Concrete Materials ASTM 2023 Edition

ASTM C33-23 — Standard Specification for Concrete Aggregates

The governing US specification for fine and coarse aggregates used in concrete, covering grading limits, deleterious substance limits, soundness, abrasion resistance, alkali-silica reactivity, and organic impurity testing requirements.

Scope

ASTM C33-23 covers the quality and grading requirements for both fine and coarse aggregates used in portland cement concrete. The specification applies to natural aggregates (gravel, sand, crushed stone), manufactured aggregates (crushed rock), and recycled aggregates meeting certain quality criteria. Fine aggregate is defined as material passing the 9.5 mm (3/8 in) sieve and almost entirely passing the 4.75 mm (No. 4) sieve, with the bulk passing the 2.36 mm (No. 8) sieve and retained on the 75 μm (No. 200) sieve. Coarse aggregate is defined as material retained on the 4.75 mm sieve. The standard provides grading limits for fine aggregates (Table 1) and for eight standard size numbers of coarse aggregates (Table 2, No. 1 through No. 8). It also limits deleterious substances including clay lumps, friable particles, coal and lignite, and materials finer than 75 μm. Soundness and abrasion resistance requirements are specified through the sodium or magnesium sulfate soundness test and the Los Angeles abrasion test.

Purpose

The purpose of ASTM C33 is to ensure that aggregates used in concrete are of sufficient quality to produce workable, durable, and strong concrete. Proper aggregate grading controls the workability, water demand, and economy of concrete mixes — well-graded aggregates require less cement paste to fill voids, resulting in more economical and lower-shrinkage concrete. Deleterious substance limits protect against materials that can impair strength, durability, or appearance. Soundness and abrasion limits guard against aggregates that would deteriorate under freeze-thaw cycles or during handling and placement. The alkali-silica reactivity (ASR) provisions prevent damaging expansion due to reaction between certain siliceous minerals in aggregates and alkalis in cement.

[FIGURE — Aggregate gradation curve showing cumulative percent passing by sieve size for fine aggregate (0.075–4.75 mm) and coarse aggregate (4.75–37.5 mm) with upper and lower grading limits per ASTM C33]

Engineering Applications

Structural concrete — fine and coarse aggregates meeting ASTM C33 for all reinforced concrete elements
Pavement concrete — aggregate quality for concrete pavements subject to abrasion and freeze-thaw exposure
Precast concrete — controlled grading for consistent finish and strength in precast elements
High-performance concrete — optimised aggregate grading and quality for high-strength and low-permeability mixes
Mass concrete — larger coarse aggregate sizes (No. 1 or No. 2) to reduce cement content and heat generation
Self-consolidating concrete (SCC) — careful grading control with limited coarse aggregate content for flowability
Lightweight concrete — lightweight aggregates meeting supplementary requirements of ASTM C330

Design Philosophy

ASTM C33 follows a prescriptive specification approach, defining acceptable limits for aggregate properties rather than performance criteria. The grading limits are based on the principle of maximum density (Fuller-Thompson curve) modified for workability requirements. A well-graded aggregate with particles distributed across the full range of sizes requires less cement paste because smaller particles fill the voids between larger particles. The fineness modulus (FM) of fine aggregate provides a single-number index of grading that correlates with concrete workability — a typical FM of 2.3 to 3.1 corresponds to fine aggregate suitable for most structural concrete. Deleterious substance limits are based on empirical evidence linking specific contaminants to concrete deterioration: clay lumps and friable particles weaken the aggregate-paste bond, coal and lignite create popouts, and organic impurities interfere with cement hydration.

Important Requirements

Fine aggregate grading must conform to the limits in Table 1: 100% passing 9.5 mm, 95-100% passing 4.75 mm, 80-100% passing 2.36 mm, 50-85% passing 1.18 mm, 25-60% passing 600 μm, 10-30% passing 300 μm, and 2-10% passing 150 μm. The fineness modulus must not vary more than 0.20 from the design value. Materials finer than 75 μm (No. 200) are limited to 3% for concrete subject to surface wear, 5% for other concrete, and 7% for concrete not subject to surface wear if the fine aggregate is from a source with proven field performance. For coarse aggregates, the maximum limits for deleterious substances vary by size number and intended use. Clay lumps and friable particles are limited to 3-10% depending on aggregate type. Coal and lignite are limited to 0.5-1.0% depending on concrete appearance requirements. Soundness loss is limited to 10% (Na₂SO₄) or 15% (MgSO₄) after five cycles, and LA abrasion loss is limited to 50%.

Warning:

Aggregate sources that fail the soundness or abrasion requirements may produce concrete with poor freeze-thaw durability or excessive wear under traffic. For concrete pavements and bridge decks exposed to de-icing salts, specify the optional freeze-thaw test (ASTM C666) for the concrete itself, not just the aggregate soundness test. The ASR expansion limit of 0.10% at 16 days (ASTM C1260) is critical when using potentially reactive aggregates with high-alkali cement.

