Asphalt Mix Designer. Module 2 Physical Properties of Aggregate. Specification Year: July Release 4, July

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1 Specification Year: July 2005 Release 4, July

2 The first step in the development of an HMA mix design is to identify the materials that will be used in the pavement. In Florida the asphalt binder is established by the plans and is generally a PG But, the aggregates to be used will vary throughout the state. The process for accepting aggregates is defined in the FDOT State Materials Office Mineral Aggregate Manual prepared by the Aggregate Control Unit. This manual is included in the course materials for your reference. The Department also publishes a listing of the aggregate sources in the state and their gradations and the specific gravity of those materials. Release 4, July

3 The topics to be covered in this first block are specific gravities, inherent aggregate properties, processed aggregated properties, gradations. Release 4, July

4 The specific gravities of aggregates need to be determined separately for the fine and coarse fractions of either a stockpile or a blended material. The test procedures listed in this slide are included in your course materials. The FL DOT publishes a listing of the specific gravities for various aggregate sources throughout the state. This listing is provided with your course materials. Release 4, July

5 But, before we discuss the actual test procedures we need to define some terms. Release 4, July

6 Bulk Specific Gravity, G sb - the ratio of the mass in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air of equal density of an equal volume of gas-free distilled water at a stated temperature. In other words, the aggregate bulk specific gravity includes the volume of the water permeable voids in the aggregate (often termed the saturated surface dry or SSD volume of the aggregate. It is used for the calculation of the volume occupied by the aggregate in various mixtures containing aggregate, including portland cement concrete, bituminous concrete and other mixtures that are proportioned or analyzed on an absolute volume basis. Bulk specific gravity (dry) is usually used for computations when the aggregate is dry or assumed to be dry (such as in HMA mixtures). Release 4, July

7 The ratio of the weight in air of a unit volume of aggregate, including the weight of water within the voids filled to the extent achieved by submerging the aggregate in water for approximately 15 hours (but not including voids between particles) at a stated temperature, compared to the weight in air of an equal volume of gas-free distilled water at a stated temperature. Release 4, July

8 Apparent Specific Gravity, G sa - the ratio of the mass in air of a unit volume of an impermeable material at a stated temperature to the mass in air of equal density of an equal volume of gas-free distilled water at a stated temperature. In other words, the aggregate apparent specific gravity does not include the volume of the water permeable voids in the aggregate Release 4, July

9 Effective Specific Gravity, G se - the ratio of the mass in air of a unit volume of a permeable material (excluding voids permeable to asphalt) at a stated temperature to the mass in air of equal density of an equal volume of gasfree distilled water at a stated temperature. In other words, the effective specific gravity includes the volume of the water permeable voids in the aggregate that cannot be reached by the asphalt. We will discuss this in more detail when we discuss volumetric analysis. Release 4, July

10 The increase in the weight of the aggregate due to water in the pores of the material but not including water adhering to the outside surface of the particles, expressed as a percentage of the dry weight. The aggregate is considered dry when it has been maintained at a temperature of C for sufficient time to remove all uncombined water. Generally this is needed when doing concrete mix design. But, it can be used to estimate the amount of asphalt absorption for a mix. The ratio of asphalt to moisture absorption will vary with different aggregates and different asphalt binders. But, it will provide a mix designer with a general estimate of the amount of asphalt absorption. Release 4, July

11 This slide lists the general steps needed to determine the specific gravity of a coarse aggregates. Release 4, July

12 The apparatus consists of a balance, a sample container, a water tank, a suspended apparatus and a bowl for the initial soaking. Release 4, July

13 After the sample has been soaked for 15 to 19 hours the sample is to be removed from the water bath and rolled in a large absorbent cloth until all visible films of water are removed. Wipe the larger particles individually. A moving stream of air can be used to assist in the water removal. Take care to avoid evaporation of the water from the aggregate pores during the operation of the surface drying. After the SSD weight has been determined the sample is place in a sample container to determine its weight in water. Take care to remove all entrapped air before weighing by shaking the container while immersed. The last step is to dry the sample to a constant weight at at a temperature of C, cool in air at room temperature for 1 to 3 hr and weigh. Release 4, July

