Materials Sciences and Applications, 2011, 2, 151-162
doi:10.4236/msa.2011.23019 Published Online March 2011 (
Copyright © 2011 SciRes. MSA
Strengthening of Sewerage Systems with
Composites Plates: Numerical Optimization
Stephan Kesteloot1, Chafika Djelal1, Idriss Benslimane2, Saïd Baraka2
1Laboratoire Génie Civil et Géo-Environnement, Lille Nord de France Université d’Artois Béthune, Lille, France; 2Structure &
Réhabilitation Bagnolet, Bagnolet, France.
Received October 5th, 2010; revised December 14th, 2010; accepted February 9th, 2011.
Sewerage systems are subject to many types of degradation. In France, an estimated 10% of the total systems length
requires work due to structural degradation. At present, there is no method to localised rehabilitation of man-entry
sewers. Laboratory tests have validated localised methods that involve bonding composite plates to the sewer. Those
tests were performed on pre-damaged (multi-cracked) ovoid test pieces. The degradation observed was a longitudinal
crack opened at the crown. The tests were performed under vertical loading. Our Study concerns the application of
partial lining of sewer with composites plates. The composite strengtheners used were 1.2 mm thick pultruded carbon
plates. A series of experiments were carried out on reinforced-concrete ovoids (T180) strengthened and unstrengthened
by carbon plates in the keystone. After this test, a vertically-loaded ovoid was subjected to three-dimensional modelling
in order to determinate its structural behavior and collapse mechanism. Knowledge of the latter make it possible to
limit the area s in need of stren gthening. An ovoid strengthened by com posite plates adhered to the damaged a reas was
also modelled. Using real case data, modelling was carried ou t using a finite-element computationa l software prog ram.
This program allows cracking to be monitored until the structure collapses. Many conventional approaches using in-
tensity factors k and contour integrals J have already been reported in the literature. We used methods for restituting
energy G. Because nonlinear elasticity was being calculated, the constitutive laws of the various materials had to be
taken into account. Th ese constitutive laws describe the evolution of the materials. Moreover, those laws are subject to
deformation limits. The simulated models were homogeneously meshed with physically nonlinear, triangular elements.
The test results were then compared to those of the digital models. Partial lining of a sewer with composite plates,
compared to a traditional reinforced-concrete lining, achieves a cost reduction of about 55%.
Keywords: Sew e rage Sy st em s, Carbon Plates, Concrete, Repair, Reinforcement, Finites Elements
1. Introduction
Sewerage network represent an important heritage which
is becoming obsolete. The strategic location of those
systems and the subsurface congestion all make sewer
reconstruction impossible, very difficult or very onerous.
However, their hydraulic capacity is still satisfactory
because they were over-dimensioned when built [1].
There are various types of sewerage network; they
may or may not allow man entry and are made with dif-
ferent kind of materials (masonry, concrete).
They vary in shape, being circular or ovoid. This study
mainly deals with ovoid section reinforced-concrete
sewers. Ovoid sewerage systems (Figure 1) consist of a
vault, abutment walls and an invert.
From an era of reconstruction and construction, we
progressively proceed to an era of rehabilitation. So to
reinforce or to repair these systems, numerous techniques
using different materials or processes came into being.
However, the budget, which is granted, enables us to
handle only the works, which are on a c urative list because
of their structural state.
Taking into account this real problem, it is necessary
to settle punctual methods of repairing and reinforcement.
The carbon fiber composite plates showed their numer-
ous advantages in the building trade and public works.
Consequently, the major purpose of our research is the
use of this process on man-hole sewers which are made
with reinforced concrete or not.
After relating the most common degradations, this ar-
Strengthening of Sewerage Systems with Composites Plates: Numerical Opti mi zation
Copyright © 2011 SciRes. MSA
Figure 1. Scheme of an ovoid section.
ticle will explain the existing repairing techniques for
sewerage systems. Then, the punctual then lining with
composite plates and the results of the tests carried out in
a laboratory will be put forward. According to the first
results, an optimization of the carbon plates thanks to
numerical calculations for finites elements was consid-
ered so as to reduce the cost. The validation of this opti-
mization will then be made with tests in a laboratory.
