3.3 ASTM C40 / AASHTO T 21: Organic Impurities in Fine Aggregates

Natural sand deposits excavated from riverbeds, glacial outwash plains, and alluvial pits frequently contain decomposed vegetable matter, topsoil overburden, peat, humus, and decaying roots. When introduced into portland cement concrete mixtures, even small concentrations of organic compounds can severely interfere with cement hydration, weaken compressive strength, retard setting times, and damage surface integrity. ASTM C40 (and AASHTO T 21) provides a rapid, qualitative field and laboratory screening procedure to determine the presence of injurious organic impurities in natural fine aggregates intended for concrete.

Technicians must understand the underlying chemistry, the exact mechanical steps of the test, how to evaluate the liquid color against standard reference standards, and the required engineering follow-up protocol under ASTM C87.

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Scope and Significance: Why Organic Matter Poisons Concrete

ASTM C40 is strictly a screening test applied to natural fine aggregates. It is not typically required for manufactured sand produced by crushing fresh quarry stone, unless quarry rock layers were contaminated with surface overburden or organic seams.

The Chemical Mechanism of Organic Interference

Portland cement cures and develops structural strength through the exothermic hydration of silicate and aluminate clinker phases—most notably tricalcium silicate ($C_3S$) and dicalcium silicate ($C_2S$):

$2\text{C}_3\text{S} + 11\text{H}_2\text{O} \longrightarrow \text{C-S-H} + 3\text{Ca(OH)}_2$

When soluble organic compounds—such as humic acids, fulvic acids, tannic acid, and complex polysaccharides (sugars)—are present in aggregate:

1. Chelation of Calcium Ions: Organic acids react rapidly with calcium ions dissolved in the pore fluid, forming insoluble or semi-permeable calcium organo-complexes.
2. Barrier Formation: These organo-metallic complexes precipitate directly onto the surfaces of unhydrated cement grains, creating an impermeable chemical blanket that prevents water molecules from reaching the reactive $C_3S$ core.
3. Retardation and Strength Halting: Cement hydration is severely retarded or completely poisoned. Setting times can be delayed from hours to several days. In severe cases, the concrete remains semi-plastic and fails to attain its specified compressive strength.
4. Surface Defects: Lighter organic particles migrate toward the finished surface during vibration and bleeding, creating localized popouts, brown or black organic surface staining, and localized soft spots that spall under traffic.

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Test Apparatus, Glassware, and Chemical Reagents

ASTM C40 specifies simple but precise glassware and chemical solutions:

1. Glass Bottle

• Colorless glass graduated bottle of approximately 240 to 470 mL (8 to 16 oz) nominal capacity.
• Measured along the line of sight used for the color comparison, the maximum outside thickness of the bottle must be not greater than 63.5 mm (2.5 in.) and not less than 38.1 mm (1.5 in.). A bottle outside that band distorts the color comparison.
• Graduated in millilitres or U.S. fluid ounces. If an unmarked bottle is calibrated and scribed by the user, ASTM C40 requires marks at only three points: 75 mL (2-1/2 oz) for the standard color solution, 130 mL (4-1/2 oz) for the fine aggregate, and 200 mL (7 oz) for the NaOH solution level.
• Equipped with watertight stoppers or caps that are not soluble in the specified reagents.

2. Reagent: 3% Sodium Hydroxide (NaOH) Solution

• The chemical reagent is a 3% sodium hydroxide (NaOH) solution by mass.
• Preparation: Dissolve 3 parts by weight of ACS reagent-grade sodium hydroxide pellets in 97 parts by weight of distilled or deionized water.
  • Example: Dissolve $30.0\text{ g}$ of pure NaOH pellets into $970.0\text{ g}$ of distilled water to produce $1,000\text{ g}$ of 3% NaOH reagent solution.

3. Color Comparison Standards

To interpret the color of the liquid above the sand after the resting period, the technician compares it against an official reference standard. ASTM C40 recognizes two approved comparisons, and the standard numbers them in this order:

Comparison A — Standard Color Solution Procedure (ASTM C40 Sections 5.2 and 9.1)

A liquid chemical reference prepared by dissolving reagent-grade potassium dichromate ($K_2Cr_2O_7$) in concentrated sulfuric acid ($H_2SO_4$, sp gr 1.84) at the rate of 0.250 g per 100 mL of acid, using gentle heat if necessary to effect solution. ASTM C40 Section 5.2 requires the solution to be freshly made for the color comparison, and Section 9.1 requires it to have been prepared not longer than 2 h previously.

To run the comparison: fill a second glass bottle to the approximately 75-mL (2-1/2 fluid oz) level with the fresh standard color solution, hold the bottle holding the test sample and the bottle holding the standard color solution side by side, compare the light transmitted through the supernatant liquid above the sample with the light transmitted through the standard color solution, and record whether the supernatant liquid is lighter, darker, or equal to the standard color solution.

Comparison B — Glass Color Standard Procedure (ASTM C40 Section 9.2)

To define the color of the supernatant liquid more precisely, five mounted glass standards drawn from the Gardner color scale of Test Method D 1544 are used instead. The Gardner numbers are not 1 through 5 — they are 5, 8, 11, 14 and 16, and each corresponds to an organic plate number. Confusing the two numbering systems is one of the most common written-exam errors on this standard.

The comparison procedure is the same as Comparison A, except that the organic plate number nearest the color of the supernatant liquid above the test specimen is what gets reported. When this route is used, no standard color solution needs to be prepared at all — which is why most production laboratories own the glass set.