14 These are the equations for calculating the bulk specific gravity (G sb ), bulk specific gravity saturated surface dry (G s, SSD ), apparent specific gravity (G sa ), and water absorption capacity. The denominator in all of the specific gravity equations is the mass of water displaced by the sample. Since 1 gram of water equals 1 cubic centimeter, this value also represents the volume of water displaced by the sample. Release 4, July

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18 These are the steps necessary for determining the specific gravity of the fine aggregate. Release 4, July

19 The sample is to be spread in a pan, and exposed to a gently moving current of warm air, stirring frequently to secure homogeneous drying. Continue this operation until the test specimen approaches a free-flowing condition. The cone test for surface moisture is to be used to determine whether or not surface moisture is present on the constituent fine aggregate particles. Release 4, July

20 In the cone test the mold is held firmly on a smooth nonabsorbent surface with the large diameter down. Place a portion of the partially dried fine aggregate loosely in the mold by filling it to overflowing and heaping additional material above the top of the mold by holding the mold with cupped fingers of the hand holding the mold. Lightly tamp the aggregate into the mold with 25 light tamps of the tamper. Each drop should start about 0.2 inches above the top surface of the fine aggregate. After the aggregate has been tamped the mold is lifted vertically. If surface moisture is still present, the fine aggregate will retain the molded shape. When the fine aggregate slumps slightly it indicates that it has reached a saturated surface dry condition. Release 4, July

21 After the sample has reached the SSD state, 500 grams of SSD material is placed in a pycnometer. The pycnometer is filled with water to approximately 90 percent of its capacity. The pycnometer is rolled, inverted and agitated to eliminate the air bubbles (this normally takes 15 to 20 minutes). After this is done bring the water level in the pycnometer to its calibrated capacity. The total weight is determined (C). At the same time as the SSD sample is placed in the pycnometer a separate 500 gram sample should be put into a beaker and dried to a constant weight to determine the oven-dry weight (A). Release 4, July

22 These equations are similar to those used to determine the specific gravities of the coarse aggregate. Again, the denominator represents the volume of water displaced by the aggregate Release 4, July

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25 These are the specifications that govern coarse and fine aggregate. Release 4, July

26 The aggregate requirements for Superpave mixes are controlled by the traffic levels and the depth in the pavement structure for the roadway being built. Generally as the traffic level increases the requirement also increases. Release 4, July

27 This slide shows the distribution of the various traffic levels in Florida. Note that about 77 percent of the roads will fall in the B & C traffic categories with very few very high volume and very few low volume roads. Release 4, July

28 An ESAL is defined as one 18,000 pound wheel load on a dual axle. This definition was established in the late 1950s to be used as a tool for the design of both rigid and flexible pavements for the Interstate system. It is a standardized method for keeping track of trucks. Release 4, July

29 This is an example - a small truck will have a 0.49 ESAL and a fully loaded 18 wheeler with have an ESAL of Release 4, July

30 Source aggregate properties are those properties which are measured for the aggregate as-stockpiled and are commonly used for aggregate source acceptance control. These are properties that are inherent to a source and generally cannot be changed in the processing operations. The properties of interest to the production of HMA mixtures are toughness, soundness, deleterious materials and specific gravity. Release 4, July

31 This test subjects the coarse aggregate (in this case, retained on the 2.36 mm sieve) to impact and grinding by steel spheres. Each sphere has a mass between 390 and 445 g. The number of spheres introduced into the drum depends on the gradation of the aggregate to be tested. The number of spheres increases with increasing size of aggregate. Release 4, July

32 Once the aggregate and spheres are placed in the steel drum, the machine is rotated at between 30 and 33 rpm s for 500 revolutions. The aggregates are then removed from the drum and sieved to determine the degradation as a percent loss. The percent loss is the difference between the original mass at the required gradation and the final mass of the test sample) Release 4, July

33 Typical values. Release 4, July

34 The aggregate specifications limit maximum loss to 45%. Release 4, July

35 Weathering of aggregates is simulated by repeated immersion in saturated solutions of either sodium or magnesium sulfate followed by oven drying. The internal expansive force from the expansion of the rehydration of the soluble salts upon re-immersion simulates freeze-thaw damage. The difference between the original and final mass, expressed as a percent of the original mass is the percent loss. A weighted percentage is used when several fractions are tested. The soundness of both fine (passing the 4.75 mm sieve) and coarse aggregate can be determined using this test. Release 4, July

