RTM Underground Pipes

Composite Structures 68 (2005) 267–283

www.elsevier.com/locate/compstruct

Development of the trenchless rehabilitation processfor underground pipes based on RTM

Woo Seok Chin, Dai Gil Lee *

Mechanical Design Laboratory with Advanced Materials, Department of Mechanical Engineering, Korea Advanced Institute of Science and

Technology, ME3221, Guseong-dong, Yuseong-gu, Daejeon-shi 305-701, South Korea

Available online 26 April 2004

Abstract

In order to overcome the disadvantages of conventional excavation technology for repairing and replacing worn-out under-

ground pipes, various trenchless technologies have been developed and tried. But trenchless technologies so far developed have some

drawbacks such as high cost and inconvenience of operation.

In this study, a rehabilitation process for underground pipes has been developed using vacuum assisted resin transfer molding

(VARTM) with glass fiber fabric preform to overcome the disadvantages of present trenchless technologies. For the reliable

operation of the developed method, a simple method to apply pressure and vacuum to the reinforcement was devised with a flexible

mold technology.

From the investigation, it has been found that the developed process requires shorter operation time and lower cost with smaller

and simpler operating equipments than those of the conventional trenchless technologies.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Trenchless; Rehabilitation; Underground; RTM; CIPP; Dielectrometry

1. Introduction

Underground pipes, such as sewer and water-supply

pipes, gas pipes, shield pipes of communication cables

and electric power cables, etc., have been constructed all

over the world, and they are gradually increased in

proportion to industrialization and increase of income.

But these underground pipes have been undergone sev-eral problems such as cracks, breakages, or corrosion

due to the inappropriate design, careless management,

and the aging of pipe materials [1]. Since all of these

problems could lead to huge disaster and loss of social

fund, such as soil and water pollution, ground subsi-

dence, and explosion due to the gas leakage, the large-

scale rehabilitation of those damaged underground

pipes is very imminent. In order to solve these problems,the excavation technology has been used widely, which

replaces the damaged pipe with new one through the

*Corresponding author. Tel.: +82-42-869-3221; fax: +82-42-869-

5220/5221.

E-mail address: [email protected] (D.G. Lee).

URL: http://scs.kaist.ac.kr.

0263-8223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compstruct.2004.03.019

excavation of the ground. Since such a conventional

method has many drawbacks and irrationality induced

by the needless excavation of the ground, various

trenchless (sometime called excavation free or no-dig)

technologies without any excavation of the ground have

been developed and tried worldwide. However, these

technologies also have many disadvantages such as high

processing cost, inconvenience of operation and limitedapplications [1]. Since RTM has the capability to fab-

ricate large and complex three-dimensional anisotropic

fiber reinforced composite structures at the designed

position with low production cost, it may be plausible to

apply RTM to repair large underground pipes [2]. Al-

though there are many researches that are related to the

trenchless rehabilitation technologies and RTM [3–6],

the RTM process to repair underground pipes has notbeen attempted until now.

Therefore, in this study, a new trenchless rehabilita-

tion process of underground pipes to overcome such

problems of former trenchless technologies and ade-

quate to the situation of high traffic road, has been tried

and achieved with E-glass fiber fabric preform and

unsaturated polyester resin by vacuum assisted resin

transfer molding (RTM).

mail to: [email protected]

http://scs.kaist.ac.kr

268 W.S. Chin, D.G. Lee / Composite Structures 68 (2005) 267–283

Also several different combinations of reinforcing fi-

ber preform have been tried through material tests and

experiments. In order to remove the micro void and

excessive resin within the reinforcing element and en-hance the resin wetting efficiency, the void removal

method using porous breathing tubes has been devised

and tested. For the reliable process, the wetting and

curing status of instilled resin were on-line monitored

using a dielectric sensor and a dielectrometer. After the

rehabilitation experiment on a small scale had been

performed successfully, actual repairing experiments

have been performed at the real concrete conduits. Fi-nally, the efficiency of the developed process was eval-

uated with respect to processing variables, such as

process cost and time, which were compared with those

of the conventional trenchless technology. Also, the

design criteria for the reinforcing element were sug-

gested to assure the sufficient reinforcing effect with

minimum material cost.

2. Trenchless rehabilitation process using RTM

2.1. Introduction of developed process

The rehabilitation process composed of four steps is

studied in this paper. Fig. 1 shows the process which is a

modification of the general RTM process. The range ofinternal diameter of the underground pipes for the

Fig. 1. Repairing processes of underground pipes with RTM: (a) preprocessin

sealing; (d) removal of wrinkles and twists of the reinforcing element; (e) inje

removing voids and excessive resin within the preform.

application of the developed process is 150–1000 mm,

and the length of pipe can be up to 50 m.

Step 1: After cleaning the interior of target conduits

(underground pipes), a wire is placed through conduits bythe mobile robot as shown in Fig. 1(a). Then, as shown

in Fig. 1(b), the reinforcing element of Fig. 2 is installed

at the designated position of conduits by a winding

machine. The adhesive of suitable viscosity is pasted on

the outer protection skin, which gives lubrication effect

during the dragging process as well as adhesion between

the reinforcing element and the target conduits.

Step 2: After placing the reinforcing element in theconduit, the both ends are closed and sealed as shown in

Fig. 1(c) using two specially designed covers. At this

time, the covers and the reinforcing element are clamped

with a band clamp that is tightly fitted into the groove of

the cover in the manner as shown in Fig. 3. After sealing

both ends of the reinforcing element, the compressed air

or nitrogen gas is supplied into the inside cavity through

the air inlet of covers to expand the inner protectionskin. This makes the outer protection skin and fiber

preform contact closely to the inner surface of the

conduits, and removes wrinkles and twists in the rein-

forcing element that might occur during the placing

operation as shown in Fig. 1(d). Then the pressure in the

cavity is removed, which helps the easy resin transfer at

the next stage.

