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Someday in the future, the concrete producer may employ ultrasonic waves to monitor the progress of concrete in place. At least that's what researchers hope at the Center for Advanced Cement-Based Materials (ACBM) at Northwestern University, Evanston, Ill.

For several years, ACBM teams have been investigating the practicality of a field-friendly, nondestructive test based on ultrasonic wave technology that monitors concrete stiffness. If successful, the new test may replace the conventional maturity method that's based on hydration and chemical reactions.

Researchers suggest that their new test will help solve the fundamental industry problem of discrepancies between conventional maturity-strength lab results and in-place concrete performance, which is affected by field variables.

The ACBM team's test measures wave strength at a single point, which can help the producer who has access to only one side of forms. The prototype uses a steel mold, into which they place freshly mixed concrete. It then transmits ultrasonic waves into one side of the mold. Using a transducer mounted on the mold's interior surface (where fresh concrete contacts the mold), the team measures the strength of shear waves reflected at the steel-concrete interface.

Researchers performed the same procedure on five different mix designs: ones that used accelerators, silica fume, a superplasticizer, and retarders, and another mix for ordinary concrete.