Accelerated aging and fatigue effects on GFRP grids

Abstract

In recent decades, the use of composite materials for upgrading and consolidation of historical
masonry building has been extensively studied. Whilst there are numerous experimental results that demonstrate their effectiveness in terms of increase in capacity of the strengthened elements, much remains to be known about durability of Fiber-Reinforced Polymer (FRP) composites as related to civil infrastructure applications. In this work, mechanical characteristics of Glass Fibre Reinforced Polymer (GFRP) specimens have been evaluated. Tensile tests have been carried out on samples subjected to different treatments in water solutions (water and salt water) and at different number of load cycles (60000, 150000 and 300000). The tensile strengths of the specimens determined before and after exposure were considered a measure of the durability performance. Test results showed a
reduction of tensile strength up to 30.2% for the specimens subjected to treatment in water solution and up to 10.8% for those subjected to fatigue treatments.

Full text

IF CRASC ’15
III CONVEGNO DI INGEGNERIA FORENSE
VI CONVEGNO SU CROLLI, AFFIDABILITÀ STRUTTURALE, CONSOLIDAMENTO
SAPIENZA UNIVERSITA’ DI ROMA, 14-16 MAGGIO 2015
ACCELERATED AGING AND FATIGUE EFFECTS ON GFRP
GRIDS
M. Corradi, L. Righetti, A. I. Osofero
Northumbria University, Newcastle Upon Tyne, UK
A. Borri, G. Castori, R. Sisti
Università degli Studi di Perugia, Perugia, Italy
ABSTRACT
In recent decades, the use of composite materials for upgrading and consolidation of histor-
ical masonry building has been extensively studied. Whilst there are numerous experi-
mental results that demonstrate their effectiveness in terms of increase in capacity of the
strengthened elements, much remains to be known about durability of Fiber-Reinforced
Polymer (FRP) composites as related to civil infrastructure applications. In this work, me-
chanical characteristics of Glass Fibre Reinforced Polymer (GFRP) specimens have been
evaluated. Tensile tests have been carried out on samples subjected to different treatments
in water solutions (water and salt water) and at different number of load cycles (60000,
150000 and 300000). The tensile strengths of the specimens determined before and after
exposure were considered a measure of the durability performance. Test results showed a
reduction of tensile strength up to 30.2% for the specimens subjected to treatment in water
solution and up to 10.8% for those subjected to fatigue treatments.
1. INTRODUCTION
FRP composites are being increasingly used in historic masonry structures due to their re-
sistance to corrosion, high strength-to-weight ratio and ease of handling and application.
The use of FRP materials for the rehabilitation and consolidation of the masonry and con-
crete elements is strongly related to their ability to maintain their mechanical and physical
characteristics over the years. Recently, several studies have focused their interest on the
analysis of new techniques to reinforce masonry elements using GFRP grids (Borri et al.,
2014a, b, c; Corradi et al., 2014; Gattesco et al., 2014). Compared to traditional techniques,
instead of metal bars a GFRP grid is inserted into a low cement content mortar jacketing.
The use of composite materials provides a solution to the problems usually encountered in
traditional steel-bar based techniques: the rusting of the steel rebars, the excessive stiffness
of the reinforcement, the limited reversibility of the work.

