The Rapid Chloride Migration Test (RCMT), defined in NT Build 492, is a laboratory test that is specifically designed to measure the rate at which chloride ions penetrate concrete under an externally applied electrical potential.
The RCMT test is conducted to evaluate how well the concrete mix is able to resist the chloride ions migration, which can destroy the protective oxide layer around the steel reinforcement, leading to corrosion over time.
In RCMT, the chloride ion migration is accelerated by applying a direct voltage across a concrete specimen, creating an electric field that drives the ions movement from negative side to positive, where they eventually penetrate the concrete.
A concrete core of 100 mm diameter and 50 mm thickness is placed inside a rubber sleeve, which separates two compartments, one side is filled with sodium chloride (NaCl) solution (connected to the anode) and the other with sodium hydroxide (NaOH) solution (connected to the cathode).
The sample is placed such that the bottom is in contact with the NaCl solution, and the top is in contact with the NaOH solution. An initial voltage of 30V is applied, and the initial current is recorded. Based on this value the RCMT apparatus recommends the next voltage and test duration as per NT BUILD 492.
At the end of the test, the final temperature of the system is logged. The concrete sample is then split, and the internal surface is sprayed with silver nitrate (AgNO₃) solution, and visually examined for the depth of chloride migration.
The distance chloride ions have travelled during the test is then used to calculate the non-steady-state migration coefficient (Dnssm). This coefficient serves as a quantitative indicator of concrete’s resistance to chloride penetration. Lower values indicate better durability performance, dense micro-structures, low water–cement ratios, and the presence of supplementary cementitious materials such as fly ash, slag, or silica fume.
One major advantage of RCMT test, particularly in Quality Control, is its sensitivity to variations in concrete micro-structure that affect the chloride ion movement. This allows to differentiate between different high-performance mixes that may show similar results in RCMT results.
