Introduction - Geomaterials: Under the Microscope - page 8

Future developments
Optical microscopes have now entered the digital age and
modern microscopes tend to come with a computer
attached. The major microscope manufacturers now offer
options for automation of their optical microscopes with
a range of functions being controlled via a touch-sensitive
screen. Motors automatically move the objective, substage
condenser, and the image projection system; all of which
is optimized quite automatically for quality of
illumination, resolution, and even focus. Combined with
digital image capture systems and image analysis software
we now have a real possibility of automated sample
examination. However, an experienced petrographer will
always be required to supervise the process and check the
results. Automation offers huge improvements in
productivity and potentially significant reductions in the
unit price of examinations. Automated modal analysis is
likely to become a serious alternative for a number of
standard tests that have traditionally been performed by
cheaper (and arguably less accurate) chemical analysis
methods. For example, the mix proportions of hardened
concrete and mortars could routinely be determined by
modal analysis instead of chemical analysis.
Digital technology is changing the way that the
findings of petrographic examinations are presented.
Digital image capture allows a series of sequential
micrograph frames to be animated into movies to illustrate
talks and websites (Entwistle, 2003a). Digital movies
demonstrating petrographic features of samples could be
included within the report submitted to the client.
The recent development of the birefringence imaging
microscope offers the possibility of interesting
applications for examination of geomaterials. The
equipment consists of a motorized rotating polarizer that
is fitted below the sample stage and a digital camera that
fits on the trinocular microscope head, both attached to a
computer. As the motorized polarizer rotates, the camera
collects birefringence data at a number (between five and
fifty) of different positions. The data are then processed by
special computer software to produce various types of
false colour image. The birefringence microscope excels at
detecting strain and defects within materials. Figures 11
and 12 show examples of false colour images obtained
using the birefringence microscope. Existing applications
include quality control of industrial diamonds, silicon
carbide abrasives, and glass, study of decomposition of
biomaterials, and mapping of collagen in heart valves.
Potential applications for geomaterials include
investigation of rock microstructure (including the

investigation of bowing marble panels), detection of
alkali-silica reactive aggregates, and identification of
flaws in a variety of construction products.
Electron microscopy is one of the main
complementary techniques used in conjunction with
optical microscopy hence developments in the field of
electron microscopy concern the petrographer. The
resolving power of the electron microscope is continually
improving, with modern field emission scanning electron
microscopy (FESEM) now providing magnifications of up
to 550,000 times and resolutions down to 0.5 nm. One
disadvantage of conventional electron microscopy is that
the sample has to be viewed in a vacuum. Recent
advances have allowed hydrated samples to be imaged
using environmental scanning electron microscopy
(ESEM) or alternatively, by soft X-ray transmission
microscopy. These methods allow 'live' examination and
analysis of geomaterials undergoing reactions, for
example, hydrating cement paste or carbonating lime.
Also, the latest cryotransfer SEM allows sensitive
hydrating specimens to be set in a stable state by quick-
freezing, enabling previously impossible examinations.
Advances in electron microscopy will continue to
improve our understanding of geomaterials and reactions
that they undergo.

SAMPLING AND SAMPLE PREPARATION

S AMPLING
Samples of construction materials may be obtained during
the manufacturing production run or, alternatively, from
structures during construction or while they are in service.
The objectives of the materials investigation and details of
the proposed laboratory testing should be clearly defined,
before any sampling is attempted. A coordinated sampling
programme should be prepared by persons experienced
with the investigation of construction materials and built
structures. The number of samples required to achieve the
investigation objective will depend on the purpose of the
investigation/testing, the size of the structure, the types of
construction used, and the number of construction phases.
Sampling schemes usually comprise one of two types
(or a combination of both). The first is essentially a
random or even spread of samples across a structure or
production run, to ascertain representatively the general
materials' characteristics and quality. The second scheme
is more targeted to address specific issues, such as
investigating suspected defects identified by visual
survey. In either case, the investigator must clearly