Industrialised, Integrated, Intelligent sustainable Construction - I3con

SUSTAINABLE CONSTRUCTION HANDBOOK 2
Figure 3. Rectangular wireless sensor unit with helix antenna. Design by DTI
The sensor that is meant for cast-in applications (Fig. 3) is protected in a transparent plastic casing
with dimensions approx. 2x4x6 cm. In order to avoid water or cement paste to enter the sensor unit an
appropriate membrane material is used to block water molecules but allowing water vapour to enter
the sensor unit. The performance of the sensors is demonstrated in the full-scale applications and so
far they have survived castings in self-compacting concrete and dry mix concrete (roller compacted).
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Applications in civil structures
Helix antenna
Battery
Large structures such as civil structures (e.g. bridges and tunnels) are often designed with 100+ years
of service life. They are often placed in aggressive environments (frost, thaw, de-icing, seawater, etc.).
Sensors may be useful to monitor the critical areas for strength and durability both during construction
(short term quality assurance) and over long term considerations of maintenance issues. Since civil
structures are often important infrastructures, causing large problems in case of failure and during
repairs, it is important to be able to monitor the state of the structures with as little disturbance as
possible. Embedded wireless sensors are expected to provide better decisions for repair need and
improved prioritising assistance to consulting engineers and building owners.
During the summer of 2007 a preliminary application was installed in connection with a highway
bridge repair project under the auspices of the Danish Road Directorate (Fig. 4). The repair was
typical for Danish concrete bridges where the waterproof membrane under the asphalt pavement
becomes defective after 20-40 years of service life. Then water gets access to the reinforced concrete
bridge deck from above, bringing harmful chlorides to the concrete and increasing the risk of
corrosion. After removal of asphalt and the old damaged concrete the bridge deck is recast, followed
by new waterproofing membrane and pavement. The whole operation is carried out in two phases in
order to minimize traffic disturbance.
A total of 5 wireless moisture sensors corresponding to the one shown in Fig. 3 have been installed in
small box outs directly under the membrane. Each sensor is equipped with extra batteries in order to
ensure up to 30 years battery life (Fig. 4 and 5). The five sensors are installed along the bridge deck
gutter line over a length of 30 m. Their purpose is to monitor whether the membrane starts to leak and
indicate the time of rehabilitation. It is recognized that the sensors are probably not going to last 30
years but they will give valuable information on the practical aspects of installation and embedment
techniques. It is also demonstrated how the sensor survive heat impact from hot bitumen during
application of membrane and hot asphalt.