[!IMPORTANT] Both routes converge on the same threshold. ASTM C40 Section 10.1 states the interpretation once, in terms of both: when a sample produces a color darker than the standard color, or Organic Plate No. 3 (Gardner Color Standard No. 11), the fine aggregate shall be considered to possibly contain injurious organic impurities, and it is advisable to perform further tests before approving it for use in concrete. Note the standard's own hedged language — "possibly contain" and "advisable" — which is why ASTM C40 is a screening test and ASTM C87 is the adjudication.

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Step-by-Step Test Procedure

Adherence to exact sample preparation and timing is paramount:

Step 1: Specimen Preparation (Un-Dried Sand)

• Sample in general accordance with ASTM D75 and reduce a test sample of approximately 450 g (1 lb) in accordance with ASTM C702.

[!CAUTION] Never Oven-Dry the Aggregate: Under no circumstances should the sand sample be oven-dried or heated before testing under ASTM C40! Oven drying at 110°C will volatilize, oxidize, or chemically decompose organic compounds, giving a false "passing" result on a sand that contains harmful organic impurities. The sand must be tested in its as-received, un-dried condition.

Step 2: Filling the Bottle with Fine Aggregate

• Fill the graduated glass bottle with the un-dried fine aggregate to approximately the 130 mL (4-1/2 fl oz) level.

Step 3: Adding the Caustic Reagent Solution

• Pour the 3% sodium hydroxide (NaOH) solution into the bottle until the total volume of aggregate plus liquid reaches approximately the 200 mL (7 fl oz) mark.
• Because the 130-mL sand level is a bulk volume that already contains voids, the volume of solution actually added is larger than the 70-mL difference between the two marks; what the standard controls is the combined 200-mL level of aggregate plus liquid after shaking, which leaves a distinct liquid layer above the settled sand bed.

Step 4: Shaking the Bottle

• Stopper or cap the bottle tightly to prevent leakage.
• Stopper the bottle and shake vigorously. ASTM C40 Section 8.3 prescribes vigorous shaking but does not specify a shaking duration — do not memorize an invented time limit for the written exam.
• Vigorous shaking is what brings every sand grain into contact with the alkaline solution so that soluble organic acids are extracted into the liquid phase. Note that Section 8.2 sets the 200-mL level as the volume of aggregate plus liquid indicated after shaking, so the level is checked once the sand has settled back down.

Step 5: Quiescent Standing Period

• Place the bottle in an upright position on a level shelf in a secure, vibration-free laboratory location at room temperature.
• Allow the bottle to stand undisturbed for 24 h. ASTM C40 states the standing period as 24 hours with no published tolerance band.

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Interpretation of Results and Color Comparison

At the conclusion of the 24-hour quiescent standing period, the heavier sand particles will have settled to the bottom, leaving a layer of supernatant liquid above the sand bed:

1. Bring the bottle and the color reference standards to eye level.
2. Observe the color of the clear supernatant liquid directly above the sand.
3. Compare the color of the liquid against the Organic Plate No. 3 standard (or the freshly prepared potassium dichromate solution):

[!IMPORTANT] A Dark Color Does Not Mean Automatic Rejection: ASTM C40 is a screening test, not a definitive condemnation. Dark colors can occasionally be caused by non-injurious materials (such as small amounts of lignite, coal particles, or iron oxide minerals) that react with NaOH but do not poison cement hydration. Therefore, a sand that fails ASTM C40 may still be used if it passes follow-up strength testing under ASTM C87.

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ASTM C87 / AASHTO T 71: Follow-Up Mortar Strength Acceptance

When a fine aggregate produces a color darker than Organic Plate No. 3, ASTM C33 permits aggregate acceptance only if the sand passes ASTM C87 / AASHTO T 71 (Standard Test Method for Effect of Organic Impurities in Fine Aggregate on Strength of Mortar):

1. Mortar Cube Preparation: Two batches of 2-inch (50-mm) mortar cubes are cast using standard portland cement under standardized water-cement proportions:
   • Test Mortar: Made using the unwashed fine aggregate under investigation.
   • Control Mortar: Made using the same sand after it has been thoroughly washed with a 3% sodium hydroxide solution and rinsed with water until free of organics, or an approved reference control sand.
2. Curing and Compressive Testing: The cubes are moist-cured and tested in compression at 7 days (and optionally 28 days).
3. ASTM C33 Acceptance Criterion:

$\text{Relative Strength} = \left[ \frac{\text{Compressive Strength of Test Mortar}}{\text{Compressive Strength of Control Mortar}} \right] \times 100 \ge 95\%$

If the compressive strength of the mortar made with the unwashed test sand is not less than 95% of the strength of the control mortar, the fine aggregate is accepted for use in concrete despite its failure on the ASTM C40 color screening.

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Laboratory Safety Precautions with Sodium Hydroxide (NaOH)

Sodium hydroxide is an aggressive, caustic, alkaline chemical. In a testing laboratory, technicians must observe strict safety protocols:

• Corrosive Hazard: Solid NaOH and its 3% aqueous solution attack organic tissue through saponification and liquefactive necrosis. Contact with eyes causes rapid, irreversible corneal burns and permanent blindness.
• Personal Protective Equipment (PPE): Technicians must wear chemical splash goggles (or full-face shield), neoprene or heavy nitrile chemical gloves, and a rubber laboratory apron or lab coat. Normal prescription glasses do not provide splash protection.
• Exothermic Solution Reaction: When preparing 3% NaOH, always add sodium hydroxide pellets slowly to water with constant stirring. Adding water to dry pellets can cause explosive local boiling and chemical splattering.
• First Aid / Emergency Measures: In case of contact with skin or eyes, immediately flush the affected area with copious amounts of clean running water at an emergency eyewash or safety shower for at least 15 continuous minutes. Remove contaminated clothing immediately and seek urgent medical attention.