shear and flexural characterization of grid- reinforced ... · revised manuscript...

Revised Manuscript DOI: 10.1617/s11527-013-0207-1

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Shear and flexural characterization of grid-

reinforced asphalt pavements and relation

with field distress evolution

Francesco Canestrari – Leonello Belogi – Gilda Ferrotti – Andrea Graziani

Università Politecnica delle Marche +39 071 2204780

+39 071 2204510

[email protected]

Abstract

The use of geogrids at the interface of asphalt layers is currently adopted to improve pavement

performance in terms of rutting, fatigue and reflective cracking. Several test methods have been

proposed in order to simulate the complex mechanical behavior of reinforced pavements and assist

practitioners in the selection of the appropriate reinforcement product. A particular subject of

debate is the evaluation of geogrid effects in terms of both flexural strength and interlayer shear

resistance. In this context, an interlaboratory experiment has been organized as part of the RILEM

TC 237-SIB/TG4 with the twofold objective of comparing the predictive effectiveness of different

experimental approaches and analyzing the behavior of different geogrid types. For this purpose

two experimental reinforced test sections have been realized, the first to prepare samples for the

interlaboratory experiment, the second to analyze the geogrid field performance under heavy

traffic conditions. This paper describes the test results obtained by one participating laboratory on

double-layered asphalt samples obtained from the first experimental section and compares them

with the periodic visual observation of the reflective cracking evolution occurred in the second test

section. The laboratory tests were performed following a specific testing protocol that combines

interlayer shear tests, repeated loading tests in a four-point bending configuration and quasi-static

three-point bending tests, in order to investigate the overall performance of double-layered asphalt

systems. Results have shown that geogrid reinforcement does not noticeably influence the flexural

stiffness and strength in the pre-cracking phase, whereas the crack propagation speed can be

significantly reduced and the failure behavior may change from quasibrittle to ductile, depending

on the interlayer shear strength. Laboratory results were confirmed by periodic visual observation

of field performance in terms of reflective cracking evolution.

Keywords: asphalt concrete, interface, geogrid, reinforcement, field performance


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1. Introduction

The increase of traffic volume and axle loads, leading to premature failures in the

in-service road pavement network, is one of the reasons for the worldwide

growing interest on pavement reinforcement. Particularly, the performance of

asphalt pavements can be improved by the installation of a geosynthetic

reinforcement at the interface of asphalt concrete layers. In fact, many research

studies showed that reinforced asphalt systems obtained installing geogrids,

geomembranes and geocomposites, can be appropriately employed in order to

improve rutting resistance (Austin and Gilchrist 1996; Komatsu et al. 1998;

O’Farrell 1996), fatigue resistance (Austin and Gilchrist 1996; Brown et al. 2001;

Lee 2008; Zielínski 2008) and reflective cracking inhibition (Komatsu et al. 1998;

Montestruque et al. 2004; Penman and Hook 2008; Sobhan et al. 2004; Zamora-

Barraza et al. 2011).

However, the structural improvement provided by the installation of a

geosynthetic reinforcement at asphalt interfaces, can be negatively affected and

eventually neutralized by its potential de-bonding effect. In fact, it has been

shown that the installation of a geosynthetic reinforcement can hinder the full

transmission of horizontal shear stress at asphalt layers interfaces and reduce the

overall efficiency of the interlayer system (Brown et al. 2001; Canestrari et al.

2012a; Ferrotti et al. 2011; Zamora-Barraza et al. 2010; Canestrari et al. 2012b).

In order to provide structural improvements and load spreading ability the

reinforcement should also ensure an adequate level of interlayer shear resistance;

however, in such a case it may result in a limited capability in preventing bottom-

up crack propagation and reflective cracking. On the other side, if the

reinforcement is installed on a distressed surface and preventing crack

propagation from the underlying pavement is the main goal, its contribution to the

load carrying capacity of the pavement may be limited, or even negative.

Given this background, the reinforcement of asphalt layers with geosynthetics is

still an important subject of debate.

To investigate the impact of reinforcement installation at the interface of asphalt

layers, RILEM TC 237-SIB/TG4 “Pavement Multilayer System Testing”

organized an interlaboratory experiment, based on the construction of two full-

scale reinforced pavement sections: the first one to prepare samples for


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interlaboratory tests and the second one to perform a real scale analysis of

pavement performance.

This paper presents the results obtained by one laboratory participating to the

interlaboratory experiment and compares them with visual observations of the

distresses occurred in the second test section. The laboratory tests were performed

following a specific testing protocol combining shear and flexural tests on double-

layered asphalt concrete specimens, with reinforced and un-reinforced interface.

Particularly, interlayer bonding was evaluated by means of shear tests, while

flexural behaviour by means of repeated loading four-point bending tests and

quasi-static three-point bending tests. The obtained results were then compared

with periodic visual observations of reflective cracking in the real scale section

subjected to heavy traffic conditions, in order to find possible correlations

between laboratory tests results and field performance.

2. RILEM project description

The study described in this paper is part of the research project: “Advanced

Interface Testing of Geogrids in Asphalt Pavements” promoted by Task Group 4

“Pavement Performance Prediction and Evaluation” of RILEM Technical

Committee 237-SIB “Sustainable and Innovative Bituminous Materials”. The

main goal of this RILEM project is the comparison of experimental procedures

and devices for the mechanical characterization of geogrid reinforced interfaces in

asphalt concrete pavements and the determination of their relationship with the

actual field performance.

In order to meet these challenges, the project is based on the construction of two

full-scale pavement test sections using real scale paving equipment and geogrid

installation techniques. Section A was realized in order to prepare double-layered

asphalt pavement samples to be tested with different devices within the

interlaboratory experiment. Section B was used to evaluate field performance of

reinforced asphalt pavements.

In each experimental section, an unreinforced (UN) and two reinforced (FP and

CF) asphalt interfaces were realized, creating three different double-layered

systems. The same AC mixes were used to construct both sections: in Section A

two lifts having a thickness of 50 mm each were realized, whereas in Section B

the lower lift has a thickness of 40 mm.


