Aggregates for Concrete Design and Control of Concrete Mixtures CHAPTER 8 Design and Control of Concrete Mixtures, 16th edition, Chapter 8 – Aggregates for Concrete

Overview Aggregate Geology Aggregate Classification Characteristics of Aggregates Potentially Harmful Materials Alkali-Aggregate Reactivity Aggregate Beneficiations Handling and Storing Aggregates The module will discuss aggregates for use in concrete, including aggregate classification and the properties of aggregates.

Aggregates of Concrete Aggregates generally occupy 60% to 75% of the concrete volume and strongly influence the concrete’s fresh and hardened properties, mixture proportions, and economy. Fine aggregates (left) generally consist of natural sand or crushed stone with most particles smaller than 5 mm (0.2 in.). Coarse aggregates (right) typically consist of gravels, crushed stone, or a combination of both, with particles predominantly larger than 5 mm (0.2 in.) and generally between 9.5 mm and 37.5 mm (3⁄8 in. and 1½ in.).

Aggregate Geology Rock and Mineral Constituents in Aggregates Concrete aggregates are a mixture of rocks and minerals. A mineral is a naturally occurring solid substance with an orderly internal structure and a chemical composition that ranges within narrow limits. A rock is generally composed of several minerals. Rocks are classified as igneous, sedimentary, or metamorphic, depending on their geological origin.

Aggregate Classification Natural aggregate Manufactured aggregate Recycled-concrete aggregate Marine-dredged aggregate Aggregates are classified into three categories: normal-weight, lightweight (shown in the picture), and heavyweight.

Bulk Density Normal weight concrete: 1200 kg/m3 to 1750 kg/m3 (75 lb/ft3 to 110 lb/ft3) Lightweight concrete aggregates: 560 kg/m3 to 1120 kg/m3 (35 lb/ft3 to 70 lb/ft3) Heavyweight aggregates: Typically over 2100 kg/m3 (130 lb/ft3) The approximate bulk density of aggregate commonly used in normalweight concrete ranges from about 1200 kg/m3 to 1750 kg/m3 (75 lb/ft3 to 110 lb/ft3) while lightweight concrete aggregates range from 560 kg/m3 to 1120 kg/m3 (35 lb/ft3 to 70 lb/ft3), and heavyweight aggregates is typically over 2100 kg/m3 (130 lb/ft3).

Natural Aggregate Gravel and sand Pits, rivers, lakes, seabeds Mixture of several minerals Pits, rivers, lakes, seabeds Quality depends on parent bedrock Gravel and sand are often a mixture of several minerals. Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. The quality (or soundness) of natural aggregate depends on the bedrock from which the particles were derived and the mechanism by which they were transported.

Manufactured Aggregate Crushed rock or air-cooled slag Fine and coarse aggregate Rough, angular texture Cubical or elongated shape More uniform in size Less likely to be contaminated Manufactured aggregate is produced by crushing sound parent rock at stone crushing plants. As a result of the crushing operation, manufactured aggregates often have a rough surface texture, are more angular in nature, tend to be cubical or elongated in shape, and more uniform in size. Manufactured aggregates are less likely to be contaminated by deleterious substances such as clay minerals or organic matter.

Recycled-Concrete Aggregate Recycled concrete involves demolishing and removing the existing concrete, crushing the material in primary and secondary crushers, removing reinforcing steel and other embedded items, grading and washing, and stockpiling the resulting coarse and fine aggregate.

Recycled-Concrete Aggregate Recycled-concrete aggregate generally has a higher absorption and a lower specific gravity than conventional aggregate. Absorption values typically range from 3% to 10%. The graph is a comparison of water absorption of three different recycled aggregate particle sizes and one size of natural and lightweight coarse aggregate. Absorption rates increase as coarse particle size decreases.

Marine-Dredged Aggregate Tidal estuaries, seashore Two concerns: seashells and salt Avoid uncrushed shells Avoid using high chloride aggregates in reinforced concrete Marine-dredged aggregate from tidal estuaries and sand and gravel from the seashore may be used with caution in concrete applications when other aggregate sources are not available. Aggregate containing complete shells should be avoided as their presence may result in voids in the concrete and may lower the compressive strength. Generally, marine aggregates containing large amounts of chloride should not be used in reinforced concrete.

Aggregate Characteristics Characteristics and Tests of Aggregate The important characteristics of aggregates for concrete are listed here. Normal-weight aggregates should meet the requirements of ASTM C33, or AASHTO M 6 and AASHTO M 80. These specifications limit the permissible amounts of deleterious substances and provide requirements for aggregate characteristics.

