3.1 ASTM C117 / AASHTO T 11: Apparatus, Specimen Masses & Washing Procedures A and B

In concrete technology, mineral aggregate constitutes roughly 60% to 75% of total concrete volume. While gradation across standard sieve sizes determines paste demand and particle packing, the ultra-fine fraction—specifically mineral particles finer than the 75-µm (No. 200) sieve—exerts a disproportionate influence on fresh and hardened concrete properties. ASTM C117 (and its AASHTO counterpart AASHTO T 11) defines the standard test method for determining the amount of material finer than the 75-µm (No. 200) sieve in mineral aggregate by washing.

Understanding why a washing procedure is legally and technically mandated—rather than relying solely on standard dry sieving under ASTM C136—is fundamental to passing the ACI Aggregate Testing Technician Level 1 certification.

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Scope and Significance: Why Dry Sieving Alone Is Inadequate

Under ASTM C136 (Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates), dry aggregate is agitated mechanically through a vertical stack of woven wire cloth sieves. However, particles finer than 75 µm include silt, active clay minerals (such as montmorillonite, illite, and kaolinite), and ultra-fine dust of fracture produced during rock crushing.

In an oven-dry or air-dry state, these micro-particles behave very differently than larger sand grains or gravel particles:

1. Electrostatic Clinging: Micro-fines carry electrical surface charges that cause them to cling tenaciously to the surfaces of larger sand grains and coarse aggregate rocks.
2. Hygroscopic Moisture Films: Minute films of adsorbed surface moisture create surface tension forces that bind ultra-fines together into false aggregates or cement them into the microscopic surface indentations of larger particles.
3. Tenacious Clay Coatings: Cohesive clay minerals form durable crusts and coatings around aggregate particles that mechanical shaking in a dry sieve shaker cannot break apart.

[!IMPORTANT] The Dry Sieving Fallacy: If an aggregate sample is tested by dry sieving alone (ASTM C136), particles passing the 75-µm threshold remain stuck to larger particles or trapped in sieve meshes. Dry sieving will severely underestimate the true fines content. Washing with water separates the particles from the larger aggregate particles, disperses clay agglomerates, and brings the micro-fines into colloidal suspension so they wash completely through the No. 200 sieve.

Detrimental Effects of Excessive Fines in Concrete

Excessive material passing the No. 200 sieve degrades concrete performance in several critical ways:

• Increased Water Demand: The specific surface area of aggregate increases exponentially as particle diameter decreases below 75 µm. High surface area requires significantly more mixing water and cement paste to lubricate the aggregate skeleton, driving up the water-cementitious materials ($w/cm$) ratio.
• Reduced Compressive and Flexural Strength: Increased water demand directly lowers compressive strength. Furthermore, clay coatings physically isolate aggregate particles from the hydrating cement paste, weakening the Interfacial Transition Zone (ITZ).
• High Drying Shrinkage and Cracking: Fine silts and plastic clays dramatically increase volume change upon drying, leading to severe plastic shrinkage cracking, drying shrinkage cracking, and curling in concrete flatwork.
• Reduced Abrasion Resistance and Surface Scaling: Light fines migrate to the surface of flatwork during bleedwater evaporation, forming a weak, porous surface layer (laitance) that dusting and scales under traffic.

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Test Apparatus and Materials

Executing ASTM C117 requires standard laboratory equipment meeting strict dimensional and calibration tolerances:

1. Nested Sieves

A nest of two sieves is required:

• Lower Sieve: A No. 200 (75-µm) woven wire cloth sieve conforming to the physical specifications of ASTM E11.
• Upper (Protective) Sieve: A coarser sieve placed directly above the No. 200 sieve. ASTM C117 specifies a nest of exactly two sieves, and names the upper sieve as the 1.18-mm (No. 16) sieve.

[!WARNING] Why the Protective Sieve Is Essential: The wire diameter of a No. 200 sieve cloth is approximately 0.050 mm (50 µm)—thinner than a human hair. Pouring coarse gravel or angular sand directly onto an unprotected No. 200 sieve cloth will quickly stretch, distort, tear, or puncture the fragile wire mesh, rendering the sieve out of specification.

2. Balance

ASTM C117 Section 5.1 states the requirement directly: a balance or scale readable and accurate to 0.1 g or 0.1% of the test load, whichever is greater, at any point within the range of use.

3. Wash Vessel / Container

A container of sufficient size to contain the aggregate sample covered with water, allowing vigorous agitation of the contents without accidental overflow, splashing, or loss of any portion of the sample or wash water.

4. Drying Oven

A thermostatically controlled, forced-draft or ventilated drying oven capable of maintaining a uniform temperature of 110 ± 5°C (230 ± 9°F) throughout the heating chamber.

