2.3.5.1 Free chloride content
The determination of free chloride content usually involves two stages – extraction
and analysis.
Extraction:
There are two main methods that have been reported for use in extracting free
chlorides from concrete specimens. The first approach involves placing cylindrical
specimens on a pore press, and applying a load until the pore fluids are squeezed
out. This approach seems to give the exact representation of the free chloride content
[270]. However, a major setback to this approach is that pore presses are not readily
available; hence they have to be constructed as a special rig.
The second approach involves leaching the free chlorides from concrete specimens.
The free chlorides obtained by this process are usually referred to as „water-soluble
chloride‟. Arya et al. [270] reported that free chloride contents obtained from leaching experiments gave an underestimation of the free chloride content of the
techniques have been reported [271-273] concerning the extraction of free chlorides,
and the accuracy of each technique tends to depend on several factors such as the
solvent type, exposure time, and temperature. Arya et al. [273] used several leaching
methods and observed that the methods were dependent on the total chloride
content. Amongst the methods used were:
48 hour standing at 20°C – suitable for total chloride content of 1.0 to 1.5% by weight of cement
72 hour standing at 20°C – suitable for total chloride content of 1.5 to 2.0% by weight of cement
6 hour continuous stirring – was suitable for total chloride content of 1.5 to 2.5% by weight of cement
5 mins boiling and allow to stand for 55 mins – was suitable for total chloride content of 2.0 to 2.5% by weight of cement.
In a subsequent study, Arya et al. [270] observed that the extraction technique which
involved boiling for 5 mins and allowing to stand for 55 mins gave similar free
chloride contents to that of the expressed pore solution. However, there is the
tendency for boiling to liberate some of the chemically bound chlorides, thereby
increasing the free chloride content. Since chloroaluminate compounds are known to
be insoluble in cold water, the extraction techniques carried out at room temperature
is expected to liberate only the free or water soluble chlorides and is therefore
Analysis:
After the free chlorides have been extracted, they can be analysed either by the use
of a potentiometric titration device [164, 182, 270] or by the use of an ion
chromatography-inductively coupled plasma (IC/ICP) device [173, 265, 274].
2.3.5.2 Total chloride content
The approach adopted for the determination of the total chloride content in concrete
depends on whether the chlorides are allowed to diffuse naturally into the concretes
or accelerated by the application of electrical fields.
Natural diffusion:
These natural diffusion test methods are also referred to as „salt ponding tests‟. It can take different forms, but the two most common forms are shown below in
Figure 2.14. In the first arrangement, the salt solution is placed on top of the
concrete specimen so that the chlorides can diffuse unto the concrete specimen
unidirectional under gravity. In the second arrangement, the concrete sample is
immersed completely into the salt solution but all sides of the concrete sample
except the exposed face is coated with a substance that is impermeable by chlorides.
In both arrangements, the concrete samples would usually be exposed for periods up
to 90 days, for a 3% NaCl solution [275]. BS EN 12390:2015 [275] also prescribes a
Figure 2.14: Typical set-ups for salt ponding test (taken from [181])
At the end of the ponding period, powders are either drilled from various depths or
slices of concrete are collected from the exposed face downwards. The drilled
powders or ground concrete slices can then be analysed for total chloride contents
using the method specified in BS 1881-124:1988 [276] or RILEM TC 178-TMC
[277] for determining acid-soluble chlorides. Dhir et al. [278] reported that the acid
extraction technique underestimated the total chloride content. They observed that
using different acid concentrations, temperature and exposure times resulted in
different total chloride contents, and recommended that for accurate determination
of the total chloride content, XRF should be used. However, the method
recommended by RILEM TC 178-TMC [277] was based on the results from a
round-robin test that involved 30 laboratories and 64 independent measurements.
When the total chloride contents at various depths from the exposed face of the
sample have been obtained, a chloride profile is plotted and fitted into Fick‟s 2nd law
Accelerated diffusion:
Accelerated diffusion usually involve the use of electric fields to drive chlorides
through concrete, in which case, the concrete acts as a diffusion membrane between
a chloride solution and a neutral solution (see Figure 2.15 below). There are several
variants of this method. In some instances, the current flowing through the concrete
is measured [279, 280], or the charge passed [281], the resistivity [282, 283] or
conductivity [284] of the diffusion membrane or the electrical impedance [285]. The
Nernst – Planck equation shown previously in Section 2.3.1, is mostly applied in
calculating the chloride diffusion coefficient in accelerated diffusion tests.
Figure 2.15: Typical set-up for an accelerated chloride migration test (taken from [181])
2.3.5.3 Bound chloride content
Tang and Nilsson [243] developed a method for determining bound chloride content.
This method is based on the principle of equilibrium and has been used by several
researchers [18, 182, 245]. The method involves wet-crushing mortar or cement
paste samples to particles ranging in size from 0.25 to 2.0 mm, saturating the
crushed specimens with NaCl solution and allowing the resulting solution to stand
for a period between 10 to 14 days to reach equilibrium. After equilibrium is
the initial concentration of the NaCl solution, the content of bound chlorides can
then be determined using the expression:
𝐶𝑏 = 35.45𝑉 𝐶0− 𝐶1 𝑊
(2.18)
Where Cb is the bound chloride content in mg/g-sample, V is the volume of solution
in ml, C0 is the initial concentration of the chloride solution in mol/l, C1 is the equilibrium concentration of the chloride solution in mol/l, and W is the weight of
the dry sample in grams.
Another method that can be used for determining bound chlorides content is QXRD.
Arya and Newman [270] showed that QXRD can detect the presence of Friedel‟s
salt. However, this method may not give an entire representation of the bound
chlorides in the concrete, since the C4AF and C-S-H are also capable of binding
some of the chlorides.