Why You Should Avoid Using Sleeves When Testing Cast Concrete Samples for Thermal Resistivity (ASTM D5334 & IEEE 442)
When performing thermal resistivity testing of concrete, accuracy is critical. Engineers rely on precise data to design reliable grounding systems, evaluate heat dissipation in mass concrete pours, and correctly size geothermal energy systems. ASTM D5334—the standard test method for thermal conductivity of soil and soft rock—can be applied to concrete testing, but one common practice can compromise results: embedding a sleeve in the concrete to hold the probe. Here’s why that approach is problematic.
The Temptation: Sleeves for Convenience
Using a pre-installed sleeve may seem like an easy solution. Testers often believe that sleeves allow them to:
Insert the probe without risking damage to the concrete
Avoid additional labor from drilling holes.
Avoid disturbing the concrete surface
While convenient, this method introduces a hidden problem that can significantly skew results.
The Hidden Problem: Shrinkage Gaps
Concrete shrinks slightly as it cures—typically 0.05–0.1% for standard mixes. When a rigid sleeve is embedded, the concrete can pull away from the sleeve, forming a microscopic annular gap. This tiny void acts as an insulating air pocket.
Thermal resistivity probes are highly sensitive to contact with the surrounding material. Air has a much higher thermal resistivity than concrete, so even a microscopic gap around the sleeve can dramatically inflate measured resistivity. The probe doesn’t “see” the true properties of the concrete—it measures the insulating effect of the air gap instead.
Why ASTM D5334 Discourages Sleeves
Even though ASTM D5334 was written for soils, the principles apply to concrete:
Direct Contact is Critical: Accurate thermal measurements require intimate contact between the probe and the material. Any artificial interface, like a sleeve, compromises accuracy.
Ensures Repeatability: Due to the nature of the thermal conductivity dry out curve, concrete samples need to be oven dried. This causes the concrete to shrink and pull away from the sleeve, introducing air gaps. Your first as-received reading will be accurate, but as the sample dries and more readings are taken, error is introduced along the way.
Proper Method: Direct Probe Insertion
For accurate results, the probe should be inserted directly into the concrete. Here’s how to do it correctly:
Drill a hole approximately the same diameter as the probe. This ensures close contact without forcing the probe, which is delicate and can be damaged if inserted into a smaller hole.
Apply thermal paste or grease between the probe and the hole to fill any micro-voids and improve heat transfer.
Avoid disturbing the sample during insertion to maintain the integrity of the measurement.
Following this method ensures that the probe contacts the concrete properly, providing accurate and repeatable thermal resistivity readings.
Conclusion
While sleeves might seem convenient, they create a hidden air gap as concrete shrinks during curing. This microscopic gap acts as an insulator, artificially inflating resistivity measurements and compromising your data. For reliable results, always insert the probe directly into a hole that matches its diameter and use thermal paste as needed. Taking these precautions ensures accurate, reproducible measurements that engineers can confidently use in design and analysis.