1.1. Most Common Type of Degradation
The deteriorations can be located around periphery of the
structure or in specific locations, on intrados or extrados.
The m ain deter ioratio ns are num erous. T hey are descri bed
1.1.1. Superficial Disorders
Superficial degradations generally affect the coating on
the intrados side. Entailing no sh ort-term prejudice, those
disorders can entail important structural damages if they
are not treated very q uic kl y .
We can have:
Poor placement of the plate during adhesion;
Faience deterioration, representing a pattern of
decimetric cracks;
Chipping, leading to superficial splinters;
Detachment of the coating;
Wearing of the coating.
These superficial disorders can then entail parasitical
infiltrations of water or exfiltrations of the effluents in the
1.1.2. Cracks
Cracking reflects a normal behavior of the reinforced
concrete [2]. A visible cracking can be a sign of a mal-
function and on this account, it must be subjected to a
serious analysis to determine its origin. According to
their origins, the cracks present a feature and a typical
line. Moreover, their shape and their direction can give
information about the causes of the disorder.
The causes of these cracks are numerous and varied,
such as a simple penetration by roots or even a structural
failure of the collector due to overloading.
1.1.3. Structural Degradations
The deformations [3] of the buried systems are the con-
sequences of structural disorders. These are generally
leaded to functional disturbances such as bad draining or
even infiltrations or exfiltrations. These deteriorations are
very serious and can lead to the whole replacing of the
deformed pipe. They are mainly due to:
A vertical overloading which creates a vertical
moving of the vault and an horizontal moving of
the abutment walls;
Ground movements (differential settlings, presence
of gaps, swelling of the soil….).
These deformations include vault subsidence, conver-
gence and divergence of the walls, bulging or a structural
tilt of the systems and invert su bsidence.
Moreover, a split of the system can happen under the
effect of the internal pressure, all this leading to numerous
longitudinal crack s. This split is in fact a breaking of the
system under the effect of extreme internal pressures.
Following this list of the most common types of deg-
radation, a description of the repairing tec hniques is m ade.
1.2. Repairing Techniques of the Sewerage
The projected-concrete or mortar liner [4 ], or tub ing with
Figure 2. Possible movements of sides which can create
Strengthening of Sewerage Systems with Composites Pla t e s: Numerical Optimiz at ion
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Figure 3. Divergence of walls.
prefabricated elements [5] can be described as structuring
techniques, whether there is a mechanical contribution.
Moreover, the collector gets back its functions of water-
proofness and its hydraulic capacities. Those methods
bring to the system a new “skin”. The anticorrosive and
anti-abrasive are thus performed.
Thanks to these techniques, a reduction of the geomet-
ric anomalies is observed. It is thus accompanied by a
significant reduction of the useful section of the system,
but not necessari ly by a reduct ion of i ts hyd raulic capacity
(decreasing of the roughness factor). The structural aim of
those strengthening methods is to enable the use of this
system, while preserving the safety aspects. In main cases,
the reinforcement only contributes to the resumption of
new permanent or variable loadings, apart from the case
of the reduction of the rigidity of the existing system.
The current str engt heni ng t ec hni que s fi t o ver t he wh ole
periphe ry o f th e s yst em or over a hal f sy st e m (Hul l PRV ).
The hypothesis of calculations [6] depend on a sufficient
length of the system. The temporary reinforcements are
thus exclude d. Moreover, only the systems, which ha ve no
deformation and can make them unfitted for a present or
future use, can be rehabilitated. Thus, it is necessary for
then to settle a new strengthening method of a punctual
type so as to re duce the costs and t o determinate the places
which must be strengthened.
2. Full Scale Test Series in the Laboratory
Tests were co nduct ed wit h a s ing le verti cal l oadi ng on t he
crown, without the walls being blocked or having loads
applied to them. This type of loading corresponds to the
weight of the backfill above sewers and to the surface
loads. In our test series, the resulting horizontal load of the
soil thrusts is not considered, in order to highlight the area
under greatest stress, i.e. the crown. This loading is there-
fore the least favourable.
Figure 4. Hull of a tubing with prefabricated sections.
Figure 5. Dry shortcrete (without prewating).
Figure 6. Wet shortcret (dense flow).
