3.2 ASTM C117: Calculation, Reporting Rules & ASTM C33 Fines Limits

Mathematical Formula and Worked Calculation

The percentage of material finer than the 75-µm (No. 200) sieve by washing is calculated using the following equation:

$A = \left[ \frac{B - C}{B} \right] \times 100$

Where:

• $A$ = percentage of material finer than the 75-µm (No. 200) sieve by washing. Report it to the nearest 0.1%, except if the result is 10% or more, in which case report it to the nearest whole number (ASTM C117 Section 11.1.1). Section 11.1.2 additionally requires the report to state which procedure, A or B, was used.
• $B$ = original dry mass of the test sample before washing, in grams (g).
• $C$ = dry mass of the test sample after washing, in grams (g).
• $B - C$ = mass of material finer than the 75-µm sieve lost during the washing operation, in grams (g).

Realistic Laboratory Calculation Example

A certified technician is performing an acceptance test on a natural fine aggregate (concrete sand). The test data recorded in the laboratory notebook is as follows:

• Nominal Maximum Aggregate Size (NMAS): No. 4 (4.75 mm)
• Specified Minimum Sample Mass: 300 g
• Initial Oven-Dry Sample Mass ($B$): $542.8\text{ g}$
• Oven-Dry Sample Mass After Washing ($C$): $527.6\text{ g}$

Step-by-Step Solution:

1. Calculate the mass lost by washing ($B - C$): $B - C = 542.8\text{ g} - 527.6\text{ g} = 15.2\text{ g}$
2. Apply the ASTM C117 formula: $A = \left[ \frac{15.2}{542.8} \right] \times 100 = 2.80029\%$
3. Round according to ASTM reporting rules:
   • ASTM C117 Section 11.1.1 requires reporting to the nearest 0.1% when the result is below 10%.
   • Reported Result: 2.8% material finer than the No. 200 sieve by washing, with a statement that Procedure A was used.
   • Contrast: had the same specimen lost 62.4 g, the result would be $ (62.4 / 542.8) \times 100 = 11.5%$, and because that is 10% or more it would be reported as 12% — a whole number, not 11.5%.

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ASTM C33 Specification Limits

ASTM C33 / C33M (Standard Specification for Concrete Aggregates) establishes legal threshold limits for material finer than the 75-µm (No. 200) sieve. The permissible threshold depends on aggregate type and whether the concrete is exposed to abrasive wear:

[!TIP] Exam Trap: On the written exam, pay close attention to whether the question specifies "concrete subject to abrasion" (limit 3.0% for sand) or "other concrete" (limit 5.0% for sand), and whether the aggregate is natural or manufactured crushed stone.

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How the C117 Result Feeds the ASTM C136 Report

ASTM C117 Section 4.1 explains why this test is run before dry sieving rather than instead of it: material finer than 75 micrometres separates far more efficiently and completely by wet sieving than by dry sieving, so when an accurate determination is wanted, C117 is used on the sample prior to dry sieving under C136. The results are then included in the C136 calculation, and the total amount finer than 75 micrometres — the wash loss plus the amount obtained by dry sieving the same sample — is reported with the C136 results.

The standard then adds a diagnostic that doubles as a laboratory quality check:

Usually, the additional amount of material finer than 75 micrometres obtained in the dry sieving process is a small amount. If it is large, the efficiency of the washing operation should be checked. It could also be an indication of degradation of the aggregate.

So the dry-pan fraction is not merely an extra number to add in. In the worked combined analysis in Section 4.3 of this guide, the wash recovered 14.0 g and dry sieving recovered a further 3.5 g — a fifth as much, which is normal. A dry pan holding as much as the wash loss means either the technician stopped decanting before the water ran clear, or the aggregate is breaking down under the shaker.

The ASTM C33 Weighted Allowance for Coarse Aggregate

The 1.0% coarse aggregate limit has a second relief clause that is easy to miss. ASTM C33 Table 3 footnote C permits the limit to be raised in either of two ways:

1. To 1.5%, if the material finer than the 75-micrometre sieve is essentially free of clay or shale — the dust-of-fracture case.
2. By weighted calculation, if the source of the fine aggregate to be used in the concrete is known to contain less than the maximum amount passing the 75-micrometre sieve. The permitted coarse aggregate limit becomes:

$L = 1 + \left[\frac{P}{100 - P}\right](T - A)$

where $P$ is the percentage of sand in the concrete as a percent of total aggregate, $T$ is the ASTM C33 Table 1 limit for the amount permitted in the fine aggregate, and $A$ is the actual amount in the fine aggregate.

Worked example. A mix uses 40% sand by mass of total aggregate ($P = 40$). The concrete is not subject to abrasion, so the Table 1 fine aggregate limit is $T = 5.0%$. The sand actually tests at $A = 1.8%$ minus-No. 200.

$L = 1 + \left[\frac{40}{60}\right](5.0 - 1.8) = 1 + 0.6667 \times 3.2 = 1 + 2.13 = \mathbf{3.1\%}$

The coarse aggregate may carry up to 3.1% minus-No. 200 instead of 1.0%, because the unusually clean sand leaves room in the total. As C33 puts it, this provides a weighted calculation designed to limit the maximum mass of material passing the 75-micrometre sieve in the concrete to what would be obtained if both the fine and coarse aggregate were supplied at the maximum tabulated percentage for each.