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From Section A, slab samples of different sizes (52×52 cm and 65×65 cm) were

cut in order to fit the needs of the laboratories participating to the interlaboratory

test (Fig. 1) whereas Section B was instrumented (Graziani et al. 2011) in order to

evaluate stress and strain at specific locations inside the pavement (currently in

progress). In addition, periodic visual inspections allow to assess distress

evolution in Section B.

3. Objectives

This paper presents the results obtained by one single laboratory which carried out

tests within the RILEM interlaboratory experiment on asphalt specimens sampled

from the Section A.

The objective of this study is the comparison of the effect of two geogrid types on

the flexural and shear behavior of reinforced double-layered systems. The test

protocols were selected to study different aspects of real scale applications where

the structural reinforcement provided by geogrids can be compromised if a

significant shear strength reduction (de-bonding effect) at the interface between

asphalt layers occurs.

In addition, results of laboratory tests were compared to the evolution of reflective

cracking on Section B subjected to real scale heavy traffic and climatic actions.

4. Materials

The Asphalt Concrete (AC) was a typical Italian dense graded mix formulation,

with 12 mm maximum aggregate dimension (AC 12) and 70/100 penetration

bitumen dosed at 5.5 % by aggregate weight. A summary of the quality control

tests performed during and after construction is reported in Table 1 and in

Figure 2.

An SBS polymer-modified tack coat emulsion (Table 2), classified as C 69 BP 3

according to EN 13808, was applied on the surface of the lower layer with a rate

of 0.25 kg/m2 of residual binder in both the reinforced and unreinforced

interfaces.

Two different geogrids were installed (Fig. 3). The Carbon Fiber/Glass geogrid

(CF) is pre-coated with bitumen and characterized by carbon fibers roving in the

transversal direction and glass fibers roving in the longitudinal direction, with a

square 20 mm mesh. The product is sanded on the upper side, whereas a burn off


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film is applied on the underside. The main characteristics of the CF geogrid are

shown in Table 3.The Glass Fiber Reinforced Polymer geogrid (FP) is obtained

by weaving continuous alkaline-resistant pre-tensioned glass fibers, covered with

a thermosetting epoxy resin (vinylester). The grid has flat transversal strands

woven into longitudinal twisted strands, with a square 33 mm mesh (Table 3).

Apart from the constituent material and mesh size, the main difference among the

two geogrids is their torsional rigidity, also called aperture rigidity (Kinney and

Yuan 1995). In particular the FP geogrid is extremely stiff as twisting and

distorting its square mesh is practically impossible, on the contrary the CF geogrid

mesh is extremely flexible and deformable.

5. Testing equipment and laboratory experimental

program

5.1 Testing devices and procedures

5.1.1 Ancona Shear Testing Research Analysis (ASTRA) tests

The ASTRA device (Fig. 4), compliant with the European Standard prEN 12697-

48 and the Italian Standard UNI/TS 11214, is a direct shear box, similar to the

device commonly used in soil mechanics. The whole apparatus is located in a

climatic chamber with temperature and relative humidity control and the specimen

is installed in two half-boxes separated by an unconfined interlayer shear zone

(Canestrari and Santagata 2005). During the test, a constant horizontal

displacement rate of 2.5 mm/min occurs while a constant vertical load,

perpendicular to the interface plane, is applied in order to generate a given normal

stress (σn). This test returns a data-set where interlayer shear stress (τ), horizontal

(ξ) and vertical (η) displacement are reported, allowing the calculation of the

maximum interlayer shear stress (τpeak), for each specimen. Carrying out ASTRA

tests at various stress levels σn, it is possible to obtain a complete assessment of

failure and residual properties (post-peak) of interfaces (Canestrari and Santagata

2005; Canestrari et al. 2005) according, respectively, to the following equations:

pn0peak tanc Φστ ⋅+= (1)

resnres tanΦστ ⋅= (2)


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where τpeak and τres are the peak and the residual interlayer shear resistance,

respectively, c0 is the pure shear resistance, Φp and Φres are the peak and the

residual friction angle, respectively.

5.1.2 Repeated four point bending (4PB) tests

Repeated loading tests were carried out in four-point bending configuration (4PB)

using a device specifically developed to test large beam specimens as required by

geogrid reinforced systems (Fig. 5).

The 4PB device is basically composed by a temperature controlled chamber, a

loading frame and a hydraulic actuator able to apply haversine loading waves up

to a peak load value of 4.0 kN, with a maximum frequency of 5 Hz. The vertical

displacement at the mid-span of the beam is measured during the test through a

strain gauge based extensometer.

The beams tested in this study, cut from double-layered slabs, have a cross section

width W = 90 mm and a total height H = 75 mm (30 mm the lower layer and 45

mm the upper layer).

The performance of double-layered systems in the cyclic 4PB test is analyzed in

terms of permanent deformation and damage associated to crack propagation.

Permanent deformation is measured by means of the accumulated mid-span

deflection at each load cycle. Since reinforced beams often do not reach a

complete collapse during the test, a failure criterion to compare different interface

types (Virgili et al. 2009) was defined by the number of load cycles (or time)

required to reach the flex point in the permanent deformation curve (Fig. 6).

Damage occurring to the tested specimen can be evaluated by the reduction of the

apparent stiffness modulus appE calculated as follows:

app,r

appappE

ε

σ= (3)

where σapp and εr,app are, respectively, the apparent values of stress and recovered

strain, calculated from the linear elastic beam theory, and therefore:

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223

r,0

0app HW12

)a4L3(LPE⋅⋅

−⋅=

δ (4)

where P0 is the amplitude of the applied haversine load, δr,0 is the amplitude of the

periodic recovered displacement response, H is the beam height, L is the beam


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span, W in the beam width and a is the distance between the beam support and the

point of load application (Fig. 5).

The calculated stiffness modulus is obviously an apparent value for two reasons:

first, the span of the beams is about three times their height (Fig. 5) and therefore

the application of the beam theory is not rigorous; second, equation (4) is applied

also in the post-cracking phase of the tests.