Step 3: A predetermined amount of unsaturatedpolyester resin is injected into the fiber preform with a

g; (b) placing of the reinforcing element; (c) attaching of the covers and

ction of polyester resin; (f) wetting of resin into the fiber preform and

Fig. 3. Clamping method of covers and the positions for pasting

adhesive.

Table 1

Material properties of the cured unsaturated polyester resin (Aekyung

PC670)

Property Value

Flexural strength (MPa) 104

Flexural stiffness (GPa) 3.9

Tensile strength (MPa) 54

Tensile modulus (GPa) 4.1

Tensile strain (%) 1.5

Curing conditions: MEKPO (of 55% peroxide) 1%+6% Co-Naph

0.1% Room temperature/24 h+ 60 �C/5 h.

Inner Protection Skin

Glass-fiber Preform

Outer Protection Skin

Fig. 2. Configuration of the reinforcing element.

W.S. Chin, D.G. Lee / Composite Structures 68 (2005) 267–283 269

RTM machine through the resin inlet of the cover as

shown in Fig. 1(e). Due to the low viscosity of the

unsaturated polyester resin, it is easy to transfer theresin into the fiber preform under low pressure.

Step 4: After injecting the resin, the compressed air or

nitrogen gas is fed into the cavity again to make the

injected resin wet the fiber preform uniformly as well as

bringing the reinforcing element into close contact with

the inner surface of the conduit as shown in Fig. 1(f).

The process is performed while a vacuum is applied to

the reinforcing element through the air vent of the cover,which helps the resin wetting and removes the micro

void and excessive resin within the fiber preform. Also

the volatiles produced during the cure of polyester are

evacuated through the air vent of the cover. When the

surplus resin and void within the fiber preform are

completely removed, the wetting of the fiber preform

and the resin flow are ceased, followed by resin cure. In

order to prevent the resin within the fiber preform fromflowing down due to the gravity, the pressure in the

cavity must be maintained. The temperature increase in

the cavity using some devices, such as the hot air blower

fan equipped with a compressor can decrease the curing

time of resin. After the injected resin is completelycured, the covers at both ends of the reinforcing element

are removed to complete the entire processes.

2.2. Material selection and tests

Since the underground structures are usually exposed

to chemically harsh environment and subjected to heavy

compressive load, the selection of repairing materials

considering reinforcing effects is very important. Also

the costs, such as material cost, processing cost, etc.,should be considered carefully because large amount of

material is required for repairing huge underground

structures and the short operation time is important not

to induce any traffic congestion. Therefore, in this study,

the unsaturated polyester resin with low viscosity, which

is five times cheaper than epoxy, was selected and used.

The PC670 (Aekyung Chemical, Daejeon, Korea) is

orthophthalic type unsaturated polyester resin with alow viscosity before cure (0.2 Pa s), which is transparent

and has the possibility of filler addition to reduce cost

and strengthen toughness. Table 1 shows the material

properties of PC670 used in this study.

The reinforcing element for repairing underground

pipes is composed of the reinforcing fiber preform and

two inner and outer protection skins as shown in Fig. 2.

These two protection skins protect the glass fiber pre-form from the internal surface of underground conduits

and subterranean water or sewage left within the con-

duit. Also the inner and outer skins encapsulate the fiber

preform in order to act as the mold during RTM pro-

cess. The protection films not only protect the fiber

preform but also sustain the tensile load up to 80 kN

that is induced by the air pressure of inside cavity (the

maximum air pressure applied to the conduit of 1000mm diameter is 0.1 MPa) and the traction force during

the process [7]. The material for protection skins should

have chemical stability when contacted to unsaturated

polyester resin because the styrene monomer used for

hardening of polyester, may react with the skin material.

In this study, the Pro-Sol film, a kind of tarpaulin films


(LG Chemical, Seoul, Korea), was used for the protec-

tion skins because the Pro-Sol film had superior tensile

properties as well as chemical stability when contacted

with the styrene monomer in polyester resin [1,7]. Thethickness of Pro-Sol film used in this study was 0.55 mm

with the tensile strength of 100 MPa.

Among the three stages of RTM process (preforming,

resin wetting, and curing), the resin wetting is the core

process of RTM because the product quality is mostly

governed by the degree of wetting [8]. The resin wetting

during RTM process, which is viscous flow through the

porous media, may be expressed by Darcy’s law as fol-lows.

U ¼ �KlrP ð1Þ

where, K is the permeability of fiber preform, l the

viscosity of resin, and DP the pressure gradient. There-

fore, the resin wetting time is dependent on the perme-

ability of fiber preform. When the viscosity of resin and

the available maximum pressure gradient are given, the

permeability of fiber preform, K, should be maximized

to reduce the overall process time. Also the strength offiber preform should be large because the fiber preform

forms a polymer matrix composite after the rehabilita-

tion process, which should sustain the external load

subjected by the ground and traffic.

