These typically have specific gravities between 2.0 and 3.0. They are usually distinguished by size as follows:
Boulders Larger than 6 in
Cobbles 6 to 3 in
Coarse aggregate 3 in to No. 4 sieve
Fine aggregate No. 4 sieve to No. 200 sieve
Mineral filler Material passing No. 200 sieve
Used in most concrete construction, normal-weight aggregates are obtained by draining riverbeds or mining and crunching formational material. Concrete made with normal-weight fine and coarse aggregates generally weights about 144 lb / ft3.
Boulders and cobbles are generally not used in their as-mined size but are crushed to make various sizes of coarse aggregate and manufactured sand and mineral filler. Gravels and naturally occurring sand are produced by the action of water and weathering on glacial and river deposits. These materials have round, smooth surfaces and particle-size distributions that require minimal processing.
These materials can be supplied in either coarse or fine-aggregate sizes.
Fine aggregates have 100% of their material passing the 3⁄8-in sieve. Coarse aggregates have the bulk of the material retained on the No. 4 sieve.
Aggregates comprise the greatest volume percentage in portland-cement concrete, mortar, or asphaltic concrete. In a portland-cement concrete mix, the coarse and fine aggregates occupy about 60 to 75% of the total mix volume. For asphaltic concrete, the aggregates represent 75 to 85% of the mix volume. Consequentially, the aggregates are not inert filler materials. The individual aggregate properties have demonstrable effects on the service life and durability of the material system in which the aggregate is used, such as portland-cement concrete, asphaltic concrete, mortar, or aggregate base.
The acceptability of a coarse or fine aggregate for use in concrete or mortar is judged by many properties including gradation, amount of fine material passing the No. 200 sieve, hardness, soundness, particle shape, volume stability, potential alkali reactivity, resistance to freezing and thawing, and organic impurities. For aggregates used in general building construction, property limits are provided in ASTM C33, ‘‘Specification for Concrete Aggregates,’’ C637, ‘‘Specification for Aggregates for Radiation-Shielding Concrete,’’ and C330, ‘‘Specification for Lightweight Aggregates for Structural Concrete.’’ For other types of construction, such as highways and airports, standards written by various trade or governmental organizations are available.
Gradation of Aggregates
The distribution of aggregate sizes in a concrete mix is important because it directly influences the amount of cement required for a given strength, workability of the mix (and amount of effort to place the mix in the forms), in-place durability, and overall economy. ASTM C33 provides ranges of fine- and coarse-aggregate grading limits. The latter are listed from Size 1 (31⁄2 to 11⁄2 in) to Size 8 (3⁄8 to No. 8). The National Stone Association specifies a gradation for manufactured sand that differs from that for fine aggregate in C33 principally for the No. 100 and 200 sieves. The NSA gradation is noticeably finer (greater percentages passing each sieve). The fine materials, composed of angular particles, are rock fines, as opposed to silts and clays in natural sand, and contribute to concrete workability.
The various gradations provide standard sizes for aggregate production and quality-control testing. They are conducive to production of concrete with acceptable properties. Caution should be exercised, however, when standard individual grading limits are used. If the number of aggregate sizes are limited or there is not sufficient overlap between aggregates sizes, an acceptable or economical concrete may not be attainable with acceptably graded aggregates. The reason for this is that the combined gradation is gap graded. The ideal situation is a dense or well-graded size distribution that optimizes the void content of the combined aggregates (Art. 4.17). It is possible, however, to produce acceptable concrete with individual aggregates that do not comply with the standard limits but that can be combined to produce a dense gradation.
Amount of Fine Material Passing the No. 200 Sieve
The material passing the No. 200 sieve is clay, silt, or a combination of the two.
It increases the water demand of the aggregate. Large amounts of materials smaller than No. 200 may also indicate the presence of clay coatings on the coarse aggregate that would decrease bond of the aggregate to the cement matrix. A test method is given in ASTM C117, ‘‘Materials Finer than 75 m Sieve in Mineral Aggregates by Washing.’’
Coarse-aggregate hardness is measured by the Los Angeles Abrasion Test, ASTM C131 or C595. These tests break the aggregate down by impacting it with steel balls in a steel tumbler. The resulting breakdown is not directly related to the abrasion an aggregate receives in service, but the results can be empirically related to concretes exhibiting service lives.
Aggregate soundness is measured by ASTM C88, ‘‘Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magneisum Sulfate.’’ This test measures the amount of aggregate degradation when exposed to alternating cycles of wetting and drying in a sulfate solution.
Natural sand and gravel have a round, smooth particle shape. Crushed aggregate (coarse and fine) may have shapes that are flat and elongated, angular, cubical, disk, or rodlike. These shapes result from the crushing equipment employed and the aggregate mineralogy. Extreme angularity and elongation increase the amount of cement required to give strength, difficulty in finishing, and effort required to pump the concrete. Flat and elongated particles also increase the amount of required mixing water.
The bond between angular particles is greater than that between smooth particles.
Properly graded angular particles can take advantage of this property and offset the increase in water required to produce concrete with cement content and strength equal to that of a smooth-stone mix.
Potential Alkali Reactivity
Aggregates that contain certain forms of silicas or carbonates may react with the alkalies present in portland cement (sodium oxide and potassium oxide). The reaction product cracks the concrete or may create pop-outs at the concrete surface.
The reaction is more pronounced when the concrete is in a warm, damp environment.
Testing for potentially reactive aggregates is difficult, since the available tests do not yield consistent answers. Tests for aggregate potential alkali reactivity can be categorized as pre- or post-concrete and chemical or physical. Of the three preconcrete tests, one is chemical. The standard chemical test (ASTM C289) is a screening test that should only be used for an initial aggregate screening. Experience has shown the test will give false positive reactions of potentially reactive aggregates.
The old mortar bar test (ASTM C227) is very slow and may be too lenient.
The rapid immersion mortar bar test (ASTM C1260) is a harsher test but can produce results in two weeks. Potential alkali reactivity can be determined in concrete by observation or using a uranal acetate ultraviolet light test procedure. Petrographic analysis of aggregates and hardened concrete can be used to evaluate the potential for alkali silica reactivity (ASR). Long-term field experience with available aggregate sources is the best predictor of ASR.
Resistance to Freezing and Thawing
The pore structure, absorption, porosity, and permeability of aggregates are especially important if they are used to make concrete exposed to repeated cycles of freezing and thawing. Aggregates that become critically saturated and then freeze cannot accommodate the expansion of the frozen water. Empirical data show that freeze-thaw deterioration is caused by the coarse aggregates and not the fine. A method prescribed in ‘‘Test Method for Resistance of Concrete to Rapid Freezing and Thawing,’’ ASTM C666, measures concrete performance by weight changes, a reduction in the dynamic modulus of elasticity, and increases in sample length.
Impurities in Aggregates
Erratic setting times and rates of hardening may be caused by organic impurities in the aggregates, primarily the sand. The presence of these impurities can be investigated by a method given in ‘‘Test Method for Organic Impurities in Fine Aggregates for Concrete,’’ ASTM C40.
Pop-outs and reduced durability can be caused by soft particles, chert, clay lumps and other friable particles, coal, lignite, or other lightweight materials in the aggregates. Coal and lignite may also cause staining of exposed concrete surfaces.
Volume stability refers to susceptibility of aggregate to expansion when heated or to cyclic expansions and contractions when saturated and dried. Aggregates that are susceptible to volume change due to moisture should be avoided.