M. Corradi, L. Righetti, A. I. Osofero, A. Borri, G. Castori, R. Sisti
This reinforcing technique combines the benefits of both types of interventions (i.e. both
‘traditional’ and ‘modern’ ones) and involves the use of GFRP grids in the form of open fi-
ber meshes (grids) externally bonded on the masonry elements’ surfaces by means of mor-
tars; those materials are sometimes named Textile Reinforced Mortars (TRM) (Papanico-
laou et al., 2008).
Understanding the mechanism of degradation, and ability to define the variation of the me-
chanical characteristics of this material over time appears to be a fundamental problem.
Most studies on the use of FRP materials for strengthening of masonry elements are limited
to the investigation of enhanced mechanical properties. For example, GFRP grids, inserted
into an organic matrix (lime mortar), can coexist with extremely high pH values, due to the
hydration process of lime, that could potentially produce damaging of the glass fibres (Bor-
ri et al., 2014). It is therefore important to investigate the effect of environmental factors on
long-term performance of the material. Gangarao et al. (1997) subjected glass and polyester
composite materials specimens to different treatments in water solutions. The susceptibility
to dissolution in alkaline environment of the glass fibres was experimentally studied by
Uomoto (2003), Karbhari et al. (2003) and Liao et al. (1999). Test on FRP materials sub-
jected to different treatments in water solutions showed the possibility of sudden decrease
of their mechanical properties (Chu et al., 2004; Nkurunziza et al., 2004; Chen et al., 2007
and Abbasi et al., 2005).
Fatigue is another factor that could produce a decrease of the mechanical properties of the
FRP materials. It is important and extremely relevant to acquire information about the be-
haviour of FRP materials subjected to high number of load cycles in order to understand
whether fatigue produces a decay of the mechanical characteristics of the material. This in-
formation will be useful for the estimation of the time duration of the structural reinforce-
ment. Andersons et al. (1999) experimentally evaluated the reduction in strength of GFRP
due to fatigue. Several tests were also performed to evaluate the fatigue behaviour of con-
crete beams reinforced with FRP materials (Gussenhoven et al., 2005; Ekenel et al., 2006;
Gherghiu et al., 2006 and Brena et al., 2005).
This paper presents the results of an experimental campaign on GFRP specimens previously
subjected to water solution treatments for different time periods and different load cycles,
evaluating the effect of decay of the mechanical behaviour of the GFRP.
2. MATERIAL DESCRIPTION
To determine the strength and Young’s modulus, different tensile tests are carried out on the
fiberglass material. Two types of a GFRP grid have been used in this experimental work.
Both GFRP grids (Figs. 1-2) are manufactured by Fibre Net and have a mesh size of 66 x
66 mm. The material was comprised of epoxy vinyl ester resin with AR-glass (Alkali Re-
sistant) reinforcement and a zirconium content equal to or greater than 16%. The rein-
forcement was unidirectional.
Coupons were dry cut from GFRP grids with final dimensions obtained using a diamond
saw. For each grid, specimens were cut off respectively from the weft and warp directions.
The coupon shape was bar form with 4 different cross sections. The following indices were
used to identify the specimens: SC and SR respectively for circular and rectangular sections

Accelerated aging and fatigue effects on GFRP grids
(dry glass fibre section 3.8 mm
2) of grid type 1, BC and BR for circular and rectangular
sections (7.6 mm2) of grid type 2. The dimensions of GFRP specimens are given in Table 1.
3. EXPERIMENTAL PROGRAM
To evaluate the behavior of the material in presence of aggressive environment and cycles
loads, GFRP specimens were subjected to different ageing and fatigue treatments to assess
any decreases in their mechanical characteristics.
Figure 1. GFRP grid (Type 1).
Figure 2. GFRP grid (Type 2).
Figure 3. Specimens cut from GFRP grids.
Index Section shape Cross section
(mm2)
GFRP Grid
Type
Length
(mm)
Weight
(g/m2)
Direction
SC Circular 3.8 1 190 500 Weft
BC Circular 7.6 2 190 1000 Weft
SR Rectangular 3.8 1 190 500 Warp
BR Rectangular 7.6 2 190 1000 Warp
Table 1. Coupon dimensions.
The coupon dimensions were approx. 190 mm long with the clear distance between grips of
approx. 90 mm (Fig. 3). Untreated and treated specimens were tested in tension, in accord-