In addition, in view of the diagram of bending moments
[7], the crown displays significant cracking on the intra-
2.1. Description of Test Pieces
The test series was conducted on the laboratory's test slab.
The test pieces were ovoid T180 segments manufactured
to industrial standards. These are reinforced-concrete
segments with one layer of rebars in the walls and two
layers in the crown and inv ert, with no space in between.
2.2. Carbon Plates
Carbon fibre-based materials have a very low mass and
are very strong. Instead of using large sheets, the com-
posite comes in the form of 1.2 mm thick plates (Sika
Carbodur) [8]. The plates we used are made of one-di-
rectional composite materials, in the direction of the
carbon fibres (longitudinal), which are embedded in an
epoxydic matrix. We used 50 mm wide plates, giving the
substantial curve in the structure.
Strengthening of Sewerage Systems with Composites Plates: Numerical Opti mi zation
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Table 1 gives the mechanical characteristics of the
carbon plates.
2.3. Bonding Material: Glue
Glue plays a very important role in the structural streng-
thening using composite plates.
Table 2 gives the mechanical characteristics of the
2.4. Plates Adhesion on Ovoid Test Piece
The substrate was treated to remove any surface traces of
oil, grease, striking products and other soiling, as well as
the surface film of cement and laitance. All elements that
were heterogeneous or did not have minimum surface
cohesio n o f 1.50 MPa we re removed.
After sanding, the substrate was cleaned of dust either
by compressed air or a brush. The substrate must be
A = 180 cm
B = 108 cm
C = 206.5 cm
D = 133 cm
e = 13 cm
Figure 7. Geometric characteristics of the T180 ovoid sec-
Table 1. Characteristics of the strengthening plates.
Characteristics Carbon plates
Poisson’s ratio 0.2
Young’s modulus 178 950 MPa
Compressive strength 280 MPa
Tensile strength 2750 MPa
Table 2. Characteristics of the glue.
Characteristics Glue
Poisson’s ratio 0.2
Young’s modulus 12 800 MPa
Compressive strength 55 MPa
Tensile strength 2.4 MPa
plane and meet the following conditions: 2 mm under
the 20 cm rule. If this is not the case, it may be smoothed
using epoxydic paste. Once the substrate is prepared, the
strips are cut to the desired length (75 cm) using a dia-
mond-disc grinder, then cleaned with a basic solvent
prior to adhesion.
A plate adhesion procedure was developed for our
laboratory tests and for future in situ application, giving
the curvature of the test pieces and the high rigidity of
the strips.
Two aluminium brackets were arranged at the crown-
springing lines to stop the plates (Figure 8). The adhe-
sive was applied in two layers 1- 1.5 mm thick . The plates
were introduced into the test piece, and then pressed us-
ing two wooden battens running along the crow (Figure
9). The battens were held in place by two props at the
springing lines.
The plates were pressed onto the substrate using a roller
between the battens. The surplus adhesive must escape
from the plate edges.
Figure 8. Fitting the brackets.
Figure 9. Placing the carbon plates.
Strengthening of Sewerage Systems with Composites Pla t e s: Numerical Optimiz at ion
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Before testing, several gaps between the plates and
substrates were observed, mainly between the batten and
the plate stop. When the reinforced ovoid segments rup-
tured, collapse occurred at the gaps.
2.5. Test Procedure
This test procedure h ighlighted the provision of localised
reinforcement in the event of vertical overload, with the
presence of gaps at the wall-soil interface.
In order to spread the load uniformly over the crown, a
sand-filled wooden container was made. The load was
therefore applied uniformly without slippage on the
crown. The load application surface was 0.45 m wide by
1.20 m long, in order to focus the loading on the crown.
At ground level, in view of the sewer's design, a fine
mortar low thickness was placed under the test piece to
absorb ground and invert irregularities.
The INSTRON press we used has a digitally-con-
trolled hydraulic jack. The load was measured using a
force sensor of 250 kN static capacity. During testing, we
measured crown sagging and wall displacement in order
to verify the structure’s symmetry. Crown displacements
were measured using LVDT sensors connected to a data-
acquisition unit.
The purpose of the tests was to define the mechanisms
of deformation and collapse of ovoid segments subjected
to vertical loading. The vertical overload was generated
without any lateral stops on the test piece, to make it as
unfavourable as possible with regard to the tensile
stresses in the crown. In situ sewers, whose soil/structure
interface does not have decompression voids, partly re-
distribute the stresses to the soil.
