An asphalt mixture can look well compacted and still leave important questions about its mechanical performance. How does it resist tensile failure? Does moisture conditioning change its strength? Are differences between specimens caused by the mixture or the testing process?

These questions matter when laboratories compare materials and investigate pavement performance.

Indirect Tensile Strength Of Asphalt is a calculated measure of tensile resistance obtained by loading a cylindrical asphalt specimen across its diameter until failure. Although the machine applies compression, the loading arrangement generates tensile stress within the specimen.

The test is useful, but its results need context. Temperature, specimen preparation, dimensions, conditioning, and loading conditions all belong in the interpretation.

What Is Indirect Tensile Strength Of Asphalt?

Indirect tensile strength, commonly called ITS or IDT strength, describes an asphalt specimen’s resistance to tensile failure under a defined diametral loading arrangement.

A cylindrical specimen is positioned on its curved edge between suitable loading strips. Compression is applied across a diameter, generating tensile stress perpendicular to the loading direction.

The maximum recorded load and specimen dimensions are used to calculate the strength.

ASTM D6931 covers indirect tensile strength testing of laboratory-fabricated asphalt specimens and field-recovered cores. The standard identifies the results as useful for evaluating relative mixture quality alongside other testing. 

It is a mixture property measured under specified conditions—not the tensile strength of the asphalt binder alone.

Why Is the Test Called “Indirect”?

In a direct tensile test, a specimen is pulled apart through an appropriate gripping arrangement.

In the indirect test, the loading machine compresses the specimen across its diameter. The resulting internal stress distribution allows tensile resistance to be calculated from the failure load.

This arrangement avoids directly gripping and pulling the asphalt specimen.

However, the indirect result remains tied to the method’s assumptions and conditions. It should not automatically be treated as equivalent to a direct tensile strength result obtained through a different procedure.

Why Do Laboratories Measure Asphalt ITS?

Comparing Mixtures

Laboratories can compare candidate mixtures tested under consistent conditions.

For example, a programme may investigate a change in aggregate source, binder content, or treatment. The test plan should distinguish the intended variable from differences introduced during preparation.

Investigating Moisture Sensitivity

Conditioned and unconditioned specimen groups can be compared using a prescribed moisture-susceptibility procedure.

The comparison examines retained tensile strength following the selected conditioning.

Evaluating Field-Recovered Material

Suitable pavement cores may be tested where the method permits.

Their sampling locations, dimensions, condition, and preparation history should remain traceable.

Supporting a Wider Assessment

ITS is one measurement within a broader material evaluation.

A high result alone does not establish resistance to fatigue, rutting, thermal cracking, and every other pavement distress mechanism.

What Equipment Is Needed?

A complete setup generally requires:

• A loading machine suitable for the specified rate and expected forces.

• Compatible indirect tensile loading strips or fixtures.

• Appropriate load measurement and recording.

• Equipment for measuring specimen dimensions.

• Temperature-conditioning facilities.

• Additional conditioning equipment where moisture sensitivity is investigated.

The machine, fixture, and conditioning workflow must work together.

Before purchasing, identify the exact test methods and specimen sizes. A frame that can apply sufficient force may still lack the necessary control, fixture compatibility, or recording capability.

How Is Indirect Tensile Strength Calculated?

The commonly used expression is:

ITS=2PDt

Where:

For an illustrative specimen:

• Maximum load: 10,000 N

• Diameter: 100 mm

• Thickness: 63.5 mm

ITS=2(10,000)(100)(63.5)1.00MPa

This example explains the calculation. It does not define an acceptable strength or prescribe specimen dimensions for every programme.

Use the governing procedure for dimensional measurements, test conditions, calculation conventions, and reporting.

How Is the Test Performed?

1. Prepare and Identify Specimens

Prepare specimens or field cores according to the chosen method.

Record their origin, mixture identification, preparation history, and condition. Assign identifiers that remain linked to subsequent measurements.

2. Measure Dimensions and Relevant Properties

Measure diameter and thickness using the prescribed procedure.

Where required, determine density and air-void information before assigning specimens to comparison groups.

3. Complete Conditioning

Bring specimens to the required test condition.

For moisture-sensitivity work, follow the selected conditioning protocol rather than using an informal soaking period.

4. Align the Specimen

Position the specimen correctly between the loading strips.

Check alignment and contact before starting the loading sequence.

5. Load, Calculate, and Inspect

Apply the specified loading procedure, record the maximum load, and calculate ITS.