M. Corradi, L. Righetti, A. I. Osofero, A. Borri, G. Castori, R. Sisti
ance with ASTM D3039, using a Instron Tensile Machine type 3382, with a load cell of
100 kN. GFRP specimens were tested with nominally identical conditions at each end. The
end condition was made of a pair of soft timber packing pieces (tabs) glued with epoxy res-
in. All tensile tests were conducted with crosshead speed of 0.50 mm/min (displacement
control mode) at temperature of 23 °C and humidity equal to 50%. These results have been
reported solely for the purpose of qualitative evaluation of the decrease in the mechanical
property due to the different treatments described in the following paragraphs.
3.1 Untreated specimens
From the test results, the tensile strength and the Young’s modulus are calculated. 30 un-
treated GFRP specimens were tested. The tensile strength is dependent of the kind of spec-
imen because of the different ratio resin/fibre. The lowest measured tensile strength is 700.2
N/mm2 which belongs to BC type. Test results are presented in Figs. 7 and 8. Standard de-
viation and mean values were determined as given in Table 2. The three other types have a
mean tensile strength higher than 950 N/mm2. During the tensile test, all GFRP coupons
showed an approximately linear behavior up to failure (Figs. 4-7). The tensile modulus, E,
was calculated from the stress-strain data as the slope of a best fit line to the monotonic ul-
timate strength curve.
Index No. of
Specimens
Max Load
(N)
Tensile Strength
(SD)
(N/mm2)
Young modulus
E
(N/mm2)
Ultimate
Strain
(%)
SC 7 3679 968.2 (85.93) 74224 1.30
BC 4 5321 700.2 (83.92) 72236 0.97
SR 14 4480 1179.1 (78.22) 70189 1.68
BR 5 8493 1117.6 (84.99) 74453 1.51
Table 2. Mechanical characteristics of the untreated GFRP specimens (SD=standard deviation).
The average tensile capacity of the SC-type is 968.2 N/mm2 (SD=85.93 N/mm2). BC-type
exhibits a tensile capacity of 700.2 N/mm2 (SD=83.92 N/mm2), SR-type show a capacity of
1179.1 (SD=78.22 N/mm2) and BR-type exhibits a capacity of 1117.6 N/mm2 (SD=84.99
N/mm2).
0.0
1.0
2.0
3.0
4.0
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Load (kN)
Extension (mm)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
0.0 0.5 1.0 1.5 2.0 2.5
Load (kN)
Extension
(
mm
)
Figure 4. SC-type load-extension curves.
Figure 5. BC-type load-extension curves.

Accelerated aging and fatigue effects on GFRP grids
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.00.51.01.52.02.53.03.54.0
Load (kN)
Extension
(
mm
)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0.00.51.01.52.02.53.03.54.0
Load (kN)
Extension (mm)
Figure 6. SR-type load-extension curves.
Figure 7. BR-type load-extension curves.
Two different failure modes for untreated GFRP were noted: the first was a catastrophic
collapse (tensile failure of the specimen approx. in the centre) (Fig. 8) and the second was a
partial fibre-failure at the GFRP grid joint (Fig. 9). Failure henceforth was defined in this
study as the point of the tensile load versus displacement (extension) curve where either
sudden tensile load reduction was noted or a 20% reduction in load was detected in speci-
mens with gradual post-peak load reduction. Both tensile strength and Young modulus were
calculated using the dry glass fiber cross section values (3.8 and 7.6 mm2 respectively for
GFRP grid No. 1 and 2) (Figs. 1 and 2).
Figure 8. Catastrophic tensile failure.
Figure 9. Fibre intersection.
3.2 Fatigue treatments
Fatigue treatments were applied to 9 specimens prior to subjecting to tensile load. In addi-
tion, 14 untreated specimens were also tested for their tensile behavior. All the specimens
tested for tensile strength were of rectangular section with dry glass fibre section of 3.8
mm2 (Tab. 3). For ease of identification, the specimens were indexed as SR_U_0 for the
untreated and SR_F_1, SR_F_2 and SRF_3 for the treated specimens with load cycles of
60000, 150000 and 300000 respectively. The treated specimens were subjected to varying
load cycles through the use of Fatigue Instron Machine E3000 with a load cell of 3 kN as
shown in Figure 10. Loads were induced by a hydraulic piston and are subsequently trans-
ferred to the specimen through two clamping jaws (Fig. 11) and with a frequency of 7.5 Hz.