For each test, crown displacement and divergence of
the walls (outward displacement of each wall) were mea-
sured. In addition, extensiometric gauges were placed in
Figure 10. Test set-up.
stressed areas of the test piece. These areas were identi-
fied during linear numerical calculations, by taking ac-
count of the permissible tensile stress of the concrete.
The test results yielded:
The deformation mechanism of the test pieces, with
the gradual appearance of cracks;
The behavior of the test pieces during loading;
The rupture load.
The carbon plates were arranged edge to edge on the
crown. The surface to be strengthened was defined by a
numerical study [9] performed using the commercial
URUS structural design software program [10]. The se-
lected area represents 0.75 m2 to be reinforced per meter
length of sewer.
2.6. Test Series Results
For our test series, two non-reinforced ovoid segments
(controls) and two reinforced segments were subjected to
an axial vertical loading. It is useful to take account of the
onset of the first cracks in order to assess the in-service
behavior of the structure.
Two ovoid segments were tested in each case, and
high reproducibility of results was observed. Results are
therefore given for one test piece only.
Figure 12 shows the behaviour of the test pieces under
vertical loading, at the speed of 1 mm/min.
In terms of elasticity, the test pieces behave identically
until onset of the first crack (95 kN) in the control-piece
crown. Then we observe cracks developing in the extrados
of the wall midpoints, in the crown, and in the intrados of
the invert-wall join.
With the control pieces, the c urve changes after onset of
the first crack, whereas with the reinforced pieces the
initial rise continues steadily until the first detachment
occurs. This is caused by:
Poor placement of the plate during adhesion;
The thrust into the void to which the plates are sub-
Figure 11. Test slab.
Strengthening of Sewerage Systems with Composites Plates: Numerical Opti mi zation
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Table 3. Results of full-scale test.
Test Max load [kN] Load at onset of first crack Crown sag [mm]Left-wall sag [mm] Right-wall sag [mm]
ovoid 160 95 kN (crown),
120 kN (invert),
125 kN (right-midpoint) 6.15 9.42 5.49
Reinforced ovoid pieces 250 150 kN (invert) and
240 kN (right-w a ll midpoint) 7.79 5.19 0.83
Figure 12. Test piece behavior.
The rupture load for the reinforced piece is 250 kN and
for the control piece 160 kN, representing a 55% gain.
However a more fragile behaviour is observed. In addi-
tion, we observed:
A redistribution of the stresses towards the invert
(obtained by gauges and modelling), but also out-
wards at the wall midpoints;
An increase in the load at first-crack onset of
nearly 60% over the load at first-crack onset in the
unreinforced test pieces;
First-crack onset in the invert under a load nearly
15% greater than that at which the first crack ap-
peared in the unreinforced test pieces.
This delay in cracking and the r edistribution of stresses
towards the invert are essential parameters. The forces are
transferred from the structure to the plates via the adhe-
sive. Moreover, traditional sizing of sewerage structures
only takes account of the service range, which validates
use of this p rocess.
The stresses are measured on the slides using strain
The stress measured in the plate was 76.90 MPa, i.e. a
stress value of 2.7%. Moreover, we had a relative elon-
gation of 1‰. This elongation is well below the permis-
sible value for carbon plates.
All the test pieces (reinforced and non-reinforced)
ruptured identically. The cracks continued to open and
progress, reaching maximum crack openning in the
crown of approx imately 7 mm. As Figure 13 shows, co l-
lapse of the crown occurred at 45˚ relative to the soil
plane, at the springing line. The crack spread outwards
from the intrados.
Placement of the reinforcements in the crown altered
the collapse mechanism of the ovoid segments. Rupture
took the form of plate detachment from the crown con-
crete. The plates did not peel off due to the geometry of
the structure.
The plates gradually detached from the substrate
where adhesion was not total. However, ruptu re occurred
in the crown due to concrete decohesion (Figure 14).
Figure 13. Rupture of a test piece under vertical loading.
Figure 14. Strip detachment before concrete.