36 Weathering of aggregates is simulated by repeated immersion in saturated solutions of either sodium or magnesium sulfate followed by oven drying. The internal expansive force from the expansion of the rehydration of the soluble salts upon re-immersion simulates freeze-thaw damage. The difference between the original and final mass, expressed as a percent of the original mass is the percent loss. A weighted percentage is used when several fractions are tested. The soundness of both fine (passing the 4.75 mm sieve) and coarse aggregate can be determined using this test. Release 4, July

37 Minimal supplies are needed for this test. Either sodium or magnesium sulfate can be used by dissolving the appropriate amount in water. Aggregates are placed in a wire mesh basket and then submerged in the solution. Release 4, July

38 Damage to the aggregate after a number of wet-dry cycles can be seen by visual examination as well as in the change in gradation. Release 4, July

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42 This property ensures a high degree of aggregate internal friction and rutting resistance. It is defined as the percent by weight of aggregates larger than 4.75 mm with one or more fractured faces. The test procedure for measuring coarse aggregate angularity is ASTM D 5821, Standard Test Method for Determining the Percentage of Fractures Particles in Coarse Aggregate. The procedure involves manually counting particles to determine fractured faces. Release 4, July

43 The desire is to have material with crushed faces. Release 4, July

44 The specifications for coarse aggregate angularity. Note that as the traffic level increases the amount of quality requirement for the aggregate also increases. Release 4, July

45 This property ensures a high degree of fine aggregate internal friction and rutting resistance. It is defined as the percent air voids present in loosely compacted aggregates smaller than 2.36 mm. Higher void contents mean more fractured faces. The test procedure used to measure this property is AASHTO T 304 Uncompacted Void Content - Method A. In the test, a sample of fine aggregate is poured into a small calibrated cylinder by flowing through a standard funnel. By determining the weight of fine aggregate (W) in the filled cylinder of known volume (V), void content can be calculated as the difference between the cylinder volume and fine aggregate volume collected in the cylinder. The fine aggregate bulk specific gravity (G sb ) is used to compute fine aggregate volume. Release 4, July

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49 The specifications for coarse aggregate angularity. Note that as the traffic level increases the amount of quality requirement for the aggregate also increases Release 4, July

50 This characteristic is the percentage by weight of coarse aggregates that have a maximum to minimum dimension of greater than five. Elongated particles are undesirable because they have a tendency to break during construction and under traffic. The test procedure used is ASTM D 4791, Standard Test for Flat Particles, Elongated Particles, or Flat and Elongated Particles in Coarse Aggregate and it is performed on coarse aggregate larger than 4.75 mm. The procedure uses a proportional caliper device to measure the dimensional ratio of a representative sample of aggregate particles. The aggregate particle is first placed with its largest dimension between the swinging arm and fixed post at position A. The swinging arm then remains stationary while the aggregate is placed between the swinging arm and fixed post at position B. If the aggregate passes through this gap, then it is counted as a flat or elongated particle. The total flat, elongated, or flat and elongated particles are measured. Release 4, July

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54 Clay content is the percentage of clay material contained in the aggregate fraction that is finer than a 4.75 mm sieve. It is measured by AASHTO T 176, Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test. In this test, a sample of fine aggregate is placed in a graduated cylinder with a flocculating solution and agitated to loosen clayey fines present in and coating the aggregate. The flocculating solution forces the clayey material into suspension above the granular aggregate. After a period that allows sedimentation, the cylinder height of suspended clay and sedimented sand is measured. The sand equivalent value is computed as a ratio of the sand to clay height readings expressed as a percentage. Release 4, July

55 This shows a typical setup. Release 4, July

56 The measurement rod is used to mark the top of the suspended material and the top of the sand layer. Release 4, July

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58 What do they do: Release 4, July

59 Gradation is perhaps the most important property of an aggregate. It affects almost all the important factors of HMA, including stiffness, skid resistance, stability, durability, and moisture damage. The gradation of a material is determined through a sieve analysis and is probably the most common test used in materials testing. Release 4, July

