Civil Engineering Reference
In-Depth Information
the existence of a skin effect, the electromagnetic radiation at high frequencies interacts
with a conductor (such as carbon fiber) only near its surface. As a result, at a constant
volume fraction of fibers the shielding effectiveness will increase with decreasing fiber
diameter. Especially effective appears to be the incorporation of carbon filaments, rather
than conventional carbon fibers, because of their smaller diameter (Fu and Chung, 1996).
In laboratory experiments the attenuation was increased from 0.4 dB to 24.4 dB at 1.0
GHz by adding 0.5% of carbon filaments (per weight of cement) to a plain Portland
cement paste (
w/c
=0.50, cured for 28 d at RH=30%).
30.3
RADIO WAVE-REFLECTING CONCRETE
Normal Portland cement based concrete exhibits a low reflectibility and a high
transmissivity to radio waves. This ratio may be reversed by incorporating small amounts
(0.5 vol.%) of carbon filaments in the mix. In such mixes the absorbed radiation is
negligible, compared with the reflected radiation (Fu and Chung, 1998). Concretes of this
type may be potentially useful for lateral guidance in automatic highways.
30.4
CEMENTS AND CONCRETES FOR USE IN STRUCTURES DESIGNED
TO SHIELD AGAINST IONIZING RADIATION
Concrete walls with the capacity to shield against ionizing radiation may be required in
atomic power plants and facilities using radioisotopes, X-ray equipment, and the like.
The shielding capability of a concrete wall may be effectively controlled by its
thickness, and by the selection of the aggregate used. In general, the shielding capacity
(at a constant thickness of the wall) increases with increasing density of the aggregate.
Aggregates that exhibit a high shielding capability include baryte (BaSO
4
), hematite
(Fe
2
O
3
), and magnetite (Fe
3
O
4
). To increase the density even further, metallic iron in the
form of powder or chips may also be added to the fresh concrete mix. In this way
concrete densities of
d
4.0 kg/L or more may be achieved.
In most instances Portland cement is used as the binder, but the shielding capacity may
be increased even further by using a cement in which a fraction of CaO has been
substituted by BaO (see also section 15.8).
REFERENCES
Fu, X., and Chung, D.D.L. (1996) Submicron carbon filament cement-matrix composites
for electromagnetic interference shielding.
Cement and Concrete Research
26,
1467-
1472.
Fu, X., and Chung, D.D.L. (1998) Radio-wave-reflecting concrete for lateral guidance in
automatic highways.
Cement and Concrete Research
28,
795-801.
Garboczi, E.J., Schwartz, L.M., and Bentz, D.P. (1995) Modelling the influence of the
