The shape, angularity and surface texture of fine aggregates are typically inseparable factors in fine aggregate tests. Cleanliness is used to estimate the presence of clay or clay-sized particles in the fine aggregates. There are separate tests for evaluating the properties of material passing the 0.075 mm (No. 200) sieve.
The material passing a #4 sieve is considered Fine Aggregate This section will include a brief background on the history and development of fine aggregate tests for shape, angularity and texture. A limited discussion of one historical test (particle index) is also included for background. The most commonly used fine aggregate, the uncompacted voids test, is discussed in detail. Some information on selected new image analyses techniques are included since this is the direction in which aggregate testing is moving.
Historically, either direct measurements based on visual mean observations have been used to define the shape, angularity, and/or texture. Because these approaches are either subjective or time consuming, surrogates that use the ability of the fine aggregate to pack or to flow into a known volume are used.
Direct measurements of shape on fine aggregate require the use of microscope images. Several researchers have evaluated this approach in the past. Limitations included the time needed to hand calculate the required shape parameters from the images.
Other researchers used either flow or packing to represent shape. The more angular an aggregate is the lower the tendency to pack tightly either under free-flow or compactive efforts. These behaviors are also influenced by the surface texture of the aggregate. Rougher textures will also tend to inhibit packing, regardless of the shape.
Another indirect measurement of shape, texture and angularity is the ability of the fine aggregate to resist a shear load. This approach is one commonly used in geotechnical testing to determine the cohesion and angle of internal friction of soils.
One test that has been around for a number of years is the Particle Index. This test can be used to evaluate both coarse and fine aggregates. The only difference between testing these aggregate fractions is the diameter of the molds used in the testing. For fine aggregates, molds of 76, 51, and 38 mm (about 3, 2, and 1.5 inches). Molds of known volume are filled in three lifts with each lift tamped 10 times from a height of 50 mm. The volume of voids in the compacted sample is then calculated using the final height of the compacted sample. This process is repeated using 50 blows per lift. The Particle Index for a given aggregate fraction is computed using the equation shown in this slide. The results for a number of aggregate fractions are combined using a weighted average based on the percent of each size present in the whole gradation.
The uncompacted voids in fine aggregates test, also referred to as the fine aggregate angularity test, is the most commonly used method for evaluating fine aggregate shape and texture. This method represents a version of the flow tests originally used to define this aggregate property. AASHTO 304 in Missouri
There are three ways to evaluate fine aggregate using this test. The first uses a predefined gradation. This requires that the fine aggregate sample be sieved and then recombined in the amounts shown in this slide. This method is good for comparing one aggregate source to another but does not provide information about the fine aggregate as used in the mix. The second method (Method B), separates the fine aggregate stockpile into individual sizes and then tests each size individually. The results are mathematically combined using a weighted average. The third method (Method C) evaluates the fine aggregate stockpile as-received. This method can provide information about the voids in the fine aggregate as used in the mix. However, if different materials are tested and the gradations are widely different, then the results could reflect more of the gradation difference rather than the properties of the aggregates.
Previous research compared the results from Method A and Method B. Note that mathematically combining the results for each sieve size results in consistently higher values. Current fine aggregate angularity limits used in the HMA industry are set based on test results from Method A. This information was used to set the current limit of 45% to differentiate between more rounded or more angular fine aggregate shapes.
This slide presents an example of test results for various fine aggregate sources from around Alabama. The area to the south east represents fine aggregates with a fair to poor performance history. Note that the fine aggregate angularity indicate that aggregates in this area are among the highest in the state. This suggests that the reported performance problems are not related to the aggregate shape. The values in blue indicate the fine aggregate sources that did not meet the currently recommended value of 45% higher traffic volume highways.
Technological advances in digital imaging and software analysis are being used to automate or semi-automate the characterization of aggregate shape.
A number of researchers are trying a wide range of two and three dimensional imaging techniques for quantifying fine aggregate shape. One example is that currently being developed at the University of Arkansas. This method spreads the aggregate on a glass plate then uses a high resolution video camera to obtain the digital image. Digital imaging hardware and software is used to measure key aggregate shape properties. It this example, the University of Arkansas uses two parameters to characterize the fine aggregate shape: EAPP and Roughness Index. These parameters are discussed in the next two slides. There are a number of ways to obtain an image of fine aggregate. The key to characterizing shape factors is in the mathematics associated with refining and defining the image. Summarizing the various imaging methods and mathematical methods would be a good term paper research project for a graduate class. .
Minus 0.075 mm (No. 200) has always been considered to be a problem when it is present in sufficient quantities. The first test listed (washing) is the most commonly used method to determine the percentage by mass of the minus 0.075 mm material present in any given gradation. The sand equivalent is used to estimate the presence of clay or clay-sized particles in the fine aggregate. The plasticity index has been used for a number of years to limit the amount of clay materials in the fine aggregates.