In order to select the proper materials for fiber pre-

form, permeability and tensile strength of various E-glass

fiber mats, unidirectional mats (T800-E06; Dong-il

Industrial, Seoul, Korea), continuous strand mats(M8600; Keun-yung Industrial, Seoul, Korea), crowfoot

satin woven mats (Spec.#224; Hyundai Fiber, Kyoun-

gnam, Korea), chopped strand mats (CM450A; Hankuk

Fiber, Kyoungnam, Korea), and roving cloth mats

(WR580A; Hankuk Fiber, Kyoungnam, Korea) were

experimentally measured [9,10]. To reinforce the strength

Table 2

Measured permeabilities of the various E-glass fiber mats

Type of mat Volume fraction (%)

Unidirectional (T800-E06) 42.1

47.0

Continuous strand (M8600) 9.1

13.5

Stacked mat [UD/CSM/CSM/UD]T 25.8

Satin woven (Spec.#224) 42.4

48.5

54.6

Roving cloth (WR580A) 21.2

26.6

Chopped strand mat (CM450A) 18.2

22.4

UD (T800-E06): unidirectional mat, CSM (M8600): continuous strand mat.

of transverse direction of the preform, the stacked mat of

[Unidirectional mat 1 ply/continuous strand mat 2 plies/

unidirectional mat 1 ply]Total stacking sequence was em-

ployed. Tables 2 and 3 show the measured permeabilitiesof various fiber mats and the tensile strengths of cured

glass/polyester composites, respectively. From the test

results, the stacked mat with high permeability and

moderate tensile strength was selected for preform

materials.

2.3. Resin flow and wetting analysis

The resin flow during the rehabilitation process was

modeled as the circumferential flow in the fiber preform

as shown in Fig. 4 on the assumption that the radial and

longitudinal resin flows were neglected. Since the outside

pressure and inside vacuum are applied to the rein-

forcing element to wet the fiber preform with the resin,

the net pressure for the resin flow was constant during

the whole process. Considering the pressure drop due togravity, the net pressure, P , in the fiber preform may be

expressed as follows.

P ¼ Pair � Pvacuum � qgh ð2Þwhere Pair and Pvacuum represent the air and vacuum

pressures, respectively, q is the density of resin, and h is

the height difference of the resin front that can be ex-

pressed as follows.

h ¼ r � r cos h ¼ rð1� cos hÞ ð3ÞSince the resin flows in the r and z directions can be

neglected due to the thin fiber preform, the velocity uh ofthe resin front in the h direction is expressed as

uh ¼ �Khh

lrP ¼ �Khh

l@Pr@h

¼ �Khh

ld

dl½Pair � Pvacuum � qgrð1� cos hÞ� ð4Þ

Kxx (m2) · 10�10 Kyy (m

2)· 10�10

45.5 39.2

7.52 5.32

17.8 17.6

9.96 9.99

41.1 40.0

0.602 0.497

0.368 0.267

0.204 0.151

25.3 23.4

20.3 19.8

44.3 43.3

17.5 17.2

Fig. 4. Model for resin flow in the fiber preform used for resin wetting

analysis.

Table 4

Viscosity of the uncured polyester resin (PC670) w.r.t. the environ-

mental temperature

Temperature Viscosity (Pa s)

Room temperature (25 �C) 0.25

10 �C 0.75

5 �C 1.0

Table 3

Tensile properties of the glass/polyester composite materials

Type of mat Fiber volume fraction (%) Tensile strength (MPa) Young’s modulus (GPa)

Unidirectional (T800-E06) 47 630 33.9

Continuous strand (M8600) 10 55 6.87

14.5 77.2 7.4

Stacked mat [UD/CSM/CSM/UD]T 29.5 340 18.2

26.6 288 16.6

Satin woven (Spec.#224) 42.4 471 28.2

Roving cloth (WR580A) 28.5 259 13.5



where l is the length of the resin front measured along

the circumference of conduit and 06 h6 p.Using Eq. (4) and the measured values of perme-

abilities of various fiber mats, the movement of resin

front and complete wetting time were calculated with

respect to environmental temperature (viscosity of re-sin), types of fiber preform, and diameters of conduits

on the assumption that inside vacuum and outside air

pressure of 30 kPa were applied to the reinforcing ele-

ment. Tables 4 and 5 show the measured viscosity of

uncured polyester resin (PC670) with respect to the

environmental temperature and the permeabilities of

fiber mats used in the analysis, respectively. Fig. 5 shows

the predicted resin wetting time calculated under the

Table 5

Permeabilities of the various E-glass fiber mats used in the resin flow analys

Type of mat Volume fraction

Unidirectional (T800-E06) 47.0

Continuous strand (M8600) 13.5

Stacked mat [UD/CSM/CSM/UD]T 25.8

Satin woven (Spec.#224) 54.6

Roving cloth (WR580A) 26.6

Chopped strand mat (CM450A) 22.4


assumptions of constant viscosity of resin at a certain

experimental temperature and uniform permeability of

fiber preform.

As shown in Fig. 5, the stacked mat of [Unidirectional

mat 1 ply/continuous strand mat 2 plies/unidirectional

mat 1 ply]Total stacking sequence was the most appro-

priate for the shortest processing time because it assured

the shortest resin wetting time. In order to verify theseanalysis results, small-scale rehabilitation experiments

were performed in the transparent acryl pipes of inner

diameters of 180 and 300 mm using the preforms of

stacked mats and satin woven mats, respectively. From

the experiments, it was found that the predicted wetting

times had an error less than 7% [11]. Since the resin

wetting time should be regulated by controlling the

process variables, such as temperature and pressureduring the actual rehabilitation process, diagrams relat-

ing these process variables are required in order to

accomplish the repairing process reliably and effectively.

