superpave specifications. A secondary objective was to evaluate the influence of the method of the addition of hydrated lime on mechanical properties of the resulting HMA mixtures. The third objective was to compare the laboratory performance of hydrated-lime-treated mixtures containing a lower “high temperature PG graded” asphalt binder with the conventional mixtures containing a relatively higher “high temperature PG graded” asphalt binder.
 
Table 1 presents the experiment test factorial evaluated in this study. A Louisiana Superpave 19-mm Nominal Maximum Aggregate Size Level II (3-30 million ESALs) HMA mixture was designed and examined in this study. The test factorial included siliceous limestone aggregates and three asphalt binders, a neat PG 64-22 and two styrene-butadiene (SB) polymer-modified binders meeting Louisiana specifications for PG 70-22M and PG 76-22M. Based on the same mixture design, three conventional and six hydrated-lime-treated HMA mixtures were developed. The conventional mixtures contained no hydrated lime and the three aforementioned asphalt binders, respectively. Lime-treated mixtures were produced by incorporating hydrated lime into the HMA mixture in two ways: “slurry” or “paste” method when hydrated lime was mixed with the aggregate as slurry, and “dry” or “no-paste” method when dry hydrated lime was blended with the asphalt binders. For each lime-treatment method, three HMA mixtures were produced using the three identical asphalt cements (PG 64-22, PG 70-22M and PG 76-22M) utilized in the three conventional mixtures. The hydrated lime was incorporated at a rate of 1.5% of the total aggregate weight into the HMA mixture.
 
A suite of mechanistic material characterization tests were conducted to investigate the potential benefits of hydrated lime to improve the permanent deformation and fatigue performance for the aforementioned HMA mixtures (Table 1). An indirect tensile strength test (ITS), a loaded wheel tracking (LWT), simple performance tests (dynamic modulus and flow number), a semicircular bend test and a dissipated creep strain energy test were conducted. Triplicate samples were used for each test except for the LWT test where two specimens were tested. The dynamic modulus test was used to determine the permanent deformation and viscoelastic properties of the mixtures. Flow number tests were used to evaluate the permanent deformation of the mixtures. The indirect tensile strength test, semicircular bend test and dissipated creep strain energy test were used to examine the fracture properties of asphalt mixtures. Also the LWT test was used to evaluate the rut and moisture susceptibility of the mixtures. The test results were statistically analyzed.  
 
Nothing to sour on
The following conclusions were drawn from this project: 
  
In general, the addition of hydrated lime to mixtures containing PG 64-22 binder improved the rut resistance with slight improvement in fatigue endurance;   
 
The addition of hydrated lime to mixtures containing SB polymer-modified asphalt PG 70-22M and 76-22M binder improved the rut resistance. However, there was decrease in the measured fatigue properties. It is noted that despite this decrease, these mixtures did meet the minimum required values for fracture-resistant mixtures; and  
 
No substantial difference in mixture fatigue or rutting laboratory performance was observed when hydrated lime was added either in paste or no-paste method to mixtures containing PG 64-22 binder. However, the paste method resulted in mixtures that performed better in rutting than the no-paste method when PG 70-22M or PG 76-22M was used, whereas, the fatigue resistance for both methods were similar.  
 
The use of hydrated lime in HMA mixtures can reduce permanent deformation, improve fatigue endurance and reduce moisture sensitivity of asphalt concrete mixtures. In addition, hydrated lime increases the stiffness and fatigue resistance of mixtures. Therefore, based on the results of this study, specifications were developed and added to the LADOTD standard specifications for the HMA mixture and asphalt cement binder to allow the use of the hydrated lime in HMA mixtures. Hydrated-lime treatment has shown promise to improve the permanent deformation characteristics of HMA pavements. The use of hydrated lime in Louisiana’s Superpave mixes should provide for a longer life expectancy of the completed roadway structure. AT

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