Strengthening of Sewerage Systems with Composites Pla t e s: Numerical Optimiz at ion
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Once the plates detached from the crown, rupture was
immediate. Complete collapse of the structure occurred
in the same way in the unreinforced structures.
3. Numerical Study—Non Linear Finite
Element Analysis
Several analytical methods [11,12] have been developed
to predict the bending behaviour of elements reinforced
with glued-on composites. In view of the limits imposed
by these numerical methods, and particularly on account
of the complex distribution of stresses in the film of glue,
it seemed necessary to use a more precise method.
3.1. Description of the Sofware
Numerous studies of numerical methods [13,14] have
already been carried out in recent years in the field of
concrete. With the software currently available, it is pos-
sible to represent the state of the structures under load-
It is proposed that th e question shou ld be stud ied again
in the framework of fracture mechanics as the classical
approaches in the form of stress intensity factors k [15]
and contour integrals J [16] have not been fruitful. New
software has recently been developed using energy release
rate methods [17], whereby it is possible to perform the
calculations for concrete. The finite-element software
URUS Version 9 [10] will be used in the context of this
study. The finite elements used are triangular or quad-
rangular plate elements. Only the membrane effects will
be taken into account (in plane stress conditions). During
the simulations, solution in non-linear elastic conditions
will be used.
The non-linear prob lem arises because of the constitu-
tive law of the concrete. They are also limited in relation
to strain. The URUS software possesses a library of pre-
defined laws. By way of example, there is the “poly-line”
constitutive law that corresponds to portions of linear
segments linked to one another, or the “exponential” type
law. The limits are prescribed in terms of strain.
3.2. Model Used
The chosen model was uniformly meshed with non-linear
physically triangular plates. The mesh pitch was refined
during the simulations. The chosen pitch was 0.1 m. The
materials of the strengthened ovoid sections were assem-
bled by bonding the elements with nodes whose degrees
of freedom remained unrestricted. Boundary conditions
were defined by locking the invert on the extrados side to
form a 60˚ angle. To represent the structure symmetry, nil
horizontal displacement was applied to the nodes on the
axis of symmetry.
Figures 15 and 16 show the chosen model.
Figure 15. Face model mesh.
Figure 16. 3D model mesh.
The computation was then run until the iterative pro-
cess no longer conver ged.
3.3. Law of Behaviour
3.3.1. Concrete
The characteristics of the concrete of ovoid sections were
obtained from laboratory tests. Holes were drilled in an
un-strengthened ovoid section, and then single compres-
sion tests and Young’s modulus measurement tests were
conducted. Table 4 gives the characteristics of the con-
The general form [18] of uni-axial laws of behavior,
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Table 4. Characteristics of concrete.
Characteristics Concrete Substrate
Poisson’s ratio 0.2
Young’s modulus 15 400 MPa
Compressive strength 30 MPa
Tensile strength 2.4 MPa
used for the continuous media applicable to concrete, is
given in Figure 17.
3.3.2. S tr engthening: Carbon Plate s
Because the final characteristics of composite materials
depend on those of the fibers, Meier [19] chose various
criteria for a comparative study. This study found that
carbon fiber is best suited to structural strengthening.
The law of behaviour chosen for the carbon plates is
given in Figure 18.
3.3.3. Bonding Material: Glue
Deuring [20] found that the behavior of a glue film de-
composes in two areas: one elastic and nearly linear, the
other pl ast ic.
The laws of behaviour adopted for the glue are expo-
nential. This type of law makes it possible to reproduce
the linear elastic area, then the plastic area.
The law of behavior of the glue is outlined in Figure
3.4. Results of the Numerical Computations
An un-strengthened ovoid section was numerically com-
puted to obtain its collapse mechanisms and define the
areas in need of strengthening. The carbon plates were
positioned transversally and continuously on the vault.
After several numerical simulations, [21] the chosen trans-
versal surface corresponded t o a width of 0.75 metres. The
results of this strengthening method are described in the
next paragraph. Once the transversal strengthening area
was determined, a longitudinal optimisation was perfor-
med in order to limit the number of plates (inter-plate
3.4.1. Compari s o n between Un- S t rengthe ne d Ovoid
The un-strengthened ovoid section was subjected to a
loading equating to silty soil backfill 2 meters in height.
During modelling, a series of load increments was applied
to obtain its full collaps e.
