Sampling and Testing Aggregate Deposits

Results of aggregate tests will be inaccurate unless representative samples are obtained. Geologic deposits may exhibit material changes between differently identified formations and within the same formation. For effective sampling of a deposit, material changes should be delineated so that each material type can be sampled independently. Performing aggregate tests on samples from each material type will help determine the distribution of different qualities of aggregate within a deposit.

These physical test methods are the prevailing tests used for evaluation of potential construction aggregates and quality control of processing and production phases of acceptable construction aggregate. ASTM C33 (1987) describes the specifications for grading and quality of fine and coarse aggregate for use in concrete. ASTM C33 may be used by a specifier (designer, architect, engineer, etc.) to define the quality and grading of the aggregate to be used in concrete in the structure. The specifications may also be used by a contractor, concrete supplier, or other purchaser as a purchase document describing the material to be furnished )y the aggregate producer.

Many of the physical test methods listed for aggregate qualification in ASTM C33 (1987) can also be applied to valuation and quality control of aggregates used for other and products such as asphaltic concrete and aggregate base. Other official publications list the grading and additional specifications required of these construction products. The appropriate publication should be utilized concerning the specific type of aggregate use.

Laboratory Testing of Potential Construction Aggregates

Once a potential aggregate source is identified and delineated by field reconnaissance and mapping, physical testing of the potential aggregate should then be performed. Appropriate physical tests will indicate whether the material is acceptable as construction aggregate. Scree slopes and talus can be sampled for physical testing of hard quarry rock deposits. Talus may consist of some of the most resistant material of a hard rock deposit, but its longer exposure to weathering results in physical characteristics that should approximately represent the deposit majority. Sedimentary deposits should be sampled in test pits or borings that are randomly located throughout the deposit. The normally high heterogeneity of these deposits makes it difficult to obtain representative samples without utilizing a backhoe or drill rig.

The standard for testing aggregates is governed by the American Society of Testing Materials (ASTM). The most important laboratory tests for initially evaluating the physical quality of a potential aggregate deposit are the soundness test in accordance with the ASTM C88 test method and the Los Angeles rattler loss test in accordance with the ASTM C131 test method. Both test methods are described in the section, "Laboratory Tests to be Performed".

Maximum limits of material lost in the soundness and abrasion tests for aggregate acceptability are presented in ASTM and generally accepted by most contractors and federal, state, and city planning and development departments. If material losses are within these maximum limits, then the aggregate may be accepted for use in construction. Additional physical testing should be conducted to evaluate its suitability in specific construction products.

Laboratory Tests to be Performed

In addition to using the soundness test and Los Angeles Rattler loss test in initial aggregate investigations, several other laboratory tests are used in more detailed investigations of potential aggregate deposits and for quality control of proven aggregate deposits. These tests are described as follows:

1. Gradation analysis. This test is used for quality control to determine whether aggregate products meet required grading specifications. According to ASTM C33(1987), aggregate must conform to particular particle size distributions for use in concrete. Hard quarry rock deposits can not be tested for gradation until they are extracted and processed at an aggregate pit. In unconsolidated deposits the natural grading of the material should be determined to show if any specific grain sizes are deficient or whether there is an excessive amount of unusable fine material (particles smaller than the #200 standard sieve size).

2. Atterberg limits. This test is performed to determine the plasticity of an aggregate sample. A deposit that exhibits substantial plasticity is an indication that the aggregate probably needs to be washed during processing which can be very costly. To avoid this production cost, an aggregate producer can attempt to selectively mine the deposit so that significant areas of clay are not processed with the quality aggregate material. When producing aggregate base, low plasticity in the materials is normally acceptable and actually advantageous because a small amount of clay will bind the aggregate and make a base section slightly more durable and resistant to weathering.

Aggregate used for concrete and asphalt must be nonplastic. A small percentage of fines (material smaller than #200 standard sieve size) normally comprises concrete and asphalt aggregate. If a deposit proposed for concrete and asphalt aggregate production contains a large amount of fines compared to larger particle sizes, then washing should be anticipated.

3. Specific gravity and absorption. Specific gravity and absorption values of aggregate are needed to design concrete and asphalt mixes. These mixes are generally made under laboratory controlled conditions to evaluate the performance of the construction products using different specific amounts of constituents. Definitions obtained from ASTM C127 test method (1987) are outlined below:

a. specific gravity - the ratio of the mass (or weight in air) of a unit volume of a material to the mass of the same volume of water at stated temperatures. Values are dimensionless.

b. bulk specific gravity - the ratio of the weight in air of a unit volume of aggregate (including the permeable and impermeable voids in the particles, but not including the voids between the particles) at a stated temperature to the weight in air of an equal volume of gas-free distilled water at a stated temperature.

c. bulk specific gravity (SSD) - 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 in water for approximately 24 hours (but not including the voids between the particles) at a stated temperature, compared to the weight in air of an equal volume of gas-free distilled water at a stated temperature.

d. apparent specific gravity - the ratio of the weight in air of a unit volume of the impermeable portion of aggregate at a stated temperature to the weight in air of an equal volume of gas-free distilled water at a stated temperature.

e. absorption - the increase in the weight of 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 110 plus or minus 5 degrees Celsius for sufficient time to remove all uncombined water.

ASTM C127 (1987) explains that bulk specific gravity is the characteristic generally used for 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 on an absolute volume basis. Either bulk specific gravity or bulk specific gravity (SSD) is used for volume calculations depending if the aggregate is in a dry state or a saturated-surface-dry (SSD) state.

