Publications
Fine and Coarse Aggregate Absorption and
Specific
Gravity Determination Using
the AggPlus™ System
October 2, 2000
InstroTek, Inc.
The measured water absorption rate and specific gravity of aggregates is routinely used in design and construction of pavement materials and structures worldwide. The ability to measure the water absorption and specific gravity of aggregate materials with high degree of accuracy and repeatability in a short time is critical for engineers and practitioners interested in the properties of soils and aggregates.
Specifically, in the asphalt mix design industry, the bulk specific gravity and absorption of the aggregates, both fine and coarse, is crucial for quality asphalt design. Bulk specific gravity is used to determine the amount of asphalt binder absorbed by the aggregates and the percentage of voids in the mineral aggregates which are both important in design and quality control.
The current method, AASHTO T84 and ASTM C128, for fine aggregates have poor repeatability. The current method is based on the ability of the material to slump in a cone. This method requires a 24 hour saturation period for the aggregates. After full saturation, the material is progressively dried and checked in a small cone. The cone is removed and the amount of material slump indicates the saturation condition. The saturation condition of the aggregates is very operator dependent and extremely unscientific. Some agencies define the saturation condition as the weight of the material after it has slumped by a diameter of a dime from the top of the cone. This can be achieved by repetitive drying of the aggregates until the appropriate slump is achieved. If the aggregate is dried too much, the sample has to be saturated again and the drying process repeated.
Angular fine aggregates with high absorption and rough surface textures do not slump readily. Determining the saturated surface dry (SSD) weight for these samples is very difficult with the cone method specified in the current standards. Incorrect determination of this parameter can have damaging effects on the performance of the asphalt pavement.
The fundamental problem with fine aggregate SSD condition is the inability to define SSD status of the aggregate grain. Two or more particles can stack up or attach to each other not allowing the surface of each individual aggregate to reach SSD condition. This is a problem that has to be addressed with any device attempting to directly measure the SSD condition of fine aggregates.
The standards for coarse aggregates require the user to pat the aggregates with a towel and use this weight as the SSD weight of the sample. Again, this procedure is highly operator dependent. In this method if the material is not washed correctly, the towel can remove the dirt (fine) as well as water from the aggregates, indicating an inflated weight loss, hence a lower absorption rate than the true value. Furthermore, using a towel to dry the surface is a very subjective method as the operator is left to decide the degree of dryness of the aggregate.
The AggPlus method developed by InstroTek, involves two density measurements of fully dried samples. One sample vacuum evacuated and another sample at atmospheric pressure. The difference between these two densities will yield %absorption.
Two representative, completely dry samples (1000 grams each) of the aggregates is selected for this test. One sample of 1000 gram is vacuum sealed at approximately 29.5 in.Hg. The sample is placed in the water tank on top of the scale and cut open. The measurement of the dry weight and the saturated submerged weight of the aggregates will allow the operator to calculate a fully saturated density (apparent density), PV, of the aggregates. A second density, PU, is measured by taking the second dry 1000 gram sample and introducing water to a sample at atmospheric pressure until the sample is completely wet. This measurement is done to ensure all voids between aggregates is filled with water. The wet sample is placed in the water tank on top of the scale and the weight under water is determined. The knowledge of the submerged weight and the dry weight will allow for the calculation of the second density.
The difference between the first and second density is the measure of the absorption of the aggregates.
Knowing the absorption and the apparent density, PV, one can calculate the SSD condition, bulk specific gravity at SSD and bulk specific gravity dry basis of the aggregates from established equations. The following equations can be used for these calculations.
%absorption=100 * (B - A)/A (1)
Apparent Specific Gravity = Pv = Saturated Maximum Gravity = A / (A - C) (2)
Bulk Specific Gravity, SSD Basis = B / (B - C) (3)
Bulk Specific Gravity = Bsg = A / (B - C) (4)
where:
A= Mass of oven-dry sample in air, g
B= Mass of saturated surface-dry sample in air, g
C= Mass of saturated sample in water, g
This method assumes that the second density, PU, measurement under atmospheric pressure only fills the air voids between aggregates. However, in reality a small amount of water will be absorbed into the aggregates while the water is being introduced. The amount of water absorbed during the PU density measurement will depend on the absorption characteristics of the aggregates tested. For this reason a calibration is performed for each aggregate type to determine the correction to the final absorption calculation. This correction will only be performed once for each aggregate type and it will range from 0% to approximately 0.6% for materials ranging in total absorption from 0 to 4%.
An absorption correction relationship is established at the factory based on numerous different aggregates with varying absorption rates. This relation is established by exposing these aggregates to different vacuum levels and calculating the absorption rates. The absorption rate calculated for each aggregate will increase to the maximum absorption rate, as the vacuum level is increased. A relationship is established at each absorption rate and based on the calibration determination explained below, an absorption correction can be estimated for each aggregate type.