5.1.3 Three Point Bending (3PB) tests

Three Point Bending (3PB) tests were performed on prismatic beams identical to

the specimens used in 4PB tests. A scheme of the Three Point Bending (3PB) test

equipment is shown in Figure 7. During the test, the prismatic beam specimens

are subjected to a bending load under displacement control. Load and beam

deflection at the mid-span of the specimen are measured through a load cell and a

LVDT displacement transducer, respectively.

The performance of double-layered reinforced systems can be evaluated through

the maximum pre-cracking flexural load Pmax, the pre-cracking energy PE and the

fracture energy or toughness T. These parameters are shown in Figure 11: Pmax is

a measure of the flexural strength, PE represents the area under the load-

deflection curve up to the Pmax value is reached and T represents the area under the

entire load-deflection curve (Pasquini et al. 2012).

The pre-cracking energy PE takes into account the crack initiation of the beam,

whereas the toughness T provides an indication of the post-peak deformation

energy and therefore can be used to evaluate the performance of the geogrid

reinforcement in the crack propagation phase (Lee 2008). This kind of test is

appropriate to give information in terms of crack propagation allowing to perform

comparisons between different reinforced systems.

5.2 Test program

The laboratory experimental program focuses on the evaluation of shear and

flexural properties of reinforced and unreinforced double-layered bituminous

systems in order to compare their mechanical behaviour. The three interface types

(UN, CF and FP) were studied in terms of interlayer shear resistance (ASTRA

test) and repeated loading behavior (4PB test), following a specifically prepared

protocol for the investigation of geogrid performance in flexible pavements


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(Ferrotti et al. 2011). Moreover, 3PB tests were performed to study the geogrid

effect on the mechanical strength of the double-layered systems under quasi-static

flexural loading (Lee 2008, Kunst and Kirschner 1993, Kwang et al. 1996). A

summary of the laboratory test program is reported in Table 4.

The ASTRA tests were carried out under three normal stresses (σn = 0.0, 0.2 and

0.4 MPa) in order to obtain the peak and the residual friction envelopes

(Canestrari et al. 2005). Temperature effect on interlayer shear resistance was also

investigated performing tests at three temperatures (T = 10, 20 and 30 °C) on

cylindrical specimens cored from slabs stored in the lab for one year (Table 4).

Moreover, in order to assess the influence of sample age, tests were performed at

T = 20 °C on cores obtained from slabs stored in the lab for two years. Three

repetitions were carried out for each interface type.

The cyclic 4PB tests were carried out at a temperature of 20 °C in load-controlled

mode. A haversine load with a frequency of 1 Hz and three amplitudes (P0 = 1.0,

1.5 and 2.0 kN) was applied in order to obtain information at different stress

levels. A maximum test time of 10 hours was adopted, allowing the application of

up to 36,000 load cycles. Two repetitions were performed for each test type.

The quasi-static 3PB tests were carried out at a temperature of 20 °C in

displacement-controlled mode, with a constant rate of 50.8 mm/min (Judycki

1990, Kunst and Kirschner 1993, Kwang et al. 1996). Three repetitions were

performed for each test type.

6. Laboratory test results and analysis

6.1 ASTRA test results

The peak and friction envelopes of the interlayer shear resistance obtained for the

studied interface types are presented in Figure 9, while the corresponding

envelope parameters (c0, Φp and Φres) and the linear regression determination

coefficients (R2) are reported in Table 5.

In terms of peak values (τpeak), the comparison of the three systems shows that

geogrid-reinforced samples (CF and FP) provide lower interlayer resistance

compared to unreinforced systems (UN), particularly in terms of pure shear

component (c0). This is in accordance with previous experimental investigations,

carried out with various experimental devices (Brown et al. 2001; Ferrotti et al.


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2012; Zamora-Barraza et al. 2010; Zielínski 2008), where a similar interlayer de-

bonding effect was measured after the installation of geogrid reinforcement.

In all test conditions, CF systems provided higher τpeak values respect to FP

systems. This is due to the higher thickness and stiffness of the FP geogrid, which

probably inhibits the achievement of an optimal compaction of the upper AC layer

in the interface proximity, and reduces the interlocking between the two

bituminous layers in contact.

A previous statistical investigation performed on ASTRA test results (Santagata et

al. 2009) showed that τpeak = 0.38 MPa can be considered as minimum reference

value for the interlayer shear resistance of unreinforced AC interfaces, at

T = 20 °C and σn = 0.2 MPa. In this study, improved peak shear resistance values

(0.56 MPa and 0.51 MPa, respectively) were measured with UN and CF systems,

whereas the FP geogrid provided a reduced peak shear resistance (0.34 MPa). As

already showed by Millien et al. (2012), this de-bonding effect is expected to

influence bending behavior of double-layered systems.

As far as the influence of test temperature on τpeak is concerned, Figure 9 shows a

decrease of interlayer shear resistance with increasing temperature, for all

interface types. This indicates that, at higher temperatures, the geogrid influence

tends to vanish and τpeak is controlled mainly by the characteristics of the two AC

layers in contact (Fig. 9c).

The comparison between residual strength envelopes (τres) of the studied interface

types (Fig. 9 and Table 5) shows that, at all temperatures, the three double-layered

systems (UN, CF and FP) are characterized by analogous residual friction angles

(Φres). This confirms the results of previous studies (Ferrotti et al. 2011; Ferrotti et

al. 2012) and indicates that the residual interface resistance, which can be

associated to a purely frictional mechanism, is mainly controlled by the

composition of the two AC layers in contact.

Comparing results of ASTRA tests performed at 20 °C on samples of different

ages (Fig. 10 and Table 5), for all interface types the same increase of peak shear

resistance is observed for two years aged samples respect to one year aged. Since

this increase does not depend on interface type (tack-coat and geogrid) and normal

stress (friction and dilatancy contributions), it can only be explained by means of

an increase of inner cohesion of layer materials (Canestrari et al. 2005), related to

the bituminous binder hardening.


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6.2 Four point bending test results

The influence of load level and interface type on the permanent deformation

resistance of double-layered systems is showed in Figure 11 where the

accumulated mid-span deflection of the beam as a function of load cycles is

reported.

