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http://dx.doi.org/10.1007/s10921-013-0183-y
http://hdl.handle.net/10251/46457
Elsevier
Eiras Fernández, JN.; T.Kundu; Bonilla Salvador, MM.; Paya Bernabeu, JJ. (2013).
Nondestructive monitoring of ageing of Alkali resistant Glass fiber reinforced cement
(GRC). Journal of Nondestructive Evaluation - NDT and E International. 32:300-314.
doi:10.1007/s10921-013-0183-y.
Nondestructive Monitoring of Ageing of Alkali Resistant Glass Fiber
Reinforced Cement (GRC)
J. N. Eiras
a
, T. Kundu
b
, M. Bonilla
a
, J. Payá
a
a
Instituto de Ciencia y Tecnología del Hormigón (ICITECH). Universitat Politècnica de València. Camino de Vera
s/n, Valencia, E-46022 Spain.
b
Department of Civil Engineering and Engineering Mechanics, University of Arizona, Tucson, Arizona 85721,
USA
ABSTRACT
Glass fiber reinforced cement (GRC) is a composite material made of portland cement mortar and alkali resistant
(AR) fibers. AR fibers are added to portland cement to give the material additional flexural strength and toughness.
However ageing deteriorates the fibers and as a result the improvement in the mechanical properties resulted from
the fiber addition disappears as the structure becomes old. The aim of this paper is monitoring GRC ageing by
nondestructive evaluation (NDE) techniques. Two different NDE techniques – 1) nonlinear impact resonant
acoustic spectroscopy analysis and 2) propagating ultrasonic guided waves - are used for this purpose. Both
techniques revealed a reduction of the nonlinear behavior in the GRC material with ageing. Specimens are then
loaded to failure to obtain their strength and stiffness. Compared to the un-aged specimens the aged specimens are
found to exhibit more linear behavior, have more stiffness but less toughness. Finally, undisturbed fragments on the
fracture surface form mechanical tests are inspected under the electron microscope, to understand the fundamental
mechanisms that cause the change in the GRC behavior with ageing.
Keywords: Glass Fiber Reinforced Cement, Material Ageing, Ultrasonic Guided Waves, Nonlinear Impact Resonant
Acoustic Spectroscopy, Nondestructive Evaluation.
1 Introduction
Alkali resistant glass fiber reinforced portland cement is a cement-based composite material that has higher flexural
strength and toughness than plain cement [1]. However, GRC undergoes a rapid ageing process especially in humid
and alkaline environment (pH>12). This ageing can nullify the positive effects of glass fibers undergoing from a
ductile to a brittle material. The loss of mechanical properties with ageing has been attributed to two different
mechanisms - a stress corrosion cracking process in glass materials called static fatigue [2] and the growth of
hydration products, mainly portlandite around the single filaments in the strand [1]. This concern relegates GRC
mainly to nonstructural applications, such as façade panels, acoustic barriers, permanent formwork or cladding
tunnels. Recent applications of GRC as structural material have been investigated in telecommunication towers by
combining carbon fiber, glass fiber and steel reinforcement [3].
Different strategies for improving the durability of GRC have been attempted by modifying the fibers and/or by
altering the alkalinity of the matrix [7-9]. All these improvements have been evaluated by mechanical tests after
accelerated ageing. Accelerated ageing tests have been broadly accepted for testing the durability of GRC. They can
be classified as i) Deemed to satisfy tests, (EN 1170-8 [10]) where the GRC specimens are exposed to severe
conditions, and ii) predictive accelerated ageing tests [11] that are commonly used to predict the service life of the
material, in real weather conditions. For example, Purnell et al. [2] established that GRC soaked for 1 day in water
at 55ºC, corresponds to 100 days of exposure to the real weather conditions in the United Kingdom. However, it
should be noted that the correspondence between the accelerated ageing tests and the real aging conditions is still
being investigated, especially when different matrix compositions are to be compared [11-12]. Typically the ageing
process has been assessed by mechanical testing or by strand in cement (SIC) tests [13-14]. The aim of this work is
to assess the ageing process in GRC by two nondestructive testing techniques – 1) resonance frequency tests at
different impact energy levels called Nonlinear Impact Resonance Acoustic Spectroscopy (NIRAS) and 2)
Ultrasonic Guided Wave (UGW) tests. GRC specimens are subjected to accelerated ageing by placing them in water
baths at elevated temperatures. NIRAS and UGW tests are conducted on aged and un-aged specimens to study the
effect of ageing on different parameters measured by these tests. The specimens are then loaded to failure and
fragments from the fracture surface of the specimens are inspected under the electron microscope to investigate the
effect of ageing on the strength, toughness and internal composition of GRC specimens. The final objective of this
research is to be able to monitor the health (strength and toughness) of GRC by nondestructive testing, and to
understand why and how this material degrades with time.