A known mass of aggregate is placed in the washing bucket, the water is turned on and the bucket is rolled around on mechanical rollers. The flow of water is adjusted so that it flows over the nested 1.18 mm (No. 16) and 0.075 mm (No. 200) sieves. This process is continued until the water runs clear. The aggregate is then removed from the bucket and any aggregate remaining on the sieves is added to this aggregate. The sample is then place in an oven and dried to a constant weight. The amount of minus 0.075 mm (No. 200) is the difference in the original and after-washing mass. This is reported as a mass percent passing the 0.075 mm (No. 200) sieve after a mechanical sieve analysis is completed for the dried aggregate.
Three tests can be used to estimate the presence of clay or clay-sized particles in fine aggregates. These are the plasticity index (oldest), sand equivalent test (from the 1950’s), and methylene blue (relatively recent).
Plasticity of fines has been used for a number of years to control the clay content of fine aggregate.
In the Superpave mix design method, the sand equivalent is also referred to as measuring clay content. This test has both an ASTM and AASHTO designation.
The first step is to obtain a known volume of sample for testing. This is done by scooping, then leveling the fine aggregate into a tin container (usually a 3 oz. penetration tin). Care is taken so that the fine material is not lost. If necessary, the aggregate is dampened before scooping. The aggregate sample is then dried before testing as damp aggregate will generally result in lower values.
Step 4: Remove the stopper and shove the irrigation wand down into the aggregate sample so that the fines are flushed up and into the solution. Continue to add solution until the level is at 38 cm. Allow the cylinder to stand for 20 minutes.
Step 5: Record the height of the clay reading (top of suspended material). Then place the weighted foot (measurement rod) into cylinder. Read the height of the marker on the top of the measurement rod then subtract 25.4cm (10 inches) from the reading. This is the top of the sand layer.
The sand equivalent value is the ratio of the sand reading to the clay reading, expressed as a percent. If there are no clay particles present, then the sand equivalent value will be 100%. The lower the number, the more clay content.
While the HMA industry uses this method to estimate clay content, it is important to understand the results from the original research by Francis Hveem in the 1950’s. It was originally intended to be a rapid field test for evaluating the quality of stockpiles.
The flocculating solution was developed so that the presence of either bentonite or kaolinite clays would be highlighted.
Limits for this test were originally suggested for all construction materials.
While the test was designed to highlight the presence of clay, the type and amount of minus 0.075 mm (no. 200) material was also shown to influence the test results. This figure shows that while very small quantities of clay will drastically lower the sand equivalent values, various types of filler sized materials will also lower the test results. At 10 percent quartz dust no. 2, a sand equivalent value of only about 70% can be expected, even though there is no clay present.
The shape of the filler sized materials can also influence the test results. This figure shows that 10% natural dust can result in a sand equivalent value of near the current limit of 45% while it would take more that 15% of the crusher dust to reach this limit.
This figure compares the sand equivalent and the minus 0.075 mm (no. 200) percents for various types of fine aggregates. As expected, washed sands with a low percent of dust have a sand equivalent value of close to 100%. Crushed stones with a higher percentage of dust than crushed gravels have a lower sand equivalent value.
The International Slurry Seal Association recommended this French test as a way to evaluate the quantity of harmful smectite clays, organic materials, and ironhydroxides in the fine aggregates. This test has also been found to be useful for limiting problems with stripping in HMA.
The first step is to obtain a representative dry sample of material passing the 0.075 mm (no. 200) sieve. Ten grams of this material is dispersed in 30 grams of distilled water.
The next step is to prepare 200 ml of methylene blue solution. This is done by dissolving 1 gram of methylene blue in enough distilled water to produce this volume.
A drop of solution is removed before each addition and placed on a filter paper. Initially, a well-defined circle of methylene blue-stained dust is formed.
The end of the test is reached once a noticeable blue halo is observed in the clear water surrounding the dark blue center. The methylene blue value is calculated as the milligrams of methylene blue per gram of fine aggregate dust.
Research with Alabama fine aggregates indicated that historical problems with pavement performance related to the fine aggregates in the southeastern portion of the state can be traced to the presence of clay materials in the fine aggregate. Note that this figure shows fine aggregates with performance problems all have methylene blue values of greater than 10.
NCHRP 4-19 suggested the following guidelines for using methylene blue values to indicate anticipated performance due to the presence of clay or organic materials.
The size distribution of the minus 0.075 mm (no. 200) material as well as the clay content is also considered important. Historically, a hydrometer analysis is used to determine the size distribution of this fraction. However, new laser technology is being increasingly used as it is faster and appears to be at least as reliable as the hydrometer analysis.
This slide shows examples of how some states use this type of information in their specifications.
