Oxygen Permeability Test of Concrete

The oxygen permeability test is a durability test used to assess the permeability of hardened concrete to gas. It provides an Oxygen Permeability Index (OPI), which is particularly useful for evaluating the quality and durability of the concrete cover.

Reference:

IS 516 (Part 2/Sec 3): 2022, Hardened Concrete — Methods of Test: Oxygen Permeability Index.

The test is applicable to concrete having a maximum nominal aggregate size of 20 mm.

Principle of the Test

The test determines the gas permeability of concrete using oxygen. The Oxygen Permeability Index is defined as the negative logarithm of Darcy’s coefficient of permeability.

A 30 ± 2 mm thick oven-dried concrete disc, representing the cover concrete, is mounted in a falling-head permeability apparatus. Oxygen is introduced under pressure on one face of the specimen and the pressure decay with time is recorded.

A lower gas permeability corresponds to a higher OPI, indicating a denser and less permeable concrete.

Engineering Significance

Gas permeability is strongly influenced by the concrete microstructure, moisture condition and characteristics of the permeating gas. The oxygen permeability test can distinguish the effects of water-cement ratio, binder type and curing conditions. The OPI has also been reported in the standard to correlate with Torrent air permeability and accelerated carbonation depth.

The test is particularly useful for assessing the outer/cover concrete, including:

• Degree of compaction
• Interconnectedness of pore structure
• Presence of bleed voids and channels
• Overall micro- and macrostructure of the concrete surface.

Because oxygen does not react with concrete during the test, the pore structure is not chemically modified by the permeating gas.

Apparatus

The principal equipment includes:

• Water-cooled diamond-tipped core barrel — 70 mm nominal internal diameter
• Holding device for coring
• Water-cooled diamond saw
• Facing machine, where required
• Oven capable of maintaining 50 ± 2⁰C
• Oxygen permeability cell, approximately 5 L ± 5%
• Compressible rubber collars — 39 Shore A hardness
• Pressure gauge/transducer with 0.5 kPa least count
• Oxygen pressure regulator capable of up to 120 kPa
• Oxygen supply of 99.8% standard grade
• Vernier caliper with 0.02 mm least count
• Desiccator with RH controlled to a maximum of 60%, where required.

The permeability cell is operated in a controlled environment at 27 ± 2⁰C.

Test Specimens

A minimum of four specimens is required.

• From concrete cubes

The specimens consist of:

• Diameter: 70 ± 2 mm
• Thickness: 30 ± 2 mm

Concrete cubes should have minimum dimensions of 100 mm and are normally cored at 28 ± 3 days unless the project specification requires otherwise.

The first 5 mm from the face of the core is removed before preparing the test disc.

• From cast cylinders

Discs can alternatively be obtained from 75 × 150 mm cylinders. The first 5 mm from the cast surface is removed, followed by cutting 30 ± 2 mm thick discs.

• From structural concrete

Where the permeability of in-situ concrete is required, cores may be extracted directly from the structure. Importantly, coring should be perpendicular to the direction of casting to minimize the influence of bleed channels on the permeability measurement.

Specimen Conditioning

The specimens are oven-dried at:

50 ± 2⁰C for 7 days ± 4 h

After conditioning, they are cooled to 27 ± 2⁰C in a desiccator or in a room maintained at 27 ± 2⁰C with RH < 60%.

Test Procedure

After conditioning:

1. Measure the diameter and thickness of each specimen at four equally spaced locations around its perimeter.
2. Record dimensions to the nearest 0.02 mm.
3. Place the specimen inside the compressible rubber collar with the test face flush with the collar lip.
4. Insert the assembly into the rigid sleeve.
5. The sleeve internal diameter is:
   • 105 mm for 70 mm core specimens
   • 110 mm for 75 mm cylinder specimens.
6. Ensure an airtight fit using suitable sealing materials where necessary.
7. Place the specimen assembly in the permeability chamber and tighten the cover plate.

Oxygen purging and pressurization

Before testing, purge the cell with oxygen by opening both inlet and outlet valves for at least 5 seconds to remove other gases.

The valves are then operated to establish an initial pressure of:

P₀ = 100 ± 5 kPa

Record:

• Initial time, t_₀, to the nearest minute
• Initial pressure, P₀, to the nearest 0.5 kPa.

The test continues for:

6 h ± 15 min

or until the pressure falls to:

50 ± 2.5 kPa

whichever occurs first.

At least eight instantaneous pressure readings, Pₜ, must be obtained during the test.

Leakage Check

Leakage can seriously affect the result.

If the pressure drops at a rate greater than 5 kPa/min, leakage in the test arrangement is indicated and must be rectified before testing.

The permeability cell should also be periodically checked using an impermeable specimen. The cell is considered acceptable when pressure does not drop by more than 0 kPa over 24 h, starting from 100 kPa.

Calculation of Permeability

A linear regression is performed using:

ln(P_t​/P₀​​)

against time, with the regression line constrained to pass through the origin.

The slope z is:

z=∑[ln(P₀​/P_t​)]²/∑[t ln(P₀​/P_t​)]

where:

• z = slope of the best-fit line, s⁻¹
• P₀ = initial oxygen pressure
• P_t = oxygen pressure at time t.

The regression coefficient must be greater than 0.99. If it is below 0.99, the test is repeated. If the specimen repeatedly fails to achieve the required coefficient, it is discarded.

Darcy’s coefficient of permeability

For each specimen:

k=mVgdz​/RAθ

where:

k = Darcy’s coefficient of permeability, m/s

m = molecular mass of oxygen = 0.032 kg/mol

V = volume of pressurized oxygen in the cell, m³

g = acceleration due to gravity = 9.81 m/s²

d = specimen thickness, m

z = regression slope, s⁻¹

R = gas constant = 8.313 J/(K·mol)

θ = absolute temperature, K

A = cross-sectional area of specimen, m².

The oxygen volume is measured to the nearest 0.00001 m³ and may be determined from the cell dimensions or by measuring the volume of water contained in the cell at 27 ± 2°C.

Oxygen Permeability Index (OPI)

After calculating k for each specimen, the OPI is calculated from the average permeability coefficient:

OPI = -log₁₀ [(k₁ + k₂ + k₃ + …. + k_n)/n]

Thus, higher OPI values indicate lower permeability and better-quality concrete.

Concrete quality classification

Oxygen Permeability Index	Concrete Quality
> 10.0	Very good
9.5–10.0	Good
< 9.5	Poor

These limits are given by IS 516 (Part 2/Sec 3) as an informative classification.

Limitations

The method is:

• Sensitive to macro-voids and cracks
• Sensitive to specimen preparation
• Difficult to perform on concrete having a very dense microstructure, such as high-performance concrete.

Therefore, OPI results should always be interpreted together with specimen condition, preparation method and concrete history.

Test Report

The report should include, for each specimen:

• Darcy’s permeability coefficient k, to three decimal places
• OPI, to two decimal places
• Specimen identification
• Visible cracks, honeycombing or bleed paths.

Where available, also report:

• Specimen source
• Location within the cube, cylinder, core or structural member
• Concrete type, including binder and water-cement ratio
• Curing history
• Any unusual specimen preparation
• Cracks, voids or chipped edges
• Age of concrete at testing.

Conclusion

The oxygen permeability test provides a quantitative indication of the permeability of the concrete cover by measuring oxygen pressure decay through a standardized, oven-dried concrete disc. The measured permeability coefficient k is converted into the Oxygen Permeability Index (OPI), where higher OPI means lower permeability and generally better resistance of the concrete cover to ingress-related durability mechanisms.