Therefore, the resin flow analysis was performed with

respect to the environmental temperature and applied

net pressure (vacuum+air pressure) for the conduit of

specified internal diameter. Fig. 6 shows the predicted

resin wetting time of the preform of stacked mats whose

is

(%) Kxx (m2)· 10�10 Kyy (m

2)· 10�10

7.52 5.32

9.96 9.99

41.1 40.0

0.204 0.151

20.3 19.8

17.5 17.2

0

30

60

90

120

150

180

0 200 400 600 800 1000 1200Diameter (mm)

Diameter (mm)Diameter (mm)

Diameter (mm)

Diameter (mm)

Wet

ting

Tim

e (m

in)

Wet

ting

Tim

e (m

in)

Wet

ting

Tim

e (m

in)

Wet

ting

Tim

e (m

in)

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200

UD

CSM

Roving

Chopped

Stacked

UD

CSM

Roving

Chopped

Stacked

0

40

80

120

160

200

240

0 200 400 600 800 1000 1200

UD

CSM

Roving

Chopped

Stacked

0

20

40

60

80

100

120

140

160

100 300 500 700 900 1100

25

10

0

(a) (b)

(d)(c)

Fig. 5. Predicted resin wetting time of the rehabilitation process with respect to environmental temperature, types of fiber perform, and diameters of

conduits: (a) at 25 �C; (b) at 10 �C; (c) at 5 �C; (d) the resin wetting time of the preform of satin woven mat with respect to environmental temperature

and diameters of conduits.


fiber volume fraction is 25.8% as the temperature and

applied pressure are changed. By using these analysis

results, diagrams shown in Fig. 6 were prepared to

control the process variables.

3. On-line process monitoring with dielectrometry

The resin wetting and curing are two major process of

RTM because the product quality is dependent on thedegree of wetting [8] and its cure process [12]. Therefore,

it is necessary to monitor the wetting and curing status of

resin continuously during the rehabilitation process in

order to yield a satisfactory reinforcing quality in the

shortest process time. Since the Pro-Sol films used in this

study are not only opaque but also placed underground,

it is difficult to check them with the naked eye. For this

reason, dielectrometry and dielectric sensors were em-ployed to on-line monitor the resin flow and cure status

in this work, from which the applied pressure and tem-

perature were adjusted. The dielectrometry has been

known as a promising technique to in situ monitor the

entire cure process of thermosetting resin continuously.

When an alternating electric field is applied to the two

electrodes of the dielectric sensor embedded in the

composite material as shown in Fig. 7(a), the dipole and

ions within the resin, which is a dielectric material, are

aligned following an applied alternating electric field. At

this time, the combination of two electrodes and poly-

meric resin can be modeled as a parallel equivalent circuit

composed of resistance Rm and capacitance Cm as shown

in Fig. 7(b). The mobility of dipoles and ions has closerelations with the cure state and the viscosity of resin

within the composite material as shown in Fig. 7(c), and

can be expressed by the dissipation factor D, which

represents the ratio of the energy loss by movements of

dipoles and ions to the supplied electric energy. The

dissipation factor for the equivalent circuit shown in Fig.

7(b) can be obtained as follows [12].

D ¼ IR � VmIC � Vm

��

�� ¼IRIC

��

�� ¼ZC

ZR

��

�� ¼1

x � Rm � Cm

ð5Þ

where

IR electric current in resistance (A)

IC electric current in capacitance (A)

Vm voltage applied to equivalent circuit (V)

ZR equivalent impedance of resistance ðXÞZC equivalent impedance of capacitance ðXÞ

0

1

2

3

4

5

Tim

e(m

in)

10

20

30Temperature (degree C)

100

120

140

160

180

200

Pressure (kPa)

4.404.133.873.613.353.082.822.562.302.031.771.511.250.980.72

Temperature (degree C)

Pre

ssur

e(k

Pa)

10 20 30100

110

120

130

140

150

160

170

180

190

200

0

5

10

15

20

Tim

e(m

in)

10

20


100

120

140

160

180

200

Pressure (kPa)

18.0416.9615.8914.8113.7312.6511.5710.50

9.428.347.266.185.104.032.95


Pre

ssur

e(k

Pa)

10 20 30100

110

120

130

140

150

160

170

180

190

200

0

20

40

60

Tim

e(m

in)

10

20


100

120

140

160

180

200

Pressure (kPa)

51.8748.7745.6642.5539.4536.3433.2330.1227.0223.9120.8017.7014.5911.48

8.38


Pre

ssur

e(k

Pa)

10 20 30100

110

120

130

140

150

160

170

180

190

200

(a)

(b)

(c)

Fig. 6. Variation of resin wetting time with respect to environmental temperature and applied net pressure for the specified inner diameter of

conduits: (a) diameter of 0.3 m; (b) diameter of 0.6 m; (c) diameter of 1.0 m.


x frequency of an applied alternating electric field

(Hz; 1 kHz was used in this study)

The principle of dielectrometry is to monitor the cure

state by measuring this dissipation factor [1,12]. Since

the value of the dissipation factor changes when the

dielectric sensor contacts the resin, it is possible tomonitor the resin wetting as well as the cure status by

measuring the dissipation factor continuously.

In this work, a 1-channel dielectrometer and an

interdigital capacitance (IDC) dielectric sensor were

used to monitor the resin wetting and curing during the

rehabilitation process. In order to check the availability

of the proposed monitoring method, the resin wetting

experiments were performed in the transparent acryl

pipe of 180 mm diameter (the smallest inner diameterof the conduit in this study) with 1 m length and 5 mm

thickness using the reinforcing element in which one

Polymer Composite

Electrode

AC Electric Field

CmRm

ICIR

Vm

Cure state

Curing

Uncured

Fully cured

Behavior ofDipoles & Ions

Fully oriented

Random

Partially oriented

+ Dipoles - + Ions -

(a) (b)

(c)

Fig. 7. Principle and measuring method of cure status of composite with dielectric sensor: (a) measuring method; (b) equivalent circuit of dielectric

sensor and composite; (c) behavior of dipoles and ions w.r.t. the cure status of composite materials.


dielectric sensor was embedded in the middle upper

portion of fiber preform as shown in Fig. 8. In order to

observe the flow of resin with the naked eye, trans-

parent PVC film of 1.0 mm thickness was used for theouter protection skin. After manufacturing the rein-

forcing element in the manner shown in Fig. 2, it was

placed in the acryl pipe, and the two covers were at-

tached at the both ends of the reinforcing element.