M. Corradi, L. Righetti, A. I. Osofero, A. Borri, G. Castori, R. Sisti
The induced load values ranged from 1.5 to 2.5 kN (amplitude of the sinusoidal curve equal
to 1 kN).
Index No. of specimens Section type Cycles
SR_U_0 14 SR 0
SR_F_1 3 SR 60000
SR_F_2 2 SR 150000
SR_F_3 4 SR 300000
Table 3. Geometrical characteristics of the specimens subjected to fatigue treatments.
Figure 10. Fatigue Instron Machine E3000.
Figure 11. View of the specimen during the load
cycles treatments.
Index Max Load
(N)
Tensile Strength
(SD)
(N/mm2)
Ultimate
Strain
(%)
Strength
Decrement
(%)
Young’s
Modulus
decrement (%)
SR_U_0 4480 1179.1 (78.22) 1.56 - -
SR_F_1 3995 1051.3 (53.33) 1.41 10.8 6.7
SR_F_2 4309 1133.9 (55.26) 1.52 3.8 10.7
SR_F_3 4137 1088.9 (82.72) 1.26 7.6 1.1
Table 4. Mechanical characteristics of GFRP specimens subjected to fatigue treatments. SD is the
standard deviation of the tensile strength.
Specimens were subjected to tensile test to evaluate their tensile capacity and extension at
failure post treatment. For these tests, failures occurred either through the complete split of
the specimen at mid-point (Fig. 8) or at grid intersection (Fig. 9). The tensile test results,
including the maximum load, average tensile strength and ultimate strain are reported in
Table 4. The average tensile capacity of SRF_1 is 1051.3 N/mm2 (SD = 53.33 N/mm
2),
SRF_2 exhibit a tensile capacity of 1133.9 N/mm2 (SD = 55.26 N/mm2), SRF_3 show a ca-

Accelerated aging and fatigue effects on GFRP grids
pacity of 1088.9 (SD = 82.72 N/mm2) and the untreated specimens SRU_0 exhibit an aver-
age tensile capacity of 1179.1 N/mm2 (SD = 78.22 N/mm
2). This result shows an up to
10.8% decrease in the ultimate tensile strength of the tested specimens due to fatigue effect.
However, it should be noted that no clear trend between the number of load cycles for fa-
tigue treatment and reduction in ultimate tensile strength has been observed in this particu-
lar experimental campaign. Larger experimental programs, with more number of specimens
might be necessary to establish the likelihood of such trends.
The load-extension curves for varying number of load cycles during fatigue treatments are
presented in Figs. 12–14. These curves are characterized by almost perfectly linear behav-
ior up to failure with a sudden loss of load capacity post-failure.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Load (kN)
Extension (mm)
Figure 12. SR_F_1 load-extension curves.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Load (kN)
Extension
(
mm
)
Figure 13. SR_F_2 load-extension curves.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
Load (kN)
Extension (mm)
Figure 14. SR_F_3 load-extension curves.
3.2 Ageing treatments
Moisture diffuses into organic resins, leading to changes in mechanical, physical and chem-
ical characteristics. The effect of this absorption is on the matrix (resin) itself through plas-
ticization, hydrolysis and other mechanism of degradation, which may cause irreversible
changes in the resin structure.
All four different types of GFRP specimens were subjected to ageing treatment in water so-
lutions. Two different solutions were adopted to simulate exposure of specimens to field
conditions: specimens were stored in deionized water and NaCl solution for different peri-
ods of time (1, 2, 3, 5 and 7 months). The quantity of NaCl added was 35 g for 1 litre of
water. After the treatment, specimens were subjected to tensile test to assess the effect of
these ageing treatments. Specimens were tested using the test set up as defined in Section 3.
Table 5 shows various ageing treatments and the subsequent tensile test results: indices SW
and W indicate treatment in NaCl solution and in deionized water respectively; while the

