Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and plant evaluations. 

HELENA ASKS: I am working on a project that has severely corrosive soils and water. Specifically, there are samples with resistivity values less than 300. Can I still use precast?

NPCA EXPERTS ANSWER: Yes! Precast concrete structures are still a long-lasting, durable and reliable choice even in soils with low electrical resistivity. But the key lies in moving beyond resistivity alone and making decisions based on a fuller understanding of the actual chemical environment and proven mix designs.

The commonly cited 300 ohm-cm threshold for soil resistivity was originally developed for evaluating metal pipe corrosion risk. Concrete behaves differently. Its durability depends far more on actual exposure to aggressive ions — such as chlorides and sulfates — than on resistivity alone. In other words, a low resistivity reading doesn’t automatically mean concrete will fail.

When resistivity is below about 1,000 ohm-cm, geotechnical testing should guide the decision. In particular:

  • Chlorides: Levels above 500 ppm may be a concern for reinforcing steel corrosion.
  • Sulfates: Follow AASHTO LRFD Construction and/or ACI318 Chapter 19 exposure classifications to determine the proper cement type.
  • pH: Soils or groundwater with a pH below 5.5 can be aggressive toward concrete.

These parameters offer a more direct and reliable assessment of potential chemical attack, ensuring that viable and durable materials aren’t excluded based solely on one number.

The precast industry has decades of experience in designing concrete mixes that resist chemical attack. Supplementary cementitious materials (SCMs) such as fly ash, slag or silica fume, improve sulfate resistance and lower permeability. Producers have long used Type V cement with fly ash to handle “very severe” sulfate conditions. Many are now transitioning to blended Type 1L-HS cements, which meet high sulfate resistance requirements. Performance can be verified through ASTM C1012 sulfate expansion testing, where durable mixes achieve less than 0.10% expansion at 18 months under severe exposure conditions.

Pozzolans (e.g. fly ash) and slag are particularly valuable in these applications. While they do not increase resistivity, they greatly reduce permeability, helping to keep chlorides and sulfates from penetrating the concrete. When combined with the right cement type and curing process, it significantly extends service life.

Proven in the Field

Real-world experience supports the science. The Florida DOT’s Culvert Service Life Estimator shows that precast can achieve a 100-year service life in low-resistivity soils, provided chloride and sulfate levels are within acceptable limits. In Montana, there are no known cases of precast failure due to sulfate or chloride attack, even in regions like Great Falls and Glendive where geochemical conditions could be considered challenging.

Bottom line: Low resistivity soils do not automatically rule out precast concrete structures. With proper testing and mix design, precast remains a proven, durable solution — even in environments once thought to be unsuitable.