The manufacturer of the ovoid sections certified their
strength to be 138 kN. We obtained a rupture load of 157.5
kN, giving a safety coefficient of 1.15 compared to the
manufacture r's value. The behavior of the un- strengthened
ovoid sectio n is give n i n Figure 12. During the numerical
Figure 17. Constitutive law of concrete.
Figure 18. Constitutive law of composite.
Figure 19. Constitutive law of glue.
Strengthening of Sewerage Systems with Composites Pla t e s: Numerical Optimiz at ion
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simulation, the damaged areas were verified in relation to
the line of the bending-moment diagram [17].
The distribution of the tensiles areas is identical be-
tween model and experimentation. In order to validate
our laws of behavior of the carbon plates and the con-
crete, and our model, a comparison is carried out be-
tween the behavior of the model and the experimental
3.4.2. Comparison between St re ngthened Ovoid
With the strengthened ovoid section, collapse occurred at
the fifth loading step, 262.5 kN. A 60% load gain over the
un-strengthened ovoid section was thus observed. In ad-
dition, cracking was retarded. In the reference ovoid sec-
tion, the first crack appeared at 95 kN in the vault, whereas
it appeared at 160 kN in the invert of the ovoid section
continuously strengthened by the positioning of the plates.
Figure 21 shows the stresses present at collapse of the
continuously strengthened T180 ovoid section. The stre-
sses in the concrete were low, because the glue transmit-
ted the forces into the plates.
The behavior of the strengthened ovoid section is
identical to that of the un-strengthened section in terms
of concrete elasticity, i.e. until the appearance of the first
crack, in the un-strengthened ovoid section. The load-sag
curve in Figure 22 shows the behavior of a continuously
strengthened ovoid section.
Our findings show that this type of strengthening is well
suited to ovoid sewers, despite the considerable curve in
the vault.
Having obtained these first results, we optimised the
area in need of strengthening in order to limit the cost of
in-situ application.
3.4.3. Optimization of the Strengthening Plates
Different numerical strengthening cases were tested to
determine the smallest possible area in need of strength-
Figure 20. Comparison between un-strengthening ovoid sec-
Figure 21. Collapse mechanism. Mapping of the Main
Stresses of the continuously strengthening ovoid section.
Figure 22. Comparison between strengthening ovoid sec-
First, one out of two plates was applied. The numerical
results showed that it was possible to reduce the area in
need of strengthening. The area finally chosen was one-
third of the initial area. The chosen arrangement was 150
mm wide plates separated by a 300 mm gap.
The parameters determining the minimal strengthening
area were as follows:
Retardation of the first crack until 138 kN, the
permissible load of a T180 ovoid section supplied
by the manufacturer;
Load gain greater than 30% of the reference
ovoid-section load.
Figure 23 gives the mapping of the main stresses for
the strengthened ovoid section with inter-plate spacing.
The load-sag curve of the s t r engthened ovoi d w ith spa-
Strengthening of Sewerage Systems with Composites Plates: Numerical Opti mi zation
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Figure 23. Collapse Mechanism - Mapping of the main
Stresses of the continuously strengthening ovoid section.
cing (Figure 12) indicates a slight change in elasticity
around 105 kN. However, the significant change in rigid-
ity occurs at 157.5 kN, the load at which the first crack
appears in the vault between the carbon plates. The rup-
ture load is 210 kN, a 35% gain over the un-strengthen ed
ovoid section.
3.4.4. C om parison an d Conclusion
The differences in behavior between the various models
are shown in Figur e 2 4.
The three load-sag curves are identical until 95 kN,
when the first crack is observed in the un-strengthened
ovoid section. The comparison between the behavior of
the two types of strengthened area shows a reduction in
the rigidity of the stre ngthene d ovoid secti on with s pacing
at about 105 kN. However, the two sections behave
similarly until 157.5 kN. Above this loading there is a
greater difference in rigidity, due to the appearance of the
cracks between the plates. As for the reference ovoid
section, the load-sag curve shows two main domains: the
elastic domain up to 95 kN, then the plastic domain with
appearance of cracks through to structural collapse.