M. Corradi, L. Righetti, A. I. Osofero, A. Borri, G. Castori, R. Sisti
number after this index indicates the duration of the treatments in months.
Comparisons between tensile strengths for untreated and treated specimen for each type of
sample are presented in Figures 15-17 (the over bar represents a standard deviation). It
should be noted that limited number of specimens were testes for SC and BC series and re-
sults should be confirmed by larger experimental program. However, the emerging trend
seems quite interest and confirmed by comparing these results with those obtained for other
cross sections types (SR and BR).
Index No. of
coupons Treatment Time
(months)
Max
Load
(N)
Ten sil e
strength
(N/mm2)
Strength
decrease
(%)
Young’s
Modulus
decrease
(%)
SC 7 Untreated - 3679 968.2 - -
SC_SW_2 2 NaCl 2 3329 876.2 9.5 -2.09
SC_SW_3 1 NaCl 3 2900 763.3 21.2 11.02
SC_W_1 1 Water 1 3610 950.3 1.8 10.89
SC_W_5 1 Water 5 2964 780.1 19.4 13.19
BC 4 Untreated - 5321 700.2 - -
BC_SW_3 2 NaCl 3 4049 532.9 23.9 28.48
BC_W_5 1 Water 5 4710 619.8 11.5 11.46
SR 14 Untreated - 4480 1179.1 - -
SR_SW_3 4 NaCl 3 4108 1081.1 8.3 5.55
SR_SW_7 7 NaCl 7 3459 910.4 22.8 7.68
SR_W_5 2 Water 5 3125 822.5 30.2 1.65
SR_W_7 9 Water 7 3241 853.1 27.7 3.83
BR 5 Untreated - 8493 1117.6 - -
BR_SW_3 5 NaCl 3 7309 961.8 13.9 9.05
BR_W_5 2 Water 5 8136 1070.6 4.2 0.42
Table 5. Test results of GFRP specimens subjected to ageing treatments.
968.2 876.2 763.3
950.3
780.1
0
100
200
300
400
500
600
700
800
900
1000
1100
1
Tensile Strength (N/mm
2
)
SC SC_SW_2 SC_SW_3 SC_W_1 SC_W_5
Figure 15. Tensile strength of SC-type.
700.2
532.9 619.8
0
100
200
300
400
500
600
700
800
1
Tensile Strength (N/mm
2
)
BC BC_SW_3 BC_W_5
Figure 16. Tensile strength of BC-type.
Specimens subjected to treatment in deionized water experienced change in color from
green to white. This change became more evident with the increase of treatment duration
(Fig. 19). On the contrary, specimens subjected to treatment in NaCl solution did not exhib-
it any change in color (Fig. 20).

Accelerated aging and fatigue effects on GFRP grids
1179.1 1081.1 910.4 822.5 853.1
0
200
400
600
800
1000
1200
1
Tensile Strength (N/mm
2
)
SR SR_SW_3 SR_SW_7 SR_W_5 SR_W_7
Figure 17. Tensile strength of SR-type.
1117.6 961.8 1070.6
0
200
400
600
800
1000
1200
1
Tensile Strength (N/mm
2
)
BR BR_SW_3 BR_W_5
Figure 18. Tensile strength of BR-type.
Figure 19. Specimens after ageing in deionized
water (respectively 1 month, 3 months and 7
months from left to right).
Figure 20. Specimens after NaCl
solution treatments.
The NaCl solution treatment for SC series caused a 21.2% decrease in the tensile strength
after 3 months while the deionized water treatment produced a slightly smaller decrease in
tensile capacity of 19.4% after 5 months of immersion. Similar behaviour was noticed with
the BC series; tensile capacity decrease of 23.9% and 11.5% were recorded after 3 months
of immersion in NaCl solution and 5 months in deionized water treatment respectively.
Immersion in NaCl solution resulted in 22.8% decrease in the tensile strength of the SR se-
ries after 7 months while deionized water solution produced a 30.2% decrease in tensile ca-
pacity after 5 months.
For the BR series, tensile capacity decrease of 13.9% after 3 months and 4.2% after 5
months were recorded when immersed in NaCl solution and deionized water respectively.
The rapid degradation of the mechanical characteristics of the material may be due to the
direct exposure of specimens to strong ionic solutions. Master curves for tensile strength
retention versus exposure time at 23°C were obtained by fitting curve to the experimental
data as shown in Figs. 21-24. A clear trend of increased reduction in tensile strength with
increase in exposure time in both NaCl solution and deionized water is established. Howev-
er, due to limited experimental results these curves, in its present form, cannot be employed
in the prediction of the tensile strength retention at any exposure time. Further experimental
campaign, with larger data set is required to establish such relationship.