60 An open graded mix is generally used for friction courses. The well-graded (or dense graded) mix is the standard mix used for construction of HMA pavements. The Gap Graded mix is generally used for an SMA mix. Release 4, July

61 Course Name and Level Module # - Module Name Previous mix design standards prohibited certain aggregate blends thought to be problematic. These prohibited aggregate blends were those where the plotted density line passed through a restricted zone. The restricted zone resides along the maximum density gradation between the intermediate size (4.75 mm for SP-19 or 2.36 mm for SP-12.5 and SP-9.5) and the 0.3 mm size. It forms a band through which gradations should not pass. Gradations that pass through the restricted zone have often been called humped gradations because of the characteristic hump in the grading curve that passes through the restricted zone. In most cases, a humped gradation indicates a mixture that possesses too much fine sand in relation to total sand. This gradation can result in tender mix behavior, which is manifested by a mixture that is difficult to compact during construction and offers reduced resistance to permanent deformation during its performance life. These gradations are considered to possess weak aggregate skeletons that depend too much on asphalt binder stiffness to achieve mixture shear strength. These mixtures are also very sensitive to asphalt content and can easily become plastic. While the current mix design standards no longer prohibit these aggregate blends, many mix design professionals agree that they should be avoided. Release #, Month YYYY 1, 2, etc -61

62 Course Name and Level Module # - Module Name Previous mix design standards prohibited certain aggregate blends thought to be problematic. These prohibited aggregate blends were those where the plotted density line passed through a restricted zone. The restricted zone resides along the maximum density gradation between the intermediate size (4.75 mm for SP-19 or 2.36 mm for SP-12.5 and SP-9.5) and the 0.3 mm size. It forms a band through which gradations should not pass. Gradations that pass through the restricted zone have often been called humped gradations because of the characteristic hump in the grading curve that passes through the restricted zone. In most cases, a humped gradation indicates a mixture that possesses too much fine sand in relation to total sand. This gradation can result in tender mix behavior, which is manifested by a mixture that is difficult to compact during construction and offers reduced resistance to permanent deformation during its performance life. These gradations are considered to possess weak aggregate skeletons that depend too much on asphalt binder stiffness to achieve mixture shear strength. These mixtures are also very sensitive to asphalt content and can easily become plastic. While the current mix design standards no longer prohibit these aggregate blends, many mix design professionals agree that they should be avoided. Release #, Month YYYY 1, 2, etc -62

63 Shown here is a more precise depiction of the restricted zone, along with the Design Aggregate Structures or Job Mix Formulas (JMFs) for three different aggregate blends. Note that each of the blends would be considered a coarse mix (falling below the 2.36 mm Primary Control Sieve). Each of the lines also falls within the control points and avoids the restricted zone, although blends 2 and 3 are barely within the control points (blend 2 barely makes the 28% minimum for the 2.36 mm sieve and blend 3 barely passes under the 90% maximum for the 9.5 mm sieve). Release 4, July

64 Effective July, 2007, coarse mixes may not be used for Traffic Levels A-C. Release 4, July

65 Shown here are Tables 3 and 4 from AASHTO 323, specifying Gradation Control Points and the PCS Control Points for gradation classification. Release 4, July

66 The term used to describe the cumulative distribution of aggregate particle sizes is the design aggregate structure or Job Mix Formula (JMF). A design aggregate structure or JMF that lies between the control points meets the requirements of Superpave with respect to gradation. Superpave defines five mixture types as defined by their nominal maximum aggregate size. Note that the line depicting the aggregate gradation falls within the plotted control points (it s an acceptable aggregate mix): and because it passes below the Primary Control Sieve Control Point, a mix with this aggregate blend would be considered as a coarse graded mix. Release 4, July

67 Superpave uses a standard set of ASTM sieves and these definitions with respect to aggregate size (Appendix B shows sieve sizes used by Superpave): size. Maximum Size: One sieve size larger than the nominal maximum Nominal Maximum Size: One sieve size larger than the first sieve to retain more than 10 percent. Release 4, July

68 The three Superpave gradations being used by the FDOT are shown on this slide. Release 4, July

69 Images of Superpave mix Sizes. Release 4, July

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