The absorption of the aggregate is used to account for the amount of water or oil (if bituminous concrete is being produced) that will be absorbed by the aggregate. Highly absorptive aggregate is not desired for use in bituminous concrete because a high amount of oil would be needed in an asphalt mix to compensate for the excessive oil absorbed by this type of aggregate.

4. Sodium sulfate (soundness). This test is used for the initial evaluation of a potential aggregate source. As explained in ASTM C88 (1987), the test method provides a procedure for making a preliminary estimate of the soundness of aggregates for use in concrete and other purposes. The immersion and re-immersion of aggregate samples into a sodium sulfate solution simulates several freeze-thaw cycles in a short period of time. After the cycles are completed the amount of aggregate lost from the freeze-thaw effect of the sodium sulfate is determined. If the loss of aggregate is less than 10% by weight of the original weight of coarse aggregate and 12% by weight of the original weight of fine aggregate, then the aggregate deposit should be further evaluated for other physical characteristics to see if it qualifies as a construction aggregate source. Even if aggregate losses are greater than the maximum losses allowed, the precision of this test method is poor, and therefore failing results may not indicate outright rejection of aggregates without confirmation from other tests more closely related to the specific service intended (ASTM, 1987).

5. Los Angeles abrasion. This test usually accompanies the sodium sulfate test in the initial investigation of a potential aggregate source. As stated in ASTM C131 (1987), the Los Angeles test has been widely used as an indicator of the relative quality or competence of various sources of aggregate having similar mineral compositions. The results do not automatically permit valid comparisons to be made between sources distinctly different in origin, composition, or structure. Specification limits based on this test should be assigned with extreme care in consideration of available aggregate types and their performance history in specific end uses."

Utilization of this test in conjunction with the soundness test can reliably indicate the relative potential of an aggregate deposit. If material losses of aggregate samples used in both tests are within the maximum losses allowed per ASTM, then the deposit should be further investigated.

6. Alkali-aggregate reactivity. Several methods in ASTM exist for evaluating the potential reactivity of an aggregate. If there is suspicion that the aggregate used in concrete may deleteriously react with the alkalis of the cement, then specific test methods should be undertaken to determine which minerals or aggregate types could cause the harmful chemical reactions. Alkali-aggregate reactions can deteriorate the aggregate-cement bond and thereby decrease the strength of the concrete. ASTM C33 (1987) indicates that certain materials are known to be reactive with the alkalis in cements. These include the following forms of silica: opal, chalcedony, tridymite, and cristobalite; intermediate to acid (silica-rich) volcanic glass such as likely to occur in rhyolite, andesite, or dacites certain zeolites such as heunlandite: and certain constituents of some phyllites. Determination of the presence and quantities of these materials by petrographic examination is helpful in evaluating potential alkali reactivity. Some of these materials render an aggregate deleteriously reactive when present in quantities as little as 1.0% or even less.

7. Petrographic analysis. As per ASTM C295 (1987), petrographic examination serves several purposes in evaluating aggregates for concrete including:

a. Establishment of whether the aggregate contains chemically unstable minerals.

b. Identification of the portion of each coarse aggregate that is composed of weathered or otherwise altered particles and the extent of that weathering or alteration.

c. Determination of the proportions of cubic, spherical, ellipsoidal, pyramidal, tabular, flat, and elongated particles in an aggregate sample or samples.

d. Identification of potentially alkali-silica reactive and alkali-carbonate reactive constituents, determination of such constituents quantitatively, and recommendation of additional test to confirm or refute the presence in significant amounts of aggregate constituents capable of alkali reaction in concrete. Analysis of thin sections obtained from concrete can expose reactivity evidence caused by deleterious constituents.

e. Identification of contaminants in aggregates, such as synthetic glass, cinders, clinker, or coal ash, magnesium oxide, calcium oxide, etc.

Petrographic examination can also be applied to aggregate considered for other end products as well. Contamination and reactivity of aggregates are less important in products such as asphaltic concrete and aggregate base: however, evaluation of the mineral habit types and the weathered and/or altered degree of the aggregate constituents is invaluable for assessing an aggregate's potential for these end uses.

8. Organic. This test is used for an approximate determination of the presence of injurious organic compounds in fine aggregates that are to be used in cement mortar or concrete (ASTM C40, 1987). This test is normally conducted when there is strong suspicion that organic material may be present in the aggregate. ASTM C40 (1987) indicates that this test method is of significance in making a preliminary determination of the acceptability of fine aggregates with respect to the requirements of ASTM C33 specifications.

9. Clay lumps and friable particles. When an aggregate deposit is suspect of containing a significant amount of incompetent material, then ASTM C142 test method (1987) is of primary significance in determining the acceptability of aggregate with respect to the requirements of ASTM C33. A significant amount of clay lumps and/or friable particles assumed to be competent aggregate in a sample could cause construction products using such aggregate to be unexpectedly weak and undurable. Lower strength values of concrete would be one result because the normally strong bond associated with cement and aggregate would not occur between cement and incompetent material such as clay lumps and friable particles.

Additional testing related information can be downloaded from Chapter 4, Construction Details, and Chapter 6, Sampling and Testing, of the Caltrans Construction Manual at http://www.dot.ca.gov/hq/construc/manual2001/.

University of Nevada, Reno

THE CONSTRUCTION AGGREGATE POTENTIAL OF GEOLOGIC DEPOSITS STOREY COUNTY, NEVADA

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geological Engineering, By Peter Robert Kraatz, April 1989

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