5. Wetting Agent (Dispersing Agent)

Depending on the specification or the nature of the fines, two operational procedures exist:

• Procedure A (Plain Water): Uses potable water without any chemical additives.
• Procedure B (Wetting Agent): Uses water containing a dispersing agent to facilitate the separation and suspension of extremely fine particles, particularly sticky clay coatings. Common dispersing agents include:
  • Commercial liquid dishwashing detergent (a few drops per wash).
  • Sodium hexametaphosphate solution (commercial Calgon).

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Minimum Sample Mass Requirements

Field samples must first be reduced to testing size in accordance with ASTM C702 (Standard Practice for Reducing Samples of Aggregate to Testing Size). The minimum dry test sample mass depends directly on the Nominal Maximum Size (NMAS) of the aggregate:

[!IMPORTANT] Two things candidates get wrong about this table. First, it has four rows, and the smallest is 300 g — there is no 100-g row in ASTM C117, and the largest row is keyed to greater than 19.0 mm, not to 37.5 mm. Second, and more important, the table only applies when the specimen is not also going to be sieved under C136. ASTM C117 Section 6.2 states that if the same test sample is to be tested according to Test Method C136, the minimum mass shall be as described in the applicable sections of that method — which for a 19.0-mm coarse aggregate means C136's 5 kg, not C117's 2,500 g. Because a combined wash-then-sieve analysis is the normal case in a concrete laboratory, the C136 masses govern more often than this table does.

[!NOTE] Testing a reduced fraction (ASTM C117 Section 8.2). If the applicable specification requires the amount passing the 75-µm sieve to be determined on a portion of the sample passing a sieve smaller than the nominal maximum size, separate the sample on the designated sieve, determine the mass of the material passing that sieve to 0.1% of the mass of that portion, and use that mass as the original dry mass $B$ in the calculation. Note 2 explains why this exists: some specifications for aggregates of 50-mm nominal maximum size or greater set the minus-No. 200 limit on the portion passing the 25.0-mm sieve, because it is impractical to wash a sample of the size C136 would otherwise require.

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Step-by-Step Test Procedure

Following the exact chronological sequence is critical for accuracy and performance exam compliance:

Step 1: Initial Oven Drying and Mass Determination (Mass B)

1. Place the test specimen in an oven set at 110 ± 5°C (230 ± 9°F).
2. Dry the sample to constant mass — the condition reached when further heating causes, or would cause, less than 0.1% additional loss in mass (the definition ASTM C566 Section 7.4 states explicitly, and the one the aggregate standards share). The standards do not fix a waiting interval between check weighings; establish one by trial for your oven and specimen size.
3. Allow the sample to cool in an enclosed container to room temperature.
4. Determine and record the initial dry mass of the test sample to the nearest 0.1 g (or 0.1% of test mass). Designate this value as B.

Step 2: Immersion and Agitation

1. Place the dried test specimen into the wash container.
2. Add sufficient water to cover the aggregate. In Procedure A this is plain water and nothing else — ASTM C117 Section 8.3 is emphatic: no detergent, dispersing agent, or other substance shall be added to the water. Procedure B is the variant that adds the wetting agent, and Section 7.1 makes Procedure A the default unless the governing specification says otherwise or the agency directs otherwise.
3. Agitate the aggregate vigorously with your hands, a stirring rod, or a mechanical washer for a sufficient duration to completely separate all particles finer than the 75-µm sieve from the coarse particles and to bring the fine material into suspension.

Step 3: Decantation Over Nested Sieves

1. Nest the 1.18-mm (No. 16) protective sieve directly on top of the 75-µm (No. 200) sieve.
2. Pour the wash water containing suspended and dissolved solids over the nested sieves.
3. Exercise extreme care during decanting: do not transfer coarse aggregate particles from the wash vessel onto the sieves. Retain the bulk aggregate within the wash container.

Step 4: Repeat Washing and Decanting Cycles

1. Add a second charge of water to the sample in the container. Note that even in Procedure B, ASTM C117 Section 9.3 adds the second and subsequent charges without wetting agent — the agent goes in on the first charge only.
2. Agitate vigorously once again to suspend remaining fines.
3. Decant the wash water over the nested sieves.
4. Repeat this washing, agitating, and decanting operation until the wash water is visually clear when poured off.

Step 5: Returning Retained Material to the Washed Specimen

1. Using a wash bottle equipped with a fine nozzle, wash all aggregate particles retained on the upper protective sieve onto the lower No. 200 sieve.
2. Wash all material retained on the No. 200 sieve back into the container holding the washed aggregate specimen.
3. Use the minimum volume of rinse water necessary to avoid overflowing the container.

Step 6: Final Oven Drying and Mass Determination (Mass C)

1. Carefully decant any excess clear water from the container, ensuring that no settled aggregate particles are lost.
2. Place the container and washed aggregate in the drying oven at 110 ± 5°C (230 ± 9°F).
3. Dry the washed aggregate to constant mass.
4. Cool to room temperature and weigh the dried washed aggregate to the nearest 0.1 g or 0.1% of original sample mass. Designate this value as C.