The geogrid improves the permanent deformation resistance of the double-layered

systems respect to the unreinforced specimens which always reach collapse before

the planned test conclusion (36,000 cycles). In particular, at P0 = 2.0 kN, collapse

of UN systems is obtained within the first ten load cycles, thus making unfeasible

their representation in Figure 11c.

On the contrary, CF and FP reinforced systems did not always reach complete

collapse before test conclusion, therefore the flex point of the permanent

deformation curves was used as failure criteria (Virgili et al. 2009). In these cases,

in order to compare grid effectiveness, an Improvement Factor (IF) was calculated

as follows:

UNflex

iflex

N

NIF =

(5)

where iflexN is the number of cycles at the flex point of interface type i (= CF, FP)

and UNflexN is the number of cycles at the flex point of interface type UN. The IF

parameter describes the increment of specimen life, as defined by the flex point

criteria, due to the presence of a geogrid reinforcement. As shown in Figure 12

both FP and CF geogrids produce a remarkable increase of permanent

deformation resistance that leads to high IF values, especially at higher load

levels. These results confirm the positive effect of CF geogrids observed in

previous studies for porous asphalt surfacing (Kim et al. 2009). This performance

improvement could be explained considering that, at each load cycle, part of the

deformation energy provided by the external load is stored by the geogrid

reinforcement itself as reversible deformation. Consequently, the presence of the

reinforcement reduces the load fraction carried by the AC double-layer and

therefore reduces damage accumulation of the AC mixture.

The behavior of double-layered systems was also analyzed in terms of apparent

stiffness modulus (Eapp, equation 3). Initial Eapp values (data measured at the 300th

load cycle) are reported in Figure 13, whereas the Eapp evolution during the test is


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summarized in Figure 14. Experimental data show that geogrid reinforcement

results in a stiffness increase of the double-layered systems, that is more evident

with the FP geogrid, but is also observed for the CF geogrid. The higher flexural

stiffness of the reinforced interfaces during the 4PB test, suggests that double-

layered specimens behave like single-layered systems, highlighting the stiffening

contribution given by the geogrid that works effectively as reinforcement. This

implies that an appreciable level of shear stress is effectively transmitted across

the reinforced interface. Such a result may appear in contrast with the direct shear

tests results which highlighted a marked reduction of peak interlayer strength for

geogrid reinforced interfaces. However, considering the different stress level

applied in the two tests, this may actually indicate that the geogrid de-bonding

effect, particularly evident for the FP grid, is attained only when interface shear

stress reaches failure conditions.

In Figure 14 the flex point position of the permanent deformation curves

(Figure 11) is reported on the corresponding curves representing the Eapp

evolution. It has been shown by previous studies (Virgili et al. 2009, Ferrotti et al.

2011) that the flex point indicates the stage of the test when cracking begins to

significantly propagate. For UN and CF reinforced systems this behavior is

confirmed by the Eapp decrease that follows the flex point, whereas, for FP

reinforced systems, the significant Eapp decrease prior to the flex point suggests a

different failure mechanism, that is confirmed by visual observations of double-

layered specimens at the end of the test. In fact, Figure 15 shows that in CF

systems the crack moves vertically and propagates across the interface and the

geogrid, whereas in FP systems when the crack reaches the interface it starts to

propagate horizontally. This change of crack propagation mode, that is also

visible in 3PB tests (see Section 6.3), can be related to the marked reduction of

peak interlayer shear strength measured by ASTRA tests for FP specimens.

Indeed, this different type of separation mechanism allows for additional energy

dissipation and brings to longer specimen life, in control stress loading mode.

Despite this different crack propagation mode, it is worth noting that both CF and

FP grids provided a considerable increase in terms of permanent deformation

resistance and flexural stiffness of the double-layered system, working effectively

as reinforcement.


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6.3 Three point bending test results

The load-deflection curves obtained in the quasi-static 3PB test are reported in

Figure 16 and the average values of the corresponding characteristic parameters

are summarized in Table 6.

It can be observed that, up to the flexural strength point (Pmax, δP,max) the shape of

the P-δ curves is very similar for the studied double-layered systems, resulting in

similar magnitude of the pre-cracking energy PE values (Table 6). Although the

geogrids lead to somewhat higher values of Pmax, δP,max and PE, results confirm

that these parameters largely depend on the characteristics of the un-damaged AC

mixture. As already shown by 4PB tests, this confirms that, in the pre-cracking

phase, double-layered systems behave like single-layered systems and that

geogrid reinforcement has a minor impact on the crack initiation resistance.

Beyond the flexural strength point (crack initiation), both reinforced and

unreinforced systems show an initial, rapid decrease of load carrying capacity due

to the crack propagation towards the interface. Afterwards, UN systems rapidly

and steadily lose their resistance until complete failure, whereas CF and

particularly FP systems show a post-peak deformation phase, bringing to higher

toughness values (T average values for CF and FP interface types are about 2

times and 6 times higher than the UN interface type, respectively).

In the crack-propagation phase, after the initial rapid load decrease, CF systems

show a short delayed phase (Fig. 16b) and than a rapid evolution toward collapse

(no residual flexural resistance). The FP systems, after the initial rapid load

decrease and the short delayed phase, show an apparent strain-hardening behavior

(Fig. 16c) where the resistance of the double-layered system increases reaching

values similar, or even above the flexural strength Pmax.

The different behavior of the two reinforced systems can be explained by different

failure mechanisms, analogous to that previously described for the repeated

loading 4PB tests. Specifically, when the crack reaches the interface of CF

systems, its bottom-up movement is shortly delayed before it eventually

propagates across the geogrid and brings to specimen collapse (Fig. 17).

Conversely, in FP systems, when the crack reaches the interface its bottom-up

propagation is diverted and it proceeds along the interface allowing additional

energy dissipation (Fig. 18).


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In the first case, a Mode 1 crack propagation (bottom-up opening) can be

identified; in the second case the horizontal crack propagation is probably related

both to the bending of the lower layer, due to the beam curvature (Mode 1), and to

the interlayer shear stress (Mode 2).

In this sense, CF grid seems to be not so effective to prevent crack propagation

across the interface, similarly to UN interface. On the contrary, FP grid

significantly delays the crack propagation, working effectively as anti-reflective

cracking system.

