Thermal Resistivity & Conductivity Testing

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MAIL US YOUR SAMPLE AND HAVE RESULTS WITHIN 5 BUSINESS DAYS.

$625/SAMPLE, FOR A FULL DRY OUT CURVE

$375/SAMPLE FOR A SINGLE DATA POINT

FREE SAMPLE MOLDING. (Soil)

Concrete samples accepted from anywhere in the world. Soil samples accepted within US states and territories.

Thermal Resistivity (Rho) Dry Out Curves

Soil Tests Laboratory is a thermal testing company and laboratory for soil, concrete, flowable fill and thermal slurry used in underground cable and duct-bank projects. Thermal resistivity (thermal rho) measures resistance to heat flow; thermal conductivity measures how readily a material conducts heat. The properties are reciprocals when expressed in compatible units. ASTM D5334 describes the thermal needle-probe method, while IEEE 442 provides guidance for thermal resistivity measurements. A dry-out curve shows how thermal resistivity changes with sample moisture content.

Purpose of Thermal Resistivity/Conductivity Testing

The primary purpose of testing the thermal resistivity or conductivity of soil and rocks is to understand how they manage heat. This is important for applications where heat generation, dissipation, or insulation is involved. The Thermal Resistivity of a material changes with moisture content. As the soil dries, the resistivity increases. Soil Tests Laboratory will provide a “Dry out Curve,” as part of the deliverable report. This dry out curve will clearly display how the thermal resistivity of your sample changes with moisture content. Rho testing changes depending upon the material.

We specialize in thermal resistivity testing of soil, concrete, flowable fill, and thermal slurry mixes used in duct banks and cable systems.

Applications of Thermal Resistivity/Conductivity Testing

Underground Power Cable and Duct-Bank Design
- Underground power cables generate heat through electrical losses. Thermal resistivity testing measures how readily surrounding soil or backfill dissipates that heat, providing data for cable ampacity and thermal design. Thermal resistivity is different from electrical resistivity, which is used for grounding-system design.
- Soils with high thermal resistivity (i.e., low conductivity) will slow down heat dissipation, potentially leading to cable overheating. By understanding the thermal properties and rho value of the soil, engineers can choose appropriate materials, design proper burial depths, or use thermal backfill to ensure adequate heat dissipation.
Geothermal Systems (Ground Source Heat Pumps)
- For geothermal systems, heat exchange between the ground and the system is crucial for efficient operation. Thermal resistivity testing helps determine how effectively the soil or rock will transfer heat, influencing the design of ground loops and the overall efficiency of the system.
- Soils or concretes with high thermal conductivity allow for efficient energy transfer between the ground and the heat pump, improving system performance.
Pipelines (Oil, Gas, and Water Pipelines)
- In buried pipelines that transport heated materials (e.g., hot water, oil), understanding soil thermal resistivity helps in determining how much insulation or thermal protection is needed to maintain the required temperature of the transported materials or prevent excessive heat loss.
- Conversely, for cold pipelines (e.g., transporting liquefied natural gas), thermal resistivity testing helps assess whether additional insulation is required to prevent heat from entering the pipeline.
Energy Storage Systems
- In applications involving thermal energy storage, particularly in renewable energy systems or large-scale batteries, understanding the thermal properties and rho value of the ground is essential. It ensures that heat generated during energy storage or discharge is effectively managed, avoiding overheating or inefficiencies.
Construction Projects (Foundations and Embankments)
- In certain construction projects, like road embankments, building foundations, or railways, thermal properties can affect structural integrity, especially in regions with extreme temperature variations or permafrost. Understanding the thermal resistivity of the soil or rocks helps design foundations that remain stable by managing heat flow and freeze-thaw cycles.

How Thermal Resistivity Testing Is Done

The testing typically involves placing a thermal needle probe into the soil or rock to measure the rate of heat flow through the material. The test can be conducted in the field or in the laboratory, depending on the material’s characteristics and the project’s requirements.

Key Steps in Thermal Testing:

Inserting a Heat Source: A heat pulse is generated by a thermal probe inserted into the soil or concrete sample. The heat source is typically a small heater inside the probe.
Measuring Temperature Changes: Sensors in the probe measure how the temperature changes over time, as heat moves through the material.
Analyzing Data: Based on the rate of temperature rise and fall in the sample, thermal resistivity (or conductivity) is calculated. The higher the temperature rise, the greater the thermal resistivity of the material.

The most common method for testing is the Transient Line Source (TLS) or thermal needle probe method. This involves heating a thin needle and measuring the time-dependent temperature increase in the soil or rock around it.

Factors Affecting Thermal Properties of Soils and Concrete

Several factors can affect the thermal resistivity and conductivity of soil and rocks:

Moisture Content: Wet soils generally have lower thermal resistivity (higher conductivity) than dry soils because water conducts heat better than air.
Soil Composition: Soil texture alone does not determine thermal resistivity. Mineral composition, water content, density and pore structure all affect the result. Test representative samples under the conditions specified for the project rather than assuming a value from soil type alone.
Density and Compaction: Dense, compacted soils transfer heat more effectively, leading to lower resistivity and higher conductivity.
Temperature: Thermal properties can change with the temperature of the material, with some materials becoming better or worse heat conductors as temperatures rise.

Who Uses Thermal Resistivity Test Results?

Electrical and Civil Engineers: Thermal resistivity data supports heat-transfer analysis for underground power cables, duct banks and pipelines. Electrical grounding requires separate electrical soil resistivity measurements, such as the Wenner four-pin method.
Geothermal System Designers: Geothermal engineers benefit from understanding soil conductivity when designing systems that exchange heat with the ground for heating or cooling applications.
Construction and Geotechnical Engineers: Knowing the thermal properties of soils and rocks can help in designing stable foundations in regions affected by extreme temperatures or permafrost.
Environmental Consultants: For renewable energy storage or other environmental assessments, thermal resistivity data can help evaluate how heat or cold is stored and transferred in the ground.

Thermal Resistivity of Concrete and Slurries

Soil Tests Laboratory tests concrete, controlled low-strength material (CLSM), flowable fill and thermal slurry mixes used around underground power cables and duct banks. These materials can be designed to meet thermal-performance requirements, but the material name alone does not establish performance.

How should engineered backfill be evaluated?

Evaluate the proposed mix under the project’s specified preparation and moisture conditions, including dry conditions where required. Compare measured results rather than assuming that every concrete or slurry mix outperforms native soil.

What affects thermal performance?

Composition: Aggregate mineralogy and the proportions of aggregate, binder and water affect heat transfer.
Density and air voids: Placement, consolidation and pore structure influence thermal behavior.
Moisture content: Results can change as material dries. A dry-out curve characterizes that change for the tested sample.
Sample preparation: Preparation and conditioning should represent the project specification and be recorded with the results.

Single-point testing or a dry-out curve?

A single-point result describes the sample at a stated condition. A dry-out curve provides measurements at multiple moisture contents. Select the test scope with the design engineer so the reported conditions match the intended cable or duct-bank design inputs.

Planning a test

Send the material type and project testing specification when requesting a quote. Confirm sample preparation, density or compaction requirements, moisture conditions and reporting needs with the laboratory before shipping.