The US Department of Transportation (DOT) Federal Highway Administration (FHWA) sponsored a Small Business Innovation Research (SBIR) project for development of a commercially viable concrete pore solution resistivity (PSR) sensor, conducted by the Callentis Consulting Group in collaboration with Penn State University. In line with the Performance Engineered Mixtures (PEM) initiative, a concrete’s Formation Factor (FF) is used to assess its transport properties and as an indicator of long-term durability and resilience against ingress of aggressive ions. The Formation Factor has been shown to be an important parameter in service-life models to predict permeability of concrete, and chloride ion penetration that can cause corrosion of concrete and its reinforcing steel. In AASHTO R101, the Formation Factor is defined as the ratio of the electrical resistivity of the bulk concrete mixture over the resistivity of the concrete pore solution. The AASHTO T 402 and the equivalent ASTM C1876 standards were developed to measure the bulk concrete resistivity, and AASHTO T 358 to measure surface resistivity of the concrete mixture. However, there are no standard equipment or test methods for non-destructive measurement of the pore solution resistivity, and the only available methods involve labor-intensive laboratory extraction of the pore solution.

AASHTO T 402:

Uniaxial Resistivity

AASHTO T 358:

Surface Resistivity

In-situ measurement of the electrical resistivity of the pore solution along with the resistivity of bulk concrete allows for qualification of concrete mix designs before construction, quality control (QC) and quality acceptance (QA) of concrete batches placed during construction, and service-life prediction of vital concrete infrastructure such as bridges, pavements, and marine structures. Additionally, the FF sensor embedded in cylindrical samples can be used to evaluate the impact of new additives and supplementary cementitious materials on the durability of concrete mixtures. Moreover, the FF sensor can be embedded in structures and used for long-term health monitoring to evaluate changes in the chloride content inside concrete over time. Finally, the sensor’s output can be translated to concrete’s pH and used for evaluation and mitigation of the risk of alkali-silica reaction (ASR) in concrete containing reactive aggregates.

The technology developed in this project is a sensor system that allows in-situ measurement of concrete’s PSR along with the concrete mix resistivity. The sensor system includes the sensor assembly that is embedded inside concrete and a measuring device to interrogate the sensor. The sensor assembly includes the sensor matrix, attached insulated electronic components, and a placement mechanism. The sensor matrix is made of a custom-designed nano-porous body. The sensor placement mechanism is designed in a way that would be easily embedded in concrete cylinders or structures, and has probes for measuring concrete mix resistivity. The mobile measurement device is a low-cost consumer-level product. The overall sensor system and the simple data post-processing is designed for seamless application by field and lab technicians.

Technology Readiness Levels (TRL)

Phase I: Proof of Concept (July to December 2021) TRL-2 to TRL-3

The Phase I project explored candidate body (matrix) materials for the sensor, manufacturing techniques, optimum sensor geometry and packaging, and the best methods for reading and measuring the sensor output. Phase I also explored the costs, material availability, manufacturability, scale-up, marketability, technology transfer, and industry implementation of the proposed sensor system. Phase I output was a proof-of-concept report summarizing the above and offering two best prototype designs to be produced and tested in Phase II. The project team has also filed a patent for the developed technology.

Phase II: Technology Development (July 2022 to September 2024) TRL-3 to TRL-6

Phase II will include the development and demonstration of market-ready prototypes for user testing and commercialization. Phase II will perform further refinement of the technology, design, and fabrication of the sensor prototypes. Phase II will also include experimental verification of the technology, including third-party testing, precision and bias evaluation, and development of a user’s guide and draft standard test method to facilitate effective implementation of the FF sensor.

Phase IIB: Technology Refinement and Demonstration (TBD) TRL-6 to TRL-8

Phase IIB will include refinement of the technology for enhanced ruggedness and extended shelf life of the sensor system. In addition, Phase IIB will improve and scale up the manufacturing process. Phase IIB will also involve demonstrations of more prototypes in pilot projects, by the FHWA and several State DOTs. Further precision and bias evaluation, and refinement of the user’s guide and draft standard test method will be conducted to facilitate effective implementation of the FF sensor.

Phase III: Commercialization (TBD) TRL-8 to TRL-9

Phase III will include execution of the Commercialization Strategy Plan that was developed in Phase II and refined in Phase IIB. This involves presentation of the demonstration and pilot project results in evaluating the sensor precision and bias at various industry events, outreach to various stakeholders across multiple industries to disseminate relevant information regarding the sensor technology, business development and marketing efforts to expand the customer base, partnership efforts for sales and distribution of the sensor systems, and collaborations for manufacturing at scale and in multiple facilities.