Aggregate Characteristics Characteristics and Tests of Aggregate

Grading Grading is the particle-size distribution of an aggregate as determined by a sieve analysis. The range of particle sizes in aggregate is illustrated in this picture.

Grading The aggregate particle size is determined using wire-mesh sieves with square openings. The seven standard ASTM C33 (AASHTO M 6/M 80) sieves for fine aggregate have openings ranging from 150 μm to 9.5 mm (No. 100 sieve to 3⁄8 in.). The 13 standard sieves for coarse aggregate have openings ranging from 1.18 mm to 100 mm (0.046 in. to 4 in.). Size numbers or grading sizes for coarse aggregates apply to the amounts of aggregate in percentages that pass through an assortment of sieves.

Grading Limits This graph shows the limits specified in ASTM C33 for fine aggregate and for one commonly used size number (grading size) of coarse aggregate. In general, aggregates that do not have a large deficiency or excess of any size and give a smooth grading curve will produce the most satisfactory results in concrete.

Fine-Aggregate Grading Requirements of ASTM C33 or AASHTO M 6/M 43 permit a relatively wide range in fine-aggregate gradation. Fine-aggregate grading within the limits of ASTM C33 (AASHTO M 6) is generally satisfactory for most concretes. The ASTM C33 (AASHTO M 6) limits with respect to sieve size are shown here.

Coarse-Aggregate Grading The coarse aggregate grading requirements of ASTM C33 (AASHTO M 80) permit a wide range in grading and a variety of grading sizes, as shown in this table.

Coarse-Aggregate Grading The maximum size of coarse aggregate used in concrete has a direct bearing on the properties of concrete. These graphs show that, for a given water-cement ratio, the amount of cement (or water) required decreases as the maximum size of coarse aggregate increases.

Maximum Size vs. Nominal Maximum Size Maximum size – 100% passing Nominal maximum size – typically 85% to 95% passing The maximum size of an aggregate is the smallest sieve that all of a particular aggregate must pass through. The nominal maximum size of an aggregate is the smallest sieve size through which the major portion of the aggregate must pass, typically 85% to 95% passing.

Nominal Maximum Sizing Dmax = 1⁄5 B, ¾ S, ¾ C, 1⁄3T Requirements for limits on nominal maximum size of aggregate particles are covered by ACI 318. Using this illustration, the nominal maximum size of aggregate should not exceed: one-fifth the narrowest dimension of a vertical concrete member, Dmax = 1⁄5 B; three-quarters the clear spacing between reinforcing bars and between the reinforcing bars and forms, Dmax = 3⁄4 S and 3⁄4 C; and one-third the depth of slabs, Dmax = 1⁄3 T.

Combined Aggregate Grading The effect of a collection of various sized aggregate particles in reducing the total volume of voids is best illustrated in these figures. A single-sized aggregate results in higher voids. When the two aggregate sizes are combined, the void content is decreased. If several additional sizes are used, a further reduction in voids would occur. The paste requirement for concrete is dictated by the void content of the combined aggregates.

Combined Aggregate Grading In reality, the amount of cement paste required in concrete is somewhat greater than the volume of voids between the aggregates, shown here. The illustration on left represents coarse aggregates, with all particles in contact. The illustration on the right represents the dispersal of aggregates in a matrix of paste. The amount of paste necessary is greater than the void content in order to provide workability to the concrete. The actual amount is also influenced by the cohesiveness of the paste.

Combined Aggregate Grading This graph illustrates a theoretical ideal uniform gradation (well-graded aggregate). Well-graded aggregate, having a balanced variety of sizes, maximizes the aggregate volume to the greatest extent. The higher sand and paste requirements may cause higher water demand; resulting in poor workability, and possibly higher shrinkage. Strength and durability may also be affected.

Workability Factor Shilstone 1990 This figure shows the workability factor for combined aggregate gradations. The optimum mixture should plot in or near the trend bar. Workability factor increases as the coarseness factor decreases. The workability factor is considered the minus 2.36 mm (No. 8) sieve particles. The coarseness factor is considered the percentage of plus 2.36 mm (No. 8) particles retained on a 9.5 mm (3/8) sieve. Shilstone 1990

Tarantula Curve http://www.optimizedgraded.com/ Cook and others (2013) found that the desirable overall combinations that fall within the Tarantula Curve provide improved workability and resistance to segregation. A set of suggested limits was developed for pavements by comparing the workability and aggregate gradation of more than 500 different mixtures with 8 different aggregate sources. http://www.optimizedgraded.com/

Gap-Graded Aggregates Certain particle sizes omitted, typically one coarse aggregate size Excess coarse aggregate – honeycomb, segregate Excess fine aggregate – high water demand, shrinkage Properly proportioned mixtures are readily consolidated with vibration In gap-graded aggregates certain particle sizes are intentionally omitted. Typical gap-graded aggregates consist of only one size of coarse aggregate with all the particles of fine aggregate able to pass through the voids in the compacted coarse aggregate. Care must be taken in choosing the percentage of fine aggregate in a gap-graded mixture. A poor choice can result in concrete that is likely to segregate or honeycomb because of an excess of coarse aggregate. Also, concrete with an excess of fine aggregate could have a high water demand resulting in higher shrinkage. When properly proportioned, however, these concretes are readily consolidated with vibration.