Then the resin wetting experiment was performed fol-

Fig. 8. Reinforcing element with a breathing tube and a dielec

lowing the general rehabilitation process of Fig. 1.

From the experiments, it was found that the proposed

monitoring method detected well the resin wetting as

well as curing as shown in Fig. 9, in which the increaseof measured dissipation factors at the initial stage oc-

curred when the dielectric sensor contacted the resin.

The resin wetting time measured by the dielectric sensor

was about 7.5 min and it took about 3 h to complete

the resin curing at 50 �C.

tric sensor whose dimension is 9 mm· 250 mm· 168 lm.

0.4

0.6

0.8

1

0 100 200 300

Dis

sipa

tion

Fact

orD

issi

patio

n Fa

ctor

Time (min)

Time (min)

0.4

0.5

0.6

0.7

0 5 10 15 20

Curing Completed

Wetting Completed

Fig. 9. Dissipation factor of unsaturated polyester resin during the

rehabilitation process measured by the dielectrometer and dielectric

sensor.


4. Rehabilitation experiments with developed process

Using the above results and the developed process,

several real rehabilitation experiments, whose schematicdiagram is shown in Fig. 10, were performed at the

concrete pipe (reinforced spun concrete pipe) used for

Fig. 10. Schematic diagram of the repairing-reinforcing experiment with g

electrometry.

underground applications. The diameter and length of

the three concrete pipes were 300 mm and 10 m, 600 mm

and 10 m, 600 mm and 30 m, respectively. After the

reinforcing elements were prepared using the stackedmats and Pro-Sol films in the manner of Fig. 2 consid-

ering the dimensions of the target concrete pipes, a

porous breathing tube of 8 mm inside diameter was in-

serted into the upper position of the reinforcing element

as shown in Fig. 11 [7]. The breathing tube was con-

nected to the air vent of the reinforcing element to apply

a vacuum, which enhanced the efficiency of void re-

moval much. For the process monitoring, one dielectricsensor was embedded in the fiber preform and connected

to the dielectrometer whose sensor line was connected

through the breathing tube. Since the length of the pipes

is somewhat long, the method for supplying the resin

uniformly along the reinforcing element was needed.

For this end, two resin injection pipes with small pin-

holes were installed along the reinforcing element as

shown in Fig. 11, which was connected to RTM ma-chines to inject unsaturated polyester resin into the fiber

preform. By regulating the distance between pinholes,

the resin injection amount per unit length was made

uniform [7]. Since the breathing tube and the resin

injection tube were installed in the reinforcing element in

the manner as shown in Fig. 12, they could be easily

removed by pulling out one end of them before curing of

resin.After installation of the reinforcing element, its both

ends were closed using specially designed cover assembly

that are composed of a steel ring, a steel disk, and an

acryl disk as shown in Fig. 13. Fig. 14 shows the cross

sectional view of the reinforcing element after installa-

tion. The cover assembly is equipped with the pressure

gauge for checking an internal air pressure as well as the

lass fiber fabric and unsaturated polyester resin using RTM and di-

Fig. 11. Configuration of the reinforcing element containing the porous breathing tube and the resin injection pipes with pinholes.

Fig. 12. Pipe removal after the completion of resin wetting.


air inlet for pressurizing the internal cavity. After

attaching the cover assembly to the reinforcing element,

the multi-joint clamp made of chain-link as shown in

Fig. 15 was tightened using the steel ring with inner and

outer protection films by fitting into the groove of steel

ring. The groove on the circumferential face of the steel

ring increased the load transfer capability and sealing

effect of the resin, and the transparent acryl disk offeredthe visibility of inside status. In order to seal the gap

between the inner and outer protection skins and the

internal cavity of reinforcing element, thermosetting

adhesive, DP460 of 3M whose material properties are

given in Table 6, was pasted on the interface of two

skins, and a circumferential groove of steel ring. The

resin reservoir shown in Fig. 10, which was placed be-

tween the reinforcing element and a vacuum pump,prevents the expelled resin from going into a vacuum

pump directly and separates voids from the resin

through breathing tube. Since the amount of the bleeded

resin could be measured by the gauge on the resin res-

ervoir, it was possible to predict a fiber volume fraction

of the cured reinforcing element through a simple cal-

culation. After removing the wrinkles and twists in the

reinforcing element by expanding the inner protectionfilm, a predetermined amount of unsaturated polyester

resin (130% of the required amount) was injected into

the fiber perform with the RTM machine, which mixed

the resin with cure catalyst (MEKPO; methyl-ethyl-ke-

tone-peroxide) at predetermined rate. Then the resin

wetting and curing were followed in sequential order to

complete the entire rehabilitation process. The RTMmachines (high volume H.I.S.e hand-held casting unit;

Venus-Gusmer, Washington, USA) were actuated by

high pneumatic pressure and their detailed specifications

are given in Table 7. Test conditions and specifications

of each experiment are depicted in Table 8. Since the

long pipes with large diameters were tested, fairly long

times were required to finish the resin wetting. There-

fore, a little amount of the cure retardant for slowingdown the cure reaction was used in order to obtain the

sufficient process time considering the gelation time of

the unsaturated polyester resin (PC670) listed in Table 9.

