Why Chloride Migration Testing:
Due to very high construction costs and the socio-economic importance of civil engineering structures, the durability demand for civil engineering structures is increasing, hence pushing for longer and longer service life design.
The service life and durability of reinforced concrete structures are affected by the degradation of concrete and corrosion of reinforcement. One of the main aspects of the durability design approach is to restrict the movement of the aggressive ions into the concrete (i.e. chloride ions). Chloride-initiated corrosion is one for the major causes of early age deterioration of concrete structures. Figures 1a and 1b below show the effect of corrosion on reinforcement and subsequent delamination and spalling of concrete due to corrosion.
Figure 1: Chloride-induced corrosion of reinforcement in concrete structure
Available Test Methods:
Chloride ingress into the concrete is usually a long-term and complex phenomenon involving diffusion, capillary absorption, and permeation. For this reason, tests to determine the rate of chloride ingress through concrete provide valuable information on the long-term durability performance of a structure. Test methods used in Ireland and in Europe are EN 12390-18:2021 ‘Testing hardened concrete – Part 18: Determination of the chloride migration coefficient’ and NT Build 492 ‘Concrete, mortar, and cement-based repair materials: Chloride migration coefficient from non-steady state migration experiments’. These tests are used to determine the chloride migration coefficient for concrete and mortar.
Both EN 12390-18:2021 and NT Build 492 test methods are based on the non-steady state migration of chloride ions. The test results are used as a durability indicator with respect to the resistance of the concrete to chloride penetration.
Infrastruct AMS Ltd provides both EN 12390-18:2021 and NT Build 492 test methods for determining the chloride migration coefficient of concrete. We use the Proove’it system, this system was developed by Germann Instruments in Denmark. Figure 2 shows Proove’it chloride migration test set-up.
How the Chloride Migration tests are performed:
Migration testing is performed on concrete cylindrical specimens of 100 mm diameter prepared at project site. The specimens are cured for 28 days before the performing the tests (recommended). However, the tests can be performed on the cored specimens as well.
After the required pre-conditioning, where necessary, the test specimen is placed in designed cells with reservoirs at each end. The reservoirs are filled with a chloride free and chloride containing solution (different solutions are prepared for different tests). An electric voltage is applied between two external electrodes to rapidly force the chloride ions into the concrete specimen. Figure 3 shows a typical cell arrangement according to the NT Build 492 test method.
After a defined period of time, the specimen is removed from the cells and split. The penetration depth of the free chloride ions is determined using a suitable colour indicator solution. The chloride migration coefficient is then calculated based on the measured depth of penetration in the concrete, the magnitude of the applied voltage and other measured parameters. Figure 4 shows the boundary of the free chloride (white / blueish colour) within the concrete specimen.
How the test is useful to Engineers:
The chloride migration coefficient is an indicator for the durability and long-term performance of concrete. The test methods described above will help designers, concrete technologists concrete producers and engineering consultants to assess the durability performance of concrete before construction commences, during the construction phases and after placement.
Migration testing will help to improve the durability assessment for concretes in civil engineering structures.
For more information on the chloride migration testing on concretes in Ireland, please contact Dr Thomas Callanan in Infrastruct Asset Management Services Limited.