Calibration has to be performed once for each aggregate type. Two samples will be tested for density at atmospheric pressure and averaged. The remaining two samples will be tested at setting 1 of the AggPlus unit. The machine will create a reduced amount of vacuum in the sample. The sample will be opened under water and densities measured and averaged. Based on the relationships established at the factory, a correction will be calculated automatically by the AggSpec software and applied to the final %absorption calculation.
Once the percent absorption and apparent density are calculated, equation (1) and (2) can be rearranged to calculate B, and C, respectively.
B = (%absorption * A / 100) + A
C = A - (A / Apparent Density)
The values for B and C can now be used to calculate Bulk Specific Gravity at SSD and Bulk Specific Gravity dry basis can be calculated from equation (3) and (4).
The calculation for apparent specific gravity, percent absorption and bulk specific gravity is performed by the computer program AggSpec™ provided with the AggPlus software.
Advantages:
1. This method bypasses direct determination of the mass at SSD (B value) which is very difficult to define with fine aggregates.
2. The results are repeatable and are based on fundamental parameters that can be well defined.
3. The determinations are not empirical and calibration for absorption correction is based on each specific material used and not a general or an average relationship.
4. The time required to perform this test is in the order of approximately 10 minutes.
5. Twenty four hour saturation is not required.
6. This method can be used with coarse and fine aggregates and does not depend on aggregate angularity.
| Sample Type: Chattanooga Sand, InstroTek AggPlus Method | |||
| Sample | Apparent Density | %Absorption | Bulk Specific Gravity |
| 1I | 2.654 | 2.02 | 2.519 |
| 2I | 2.654 | 2.03 | 2.518 |
| 3I | 2.657 | 2.07 | 2.518 |
| 4I | 2.654 | 1.69 | 2.540 |
| 5I | 2.654 | 1.7 | 2.539 |
| 6I | 2.657 | 1.74 | 2.540 |
| 7I | 2.654 | 1.77 | 2.535 |
| 8I | 2.654 | 1.78 | 2.538 |
| 9I | 2.657 | 1.82 | 2.538 |
| Average | 2.655 | 1.85 | 2.531 |
| Std. Dev. | 0.002 | 0.151 | 0.010 |
| Sample Type: Chattanooga Sand, Performed by NCAT Cone method | |||
| Sample | Apparent Density | %Absorption | Bulk Specific Gravity |
| 1 | 2.670 | 2.44 | 2.506 |
| 2 | 2.667 | 2.06 | 2.529 |
| 3 | 2.632 | 1.96 | 2.503 |
| 4 | 2.677 | 2.15 | 2.532 |
| 5 | 2.653 | 1.61 | 2.545 |
| 6 | 2.684 | 2.04 | 2.545 |
| 7 | 2.656 | 2.06 | 2.518 |
| 8 | 2.648 | 1.59 | 2.535 |
| 9 | 2.652 | 1.75 | 2.534 |
| 10 | 2.675 | 1.67 | 2.561 |
| Average | 2.661 | 1.93 | 2.531 |
| Std. Dev. | 0.016 | 0.27 | 0.018 |
| Sample Type: Stockton Sand, InstroTek AggPlus Method | |||
| Sample | Apparent Density | %Absorption | Bulk Specific Gravity |
| 1I | 2.663 | 1.04 | 2.591 |
| 2I | 2.661 | 0.88 | 2.600 |
| 3I | 2.663 | 0.91 | 2.600 |
| 4I | 2.661 | 1.01 | 2.591 |
| 5I | 2.669 | 0.88 | 2.608 |
| 6I | 2.673 | 0.93 | 2.608 |
| Average | 2.665 | 0.94 | 2.600 |
| Std. Dev. | 0.005 | 0.068 | 0.008 |
| Sample Type: Stockton Sand, Performed by NCAT Cone Method | |||
| Sample | Apparent Density | %Absorption | Bulk Specific Gravity |
| 1 | 2.642 | 0.78 | 2.589 |
| 2 | 2.641 | 0.76 | 2.589 |
| 3 | 2.637 | 0.66 | 2.591 |
| 4 | 2.632 | 0.68 | 2.585 |
| 5 | 2.647 | 0.73 | 2.597 |
| 6 | 2.645 | 0.63 | 2.602 |
| 7 | 2.650 | 0.62 | 2.608 |
| 8 | 2.654 | 0.71 | 2.605 |
| Average | 2.644 | 0.70 | 2.596 |
| Std. Dev. | 0.007 | 0.060 | 0.008 |
| Sample Type: Crushed Glass, InstroTek AggPlus Method | |||
| Sample | Apparent Density | %Absorption | Bulk Specific Gravity |
| 1I | 2.514 | 0.04 | 2.511 |
| 2I | 2.510 | 0.06 | 2.507 |
| 3I | 2.514 | 0.11 | 2.507 |
| 4I | 2.510 | 0.00 | 2.510 |
| Average | 2.512 | 0.05 | 2.509 |
| Std. Dev. | 0.002 | 0.046 | 0.002 |