ASTM D2726 and ASTM D3549 - How to Find the Bulk Specific Gravity and Volumetric Thickness of Asphalt Cores
Significance and Use of ASTM D2726 and ASTM D3549
These two tests cover the process of getting the bulk specific gravity, density, and thickness of compacted asphalt core sections. The results of these tests will be used to find the unit weight of each compacted asphalt mixture, and can also be used with ASTM D3203 to determine the percent of air voids in an asphalt core. Those values will then be used to determine the relative degree of compaction of the asphalt. This helps the client know how effective their equipment is at compacting the asphalt, and whether or not the road is dense enough to pass the requirements of the specifications. If asphalt is not compacted enough in certain parts of the road, potholes can appear, so having well-compacted asphalt is important for keeping roads in good shape.
Both tests have been combined into one procedure to get the more accurate volumetric thickness, accounting for irregularities in shape from one side to the other, instead of measuring several sides and averaging those measurements. Since specific gravity has no units, it must be converted to density in order to do calculations that need units, so this procedure has you convert specific gravity to density using the unit weight of water at around 77 degrees Fahrenheit.
Equipment You Will Need for ASTM D2726 and ASTM 3549
- Scale – must be able to suspend the chain and cage from below, and display at least 4 significant figures. To create this suspended apparatus, you should have your scale on a shelf with a hole drilled in it that you can put the chain through, and put your water tank below it. See the picture below for an example setup.
- Chain and cage apparatus – the cage must be large enough to contain the asphalt core, and be able to be suspended in a stable manner. The chain can be hooked onto opposite sides of the cage to keep it steady.
- Water bath – must have an overflow outlet to maintain a constant water level in the tank. The water bath must also be able to keep a constant temperature of 77±1 degrees Fahrenheit. A heater for the tank is optional but helpful in cold temperatures.
- Drying oven- must be able to maintain a temperature of 230 ± 9 degrees Fahrenheit.
- Thermometer – must be readable to the nearest degree and be calibrated with a traceably certified thermometer.
ASTM D2726 and ASTM D3549 Procedure
Upon receiving the asphalt cores, the first thing you want to do is make sure that the person who brought the core in labeled the bag with the street name and station number where the core was drilled at. If this is not on there, you need to follow up with them and get the correct information, because if you have multiple cores you want to be able to tell them apart.
Take each core out of the bag and wash off as much drilling mud as you can. Put it back on top of the bag and let it air dry in front of a fan for a few hours until it is dry to the touch.
Once each core is dry, label it with a Sharpie. Write the street name and station number clearly on the side.
Measure the diameter of each core at the center, twice. The second measurement will be 90 degrees off from the first one. Write these measurements down, and then calculate the area of the circular surface of the core using the average of those two measurements as your diameter.
Average diameter, AD = (D1+D2)/2
Area of the core's circular side, E = Pi x (AD/2)^2
5. Set up your apparatus by hooking the chain onto the cage and hooking the chain onto the bottom of the scale, then suspend the cage and chain in your tank of 77 degree water. Make sure that the temperature of the water is 77 degrees before you continue. Zero out the scale.
6. Soak your asphalt core in water for 3 to 5 minutes, then put it in the cage and allow it to stabilize its weight reading for a few minutes. Record the weight of the immersed core.
7. Once you have the immersed weight for each of your specimens, remove the chain and the cage from the tank and put them away. Re-zero the scale, making sure that nothing connected to the scale is touching the water, or buoyancy will affect your next measurement.
8. Pull the core out of the water and quickly blot its surface dry with a towel, and then get the saturated surface-dry, or SSD, weight. You will need to do this within one minute.
9. Dry the specimen to an oven-dry state, by heating it to a temperature of 230 ± 9 degrees Fahrenheit. You will need to weigh a pan that is large enough to hold the core, and put the core on it. DO NOT put it in by itself or pieces of asphalt will go everywhere, and will stick to your oven. This will change the characteristics and shape of the sample, making it unsuitable for further testing, so make sure everything else that needs to be tested with the sample has already been completed accurately before you put it in the oven. The ASTM says (in paragraph 10.1.3, note 7) that drying the specimen at a reduced temperature in order to keep it intact does not meet the requirements of the test.
10. Allow the specimen to cool for at least 1 hour after taking it out of the oven, and then weigh it, subtracting the weight of the pan it is in. This is your oven dry weight.
11. Perform the calculations below, making sure you have the correct units. Turn your results in to your engineer, who will take the Rice number for that particular mix of asphalt and calculate the density of each core and make a report for the client.
A = Dry Weight
B = Saturated Surface Dry Weight
C = Immersed Weight
Bulk Specific Gravity, BSG = A/(B-C)
Density, D = BSG x 62.24
% Water Absorbed by Volume, W= ((B-A)/(B-C)) x 100
Volume in cubic inches, V = ((B-C)/62.24) x 1728
Volumetric Thickness = V/E
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