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Aggregates: Types, Properties and Gradation In this lecture; ---------------------
A- Source and types. B- Properties, Specifications and Tests.
C- Gradation. D – Blending (Combining). Information listed in this lecture is mainly taken from the Iraqi General Specifications for Roads and Bridge (SCRB, 2007), Standard Specifications for Engineering Materials and Methods of Sampling and Testing (AASHTO, 2013), Handbook of Highway Engineering (Fwa, 2006), http://www.pavementinteractive.org (Accessed on 2015) and Highways (O’Flaherty, 2007) and Aggregate properties (NCHRP, Report 539, 2005).
A- Source and Types In ASTM D8 (ASTM, 2003), aggregate is defined as “a granular material of mineral composition such as sand, gravel, shell, slag, or crushed stone, used with cementing medium to form mortars or concrete or alone as in base courses, railroad ballasts, etc.” These aggregates can be divided into three main categories — natural, processed, and synthetic (artificial) aggregates. Natural aggregates are mined from river or glacial deposits ()رواﺳ ب ﺟﻠﯾدﯾ ﺔ. Gravels and sands are examples of natural aggregates. According to the AASHTO Specs M146, stone is crushed or naturally angular particles of rock while gravel is rounded particles of rock; both should pass a 75-mm (3-in.) sieve and be retained on a 2.00mm (No. 10) sieve. In Iraq, the control sieve is No. 4 (4.75 mm) sieve.
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Sand is granular material resulting from the disintegration, grinding, or crushing of rock and that will pass the 2.00-mm (No. 10) sieve (No.4 in Iraq) and be retained on the 0.075 mm (No. 200) sieve. Silt-clay (minus 75-µm (No. 200) material)—fine soil particles that will pass the 0.075 mm (No. 200) sieve. Processed materials include gravel or stones that have been crushed, washed, screened, or otherwise treated to enhance the performance of the hot mix asphalt concrete (HMAC). Processed materials tend to be more angular and better graded. Synthetic (artificial) aggregates are not mined or quarried. Rather, they are manufactured through the application of physical and/or chemical processes as either a principal product or a by-product. They are often used to improve the skid resistance of HMAC. Blast furnace slag, lightweight expanded clay, shale are examples of artificial aggregates. In addition to the use of traditional aggregates mentioned above, there has been an increase in the use of waste products in HMAC. Scrapped tires and glass are the two most commonly used waste products that have been “disposed of ” in HMAC.
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Dr. Firas Asad
Highway Eng.
Aggregates of Road Pavements
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B- Properties, Tests and Specifications Mineral aggregates make up between 80% and 90% of the total volume or 94% to 95% of the mass of hot mix asphalt (HMA). For this reason, it is important to maximize the quality of the mineral aggregates to ensure the proper performance of our country’s roadways. The quality of mineral aggregates for road-paving materials has been specified by the toughness, soundness (durability), cleanliness, particle shape, angularity, surface texture, and absorption. This section will mainly review the key aggregate’s properties considered by the Superpave mix design method. The Superpave mix design method was recently designed -1993- to replace the Hveem and Marshall methods. The important aggregate’s physical properties, tests and specifications were determined by asking an expert group in order to determine the consensus and source aggregate properties. The consensus properties— including coarse and fine aggregate angularity, flat and elongated particles, and clay content— were chosen to ensure that the aggregate quality was sufficient to provide satisfactory HMA performance for the design traffic level. In contrast, the source properties - including toughness (Los Angeles abrasion), soundness, and deleterious materials—were to be set by the agency. This was done to allow for variances in locally available materials. In addition to the Superpave consensus and source properties, there are other properties that may influence the performance of aggregates in HMA — particle index, plasticity index (PI), affinity ( )أﻻﻟﻔـــﺔfor asphalt, and absorption. 1- Consensus (required)
• CAA; FAA;Flat & Elongated and Clay content
2- Source (optional)
• Toughness; Soundness, and Deleterious materials • Particle index; PI; Ashpalt affinity and Absorbtion
3- Other properties
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1- Consensus aggregate properties (required) a) Coarse Aggregate Angularity (CAA) CAA is defined as the percent by weight of aggregates retained on the No. 4 (4.75 mm) sieve with one or more fractured face. This property is determined using ASTM D5821 “Standard Test Method for Determining the Percentage of Fractured Particles in Coarse Aggregate”. In this procedure, individual aggregates are manually examined for the presence of fractured faces. Once the aggregates are sorted by the number of fractured faces, their percentages of aggregate with at least one or two fractured faces are calculated. A fractured face is defined as any angular, rough, or broken surface of an aggregate particle that occupies more than 25% of the outline of the aggregate particle visible in that orientation. The percent of fractured particles in coarse aggregate can provide an indication of inter-particle shear friction or stability.