From the experiments, it was found that large three-

dimensional composite structures could be constructed

inside of the large concrete pipes without any dry region

as shown in Fig. 16. Also, it was found that the void

removal method through the porous breathing tubesand the resin injection pipes with small pinholes were

very effective to expel the micro-void.

5. Comparison of processing cost and time

There are about 80 sorts of trenchless technologies so

far developed all over the world and largely classified

into four kinds: slip-lining, cured-in-place pipes (CIPP)

lining, close-fit lining, and spirally wound pipes lining

[11]. Among them, CIPP has been very successful

through the achievements of Insituform (Insituform�

Technologies, Ltd., Chesterfield, United Kingdom) andPaltem/Phoenix, and has attracted many modifications

and improvement. Since the CIPP products probably

take 35–40% of the global sewer market, the trenchless

rehabilitation method developed in this study was

compared with the two conventional CIPP lining pro-

cesses (one was Paltem/Phoenix lining process and the

other was D-Ins� lining process) with respect to cost,

Fig. 13. Photographs of cover for sealing the both ends of the reinforcing element for 600 mm pipe: (a) steel ring, (b) assembly of steel ring and steel

disk, (c) assembly of cover (steel ring+ steel disk+acryl disk) and multi-joint clamp.

Fig. 14. Cross sectional view of the reinforcing element and concrete pipes after installation.


time, and reinforcing effects. The Paltem/Phoenix liningprocess is a cured-in-place lining process that was

developed by Tokyo Gas (Tokyo Gas Co., Ltd., Tokyo,

Japan) and Ashimori Industry (Ashimori Industry Co.,

Ltd., Osaka, Japan) for lining gas pipes, in which a

polyethylene resin coated woven hose is bonded with

epoxy resin to the pipe to be repaired in inversed shape

by water or air pressure [13]. From the beginning, this

process was named Paltem (pipeline automatic liningsystem), and then became known as Phoenix in Europe

because Osaka Bosui (Osaka Bosui Construction, Co.,

Osaka, Japan) participated in this development shared it

with a French company Le Jointe Interne [13].

The D-Ins� lining process (Samil Setec, Seoul, Korea)

is a modified Insituform process, in which the polyester

fabric felts impregnated with polyester, or vinylester

Table 7

Specifications of the RTMmachine (Venus-Gusmer, Inc., Washington,

USA)

Property Value

Mass injection output 2.3–13.6 kg/min.

Injection capacity 0.408 kg/stroke

Air consumption 0.28 m3/min.

Catalyst mixing ratio (volume) 0.5–3.0%

Catalyst jug 2 gallon (7.57 l)

Fig. 15. Multi-joint clamp: (a) photograph of multi-joint clamp with roller chain links, (b) assembly of cover and clamp, (c) detailed drawing of the

multi-joint clamp, (d) roller chain rink and detailed draft of joining part.

Table 6

Material properties of the cured epoxy adhesive (3M DP460)

Properties Value

CTE (10�6 m/m �C) 59.0 (below Tg)159.0 (above Tg)

Poisson ratio 0.4

Density (kg/m3) 1100

Elastic modulus (GPa) 2.7

Tensile strength (MPa) 37

Tg: glass transition temperature.


resin are used and also installed in the conduit in the

inversed shape by water pressure.

The mechanical properties of reinforcing element

developed in this study (RTM liner) were tested and

compared with that of Paltem-SZ liner and D-Ins� liner,

and the production cost and processing time were com-

pared only with those of D-Ins� process. For the con-

venience, the reinforcing element of this study was named

‘‘RTM liner.’’ The tensile properties of the RTM linerwere measured with INSTRON 4206 according to

ASTM D3039 and their flexural properties were mea-

sured with the same machine according to ASTM D790.

Tables 10 and 11 show the comparison results of mecha-

nical properties and the production cost and processing

time, respectively. As listed in Table 10, it has been found

that the RTM liner of this study has much superior

mechanical properties to those conventional liners forsewer rehabilitation. The comparison results of produc-

tion cost and processing time in Table 11 were carried out

on the assumption that the extra cost was same for both

cases and the processes were applied to repair the conduit

of 300 mm inner diameter. From the results, it was found

that the margin of repairing cost of developed process

was about 85 US $, and the margin of repairing time was

about 16 h. Comparing with the excavation technology,this newly developed process could cut cost of excavation

technology by more than 40%. If the diameter of target

conduits is increased, the effect of cost saving will be

further enhanced.

Table 9

Gelation time of the unsaturated polyester resin (PC670) with respect to the amount of cure catalyst (MEKPO)

Temperature (�C) MEKPO (methyl-ethyl-ketone-peroxide) Remarks

0.6 0.8 1.0 1.26 Unit: %

20 46 41 32 28.5 Unit: min

25 31 25 21.5 17

30 18 14.5 12 10.5

36 11.5 9.5 8.5 8

Fig. 16. Real rehabilitation experiment using concrete pipes of 600 mm inner diameter: (a) initial setup of the equipments; (b) resin injection (RTM

machine#1) and application of vacuum; (c) resin injection (RTM machine#2); (d) photograph of the real construction experiment; (e) photograph of

the repaired concrete pipes; (f) inner surface of repaired concrete pipes.