6.4 Relation between flexural and shear properties

In the previous sections the performance of reinforced double-layered AC samples

obtained from Section A was evaluated under different experimental conditions.

Both CF and FP geogrids produced a reduction of interlayer shear resistance

(ASTRA test), an increase of initial stiffness in the pre-cracking phase (4PB test)

and an increase of cyclic and quasi-static bending resistance (4PB and 3PB tests).

In particular the “thin and flexible” CF geogrid provided better interlayer shear

performance respect to the “thick and stiff” FP geogrid for which the ASTRA

tests clearly highlighted a de-bonding effect.

The 4PB and 3PB performance improvement provided by the CF geogrid can be

explained with a delayed Mode 1 crack propagation in the vertical direction

respect to UN systems. Nevertheless, the failure mechanism of CF and UN

specimens is similar and the Fracture Process Zone (FPZ) has similar size; in both

cases tested beams behaved as quasi-brittle structures (RILEM 2004). On the

other side, the FP geogrid brings to a significant horizontal crack propagation

phase (Mixed mode) in 4PB and 3PB tests, because of the reduced interlayer shear

strength. As a consequence, the failure mechanism of the FP double-layered

asphalt systems becomes ductile (or plastic), as the FPZ extends to a large part of

the interface.

7. Comparison with field distress evolution

In this section an attempt is made to relate the results of laboratory tests on

double-layered specimens obtained from Section A to the field performance of the

same double-layered systems on Section B. The comparison is focused on the


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crack-propagation behaviour observed in the lab and on the reflective cracking

performance observed in the field.

In this sense, during the construction of Section B, areas with “simulated” cracks

were prepared in the lower asphalt layer (Fig.19a) in order to evaluate the grids

performance in terms of reflective cracking inhibition. The simulated cracks were

obtained by full-depth saw cuts with an interaxis of 0.4 m (Fig. 19b).

As shown in Figure 20a and Figure 20b, a similar reflective cracking pattern

developed in UN and CF sub-sections, starting about six months after construction

and rapidly growing in the following weeks. In both sub-sections the typical

double-asymmetric crack pattern due to traffic loading developed above

transverse cuts, whereas single cracks developed above longitudinal cuts. One

year later these two zones were completely failed (Fig. 20c and Fig. 20d).

On the other side, after the same 22 months period, a double-asymmetric crack

pattern was barely visible in the FP reinforced sub-section (Fig. 20e) subjected to

the same traffic and environmental conditions, highlighting the ability of the FP

geogrid to extend service life of asphalt surfacings when cracks are present in the

underlying asphalt layers. This capability can be related to the post-cracking

behaviour previously observed in the bending tests where FP systems showed

better performance. In addition, it can be observed that the de-bonding effect

highlighted by ASTRA tests did not reduce the overall pavement performance in

this area.

However, a detailed analysis of the geogrid reinforced systems performance in

Section B is currently in progress, and will consider stress, strain and temperature

data measured by the installed sensors.

8. Conclusions

The research described in this paper focuses on two effects of geogrid

reinforcement on the behavior of asphalt pavements: interlayer shear resistance

and flexural resistance. Three interface types were considered: unreinforced (UN),

Carbon Fiber/Glass pre-bituminised (CF) geogrid and Fiber Reinforced Polymer

(FP) geogrid. Double-layered AC samples were obtained from an experimental

test section where real scale equipment and installation techniques were

employed. ASTRA test carried out to investigate interlayer shear resistance

showed that the presence of a geogrid leads to a peak strength reduction, whereas


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the residual (post-peak) interlayer friction is not influenced by the reinforcement.

The peak de-bonding effect is particularly evident with the FP geogrid which is

characterized by higher thickness and torsional stiffness respect to CF geogrid.

However, the geogrid influence tends to vanish at higher temperatures, where the

interlayer shear resistance appears to be controlled only by the characteristics of

the two AC layers in contact. Binder aging brings to an increase of the peak

strength, especially the pure shear component, for both reinforced and

unreinforced interfaces.

Repeated loading 4PB tests carried out on double-layered systems showed that

geogrids lead to a remarkable increase of permanent deformation resistance,

especially at higher load levels. In the pre-cracking phase, both geogrid types

produce a flexural stiffness increase, which suggests that the de-bonding effect

highlighted by the ASTRA tests, especially for the FP geogrid, is attained only in

proximity of, or at shear failure conditions. On the contrary, at lower number of

cycles, shear stress is effectively transmitted across the reinforced interface. In the

cracking phase, the two geogrid types lead to different failure mechanisms. In

particular, the vertical cracks movement is delayed by the CF geogrid but they

eventually get across the inteface leading to failure, whereas the FP geogrid

induces an horizontal propagation along the interface allowing for additional

energy dissipation and longer specimen life. Despite this different crack

propagation mode, it is worth noting that both CF and FP provided a considerable

increase in terms of permanent deformation resistance and flexural stiffness of the

double-layered system, working effectively as reinforcement.

Quasi-static 3PB tests confirmed that the presence of the geogrid does not

noticeably influence the crack initiation resistance, whereas CF and particularly

FP systems show a post-peak deformation phase, bringing to higher toughness

values. In the crack-propagation phase, similarly to 4PB tests, the two geogrids

induce different failure mechanisms. The CF geogrid shortly delays crack

propagation in the vertical direction but the failure mechanism and the fracture

process zone remains analogous to unreinforced systems. On the other side, due to

the reduced interlayer shear strength, the FP geogrid brings to a significant

horizontal crack propagation phase and consequently the failure mechanism

becomes more ductile. In this sense, CF grid seems to be not so effective to

prevent crack propagation across the interface, similarly to UN interface. On the


16

contrary, FP grid significantly delays the crack propagation, working effectively

as anti-reflective cracking system.

Observation of the in-situ performance of the two geogrids revealed a post-

cracking behavior coherent with laboratory test results. In particular a similar

reflective cracking pattern developed in the field in UN and CF reinforced

pavements, whereas the FP geogrid provided an improved reflective cracking

resistance. In addition, the de-bonding effect highlighted by ASTRA test did not

reduce the overall pavement performance, in case of reinforcements installed as

anti-reflective cracking systems.