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b) Fine Aggregate Angularity (FAA) FAA is defined as the percent of air voids present in a loose uncompacted aggregate sample that passes the No. 8 (2.36 mm) sieve. This property is determined using AASHTO T304 “Standard Method of Test for Uncompacted Void Content of Fine Aggregate — Method A”. In this procedure, a nominal 100 cm3 calibrated cylinder measure is filled with fine aggregate through a funnel placed at a fixed height above the measure. When the measure is overfilled, it is struck off ( )ازاﻟ ﺔ اﻟزاﺋ دand the mass of aggregate inside the measure is determined by weighing.
The uncompacted void content is the difference between the volume of the cylindrical measure and the absolute volume of the fine aggregate (calculated from its mass and bulk dry specific gravity). For fine aggregate with a given gradation, the higher the void content, the higher the assumed angularity and rougher the surface. High angularity results in high internal friction of aggregate and hence high shear strength (τ = c + б tan ф). Generally, angular aggregates are harder to compact due to the aggregate-interlock (low workability), which gives the mix greater shear strength. These mixes tend to be more stable and resistant to rutting. Lecture 13
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c) Flat and Elongated Particles Flat or elongated particles are defined as aggregate having a ratio of width to thickness or length to width greater than a specified value. This property is determined using ASTM D4791 “Standard Practice for Flat Particles, Elongated Particles, or Flat and Elongated Particles in Coarse Aggregate”. This test is conducted on aggregates retained on the No. 4 (4.75 mm) sieve. In this procedure, a proportional calliper ( )اداة ﻗﯾ ﺎس ﻛﺎﻟﻔﯾرﻧﯾ ﺔis used to measure the dimensional ratio of a representative sample of coarse aggregate. Aggregates exceeding the 5 to 1 ratio are considered flat and elongated in the Superpave mix design system. The percent of flat or elongated aggregates is reported as a percentage of total aggregates tested.
Aggregates used in HMA mixes should be cubicle rather than disproportionate in their dimensions. Aggregates particles that are significantly longer in one dimension than in the other one or two dimensions have a propensity to break during the construction process or under traffic loading.
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Aggregates of Road Pavements
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d) Clay content Clay content is defined as the percentage of clay material contained in the aggregate fraction that passes the No. 4 (4.75 mm) sieve. This property is determined using AASHTO T176 “Standard Method of Test for Plastic Fines in Graded Aggregates and Soils by Use of the Sand Equivalent Test”. In this procedure, a sample of fine aggregate is placed in a graduated cylinder with a flocculating solution. The cylinder is then agitated to loosen the clayey fines within and surrounding the aggregate particles. After allowing the constituents اﻟﻣﻛوﻧ ﺎتto settle for a specific length of time, the height of suspended clay and sedimented aggregate is measured. The sand equivalent value is the ratio of the sand reading to the clay reading as a percentage. A low sand equivalent value, or high clay content, means that there is “dirt” on the surface of the aggregates. This “dirt” can reduce the bond between the aggregate and asphalt cement. Consequently, the mix would have a greater tendency for stripping. Cleaner aggregate with higher sand equivalent values will enhance the performance of HMA. According to SCRB (2009) sand equivalent value should not be less than 45%
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2- Source aggregate properties (optional) a) Toughness Toughness is the percent loss of material from an aggregate blend during the Los Angeles Abrasion ( اﻟﺗﺄﻛ ل/ )اﻟﺑ ريtest. This property is determined using AASHTO T96 (ASTM C131) “Standard Method of Test for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine”. The Los Angeles (L.A.) abrasion test is a common test method used to indicate aggregate toughness (impact resistance) and abrasion characteristics (hardness). These characteristics are important because the aggregate in HMA must resist crushing اﻟﺳ ﺣق, degradation ﺗﺗﻔﺗ تand disintegration ﺗﺗﺻ دعit might face during stockpiling, producing (in plant), placing, and compacting. Aggregates not adequately resistant to abrasion and polishing may cause premature structural failure and/or a loss of skid resistance.
