27-09-2012, 01:19 PM
Temperature and moisture monitoring in concrete structures using embedded nanotechnology/microelectromechanical systems (MEMS) sensors
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Abstract
The curing and damage process of concrete is significantly affected by the temperature and moisture level. Moisture transfer during
construction could prevent concrete from developing its full strength and might lead to high shrinkage stresses. In addition, self-desiccation
due to temperature of the hydrated cement paste causes an additional decrease of moisture content at early ages, thereby influencing
the properties of the young concrete as well as its long-term behavior, i.e., deformation caused by self-generated stresses, stability
and durability. Temperature and high moisture content can also promote deterioration processes of concrete structures and could jeopardize
their integrity and long-term durability. The monitoring of temperature and moisture level will provide crucial information about
the hardening and setting process of portland cement concrete as well as the progress of deterioration mechanisms such as corrosion of
steel reinforcement, freeze–thaw cycles, carbonation and alkali-aggregate reaction. A new technique that monitors the moisture level and
temperature is presented in this paper. The proposed technique uses nanotechnology/microelectromechanical systems (MEMS) to measure
temperature and internal relative humidity (RH) using microcantilever beams and moisture-sensitive thin polymer. The durability
and the sensing capability of the proposed MEMS were investigated analytically and experimentally. Based on the obtained results, it
was found that the proposed MEMS survived the concrete corrosive environment and internal and external stresses. Also it was found
that the MEMS outputs reflect the change in the concrete properties and can be used to measure moisture content and temperature effectively
and with a high sensitivity.
Introduction
Concrete strength and durability depend mainly on the
temperature and the dynamics of moisture transport [1–3].
Concrete material properties change with the time, and these
properties (i.e., concrete strength, modulus of elasticity,
creep, and shrinkage) are significantly influenced by the heat
of hydration and moisture content in the concrete at early
ages [4]. For example, moisture diffusion during curing
may prevent concrete from developing its full strength and
might lead to high shrinkage stresses [5,6]. In addition,
self-desiccation due to temperature of the hydrated cement
paste causes an additional decrease of moisture content at
early ages, thereby influencing the properties of the young
concrete as well as its long-term behavior [6].
Technologies enabling nanotechnology/MEMS in concrete
structures
MEMS represent one of today’s most exciting areas of
microelectronics activities. MEMS technology has brought
together innovations from many areas of microelectronics
to develop rapidly into a discipline of its own. MEMS refer
to a collection of microsensors and actuators which can
both, sense their environment and have the ability to react
to changes in that environment with the use of microcircuit
control. They include, in addition to the conventional
microelectronics packaging, integrating antenna structures
for command signals into micro-electromechanical structures
for desired sensing and actuating functions. MEMS
combine the signal processing and computational capability
of analog and digital integrated circuits with a wide variety
of non-electrical elements, including pressure, temperature,
chemical, stress/strain, and accelerometer sensors.
MEMS sensor principal
The behavior of the proposed microcantilever sensor
under concrete moisture generated stresses can be analyzed
theoretically using mechanics of materials to obtain
a relationship between the microsensor output and the
applied stress. We will use the stress sensitivity of the
nano-resistor to study the change on the cantilever surface.
Fig. 1 shows a MEMS cantilever equipped with a
nano-water vapor polymer film. Fig. 1a shows the cantilever
with an expanding film on the top surface. As shown,
the cantilever bends downwards and expands until the
cantilever beam’s stress balances the stress in the thin film.
The stress in the film rf is compressive as the expansion is
hindered by the supporting cantilever, where the bond
constrains this film surface, preventing it from displacing.
This constraint, known as a full shear constraint, produces
shear stresses rs at the film/beam interface which
cause the cantilever beam to deflect. This deflection is
measured as resistance change in the embedded strain
gauges, and is linearly proportional to the shear stress
rs.
Fabrication of MEMS sensor
The manufacturing of micro-machined temperature/
moisture sensor was performed using a combination of
standard and customized semiconductor processing steps.
Manufacturing begins with standard complimentary metal
oxide semiconductor (CMOS) processing steps. The microsensor
starts out as a blank silicon wafer. Typical CMOS
steps performed on the wafer include chemical vapor deposition
(CVD), oxidation, doping, diffusion and metallization.
Photolithography and chemical wet etching are used
to pattern and form the silicon platform and measurement
structures of the sensor. A hybrid CMOS process is performed
to deposit, pattern and activate the polymeric sensing
element.
Conclusions
In this paper, the feasibility of embedding MEMS into
concrete for temperature and moisture measuring was
studied analytically and experimentally. The theoretical
study showed that the change in the resistance of the sensor
due to moisture depends mainly on the cantilever beam
thickness and modulus of elasticity, and the magnitude of
the shear stress at the cantilever/polymer interface. Also
it was found that only the cantilever beam thickness and
stiffness influence the MEMS sensitivity, and this sensitivity
is independent of the cantilever length. Short experimental
tests indicated that the MEMS survived the concrete curing
and internal/external stresses, and their outputs reflect the
change in the concrete’s moisture content and temperature.
However, long-term experimental tests are required to evaluate
the long-term durability of the proposed device when
embedded into concrete.