One type of laser particle size analyzer is manufactured by Coulter. The first step in using this equipment is to prepare the sample. The next step is to place the sample in the sample chamber.
The third step is to set up the equipment and start the test from the computer-controlled software (not shown).
Data is automatically collected and analyzed by the computer software.
The voids contents in the minus 0.075 mm (no. 200) material appears to be useful in characterizing the dust portion of aggregates when used in HMA asphalt. Two tests that evaluate the voids content are the Ridgen void test (original and modified by Pennsylvania State University) and the German filler test.
As with the fine aggregate shape and texture tests, the volume of voids in the dust are also a function of shape size, gradation and surface texture.
The Rigden voids are used to estimate the amount of free binder left to coat and bind the remaining aggregate after the voids in the dust have been filled (fixed binder). This slide presents the volumetric concepts that define these terms.
The second test used to evaluate the voids content in the dust is the German filler test. This test determines the amount of dust needed to completely absorb 15 grams of hydraulic oil.
Block 7 fine aggregates 13
Senior/Graduate HMA Course Fine Aggregates Shape, Angularity, and Surface Texture Cleanliness Properties of Minus 0.075 mm (#200)Aggregates Fine Aggregates 1
Fine Aggregate Shape, Angularity and Texture • General Concepts • Particle Index • Fine Aggregate Angularity • Uncompacted Voids • Image AnalysisAggregates Fine Aggregates 2
General Concepts • Direct measurements • Visual • Indirect measurements • Packing volume • FlowAggregates Fine Aggregates 3
Background • Direct Measurements • US Corps of Engineers • Flat and elongated particles in fine agg. • Microscope evaluation • length:width = 1:3 • Laughlin Method • Developed for PCC • Enlarged photographs • Radii of curvature of particles and inscribed circle • Roundness of particles then computedAggregates Fine Aggregates 4
Background (continued) • Indirect methods • Ishai and Tons Method • Relates flow test to geometric irregularities of particles • Size of orifice depends on agg. size • Specific rugosity by packing volume • Flow test used as direct measurement of packing specific gravity of one-sized particlesAggregates Fine Aggregates 5
Background (continued) • Indirect methods • Direct shear test • Direct shear box used to determine angle of internal friction under different normal stress conditionsAggregates Fine Aggregates 6
Particle Index ASTM D3398 • Vol. of voids between packed, uniform-size aggregate particles indicate combined effect of shape, angularity and surface texture • 76, 51, and 38 mm diameter molds • each of three layers tamped 50 mm above surface • 10 blows/layer • 50 blows/layer • Ia = 1.25 V10 - 0.25 V50 - 32.0 • Particle index increases with angularityAggregates Fine Aggregates 7
Fine Aggregate Angularity (ASTM C1252 or ASSHTO 304)• Void volume indicator of shape, surface textureAggregates Fine Aggregates 8
Fine Aggregate Angularity (ASTM C1252) • Uncompacted voids in fine aggregate • Method A (specific gradation) • 44 g of 2.36 - 1.18 mm • 57 g of 1.18 - 0.60 mm • 72 g of 0.60 - 0.30 mm • 17 g of 0.3 - 0.15 mm • Method B • Individual sieve sizes • Method C • As-receivedAggregates Fine Aggregates 9
Fine Aggregate Angularity (ASTM C1252) Examples of Test Results Method Type Agg. 95% Confidence Limits A Natural 39.5 - 45.5 Manufact. 42.8 - 53.4 B Natural 43.0 - 49.2 Manufact. 46.8 - 57.0Aggregates Fine Aggregates 10
Uncompacted Voids Results for Alabama Pit Run Sands 40.1 (Method A) 43.2 Unexpected state-wide 46.0 variation 46.1 Problems with aggregates in south east part of state 46.1 44.9May be due to other factors 43.6 that shape and angularity 46.4 44.1 47.2 46.6*Aggregates Fine Aggregates 11
Advanced Topics on Shape and Texture • Image Analysis • Microscopic EvaluationAggregates Fine Aggregates 12
Image Analysis • University of Arkansas • Agg spread on glass plate • High resolution video camera • Modern digital imaging hardware, analysis techniques and computer analysis used • Uses two parameters • EAPP • Roughness IndexAggregates Fine Aggregates 13
Plastic Fines in Fine Aggregate • Mineral Finer than 0.075 mm in Mineral Aggregate by Washing • Sand Equivalent • Plasticity IndexAggregates Fine Aggregates 14
Minus 0.075mm by Washing (ASTM C117)• Only measures quantity not quality of minus 0.075 mm (#200).Aggregates Fine Aggregates 15