In this procedure, aggregate is degraded (broken apart to smaller sizes) through abrasion, impact, and grinding ( )اﻟﻄﺤ ﻦin a rotating steel drum ( )ﺑﺮﻣﯿ ﻞcontaining steel spheres. The LA abrasion loss is the difference between the original and final mass of the sample after washing off the No. 12 (1.70 mm) screen. Typical test values range from 10% for extremely hard rocks (e.g. basalt) to 60% for soft rocks (e.g. limestone). Lecture 13
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b) Soundness The soundness test determines an aggregate’s resistance (durability) to disintegration by weathering and, in particular, freeze-thaw cycles. Aggregates that are durable are less likely to degrade in the field and cause premature HMA pavement distress and potentially, failure. Soundness is the percent loss of material from an aggregate blend during the sodium sulfate Na2SO4 or magnesium sulfate MgSO4 soundness test. This property is determined using AASHTO T104 “Standard Method of Test for Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate”. In this procedure, aggregate samples are put through repeated cycles of immersion in saturated solutions of sodium or magnesium sulfate followed by oven drying. The percent loss of material is determined by taking the difference between the original and final masses expressed as a percentage of the original mass. This test evaluates the aggregate’s ability to resist breaking down or disintegrating due to weathering (i.e., wetting and drying and/or freezing and thawing). ASTM D692 “Standard Specification for Coarse Aggregate for Bituminous Paving Mixtures” specifies a 5cycle weighted loss of not more than 12% when sodium sulfate is used or 18% when magnesium sulfate is used.
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c) Deleterious Materials The percentage of deleterious اﻟﺿ ﺎرةmaterials in blended aggregate is determined using AASHTO T112 (ASTM C142) “Standard Method of Test for Clay Lumps ﺗﻛ ﺗﻼت and Friable Particles in Aggregate”. In this procedure, aggregates are individually subjected to finger pressure (while soaking) to determine materials that are friable or clay lumps. The percent of clay lumps and friable particles is determined by taking the difference between the original and final mass retained on a No. 200 (0.075 mm) sieve, after wet sieving, expressed as a percentage of the original mass. The percent of deleterious materials can range from 0.2 to 10%. ========================= 3- Other properties a) Particle Index (Shape and Texture) Particle index is an overall measure of aggregate particle shape and texture. This property is determined using ASTM D3398 “Standard Test Method for Index of Aggregate Particle Shape and Texture”. In this test method, the percent voids in the aggregate compacted in two stages according to a specified procedure is used to calculate the particle index (Ia) value. Typically, rounded particles with smooth surface textures may have a particle index of 6 or 7 while a highly-angular crushed particle with rough surface textures can have particle indices of 15 to 20 or higher. Surface texture, similar to particle shape, influences the workability and strength of HMA. Aggregates with rough textures, such as crushed limestone or gravel, tend to form stronger bonds with asphalt cement and increases the strength and asphalt cement demand of a mix. On the other hand, aggregates with smooth textures, such as river gravels and sands, tend to form weaker bonds with asphalt cement which
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leads to reduced strength and decreased asphalt cement demand. However, smooth aggregate surface textures may provide more workability. b) Plasticity Index The Plasticity Index (PI) is a measure of the degree of plasticity of fines (material passing the No. 200 sieve). It can provide an indication of the amount and type of fines. This property is determined using ASTM D4318 “Standard Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils”. The PI is defined as the difference between the liquid limit (LL) and plastic limit (PL). c) Affinity for Asphalt An aggregate’s affinity for asphalt cement is its propensity اﻟﻣﯾ لto attract and remain attached to asphalt cement. Asphalt cement must coat the aggregate, stick to the aggregate, and resist stripping of the asphalt film in the presence of water. It is important to recognize that some aggregates appear to have a greater affinity for water than for asphalt cement. These hydrophilic (water-loving) aggregates have a tendency to get stripped (asphalt film gets detached from the aggregate) with exposure to water. On the other hand, hydrophobic (water-hating) aggregates have a greater affinity for asphalt cement. Limestone and dolomite are examples of hydrophobic aggregates. d) Absorption Absorption is a measure of an aggregate’s porosity. While porosity is generally associated with the absorption of water, a porous aggregate also tends to absorb asphalt cement. Porous aggregates have a greater asphalt cement demand and require additional asphalt cement for a comparable mix. Therefore, highly porous aggregates are generally not used for HMA unless the aggregates possess certain desirable qualities that outweigh the cost of additional asphalt cement. Lecture 13
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Highway Eng.
Aggregates of Road Pavements
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C – Aggregate’s gradation According to Iraqi roads’ specifications, Coarse Aggregate is that portion of the combined aggregate retained on the 4.75 mm (No. 4) sieve used for asphalt concrete. Fine Aggregate is that portion of the combined aggregate passing the 4.75 mm (No. 4) sieve shall consist of stone screenings (dust) and natural sand. Natural sand shall not exceed 25% of the portion fines than 2.36 mm for surface and binder courses. Mineral Filler shall consist of limestone or other stone dust, portland cement, hydrated lime or other non-plastic mineral matter from approved sources. It shall conform to the grading requirements shown in Table R9/1. The plasticity index as determined by ASSHTO T90 shall not be greater than 4. The amount of filler to be added shall be only that amount which is necessary to the grading requirements for the completed mixture prescribed.