Table 8

Test conditions of each rehabilitation experiment

Descriptions (diameter· length) 300· 1000 (mm) 600· 1000 (mm) 600· 3000 (mm)

Preform [UD1/CSM3/UD1] [UD2/CSM4/UD2] [UD2/CSM4/UD2]

Injected resin (kg) 44 120 400

Resin:MEKPO 100:1 (volume) 100:0.75 100:0.75

Retardant No No 15000 ppm

Environmental temperature (�C) 5 14 16

Viscosity of resin (Pa s) 0.8–1.0 0.4–0.5 0.3–0.4

Inner pressure (kPa) 20–30 20–30 20–30



Table 10

Comparison of the mechanical properties between the reinforcing element developed in this study and conventional liners for sewer rehabilitation

RTM liner Paltem-SZa D-Ins�b

Resin system Unsaturated polyester Unsaturated polyester Unsaturated polyester

Felt E-glass fiber Chopped strand glass

fiber+ polyester fabric

Unwoven polyester fabric

Tensile strength 340 60 23

Flexural strength 420 110 53

Flexural modulus 15.8 6.0 1.9

aMaterial data from Ashimori Industry, Japan.bMaterial data from Samil Setec, Korea.

Table 11

Comparison of repairing cost with CIPP (cured-in-place pipes) lining process

Cost RTM method (US $/m) D-Ins� lining process (US $/m)

Material 38 72

Process 95 146

Extra Same ðaÞ Same ðaÞTotal 133þ a 218þ aMargin 85

• Material cost: reinforcing element, resin, adhesive, tubes, etc.

• Extra cost: inspection, cleaning, perforation of junction, etc.

• Repairing cost of excavation technology: 380 US $/m.

• 1 US$ ¼ 1300 won.


6. Design criteria of the reinforcing element

As mentioned previously, the RTM liner of this study

has much higher mechanical properties than those of

conventional liners, which might be somewhat over-designed, consequently there is a possibility of further

material cost saving. Therefore, it is necessary to

develop some design criteria of the RTM liner which can

guarantee sufficient material properties with minimum

cost. On the assumption that the RTM liner is com-

posed of unidirectional mats (T800-E06) and continuous

strand mats (M8600) only, the design criteria of RTM

liner was suggested in order to reduce the material costwith sufficient reinforcing effect.

The American Society for Testing Materials (ASTM)

has produced the specifications drafted by committees

of engineers drawn from users and suppliers. These

specifications are issued provisionally and revised in

accordance with the comments of the industry received

by ASTM. In 1989 ASTM produced F1216-89 which

was subsequently revised in 1991 and 1993. The currentissue ASTM F1216-93 covers the design and installation

issues and performance of installed liners [13]. Under-

ground pipes are largely classified into two kinds by

their application: gravity pipes and pressure pipes, and

each of them are divided into partially deteriorated pipes

and fully deteriorated pipes according to their damaged

condition. ASTM F1216 suggests some design consid-

erations to determine the thickness of CIPP (liner) forrepairing underground pipes case-by-case (totally four

cases). In this study, the RTM liner for rehabilitating the

fully deteriorated gravity pipes of 300 mm inner diam-

eter was considered and its design criteria were sug-

gested. Since a fully deteriorated pipe is structurally

unsound and cannot support soil and outside load, the

RTM liner for this case should be designed to supporthydraulic, soil and outside loads. ASTM F1216 suggests

that the following equation should be used to calculate

the minimum thickness of CIPP liner for repairing the

fully deteriorated gravity pipes [14].

E

12ðD=tÞ3P 0:00064 ðSI unitsÞ ð6Þ

where E is the initial modulus of elasticity (MPa), D is

the mean inside diameter of original pipe (mm) and t isthe thickness of CIPP liner (mm).

Since the RTM liner is basically the combination ofthe unidirectional mat and continuous strand mat, its

total thickness is the summation of the thickness of used

mats. However, since the thicknesses of fiber mats be-

fore resin wetting are different from those after curing

followed by resin wetting, their effective thicknesses (the

thickness that each fiber mat occupies within the cured

composite made of those mats) after curing should be

known to design the liner using Eq. (6). The elasticmodulus of the arbitrarily stacked composite made of

unidirectional mats and continuous strand mats can be

calculated through the rule of mixture (ROM) if their

effective thicknesses are known, because the fiber vol-

ume fractions of each layer can be determined from their

effective thicknesses. To obtain the effective thickness of

each fiber mat, the following procedure was devised.

y = 8.9755E-09x - 5.3147E-09

5.0E-10

1.5E-09

2.5E-09

3.5E-09

4.5E-09

0.6 0.7 0.8 0.9 1.0 1.1 1.2

Per

mea

bilit

y (

m2 )

Per

mea

bilit

y (

m2 )

e

ex

+=

1

3

y = 1.4203E-11x + 4.9486E-10

5.0E-10

1.5E-09

2.5E-09

20 40 60 80 100

e

ex

+=

1

3

(a)

(b)

Fig. 17. Plot of the permeabilities versus x ¼ e3=ð1þ eÞ: (a) unidirec-tional mat (T800-E06); (b) continuous strand mat (M8600).


The most well-known permeability modeling for

Newtonian flow through various porous media is the

Kozeny–Carman equation which considers the porous

medium as a bundle of capillaries, which was knownthat this equation could predict the permeability in the

fiber direction pretty well [15]. Generally, the Kozeny–

Carman equation is

Ki ¼r2f4Ci

ð1� vfÞ3

v2f¼ r2f

4Ci

e3

1þ eð7Þ

where Ki is the permeability in the i-directionði ¼ x; y; zÞ, rf is the fiber radius, Ci is the Kozeny con-

stant to be determined experimentally, vf is the fibervolume fraction, and e is the void ratio which is ex-

pressed by the following equation.

e ¼ 1

vf� 1 ð8Þ

Since the permeability is proportional to the function of

void ratio, it was possible to derive the empirical relation

between the permeability and void ratio using the per-meabilities and the fiber volume fractions of unidirec-

tional mats and continuous strand mats listed in Table

2. From the plot of the permeabilities versus x ¼ e3=ð1þ eÞ shown in Fig. 17, two experimental equations for

K (one is for unidirectional mat and the other is for

continuous strand mat) were obtained as follows.

