Instantly calculate air content percentage of fresh concrete using the gravimetric method (ASTM C138), check acceptance against ACI 318 target ranges, and determine if your air-entrained concrete mix meets US specification — for any aggregate size and exposure class.
Select a method, enter your test data, and instantly calculate air content % with ACI 318 acceptance status.
Weigh filled unit weight measure per ASTM C138 — typical normal-weight concrete: 140–150 lb/ft³
Calculate from mix design: Total batch weight (lb/yd³) ÷ [Absolute volume (ft³/yd³) × 27] — see formula below
Used to look up ACI 318 Table 19.3.3.1 target air content for your exposure class
F2 = bridge decks, driveways, exterior flatwork with deicers · F1 = exterior walls, footings, no deicers
Direct dial reading from Type A or Type B air meter — read before applying aggregate correction factor
From calibration test on aggregate per ASTM C231 Section 10 — typically 0–1% for normal-weight aggregates
Select nominal maximum size of coarse aggregate used in this mix
F2 requires highest air content — F0 does not require air entrainment
Direct reading from the graduated neck of the roll-a-meter (ASTM C173) before alcohol correction
Each cup of alcohol added = add 1% to the scale reading (ASTM C173 Section 10)
ASTM C173 is preferred for lightweight aggregate concrete where C231 pressure method is not valid
Select the exposure class from the project's approved mix design documents
Concrete air content is the total percentage of the concrete volume occupied by air voids — including both intentionally entrained air bubbles from air-entraining admixtures (AEA) and incidentally trapped air. In the United States, air-entrained concrete is required by ACI 318, ACI 301, and virtually every state DOT specification for any concrete exposed to freezing and thawing cycles — which includes all exterior flatwork, pavements, bridge decks, and structures in approximately 40 of the 50 US states. The air content test per ASTM C138 or ASTM C231 is one of the three mandatory acceptance tests for fresh concrete (along with slump and temperature) on most US construction projects.
When water in concrete pores freezes, it expands approximately 9% in volume, generating internal hydraulic pressures that can exceed the tensile strength of the paste matrix. Air-entraining admixtures create billions of microscopic, uniformly distributed air bubbles (0.01–1.0 mm diameter) that act as pressure relief valves — water expelled from freezing capillary pores moves into nearby air voids rather than fracturing the paste. ACI 318 Table 19.3.3.1 specifies minimum total air content from 3.0% to 7.5% depending on nominal maximum aggregate size and exposure class (F1 or F2) — and ASTM C138 / C231 are the field tests used to verify compliance at the point of discharge on every qualifying US concrete pour.
The gravimetric method calculates air content by comparing the measured unit weight of a fresh concrete sample to the theoretical air-free unit weight calculated from mix design batch weights and specific gravities. Per ASTM C138: Air % = [(T − D) ÷ T] × 100, where T = theoretical unit weight and D = measured unit weight. For normal-weight concrete, every 1.5 lb/ft³ decrease in unit weight equals approximately 1% air content — making this a quick field check even without formal calculation.
The pressure method (Type A and Type B meters) is the most widely used US field method for air content — it is fast, portable, and directly reads air percentage on a calibrated gauge. It works on Boyle's Law: a known pressure is applied to the concrete specimen, compressing the air voids, and the change in volume is read as air content. Important limitation: ASTM C231 pressure methods cannot be used for concrete containing lightweight, porous, or slag aggregates — the volumetric method (ASTM C173) must be used instead.
The volumetric method (ASTM C173) is the only US-approved field method for air content testing of concrete containing lightweight aggregate, slag, natural pumice, or other highly porous aggregates where the pressure method gives false high readings. A measured concrete sample is placed in the roll-a-meter, water is added to fill to a calibrated mark, and air is displaced and measured directly by the volume of water required. Alcohol is added to break foam, with 1% added per cup of alcohol used.
The gravimetric air content calculation per ASTM C138 requires calculating the theoretical air-free unit weight from the mix design batch weights and absolute volumes, then comparing to the measured unit weight of fresh concrete. This is the fundamental method used by all US concrete labs, and forms the basis for understanding air content regardless of which field test method is used.
While adequate air content is essential for freeze-thaw durability, excess air content directly reduces concrete compressive strength. The approximate rule per ACI 318 and research data: every 1% increase in air content above the design target reduces 28-day compressive strength by approximately 5%. For a 4,000 PSI design mix, air content that is 3% above target (e.g. 9% measured vs. 6% design) could reduce strength to approximately 3,400 PSI — potentially causing cylinder test failures. This is why the ±1.5% tolerance exists and why consistently measuring and controlling air content is critical on every US concrete project.
