Understanding the Texas Overlay Test: What It Is and Why It Matters

May 23, 2025

When it comes to evaluating pavement performance, few tests are as critical or insightful as the Texas Overlay Test (TOT). Originally developed by the Texas Department of Transportation (TxDOT), this test has become a key standard in assessing the crack resistance of asphalt mixtures — especially for roads subjected to heavy traffic and extreme temperature variations.

In this blog post, we’ll break down what the Texas Overlay Test is, how it works, and why it’s essential for ensuring long-lasting roadways.

What Is the Texas Overlay Test?

The Texas Overlay Test is a mechanical fatigue test used to evaluate an asphalt mix’s resistance to cracking under repeated loading. It simulates the conditions experienced by pavement when vehicles, especially trucks, pass over it again and again.

The core idea is simple: determine how many cycles an asphalt sample can withstand before cracking, giving a direct measurement of fatigue life and crack propagation resistance.

Traditional asphalt testing methods often failed to capture real-world fatigue behavior. Roads were cracking prematurely, leading to expensive repairs and reduced safety. The Texas Overlay Test emerged as a way to:

Better mimic actual pavement conditions
Understand how different materials respond to cyclic tensile stresses
Optimize asphalt mix designs for performance, not just cost

How the Texas Overlay Test Works

A rectangular asphalt specimen (typically 150 mm × 75 mm × 38 mm) is either molded or sawed from a compacted slab, then bonded between two steel plates using high-strength epoxy. This assembly is placed into a mechanical test frame configured for cyclic fatigue loading.

The setup includes:

An actuator that applies cyclic vertical displacement of ±0.315 mm (total stroke of 0.63 mm), simulating crack opening and closing
A cycle frequency of 0.1 Hz, with the lower plate held stationary
Test temperature maintained at 25°C (77°F) to replicate service conditions

As the actuator repeatedly displaces the upper plate, the system records the applied force and resulting displacement throughout the test. The asphalt specimen experiences tensile stresses similar to those caused by real-world traffic loads.

Failure Criteria

The test continues until one of the following occurs:

The peak load degrades by 93% compared to the first cycle, indicating substantial crack propagation
A pre-set maximum number of cycles is reached (typically 1000 cycles)

Key Output Metrics

The Texas Overlay Test provides several critical indicators:

Maximum Load – The peak tensile force recorded during the test
Cycles to Failure – Total number of cycles endured before meeting the failure criteria
Critical Fracture Energy – Calculated as:

G_c = ^W⁄_{b · h}

Where:

- W = total energy dissipated (area under load–displacement curve),
- b = specimen width,
- h = specimen height

Crack Progression Rate (n) – Derived by fitting a power-law curve to the decline of peak load vs. number of cycles:

These measurements deliver insight into:

Fatigue performance of different asphalt mixtures
Resistance to reflective cracking, especially over rigid or stabilized bases
Impact of material innovations, such as modified binders or high RAP content

As transportation agencies prioritize durability, cost-efficiency, and sustainability, the Texas Overlay Test remains a vital tool for material selection and pavement design validation.

Conclusion

Cracking remains one of the most critical failure modes in asphalt pavements and understanding it starts with proper testing. The Texas Overlay Test provides a fast, effective way to quantify crack resistance, helping engineers design longer-lasting roads and optimize material selection for performance and durability.

For equipment manufacturers developing the Texas Overlay Testers, the TACTUN platform offers a powerful advantage. TACTUN enables seamless control of test machinery through its industrial controller and no-code software builder, allowing manufacturers to:

Configure and automate test sequences with precision
Collect high-resolution force-displacement data in real time
Generate fully branded, Windows-based software installers for different test setups
Adapt easily to various machine configurations or actuator types with custom controller boards

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