Civil Engineering Reference
In-Depth Information
1.8 Infra-red thermography
This technique uses infra-red photographs taken from a structure which
has been heated, as it cools. The heating is normally performed by the sun
in daytime and the photographs are best recorded in the evening as the
structure cools. Infra-red thermography offers many potential advantages
over other physical methods for the detection of delamination in bridge
decks. Areas of sound and unsound concrete will exhibit different thermal
characteristics and thus have different surface temperatures as the structure
cools. Delaminated areas, for example, will have a different temperature
gradient compared with sound areas. Water-saturated concrete will appear
quite different to dry concrete. The temperature differences are small but
are readily visible on the highly sensitive modern infra-red cameras, such as
the RAZ-IR™. The images can be downloaded to a PC for further analysis.
The surface of the structure needs to be viewed from a reasonable
distance and so cannot be recorded whilst, for example, standing on a deck.
Some success has been achieved working from a tall truck at a height of 20
m, provided that the temperature differences were at least 2°C. Working
from an aircraft or helicopter avoids the need for lane closures but has not
consistently shown good results.
Figures 1.30
and
1.31
demonstrate the
usefulness of IR photography in revealing delamination in a silo tower.
Holt and Eales (1987) have also described the successful use of
thermography to evaluate effects in highway pavements with an infra-red
scanner and coupled real-time video scanner mounted on a 5 m high mast
attached to a van.
The vehicle is driven at up to 15 mph and images are matched by computer.
Procedures for infra-red thermography in the investigation of bridge deck
delamination are given in ASTM D4788 (ASTM, 2007).
Hidden voids or ducts can also sometimes be detected, and techniques
have been developed to detect reinforcing bars which have been heated
by electrical induction. More recent development of 12-bit equipment has
improved the sensitivity to within ±0.1°C. This has enabled high-definition
imaging and accurate temperature measurement on buildings. The smallest
detectable area is reported to be 200 × 200 mm.
Infra-red thermography can also be used to reduce heat losses at hot spots
by identifying missing thermal insulation.
1.9 Testing for reinforcement corrosion
1.9.1 Half cell potential testing
Steel embedded in good-quality concrete is protected by the high alkalinity
pore water which, in the presence of oxygen, passivates the steel. The loss of
alkalinity due to carbonation of the concrete or the penetration of chloride
ions (arising from either marine or de-icing salts, or in some cases present