Following these numerical computations, an extensive
series of experiments was conducted to compare the ob-
tained results to the numerical computations. Two un-
strengthened ovoid sections were tested to verify the law
of behavior applied to concrete and to d etermine the col-
lapse mechanism. Then two ovoid sections continuously
strengthened in the vault were loaded to collapse. Appli-
cation of composite plates on the vault achieves a 60%
load gain and retards cracking.
Figure 24. Comparison of the numerical curves.
For this paper, only the tests on strengthened ovoid
sections with inter-plate spacing were compared to the
numerical simulations.
4. Comparison of Test and Computations
The ovoid sections were vertically loaded. The load was
applied to the vault, and eq uated to the weight of backfill
soil above the sewers. The jack load was transmitted to
the structure by means of a wooden shuttering filled with
sand, to uniformly distribute the load on the vault. (Fig-
ure 25) The resultant horizo ntal thru st of the soil was not
considered, in order to highlight the area under greatest
stress, i.e. the vault.
Two T180 ovoid section s with inter-plate spacing were
tested. The results were similar, so only one of the tests
was used for this paper.
Cracks appeared at 168 kN, and first developed be-
tween the carbon plates. Cracks also appeared in the in-
vert inside and at the midpoint of the walls on the outside.
Gradual opening of the cracks from the inside towards
the outside was o bserved. These cracks caused the struc-
ture to collapse. (Figure 26)
Collapse occurred between the extreme point of loading
and the edge of the strengthened area. We obtained the
same collapse mechanism for the un-strengthened and
continuously-strengthened sections.
The maximum recorded loading was 192 kN. During
testing, the plates detached from the substrate. However,
detachm ent was caused by decohe sion in the subst rate and
not by rupture of the glue bond.
Comparison of numerical simulation and experiment
series; Figure 27 gives the curves o btained by modelling
and the experiment series.
In the elastic domain, behavior were similar. The be-
havior of the tested ovoid section showed quasi-elastic
behavior u p to 160 kN. C racks then ap peared between t he
plates, showing the plastic behaviour of the section.
The numerical cu rve showed greater structural rigidity
in the plastic domain. The difference in behaviour was
Strengthening of Sewerage Systems with Composites Pla t e s: Numerical Optimiz at ion
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Figure 25. Loading.
Figure 26. Structural collapse.
Figure 27. Comparison on the experimental and numerical
caused by the difficulty of applying the plates to the
curved surface. During testing, spaces were noted be-
tween the plates and the substrate. These spaces caused a
loss of structural ri g i d ity.
5. Conclusions
The appl ication of ca rbon plates to sewer vaults achieves a
60% gain in rupture loads, and retards the appearance of
the first cracks. The gaps in our understanding of ovoid-
section behavior were filled by finite element modelling.
However, because our computations were based on per-
fect adhesion, we obtained slightly different behavior
when cracks appeared. For the application of carbon
plates in the next stage of our experiment series, the
strengthening plates will be placed by inflating a balloon.
Use of this strengthening method for sewer renovation
reduces job cost by 60%. In addition, the section of the
sewer is not r educed.
During loadin g, inte r-plat e spaci ng m akes it possi ble to
visualise the concrete cracking in the vault.
A series of tests was carried out on concrete sample
reinforced by FRP in various environments. During join-
ing in saturated environment, the results of the tests
showed that it was necessary to apply a safety coefficient
related to the problem of adherence to wet facing [21].
Our project partner companies decided to validate this
process by a field test. The site was located in Dépar-
tement du Val de Marne, in the municipality of Saint
Maur des Fossés, on Quai du Port au Fouarre. The struc-
ture was a drainage sewer (2.30 m high and 1.30 m wide).
After the work, the area rehabilitated by the process will
undergo regular inspections to monitor the behavior and
Figure 28. Field test.
Strengthening of Sewerage Systems with Composites Plates: Numerical Opti mi zation
Copyright © 2011 SciRes. MSA
durability of the materials. Twenty months after adhesion
was completed, no deterioration in the reinforcement
(plate or adhesive) was observed. Moreover, no detach-
ment was observed. Routine inspections were held to
monitor the performance of the reinforcement.
6. Acknowledgements
This study was undertaken thanks to the financial supp ort
of the Companies Structure & Réhabilitation, Sika and
valentin Environnement et Travaux Publics.
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