The table below shows the ACI 318 Table 19.3.3.1 required total air content ranges for air-entrained concrete by nominal maximum aggregate size and exposure class. These are the target values used on every US DOT, bridge, pavement, and exterior concrete project requiring air entrainment. The acceptance tolerance for most US DOT specifications and ACI 301 is ±1.5% of the design air content value.
| Nominal Max Agg Size | F2 — Severe Exposure (%) | F1 — Moderate Exposure (%) | F0 — No Exposure (%) | Typical Use — Severe (F2) | ACI 318 Reference |
|---|---|---|---|---|---|
| 3/8 in. (9.5 mm) | 7.5% | 6.0% | Not Required | Pavements, bridge decks with deicers | Table 19.3.3.1 |
| 1/2 in. (12.5 mm) | 7.0% | 5.5% | Not Required | Driveways, exterior flatwork | Table 19.3.3.1 |
| 3/4 in. (19 mm) | 6.0% | 5.0% | Not Required | Walls, foundations, common flatwork | Table 19.3.3.1 |
| 1 in. (25 mm) | 6.0% | 4.5% | Not Required | Structural members, footings | Table 19.3.3.1 |
| 1½ in. (37.5 mm) | 5.5% | 4.5% | Not Required | Mass concrete, large footings | Table 19.3.3.1 |
| 2 in. (50 mm) | 5.0% | 4.0% | Not Required | Dam fills, very large mass pours | Table 19.3.3.1 |
Concrete temperature significantly affects air content — warmer concrete loses air. Air content decreases approximately 0.5–1.0% for every 10°F increase in concrete temperature above 70°F. On hot summer pours in US southern states, air content at the batch plant may be set 1–2% higher than the specification target to account for air loss during transit. Always test air content at the point of discharge (truck chute), not at the batch plant.
Air entrainment builds up with mixing. Ready-mix trucks typically require 70–100 revolutions at mixing speed to develop proper air content after adding AEA. Over-mixing (more than 300 revolutions total, or more than 90 minutes per ASTM C94) can collapse air void structure, reducing air content below specification. The air content of the first few cubic yards discharged from a ready-mix truck is often different from the main load — always test the middle portion of the discharge per ASTM C172 sampling requirements.
Internal vibration during concrete placement expels entrapped (large) air voids but should not significantly reduce entrained (micro-bubble) air if done correctly. Over-vibration — leaving the vibrator in one spot too long or vibrating air-entrained concrete with a high-frequency vibrator at low amplitude — can destroy the micro-air bubble structure. US best practice is to use immersion vibrators at 6–8 second insertions, 18-inch spacing, to consolidate without destroying air entrainment.
In the USA, concrete air content testing (ASTM C138 and C231) is typically performed by personnel certified through the ACI Concrete Field Testing Technician — Grade I program, or equivalent state DOT certification. Most US state DOT projects, federal-aid highway projects, and commercial construction quality control programs require air content tests to be conducted and signed by an ACI Grade I or higher certified technician to be accepted by the engineer of record.
ASTM C138 and C231 require that the air content test begin within 5 minutes of obtaining the composite sample per ASTM C172. Air content begins decreasing as soon as concrete is sampled due to ongoing hydration and CO₂ absorption. On summer pours above 85°F, this 5-minute limit is critical — a 10-minute delay can result in a reading 0.5–1.0% lower than the actual discharge air content, potentially causing a false rejection of a compliant load. Keep test equipment staged and ready before each truck arrival on high-volume DOT pours.
The table below compares the three ASTM-approved field methods for measuring air content of fresh concrete in the USA, covering applicable aggregate types, accuracy, equipment, time required, and when each method is required or preferred by US specifications.
| Test Method | ASTM Standard | Principle | Aggregate Suitability | Field Time | Accuracy / Notes |
|---|---|---|---|---|---|
| Gravimetric | ASTM C138 | Unit weight vs. theoretical air-free weight | All aggregates — universal | 10–15 min | Requires accurate mix design batch weights · Also gives yield |
| Pressure (Type A) | ASTM C231 | Boyle's Law — volume change under pressure | Normal-weight only — no LWA | 5–10 min | Direct gauge reading · Fastest common method in the field |
| Pressure (Type B) | ASTM C231 | Boyle's Law — air chamber pressurized | Normal-weight only — no LWA | 5–10 min | Most common US field meter · Calibrated dial reading |
| Volumetric (Roll-a-Meter) | ASTM C173 | Water displacement of air voids | All aggregates — including LWA & slag | 15–20 min | Required for LWA concrete · Alcohol addition for foam |
The ASTM C231 pressure method (Type A and Type B air meters) cannot be used for concrete containing lightweight aggregate, blast furnace slag, pumice, natural zeolite, or other porous aggregates. These porous aggregates compress under pressure like air voids, giving a falsely high air content reading that can mask a true air deficiency in the mix. For any concrete mix using lightweight aggregate (e.g. Norlite, Buildex, Solite) or slag aggregate, the ASTM C173 volumetric (roll-a-meter) method is the only valid US test method. This is specified in ASTM C231 Section 1.2 and must be observed on all US project specifications.
Official ASTM standards, ACI codes, and technical guides for air content testing in the United States
Official source for ASTM C138 (Unit Weight & Air — Gravimetric), C231 (Pressure Method — Type A & B), C173 (Volumetric — Roll-a-Meter), and C457 (Microscopical Determination of Air Void System) — the governing standards for all concrete air content testing on US construction projects nationwide.
View ASTM Concrete StandardsACI 318 — Building Code Requirements for Structural Concrete — is the primary US structural design code, Table 19.3.3.1 specifies required air content by exposure class and aggregate size for all US air-entrained concrete. ACI 301 (Specifications for Structural Concrete) provides the ±1.5% acceptance tolerance used on commercial and institutional projects across all 50 US states.
View ACI 318 CodeThe ACI Concrete Field Testing Technician — Grade I certification is the industry-standard qualification for US concrete field testing personnel performing ASTM C138, C231, C173, C143, and C1064 tests. Required on most US DOT, federal-aid highway, and commercial structural concrete projects — accepted in all 50 US states as proof of testing competence.
View ACI Certifications