Technical Questions

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Chloride concentration in soil and exposure classes

Q. I’m designing a structure that will be exposed to chlorides in soil. The chloride concentration in the soil is lower than the concentration in seawater, so can I specify concrete with a strength value between 2500 and 5000 psi (17.2 and 34.5 MPa), based on C1 and C2 exposure classes, respectively?


A. ACI 318-19 does not take into account the amount of salt present. Exposure to any amount of salt is considered a concern for corrosion. In the case you describe, if the soil is moist, the chlorides will penetrate the concrete toward the reinforcing steel and can initiate corrosion, even at concentrations well below that of seawater. Therefore, the project described in the question clearly falls into Exposure Class C2, which requires a maximum water-cementitious materials ratio (w/cm) of 0.40 and a minimum specified compressive strength of 5000 psi.


References:  ACI 318-19

Topics in Concrete:  318 Building Code; Corrosion; Durability

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