Document the failure appearance and any unusual behaviour before accepting the result.

Understanding Tensile Strength Ratio

Tensile strength ratio, or TSR, compares the average strength of conditioned specimens with the average strength of an appropriate unconditioned group:

TSR=AverageconditionedITSAverageunconditionedITS100

For example, conditioned strength of 0.80 MPa and unconditioned strength of 1.00 MPa give a TSR of 80%.

That percentage requires interpretation under the applicable specification.

FHWA’s moisture-susceptibility protocol describes dividing specimens into groups, conditioning one group, and testing both in indirect tension. The conditioning procedure is an essential part of the comparison. 

Report the individual strengths as well as the ratio. Similar ratios can arise from groups with substantially different absolute strengths.

7 Mistakes That Can Make Results Misleading

1. Comparing Different Test Temperatures

Asphalt response depends on test conditions.

A result obtained at one temperature should not be compared casually with another obtained under different conditions.

Identify the specified temperature, conditioning procedure, and permitted handling time. Keep those details visible in the report.

Avoid adopting a temperature simply because it was used for the laboratory’s previous programme.

2. Using Incorrect Specimen Dimensions

Diameter and thickness appear directly in the calculation.

Measure each specimen rather than copying nominal dimensions into every record. Verify units before calculating.

If a specimen does not satisfy the method’s dimensional requirements, resolve that issue before testing rather than relying on a spreadsheet to compensate.

3. Overlooking Loading-Strip Alignment

Poor positioning can alter the loading arrangement.

Check that the specimen and strips are compatible and correctly seated. Inspect contact surfaces and replace damaged components as required.

Record abnormal failures or movement during loading. Do not assume every peak load represents a valid test.

4. Changing Preparation Between Comparison Groups

Comparisons become difficult when specimen production varies without explanation.

Use documented preparation and grouping procedures. Retain the density, air-void, conditioning, and dimensional information required by the method.

For moisture comparisons, the groups should be established according to the protocol—not selected afterward to produce a favourable ratio.

5. Confusing Strength with Other Indirect-Tension Properties

The same general loading geometry can support different investigations.

FHWA documentation distinguishes procedures for creep compliance, resilient modulus, and tensile strength. These require their own loading and measurement arrangements. 

Owning an indirect-tension fixture does not automatically provide every related test capability.

6. Treating the Highest ITS as the Best Mixture

Peak strength answers a specific question under defined conditions.

It does not fully describe cracking progression, repeated-load response, or performance across temperatures.

Compare ITS with the other measurements required by the design or evaluation framework. Avoid ranking mixtures from one number without considering the investigation objective.

7. Applying a Universal Acceptance Limit

There is no single ITS or TSR threshold suitable for every project.

Use the governing specification, test method, and acceptance framework.

State those requirements before testing begins. This reduces the risk of interpreting a result against an unrelated threshold after the data are available.

Choosing Equipment for Your Laboratory

Begin with the required method and outputs.

Confirm specimen dimensions, anticipated loads, loading control, temperature conditioning, fixture geometry, and data-recording requirements.

Ask whether the system retains the load history and allows export of the original measurements.

Also review calibration documentation, maintenance access, software licensing, and training.

A useful demonstration should follow the complete workflow—from specimen positioning to the final calculation—rather than showing only that the machine can apply force.

Discussing Your Requirements with Avantech

When approaching Avantech about Indirect Tensile Strength Of Asphalt testing, provide the selected methods, specimen types, expected throughput, conditioning requirements, and reporting needs.

Avantech’s advanced pavement testing listings include indirect tension and creep-compliance equipment. Confirm the proposed configuration against your specific strength-testing procedure and required accessories. 

Request an itemised proposal covering the frame, fixtures, instrumentation, conditioning facilities, software, commissioning, and support.

Frequently Asked Questions

What is Indirect Tensile Strength Of Asphalt?

It is tensile resistance calculated from the maximum diametral compression load and the dimensions of a cylindrical asphalt specimen.

Is ITS the same as TSR?

No. ITS is a strength value. TSR compares average conditioned and unconditioned strengths as a percentage.

Can pavement cores be tested?

Yes, where their preparation and condition satisfy the applicable method.

Does ITS directly predict pavement life?

No. It contributes information to a wider assessment and does not independently establish service life.

Can Marshall stability equipment perform the test?

Some suitable machines can accommodate indirect-tension fixtures. Confirm loading control, measurement capability, and fixture compatibility before use.