| The water to cement ratio is the single most
important engineering parameter in the use of concrete. The
water to cement ratio determines the strength and the durability
of concrete.
Optical microscopy of fluorescent impregnated thin
sections is a well-proven method for w/c determination. The
method has been used with great accuracy for more than 20 years.
The thin section is
analyzed under an optical microscope using a combination of a
blue excitation filter and a yellow blocking filter. This is the
fluorescent light mode in which epoxy filling air voids and
cracks appears yellow, cement paste as shades of green and
aggregate black. The shade of green of the cement paste depends
on the capillary porosity.
A sample with low
w/c appears dark green, i.e. has less fluorescence intensity due
to a low amount of epoxy within the paste. A sample with high
w/c appears light green, i.e. has high fluorescence intensity.
These shades of green (fluorescence intensity) are used to
determine the w/c by comparing the fluorescence intensity of the
cement paste with standards of known w/c.
Example
of the use fluorescence microscopy for w/c determination
Determination of w/c of precast concrete
railroad ties was performed on 522 thin sections representing 127 ties.
The ties were in field service for a number of years before
determination of w/c.
The average w/c of the ties
was determined to 0.42, a number which was only 0.02 lower than the
average w/c (0.44) used in the production. The standard deviation of the
measurements was 0.026.
The water to cement ratio of
hardened concrete can be determined by optical fluorescence microscopy.
To obtain reliable determinations an experienced petrographer is needed.
Moreover, the thin sections used have to be of a uniform, high quality
with full impregnation and well-defined thickness.
You can find more
information on w/c determination by fluorescence microscopy in: Jakobsen
et al., 2000: Determination of Water
to Cement Ratio in Hardened Concrete by Optical Fluorescence Microscopy,
ACI SP 191.
Concrete Experts
International offers:
|