Acknowledgements

The research described in this paper was partly funded by the Italian Ministry of Instruction,

University and Research (MIUR).

References Austin R.A., Gilchrist A.J.T. (1996). Enhanced performance of asphalt pavements using

geocomposites. Geotext. Geomembranes 14: 175-186.

Brown S.F., Thom N.H., Sanders P.J. (2001). A study of grid reinforced asphalt to combat

reflection cracking. J. Assoc. Asphalt Pav. 70: 543-569.

Canestrari F., Santagata E. (2005). Temperature effects on the shear behaviour of tack coat

emulsions used in flexible pavements. Int. J. Pavement Eng. 6 (1): 39-46.

Canestrari F., Ferrotti G., Partl M.N., Santagata E. (2005). Advanced Testing and Characterization

of Interlayer Shear Resistance. Transp. Res. Record. 1929: 69-78.

Canestrari F., Pasquini E., Belogi L. (2012a). Optimization of geocomposites for double-layered

bituminous systems. Proceedings of the 7th RILEM International Conference on Cracking in

Pavements, Delft, pp. 1229-1239.

Canestrari F., Ferrotti G., Lu X., Millien A., Partl M.N., Petit C., Phelipot-Mardelé A., Piber H.,

Raab C. (2012b). Mechanical Testing of Interlayer Bonding in Asphalt Pavements. RILEM

State-of-the-Art-Report: Advances in Interlaboratory Testing and Evaluation of Bituminous

Materials 9: 303-360. doi: 10.1007/978-94-007-5104-0

Ferrotti G., Canestrari F., Virgili A., Grilli A. (2011). A strategic laboratory approach for the

performance investigation of geogrids in flexible pavements. Constr. Build. Mater. 25 (5):

2343–2348.

Ferrotti G., Canestrari F., Pasquini E., Virgili A. (2012). Experimental evaluation of the influence

of surface coating on fiberglass geogrid performance in asphalt pavements. Geotext.

Geomembranes 34: 11-18.


17

Graziani A., Virgili A., Belogi L. (2011). Instrumented Test section for the Evaluation of Geogrids

in Asphalt Pavements. Proceedings of the 5th International Conference on Bituminous Mixtures

and Pavements, Thessaloniki, pp. 1107-1117.

Judycki J. (1990). Bending test of asphaltic mixture under statical loading. Proceedings of the 4th

RILEM International Symposium, Budapest.

Kim H., Sokolov K., Poulikakos L.D., Partl M.N. (2009). Fatigue Evaluation of Carbon FRP-

Reinforced Porous Asphalt Composite System Using a Model Mobile Load Simulator. Transp.

Res. Record. 2116: 108-117 .

Kinney, T.C., Yuan, X. (1995). Geogrid aperture rigidity by in-plane rotation. Proceedings of

Geosynthetics ’95, Nashville, pp. 525-537.

Komatsu T., Kikuta H. Tuji Y., Muramatsu E. (1998). Durability assessment of geogrid-reinforced

asphalt concrete. Geotext. Geomembranes 16: 257-271.

Kunst P.A.J.C., Kirschner R. (1993). Comparative laboratory investigations on polymer asphalt

inlays. Proceedings of Geosynthetics ’93, Vancouver.

Kwang W.K., Yong-Churl P., Kyu-Seok Y. (1996). Tensile reinforcement of asphalt concrete

using polymer coating. Constr. Build. Mater. 10 (2): 141–146.

Lee S.J. (2008). Mechanical performance and crack retardation study of a fiberglass-grid-

reinforced asphalt concrete system. Can. J. Civil Eng. 35 (10): 1042-1049.

Millien A., Dragomir M. L., Wendling L., Petit C., Iliescu M. (2012). Geogrid interlayer

performance in pavements: tensile-bending test for crack propagation. Proceedings of the 7th

RILEM International Conference on Cracking in Pavements, Delft, pp. 1209- 1218.

Montestruque G., Rodrigues R., Nods M., Elsing A. (2004). Stop of reflective crack propagation

with the use of PET geogrid as asphalt overlay reinforcement. Proceedings of the 5th

International RILEM Conference on Reflective Cracking in Pavements, Limoges.

O’Farrell D.J. (1996). The treatment of reflective cracking with modified asphalt and

reinforcement. Proceedings of the 3th International RILEM Conference on Reflective Cracking

in Pavements, Maastricht.

Pasquini E., Bocci M., Ferrotti G., Canestrari F. (2012). Laboratory characterisation and field

validation of geogrid-reinforced asphalt pavements. Road Mater. Pavement. on-line version:

http://dx.doi.org/10.1080/14680629.2012.735797 – doi: 10.1080/14680629.2012.735797.

Penman J., Hook K.D. (2008). The use of geogrids to retard reflective cracking on airport

runways,taxiways and aprons. Proceedings of the 6th RILEM International Conference on

Cracking in Pavements, Chicago, pp. 713-720.

RILEM TC QFS (2004). Quasibrittle fracture scaling and size effect - Final report. Mater. Struct.

37 (272): 547–568.

Santagata, F.A., Ferrotti, G., Partl, M.N., Canestrari, F., (2009). Statistical investigation of two

different interlayer shear test methods. Mater. Struct. 42: 705-714.

Sobhan K., Crooks T., Tandon V., Mattingly S. (2004). Laboratory simulation of the growth and

propagation of reflection cracks in geogrid reinforced asphalt overlay. Proceedings of the 5th

International RILEM Conference on Reflective Cracking in Pavements, Limoges.


18

Virgili A., Canestrari F., Grilli A., Santagata F.A. (2009). Repeated load test on bituminous

systems reinforced by geosynthetics. Geotext. Geomembranes 27 (3):187-195.

Zamora-Barraza D., Calzada-Peréz M., Castro-Fresno D., Vega-Zamanillo A. (2010). New

procedure for measuring adherence between a geosynthetic material and a bituminous mixture.

Geotext. Geomembranes 28: 483-489.

