Dry-Out Curve Testing vs On-Site Thermal Resistivity: Which Provides Accurate Data?
For engineers designing data centers, electrical infrastructure, and underground utilities, accurate thermal resistivity measurements are essential. Two common approaches exist: laboratory dry-out curve testing and on-site (in-situ) testing. Understanding the differences can mean the difference between a safe, efficient design and one prone to overheating or failure.
What Is Thermal Resistivity?
Thermal resistivity (ρ) measures how well a material resists heat flow. Materials with low resistivity conduct heat efficiently, while high resistivity slows heat transfer.
For soil, concrete, and engineered slurry backfill:
Low resistivity allows heat from underground cables, data centers, or transformers to dissipate efficiently.
High resistivity traps heat, reducing cable capacity, causing equipment stress, and increasing cooling costs.
Dry-Out Curve Testing: The Laboratory Standard
The dry-out curve shows how thermal resistivity increases as moisture content decreases. This is especially important for soil, concrete, or slurry backfill used in critical electrical or data center projects.
Laboratory dry-out curve testing typically follows ASTM D5334 or IEEE 442 standards:
Collect undisturbed samples from soil, concrete, or slurry.
Gradually reduce moisture under controlled lab conditions.
Measure thermal resistivity at each moisture level.
Plot the results to create a dry-out curve, providing engineers with a complete profile of heat transfer capability over time.
Benefits of dry-out curve testing:
Predictable and repeatable data
Full moisture range coverage
Applicable to concrete and slurry, which cure and dry over time
Compliance with ASTM D5334 and IEEE 442 standards
On-Site (In-Situ) Testing: Quick but Limited
In-situ thermal resistivity testing provides a snapshot of current conditions but has significant limitations:
Captures only current moisture content
Subject to environmental variables like temperature, sun exposure, or rain
Non-uniform materials (soil layers, concrete aggregates, slurry) can skew results
Cured concrete is difficult to test reliably in-situ
Example from the field:
“We once ran on-site resistivity tests on the existing trench soil across multiple data center trench locations. Trenches had been dug the evening prior. The next day was hot, and therefore the soil on the trench walls began drying out. Readings taken directly from the trench walls changed by nearly 25% when comparing morning data to evening data .”
-Nolan Ferguson PE
This illustrates why lab-based dry-out curve testing is preferred for engineers designing critical infrastructure.
Why On-Site Conductivity Testing for Concrete or Slurry Is Poor for QC
Some contractors or engineers may attempt on-site thermal conductivity testing of concrete as part of a quality control (QC) check. In practice, this approach is virtually ineffective:
Freshly poured concrete: Conductivity can be measured immediately after pouring. At this stage, the concrete has high water content, which artificially reduces thermal resistivity. Any reading taken at this stage do not reflect the properties of the concrete once it cures, so it provides little meaningful QC data.
Cured concrete: Testing hardened concrete in-situ is often impractical or inconsistent, especially for floors, vaults, or duct banks already in place. Variations in mix, compaction, and ambient conditions make readings unreliable for QC purposes.
Probe insertion challenges: Unless a sleeve is installed during pouring, a hole must be drilled into the concrete to insert a thermal probe. This adds additional labor and depending upon application could compromise the backfill, making on-site testing even less practical.
The takeaway: on-site conductivity testing cannot reliably verify the thermal performance of concrete as part of a QC program.
Why Engineers Prefer Dry-Out Curve Testing
For data centers, electrical vaults, duct banks, and slurry backfill, dry-out curve testing provides:
Reliable, long-term design data
Accurate cable spacing and ampacity calculations
Predictable concrete and slurry performance after curing
Verified compliance with industry standards
Unlike in-situ testing, which can be distorted by weather, surface moisture, or curing stage, lab testing gives engineers confidence in thermal management and safety.
Applications of Dry-Out Curve Testing
Data Center Foundations: Ensures heat from servers and backup power is properly conducted away.
Underground Electrical Duct Banks: Confirms slurry or concrete backfill provides proper heat dissipation.
High-Voltage Substations and Vaults: Prevents overheating of transformers and bus ducts.
Geothermal or HVAC Systems: Optimizes heat transfer in soil or concrete foundations.
Conclusion
While on-site thermal resistivity testing is convenient, it often produces inconsistent or misleading results, especially for concrete and slurry used in data centers and electrical infrastructure. Laboratory dry-out curve testing provides accurate, repeatable, and ASTM-compliant data that engineers can rely on for safe, efficient, and long-lasting designs.
Need precise thermal resistivity data for your project? Our lab offers full dry-out curve testing for soil, concrete, and slurry, giving you the confidence to design critical infrastructure with optimal heat management. Contact us today to schedule testing.
