Concrete Resistivity Impact on the Design of Impressed Current Cathodic Protection Systems

Abstract

The impact of concrete resistivity on the design of cathodic protection systems has been noted in all global concrete cathodic protection standards as a significant issue for design consideration. However, none of the standards include specific, relevant guidelines related to the consideration of resistivity data in the design of cathodic protection systems.

The research work in this thesis includes various experiments related to the measurement of concrete resistivity in atmospheric conditions, long-term concrete resistivity development over time, and the assessment of concrete resistivity on the design of cathodic protection systems. Six experiments were conducted as part of this thesis.

Experiment 1 involved the development of concrete material compositions to achieve a wide range of concrete resistivity levels, representative of a full range of real-world concrete resistivity conditions. With the use of admixtures, nine compositions were trialed achieving concrete sample resistivities from a low of 1.6kΩcm to a high of 2000kΩcm within a short period of 57 days. Compositions were limited to widely available admixtures allowing them to be easily recreated for concrete resistivity experiments in this thesis and in future works.

Experiment 2 explored the effect of water saturation on concrete resistivity measurements. The AASHTO Standard [1] adopted for the measurement of concrete requires water saturation as a method to minimise contact surface resistance between the Wenner probe equipment and the concrete surface. Although the Standard [1] was designed for chloride permeability testing, it has been widely adopted for all concrete resistivity testing, particularly in laboratory settings to achieve sufficient electrolytic contact between the Wenner Probe and concrete. The purpose of Experiment 2 was to assess if there is an impact on resistivity measurements of atmospherically exposed concrete when water saturated. Experiment 2 identified that water saturation resulted in a significant decrease in resistivity measurements and identifies the need for a new methodology for testing concrete located in atmospheric conditions, without the need for water saturation.

Experiment 3 involves the development of a new concrete resistivity testing methodology for concrete in atmospheric conditions, without the need for surface wetting or water saturation (a solution to the issue identified in Experiment 2). This experiment presents a new method minimising the impact of concrete surface variability by establishing an alternative, reliable electrolytic contact between the Wenner equipment probes and concrete. Based on the results in this experiment, a 15mm probe depth was identified to provide the most consistent resistivity measurements of concrete in varied exposure conditions. Through experimental laboratory testing, resistivity measurements were found to decrease by up to 8% between surface and probe measurements. The output from this experiment can contribute to the development of guidelines and additions of the current Standard for the measurement of concrete resistivity via the Wenner probe in atmospheric conditions.

Experiment 4 assesses the level of concrete resistivity increase over time for four commonly used repair mortars used in conjunction with cathodic protection. The testing was conducted over a period of 564 days under saturated and outdoor atmospheric exposure conditions. The experiment indicated that concrete resistivity continues to increase with time under both saturated and atmospheric outdoor conditions. The conclusion from this experiment is that the use of repair mortar based on published short term resistivity data (28 days) and under saturated conditions is misleading. The suitability of polymer-modified repair mortars in conjunction with cathodic protection must be verified based on long-term test data under atmospheric outdoor conditions. Testing in accordance with the methodology trialed in Experiment 3 provides a solution which can be adopted by the industry to test concrete in representative atmospheric conditions.

Experiments 5 and 6 involved the assessment of the impact of anode-to-rebar spacing and concrete resistivity on the current output of impressed current cathodic protection systems. A trend line representing this correlation was developed based on two laboratory testing programs over 63 days (Experiment 5) and 564 days (Experiment 6). The trend line was verified using data extracted from an operating impressed current cathodic protection system (Experiment 7). The experiments indicate a significant impact of anode-to-rebar spacing at different levels of concrete resistivity on the current output of impressed current cathodic protection systems. The developed trend line generated from this work is the first reported research data correlating concrete resistivity, anode-to-rebar spacing and current output for impressed current cathodic protection systems. This trend line can be considered as an effective tool for the design of impressed current cathodic protection systems.

This research presents the first major work on the topic of concrete resistivity and its relationship with impressed current cathodic protection through laboratory trials supported by data from a real operational structure with an operating impressed current cathodic protection system. The results from this thesis can contribute to the improvement of existing concrete resistivity testing Standards and provide invaluable data to the international cathodic protection design Standards.