10 Major Factors That Affect Thermal Resistivity in Concrete, Surry and Flowable Fill

Concrete plays a crucial role in underground power systems, geothermal installations, and duct bank construction — where heat must flow efficiently away from buried conductors. The thermal resistivity of concrete (ρ, measured in °C·cm/W) defines how well it transfers or resists heat.

When resistivity rises too high, heat cannot escape, causing cable overheating and reduced system lifespan. That’s why engineers measure concrete’s thermal properties per IEEE 442 or ASTM D5334, the industry standards for thermal resistivity testing.

Below are the biggest factors that affect concrete’s resistivity and how to control them for consistent, low-resistance backfill performance.

1. Moisture Content

Moisture is the single most important factor affecting thermal resistivity.
Water conducts heat (~0.6 W/m·K) far better than air (~0.025 W/m·K). When concrete dries, air replaces water in the pore spaces, causing resistivity to increase sharply.

ConditionTypical Thermal Resistivity (°C·cm/W)Fully saturated25 – 40Partially moist (5–10 %)40 – 60Oven-dry100 +

Maintaining adequate moisture during curing and after installation ensures stable, long-term performance.

2. Density and Porosity

Concrete’s density determines how much solid material can transfer heat.

• Higher density → fewer air voids → lower resistivity

• Lower density → more air voids → higher resistivity

Dense, low-air mixes may show resistivity near 35–45 °C·cm/W when saturated, while lightweight concrete can exceed 100 °C·cm/W. Porosity directly affects moisture retention, making dense mixes preferable for thermal applications.

3. Aggregate Type and Gradation

Aggregates make up most of the concrete volume and significantly affect thermal conductivity.

• Quartz and siliceous aggregates have high conductivity

• Limestone or lightweight aggregates have lower conductivity

• Well-graded aggregates reduce voids and resistivity

Optimizing aggregate type and gradation improves compaction, reduces air content, and enhances heat transfer through the mix.

4. Cement Paste Composition

The cementitious matrix binds the aggregates and influences microstructure.

• High cement content increases density and reduces resistivity

• Supplementary materials like fly ash or slag refine the pore structure and can lower resistivity

• Poor curing or incomplete hydration causes microcracks, increasing resistivity

Well-hydrated, dense paste is key to long-term thermal stability.

5. Water-to-Cement Ratio (w/c)

The w/c ratio controls both porosity and durability.

• High w/c (> 0.6) → more pores → higher resistivity

• Low w/c (< 0.4) → denser matrix → lower resistivity

• Optimum for thermal backfills: 0.45 – 0.55

A balanced ratio ensures good workability while maintaining low thermal resistivity over time.

6. Compaction and Curing Quality

Field practices are just as critical as mix design.

• Poor compaction traps air and raises resistivity

• Proper curing (7–14 days moist or sealed) retains moisture for full hydration

• Rapid surface drying can double resistivity within weeks

Both IEEE 442 and ASTM D5334 recommend documenting curing and moisture conditions when performing thermal testing.

7. Temperature and Service Conditions

Thermal resistivity changes with temperature:

• As temperature increases, resistivity typically decreases slightly

• Beyond ~80 °C, dehydration can cause permanent increases in ρ

• Seasonal moisture and temperature fluctuations can alter long-term values

Testing is typically performed at 20–30 °C to represent in-service conditions.

8. Long-Term Moisture Migration

Buried concrete continuously exchanges moisture with surrounding soil.

• In arid environments, drying dominates → resistivity increases

• In saturated soils, concrete may absorb water → resistivity stabilizes or decreases

IEEE 442 recommends testing both as-received and oven-dry samples to estimate long-term field performance.

9. Chemical Composition and Admixtures

Certain admixtures can change how concrete conducts heat.

• Salts or chlorides slightly reduce resistivity through ionic conduction

• Pozzolans (fly ash, silica fume) refine the microstructure and reduce ρ

• Air-entraining agents increase ρ by adding micro-voids — undesirable for thermal backfills

Select additives carefully to maintain both structural integrity and low resistivity.

10. Testing Methodology and Equipment

Accurate measurements depend on proper testing techniques.

• ASTM D5334 uses a thermal needle probe to measure heat flow

• IEEE 442 allows both needle probe and guarded heat-flow methods

• Uneven moisture or poor probe contact can produce artificially high results

For reliable results, always record conditioning, probe type, and specimen density.

Conclusion

Concrete’s thermal resistivity is primarily driven by moisture, density, and pore structure — all controlled by the mix design and field handling.

When performing IEEE 442 or ASTM D5334 testing, consistent curing, controlled moisture, and proper sampling are essential.

By designing mixes with low resistivity and maintaining in-situ moisture, buried concrete systems can achieve long-term thermal stability and reliable heat dissipation, ensuring performance for decades.