M. Corradi, L. Righetti, A. I. Osofero, A. Borri, G. Castori, R. Sisti
0
200
400
600
800
1000
1200
0 30 60 90 120 150 180
Tensile strength (N/mm2)
Exposure time (days)
Deionized
water
NaCl
solution
Figure 21. Tensile strength versus exposure time
for SC specimens.
0
100
200
300
400
500
600
700
800
0 306090120150180
Tensile strength (N/mm2)
Exposure time (days)
Deionized
water
NaCl
solution
Figure 22. Tensile strength versus exposure time
for BC specimens.
0
200
400
600
800
1000
1200
1400
0 30 60 90 120 150 180 210 240
Tensile strength (N/mm
2
)
Exposure time (days)
Deionized
water
NaCl
solution
Figure 23. Tensile strength versus exposure time
for SR specimens.
0
200
400
600
800
1000
1200
0 306090120150180
Tensile strength (N/mm
2
)
Exposure time (days)
Deionized
water
NaCl
solution
Figure 24. Tensile strength versus exposure time
for BR specimens.
During the tensile test, all the GFRP specimens showed an approximately linear behavior
up to failure and failed through the rupture of fibres (Figs. 25 and 26).
0.0
1.0
2.0
3.0
4.0
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.
0
Load (kN)
Extension (mm)
SC_SW2a
SC_SW2b
SC_SW3
SC_W1
SC_W5
Figure 25. Load-displacement for SC-type spec-
imens subjected to ageing treatment.
0.0
1.0
2.0
3.0
4.0
5.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Load (kN)
Extension (mm)
BC_SW3a
BC_SW3b
BC_W5
Figure 26. Load-displacement for BC-type spec-
imens subjected to ageing treatment.

Accelerated aging and fatigue effects on GFRP grids
4. CONCLUSIONS
This study involved the durability behavior of GFRP grids exposed to various environmen-
tal conditions. A series of experimental tests were performed in order to obtain insight into
GFRP degradation mechanisms upon prolonged exposure to fatigue and ageing treatments.
Tension-tension axial fatigue data for AR-glass FRP composites with limited frequency of
fatigue load (7.5 Hz) without environmental concerns are summarized herein.
Specimens immersed in deionized water show a high decrease (27.7%) in tensile strength
over the 7-month period of immersion. It is seen that immersion in deionized water causes a
significant decrease in both the tensile strength and normal elastic modulus (Young’s
Modulus) of the GFRP. However the decrease of Young modulus is smaller compared to
tensile strength.
Tensile tests showed that GFRP specimens had a maximum retention of tensile properties
of approx. 22.8% after immersion in a NaCl solution for 7 months. However for SC and
BC-type since the number of specimens tested was very limited and results should be con-
firmed by a larger experimental programme. Test results are in line with researchers reports
of degradation of the GFRP rebars or sheets varying from 10% to 47% depending upon the
parameters selected for durability tests viz. alkalinity, moisture, temperature, stress and du-
ration of the tests. The application of these materials for masonry retrofitting is not highly
affected by this behavior in consideration on the low stress level typical of masonry struc-
tures.
First results of fatigue treatment showed that fatigue did not produce significant damage in
GFRP composites and residual physic-mechanical properties did not show a significant de-
crease in both tensile strength and Young modulus.
ACKNOWLEDGMENTS
We appreciate the financial support of Reluis. We thank Cecilia Zampa of Fibrenet for sup-
plying GFRP grids and for her valuable technical suggestions.
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