Laboratory Procedure for the Determination of Lime in Hot Mix Asphalt
Lime is added to hot-mix asphalt to mitigate stripping and other modes of moisture damage. This laboratory method to determine whether lime is in hot mix asphalt was developed at the Federal Highway Administration’s Turner-Fairbank Highway Research Center. It is purely an experimental procedure. It has some shortcomings, which are described in the paper “Determination of Lime in Hot Mix Asphalt,” Arnold Rozario-Ranasinghe and Youtcheff, Transportation Research Board (TRB), 2006.
The test method cannot be used to estimate the lime content, if the aggregate contains limestone since the limestone will be dissolved by the acid and yield a very high result. If limestone is present it is possible to run a background analysis on the raw aggregate, alternatively the less accurate FTIR method, which is also described in the TRB paper, could be used.
While the method is accurate at lime levels of 0.25 percent and 0.5 percent, it tends to underreport at the 1 percent level, typically it will show 0.8 percent lime to be present. Some rigorous testing needs to be done especially with commercially made hot mix samples and with cores taken from existing highways which are known to contain the correct amount of lime.
- The sample is obtained from the core by drilling and analyzing the drilling dust. Stand the core on a thick piece of wood and use a heavy duty hammer drill, (we used a Hilti), with a 3/8 inch carbide bit to drill the sample. A DIY handyman type drill will not work. Wear safety glasses and protective gloves. It is possible for the drill operator to steady the core on the floor with one foot, wear safety boots or have an assistant hold the core somehow. Depending upon the number of samples it may be worthwhile making a frame to hold the core. Drill to the depth of the drill flutes. This will yield about 15 grams of dust. Discard any lumps of the core that may break off. This sample can be analyzed on its own. If a more accurate estimate for the whole core is needed, (should be the same but it depends upon how well the lime was dispersed in the hot mix), then drill the core many times, collect the dust, which will be as much as 300-400 grams, and quarter it a couple of times until you have 10-15 gram samples for analysis. There is no need to analyze more than one sample since the quartering should remove any variation. Although for the first few tests, it might be a good idea to run several samples, to confirm the reproducibility of the method.
- All glassware should be cleaned and washed with distilled water. Contamination from tap water will affect the analysis since tap water contains calcium.
- Take the 15 or so grams of dust, weigh it accurately and place it in a 250ml conical flask. Add 100mls of 4 percent acetic acid and boil the mixture for 30-45 minutes. Filter the mixture through a Buchner filter funnel and wash the insoluble dust with distilled water. Transfer the filtrate to a 250ml volumetric flask. Rinse out the filter flask with a little distilled water and add the washings to the contents of the volumetric flask. Make up to 250mls with distilled water. Stopper the flask and shake it to mix the contents.
- Using an accurate pipette (we used an Eppendorf automatic pipette) to take a 1.0ml aliquot from the volumetric flask, transfer this to a 100ml volumetric flask and make up to 100mls with distilled water. Stopper the flask and shake it well to mix the contents.
- Determine the level of calcium ions present. Use Atomic Absorption Spectroscopy or Ion Exchange Chromatography. Either will give the same answer. Be sure to run calcium standards before and after the sample analysis. The calcium ion concentration in the sample should be in the range of 1-5 milligrams per liter. If it is way outside this range it indicates the possible presence of limestone aggregate. If using Atomic Absorption Spectroscopy do not add lanthanum salts. Theories exist that calcium is subject to interference with Atomic Absorption Spectroscopy and this can be overcome by adding lanthanum salts. We found that doing so gave an artificially high lime result. See the TRB paper for details.
- If the weight of drilling dust is X grams and the calcium content is Y mg/liter:
- Concentration of calcium is Y mg/liter. The dilution was original sample extracted and made up to 250mls, then 1 ml of this diluted to 100 mls. So the calcium content of the original solution is:
Y × 100 ÷ 4 milligrams per liter ( we only have a quarter liter of solution) = Y × 25
- Lime is Calcium Hydroxide Ca(OH)2. It has an approximate molecular weight of 40 + 2(16+1) = 74. So to convert milligrams per liter of calcium to milligrams per liter of lime, multiply by 74 ÷40 or 1.85. So the lime content in milligrams per liter is:
Y × 25 × 1.85 = Y × 46.25
- Since the lime is added to the mix as a percentage of the aggregate then we need to calculate how much aggregate is present in the sample. If the weight of the drilling dust is X grams and the mix contained B percent binder then the weight of the aggregate in the dust sample is X multiplied by ((100-B) ÷100) grams or X multiplied by 1000 × ((100 - B) ÷100) milligrams.
Putting all this together, the percent lime present in the hot mix is:
100 × (Y × 46.25) ÷ 1000 × ((100 - B) ÷100)
(Y × 46.25) ÷ 10X × ((100 - B) ÷100)
Any problems, or for more information please call.
Chemistry Laboratory Manager
Turner Fairbank Highway Research Center
8300 Georgetown Pike
McLean VA 22101
October 5, 2006