Fineness Modulus The fineness modulus (FM) of either fine, coarse, or combined aggregate according to ASTM C125 is calculated by adding the cumulative percentages by mass retained on each of a specified series of sieves and dividing the sum by 100. Fineness modulus is an index of the fineness of an aggregate. In general, the higher the FM, the coarser the aggregate. An example of how the FM of a fine aggregate is determined (with an assumed sieve analysis) is shown in this table.

Particle Shape and Surface Texture The particle shape and surface texture of aggregate influence the fresh concrete properties more than the properties of hardened concrete. Rough-textured, angular, elongated particles require more water to produce workable concrete than do smooth, rounded, compact aggregates. In turn, aggregate particles that are angular require more cement to maintain the same water-cementing materials ratio.

Particle Shape and Surface Texture Flat and elongated aggregate particles should be avoided or at least limited to about 15% by mass of the total aggregate. A particle is called flat and elongated when the ratio of length to thickness exceeds a specified value. A number of automated test machines are available for rapid determination of the particle size distribution of aggregate. Shown here is a videograder.

Bulk Density and Voids Bulk density – mass of aggregate in unit volume Includes voids 30% - 45% voids in coarse aggregates 40% - 50% voids in fine aggregates Angularity increases voids The bulk density or unit weight of an aggregate is the mass or weight of the aggregate required to fill a container of a specified unit volume. The volume is occupied by both aggregates and the voids between aggregate particles. Void contents range from about 30% to 45% for coarse aggregates to about 40% to 50% for fine aggregate. Angularity increases void content while larger sizes of well-graded (uniform) aggregate and improved grading decreases void content.

Density and Relative Density Density = Relative density x density of water Typically between 2400 and 2900 kg/m3 (150 and 181 lb/ft3) Relative density typically between 2.4 and 2.9 The density of aggregate particles used in mixture proportioning computations is determined by multiplying the relative density of the aggregate times the density of water. Most natural aggregates have particle densities of between 2400 and 2900 kg/m3 (150 and 181 lb/ft3). Most natural aggregates have relative densities between 2.40 and 2.90.

Absorption and Surface Moisture The moisture conditions of aggregates are shown here. Ovendry—zero moisture content, fully absorbent; Air dry—dry at the particle surface but containing some interior moisture, less than potential absorption; Saturated surface dry (SSD)—neither absorbing water from nor contributing water to the concrete mixture, equal to potential absorption; Damp or wet—containing an excess of moisture on the surface (free water).

Bulking Bulking is the increase in total volume of moist fine aggregate over the same mass in a dry condition. Surface tension in the moisture holds the particles apart, causing an increase in volume. The figure on the left illustrates how the amount of bulking of fine aggregate varies with moisture content and grading; fine gradings bulk more than coarse gradings for a given amount of moisture. The graphs on the right show similar information in terms of weight for a particular fine aggregate.

Resistance to Freezing and Thawing The frost resistance of an aggregate is related to its porosity, absorption, permeability, and pore structure. Popouts generally appear as conical fragments that break out of the concrete surface. The offending aggregate particle or a part of it is usually found at the bottom of the resulting void.

D-Cracking The cracking of concrete pavements caused by freeze-thaw deterioration of the aggregate is called D-cracking. D-cracks are closely spaced crack formations oriented parallel to transverse and longitudinal joints that later multiply outward from the joints toward the center of the pavement panel, shown on the left. With continued freezing and thawing cycles, cracking of the concrete starts in the saturated aggregate, shown on the right.

Abrasion and Skid Resistance ASTM C131, Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine (AASHTO T 96) ASTM C535, Standard Test Method for Resistance to Degradation of Large-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine. Siliceous content > 25% for good skid resistance The most common test for abrasion resistance is the Los Angeles abrasion test (ASTM C131 or ASTM C535) - rattler method. In this test, a specified quantity of aggregate is placed in a steel drum containing steel balls, the drum is rotated, and the percentage of material worn away is measured.