Key Parameters

Fine Aggregate Grading Limits

Sieve Size	Percent Passing
9.5 mm (3/8 in)	100
4.75 mm (No. 4)	95–100
2.36 mm (No. 8)	80–100
1.18 mm (No. 16)	50–85
600 μm (No. 30)	25–60
300 μm (No. 50)	10–30
150 μm (No. 100)	2–10

Coarse Aggregate Size Numbers

Size No.	Nominal Size (mm)	Typical Application
1	90 to 37.5	Mass concrete, large structural sections
2	63 to 37.5	Mass concrete, heavy foundations
3	50 to 25.0	Heavy structural concrete
4	37.5 to 19.0	General structural concrete, pavements
5	25.0 to 12.5	Normal-weight structural concrete, slabs
6	19.0 to 9.5	Thin sections, precast, reinforced concrete
7	12.5 to 4.75	Thin slabs, architectural concrete
8	9.5 to 2.36	Very thin sections, topping, decorative concrete

Fineness Modulus:  FM = Σ(cumulative % retained on No. 4, 8, 16, 30, 50, 100) / 100

Typical FM range for concrete sand:  2.3 to 3.1

Los Angeles Abrasion:  Loss (%) = (initial mass − final mass) / initial mass × 100

Maximum LA loss for concrete aggregates:  ≤ 50%

Soundness (Na2SO4):  Max weighted loss ≤ 10% after 5 cycles

Soundness (MgSO4):  Max weighted loss ≤ 15% after 5 cycles

ASR Expansion (ASTM C1260):  Max expansion ≤ 0.10% at 16 days

Practical Engineering Notes

Note:

The fineness modulus (FM) is a critical parameter for concrete mix proportioning. A low FM (below 2.3) indicates a fine sand that increases water demand and may produce sticky concrete. A high FM (above 3.1) indicates a coarse sand that can cause segregation and harshness. Most structural concrete is designed for FM between 2.6 and 2.9. If the available sand falls outside this range, blending two sources or adjusting the coarse aggregate proportion can optimise the combined grading.

Field Tip:

The simplest field quality check for fine aggregate is the organic impurities test (ASTM C40): a 75 mL sample is placed in a glass bottle with 130 mL of 3% NaOH solution, shaken, and allowed to stand for 24 hours. A colour darker than the standard colour plate indicates possible organic contamination, which requires further testing (ASTM C87 mortar strength test). Always stockpile aggregates on a well-drained, clean surface to avoid contamination from the subgrade.

Typical Workflow

Identify aggregate sources and obtain representative samples per ASTM D75 or C702.
Perform sieve analysis (ASTM C136) for fine and coarse aggregates; compute FM for fine aggregate.
Conduct specific gravity and absorption tests (ASTM C127 for coarse, C128 for fine).
Determine deleterious substance content (clay lumps, friable particles, coal, lightweight materials, No. 200 wash).
Perform soundness test (ASTM C88) and LA abrasion test (ASTM C131/C535) if required.
Verify ASR potential using ASTM C1260 (accelerated mortar bar) or C1567 with SCMs.
Design the concrete mix with verified aggregate properties using Concrete Mix Design Calculator.
Establish acceptance testing frequency per project specifications (typically 1 test per 200-500 m³ of concrete).

Common Mistakes

Ignoring FM variability — sand FM from the same source can vary by season due to processing changes; excessive FM variation changes water demand and concrete yield.
Using surface-dry aggregate weights for batching — aggregate moisture content (free surface moisture) must be measured daily and batched weights adjusted accordingly.
Not accounting for bulking of fine aggregate — surface moisture on fine aggregate causes bulking (volume increase up to 30% for very fine sand), leading to under-yield in volumetric batching.
Over-reliance on soundness tests — the sulfate soundness test does not always correlate with field freeze-thaw performance; concrete freeze-thaw testing (ASTM C666) is more reliable.
Neglecting ASR testing for new sources — even aggregates from established sources can vary; test for ASR potential annually or when the quarry face changes significantly.

Best Practices

Blend aggregates with different gradations to achieve a combined grading curve close to the 0.45 power maximum density curve for optimal workability and economy.
When using recycled concrete aggregates, test for contaminants (gypsum, brick, asphalt, chlorides) and verify compliance with the supplementary requirements for recycled materials.
Specify a maximum FM range of ±0.15 from the design value to maintain consistent concrete properties throughout the project.
Use the Sieve Analysis Calculator for automated gradation analysis and FM computation.
For high-strength concrete above 50 MPa, limit coarse aggregate maximum size to 12.5-19 mm and require LA abrasion loss below 40% for better aggregate-paste bond.

Limitations

ASTM C33 does not cover lightweight aggregates (ASTM C330), heavy-weight aggregates (ASTM C637), or aggregates for use in radiation shielding.
The specification does not address aggregate shape and texture (angularity, sphericity, surface texture), which significantly affect concrete strength and workability.
No limits are provided for aggregate chloride content, which may be important for prestressed concrete or epoxy-coated reinforcement applications.
The LA abrasion limit of 50% is not adequate for all applications; for concrete pavements and heavy-wear surfaces, specify a more restrictive limit (typically 40% or lower).

Related CivilFlow Calculators

Related Formulas

See the Concrete Design Formulas section for aggregate properties, fineness modulus, and concrete mix design equations.

Related Handbook Chapters

Refer to the Engineering Handbook for concrete materials guidance including aggregate selection, mix design, and quality control.

Related Learn Pages

Related Glossary Terms

Visit the Glossary for definitions of fineness modulus, gradation, deleterious substances, soundness, ASR, and other aggregate testing terms.

References

ASTM C33-23, "Standard Specification for Concrete Aggregates," ASTM International, 2023.
ASTM C136-19, "Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates," ASTM International, 2019.
ASTM C88-13, "Standard Test Method for Soundness of Aggregates by Use of Sodium Sulfate or Magnesium Sulfate," ASTM International, 2013.
ASTM C131-20, "Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine," ASTM International, 2020.
ASTM C1260-21, "Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)," ASTM International, 2021.
Kosmatka, S.H. and Wilson, M.L., "Design and Control of Concrete Mixtures," 16th Ed., PCA, 2016.