Zamora-Barraza D., Calzada-Peréz M., Castro-Fresno D., Vega-Zamanillo A. (2011). Evaluation

of anti-reflective cracking systems using geosynthetics in the interlayer zone. Geotext.

Geomembranes 29: 130-136.

Zielínski P. (2008). Fatigue investigations of asphalt concrete beams reinforced with geosynthetics

interlayer. Proceedings of the 6th RILEM International Conference on Cracking in Pavements,

Chicago, pp. 751-759.


19

List of tables

Table 1 Quality control tests on experimental section A

Table 2 Properties of the residual binder of the modified emulsion used as tack-coat

Table 3 Characteristics of CF and FP geogrids

Table 4 Summary of laboratory experimental program

Table 5 Envelope characteristic parameters of ASTRA tests

Table 6 Summary of 3PB test results


20

List of figures

Fig. 1 Section A: a) layout of each experimental sub-section; b) detail of prepared slabs from sub-

section UN

Fig. 2 Average grading of the employed AC12 mixture

Fig. 3 Detail of the installed geogrids: a) Glass Fiber Polymer Geogrid (FP); b) Carbon Fiber

geogrid (CF)

Fig. 4 Scheme of ASTRA equipment

Fig. 5 Scheme of Repeated Loading test in Four Point Bending (4PB) configuration

Fig. 6 Analysis of repeated loading 4PB test results

Fig. 7 Scheme of Three Point Bending (3PB) test

Fig. 8 Analysis of 3PB test results: a) Pre-cracking energy (PE); b) Fracture energy (T)

Fig. 9 ASTRA test results – Peak and friction envelopes

Fig. 10 ASTRA test results – Interlayer shear resistances at different ages

Fig. 11 Repeated loading 4PB test results – permanent deformation behavior at load levels: a) 1.0

kN; b) 1.5 kN; c) 2.0 kN

Fig. 12 Increase of permanent deformation resistance in 4PB test respect to the Unreinforced

interface type (UN)

Fig. 13 Initial values of apparent stiffness

Fig. 14 Repeated loading 4PB test results – Apparent stiffness evolution at load levels: a) 1.0 kN;

b) 1.5 kN; c) 2.0 kN

Fig. 15 Failure mechanisms of reinforced specimens in repeated loading 4PB test: a) FP geogrid;

b) CF geogrid

Fig. 16 Three Point Bending test results

Fig. 17 Failure mechanisms of CF reinforced specimens in 3PB test

Fig. 18 Failure mechanisms of FP reinforced specimens in 3PB test

Fig. 19 Section B: a) layout of each experimental sub-section with position of instrumented area,

de-bonding and simulated cracks; b) detail of simulated cracks pattern in the lower layer

Fig. 20 Visual surveys of pavement condition above simulated cracks (section B)

1

Table 1 Quality control tests on experimental Section A

Lower Layer Upper Layer Interface Core Thickness Air Voids Thickness Air Voids ID N. mm % mm % UN 1 51 10.4 50 10.3 UN 2 49 11.4 48 10.4 CF 1 48 9.0 44 11.2 CF 2 56 10.3 43 11.8 CF 3 56 9.0 44 10.6 CF 4 48 10.4 42 11.5 FP 1 65 8.8 53 9.6 FP 2 57 10.3 53 9.9 Average 54 10.1 47 10.7

Table 2 Properties of the residual binder of the modified emulsion used as tack-coat

Penetration @ 25 °C dmm

Viscosity @ 160 °C Pa·s-1

Ring&Ball Temperature °C

Fraass Breaking Point °C

55-65 0.2-0.8 65-75 < -18

Table 3 Characteristics of CF and FP geogrids

Geogrid Direction Material Grid size mm

Tensile Modulus N/mm2

Elongation at rupture %

Tensile force mesh kN/m

CF Longitudinal Glass fiber 20 73,000 3-4.5 111 Transversal Carbon fiber 20 240,000 1.5 249

FP Longitudinal Glass fiber

Reinforced Polymer 33 23,000 3 211

Transversal Glass fiber Reinforced Polymer 33 23,000 3 211

2

Table 4 Summary of laboratory experimental program

Test type ASTRA Repeated loading 4PB Quasi-static 3PB Specimen type Cylindrical core Prismatic Beam Prismatic Beam Specimen dimensions [mm] D = 95 L0=305;W=90;H=75 L0=305;W=90;H=75 Interface type UN – CF – FP UN – CF – FP UN – CF – FP Sample age [years] 1 2 1 1 Test temperature [°C] 10 – 20 – 30 20 20 20 Normal stress σn [MPa] 0.0 – 0.2 – 0.4 0.0 – 0.2 – 0.4 - - Test speed [mm/min] 2.5 2.5 - 50.8 Test Frequency [Hz] - - 1 - Maximum sinusoidal load P0 [kN] - - 1.0 – 1.5 – 2.0 - Repetitions for each test configuration 3 3 2 3 Total number of specimens 81 27 18 9

3

Table 5 Envelope characteristic parameters of ASTRA tests

Interface type Sample age

T c0 φp R2 φres R2

years °C MPa degrees degrees

UN 1 10 0.909 39.09 0.88 40.21 0.98 20 0.374 38.12 0.88 40.52 0.97 30 0.115 39.25 0.96 41.94 0.97

CF 1 10 0.841 39.73 0.83 40.21 0.99 20 0.344 35.10 0.93 40.71 0.99 30 0.106 34.17 0.97 40.55 0.99

FP 1 10 0.404 41.91 0.83 39.52 0.99 20 0.145 38.86 0.92 40.01 0.99 30 0.065 37.25 0.98 40.18 0.99

UN 2 20 0.499 37.14 0.82 44.07 0.97 CF 2 20 0.471 34.67 0.83 42.97 0.92 FP 2 20 0.251 36.45 0.91 44.83 0.87

Table 6 Summary of 3PB test results

Interface type Pmax δP,max PE T kN mm Nm Nm UN 4.24 1.59 4.19 7.88 CF 4.77 1.74 5.01 16.00 FP 4.79 1.75 5.61 44.66

1

a) 65 x 65 cm 65 x 28 cm52 x 52 cm

4.0

m

0.0

m

2.5

m

10.0

m

8.0

m

Slabs

b)