Aggregate gradation is the distribution of particle size expressed as a percentage of the total sample weight. This property is determined using AASHTO T27 (ASTM C136) “Standard Method of Test for Sieve Analysis of Fine and Coarse Aggregate”. In this test, aggregate passed through sieves with progressively smaller openings. The mass of aggregate retained is then used to determine the percent of aggregate retained on and/or passing each sieve. Gradations can be represented graphically Lecture 13
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using “percent passing” (by weight) as the ordinate and particle size as the abscissa. Fuller and Thompson (1907) developed one of the best-known grading charts in the early 20th century. The equation for Fuller’s maximum density curve is as follows: P = (d/D)0.45 * 100 where d is the diameter of the sieve in question, P total percent passing or finer than the sieve, and D is the maximum size of the aggregate. Maximum size is the smallest sieve through which 100 percent of the aggregate sample particles pass. For example, for surface course; P(#4) = (4.75/19)0.45 * 100 = 53.58 % for surface course; P(#200) = (0.075/19)0.45 * 100 = 8.28 % for binder course; P(#4)
= (4.75/25)0.45 * 100 = 47.36 %
for binder course; P(#200) = (0.075/25)0.45 * 100 = 7.32 % --------------------------------------------------------Types of Aggregate Gradation Dense or Well-Graded: The most common HMA and PCC mix designs in the U.S. tend to use dense graded aggregate. Typical gradations are near the 0.45 power curve but not right on it. Generally, a true maximum density gradation (exactly on the 0.45 power curve) would result in unacceptably low VMA (voids in mineral aggregate).
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Gap Graded: Refers to a gradation that contains only a small percentage of aggregate particles in the mid-size range. The curve is flat in the mid-size range. Some PCC mix designs use gap graded aggregate to provide a more economical mix since less sand can be used for a given workability. HMA gap graded mixes can be prone to ( )ﻋرﺿﺔ اﻟﻰsegregation during placement. Open Graded: Refers to a gradation that contains only a small percentage of aggregate particles in the small range. This results in more air voids (permeable aggregate) because there are not enough small particles to fill in the voids between the larger particles. The curve is near vertical in the mid-size range, and flat and near-zero in the small-size range. Uniformly Graded: Refers to a gradation that contains most of the particles in a very narrow size range. In essence, all the particles are the same size. The curve is steep and only occupies the narrow size range specified.
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D – Aggregate Blending (Combining) The first phase in any asphalt mix design is the selection and combination of aggregates to obtain a gradation within the specification limits. This sometimes is referred to as mechanical stabilization. Several aggregate gradations are usually proportioned for three reasons: (1) to obtain the desired gradation; (2) single natural or quarried materials not enough and (3) it is economical to combine natural and process materials. According to the Iraqi specification (SCRB, 2007, R9), the asphalt concrete mixtures for base course (type I), binder course (type II) and surface course (type IIIA or IIIB) shall be composed basically of coarse aggregate, fine aggregate, mineral filler (if needed), and asphalt cement. The several mineral constituents shall be sized, uniformly graded and combined in such proportions that the resulting blend meets the grading requirements for the specific type under contract. The requirements for the asphalt concrete mixtures shall conform to grading shown in the Table below.
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The Job Mix Formula (JMF) Generally, a job-mix formula consists of two parts: (1) The combined gradation of the aggregates to be used in the production of the asphalt concrete mixture; (2) The asphalt content necessary to produce a satisfactory mix meeting all the specification requirements (next lectures). The Iraqi SCRB (2009, R9) stated that contractor will be allowed the tolerances from the approved job-mix formula shown in Table R9/4.
How to Combining Aggregates to Achieve the Target Blend?
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Proportioning Determinations
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ﺗﺣدﯾد ﻧﺳب اﻟﻣزج
(1) Combining Two Aggregates
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Figure 3.10 Trial-and-Error calculations for combining two aggregates.
(2) Combining Three Aggregates or More The same procedure adopted in blinding two aggregates can be followed to determine the adequate proportions for combining three or more aggregates. For example, when three aggregates needs to be combined (Coarse agg. A, Fine Agg. B and Mineral Filler C) the following Basic Formula and Trial-and-Error procedure should be applied. P = aA + bB + cC a+b+c=1 =================================== Lecture 13
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Dr. Firas Asad