KUD ¼ 8:9755� 10�9x� 5:3147� 10�9

KCST ¼ 1:4203� 10�11xþ 4:9486� 10�10ð9Þ

Since the ply weight of each glass fiber mat and the

density of E-glass fiber are known, the fiber volume

fractions of the unidirectional mat and continuous

strand mat with an arbitrary thickness can be calculated.

Then the permeabilities of each fiber mat at the specifiedthickness can be determined from Eq. (9).

In order to determine the effective thickness of two

fiber mats, the measured permeability of the stacked mat

listed in Table 2 and the concept of equivalent perme-

ability were used. When the permeability for flow in the

horizontal direction changes from layer to layer as shown

in Fig. 18, which is similar to the flow in the stacked mat,

the equivalent permeability of n layers can be formulatedas follows [16].

Keq ¼1

h

Xn

i¼1

Kihi ð10Þ

where, Keq and h are the equivalent permeability andtotal thickness of n layers and Ki and hi are the perme-

ability and thickness of the individual layers, respec-

tively. It was known that Eq. (10) agrees pretty well to

the experimental result irrespective of staking sequence

when the total thickness of preform is relatively thin and

the through-thickness flow is negligible [6]. If the per-

meabilities of the individual layer are known, the

equivalent permeability of the layered preform can be

determined from Eq. (10). After transforming the vari-

able x in Eq. (9) into the function of thickness of each

mat because the void ratio can be expressed by means of

the effective thickness, it can be combined with the Eq.

(10). Since the permeability ðKeqÞ and the total thickness

ðhÞ of stacked mat are known, the effective thickness of

unidirectional mat and continuous strand mat can bedetermined by solving Eqs. (9) and (10) simultaneously.

From the calculation, it was found that 1 ply of the

unidirectional mat (T800-E06) and the continuous

strand mat (M8600) have the effective thickness of 0.868

and 1.132 mm, respectively.

In order to determine the thickness of RTM liner

using the Eq. (6), its modulus should be known. Since

the cured RTM liner is the mixture of unidirectionalmats, continuous strand mats and polyester resin, its

equivalent modulus can be formulated by employing the

rule of mixture (ROM) as following.

E ¼ vf ;UDEf ;UD þ vf;CSTEf ;CST þ vmEm ð11Þ

where, vf ;UD and Ef;UD are the fiber volume fraction and

modulus of unidirectional mats, respectively, vf ;CST and

Ef ;CST are those of continuous strand mats, respectively,

and vm and Em are those of matrix (polyester resin),

respectively.

Fig. 18. Horizontal resin flow through the porous media composed of n layers with different permeabilities.


From the material properties given in Tables 1 and 3,

Ef ;UD (¼ 67 GPa) and Ef ;CST (¼ 30 GPa) were obtained

through ROM. Since the effective thickness of each mat

is known, the equivalent modulus of RTM liner com-posed of those mats can be determined if the number of

each mat is specified. Using Eqs. (6) and (10) and the

previous result, other stacking sequences for RTM liner

were suggested. When one ply of the unidirectional mat

and two plies of the continuous strand mats were se-

lected, vf;UD and vf;CST are 0.107 and 0.116, respectively.

This gives the equivalent modulus of 14 GPa and

E=12ðD=tÞ3 of 0.001341, which is two times larger thanthe limit value of 0.00064. Another solution is to use

three plies of the continuous strand mats. This gives the

equivalent modulus of 8.2 GPa and E=12ðD=tÞ3 of

0.000997, which is also larger than the limit value. Since

the price of unidirectional mat ($2.4/m2) is about 1.5

times more expensive than that of continuous strand

mat, it is evident that the newly suggested stacking se-

quences are more cost effective than the original one.Because large amount of fiber mats are required to re-

pair huge and long underground conduits, the effect of

cost saving will be large.

Since the design criteria of RTM liner suggested in

this study uses just two materials, such as unidirectional

mats and continuous strand mats, it will be possible to

extend the range of design parameter (wide variance of

materials) by performing the previous design procedurewith more diverse materials, which will lead to the fur-

ther enhanced cost saving.

7. Conclusions

In this study, a new trenchless rehabilitation process

of underground pipes, which not only overcomes the

problems of former trenchless technologies, but also is

adequate to the situation of high traffic road, has been

tried with E-glass fiber fabric and unsaturated polyester

resin by vacuum assisted resin transfer molding (RTM).

E-glass fiber reinforced composites were used for rein-

forcing damaged underground pipes and the RTM

technology was modified for fabricating large under-

ground composite structures. Also the reinforcing ele-ment for repairing the interior of damaged underground

pipes has been developed through various material tests

and experiments. For the reliable rehabilitation process,

the glass fiber preform was covered with tarpaulin films

that worked as a flexible mold and protection skins and

a porous breathing tube was used to remove the volatile

and micro void within the reinforcing element. After

actual repairing experiments have been performed atthe real concrete pipes, the efficiency of the developed

process was evaluated and compared with those of the

conventional trenchless technology. From the compar-

ison of processing cost and processing time, it was

found that the developed process was very effective in

view of cost and time. Finally, the design criteria of the

reinforcing element which assure the sufficient rein-

forcing effect with minimum material cost have beensuggested.

Acknowledgements

This work was supported financially by the Korean

Government under NRL (National Research Labora-

tory) projects and, in part, by BK 21 Project. The au-thors would like to thank to their financial support.

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RTM Underground Pipes