Geoscience Reference
In-Depth Information
thin sections visually and can be quantified by
measuring the number of unhydrated clinker grains
encountered on a traverse of at least 120 mm in length.
French suggests that the results are accurate to ±0.02. A
similar method, described by Ravenscroft (1982) involves
determination of the unhydrated clinker by point-
counting. This method is more time consuming and uses
highly polished and etched specimens of cement paste
that are examined in reflected light.
It is useful to know that a number of different methods
using scanning electron microscopy and associated
microanalysis have been developed for determining W/C.
These are yet to be widely used and variously involve:
• Comparison of chemical (oxide) composition of the
hardened cement matrix determined using the EDS
(energy dispersive X-ray spectroscopy) attached to
the electron microscope with standard specimens
(French, 1991a).
• Examination of the grey level observed in
backscattered imaging mode. The grey level appears
darker as W/C increases (Gonçalves
et al
., 2007).
• Quantifying the volumetric fractions of capillary
pores, hydration products, and unreacted cement
using high resolution FESEM in the backscattered
electron mode. The free W/C is calculated from the
obtained data and the volumetric expansion
coefficient of cement hydration (Wong & Buenfeld,
2007).
184
A
IR VOIDS
When freshly mixed concrete is placed, it is typically
compacted (by vibration) to eliminate entrapped air
bubbles and improve aggregate packing. However, some
air bubbles will invariably be retained to become
entrapped air voids in the hardened concrete. In addition,
air-entraining chemical admixtures may be added to the
fresh concrete to improve frost resistance of the hardened
concrete, by facilitating the formation of entrained air
voids. Also, the use of water-reducing chemical
admixtures (superplasticizers) may result in a lesser
degree of air entrainment.
Entrapped air voids (
184
) are mostly >1 mm across
and are typically irregular in shape. They are irregularly
distributed in concrete and often increase in number and
size towards the concrete surface. In contrast, entrained
air voids (
185
) are between 10 μm and 1 mm in diameter,
are characteristically spherical in shape, and are
uniformly distributed throughout the concrete.
Deliberately entrained air may form 4-5% of the volume
of concrete.
The degree of concrete compaction is normally
assessed by estimating the excess voidage. The excess
voidage is the volume percentage of entrapped air voids
present over and above that found in the same concrete
which has been as fully compacted as possible. This is
normally estimated visually in hand specimen by
comparison with standard specimens, or more commonly
standard photographs, such as those provided in
Concrete Society Technical Report No. 11 (1987). Once
the excess voidage has been estimated the degree of
184
Surface of a finely ground slice of concrete (100 ×
75 mm area) that is obliquely illuminated to pick out
the air voids (in shadow). Most of the air voids in view
are entrapped air voids and the compaction is good
(excess voidage in the 0.5-3.0% range). Crushed
limestone aggregate appears light brown while the
cement matrix is shown medium brown.
185
185
Air-entrained concrete exhibiting numerous small,
spherical entrained air voids (yellow). Crushed granite
aggregate appears mottled and the cement matrix is
shown dark brown; PPT, ×35.
