Fluorescence, a handy technique for visualization of cracks, capillary porosity, and
entrained/entrapped air voids in concrete.
Also commonly used in
biology/medicine to
mark/tag proteins .
...and in detection of
cracks/flaws in metals. Spray on
the dye penetrant solution, and
illuminate with UV light
Fluorescent dyes are excited by a
shorter wavelength, and emit light at
a longer wavelength.
A) Electrons in orbitals in the dye
molecule absorb energy, and
move to a higher energy orbital
B) When an electron returns to
its normal state, it releases
energy, in this case visible light,
that we see as fluorescence.
Pictures from Steve Haddock, http://www.mbari.org/staff/haddock/
There are a wide variety of fluorescent dyes. The dyes used for concrete
microscopy are not water-soluble, and referred to as “solvent” dyes. The dye in
the epoxy used to impregnate the thin sections you are looking at is called
Solvent Yellow 43.
The resin (part A) for the epoxy used at U of T is dosed 1% by weight with solvent
yellow 43, and stirred for a few hours (with a magnetic stir-bar) until all of the dye has
dissolved. The solubility of the dye in the resin hasn’t been carefully measured, but
isn’t much higher than 1%. At a dosage of 1%, the hardened epoxy is visibly yellow.
Some of the epoxy impregnated thin sections you are looking at today had dosages
less than 1%. These thin sections will not fluoresce very brightly, and you might want
to trade thin sections with your neighbor.
The manufacturer (DAY-GLO) didn’t have any information about the emission
spectra for Tigris Yellow 43, but did provide this absorption chart, showing what
wavelengths it readily absorbs:
There are big peaks around 280 nm (UV-C), and in the visible light violet-blue range. It
would be nice to know the emission spectrum too, but would need to visit Physics
Dept., where there is maybe a spectrophotometer.
Last week we saw the dye fluoresce under long-wave UV light (~315-400nm) in some
sections cut from prisms of concrete ponded with potassium acetate deicer (picture from
MASc thesis of Sonia Ghajar-Khosravi).
Last week, we also saw polished slabs from cores sent to U of T from an airport
runway that had experienced damage from a fire. The damage didn’t penetrate very
far into the concrete...
Polished slab prepared from core taken from area not exposed fire
(slab dimensions 3” x 4”).
Polished slab prepared from core taken from area where top surface had spalled away.
UV light isn’t good for your skin, or your eyes. When working with UV light, you should
cover your skin, and protect your eyes with proper eyewear. The light we are using with
your microscopes is not UV light, just visible spectrum from a tungsten source.
We are using a blue filter to isolate the blue light that efficiently excites the yellow dye,
which fluoresces a bright green. We are using a yellow filter to block the blue light
before it reaches our eyes, so all that remains is the green light from capillary pores,
cracks, and air voids.
The first number on your blue filter is the mid-point of the wavelength (in nm) that the
filter allows to pass through. The second number is the width/spread of the
wavelengths allowed through.
The number on your yellow filter is the point at which longer wavelengths are allowed
to pass through – the yellow filter is a “long-pass” filter (hence the LP).
Most microscopes set up for fluorescence use reflected light, or epifluorescence – we
are using transmitted light, which works, but not as well. See Chapter 13 of Hollis
Walker’s “Petrographic Methods of Examining Hardened Concrete: A Petrographic
Manual” for more details.
The chart below shows lines for a dichroic mirror used in an epifluorescent
micrposcope. The teaching microscope can do both transmitted and reflected light, but
hasn’t been set up yet for fluorescence.
Epifluorescent (reflected) light microscope
Dichroic mirrors reflect light from the source down towards the sample, and allow light
travelling from the sample back through the dichroic mirror, through the yellow
blocking filter, and to the eye.
Many of the figures in this .ppt from Handbook of Optical Filters
http://www.chroma.com/sites/all/themes/chroma-blue/uploads/files/HandbookofOpticalFilters_0.pdf
The blue filters purchased for your microscopes are not ideal, they are too small. So,
there isn’t enough light to cause the dye to glow bright green. To compensate for this,
you will use the converging lens below the microscope stage to concentrate the light.
This won’t be sufficient for the low power (4x) objective, but will work for the higher
power objectives.
Coverging lens, flip it into
position, and raise the substage
so that the lens is closer to
your sample.
With fluorescent microscopy, entrained air voids, cracks, and capillary porosity are all
easily observed. The brightness of the cement paste can be used as an indirect
measure of the amount of water used to produce the concrete, and forms the basis of
a European standard (will link in blackboard):
http://www.nordicinnovation.net/nordtestfiler/build361.pdf
This image borrowed from TU Delft Concrete Microscopy Course
MORE EXAMPLES....
Transmitted light, hydrated cement paste, 24 hours.
Crossed-polars, bright lath-like xtals are Ca(OH)2.
Epifluorescent mode, capillary pores visible.
Back-scattered electron image
Large belite-rich cement grains in paste between two coarse aggregate particles
Large belite-rich cement grains in paste between two coarse aggregate particles
Large belite-rich cement grains in paste between two coarse aggregate particles
Close-up of belite-rich cement grains – field of view 0.627 mm
Close-up of belite-rich cement grains – field of view 0.627 mm
Close-up of belite-rich cement grains – field of view 0.627 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 2.54 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 2.54 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 2.54 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 1.245 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 1.245 mm
Ettringite filled air void, and crack filled with ASR gel, field of view 1.245 mm