2 Background
2.1 Nonlinear Impact Resonant Acoustic Spectroscopy
In general, cement based materials as a result of their intrinsic heterogeneities, can be classified as Nonlinear
Mesoscopic Elastic (NME) materials [15]. This particular behavior is manifested as a frequency shift in their
resonant frequencies known as fast dynamic effect. Experimental findings have demonstrated that fast dynamic
effect is related to hysteresis in the strain-stress relationship [16]. After Guyer et al. [17] a phenomenological model
to describe the hysteresis in the Preisach-Mayergoyz space [18] can be written as:
( )
[ ]
)(1
2
εεαδεβε
signEE
o
+∆+++=
(1)
where E
0
is the linear elastic modulus, β and δ are the cubic and quartic anharmonicities, ε is strain, Δε is the strain
amplitude, is the strain rate due to hysteresis, and sign is the sign function which is equal to 1 if > 0, -1 if < 0
and 0 if = 0. The hysteresis nonlinearity parameter α is a measure of the material hysteresis and is related to the
fast dynamics as follows [19].
εα
∆=
−
·
o
o
f
ff
(2)
where f
0
is the linear resonance frequency and f is the resonance frequency with increasing strain amplitude. NIRAS
technique requires exciting the resonant frequencies at different energy levels. The main advantage of NIRAS
measurements is that multiple modes are generated with a single impact and their corresponding dynamic nonlinear
parameter α can be obtained. It has been demonstrated that α is a sensitive indicator of damage in cement based
materials subjected to alkali silica reaction [20-21], carbonation [22] and compressive mechanical damage [23].
Boundary conditions and sample shape differ in our study from those of others. In this study we report the hysteretic
parameter α for various vibration modes and its sensitivity to the ageing process of GRC.
2.2 Ultrasonic Guided Waves – Linear and Nonlinear Techniques
Traditionally, linear ultrasonic inspection technique is used for detection of material damage or anomalies.
Macroscopic anomalies such as cracks, notches, inclusions and corrosions can be detected in this manner by
propagating ultrasonic bulk waves or guided waves [24-25] through the specimen. The wave is reflected by the
anomaly or transmitted through it undergoing mode conversion because of the linear interaction between the
Código de campo cambiado
Código de campo cambiado
anomaly and the propagating wave. Only macroscopic anomalies having dimensions in the order of the wavelength
or larger can be detected in this manner while anomalies that are much smaller than the wavelength remain hidden to
the linear ultrasonic technique. However, the presence of smaller anomalies can be detected by the nonlinear
ultrasonic method. The nonlinear ultrasonic techniques are classified primarily under two categories – (i) those
based on the generation of higher harmonics [26] and (ii) those based on the generation of the side bands [27].
If an ultrasonic signal of frequency ω is sent through a linear specimen its frequency remains unchanged. However,
a nonlinear specimen alters the frequency of the propagating wave. The signal propagating through a non-linear
specimen contains frequency components that are different from the original frequency ω. When the received signal
frequency
(
=
) is an integer multiplier of the original frequency ω, then the generated signals are called
higher harmonics. The degree of nonlinearity of the material can be related to the strength of the higher harmonic
signals using the β-factor [25]. If the high frequency signal is modulated by a high amplitude low frequency signal
then the spectral plot of the received signal shows several smaller frequency peaks at
(= ±
) near the high
frequency peak at ω. These frequencies
are not integral multipliers of ω and are called sidebands. The degree of
nonlinearity is related to the strength of the sidebands. Higher the material nonlinearity stronger are the sidebands.
3 Experimental Investigation
3.1 Specimen Fabrication
GRC specimens of dimension 225x50x10 mm were produced following the European standard EN 1170. The
samples were made from cement of type CEM I/52.5R EN 197-1 and siliceous aggregate with cement-aggregate
proportion of 1:1 and a fineness modulus of 3.1. Water to cement ratio was taken as 0.35 and Glenium ACE 32
superplasticizer was added (0.43% of cement weight) in order to obtain a slump of 165 mm as instructed in EN
1170-1 [28]. Non dispersible AR Glass fibers CemFil with a length of 12 mm were added to the mortar. The fiber
weight was 3% of the mortar weight.
The specimens were stored at 20ºC and relative humidity of 100%. After more than 28 days of curing, the specimens
were aged in a hot water bath at 65ºC. Two different types of nondestructive inspections were conducted to assess
the ageing process: NIRAS and UGW. Then mechanical testing and scanning electron microscopy observations
were performed.