Fig. 1 Section A: a) layout of each experimental sub-section; b) detail of prepared slabs from sub-

section UN

Fig. 2 Average grading of the employed AC12 mixture

2

a)

b)

Fig. 3 Detail of the installed geogrids: a) Carbon Fiber/Glass geogrid (CF); b) Glass Fiber Polymer

Geogrid (FP)

Fig. 4 Scheme of ASTRA equipment

Fig. 5 Scheme of repeated loading test in Four Point Bending (4PB) configuration

3

0

5

10

15

0 10000 20000 30000 40000

Vert

ical

def

lect

ion

[mm

]

Number of load cycles

Flex point

Hardening phase Damage phase

Fig. 6 Analysis of repeated loading 4PB test results

Fig. 7 Scheme of Three Point Bending (3PB) test

a)

Load

Deflection

Experimental dataPre-cracking energy (PE)

Pmax

b)

Load

Deflection

Experimental data

Fracture energy (T)

Pmax

Fig. 8 Analysis of 3PB test results: a) Pre-cracking energy (PE); b) Fracture energy (T)

4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6

W pea

k[M

Pa]

Vn [MPa]

T=10 °C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6W p

eak

[MPa

]Vn [MPa]

T=20 °C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6

W pea

k[M

Pa]

Vn [MPa]

T=30 °C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6

W pea

k[M

Pa]

Vn [MPa]

T=10 °C

Peak UN Peak CF Peak FPPeak envelope UN Peak envelope CF Peak envelope FPFriction envelope UN Friction envelope CF Friction envelope FP

Fig. 9 ASTRA test results – Peak and friction envelopes

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Peak

she

ar re

sist

ance

-2

year

s ag

ed [M

Pa]

Peak shear resistance - 1 year aged [MPa]

T=20 °C

Fig. 10 ASTRA test results – Interlayer shear resistances at different ages

5

a)

0

5

10

15

0 10000 20000 30000 40000

Vert

ical

def

lect

ion

[mm

]


Unreinforced (UN)

Carbon Fiber (CF)

Glass Fiber (FP)

P0 = 1.0 kN

b)

0

5

10

15

0 10000 20000 30000 40000

Vert

ical

def

lect

ion

[mm

]


Unreinforced (UN)

Carbon Fiber (CF)

Glass fiber (FP)

P0 = 1.5 kN

c)

0

5

10

15

0 10000 20000 30000 40000

Vert

ical

def

lect

ion

[mm

]


Carbon fiber (CF)

Glass fiber (FP)P0 = 2.0 kN

Fig. 11 Repeated loading 4PB test results – permanent deformation behavior at load levels:

a) 1.0 kN; b) 1.5 kN; c) 2.0 kN

6

0

5

10

15

20

1.51.0

Impr

ovem

ent F

acto

r, IF

Load level [kN]

Carbon Fiber (CF)Glass Fiber (FP)

Fig.12 Increase of permanent deformation resistance in 4PB test respect to the Unreinforced

interface type (UN)

0

500

1000

1500

2000

2.01.51.0

Initi

al v

alue

of E

app

[MPa

]

Load level [kN]

Unreinforced (UN)Carbon Fiber (CF)Glass Fiber (FP)

Fig. 13 Initial values of apparent stiffness

7

a)

200

2000

100 1000 10000 100000

Appa

rent

Stif

fnes

s M

odul

us [M

Pa]

Load cycles

Unreinforced (UN) Carbon fiber (CF) Glass Fiber (FP)

flex pointspecimen COLLAPSE

P0 = 1.0 kN

b)

200

2000

100 1000 10000 100000

Appa

rent

Stif

fnes

s M

odul

us [M

Pa]

Load cycles

Carbon Fiber (CF) Unreinforced (UN) Glass Fiber (FP)


P0 = 1.5 kN

c)

200

2000

100 1000 10000 100000

Appa

rent

Stif

fnes

s M

odul

us [M

Pa]

Load cycles

Carbon Fiber (CF) Glass Fiber (FP)


P0 = 2.0 kN

Fig.14 Repeated loading 4PB test results – Apparent stiffness evolution at load levels: a) 1.0 kN;

b) 1.5 kN; c) 2.0 kN

8

Bottom-up crackBottom-up crack

FP geogrid strands

Crack propagatesalong the interface

CF geogrid

Crack propagatesacross the interface

Fig. 15 Failure mechanisms of reinforced specimens in repeated loading 4PB test: a) FP geogrid;

b) CF geogrid

9

0

1

2

3

4

5

6

7

0 5 10 15

Load

P[k

N]

Deflection G [mm]

UN

a)

0

1

2

3

4

5

6

7

0 5 10 15

Load

P [k

N]

Deflection G [mm]

CF

b)

0

1

2

3

4

5

6

7

0 5 10 15

Load

P [k

N]

Deflection G [mm]

FP

c)

Fig. 16 Three Point Bending test results

10

0

1

2

3

4

5

6

7

0 5 10 15

Load

P [k

N]

Deflection G [mm]

CF1. Pre-cracking2. Delayed propagation3. Failure

1 2

3

1 2

3G [mm]

Fig. 17 Failure mechanisms of CF reinforced specimens in 3PB test

0

1

2

3

4

5

6

7

0 5 10 15

Load

P [k

N]

Deflection G [mm]

FP

1. Pre-cracking2. Delayed propagation3. Failure

12

3

1 2

3G [mm]

Fig. 18 Failure mechanisms of FP reinforced specimens in 3PB test

11

a)

0.0

m

1.0

m

2.0

m

12.8

m

14.0

m

15.0

m

7.0

m

8.6

m

De-bonding Simulated cracks

Instrumentedarea

3.8

m1.

2 m

5.0

m

0.4 m

0.4

m

Reference Wheel Path

b)

Fig. 19 Section B: a) layout of each experimental sub-section with position of instrumented area,

de-bonding and simulated cracks; b) detail of simulated cracks pattern in the lower layer

12

Fig. 20 Visual surveys of pavement condition above simulated cracks (section B)

shear and flexural characterization of grid- reinforced ... · revised manuscript...

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