Sample Preparation
Complete laboratory personal protective equipment during sample preparation of concrete slabs and thin sections for petrographic examination is a must.
Quick Links:
A Guide to Core Sectioning
Featuring MJ, one of our lab technicians, the proper sectioning of the core samples and their importance are discussed.
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Core Cutting (in action)
See core cutting in action. Featuring Vannie, one of our lab technicians, see how important PPEs are in sample preparation.
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Housing the Samples
Watch how our team meticulously prepares the housing for the core samples. This is a crucial part of the process as it will act as mold to hold and confine the chemicals added to the concrete sample.
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Impregnation
Fluorescent dye or fluorescent powders are mixed with epoxies during sample mounting and impregnation. Once mixed with the epoxy, the solution will infill and penetrate the sample. Along with the use of UV light or fluorescence light setting in the microscope it will highlight the presence of discontinuities present in the sample such as cracks and air voids. It also has a very important role in observing the apparent water-cement ratio of the hardened concrete.
Tips in mixing dyed epoxies:
Proper mixing of dyed epoxies and sample impregnation are crucial steps in the preparation of concrete samples.
Materials (consumables) needed:
Resin
Hardener
Fluorescent Dye
Mixing Procedure:
Weigh resin and hardener in separate containers. Make sure that the containers used and the scales are clean to prevent contamination.
Prepare fluorescent dye about 0.5-0.1 wt% of total solution. Mix resin and fluorescent dye until there is no undissolved dye particles remain.
Add the hardener. Mix the solution, but do not mix vigorously; too violent mixing may lead to excess air entrainment or even heat.
Now you’re ready to impregnate concrete samples!
#Mixing #Impregnation
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Cutting and Grinding
To ensure that the samples reach the required thickness without damaging the concrete, the sections are cut and ground thrice.
Final Polishing
To ensure that the sections have an even thickness, they are polished on a glass plate. Using silicon carbide grits and sandpapers, the samples are polished one last time.
Covering and Final Touches
A cover slip is a very thin glass that is mounted on the concrete thin section sample for petrographic analysis. Cover slips prevent exposure of sample to the atmosphere, thus preventing carbonation during and after sample preparation. Thin section samples with cover slips also appear clearer and more defined under the microscope as light scattering is prevented by this very thin glass.
How to mount a cover slip on concrete thin section?
Make sure that the thin section sample is clean and clear of any debris from grinding and polishing
Affix glass cover slip using UV curing adhesive. Do not leave excess adhesive on the surface
Drive out all bubbles present in the sample
Be careful in distributing weight to prevent cracking
Place a weight on top of the sample and expose to ultraviolet light for 10-15 minutes.
Other Information
Concrete Marker Minerals
When preparing concrete thin sections, the thickness of 0.03 mm or 30 microns must be achieved in order to have proper analysis. To do this, we use marker minerals which are commonly present and abundant to guide us in identifying what thickness range we are currently in.
Two of the most commonly used marker minerals in concrete thin sections are quartz and plagioclase. We can say that our thin section is almost or already 0.03 mm in thickness once their interference color under cross polars become first order gray or white. This means that our thin section is nearly finished. Without some of their distinguishing characteristics, these two minerals can be confused with one another. Here are some of their key characteristics:
Quartz
Maximum Interference color : first order gray and white in XPL
Habit: most commonly anhedral or equant
Twinning: not visible
Color: colorless in PPL
Others: can have undulatory extinction
Plagioclase
Maximum Interference color : first order gray or white in XPL
Habit: Tabular or lathlike; euhedral to anhedral
Twinning: Penetration and/or Polysynthetic twinning
Color: colorless in PPL, may appear cloudy
Others: can have zoning
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Reference:
Petrographic Data File. (n.d.). Retrieved October 01, 2020, from http://www.science.smith.edu/geosciences/petrology/petrography
Photos:
Vale, L. (n.d.). Alex Strekeisen. Retrieved October 01, 2020, from https://www.alexstrekeisen.it/english/vulc/quartz.php
Vale, L. (n.d.). Alex Strekeisen. Retrieved October 01, 2020, from http://www.alexstrekeisen.it/immagini/vulc/plagioclasi2012(8).jpg
UV Light and UV Curing Adhesives
The UV Light is used for faster drying/curing of the UV curing adhesive which is applied in attaching the cover slip in the sample.
Unlike most epoxies that will harden after almost 1 hour, this adhesive will only take 10-15 minutes to harden without heating. The color of the adhesive is also transparent so it will not affect the sample when viewed under the microscope.
Another use of the UV Light is during microscopic analysis. It is used to easily detect airvoids and cracks in the sample containing the fluorescent dye epoxy solution.
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