Strength Tensile strength – 2 MPa to 15 MPa (300 psi to 2300 psi) Compressive strength – 65 MPa to 270 MPa (10,000 psi to 40,000 psi) Rarely tested Aggregate tensile strengths range from 2 MPa to 15 MPa (300 psi to 2300 psi) and compressive strengths from 65 MPa to 270 MPa (10,000 psi to 40,000 psi). The strength of an aggregate is rarely tested and generally does not influence the strength of conventional concrete as much as the strength of the paste and the paste-aggregate bond.

Shrinkage Shrinkage Characteristics of Aggregates Different aggregate types have different compressibility, modulus of elasticity, and moisture-related shrinkage characteristics that influence the same properties in concrete.

Shrinkage Aggregates with high absorption may have high shrinkage on drying. Quartz and feldspar aggregates, along with limestone, dolomite, and granite, are considered low shrinkage aggregates; while aggregates with sandstone, shale, slate, hornblende, and graywacke are often associated with high shrinkage in concrete (pictured above). Carlson 1938

Fire Resistance and Thermal Properties Dependent on mineral constituents Lightweight aggregates outperform normal-weight aggregates Calcareous aggregates outperform siliceous aggregates Coefficient of thermal expansion – 0.55 x 10-6 /°C to 5 x 10-6 /°C (1 x 10-6 /°F to 9 x 10-6 /°F) The fire resistance and thermal properties of concrete depend to some extent on the mineral constituents of the aggregates used. Manufactured and some naturally occurring lightweight aggregates are more fire resistant than normal-weight aggregates due to their insulating properties and high-temperature stability. In general, concrete containing a calcareous coarse aggregate performs better under fire exposure than a concrete containing quartz or siliceous such as granite or quartzite. The coefficient of thermal expansion of aggregates ranges from 0.55 x 10-6 per °C to 5 x 10-6 per °C (1 x 10-6 per °F to 9 x 10-6 per °F).

Potentially Harmful Materials Harmful substances that may be present in aggregates include organic impurities, silt, clay, shale, iron oxide, coal, lignite, and certain lightweight and soft particles.

Potentially Harmful Materials Aggregates can occasionally contain particles of iron oxide and iron sulfide that result in unsightly stains on exposed concrete surfaces. The aggregate should meet the staining requirements of ASTM C330 (AASHTO M 195) when tested according to ASTM C641.

Potentially Harmful Materials Aggregates are potentially harmful if they contain compounds known to react chemically with portland cement and produce any of the following: (1) significant volume changes of the paste, aggregates, or both; (2) Interference with the normal hydration of cement, and (3) otherwise harmful reaction products.

Alkali-Silica Reaction Classification of aggregate reactivity The reactivity of an aggregate can be classified according to guidance provided in ASTM C1778. The aggregate is tested using ASTMC1293, Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction (Concrete Prism Test), commonly called CPT. Because this test takes one year to complete, a rapid assessment test, in the form of ASTM C1260 (AASHTO T 303), Potential Alkali-Reactivity of Aggregates (Mortar-Bar Method), can be used to produce results in sixteen days. ASTM C1778, Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete

Alkali-Carbonate Reaction Rare due to general unsuitability of reactive aggregates Tested through: Petrographic examination (ASTM C295) Rock cylinder method (ASTM C586) Concrete prism test (ASTM C1105) Reactions observed with certain dolomitic rocks are associated with alkali-carbonate reaction, or ACR. ACR is relatively rare because aggregates susceptible to this reaction are usually unsuitable for use in concrete for other reasons, such as strength potential. The three test methods commonly used to identify potentially alkali-carbonate reactive aggregate are petrographic examination (ASTM C295); rock cylinder method (ASTM C586); and concrete prism test (ASTM C1105).

Aggregate Beneficiation Improving quality through processing Heavy media separation Jigging Rising-current classification Crushing When necessary, to eliminate deleterious aggregates, aggregates may improve quality through processing. Beneficiation includes processing methods such as heavy media separation, jigging, rising-current classification, and crushing. In heavy media separation, aggregates are passed through a heavy liquid. The heavier particles sink to the bottom while the lighter particles float to the surface. Jigging separates particles with small differences in density by pulsating water current. Rising-current classification separates particles with large differences in density. Crushing is also used to remove soft and friable particles from coarse aggregates.

Handling and Storing Aggregates Aggregates should be handled and stored in a way that minimizes segregation and degradation and that prevents contamination by deleterious substances. Stockpiles should be built up in thin layers of uniform thickness to minimize segregation.

Handling and Storing Aggregates Bulkheads or dividers should be used to avoid contamination of aggregate stockpiles. Partitions between stockpiles should be high enough to prevent intermingling of materials.

Summary Geology Classification Characteristics of Aggregates Potentially Harmful Materials Alkali-Aggregate Reactivity Beneficiation Handling and Storage

?