Clay Content • Sand equivalent • Plasticity index • Methylene blueAggregates Fine Aggregates 16
Plasticity Index• Atterberg limits • Used to determine • Liquid limit • Plastic limit • Plasticity index • LL - PLAggregates Fine Aggregates 17
Plasticity Index • Non-plastic for highway construction • PI < 4 to 6Aggregates Fine Aggregates 18
Clay Content (ASTM D2419)• Percentage of clay in material finer than 4.75 mm sieve ASTM D2419 or AASHTO T 176 • Sand equivalent test methodAggregates Fine Aggregates 19
Clay Content (ASTM D2419)• Step 1: Obtain a known volume of fine aggregate; if the sample is not dry, dry it to a constant mass before testing• Step 2: • Prepare working solution • Add sample to cylinder • Use wand to add solution to cylinder• Step 3: Stopper the cylinder and agitateAggregates Fine Aggregates 20
Bottle of Solution on Shelf Above Top of Cylinder Hose and Irrigation Tube Measurement Rod• Step 4: Irrigate the sample to flush the fines intoAggregatessolution the Fine Aggregates 21
Clay Content (ASTM D2419) • Step 5: After 20 minutes, determine the height of the sand and suspended clay particles Marker on Measurement Rod Top of Suspended Material Top of Sand LayerAggregates Fine Aggregates 22
Clay Content - Background• Francis Hveem of Caltrans • 1952 • Rapid field test to evaluate the effective volume of clay • Measurements based on volume rather than weight (or mass)Aggregates Fine Aggregates 24
Development of Solution • Strength of flocculating solution selected so that 5% of bentonite would give same SE reading as 25% of kaolinite after 20 minutes • Not critical for natural soils • Working sol’n of 0.05N CaCl2 adopted • Small amount of glycerin for stabilizing • Formaldehyde to prevent mold formationAggregates Fine Aggregates 25
Suggested Limits (1952) • Bituminous mixtures • Original limit was 60 • Secondary limit of 50 proposed for slightly greater tolerance • Bases • Not less than 30 • PCC • Minimum of 80 to 85Aggregates Fine Aggregates 26
Effect of Dust on SE Values 100 Bentonite 80 Kaolinite Quartz Dust #1Sand Equivalent, % 60 Limestone Dust Quartz Dust #2 40 20 0 0 20 40 60 80 100 Percent Clay or Dust Mixed with Ottowa Sand, %Aggregates Fine Aggregates 27
P200 and Sand Equivalent SE% P200, % 100 80 Percent, % 60 40 20 0 Washed Crushed Crushed Pit Run Sands Gravels Stones SandsAggregates Fine Aggregates 29
Methylene Blue• ISSA recommended method • Quantifies amount • Harmful clays (smectite) • Organic matter • Iron hydroxidesAggregates Fine Aggregates 30
Methylene Blue• Step 1: 10 grams of Minus -0.075 dispersed in 30 grams distilled waterAggregates Fine Aggregates 31
Methylene Blue• Step 2: 1 gram methylene blue in distilled water and enough distilled water to make 200 ml of solution• Step 3: Titrated in 0.5 ml aliquotes from burette • Fines solution stirredAggregates Fine Aggregates 32
Methylene Blue• Step 4: After 1 minute of stirring, drop removed with glass rod and placed on filter• Step 5: End point is reached when a permanent light blue “halo” is observed in the clear ringAggregates Fine Aggregates 33
Methylene Blue• MB value is reported as the mg of methylene blue per gram of fine aggregate • Example: MB value = 5.3 mg/gAggregates Fine Aggregates 34
Methylene Blue Results 2.0for Alabama Pit Run Sands 7.3 1.1 High MB for pit run sands indicate presence of clay minerals 11.0 High MB are found in south 13.9 4.7* ease, where potential for accumulation of smectite 16.9 greatest 11.6 7.0* 18.4 11.7 Aggregates Fine Aggregates 35
Methylene Blue • General guidelines for methylene blue valuesMethylene Blue Expected HMA Performance mg/g 5-6 Excellent 10 – 12 Marginally Acceptable 16 – 18 Problems or possible failure 20+ Failure Aggregates Fine Aggregates 36
Properties of Minus 0.075 mm • Traditional • Size distribution by hydrometer • New • Laser evaluationAggregates Fine Aggregates 37
Hydrometer Analysis Add soil, shake Let stand and test periodicallyAggregates Fine Aggregates 38
Hydrometer Analysis• Examples of HMA specifications • Michigan • Not more than 60% nor less than 10% passing the 10 µm • Minnesota Particle Size % Finer 20 µm 35 - 100 5 µm 10 - 40 1 µm 1 - 25Aggregates Fine Aggregates 39
German Filler• Measures the amount of mineral filler needed to absorb 15 grams of hydraulic oil• Steps: • Combine 15 g oil and 45 g filler, mix • Form ball, if it holds shape, add 5 g more of filler • Repeat until mixture loses cohesion • At this point, all of oil is fixed in voids of No. 200 • Report amount of No. 200 added • Related to Rigden voidsAggregates Fine Aggregates 46